MATTER Deliverable D3.4 v1 final · Document type Deliverable Work Package WP 3.2 Document number D3.4 – Revision 0 Date of completion 17/07/2014 Level of Confidentiality PU PP

MATTER

MATERIALS TESTING AND RULES – Grant agreement for collaborative Project

Co-funded by the European Commission under the Euratom Research and Training Programme on Nuclear Energy

within the Seventh Framework Programme

Grant Agreement no. 269706 Start date: 01/01/2011 Duration: 48 Months

Guidelines for Corrosion Testing

in Liquid Metals (Pb, LBE)

Deliverable D3.4

Authors: Carsten Schroer (KIT) Contributors: Approval:

MATTER – Deliverable D3.4 EURATOM FP7 Grant Agreement no.269706

III

MATTER project

EC Scientific Officer: Mykola Džubinský

Document title Guidelines for Corrosion Testing in Liquid Metals (Pb, LBE) Author(s)

Carsten Schroer (KIT)

Number of pages 42 Document type Deliverable Work Package WP 3.2 Document number D3.4 – Revision 0 Date of completion 17/07/2014

Level of Confidentiality PU PP RE CO

Document Version Final Summary

The guidelines for corrosion testing in liquid Pb or LBE presented and discussed are mainly based upon the long lasting experience of the partners of Task 3.2 of the MATTER project, supplemented by knowledge retrieved from the technical literature. They include the characterisation and monitoring of testing conditions, examples of experimental devices and testing facilities, as well as post‐test examination and quantification of corrosion damage. Results of the ongoing round robin on corrosion testing cannot yet be presented, but will be reported and discussed in form of a supplement to D3.4 by the end of the MATTER project.

Revisions Rev. Date Short description Main author (WP Leader) DP Leader

01

02


IV

Distribution list

Name Organisation Comments Džubinský, Mykola EC Scientific Officer Agostini, Pietro ENEA Project Coordinator Utili, Marco ENEA Scientific Responsible

Domain Leader – 4 Workpackage Leader – 15

Gavrilov, Serguei SCK CEN Domain Leader – 1

Aiello, Giacomo CEA Domain Leader – 2

Jürgen Konys KIT Domain Leader – 3 Workpackage Leader – 8

Gelineau, Odile AREVA Workpackage Leader – 1

Yong, Dai PSI Workpackage Leader – 2

Lambrinou, Konstantza SCK CEN Workpackage Leader – 3

Ancelet, Olivier CEA Workpackage Leader – 4

Aktaa, Jarir KIT Workpackage Leader – 5, 7

Barbieri, Giuseppe ENEA Workpackage Leader – 6

Nilsson, Karl‐Fredrik JRC Workpackage Leader – 9

Decarlan, Yan CEA Workpackage Leader – 10

Maday, Francoise ENEA Workpackage Leader – 11

Malerba, Lorenzo SCK CEN Workpackage Leader – 12

Moreno, Anna ENEA Workpackage Leader – 13

Sarra, Simona ENEA Workpackage Leader – 14

Colombarini, Mara ENEA Assistant for the Project Organization


V

Table of contents

1 Introduction ........................................................................................... 1

2 Characterisation and monitoring of experimental conditions .............. 2

Sample material and specimens ............................................................................................ 2 2.1

Testing conditions .................................................................................................................. 4 2.2

3 Oxygen content ..................................................................................... 7

Sensors for oxygen measurement ......................................................................................... 8 3.1

Evaluation of sensor output ................................................................................................ 10 3.2

Oxygen solubility in Pb or LBE .............................................................................................. 13 3.3

Documentation and reporting of oxygen content ............................................................... 15 3.4

Control of oxygen content ................................................................................................... 17 3.5

4 Experimental devices and facilities ...................................................... 19

Mounting, loading and unloading of specimens ................................................................. 19 4.1

Static oxygen‐containing Pb or LBE ..................................................................................... 22 4.2

Flowing oxygen‐containing Pb or LBE .................................................................................. 23 4.3

5 Post‐test examination .......................................................................... 26

6 Quantitative analysis of corrosion ....................................................... 27

Modes of steel corrosion in oxygen‐containing Pb or LBE .................................................. 27 6.1

Gravimetric versus metallographic quantification .............................................................. 29 6.2

Metallographic method ....................................................................................................... 30 6.3

6.3.1 Pre‐test measurements ............................................................................................... 31 6.3.2 Post‐test measurements .............................................................................................. 33 6.3.3 Evaluation of measurements ....................................................................................... 37 6.3.4 Alternative determination of material recession ........................................................ 37

7 Conclusions .......................................................................................... 38


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1 Introduction

Fundamental knowledge about the deterioration of structural materials in the presence of liquid lead (Pb) or lead‐bismuth eutectic (LBE) is essential for the construction and safe operation of lead‐cooled reactors—pilot plants to be built in the near future as well as industrial‐scale reactors in the long term. Given that the structural materials are metallic, namely steels, elements of the solid metal will dissolve in the liquid metal. Depending on whether this mass transfer affects the metallic elements in proportion to their concentration in the solid material or is selective for specific constituents, the result is either a recession of the material surface or development of a near‐surface depletion zone. Both reduces the thickness of the cross section capable of bearing the mechanical load acting on the material, and may compromise other functions the material needs to fulfil as part of its service in the plant. Furthermore, a liquid metal in immediate contact may cause weakening of a solid metallic material beyond the thinning of the cross section that resulted from solution. A prominent example is the so‐called liquid‐metal embrittlement (LME) that occurs for specific combinations of liquid metal and solid metallic material, especially at low temperature and simultaneous action of mechanic stress in the solid material. The latter is not primarily subject of the guidelines for plain corrosion testing introduced and discussed in the following, but some of the recommended practices are principally applicable to mechanical testing in liquid‐metal environment as well. In flowing liquid metal, corrosion may be superimposed by erosion, depending on the local flow pattern. As important as understanding and quantifying the corrosion susceptibility of candidate materials is testing potential methods of minimising otherwise prohibitive degradation caused by interactions with the environment. This may be a modification of the material surface, e.g., a coating with superior corrosion resistance, or deliberate addition of substances to the corrosive medium that inhibit the detrimental interactions with the material. At high temperature, the inhibition results from the formation of new compounds with the material constituents rather than plain adsorption onto the surface, which, in principle, is another form of degradation or corrosion. But, if successful, the provoked corrosion mode is milder, less severe than in the absence of the added inhibitor. Examples of inhibitors tried to minimise the solution of steels in liquid Pb [1] or bismuth (Bi) [1,2] are zirconium (Zr) and titanium (Ti). The idea is to form a thin protective layer that consists of nitrides or carbides of the inhibitors on the steel surface. The nitrogen (N) and carbon (C) in the steel serves as the source of these non‐metals required for scale formation. Traces of oxygen (O) solved in the liquid metal would result in oxidation of the inhibiting metals, which is prevented by further addition of magnesium (Mg). As main steel elements like iron (Fe) and chromium (Cr) are less noble than Pb or Bi that is the other constituent in LBE, also oxygen may be used as an inhibitor. In this case, the inhibition of steel solution results from the formation of surface oxides of the steel elements. The oxygen chemical potential (concentration) has to be maintained at an optimum level that is sufficiently high for establishing a covering oxide scale, but low enough so that the oxidation rate is not too high and, especially, precipitation of Pb‐ or Bi‐oxides does not occur at any temperature within the system. Irrespective of whether or not oxygen is deliberately added to favour oxidation over solution, the oxygen content of Pb or LBE is an important parameter of testing metallic materials in these liquid metals,


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because precipitation of oxides of once solved metals counteracts saturation with these metals, promoting their continuous solution. The guidelines for corrosion testing in liquid Pb or LBE presented and discussed in the following are mainly based upon the long lasting experience of the partners of Task 3.2 of the MATTER project, supplemented by knowledge retrieved from the technical literature. They include the characterisation and monitoring of testing conditions, examples of experimental devices and testing facilities, as well as post‐test examination and quantification of corrosion damage.

2 Characterisation and monitoring of experimental conditions

As for any material test, the actual (not only nominal!) experimental conditions with respect to both characteristics of the sample material and the performed test need to be assessed and reported as detailed as possible. It is clear that unavailability of appropriate analytical instruments or the effort in terms of time and money requires making compromises. However, especially corrosion tests may take unexpected turns even for the experienced investigator, resulting in observations not anticipated, indicating potential factors of influence not considered before the experiment was actually performed. Some data proving valuable in hindsight will then not be accessible anymore.

Sample material and specimens 2.1

The sample material is fully characterised by the chemical composition, microstructure as well as residual stress resulting from the treatment during the course of the production process. While the overall chemical composition determines in which relative amounts material elements are principally available for participating in corrosion processes, the microstructure and residual stress influence the supply to the material surface, where corrosion takes place. At elevated temperature, the microstructure and stress state may change with time, concurrently with the progress of corrosion. It should be noted that corrosion possibly is sensitive to variations in element concentration within the tolerance of the material specification, especially if the underlying elementary processes additionally depend on other material characteristics like microstructure. The absolute amounts of material elements available are determined by the chemical composition along with the geometric dimensions or mass of the specimen employed in the test. Corrosion affects the material from the surface, so that the surface‐to‐volume ratio or the smallest geometric dimension of the specimen is of some importance for the exhaustion of the material elements participating in corrosion processes, especially if scale formation (e.g., oxide scale formation) dominates [3–5]. Different performance of thin and thick metallic specimens in scaling processes may not only result from the different boundary conditions with respect to the depletion of material elements, but also from the higher capability of thin specimens to deform, minimising mechanical stress that builds up in the growing scale [6]. In practice, the geometry of


3

the specimens cannot always be freely chosen, but depends on the form of the sample material (sheet, plate, rod, tube … of given thickness or diameter) available for testing as well as limits imposed by the capabilities to fix specimens in or introduce into the testing device or facility. Typical geometries of specimens employed in high‐temperature corrosion tests are rectangular coupons, circular discs, cylindrical rods as well as ring‐ and arc‐shaped specimens cut from tubes and pipes, respectively [7]. Sharp edges generally are sites of preferential corrosion. The curvature of the exposed surface may play a role when scale formation is vital. Similar to deformation and residual stress in the volume resulting from the production route of the sample material, the surface finish of specimens by mechanical milling, turning, grinding or polishing affects the material state in a near‐surface zone (to minimum extent in the case of polishing). The final cleaning, especially degreasing, of the specimen surface is usually done as preferred in the respective laboratories, mainly ultrasonically in organic solvents like acetone or isopropyl alcohol. Subsequent hot‐washing and vapour degreasing in purified (pro analysi) isopropyl alcohol have been recommended in guidelines for high‐temperature corrosion testing [7]. Near‐surface deformation and detergent molecules adhering onto the surface may have an impact especially on the initial stage of corrosion processes. The progress of corrosion in following stages naturally depends on the outcome of the initial stage. The same is of course also true with respect to the practically inevitable contamination, e.g., formation of adsorption layers on a freshly prepared metal surface, during storage and handling of the specimen between specimen preparation and starting the experiment. For improved inter‐comparability of experimental results from different laboratories, the following characteristics of the employed specimens should be assessed and documented:

(I) Material Analysed chemical composition. Final thermal or mechanical treatment. Microstructure (at least qualitatively with help of micrographs after etching).

