Corrosion study of HK-40m alloy exposed to molten sulfate or vanadate

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Corrosion Science 50 (2008) 650–663

Corrosion study of HK-40m alloy exposed to molten sulfate/vanadatemixtures using the electrochemical noise technique

C. Cuevas-Arteaga *

Centro de Investigacion en Ingenierıa y Ciencias Aplicadas – UAEM, Av. Universidad 1001, Col. Chamilpa,

C.P. 62209, Cuernavaca, Morelos, Mexico

Received 23 October 2006; accepted 14 November 2007Available online 15 January 2008

Abstract

Corrosion performance of HK-40m alloy obtained from electrochemical noise technique and polarization curves during 24 h of expo-sure in high sulfate (80 mol% Na2SO4–20 mol% V2O5) and high vanadate (80 mol% V2O5–20 mol% Na2SO4) molten salts at 700 �C arereported. Electrochemical noise signals were analyzed in the time and frequency domain. A statistical analysis obtaining the resistancenoise, the current standard deviation and the localization index are presented as well as the determination of corrosion rates. Corrosionrates were supported by X-ray diffraction analysis of corrosion products and scanning electron microscopy analysis of corroded samples.Results from optical microscope examination of the corroded samples showed that HK-40m alloy suffered inter-granular corrosion whenwas exposed to the high vanadate salt, whereas exposed to the high sulfate salt, HK-40m corroded through a mixed corrosion process. Acorrosion mechanism of HK-40m alloy was obtained together with the corrosion rate, showing the different behavior when exposing thealloy to a high vanadate and high sulfate molten salts.� 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Molten salts; C. High temperature corrosion; C. Hot corrosion; C. Localization corrosion

1. Introduction

Fuel ash corrosion is a form of attack caused by moltenvanadium and sulfur compounds. These compounds meltbetween 500 and 600 �C and higher, causing destructionof the protective oxide scales of the metal alloy, inducinga degradation of the materials. This phenomenon can beseen in boilers, gas turbines and furnaces, these equipmentsuse residual fuel oils, which during combustion producecompounds such as Na2SO4 and V2O5, and other morecomplex mixtures formed from these two primary salts.This type of corrosion is called high temperature corrosionby molten salts or hot corrosion [1], occurring on theheated surfaces of superheaters or reheaters, which arebetween 600 and 650 �C. This type of corrosion is also

0010-938X/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.corsci.2007.11.011

* Tel.: +52 777 3 29 70 84; fax: +52 777 3 29 79 84.E-mail address: [email protected]

called vanadium attack, since there is evidence that vana-dium contributes must to the corrosion of metallic surfacesat high temperatures [2,3].

Many studies of high temperature corrosion by moltensalts have applied electrochemical polarization techniquessuch as linear polarization resistance (Lpr) and polariza-tion curves (PC) to determine corrosion rates of alloysmainly exposed to Na2SO4 and V2O5 mixtures at differentcompositions [4–10]. These works obtained corrosion ratesfrom the application of the Tafel extrapolation method tothe polarization curves and from the linear polarizationresistance technique. The application of some other electro-chemical techniques to investigate the hot corrosion sys-tems has been reported. Some of those techniques areelectrochemical impedance spectroscopy (EIS) and electro-chemical noise (EN). The application of EIS has been insome way limited to hot corrosion; nevertheless it has beenpossible to identify some controlling mechanisms as chargetransfer or diffusion, and the solution resistances [11–16].

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C. Cuevas-Arteaga / Corrosion Science 50 (2008) 650–663 651

With respect to electrochemical noise, normally this tech-nique has been used to determine the type of corrosionthrough the potential noise, especially localized corrosion;while the current noise has provided data related with thecorrosive activity, the current noise patterns can also indi-cate the corrosion mechanism (general or localized). Elec-trochemical noise technique not only has been used tostudy aqueous corrosion but also molten salt corrosion[13,17–19].

In 1980, Harada and Kawamura [20] reported that therewas not yet a material resistant to the corrosive effects ofmolten salts. This statement may still be true, even thoughmany types of materials containing different concentrationsof chromium, nickel, aluminum and silicon, have beenstudied to obtain their performance in high temperatureenvironments. The modified HK-40m alloy contains3 wt% more chromium and 0.8 wt% more silicon respectto the commercial HK-40 ASTM A351, having the compo-sition: Fe–28Cr–20Ni–2.04Si–0.64Mn–0.38C [21]. For itshigh chromium content and its silicon content, HK-40malloy promises to have a good performance when exposedin acidic molten salts containing vanadium and sulfur.

In the present work, the electrochemical current andpotential noise techniques and polarization curves havebeen applied to study the modified HK-40m alloy exposedfor 24 h under two different concentrations of molten salts:80 mol% Na2SO4–20 mol% V2O5 and 80 mol% V2O5–20 mol% Na2SO4 at 700 �C. Corrosion studies were sup-ported by X-ray diffraction (XRD) and scanning electronmicroscopy (SEM) analysis.