(II) Specimen geometry Geometric dimensions. Resulting volume and mass. Exposed surface area.

(III) Surface finish Final treatment. Peak‐to‐trough roughness (Rt) and centre line average roughness (Ra) [7].

(IV) Cleaning procedure (V) Storage of specimens after preparation

Some recommendations are:

(a) Choose specimen geometry with mainly flat surfaces, a minimum of corners and length of edges per unit surface area exposed (e.g., discs [7]).

(b) Smallest geometric dimension minimum 1–2 mm. (c) Provide a surface area between 4–6 cm² [7] (may be understood as minimum). (d) Round off corners and edges slightly [7].


4

There is no systematic investigation of thin‐specimen effects for corrosion (oxidation) in Pb or LBE available so far. Indications of an effect have been found primarily for oxidation at 900°C [6] or higher [3,6]. The 1–2 mm minimum thickness stated as a recommendation is an estimate at or above which thin‐specimen effects seem unlikely to occur. This means that for significantly thinner specimens, and also for long‐term exposure, exhaustion of material elements or deformation of the specimen might have to be taken into account if scaling was the dominating form of corrosion. For calculating the exposed specimen surface, a determination of the geometric dimensions using a vernier calliper (sliding gauge) or hand‐held micrometer is, in general, sufficiently accurate. If the change of a characteristic length is used for quantification of corrosion processes, more precise measurements may be required (see Section 6.3).

Testing conditions 2.2

In view of the corrosion phenomena that can generally be expected for (metallic) materials exposed to Pb or LBE, the following factors of influence are identified as important in addition to the specific combination of liquid metal and material under consideration:

(I) Temperature Thermo‐physical and chemical properties of the liquid metal and material (elements), namely solubility limits or oxide stability. Kinetics of corrosion processes and diffusion in the liquid metal.

(II) Liquid‐metal volume/mass or mass Capacity for solution of material elements. flow

(III) Flow velocity Mass transfer coefficients. Convective mass transport. Erosion.

(IV) Free liquid‐metal surface with Potential sink or source of non‐metals (oxygen). cover gas/gas composition Precipitation of solved elements by reaction with gas components to form solid compounds.

(V) Concentration of solved Driving force for solution. material elements

(VI) Concentration of solved oxygen Relative driving force for solution or oxidation of material elements. Correspondingly strong impact on observed corrosion mechanisms.

(VII) Liquid metal containment/ Source of contamination of the liquid metal. container material Potential sink for solved elements.

This list is possibly incomplete or some factors may be negligible in the particular case, but it gives an idea of the potential complexity of materials testing in liquid Pb or LBE. The reliable evaluation of experimental results and estimation of the ranges of validity require that potential factors of influence that cannot be properly controlled are at least measured and monitored during the experiment. A rough quantitative assessment may already be helpful where specific constraints prohibit precise measurements.


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The sensitivity of corrosion processes to variations around the nominal temperature depends on the thermal activation (activation energy) of the rate‐determining elementary step in the corrosion mechanism. Preferably, the temperature adjacent to specimens is continuously monitored during the course of the experiment, e.g., in the centre of a single‐specimen arrangement and, additionally, in the marginal positions when a number of specimens are exposed simultaneously. Indicative allowable temperature variations are ±3°C at T < 600°C or ±4°C at 600°C <T < 800°C as for mechanical tests at elevated temperature according to DIN EN 10 002‐5: 1991 [8]. Alternatively, the temperature profile inside the testing device or facility, at given temperature indicated by the sensor (thermocouple) used for temperature control may be determined. As the overall thermal mass is important for the temperature distribution, respective measurements are best performed with all the equipment required for the test and designated number of specimens installed. It is clear that the insulation of the device and position where temperature is controlled should not be changed in‐between determination of the temperature profile and conduction of the test. If saturation of the bulk of the liquid metal (global saturation) with solved material elements is approached during the course of the test, the corrosion rate may decrease or processes other than solution are promoted simply because of the limited volume of liquid metal available. In general, global saturation with a particular element, especially the major constituent of the investigated materials, is approached the earlier the larger the overall surface area of specimens tested simultaneously, or the smaller the liquid metal volume or volume flow provided for the corrosion test. Solution on the inner wall surface of the containment of the liquid metal, from the specimen support or other internal equipment of the testing device or facility may also

Table 1 Equations for calculating the saturation concentration of selected metals in liquid Pb or LBE

Solute Solvent Equation* Validity range Source

Fe Pb log cFe;s 1.8244860

T 600°C < T <1300°C

[9] See also [10].

Cr Pb log cCr;s 3.88 6949

T 900°C < T <1200°C [11]

Ni Pb log cNi;s 1.301381

T 370°C < T <800°C [11]

Fe LBE log cFe;s 2.0124382

T 550°C < T <779°C

Interpolated from data in [9].

Cr LBE log cCr;s –0.026949

T 400°C < T <550°C [12]

Ni LBE log cNi;s 1.701000

T

480°C < T <550°C Confirmed at 415°C < T <535°C

[12] [13]

* Saturation concentration cs in mass%. Temperature T in K.


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contribute to saturation with some elements or other, unless practically inert materials were chosen for these parts of the experimental set‐up. A persistent driving force for solution of material elements can be produced by introducing a temperature gradient in the system, as, e.g., implemented by the cold leg of liquid‐metal loops used for exposure to flowing liquid metal, where solved elements partially re‐deposit. An alternative sink for solved elements is the formation of solid oxides, either immediately at the material surface (scale formation) or in some distance, depending on the oxygen concentration profile in the liquid metal. The free liquid‐metal surface is a preferred site of solid‐oxide formation, if covered by oxygen‐containing gas. Similar behaviour may be expected from other non‐metals (N, C …) which, however, did not yet become especially apparent, probably because of comparatively low solubility in Pb or LBE and, in general, higher stability of solid oxides than nitrides or carbides. Furthermore, precipitation of an intermetallic phase from elements of different materials tested simultaneously or with participation of metals introduced via non‐inert experimental equipment has to be taken into account, as well as plain absorption (e.g., transfer of Ni from austenitic to ferritic steels after intermittent solution in the liquid metal). Equations recommended for estimating the saturation concentration, cs, of major steel elements in the absence of significant amounts of solved non‐metals or formation of intermetallic phases are presented in Table 1. The actual equilibrium concentration, ceq., that constitutes the upper limit of enrichment may be lower, depending on the concentration (activity) of the dissolving element in the particular material in contact with the liquid metal. Analogously, absorption is likely to start if the concentration of an element solved in the liquid metal passes a threshold that depends on the concentration of this element in the absorbing solid. When planning experiments in static liquid metal, the provided volume of liquid metal may be chosen in proportion to the mass of the elements with high solubility introduced in form of the tested materials, so that the maximum possible enrichment in the bulk of the liquid metal corresponds to only a small percentage of the saturation concentration. If the walls of the liquid‐metal containment or other components of the specific experimental set‐up are practically inert, only the ratio of exposed specimen surface to available liquid‐metal volume is decisive for the average enrichment of the material elements, in addition to the envisaged exposure time. If it turns out impossible to exclude global saturation with one or more material elements in advance, especially when the respective solubility is low or because of non‐inert equipment, the testing conditions with respect to limits imposed on solution processes may be characterised using reference experiments on the pure elements (metals). Alternatively, the chemical analysis of the liquid metal for solved material elements performed after the test, or estimating the amount of dissolution from observed section or mass loss of the specimens (in a practically inert set‐up) will give an indication for the level of saturation at the end of the experiment. However, it should be noted that the low solubility of some material elements may require non‐standard methods of chemical analysis, and particularly careful probing of the liquid‐metal volume. Experiments performed in flowing Pb or LBE, in a liquid‐metal loop (usually a steel construction of larger scale) with comparatively large surface of non‐inert walls of the tubing or other components of the facility, naturally are harder to characterise with respect to the concentration of solved metallic elements than tests in static liquid metal. The mass flow of circulating liquid metal and temperature gradient along the loop, or the free liquid‐metal surfaces covered by potentially


7

differently composed gas are factors that have to be considered, in addition to the size and chemical composition of solid surfaces exposed to the liquid metal. Because of the prominent role solved oxygen may play for the quantitative and also qualitative performance of metallic materials in liquid Pb or LBE, oxygen measurement and control, as well as the characterisation of the testing conditions with respect to solved oxygen are treated in a separate section.