2. Experimental procedure

Samples were made of a sheet of modified HK-40m andcut as small rectangular parallelepipeds sized5 � 4 � 2 mm, ground to 600 grit silicon carbide paper,rinsed with distilled water, degreased with acetone anddried under a warm air stream. This specimens were theworking electrodes, to which one 80 wt% Cr–20Ni wire(150 mm long, and 1 mm in diameter) was spot welded.This wire was used as electrical connection between theworking electrode and the potentiostat. For isolating the80Cr–20Ni wire from the molten salts, ceramic tubes wereused; the gap between the ceramic tube and the electricalconnection wire was filled with refractory cement. The80 mol% Na2SO4–20 mol% V2O5 (high sulfate) and80 mol% V2O5–20 mol% Na2SO4 (high vanadate) mixtureswere prepared from analytical grade reagents. Knownamounts of the solid salts (500 mg/cm2 of the initial areaof the specimen) were introduced into a 20 ml silica cruci-ble, to give a melt depth of about 1.3 cm. Then the silicacrucible was set inside an electrical tube furnace to reachthe test temperature of 700 �C, which was measured con-stantly during the tests using a type K thermocouple andwas controlled to ±5 �C respect to the test temperature.The crucible together with the solid corrosive salt wasreplaced for each experiment. It is important to point out

that in high temperature corrosion phenomenon, alloysare covered with a thin molten salt layer and with a corro-sive gas atmosphere, whereas the experimental procedurecarried out in this work were deep melt tests in static air;therefore the experimental conditions and corrosion ratesobtained are not representative of fireside corrosion condi-tions. However, bulk molten salt tests are a viable proposi-tion for material corrosion evaluation.

For the electrochemical noise technique, the electro-chemical cell was formed by a three-electrode setup, includ-ing two ‘‘identical” working electrodes and a 5 mmdiameter/150 mm long of platinum wire as the referenceelectrode. The stability of the platinum reference electrodehas been proved and previously reported [22], also it hasbeen utilized in some others works [9,11,17,18]. This refer-ence electrode has been used under similar molten salt con-ditions [17,22,23]. For polarization curves, theelectrochemical cell was constituted by the working elec-trode (HK-40m), and two 5 mm diameter/150 mm longof platinum wires as auxiliary and reference electrodes.All platinum electrodes were cleaned, abraded on 600 gritSiC paper, washed and dried before being isolated in cera-mic tubes and sealed with refractory cement, leaving 5 mmlong free to contact with the corrosive salt. Once the silicacrucible containing the molten salt was set inside the elec-trical tube furnace, and the desired temperature wasreached, the electrochemical cell was introduced insidethe crucible, and then the corresponding cables of thepotentiostat were connected to the electrochemical cell,and tests were begun. Instruments connections were madewell away from the furnace to avoid thermocouple effects.

The simultaneous electrochemical current and potentialnoise were carried out through an ACM Instruments zero-resistance ammeter (ZRA) coupled to a personal computer,which was used to control and store the data for furtheranalysis. The electrochemical noise data were recordedwith a sampling frequency of 1 Hz; 2048 measurementswere obtained each hour during 24 h of immersion. Dataprocessing included trend removal prior to obtaining theimpedance noise spectra using the Fast Fourier Transform(FFT) algorithm. Polarization curves were accomplishedpotentiodynamically polarizing the specimen at ±150 mVwith respect to the corrosion potential at a scan rate of1 mV/s. The selection of this scan rate has been previouslypublished elsewhere [22], where a graph showed somesimilar polarization curves at different sweep rates (0.2,0.5 and 1 mV/s). The anodic branch remained the samefor the three different sweep rates, and the cathodic branchonly showed a small difference at the smallest sweep rate athigh cathodic overpotential (1000 mV). The sweep rateof 1 mV/s has been also reported in any others works[2,4–6,17,23]. The tests started after 40 min of exposingthe specimen to the corrosive salts and when the corrosionpotential reached a stable condition. Electrochemicalpolarization curves were made using and ACM Instru-ments Auto DC potentiostat, also controlled by a personalcomputer.

652 C. Cuevas-Arteaga / Corrosion Science 50 (2008) 650–663

The weight-loss method was also carried out accordingto the experimental procedure reported elsewhere [22]. Thismethod was applied for 24 h to obtain the mass loss of HK-40m under both test conditions. In order to determine thekind of corrosion compounds developed during the corro-sion process, the corrosion products obtained after clean-ing the exposed specimens from the weight-loss techniquewere kept for X-ray diffraction analysis (XRD) utilizing adiffractometer operating with Cu Ka radiation, and resultswere interpreted using the Powder Diffraction Data File[24]. Also, the cleaned specimens were observed under theoptical microscope to examine the morphological surfaceand decide the type of corrosion attack.

Scanning electron microscopy (SEM) technique wasused to characterize the exposed samples from the electro-chemical noise technique; such analysis was carried outusing a Microspec WDX-3PC system connected to a ZeissDSM 960 scanning electron microscopy. The two speci-mens exposed for 24 h to the two different molten saltswere mounted (without de-scaling) in bakelite, metallo-graphically polished and the cross section was analyzedto investigate the morphology and distribution of reactionproducts. Through SEM analysis, an electron image of themetal–scale interface and X-ray mappings of the mean ele-ments of each system were obtained.

3. Results

3.1. Microstructural observations

Fig. 1 shows a micrograph of a cross section of themetal–scale interface of the corroded HK-40m specimenexposed to the high-sulfate molten salt at 700 �C during24 h after applying the electrochemical noise technique.X-ray mappings of chromium, nickel, iron, sulfur andsodium are also presented. According to the chromiummapping at the metal surface there is a concentrated55lm chromium rich layer. Underneath this layer is a chro-mium depleted zone of the alloy, which is replaced by avery well defined 30lm thickness film of nickel compound.Above this defined nickel layer, which is on the metal sur-face, there is certain concentration of nickel together withthe chromium layer. Supported by the X-ray diffraction(XRD) results of corrosion products (Fig. 6), it can beinferred that at the beginning of the corrosion phenomenonthere was a Cr mass loss by the effect of oxidation; after-ward such chromium was transformed in a passive layerof Cr2O3 as a reaction of the alloy to be protected, then thisinitially dense and coherent passive layer of chromiumoxide was evenly dispersed with a trend to be dissolved,as can be seen to the left of the corresponding mapping.It is possible that the place initially occupied by the chro-mium inside the alloy has been replaced by the nickel,which has probably diffused from the matrix to themetal–oxide interface enhancing the alloy surface of nickel,or maybe the Cr mass loss to form the corrosion layer ofCr2O3 produced the Ni enrichment alloy surface. The chro-