3 Oxygen content

Oxygen solved in liquid Pb or LBE may actively participate in corrosion processes by entering the material (especially via grain boundaries, dislocations or other defects of the material microstructure) or formation of solid oxides. From a thermodynamic point of view, the first is likely to occur if the oxygen chemical potential, µO, or thermodynamic oxygen activity, aO, is higher in the liquid metal than in the material. The latter is possible only if aO passes a certain threshold above which solid oxides of the elements present in the investigated material (metallic alloy) become thermodynamically stable. For a particular solid binary oxide MexOy, the required aO is minimum at maximum activity of the involved metal, aMe, and increases with decreasing aMe as expressed by aMe

xaOy K (1)

(x, y and K denote the stoichiometry factors of the regarded binary oxide and constant solubility product of this oxide in the liquid metal, respectively). If oxygen is uniformly distributed in the liquid metal, at a concentration higher than corresponding to the effective threshold of aO, solid oxide is likely to form at the material/liquid‐metal interface where aMe is generally higher than in the bulk of the liquid metal. However, if an oxygen activity gradient exists, the local aO at this interface may be insufficient for solid oxide formation so that, given a significant solubility, the metal Me first dissolves in the liquid metal. The developing gradient of aMe in the liquid metal has opposite sign in comparison with the oxygen gradient. Hence, it is possible that Eq. (1) is fulfilled in some distance from the material/liquid‐metal interface, where solid MexOy then precipitates. Comparatively high mobility of Me in the liquid metal and an oxygen‐supplying cover gas make the free liquid‐metal surface a likely place for MexOy formation (less constraints on particle nucleation than in the liquid‐metal volume), especially when the liquid‐metal volume is small. The consumption of Me and oxygen in some distance from the material surface steadies the respective activity gradients and counteracts saturation of the liquid metal with Me. The net result is the promotion of the solution (removal) of Me from the material to which also the formation of ternary or higher oxides with participation of further material elements or constituents of the liquid metal may contribute. The deliberate addition of oxygen to Pb or LBE for the purpose of corrosion protection aims at sustaining an oxygen activity in the vicinity of the material that is sufficiently high for formation of a continuous scale of solid oxides on the material surface, minimising transfer of material elements to the liquid phase. But solid oxide precipitation at opposing gradients of metal and oxygen activity in the liquid metal may apply also for material


8

elements that still dissolve from the oxide scale, analogously to oxide‐scale fluxing in molten sulphate that was intensively studied in the past [14]. It should be emphasised that solid oxide formation on the material surface, at limited supply from the bulk of the liquid metal, may create the oxygen gradient that shifts the site of further oxide formation away from the material surface, and promotes solution of elements from the material, especially when the surface oxide does not (yet) form a continuous scale.

Sensors for oxygen measurement 3.1

Sensors developed for measuring oxygen in liquid Pb or LBE work according to electrochemical principles, making use of the difference in electric potential or voltage that establishes when different oxygen chemical potentials prevail at opposing surfaces of an oxygen‐ion conducting solid electrolyte (potentiometric sensor, Figure 1). The potentiometric measurement requires a

high‐impedance voltmeter (> 1 G) or a compensation technique that minimises the current I flowing in the electric circuit. For I = 0 and negligible electron transfer in the solid electrolyte (ie = 0), the indicated voltage U corresponds to the theoretical sensor output E* (zero‐current potential) that, however, may be superimposed by a thermoelectric voltage Uth, resulting from the use of different metals as electric leads [15]: U E* Uth (2) The oxygen chemical potential in the liquid metal µO;LM follows from the Nernst equation that, in the most general notation, reads as

E*μO;Ref μO;LM

2F

(3)

F denotes the Faraday constant. µO;Ref is the known oxygen chemical potential at the reference electrode of the sensor. In the presented form, Eq. (3) is valid for the reference electrode being connected to the high‐voltage input terminal of the voltmeter (Figure 1). Oxygen sensors that were specifically designed for use in liquid Pb or LBE are described in the technical literature [16–19]. The particular sensor consists of the reference electrode – either a metal/metal‐oxide couple or a gas electrode with a rigid wire serving as electric lead in both cases – that is encapsulated and in intimate electric contact with the solid electrolyte. The electric lead of the reference electrode and the metallic (steel) housing of the sensor that is joined to the electrolyte are electrically insulated from each other. The head of the sensor housing provides the terminals (plug) for transmission of the sensor output, i.e., the voltage that adjusts between the reference electrode and the sensor housing. Accordingly, the sensor housing has to be in electric contact with the liquid metal. The latter may be easily accomplished for a fully metallic experimental set‐up, but needs an auxiliary electrode when the liquid metal is, e.g., contained in a ceramic crucible. The thermoelectric voltage mentioned above depends on the materials used for the electric lead of the reference electrode (Lead A in Figure 1) and metallic container of the liquid metal or auxiliary electrode (Lead B), the temperature of the oxygen measurement (T1) and the


9

temperature T2. At temperature T2 and below, the electric lead on both sides of the electrolyte consists of the same material (Lead C). The electrochemical measurement of oxygen chemical potential can be regarded as highly accurate, and is a standard method of determining thermochemical data. However, it is recommended to test proper function of the sensor and interplay with the measuring instrument (voltmeter) used for indicating the sensor output. Detailed discussions of potential sources of error, required and proven accuracy of sensors can be found elsewhere [15,17–19]. The sensors are at best tested at the envisaged operating temperatures in the device or facility that will be used for studying the material performance in the liquid metal, or a situation that comes as close as possible. A basic test against the oxygen chemical potential of metal/metal oxide equilibria, e.g., Pb/PbO, adjusted in the Pb alloy under consideration is sufficient [15,17]. Particular care should, however, be taken that the simulated oxygen potential coincides with equilibrium conditions and not a temporarily steady state close to equilibrium.

Figure 1: Schematic illustration of potentiometric oxygen measurement [15].


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Evaluation of sensor output 3.2

In general, the oxygen chemical potentials µO that occur in Eq. (3) are defined with reference to a certain standard state. The deviation from the chemical potential in this standard state, µO

0 , is

considered by introducing the oxygen activity aO, i.e., µO µO

0 (T) RT lnaO (4)

where R and T are the universal gas constant and absolute temperature in Kelvin, respectively. It is clear that, in Eq. (4), the activity is unity in the standard state of oxygen. For an ideal gas phase, the oxygen partial pressure pO2 is the equivalent of the oxygen activity

1, as expressed by

2µO µO2µO2

0 (T, p0) RT ln pO2

p0

(5)

Factor 2 on the right hand side has to be introduced since the oxygen dimere O2 is regarded. µO2

0 is

the chemical potential of pure gaseous O2 at p0 = 1 bar (standard pressure) and prevailing T. The conversion of aO into pO2, and vice versa, is possible if the partial pressure that corresponds to

μO µO0 (or aO = 1) is known:

aOpO2

pO2;aO 1

(6)

If pure gaseous O2 at 1 bar and temperature of the measurement is assumed as the standard state of oxygen for both the reference electrode and oxygen solved in the liquid metal, combining Eqs. (2), (3) and (5) gives

U Uth

RT

4F ln

pO2;Ref

pO2

(7)

pO2;Ref

is the oxygen partial pressure at the reference electrode, with clear correspondence to the

composition for a gaseous reference system. For a reference system consisting of a metal/metal‐oxide couple, pO2;Ref

is the oxygen pressure that corresponds to the chemical equilibrium of the

respective metal and oxide, and follows from the Gibbs free energy of formation of the oxide. The pO2

associated to the liquid metal (the subscript LM is omitted here and in the following) is

equivalent to the oxygen partial pressure that would prevail in a gas phase that is in chemical equilibrium with the liquid metal at the site of the oxygen measurement. Substituting aO for pO2

results in 1 In general, the fugacity is the equivalent of activity in the gas phase. Identifying the fugacity with partial pressure is an approximation that is strictly valid only for an ideal mixture of ideal gases. Fugacity and partial pressure may be considered synonymously as long as no explicit values are inserted, so that the more familiar partial pressure is used in the derivation of equations.


11

U Uth

RT

4F ln

pO2;Ref

aO2 pO2;aO 1

(8)

There is a direct link between the stability of a (solid) oxide and the pO2

in Eq. (7) via the Gibbs free

energy of formation of the oxide or, more precisely, the corresponding thermodynamic equilibrium constant. Hence, stating U – Uth along with the reference electrode of the particular sensor is already sufficient for a basic characterisation of the conditions with respect to the potential for formation of solid oxides in the liquid metal. For improved inter‐comparability of oxidation potentials resulting from measurements performed with different types of oxygen sensors, i.e., different reference electrodes, it seems advantageous to define a standard reference electrode (SRE), equivalent to the common practice in aqueous corrosion where such potentials are usually specified with reference to the standard hydrogen electrode. According to Eq. (7), the conversion to a SRE is

USRE U Uth

RT

4F ln

pO2;Ref

pO2;SRE

(9)

with pO2;SRE

denoting the oxygen partial pressure assumed for the SRE.

The conversion of sensor output to either molar or mass concentration of oxygen is mandatory only if quantitative information on the amount of solved oxygen or the change in this amount with time and site of measurement is required. The basis for this conversion is the validity of Henry’s and Sievert’s law up to the saturation concentration of oxygen at the temperatures under consideration so that cOcO;s

xOxO;s

aOaO;s

(10)

or

cOcO;s

pO2

pO2;s

(11)

aO;s and pO2;s

denote the oxygen activity at saturation and oxygen partial pressure in a gas phase that is in equilibrium with oxygen‐saturated liquid metal, respectively. cO and xO are the mass and molar concentration of solved oxygen, respectively, with cO;s and xO;s corresponding to oxygen saturation. As oxygen is a light element in comparison with the major constituents in Pb alloys and, additionally, oxygen solubility in the liquid alloys is quite low, the ratio of oxygen molar concentrations in Eq. (10) has nearly the same value as the respective ratio of mass concentrations which is also used in Eq. (11). pO2;s

is the threshold oxygen partial pressure for formation of the

most stable oxide of the liquid‐metal constituents that, for liquid Pb, clearly is lead monoxide

(PbO). From a thermodynamic point of view, the red‐coloured modification ‐PbO is likely to form


12

at < 489°C, while yellow ‐PbO is more stable at higher temperature, up to the melting point of PbO at 886°C [20]. It can be assumed that PbO is also the oxide in equilibrium with oxygen‐saturated LBE, for which there is experimental evidence from electrochemical measurements performed at 360–740°C [21] and equilibration of PbO (at 600–800°C, [22]) or ternary Pb‐Bi oxides (at 550°C, [23]) with oxygen‐saturated LBE. For known standard Gibbs free energy of formation of

PbO, ∆fGPb/PbO0 , and Pb activity, aPb, pO2;s

calculates as

ln pO2;s

∆fGPb/PbO0

RT2 ln aPb

(12)

where ∆fGPb/PbO0 refers to the formation of the pure oxide from pure liquid Pb and 1 mol pure

gaseous O2 at 1 bar and temperature under consideration. The term containing aPb on the right‐hand side of Eq. (12) vanishes for liquid Pb, but has to be considered for Pb alloys like LBE. Thermodynamic data for calculating pO2;s

is compiled in Table 2.

The equations introduced so far allow for estimating the mass concentration of solved oxygen, cO, and, analogously, the molar concentration xO, from the output of an electrochemical oxygen sensor besides a factor that corresponds to the inverse of the respective saturation concentration

Table 2 Thermodynamic data for calculating pO2;s

Validity range Source

Standard Gibbs free energy of formation of ‐PbO:

∆fGPb/α‐PbO0 438.11

kJ

mol0.1326

kJ

mol KT T < 489°C [20]

Standard Gibbs free energy of formation of ‐PbO:

∆fGPb/β‐PbO0 434.88

kJ

mol0.1374

kJ

mol KT 489°C < T < 886°C [20]

∆fGPb/β‐PbO0 437.96

kJ

mol0.19926

kJ

mol KT 339°C < T < 838°C [24]

Only yellow oxide was observed in this temperature range.