mium oxide is one of the most protective compounds whenmolten salts are present [3,22]. With respect to the ironmapping, a weak and discontinuous layer mixed with thedissolved nickel and chromium layers is seen, may be inthe form of the NiFe2O4 spinel as evidenced in the XRDresults (see Fig. 6). Different metal oxide layers areobserved very close to the metal surface, lacking evidenceof significant oxide dissolution throughout the thick corro-sion products. The high concentrations of sulfur in the cor-rosion products scale is evident, and in a lesser extent ofsodium, which is supported by the unreacted Na2SO4

found in the corrosion products analyzed by XRD tech-nique. There is no evidence of internal degradation by sulf-idation at this time, although the chemical interactionbetween the main elements of the alloy with sulfur in theformation of metallic sulphides, at the metal–scale interfacewould be possible.

Fig. 2 shows a SEM micrograph of the interface ofmetal–corrosion products for HK-40m after exposing24 h to the high-vanadium molten salt at 700 �C, and itsrespective mappings of the principal elements as chro-mium, nickel, iron and vanadium and sulfur as elementsconstituting the molten salt. Different from the previouscase, the mapping of chromium for the high-vanadiumexposition presents a discontinuous and non coherentchromium layer on the metal surface, and an appreciableamount of dissolved chromium far away from the metal-corrosion products scale, as an indication that the high-vanadium molten salt is more aggressive than the highsulfate salt, leading to the dissolution of an initially protec-tive chromium oxide layer, as can be seen in the corre-sponding chromium mapping. Nevertheless, the mappingof nickel shows a more regular and defined layer just atthe metal surface, and a small amount of dissolved nickeloxide just over the defined nickel layer. With respect tothe mapping of iron, it seems not to have had an importantparticipation during the 24 h of exposition. In this case, thethick corrosion product presented a high concentration ofvanadium and a certain concentration of sulfur over thenickel layer and the metallic surface, where maybe therewas a chemical interaction between the chromium oxideand the sulfur in the formation of chromium sulfides. Ina previously reported work [22], related with the applica-tion of the Lpr technique for the study of HK-40m exposedto the same molten salt during 10 days at the same temper-ature, SEM results showed the depletion of chromium inthe substrate due to oxidation accompanied by dissolution,and a sulphide zone in the corrosion products region asso-ciated mainly with chromium. The corrosion mechanismreported in that work is in accord with the results presentedin this research with respect to the chromium mapping;with respect to the nickel mapping, it was noticed the pres-ence of a continuous nickel film in the metal–oxide scaleinterface, taking the place of the initially formed Cr2O3,and it was assumed that nickel had diffused from the sub-strate to the scale, to form a film, which after 10 days ofexposure appeared as a protective layer, providing good

Fig. 1. Electron image of the metal–scale interface and X-ray mappings of Cr, Ni, Fe, S and Na of HK-40m exposing 24 h to 80 mol% Na2SO4–20 mol%V2O5 at 700 �C.


corrosion resistance when molten salts have a high concen-tration of vanadium and low concentration of sodium,which was according to the observations reported by Wil-son and Pfeil [25,26].

As was mentioned in Section 2, the corroded specimensfreed from the adherent corrosion products from theweight-loss method were observed through the optical

microscope to obtain their morphology, whereas the corro-sion products were analyzed by XRD technique. Figs. 3and 4 are optical micrographs at 100 and 200�. Fig. 3shows the corroded specimen exposed to the high sulfatemolten salt, which shows a uniform degradation as themain corrosion process, but also a mixed corrosion pro-cess, observing small sites attacked in a low degree of local-

Fig. 2. Electron image of the metal–scale interface and X-ray mappings of Cr, Ni, Fe, V and S of HK-40m exposing 24 h to 80 mol% V2O5–20 mol%Na2SO4 at 700 �C.


ized corrosion. Fig. 4 presents micrographs of the corrodedspecimen exposed to the high vanadium molten salt; in thiscase, a particularly generalized corrosion process through-out the surface is observed, where a significant amount oflocal attacks are present. Although numerous local attacksare seen, this morphology is not the typical of pitting cor-rosion, in which the passivated material suffers the rupture

of the passive film, generating active zones in small areaswith high corrosion rates, where the nucleation and subse-quent formation of cavities or pits are present [27]. Thelocalized corroded sites (hollows) of the HK-40m seem tohave formed due to an inter-granular corrosion process,with the following grain dropping. The evidence ofsome revealed grain faces is present on the micrographs,

Fig. 3. Optical images of HK-40m exposed to 80 mol% Na2SO4–20 mol% V2O5 at 700 �C for 24 h.

Fig. 4. Optical images of HK-40m exposed to 80 mol% V2O5–20 mol% Na2SO4 at 700 �C for 24 h.


observing the preferential corrosion at or adjacent to thegrain boundaries of the metal; nevertheless, although itseems a pronounced inter-granular attack, there is alsosome appreciable grain-face corrosion, since there are somewide grooves that probably formed by corrosion of thegrain faces, therefore, if this corrosion process continued,the grooves grow up and grains drop out readily. Whethercorrosion is predominantly by inter-granular or by generalattack depends on the difference between the rate of corro-sion of the grain boundary zones and that of the grainfaces; this difference in rates is determined not only bythe metallurgical structure and composition of the bound-ary but also by the characteristics of the corroding mixture.

Thermal exposures during welding or fabrication canchange the composition of grain boundaries of alloy byequilibrium segregation of impurities and/or alloying ele-ments and, most frequently by the formation of precipi-tates, such as chromium carbides; this changes can makethe grain boundaries susceptible to rapid and preferentialattack (sensitization) in many, mostly acid environmentsin which the materials are otherwise considered to havean acceptable degree of corrosion resistance [28]. Thus, as

a result of structural imperfections, the chemical composi-tion, and consequently the corrosion resistance of grainboundaries may be appreciably different from those ofthe interior of grains. From metallographic examination,EDX and XRD analysis, the microstructure and composi-tion of HK-40m alloy was determined by Haro et al. [21],who reported that the microstructure of HK-40m consistedof an austenitic matrix saturated of carbon, containingchains of eutectic carbides which present a laminar struc-ture or in skeleton form (outlining the grain boundaries),typical of the eutectic carbides. The dominant phases wereaustenite (Fe,C) and eutectic carbides of the type Cr7C3

and Cr23C6. These microstructural characteristics of theHK-40m together with the exposition to the aggressivehigh vanadium molten salt certainly caused the inter-gran-ular corrosion of the HK-40m alloy.

Even though the specimens were cleaned before observ-ing their surface, it was possible to find some few corrosionproducts, on which an EDX analysis was made. Fig. 5shows the EDX of adherent corrosion products of HK-40m exposed to the high sulfate (a), and the high vanadium(b) molten salts at 700 �C. The EDX spectrum of corrosion

0 500 1000 1500 2000

0

wt. %Si = 1.69 S = 1.38Cr = 41.24Fe = 29.21 Ni = 26.46

SSi

Ni

Fe

Cr

KeV0 500 1000 1500 2000

0

wt . %Si = 2.99V = 18.37Cr = 57.67Fe = 15.76 Ni = 5.2

Si

V

Ni

Fe

Cr

KeV

Fig. 5. EDX spectral plots of corrosion products of HK-40m exposed to: (a) high sulfate and (b) high vanadium at 700 �C.

Fig. 6. X-ray diffraction results of corrosion products of HK-40m exposed24 h to 80 mol% Na2SO4–20 mol% V2O5 at 700 �C.


products derived from the exposure to high sulfate pre-sented chromium and nickel in higher concentrations thanthe corresponding concentrations in the HK-40m, alsolower concentrations of iron and silicon were observed,with a low concentration of sulfur. This result is in agree-ment with the X-ray mappings of Cr and Ni presented inFig. 1, where a very significant chromium layer covered adefined nickel film. The presence of sulfur can be evidenceof the chemical interaction between Cr, Ni and maybe Si inthe formation of metallic sulphides in metal–scale interface.The spectrum of corrosion products derived from the expo-sition to high vanadium also shows a high concentration ofchromium and a significant concentration of vanadium asthe main compound of the corrosive molten salt. The con-centration of silicon is also higher than the original concen-tration of the HK-40m. The high concentration of Crcorroborates the preferential dissolution of this elementin presence of vanadium, and the concentration of siliconindicates the most important participation of this elementwhen exposing the HK-40m in high vanadium molten salt,although the X-ray mapping analysis presented no evi-dence of silicon in the metal–scale interface.

3.2. X-ray diffraction results

Fig. 6 presents the X-ray diffraction analysis results ofthe corrosion products obtained from the exposition ofHK-40m to the high sulfate molten salt at 700 �C during24 h. This spectrum showed that during corrosion, chro-mium oxide (Cr2O3), iron oxide (Fe2O3) and the spinel(NiFe2O4) were formed. Unreacted Na2SO4 was alsofound. These results were consistent with the observationsby SEM, where these species and an important concentra-tion of sulfur and sodium could be seen. The X-ray diffrac-tion analysis of corrosion products obtained from theexposition of HK-40m exposed to the high vanadium mol-ten salt at 700 �C has been published in a previous work[22]. The reported compounds were NiO, FeCr2O4, Cr2O3

and Na8V24O63, which is in accord with the SEM observa-tions. The Na8V24O63 compound has been detected when

the 80 mol% V2O5–20 mol% Na2SO4 molten salt is exposedto temperatures higher than 600 �C [29,30]. The formationof Na–V–O compounds when the 80 mol% V2O5–20 mol%Na2SO4 mixture is exposed to high temperature is dis-cussed afterward.

3.3. Electrochemical measurements

Fig. 7 shows the potentiodynamic polarization curve forHK-40m exposed to 80 mol% Na2SO4–20 mol%V2O5 at700 �C. The graph was taken once Ecorr was stable, whichwas approximately after 40 min. This curve showed a cor-rosion potential of �55 mV. Applying the Tafel extrapola-tion method, the corrosion current density was obtained as1 mA/cm2. The Tafel slopes determined by the samemethod were ba = 147.10 mV/decade and bc = 99.42 mV/decade. A polarization curve for HK-40m exposed to thehigh vanadium salt at 700 �C has been already reported[22]; through the corresponding curve, the Tafel slopeswere obtained as ba = 78 mV/decade and bc = 76 mV/dec-ade; whereas the corrosion potential was �8 mV, and the

0.1 1 10-300

-200

-100

0

100

200

Current Density (mA/cm2)

Po

ten

tial

(m

V)

Pt

Ref

eren

ce E

lect

rod

e

Fig. 7. Polarization curve for HK-40m exposed to 80 mol% Na2SO4–20 mol% V2O5 at 700 �C.