Pb activity in liquid LBE:

ln aPb 0.82912166.80

T

With T in Kelvin.

Above the melting point of LBE.

Derived from data compiled in [25]. See also [10,26].


13

at the temperature of the measurement. It is clear that the reliability of oxygen concentrations calculated from the sensor output strongly depends on the accuracy with which the saturation concentration is known or the reliability of the value assumed for the calculations. Thus, equations proposed for estimating oxygen solubility in Pb or LBE deserve a detailed discussion.

Oxygen solubility in Pb or LBE 3.3

The numerous experimental investigations on oxygen solubility in liquid Pb that were performed before 1998 have been reviewed and analysed by Risold et al. [20]. Interpolation of oxygen solubility in liquid Pb given for the melting point of Pb (327.5°C) and the temperature at which a PbO‐rich melt starts to form (884°C; Table 8 of the cited reference), gives

Pb: log cO;s 3.144971

T

(13)

for oxygen solubility in mass% (with T in Kelvin). An apparently slight influence of whether ‐ or ‐PbO forms in equilibrium with the oxygen‐saturated melt was neglected. More recent measurements of oxygen solubility in the temperature range from 542 to 817°C performed by Ganesan et al. [21] give

Pb: log cO;s 3.215100

T

(14)

At >550°C, the difference in the calculated oxygen concentration between these equations is of the order of 10–20% (Figure 2). Extrapolating Ganesan’s data to the melting point of Pb, however, results in by about 30% lower values in comparison to Risold’s analysis. Thus, a reasonable correspondence and, therefore, reliability of both equations, can be stated only for the higher temperatures. At lower temperatures, the reliability of the available data on oxygen solubility in liquid Pb is unclear [20]. Other equations proposed for estimating the oxygen solubility in liquid Pb (see, e.g., [27,28]) can finally be traced back to experimental data included in Risold’s analysis. The equation stated by Müller et al. [28] results from a re‐evaluation of the data compiled by Risold et al., but considers oxygen measurements at lower and higher temperature as equally reliable [29]. A fair correspondence to Eqs. (13) and (14) is limited to temperatures between about 650 and 750°C, while the predicted oxygen solubility is significantly higher in comparison to Eq. (13) or (14) at lower temperature (Figure 2)2. As oxygen is a light element and oxygen solubility is fairly low, the conversion of mass concentration (mass‐%) into molar concentration (at‐%) in liquid Pb is

Pb: xO cO MPb

MO

(15)

2 Yet another equation has been proposed by Efanov et al. [30], i.e., log cO;s = –1.36 – 1274/T. Oxygen concentration calculated from this equation corresponds to results from Risold et al. or Ganesan et al. quite well at temperatures between about 550 and 600°C. Thus, it may be argued that the equation presented by Efanov et al. bases on measurements in this temperature range. Deviations from other equations increase to two and three orders of magnitude at 850°C and the melting point of liquid Pb, respectively.


14

Figure 2: Oxygen solubility cO;s in liquid Pb as a function of temperature according to the evaluation of experimental data performed by Risold et al. [20], the equation stated by Müller et al. [28] or more recent measurements by Ganesan et al. [21].

where MPb and MO denote the molar mass of Pb and oxygen, respectively. Experimental data on oxygen solubility in LBE that was available before 2007 cover the temperature range from 360 to 800°C. The evaluation of this data on the basis of the typical temperature dependence gives

LBE: log cO;s 2.624416

T

(16)

for the oxygen solubility in LBE as a function of the absolute temperature in Kelvin [10,31]. Lim [26] arrives at a similar result in his analysis of data produced by Ganesan et al. [21], i.e.,

LBE: log cO;s 2.994711

T

(17)

The predictions from Eqs. (16) and (17) deviate from each other by less than 10% at about 450–600°C. The difference increases to about 20% at 350 and 750°C, respectively (Figure 3). Another equation usually referred to as Orlov’s equation considers only data obtained at 470 and 520°C (open squares in Figure 3), so that the correspondence with other measurements of oxygen solubility is quite good in this temperature range, but deviations become considerable at both higher and lower temperature. A completely different approach was followed by Müller et al. [28],


15

Figure 3: Oxygen solubility cO;s in LBE as a function of temperature including experimental data produced by Orlov et al. [32], Ganesan et al. [21] and Kishimoto et al. [22]; results from data evaluation by Schroer et al. [10,31] and Lim [26]; as well as the equation derived by Müller et al. [28].

who derived the oxygen solubility in LBE from the oxygen solubility in pure Pb and Bi. The values calculated from the resulting equation, however, clearly underestimate the experimentally determined oxygen solubility, by about half an order of magnitude or more (Figure 3). The conversion of mass concentration (mass‐%) into molar concentration (at‐%) in liquid LBE is

LBE: xO cO 0.45 MPb 0.55 MBi

MO

(18)

where MPb, MBi and MO denote the molar mass of Pb, Bi and oxygen, respectively.

Documentation and reporting of oxygen content 3.4

For characterising the exposure conditions of materials tested in liquid Pb or LBE with respect to the stability of certain oxides, it is sufficient to state the output of an oxygen sensor, temperature of oxygen measurement and type of reference electrode. Due to the particular importance of oxide scale formation and also the type of oxides formed for further progress of corrosion, comparatively short‐term variations in oxygen content may have long‐term effects on the observed material performance [33], so that it is recommended to monitor and record oxygen data over the entire runtime of the exposure experiment, and, especially, document observed


16

Figure 4: Example of documenting oxygen sensor output and calculated oxygen concentration as a function of the runtime of an experiment during which different material specimens were exposed simultaneously or consecutively for various times.

variations. An instructive example how sensor output observed during the course of the exposure of particular specimens may be summarised, is presented in Figure 4. Estimating the corresponding oxygen concentration is mandatory only if oxygen consumption, transfer and transport or, more generally, oxygen mass balances need to be considered, explicitly or implicitly. The latter is, e.g., the case if, in a non‐isothermal experimental set‐up, oxygen content is measured in a distance from the exposed specimens where temperature significantly differs from the testing temperature of the investigated materials. In view of the ambiguity that still exists for some of the thermochemical data required for the conversion to oxygen concentration, especially the saturation concentration in Pb or LBE, details on the oxygen metrology applied and conversion factors used are mandatory additional information when reporting calculated oxygen concentration. Otherwise, a fair estimation of the reliability of the stated concentration and inter‐comparability of studies performed in different laboratories are questionable. Data needed for conversions besides oxygen solubility can be regarded comparatively accurate, possibly with the exception of aPb in LBE (Table 2). The following list is a suggestion which mathematical relations to use for calculations of oxygen concentration in liquid Pb or LBE from the output of electrochemical oxygen sensors, but may be subject to amendments based on future re‐determination of the underlying thermochemical properties:


17

(I) Calculation of oxygen partial pressure from the output of an electrochemical sensor

log pO2

p0 log

pO2;Ref

p020,159

K

VU Uth

T (19)

Eq. (7) with R 8.31451 J mol–1 K–1 and F 96485.31 C mol–1. U – Uth in Volt. T in Kelvin. (II) Specific equations for most commonly used reference electrodes

Pt air:⁄ log pO2

p00.67890 20,159

K

V

U Uth

T (20)

With concentration of O2 in dry air of 20.946 vol%.

Bi Bi2O3⁄ :log pO2

p010.231

0.67890 K

T20,159

K

V

U Uth

T (21)

With reference oxygen partial pressure calculated from the standard Gibbs free energy

∆fGBi/‐Bi2O3

0 388.9 kJ

mol0.1959

kJ

mol KT

for formation of ‐Bi2O3 from pure liquid Bi and 1 mol pure gaseous O2 at 1 bar and temperature under consideration. T in Kelvin. Range of validity: 299–715°C [34]. (III) Oxygen mass concentration, cO, as a function of oxygen partial pressure for liquid Pb and LBE

Pb: logcO

mass%

1

2 log

pO2

p01.9940

6338.1 K

T (22)

Eqs. (11) and (12) with cO;s according to Eq. (14) and ∆fGPb/PbO0 for ‐PbO from source [24] (Table

2). R 8.31451 J mol–1 K–1 and aPb 1. Eq. (14) as well as the assumed ∆fGPb/PbO0 are in good

agreement with the results of other studies [21,24]. Especially for cO;s in Pb, a thorough re‐evaluation of existing data or supplemental measurements at T < 550°C seem useful.

LBE: logcO

mass%

1

2 log

pO2

p02.9441

6949.8 K

T (23)

Eqs. (11) and (12) with cO;s according to Eq. (16) and ∆fGPb/PbO0 for ‐PbO from source [24] (Table

2). R 8.31451 J mol–1 K–1 and aPb calculated using the equation presented in Table 2. Eq. (16) meets the singular experimental data point available at low temperature better than Eq. (17) (Figure 3).