0 500 1000 1500 20007.8

8.0

8.2

8.4

8.6

8.8 23 h

20 h8 h

1 h

Po

ten

tial

(m

V)

Time (s)

Fig. 9. Time series of electrochemical potential noise for HK-40m exposedto 80 mol% Na2SO4–20 mol% V2O5 at 700 �C.


corrosion current density 2.7 mA/cm2. For the presentstudy, the polarization curves are important because theanodic and cathodic slopes ba and bc were determined tobe used in the calculation of corrosion rates from the elec-trochemical noise through the resistance noise Rn, using theStearn–Geary equation. The Tafel regions in the polariza-tion curves are not well defined, probably reflecting theimportance of mass-transfer conditions, especially in thecase at high sulfate, where the Tafel slopes were the largestvalues, indicating that the diffusion of the species can havea significant influence on the rate controlling step. Tafelslopes lower than 100 mV/dec are typical for activationcontrolled systems, whereas Tafel slopes with larger valuesare typical for systems which are not purely activation ordiffusion controlled [31]. This difference can be due to thefact that the corrosive ionic substances containing mainlythe basic species Na2O (high sulfate molten salt) transportby ionic movement, whereas the semi-conducting moltensalts (high vanadium molten salts) involve a more complexmechanism where diffusion assisted by an additional elec-tron transport is present [25]. These characteristics of mol-ten salts will be discussed further.

0 500 1000 1500 20000.05

0.06

0.07

0.08

0.09

0.10

0.11

23 h

20 h

8 h

1 h

Cu

rren

t (m

A/c

m2 )

Time (s)

Fig. 8. Time series of electrochemical current noise for HK-40m exposedto 80 mol% Na2SO4–20 mol% V2O5 at 700 �C.

The electrochemical noise time series were used to ana-lyze the changes in corrosion activity for HK-40m underthe experimental conditions. Figs. 8 and 9 show the electro-chemical current density and potential noise signals, whileFig. 10 presents the impedance spectrum at different timesof immersion (1, 8, 20, 23 h) for HK-40m exposed to80 mol% Na2SO4–20 mol% V2O5 at 700 �C. In general,the current noise exhibits low amplitude transients accom-panied with random oscillations. The larger intensity of thetransients was 0.001 mA/cm2, except at 1 h, where thehighest value was 0.004 mA/cm2. From the beginning until1000 s, the time series at 1 h has some current transients insuccessive 200 s, which is evidence for the nucleation oflocalized sites or the rupture and recovery of the passivefilm; after that, there was not propagation of transients.At 8 and 20 h there are also some transients of lower ampli-tude than that at 1 h. At the end of experiment, there wasless corrosion activity; hence the oscillations appeared to beof lower intensity and the current magnitude was lowerthan at the others times. The current density values areall positive, in a range of 0.053–0.105 mA/cm2, indicatingthe preferential dissolution of one electrode only [32]. Also,

1E-3 0.01 0.10.1

1

10

100

1000

1 h8 h20 h23 h

Oh

ms.

cm2

Frequency (Hz)

Fig. 10. Spectral analysis of electrochemical resistance noise for HK-40mexposed to 80 mol% Na2SO4–20 mol% V2O5 at 700 �C.

0 500 1000 1500 2000720

750

780

810

840

870

900

19 h

22 h 10 h

1 hPo

ten

tial

(m

V)

Time (s)

Fig. 12. Time series of electrochemical potential noise for HK-40mexposed to 80 mol% V2O5–20 mol% Na2SO4 at 700 �C.


the current values trended to decrease with time, as resultof the alloy was protecting and therefore tending to passiv-ity due to the formation of a protective corrosion layer ofCr2O3, as was seen in the Cr mapping. From the aboveanalysis, it can be said that oscillations of electrochemicalmeasurements are associated with a continuous change ofmetal surface due to the exposure to the molten salt at hightemperature. According to the behavior of noise patternsunder the exposition to high sulfate, it can be inferred thatHK-40m suffered a kind of uniform corrosion with isolatedlocalized events or the rupture and recovery of passive film.The potential current noise has similar characteristics: low-amplitude, high-frequency stochastic oscillations withsome transients, which have a good correspondence withcurrent time series. The open-circuit potentials werebetween 8.8 mVpt and 7.9 mVpt. Also, an increase of poten-tial with time indicates the formation of a passive film, suchas the current time series indicated.

Fig. 10 presents the spectral noise impedance plots,which were obtained using an algorithm based on the FastFourier Transform (FFT) of spectral analysis. The resolvedfrequency bandwidth of interest lies between 0.5 and500 mHz. The spectral noise impedance showed that atlow frequencies the amplitude impedance was increasingwith time until 20 h, after that the amplitude decreased.The spectral noise impedance is essentially independentof frequency. The magnitude of noise impedance alongthe bandwidth was from 0.7 to 1000 X cm2, being the aver-age of the first 10 values at the lowest frequency of 44, 69,45 and 19 X cm2 at 1, 8, 20 and 23 h, respectively. Theseresults suggest a passive condition and similar corrosionmechanisms along the experimental time.

Fig. 11 shows the time series of electrochemical currentnoise for HK-40m exposed to 80 mol% V2O5–20 mol%Na2SO4 at 700 �C. In the case at high sulfate, the magni-tude of current density values decreased with time(Fig. 8). In the case at high vanadium, the current densityvalues increased with time, from 0.1 mA/cm2 to 1.25 mA/cm2, whereas the highest value at the high sulfate case

0 500 1000 1500 2000

0.0

0.3

0.6

0.9

1.2

1.5

22 h

19 h

10 h

1 h

Cu

rren

t (

mA

/cm

2 )

Time (s)

Fig. 11. Time series of electrochemical current noise for HK-40m exposedto 80 mol% V2O5–20 mol% Na2SO4 at 700 �C.

was 0.105 mA/cm2. After 1 h immersion, some few lowintensity anodic transients were seen, together with sto-chastic oscillations. At 10 and 19 h, a similar behaviorwas observed. Nevertheless, at 22 h a major corrosionactivity showed more anodic and cathodic transients ofhigher amplitude. These results suggest that the corrosionprocess for HK-40m could be different for the exposures(high sulfate and high vanadium).

Fig. 12 presents the time series of electrochemical poten-tial noise at the same conditions. At the open-circuit, thepotential lies between 725 mVpt and 870 mVpt, and thenoise pattern showed white noise, especially at 10, 19 and22 h; at the 1 h, some low amplitude anodic and cathodictransients were seen. The most important feature of thepotential series is that the values are much nobler than thatat the high sulfate exposure. The spectral noise impedance(Fig. 13) showed similar characteristics than that observedfor the high sulfate, being the magnitude of the impedancenoise of 1.5– 7000 X cm2. In general, the noise impedancedecreased at low frequencies, which evidence that corrosionrate increased with time, such as the current time series

1E-3 0.01 0.1

1

10

100

1000

10000 1 h10 h19 h22 h

Oh

ms.

cm2

Frequency (Hz)

Fig. 13. Spectral analysis of electrochemical resistance noise for HK-40mexposed to 80 mol% V2O5–20 mol% Na2SO4 at 700 �C.

0 5 10 15 20 25

1E-3

0.01

0.1

High sulfate

High vanadium

Exposure time (h)

Sta

nd

ard

dev

iati

on

mA

/cm

2

Fig. 14. Current standard deviation for HK-40m exposed to both moltensalts at 700 �C.

0 5 10 15 20 25

0.0000

0.0004

0.0008

0.0012

0.0016

0.002080 mol% Na2SO4-20V2O5

80 mol% V2O5-20Na2SO4

Wei

gh

t lo

ss (

g/c

m2 )

Exposure time (h)

Fig. 15. Weight-loss along 24 h obtained from the electrochemical noisetechnique for HK-40m exposed to both molten salts at 700 �C.


indicated. The impedance spectra reflected a similar corro-sion processes of metal surface immerse in the molten saltsolution through the test. A high noise impedance flat spec-trum independent of frequency was observed, with a slightdecrease of impedance with time, suggesting a less protec-tive condition of the corrosion product layer.

Fig. 14 presents the noise current standard deviation asa function of time for HK-40m alloy exposed to both mol-ten salts at 700 �C. For the high sulfate case, this parameterpresented very small values, being in an interval of 0.0018–0.01 mA/cm2, confirming only some few transients in thecurrent time series. At the high vanadium case, the noisecurrent standard deviation is positioned in the range of0.032–0.162 mA/cm2, having an oscillatory behavior untilthe 11th h, and then remained almost constant throughoutthe experimental time, except at the end of the test, wherean important decrease was seen. The standard deviation athigh vanadate was almost two orders of magnitude higherthan that for high sulfate, indicating a major localizedactivity for the case at high vanadate, such as shown bythe current–time series.

Fig. 15 presents the corrosion rates for HK-40mexposed to the high sulfate and high vanadate molten saltsat 700 �C determined from the electrochemical noise datafor a period of 24 h. The resistance noise Rn was calculatedas the ratio of the standard deviation in the voltage signalto the standard deviation in the current signal [33,34]. Toobtain the corrosion rate as a mass loss measurement, theStern–Geary equation evaluated from the Tafel slopes pro-vided the corrosion current density Icorr (mA/cm2). Fara-day’s law was applied to determine the mass loss [34,35–37]. The Tafel slopes used for this calculation were thoseexperimentally obtained from the polarization curves.The full mathematical procedure for the determination ofmass loss have been reported elsewhere [17,23], whichwas supported by the standards ASTM G102 [36] andG16 [37]. The magnitude of the mass loss for the high sul-fate case was between 0.007 and 1.03 mg/cm2, whereas forthe high vanadate case it was between 0.139 and 2.09 mg/cm2. Even though both plots seem alike, most of the data

for high vanadate are higher, but the behavior is similar,observing some oscillations with time. Also, the tendencyof the corrosion rates decreased with time, which meansthe material was protected. From the electrochemical noisetechnique, the accumulative mass loss during the 24 h forthe high sulfate exposure was 0.0081 g/cm2, while for thehigh vanadate exposure was 0.011 g/cm2, a one order ofmagnitude of difference. From the weight-loss method,the mass loss during 24 h of exposure for the high sulfatewas 0.0093 g/cm2, and for the high vanadate was 0.03 g/cm2. A comparison of the mass-loss from the electrochem-ical noise technique and from the weight loss methodshows that both values are in the same order of magnitude,and that the values from the ENT were lower. This differ-ence may be due to the uncertainty of very low values ofmass loss generated in very small specimens in theweight-loss method. Other explanation could be the impli-cations in the use of the Faraday’s Law and the Stern–Geary equation (to convert electrochemical data in massloss), which assume that uniform corrosion is occurringand the corrosion reactions are activation controlled.Given that uniform corrosion was not seen in the corrodedsamples, and the Tafel regions in the polarization curveswas not well defined, some errors could be introduced inthe calculation of the mass loss.

From the corrosion rates results and from SEM analysisincluding microscopic morphology, it can be stated that theHK-40m alloy suffered a more intensive corrosion processwhen exposed to the high vanadium molten salt. Given thatthe HK-40m alloy was exposed to two different mixturescontaining V2O5 and Na2SO4 with 20 and 80 mol% ofV2O5, this statement is supported by the fact that the oxidelayer on the alloy suffers a dissolution by the effect of cor-rosive species, which destroys the capacity of this oxidelayer to protect the alloy of rapid oxidation (see Cr map-ping). On the other hand, the fluidity, which is related withthe viscosity of the high vanadium molten salt, is higherthan that of the high sulfate molten salt, since the meltingpoints of vanadium pentoxide and sodium sulfate are 670and 884 �C, respectively [38]. This higher fluidity of high