Control of oxygen content 3.5

A straight forward way of influencing the amount of oxygen solved in liquid Pb or LBE is guiding a stream of oxygen‐containing gas over the free liquid‐metal surface. If the oxygen partial pressure in the gas is higher than corresponding to the current activity of oxygen solved in the liquid phase,


18

oxygen is transferred from the gas stream to the liquid metal and vice versa. Such gas/liquid oxygen transfer has been applied to material tests in static Pb or LBE contained in ceramic crucibles [35] as well as conditioning a relatively large mass of LBE circulating in a loop [36]. By altering the oxygen content of the gas systematically in response to the output of an oxygen sensor, oxygen solved in the liquid metal can be controlled to a pre‐defined level. In general, oxygen will have to be added continuously so as to compensate for the consumption by oxidation of metallic sample materials or containment of the liquid metal, while, during the course of a pre‐conditioning phase, excess oxygen may have to be removed. In the latter case, supply of oxygen‐lean gas, e.g., with high percentage of hydrogen (H2), is required. In order to maintain the desired content of solved oxygen at reasonable flow of gas, the oxygen partial pressure in the gas that is introduced into the testing device/facility can be relatively high, most likely because of rapid diffusion in the gas space above the liquid phase [36]. Good experience has been made with using technical argon (Ar) as a carrier gas, to which a certain amount of air is intermittently added in response to the measured oxygen content [15,36]. Bubbling through the liquid‐metal volume is an alternative to guiding the gas over the liquid‐metal surface [37]. The appropriate volume flow of gases naturally depends on experimental parameters like temperature or size of metallic surfaces exposed to the liquid metal (as well as the hold‐up volume of gas above the liquid‐metal surface), but usually is in the range of 5–500 Nccm/min for the carrier gas, and 0.5–5 Nccm/min for intermittent addition of air or another gas mixture with comparable oxygen content. For automatic oxygen control, it proved useful to always supply a small amount of H2 to the carrier gas. As safety regulations usually limit the hydrogen content in gases for use in regular laboratories to 5 vol%, pre‐conditioning larger volumes of Pb or LBE in reasonable time may require a flow rate of hydrogen‐containing gas in the order of 1000 Nccm/min or more. Especially in a larger loop facility, intermittent addition of hydrogen to the gas may be required, e.g., for removal of oxygen released from oxides that deposited along the flow path of the liquid metal. An alternative to gas/liquid oxygen transfer is the supply of oxygen to liquid Pb or LBE via solid oxides, especially PbO [38,39]. This method is routinely used to control the amount of solved oxygen in loop facilities operated in Russia. In dependence of the actual output of an oxygen sensor, the circulating liquid metal is guided through a bed of PbO pebbles to compensate for the oxygen consumption in the loop. For the respective oxygen‐transfer device residing in a bypass of the main liquid‐metal duct, a variable percentage of the liquid‐metal flow may be redirected through the solid oxide, either intermittently or continuously. Another option is to provide the solid oxide in a perforated and movable cage that may be introduced into or pulled out of the main flow in dependence of the actual need for oxygen. Solid/liquid oxygen transfer as reported so far requires flowing liquid metal. An adaptation to smaller experimental set‐ups with static liquid metal seems theoretically possible, but, in practice, may require significantly higher experimental effort than using oxygen‐containing gas. A second mass‐transfer device is needed for enabling the removal of excess oxygen, e.g., by introducing hydrogen‐containing gas or a metallic oxygen getter.


19

4 Experimental devices and facilities

Smaller testing devices or larger facilities that have been operated in laboratories specialising in investigations on the material performance in liquid Pb or LBE, were mainly developed for characterising corrosion of the tested materials for a fixed set of experimental parameters (temperature, chemical potential/concentration of oxygen, flow velocity …), but also allow for simulating transient conditions with respect to one or more of these parameters. The finally chosen practice often is a compromise between accuracy of testing, i.e., significance of the findings for the materials behaviour under the nominal conditions, and the feasibility and efficiency of the experiments. E.g., the simultaneous exposure of several specimens of different materials (steels) clearly improves the time‐efficiency of testing, especially in the lack of a definite candidate material for later application in the plant, at the cost of a hardly quantifiable contamination of the liquid metal with solved steel constituents. The same will be true when non‐inert experimental equipment is in contact with the liquid metal. Performing short‐ and long‐term exposures at the same time requires disturbing or interrupting periodically the long‐term exposures for exchanging specimens with shorter exposure time, with potential effects on the integrity of the formed corrosion (oxide) scales resulting from intermittent cooling, handling and re‐heating.

Mounting, loading and unloading of specimens 4.1

The high mass density of liquid Pb or LBE in comparison to the exposed materials requires specific means of positioning and fixing the specimens appropriately in these liquid metals. The method that appears most viable will depend on the available sample material, i.e., the shape or size specimens that may be produced. Amongst other factors, the mounting of specimens chosen for the test determines how to load specimens into or unload from the testing device or facility. Some instructive examples will be discussed below. Figure 5 shows how specimens may be fixed in ceramic crucibles, for exposure to static liquid metal under cover gas. The restrictions with respect to the shape of tested specimens are a minimum. Filling the crucible with liquid metal and arranging the specimen is done at best in a glove box or similar, under appropriate atmosphere. A ceramic pin inserted through small bore

Figure 5: Flat material specimen fixed in a ceramic crucible.


20

Figure 6: Arrangement of specimens provided with bore holes using rigid wires as support [1].

holes in both the specimen and crucible keeps the specimen in the desired, upright position. Subsequently, one or more of such crucibles are transferred into the furnace and brought to exposure temperature. Providing the specimen with bore holes opens various options of stabilising the specimen in the wanted position during the exposure to the liquid metal. An arrangement of rectangular specimens and supporting molybdenum (Mo) wires that was used in exposure experiments performed in a small natural convection loop [1] is sketched in Figure 6. If it is not easily possible to pull out wires or pins after the experiment, e.g., because of solidified liquid metal, they may simply be cut and snapped off, respectively. Alternatively, screw threads provided on one or two opposing ends of the specimen may serve as the connection to a sample holder and facilitate combining several specimens for simultaneous exposure, respectively. It seems possible to weld, solder or otherwise join some material that allows for cutting a screw thread, to a sample of arbitrary geometry, but, preferably, the sample material itself is thick enough for producing specimens with screw thread(s). Cylindrical specimens

Figure 7: Cylindrical specimen provided with external and internal screw thread as well as a flattened surface for applying a wrench.


21

of 3 mm diameter and external screw thread cut along half of the specimen length were used for exposure experiments in static LBE [40]. The LBE resided in an alumina crucible heated in pre‐conditioned atmosphere, inside a glove box. The specimen was screwed to a steel lid covering the opening of the crucible. The type of specimen shown in Figure 7 is used for exposure experiments in a larger loop facility operated with LBE [41]. A number of such cylinders with both external and internal screw threads are combined and joined with a rod so as to install the arrangement of specimens in the centre of a vertical section of the tubing of the loop. When a part of the introduced specimens has reached the designated exposure time, the port for the specimen mount is opened and the rod removed. After unscrewing the respective lot of specimens or adding another lot of fresh specimens, the rod is re‐introduced into the test‐section of the loop. The port for installing and removing specimens resides inside a glove box, under controlled atmosphere, so that contamination of the LBE, especially with oxygen, is kept to a minimum. The rod‐type arrangement seems appropriate for inserting specimens into pre‐heated and pre‐conditioned liquid metal in the case of tests in either flowing or static liquid metal, at minimum disturbance of the conditions inside the experimental device or facility. Ingress of liquid metal into the screw joints may, however, loosen connections at operating temperature, as a consequence of corrosion. Unscrewing will require careful and local heating if a joint cools down below the melting point of the liquid metal. If flat specimens are to be exposed to flowing Pb or LBE, the design of a cartridge that contains the specimens and is introduced into the testing facility, may be an option. The example illustrated in Figure 8 was produced from Mo, and inserted into a horizontal section of a loop operated with LBE [42].

Figure 8: (a) Sample holder designed for taking up rectangular material coupons and (b) introduction into a horizontal section of a liquid‐metal loop [42].


22

Static oxygen‐containing Pb or LBE 4.2

Corrosion testing in static Pb or LBE needs an appropriate container for the liquid metal and fixture of the investigated specimens. Actively influencing the chemical potential (concentration) of solved oxygen requires separating the liquid metal from the laboratory atmosphere. At best, the oxygen potential is continuously monitored using a sensor inside the liquid‐metal pool. In the COSTA facility depicted in Figure 9, the separation of the liquid metal from the laboratory atmosphere is achieved by inserting ceramic crucibles loaded with liquid metal and specimen (Figure 5) into a quartz glass tube that resides in a horizontal furnace. The furnace provides independently heated zones allowing for performing experiments at different temperatures simultaneously. The atmosphere inside the glass tube is manipulated by introducing a gas flow variably composed of Ar (carrier gas), H2 and water vapour. The nominal pO2 follows from the

adjusted hydrogen‐to‐water ratio that is monitored by an oxygen meter in the exhaust gas line, and exposure temperature. With the assumption of chemical equilibrium between gas phase and liquid metal, this pO2 corresponds to a concentration of solved oxygen that is given by Eqs. (22)and

(23), for liquid Pb and LBE, respectively. Monitoring the actual oxygen potentials that prevail in the single crucibles would require miniaturising the oxygen sensors currently used for liquid metals and transmitting the signals to voltmeters outside the furnace.

(a)

(b)

Figure 9: (a) Side view of COSTA showing four horizontal furnaces equipped with quartz glass tubes designed for exposure of material specimens to static liquid metal under flowing cover gas, and (b) carrier for inserting and removing ceramic crucibles that contain the liquid metal and specimen.


23

Figure 10: Experimental capsule for testing cylindrical material specimens in oxygen‐containing static Pb or LBE as developed for the round robin performed in the framework of the MATTER project.

In the case of the testing device that has been developed for the round robin on corrosion in static oxygen‐containing LBE performed in the framework of the MATTER project, the crucible that contains the liquid metal is housed in a capsule made of type 316 austenitic steel (Figure 10). The lid of the capsule provides a port for introducing and removing the tested materials using a specimen mount that basically is a Ø6 mm steel rod. Other ports allow for inserting an oxygen sensor, a Mo wire as an auxiliary electrode for the oxygen measurement, a thermocouple (in a one‐end‐closed alumina tube) as well as an alumina tube for bubbling oxygen‐rich or ‐lean gas mixtures through the liquid metal. The crucible holds about 700 g LBE. The maximum gas flow that may be guided through the device is limited by spilling of the liquid metal. For the round robin experiments, 5 ml/min of either Ar or Ar‐5 vol% H2 were used. Controlling the oxygen potential in the liquid metal by varying the oxygen content of the gas in response to the sensor output is possible.

Flowing oxygen‐containing Pb or LBE 4.3

Early experiments in flowing Pb or LBE were performed in comparatively small and compact natural convection loops, made of low alloyed steels [43] or quartz glass (silica) [44]. The flow velocity that could be achieved in these loops was << 1 m/s, so that mainly conditions allowing for solution and re‐precipitation of elements in a temperature gradient between hot and cold leg were simulated, rather than a significant impact of flow immediately on the corrosion occurring at the material surface. Especially in the silica loops, the concentration of oxygen solved in the liquid


24

metal had to be kept low, by pre‐conditioning using hydrogen‐containing gas or addition of oxygen getters like Mg, so as to prevent softening and failure of the glass at elevated temperature [45]. In the case of the steel loops, the loop itself was the corrosion specimen. The target quantity was the time to plugging of the cold leg at variable maximum temperature and temperature difference,

T, along the loop, with and without addition of corrosion inhibitors, e.g., Zr [43]. The loops made of practically inert silica were loaded with dedicated specimens in the hot leg, or both hot and cold leg [1,45], that were evaluated after exposure. In the testing apparatus illustrated in Figure 11, natural convection of the liquid metal is supported

by gas lift. The set‐up is heated from the bottom so as to maintain a T between bottom and top [46]. Proper characterisation of the conditions inside the device requires computational fluid dynamics and verification by measurements, but the advantage of combining gas lift with gas/liquid oxygen transfer is obvious. Forced‐convection loops driven by either mechanical or electromagnetic pumps usually are larger facilities if compared with typical laboratory devices. The flow velocity that can be achieved is in the order of some m/s. An example is the CORRIDA loop (Figure 12), with a developed length of 36 m, and approximately 1000 kg LBE circulating at a mass‐flow rate of 5.3 kg/s during regular

Figure 11: Corrosion apparatus for liquid metal with convection partially forced by gas lift [47].