vanadium molten salt favors the diffusion of chemicalactive species. An additional increase in fluidity might beassociated with the formation of some compounds of lowermelting points when 80 mol% V2O5–20 mol% Na2SO4 isexposed to high temperature [6,20,38–40]. Some of thereported compounds are several types of sodium vanadylvanadates such as NaVO3 (meta-vanadate f.p. 560 �C),Na2S2O7 (sodium pyro-sulfate f.p. 400 �C), Na2O � V2O4 �5V2O5 (beta vanadyl vanadate, f.p. 625 �C), 5Na2O �V2O4 � 11V2O5 (gama vanadyl vanadate, f.p. 535 �C) andsome others vanadyl vanadates as NaV6O15, Na5V12O32,Na4V2O7 which melting points are lower than 630 �C. Sim-ilarly, the reduction in corrosive power of the high sulfatemolten salt can be attributed to a reduction in fluiditycaused by the predominance of the relatively high meltingpoint of some compounds such as Na3VO4 (ortho-vana-date, p.f. 850 �C) and the sodium sulfate itself [38]. Finally,it is known that 80 mol% V2O5–20 mol%Na2SO4 is veryacidic, potentially oxidant, and it has a great capabilityfor absorbing oxygen when it is in a molten state, beingthe oxygen one of the most important oxidant that acceler-ates the oxidation reactions [3,25,41,42]; hence, theexpected changes of this melt must be an important influ-ence upon the dissolution of metallic oxides. In a previouspublished work of the HK-40m [22], some vanadylvanadates were reported, which were found in corrosionproducts obtained after the exposure to 80 mol% V2O5–20 mol%Na2SO4 at 600 and 700 �C. Some of the reportedcompounds were Na2O � V2O4 � 5V2O5, Na8V24O63,5Na2O � V2O4 � 11V2O5, and NaV2O8.

Fig. 16 presents the localization index of HK-40m forboth exposures. One of the purposes of the present workwas to correlate the corrosion process and/or the type ofcorrosion of HK-40m with the localization index LI, whichin some cases can be considered as an indicator of the pre-vailing corrosion mechanism [43]. This parameter was cal-culated as the ratio between the current noise standarddeviation ri, over the root-mean-squared current value Irms

[44,45], the corresponding mathematical relationship is rep-

0 5 10 15 20 250.01

0.1

1

High sulfate

High vanadium

Mixed corrosion Zone

localized corrosion Zone

Lo

caliz

atio

n In

dex

(ad

im)

Exposure time (h)

Fig. 16. Localization index of the corrosion process for HK-40m exposedto the high vanadium and the high sulfate mixture salts at 700 �C.

resented by Eqs. (1) and (2), where n is the total data of thecurrent time series and xi the current values from 1 to n,

LI ¼ ri=I rms ð1Þ

I rms ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

n

Xn

i¼1x2

i

rð2Þ

LI = 0 is observed for systems for which the individualdata points xi show only small deviations from the meanvalue of current, while LI = 1 is observed for xi>> thanthe mean value of current. For uniform corrosion, LI val-ues lie between 0.0 and 0.01; for mixed corrosion, between0.01 and 0.1; and for localized corrosion between 0.1 and1.0 [45]. Taking into account this statement, the values oflocalization index for the high sulfate case were in the rangeof 0.019–0.05546, except for two points (15 and 22 h),which had a localization index of 0.1562 and 0.1552, thus,according to this parameter, the corrosion process corre-sponded to the mixed corrosion range (see Fig. 16). Also,an oscillatory behavior throughout the experimental timewas an indicative that the corrosion process changed frommixed to uniform corrosion. For the high vanadate case,the range was 0.1322–0.54, except for the last point(24 h), when the localization index was 0.0338, hence it ispossible to say that localized corrosion resulted for the highvanadium case. In this case, the localization index also pre-sented an oscillatory behavior with a trend to decrease withtime, as an indicative that the material was passivating,maybe with the formation of an adherent and coherentNi film. The values of the localization index in the rangeof localized corrosion (which indicates the presence of cur-rent transients) must indicate the higher corrosion ratedeveloped in the inter-granular zones of the metallic sur-face, where the anodic reactions carried out mainly bychromium oxidation, whereas the cathodic reactions werecarried out in the grain faces, thus creating a galvanic cou-ple. The difference in the type of corrosion for HK-40m al-loy exposed to the two different molten salts is evident, asprove from the visual observations of the metal surfaceafter exposition (Figs. 3 and 4).

4. Discussion

Normally, high temperature corrosion proceeds by aninitiation stage, in which an oxide scale provides protectionto the alloy. This is followed by a propagation stage, inwhich the scale is no longer protective and corrosion ofthe metal ensues, often at a rapid rate. The corrosion pro-cess of HK-40m alloy exposed to two opposite composi-tions of Na2SO4 and V2O5 has been studied by differentapproaches according to the SEM, XRD and electrochem-ical measurement results. For the attack in high sulfate, theformation of a 55 lm concentrated chromium corrosionlayer with the consequential depleted alloy was observed,which empty space was replaced by a very well-definednickel layer. In this case, a slight dissolution of bothmetallic oxides was seen. The iron appeared as a weak


and discontinuous layer mixed with nickel and chromiumlayers. In accordance with the XRD analysis, at the begin-ning of the corrosion, a protective chromium oxide (Cr2O3)was formed. Then the formation of a nickel and iron oxidelayers occurred, maybe in a form of NiFe2O, as it wasevident in the XRD results. The most significant character-istics of the corrosion mechanism when exposing theHK-40m alloy to high vanadium salt was an important dis-solution of chromium oxide, and the presence of sulfurover the surface, even though the concentration of Na2SO4