25

(a) (b)

Figure 12: (a) Schematic illustration and (b) side view of the CORRIDA loop that has been operated since 2013 with oxygen‐containing LBE at a maximum temperature of 550°C.

operation. The tubing and the other components are made of 17‐12 Cr‐Ni steel (DIN W.‐Nr. 1.4571). CORRIDA is equipped with a gas/liquid mass exchanger for oxygen transfer as well as four oxygen sensors distributed along the loop [15,36]. The sensor at the inlet of the first test‐section (Sensor 3 in Figure 12a) is used for controlling the content of solved oxygen, and characterising the exposure conditions for the specimens that reside in the test‐sections. The decrease of solved oxygen in the bulk of the flowing liquid metal while passing the specimens is monitored with help of Sensor 4, residing after the second test‐section. Both test‐sections are positioned in the hot leg, allowing for a maximum exposure temperature of 550°C that corresponds to the maximum operating temperature of the loop. The minimum temperature along the loop is 385°C at 550°C of

the hot leg, resulting in T 165°C. The minimum temperature decreases sub‐proportionally to

the temperature in the hot leg, and is around 350°C at both 450 (T 100°C) and 400°C

(T 50°C). The material tests are performed on the cylindrical specimens shown in Figure 7 that reside in the centre of the two vertical test‐sections. Up to 17 of such specimens are combined to a rod that is introduced into the pre‐conditioned LBE at reduced power of the pump. The loop has achieved a total operating time around 80,000 h, mainly at 550°C in the hot leg and 10–6 mass% solved oxygen, and an effective operation time (with specimens in the test section) of more than 60,000 h. The austenitic steel used as construction material of the loop is naturally prone to corrosion or erosion by the flowing liquid metal. Local failure of the tubing occurred first after total operation for about 29,000 h close to the Y‐shaped inlet piece of one of the test‐sections [48], probably promoted by the complex flow pattern in this position. A clear corrosion damage on a straight section of the tubing (of 2.5 mm initial thickness) was observed not before 66,000 h of operation [49].


26

5 Post‐test examination

Methods routinely applied to characterising material specimens after exposure to a corrosive environment are light‐optical (LOM) and scanning electron microscopy (SEM) supplemented by semi‐quantitative energy‐dispersive X‐ray analyses (EDX)3. Such examinations are usually applied to metallographic cross‐sections, revealing the structure of formed corrosion scales, including changes in structure of the material in a near‐surface zone, down to the upper sub‐micron range. The spatial resolution of the EDX analysis is, in general, in the range of 1 µm, but finally depends on the diameter and other parameters of the electron beam used for excitation as well as the investigated material. For the preparation of cross sections, the corroded specimens will have to be cut, mounted in resin and prepared by grinding and polishing. Standard metallographic procedures and aqueous lubricants may be used. The removal of adherent Pb or LBE prior to preparation is not recommended, as the solidified liquid metal covering the surface helps to conserve fragile corrosion scales, minimising scale spalling or detachment during cutting and grinding. At the same time, potential artefacts from stripping the corroded specimen surface are avoided. In order to prevent re‐melting and re‐distribution of solidified liquid metal in the corrosion scale, hot‐mounting resins and embedding at increased pressure should be avoided, especially for specimens with adherent LBE. The, in general, softer cold‐mounting resins may be strengthened by stirring in oxide powder (e.g., aluminia) before pouring into the mould. Emphasising structural changes in the material may require etching of the cross‐section. It is generally recommended to prepare and investigate a second cross section of the specimen, in addition to the one used for the main characterisation work, so as to check for phenomena that have not been found in the primarily examined cross section. For examining patterns of localised corrosion, phase analysis using X‐ray diffraction (XRD) or the investigation of thin surface scales not easily visualised in the cross section, a covering layer of adherent Pb or LBE needs to be removed. Thicker films of LBE may be re‐melted by immersing the specimen for a short time in hot fat, glycerine or similar agents (at around 150°C). Remnants of LBE that did not drip off during the immersion can be wiped away by carefully rubbing or dabbing at the surface with a cotton cloth or tissue paper while the specimen is still hot. A subsequent cleaning in an appropriate organic detergent (alcohol, acetone …) removes the greasy film that remains on the surface. Ultrasonic cleaning should be applied with care. Adherent Pb with higher melting point than LBE may be stripped off in molten LBE at around 350°C, followed by the procedure for removing solidified LBE. Alternatively or if further cleaning is required, a 1:1:1 mixture of hydrogen peroxide (30% aqueous solution), concentrated acetic acid (>96% aqueous solution) and ethanol removes thin films or stains of Pb or LBE during immersion for 10–20 min at room temperature. Simultaneous application of ultrasound reduces the time required for cleaning, but increases the risk of destroying corrosion (oxide) scales formed on the specimen surface as well as further corrosion of the material.

3 See [50–53] for reviews of analytical methods or preparation techniques that have general applicability to high‐temperature corrosion studies.


27

The application of transmission electron microscopy (TEM) or advanced surface analytical methods like Auger electron spectroscopy (AES), X‐ray photoelectron spectroscopy (XPS) and other is indicated when clearly sub‐micron structures or very thin surface films need to be analysed in detail. However, the gain in information has to be weighed against the higher preparatory effort in comparison with the analysis by SEM/EDX. The analysis of surface layers with AES or XPS requires cleanliness in terms of adherent Pb or LBE that may not be achievable with the introduced procedures.

6 Quantitative analysis of corrosion

The main goals of analysing corrosion experiments quantitatively are providing objective data for inter‐comparison of the performance of different materials as well as attaining a deeper understanding of the relative importance of elementary steps that determine the overall corrosion mechanism. The most appropriate method of quantification clearly depends on the particular corrosion phenomena observed. It should be emphasised that the absolute amount of corrosion and, in some cases, also the occurring corrosion modes may differ for experiments at nominally the same testing conditions, but performed in different testing devices or facilities. Also for repeated tests in the identical experimental set‐up, deviations of the quantitative characteristics by 30–40% are not unusual. The latter probably results from the number of factors that potentially influence the progress of corrosion, some of which are hard to be reproduced exactly. While a material that shows superior performance in corrosion tests is likely to behave better than the respective reference material(s) also during service in an industrial plant, the level of improvement may differ from the expectation from the tests. However, the kinetics of corrosion processes observed in laboratory tests at different boundary conditions is a valuable input to generalised models aiming at a quantitative approach to the more complex in‐service situation.

Modes of steel corrosion in oxygen‐containing Pb or LBE 6.1

In the absence of solved oxygen, the major steel elements tend to dissolve in liquid Pb or LBE. The removal in the sense of a mass transport away from the steel surface may be selective for elements exhibiting a relatively high solubility in the liquid metal. A thin surface layer of pre‐formed oxide may protect the steel from solution, but is prone to thermo‐mechanical failure or degradation caused by the liquid metal. In this case, corrosion based on the solution of steel elements is likely to start locally, at defects of the once protective surface layer. The persistence of the surface layer increases with increasing concentration in the steel of elements that form exceptionally stable oxides, like Cr, aluminium (Al) or silicon (Si). Oxygen solved in liquid Pb or LBE generally stabilises pre‐formed surface oxides and, additionally, stimulates the formation of new oxide. At not too high temperature, the material loss accompanying the formation of comparatively slow growing oxides of Cr, Al or Si are negligible (< 1 µm in material thickness) even after considerable exposure time to oxygen‐containing Pb or LBE (Figure 13a). Depending on the composition and microstructure of the steel as well as the particular exposure conditions, the presence of a thin oxide layer that protects the steel elements from both solution in the liquid


28

metal and further oxidation (protective scaling) may be a long‐lasting phenomenon observed at least locally. Oxide scales based on Fe oxides grow considerably faster than the thin protective scale. These scales typically consist of two layers (Figure 13b) which are outward growing magnetite (Fe3O4) and inward growing Fe‐Cr spinel (Fe[FexCr1‐x]2O4 with 0 < x < 1). Another feature of the, in comparison to protective scaling, accelerated oxidation may be an internal oxidation zone (IOZ), formation of which then precedes the inward growth of the spinel layer. Solution of Fe in the liquid metal and transport away from the oxide scale surface potentially causes the overall scale thickness to be lower than corresponding to the amount of steel consumed by corrosion processes. In specific cases, when transfer to the liquid metal of Fe that arrives at the oxide scale surface clearly outweighs the formation of new oxide, the outer magnetite layer may be completely missing. Especially for steels with comparatively low content of elements that form literally protective oxides, accelerated oxidation will be the general corrosion process that affects the major portion of the surface. Other steels possibly show accelerated oxidation only locally, in the form of oxide‐filled pits, even after prolonged exposure to oxygen‐containing Pb or LBE. Processes, in the course of which steel elements initially dissolve in the liquid metal, are, in general, associated to significantly higher material loss than both protective scaling and accelerated oxidation. A necessary pre‐requisite is direct contact, i.e., a common interface of liquid metal and steel. If none of the liquid‐metal constituents is capable of forming a separating layer along with the elements available in the steel, such a solution‐based corrosion is likely to affect the entire steel surface, unless an at least temporarily protective film on the steel surface has established before the exposure. In the latter case or for conditions in favour of oxidation, solution‐based corrosion starts locally, where a pre‐formed protective film or the oxide scale grown in‐situ failed. Instructive examples are shown in Figure 14.

(a) (b)

Figure 13: Modes of oxidation observed on steels after exposure to oxygen‐containing Pb or LBE: (a) Protective scaling on 316L after exposure for 10,021 h to flowing LBE at 550°C, 10–6 mass% of solved oxygen and flow velocity of 2 m/s; (b) accelerated oxidation as found on T91 after exposure for 8039 h to flowing LBE at 450°C, 10–6 mass% of solved oxygen and flow velocity of 2 m/s.


29

(a) (b)

Figure 14: Examples of solution‐based corrosion observed on steels after exposure to oxygen‐containing flowing LBE at 550°C, 10–6 mass% of solved oxygen and flow velocity of 2 m/s: (a) Selective leaching of Ni and Cr from 316L after exposure for 10,021 h; and (b) oxidation after initial solution of steel elements as found on T91 after exposure for 7518 h.

Gravimetric versus metallographic quantification 6.2

For quantifying the corrosion processes occurring during exposure to oxygen‐containing Pb or LBE, tracing the mass change or the change of geometric dimensions may be taken into account. The gravimetric method requires determining the mass before the test, which is, in general, possible with 6–7 significant digits, corresponding to a reading accuracy of 0.1–0.01 mg for a mass in the order of grams. Mass changes measured on specimens with different geometric dimensions become inter‐comparable or can be transferred to arbitrary geometries only when referring the results to the surface area that was exposed to the corrosive environment. Knowledge of the geometric dimensions with an accuracy of a few percent seems appropriate for sufficiently precise calculation of the area‐specific mass change. Conversion of area‐specific mass change into change of geometric dimensions requires the density of the tested material if mass loss of material was determined, average mass density of steel elements in the formed corrosion scales if increase in mass by formation of adherent corrosion products was measured. The metallographic quantification is performed on the cross section of the material after exposure. For this purpose, the tested specimen needs to be cut, mounted in resin and prepared by grinding and polishing, as exactly as possible parallel to the specimen dimension the change of which will be evaluated. Especially for rectangular specimens, the prepared cross section may be deliberately tilted relative to this dimension by a certain angle so as to increase changes in the characteristic length or the thickness of corrosion scales by a determined factor. In some cases, e.g., when the consumed steel elements are found quantitatively in a well‐adherent and dense corrosion (oxide) scale, it will be possible to calculate all the characteristics of corrosion from the thickness of this scale with sufficient accuracy. In general, it is, however, necessary to retrace the initial position of the material surface for determining the recession of unaffected material or


30

increase in geometric dimensions. This requires either marking the position of the initial surface appropriately [35] or measuring a characteristic length of the specimen before the experiment. The accuracy with which this dimension of the specimen needs to be determined is in the order of a few µm at a total length in the mm‐range. Equally high requirements have to be made on the production tolerance of specimens, especially with respect to the uniformity of the evaluated geometric dimension. In contrast to quantification based on measurements on the cross section of the corrosion specimen, gravimetry delivers an average measure of corrosion. Local phenomena need to be quantified by additional measurements on the cross section, evaluation of surface topography after cleaning from corrosion scales or comparison of average mass change as a function of time with the predictions from model approaches to local corrosion [54]. If the increase of specimen mass will be evaluated, adherent Pb or LBE can significantly disturb the measurement and has to be removed from the surface after the test. In view of potential solution of material (steel) elements in the liquid metal, the mass of adherent corrosion products (oxides) may not be representative of the overall material consumption so that the latter has to be determined as the mass loss of the specimen after exposure and removal of both adherent solidified liquid metal and corrosion scales. As examinations on the cross section of the tested materials as taken from the experimental device are considered mandatory for proper characterisation of occurring corrosion phenomena in terms of type, composition and structure of formed corrosion products, the cleaning required for useful mass‐change measurements necessitates providing an extra specimen per exposure time for gravimetric quantification. The metallographic method can be performed on the identical specimen (cross section) and, in principle, at the same time as the other phenomenological examinations. Furthermore, it especially allows for assessing quantitative characteristics separately for the three corrosion modes observed on steels after exposure to oxygen‐containing Pb or LBE, and is, in general, better suited for quantifying internal corrosion processes and near‐surface depletion zones in the material (possibly only after etching of the cross section or with help of qualitative EDX analysis, e.g., line scans, in the electron microscope). Depending on how precisely the initial position of the specimen surface is defined as well as the accuracy with which the evaluated cross section was prepared, the resolution of quantitative corrosion effects might, however, be poorer when using metallographic methods than by gravimetry. But this inherent disadvantage of the metallographic method becomes increasingly less important with increasing exposure time.

Metallographic method 6.3

In this section, the quantification of corrosion in liquid Pb or LBE on the basis of metallographic measurements is discussed and exemplified for cylindrical specimens. The particular method is currently being evaluated as a part of the round robin on corrosion testing in static oxygen‐containing LBE performed in Task 3.2 of the MATTER project.


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6.3.1 Pre‐test measurements

Before performing the corrosion test, each specimen has to be characterised with respect to the initial value of the geometric dimension that will later be used for quantifying the material loss. In other guidelines for corrosion testing at elevated temperature, a repeatability of this measurement in the order of ±0.2 µm and an overall accuracy of ±5 µm are considered achievable with modern instruments (e.g., digital micrometer) and the minimum required for obtaining significant values for the decrease of the characteristic dimension during the corrosion test, respectively [7]. Both may, however, be case sensitive, e.g., dependent on the size of the particular specimens tested or how severe corrosion actually was. The recommendation for overall accuracy not only includes the precision of the measurement itself, but also the uniformity along the specimen. For measuring the initial diameter of cylindrical specimens, the equipment shown in Figure 15 was developed [31,55]. It consists of a displacement transducer that is fixed in a massive stand above

the opening of a prism with known opening angle . The displacement Z indicated when the tip of the measuring instrument touches the cylindrical specimen in its highest point is compared with the displacement Zref that corresponds to the known diameter Dref. of a cylindrical calibration piece. The diameter D of the specimen then follows as

D Dref

2 sin2

1 sin2

Z Zref

(24)

The reading accuracy of the particular displacement transducer and production tolerance of the calibre used were both ± 1 µm, so that the repeatability of the diameter measurement was in the

Figure 15: Schematic illustration of equipment for measuring the initial diameter of cylindrical specimens [31].


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Figure 16: Schematic illustration of diameter measurements in the laser micrometer, with M denoting the mark for defining the angular position of the measurement.

range of 3–4 µm [55]. Comparing the results from the prism method with the diameter measured in the microscope, after preparation of a metallographic cross section shows a difference of 1 µm in the obtained mean value or 2–4 µm for single measurements. Accordingly, the combination of applying the prism method before the corrosion test and measurements on the cross section after the test, involving additional sources of error from specimen preparation, can fulfil the recommendations on minimum accuracy. The use of a laser micrometer with reading precision of ± 0.1 µm promises an additional gain in overall accuracy. The repeatability as achieved in a particular instrument is < 1 µm. Automatic manipulation of the specimen and data logging allow performing a higher number of diameter measurements in reasonable time than the prism method described above. Accordingly, the non‐uniformity of the diameter of cylindrical specimens may be readily assessed, and considered in the determination of the material loss caused by corrosion. For the round robin test conducted in Task 3.2 of the MATTER project, data on the initial diameter of cylindrical specimens was collected for 8 equidistant planes along the cylinder axis. 12 measurements were performed in each plane, rotating the specimen by an angle of 30° after each determination of the diameter. The angular


33

position is defined with reference to a marked flat surface on the specimens as illustrated in Figure 16. Accordingly, the pre‐exposure characterisation provides 6 independent measurements of the initial diameter in each plane, at angular positions 0°, 30°, 60°, 90°, 120° and 150° as well as one repetition of each of those measurements performed at the equivalent positions 180°, 210°, 240°, 270°, 310° and 330°, respectively.

6.3.2 Post‐test measurements

In preparation of the post‐test measurements, the specimen is cut vertically, close to a plane for which the initial values of the evaluated geometric dimension (diameter in the case of cylindrical specimens) have actually been determined. “Close” means that especially the thickness of the blade used for cutting is taken into account, and, if possible, another 0.2–0.3 mm allowance for abrasion during the metallographic preparation procedure, so that the visible cross section coincides with the selected plane as exactly as possible. Retrieving the angular positions along the circumference of a circular section requires an appropriate marker. Embedding a cylindrical reference piece of known diameter along with the specimen may be considered for detecting and quantifying possible tilting of the prepared cross section during grinding and polishing [7]. A full quantification of corrosion provides data for the remaining thickness/diameter of sound material and corresponding thickness of the corrosion scale. If the latter consists of several distinguishable layers, measuring the thickness of each of these separately is recommended, so that the position of interfaces relative to the initial specimen surface can be calculated. The latter may give valuable information on the growth direction and movement of interfaces during the course of the corrosion process. A particular procedure of performing such measurements on a circular cross section in the (light‐optical) microscope is outlined in Figure 17. In order to achieve an accuracy of the measurements in the order of 1 µm, it is necessary to work at 500‐fold optical magnification or higher. As the re‐measured diameters are in the order of mm, the microscope must be equipped with a movable stage, with a precision of movement in the two independent directions of 1 µm or better. Cross hairs in the ocular or superimposed on the screen of a digital microscope may be used as a stationary reference during the movement of the stage. For minimising potential sources of error, the hairs have to be aligned with the moving directions of the microscope stage (x and y in Figure 17a). Each set of measurements starts with defining the line along which measurements have to be taken, so that the distance determined between two points on the actual circumference of sound material is significant for the decrease in diameter, and the distance between two interfaces is the true thickness of a corrosion layer. The orientation of this imaginary line corresponds to one of the cross hairs if the other hair forms a tangent that touches the circumference of the original circular cross section in the point of intersection of the hairs. It is clear — and a principal problem in any metallographic quantification of corrosion — that the original circumference or initial position of the specimen surface is, in general, no longer visible. Therefore, the required positioning of the cross hairs relative to the corroded cross section is only approximately possible, which contributes to the sources of error in the measurements, especially for the determined diameter of sound


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(a)

(b)

Figure 17: Illustration of the quantification of corrosion processes on a circular cross section, using a microscope with movable stage.

material. If only a thin corrosion scale formed or for not too thick inner layers of the scale, the curvature of the scale surface and interface between layers, respectively, will be similar to the original surface and is an appropriate substitute. For stronger attack and pronouncedly non‐uniform scales, it seems helpful to make use of a larger portion of the specimen around the area of interest, at reduced magnification, from which the original curvature of the cross section becomes more obvious. In any case, pre‐aligning at lower magnification and final adjustment at


35

the magnification at which the measurements will be performed appears generally useful. It may not be necessary to mention that, for a given diameter being evaluated, both ends should be considered for proper alignment of the cross hairs, especially the one showing less severe corrosion. If the original cross section actually was elliptical rather than a circle or the cross hairs were not perfectly aligned to the moving directions of the microscope stage, it will be necessary to move the stage a comparatively small distance sy in addition to sx in the main direction of movement, in order to meet the wanted tangential positions. The measuring result, s, then is

s sx2 sy

2 (25)

The absolute value of sy above which such a misfit between the main direction of movement of the microscope stage and orientation of the evaluated diameter becomes noticeable and needs to be corrected in the measurement of the diameter of sound material is in the order of 100 µm for sx 8.000 mm or 75 µm for sx 3.000 mm. The measurements of scale thickness are generally not significantly affected. Figure 17b illustrates the measurements in detail for a corrosion scale consisting of three layers. After the preparatory adjustments, the microscope stage is moved horizontally in the depicted example, until the intersection of the cross hairs (gauge mark) coincides with the outer surface (surface of the third layer) of the corrosion scale on the right‐hand side. Starting from this point, the stage is moved from left to right until the gauge mark coincides with the interface between the third and second layer. The displacement of the stage then corresponds to the thickness of the

third layer, x3;r. The thickness of following layers on the right‐hand side (x2;r, x1;r), diameter of unaffected sound material (steel), DST, and thickness of layers of the corrosion scale on the left‐

hand side (x1;l, x2;l and x3;l) are measured by successively bringing the interfaces in compliance with the gauge mark and taking readings of the corresponding displacements of the microscope

stage. The recession of sound material, xST, follows as

∆xST1

2D0 DST

(26)

with D0 denoting the initial diameter determined before the test. The actual position of the surface of sound material, XST, with reference to the initial surface at X0 is given by XST X0 ∆xST (27) The position with reference to the to the initial specimen surface of the interface between the jth and (j+1)th layer, Xj, follows as

Xj1

2 ∆xi;r

j

i=1

DST ∆xi;l

j

i=1

D0

(28)


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In this example, the jth layer is per definition closer to the material/scale interface than the (j+1)th. For the outermost layer, Xj is the position of the scale surface. It should be noted that the data calculated from the three equations above are average values for the opposing ends of the evaluated diameter. This does not influence the average, but extreme values as well as standard deviations calculated from several sets of such measurements. A separation of material recession and position of interfaces for opposing sites of the material cross section will be possible by marking the centre point of the original cross section or computer‐based evaluation of a high‐resolution image of the complete cross section. The two sets of layer thickness determined for each diameter are independent from each other. The statistical evaluation requires a relevant number of measurements, at sites that were selected without bias and uniformly distributed over the cross section. In the case of the introduced method, the latter is achieved by starting the measurements in an arbitrarily chosen angular position, and turning the circular cross section by a fixed angle in‐between the measurements. Guidelines for corrosion testing at elevated temperature suggest to include a minimum of 24 measurements of scale thickness and recession of sound material in the statistical analysis [7], corresponding to the evaluation of 12 diameters. The discussion of the measurements in the microscope so far implies that the same type of corrosion is observed on the opposing ends of the evaluated diameter. For specimens that were exposed to oxygen‐containing Pb or LBE, that is not necessarily the case (see Section 6.1). When locally different modes of corrosion occur on the same specimen, the quantification must aim at the average or maximum for each corrosion process separately. A straightforward situation arises if one of two corrosion processes that are involved in the decrease of a particular diameter clearly dominates the other with respect to the material loss. Then, (D0 – DST) represents the recession of sound material caused by the significantly stronger of the two. For corrosion in oxygen‐containing Pb or LBE, the latter is so especially if protective scaling with material loss <1 µm is observed on one side, and, on the opposing side, solution‐based corrosion that may consume in the order of 100 µm of material after relatively short time. However, if dominance is not clear, some diameter measurements will have to be disregarded in the evaluation, which can be compensated by increasing the number of performed measurements. Disregarding particular diameter measurements may also be indicated if the classification of the observed corrosion phenomenon is ambiguous. Especially for highly irregular local corrosion, the maximum attack observed in the investigated cross section is of higher relevance than the average. If not included by measurements uniformly distributed along the circumference, the maximum damage found in the cross section has to be quantified in addition to the systematic assessment. It is a clear advantage of the systematic assessment of corrosion effects that a simple count of investigated sites showing the same type of corrosion gives an objective estimate of the frequency of occurrence or percentage of surface area affected by this process, after division by the total number of investigated sites.


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6.3.3 Evaluation of measurements

The following list summarises the characteristics of material performance in oxygen‐containing Pb or LBE that may, in principle, be retrieved from measurements in the microscope, exemplified for cylindrical specimens:

(I) Thinning or From diameter of sound material determined recession of sound material after exposure (Eq. (26)), separately for the different corrosion phenomena observed. If D0 is known for the angular positions evaluated, the local D0 may be used. Otherwise, use the best estimate of the average D0 that is available.

(II) Thickness of corrosion scales Are measured directly on the material cross as well as distinguishable layers section, with accuracy being limited solely by the of the corrosion scale applied magnification and spatial resolution of the distance measurement. Comparatively precise data. Has to be assessed for each corrosion phenomenon separately.

(III) Instantaneous position of From the recession of sound material (Eq. (27)) interfaces with reference to the and thickness of the different layers of the initial material surface corrosion scale ((Eq. (28)).

(IV) Mass of material elements consumed, From the change in diameter of sound material but missing in the adherent scale and thickness of distinguishable layers of the corrosion scale, along with the average density and chemical composition of the material and layers of the corrosion scale [56].

(V) Percentage of surface area affected From the number of sites affected, divided by by different corrosion processes the total number of sites investigated.

6.3.4 Alternative determination of material recession

The recession of sound material obtained by or analogously to the described method basically is a small difference of two comparatively large numbers, with respectively large relative error. This may become especially apparent in the result for specimens that were exposed to the corrosive environment at relatively low temperature and for relatively short time. If the overall uncertainty in the initial diameter of a cylindrical specimen and errors originating from the preparation of the evaluated circular cross section added up to, e.g., ±5 µm, a true decrease of the diameter of sound material in the same order of magnitude would correspond to a measured value around Zero or, alternatively, approximately twice as much as actually expected. Adding another ±2 µm allowing for the limited spatial resolution of distance measurements and uncertainties in the identification of the tangents in the microscope, even a small increase of the diameter may be identified, or a decrease that is by factor 2.4 higher than the expectation. In such cases, the thickness of inward growing layers of the corrosion scale that may be assessed directly from the micrograph with ±1 µm accuracy or better, probably gives a more reliable estimate of the recession of sound


38

material than calculations on the basis of the difference in diameter. The latter was the conclusion from applying the metallographic method described above to oxidation of Fe for up to approximately 8000 h in oxygen‐containing LBE at 450°C [31]. However, the identification of the thickness of inward growing layers and recession of sound material is valid only if the interface between inward and outward growing parts of the scale is practically immobile. In general, this interface may move away from the initial position of the material surface with increasing exposure time, as was, e.g., indicated for accelerated oxidation of Steel T91 under the same conditions at 450°C, by the results from both quantitative microscopy and elemental analysis of the formed oxide scales [56]. Accordingly, the question of the more appropriate way to define the material recession can be answered only after having performed and analysed the measurements of both diameter and scale thickness. A special situation arises if the general corrosion process is accompanied by clearly negligible material recession, as, e.g., observed for protective scaling in oxygen‐containing Pb or LBE (Figure 13a). The recession of sound material caused by locally occurring accelerated oxidation or solution‐based corrosion may then be quantified with sufficiently high accuracy directly with reference to the position of the practically unaffected material surface in the surrounding of the preferentially attacked site. The accuracy gets partially lost with increasing lateral dimensions of the attacked site, i.e., if the magnification in the microscope needs to be decreased so as to have the reference surface in the evaluated micrograph. This will be a minor problem for the comparatively high depth of attack involved in the solution‐based corrosion observed in Pb or LBE. However, in exceptional cases, the curvature of a circular cross section may have to be reconstructed in the micrograph using the diameter determined before the test. For locally starting accelerated oxidation, applicability of the method is confined to initial stages, in which preferentially oxidised sites still appear in the form of isolated pits.

7 Conclusions

In general, laboratory experiments cannot simulate exactly, but at best approach the complex in‐service situation that materials will face in an industrial‐scale plant. However, the analysis and quantification of material performance at well‐defined boundary conditions with respect to the parameters that are relevant to plant operation, form the basis, sometimes the only basis, for deducing the likely performance and giving a quantitative estimate of what may be expected from the tested materials in the plant. Such conclusions from laboratory experiments are naturally the more reliable the better the experimental conditions have been characterised, controlled or, if the latter was not possible, at least monitored during the course of the test. Especially when testing corrosion in metallic materials caused by liquid Pb or LBE, the oxygen potential or concentration of solved oxygen plays an important role for both the corrosion phenomena observed and the quantitative outcome. The prediction of the actual oxygen potential from the provided oxygen source is aggravated by several factors, especially the conversion into solid oxides at the interface of liquid metal and tested material or on reactive surfaces inside the testing device or facility, so that continuous measurement is recommended. Oxygen sensors for


39

use in Pb or LBE are available, and a necessary pre‐requisite for actively controlling the oxygen potential. Characterising the experimental conditions with respect to solved oxygen by stating the type of sensor, especially the reference electrode in the case of electrochemical sensors, the sensor output and temperature of the measurement is always non‐ambiguous. The calculated concentration of solved oxygen depends on the assumed saturation concentration of oxygen as a function of temperature. Currently, different correlations are used in different laboratories, questioning the inter‐comparability of results, especially if only the calculated concentration is reported. Consistent calculation of oxygen concentrations in‐between laboratories is desirable, for which suggestions have been made. The corrosion observed in the materials that have been exposed to oxygen‐containing liquid Pb or LBE may be non‐uniform, especially in the case of steels. This needs to be reflected in the procedure applied in quantifying the corrosion damage. A metallographic method that allows for determining the local recession of sound material and thickness of corrosion scale has been introduced. The proposed procedure is currently being evaluated in the round robin performed in the framework of Task 3.2 of the MATTER project. The results will be reported as a supplement to this deliverable D3.4.

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MATTER Deliverable D3.4 v1 final · Document type Deliverable Work Package WP 3.2 Document number D3.4 – Revision 0 Date of completion 17/07/2014 Level of Confidentiality PU PP