was relatively low. It is possible that during the first stageof the corrosion process, the formation of a coherent anddefined chromium oxide was present; otherwise, the corro-sion products scale would not present an important disso-lution of chromium (see Fig. 2). On the other hand, it isknown that 80 mol% V2O5–20 mol% Na2SO4 is very acidic,extremely corrosive, potentially oxidant, and it has a greatcapability for absorbing oxygen when it is in a molten state[3,25,41,42]. With respect to the transport of oxygen, Wil-son [25] indicated that oxygen transport through the mol-ten salt can be by ionic movement in a corrosive ionicsubstance containing mainly the basic specie Na2O (highsulfate), or by means of a more complex mechanism involv-ing the electron transport as in semi-conducting moltensalts. Pantony and Vasu [42,46] and Wilson [25] wereagreed in that melts high in vanadium pentoxide retainessentially semi-conducting characteristics. This differencemay be expected to exert a major influence upon the corro-sion characteristics of the two types of melts (high sulfateand high vanadium). The general mechanism of hot corro-sion involves chemical and electrochemical reactions. Insuch corrosion process an oxide scale forms on the alloysurface to provide protection to the alloy, then in a secondstage this scale suffers chemical acidic or basic dissolutionby the effect of the molten salts, which depends on the basi-city of the fused salts. The dissolution of the oxides is oneof the most important aspects of the corrosion phenome-non to determine the corrosion mechanism of pure metalsand alloys. Hence, the chemistry of the melts has been stud-ied by several authors.

Zheng et al. [16] and Otsuka et al. [47] reported the basi-city in Na2SO4–10 mol% NaVO3 melts in 1% SO2–O2 envi-ronment at 900 and 927 �C, utilizing a basicity sensor andan oxygen probe. They showed that an addition of V2O5

modified the basicity of the melt, becoming more acidic,determining that the solubilities of the metallic oxides byacidic dissolution in the melts were much higher than inpure Na2SO4. They also determined that the charge trans-fer in the molten salts via transition metal ions was bycounter-diffusion of ions with different valences containingV4+ (V2O4-derived) and V5+ (V2O5-derived); therefore, thetransport of the active oxidants in this mixture with theaddition of V2O5 was much faster than in the pureNa2SO4. They stated that the vanadate anions did enhancethe onset of hot corrosion and sulfidation, probably byfacilitating a direct contact between the melt and the basemetal at cracks/defects or grains boundaries, which proba-

bly occurred when HK-40m was exposed to the high vana-dium molten salt, where the highest corrosion rates wereseen in the grain boundaries and the formation of metallicsulphides in the metal–oxide interface. A low concentra-tion of sulphide phase in the metal–scale interface (whichhad not penetrated into the metal) has been reported else-where [22], where the HK-40m was exposed under thesame experimental conditions of high vanadium during10 days. The 80 mol% Na2SO4–20 mol% V2O5 molten saltutilized in the present study can be comparable with thatused by Zheng et al. [16] and Otsuka et al. [47], therefore,it deduces that the basicity of the melt becomes more acidicwith 20 mol% of vanadium pentoxide added in pureNa2SO4, taking into account that V2O5 species is moreacidic than NaVO3 [48], thus, the acidic solubilities ofthe main metallic oxides formed on the HK-40m surface(Cr2O3, NiO and Fe2O3) must increase more significantly.Nevertheless, the HK-40m (with 28 wt% Cr and 2% Si)seems to have developed a Cr2O3 protective film, whichcan be explained due to the lower test temperature. Thelower temperature (700 �C) provokes a lower fluidity ofthe high sulfate molten salt, enhancing a slower diffusionof oxidant species to the metallic oxides compared to thatat 900 �C in 1% SO2–O2 environment, which explains theabsence of internal sulfidation, at least at 24 h of experi-mental test. The higher values of Tafel slopes determinedfor the high sulfate molten salt confirm the influence ofmass-transfer on the rate controlling step of this corrosivesystem. It is possible that an extended period of timeimmersion and higher temperatures be necessary toobserve the effect of sulfur in the high sulfate case.

Some others studies have been made in Na2SO4–30 mol% NaVO3 molten at 900 �C in 1 atm O2 [48,49].Hwang and Rapp [48] estimated the dependencies of oxidesolubilities on melt chemistry of CeO2, Y2O3, Al2O3 andCr2O3, whereas Zhang and Rapp [49] determined experi-mentally the solubilities of CeO2, HfO2 and Y2O3. A com-parison of the solubility of CeO2 in pure Na2SO4 to that insulfate-metavanadate showed that the acidic solubilityincreased three orders of magnitude, analogous resultsfor all oxides in vanadate solutions would be anticipated,since the anion of the acidic solutes would change fromSO2�

4 to VO3�4 . For the 80 mol% V2O5–20 mol%Na2SO4

molten salt, it would be expected that the acidic solubilitiesbecome even higher, thus the dissolution of the oxideswould be much more significant, nevertheless, Hwangand Rapp [48] determined that the acidic solubilities ofM2O3 metal oxides in an extremely acidic sulfate-vanadatemelt, (where V2O5 was the dominant vanadium compound)are independent of the melt basicity at certain basicity melt,for aluminum and chromium oxides this basicity wasapproximately 17.

Unfortunately, there is no more information about thechemistry of sulfate-vanadate molten salt with a higherconcentration of vanadates, which could be comparablewith the 80 mol% V2O5–20 mol%Na2SO4 molten salt.However, it is expected that the dissolution of metallic oxi-


des in a very high composition of vanadium pentoxideindeed be higher than that for the high sulfate molten salt[3,25,41,42].

Some other investigations have been made with respectto the oxides dissolution in molten Na2SO4 in a SO2–O2

environment at 900 and 927 �C [50–52]. Otsuka and Rapp[50] made reference to the gradient criterion pointed out byRapp and Goto [51], indicating that a negative gradient inthe oxide solubility in a fused salt film may establish a self-sustained hot corrosion for a pure metal (Ni), especiallyupon metal sulfidation by means of the reaction:

4Ni + Na2SO4 = 3NiO + NiS + Na2O ð3Þ

Observing that this behavior was only possible for basicdissolution. For the 80 mol% Na2SO4–20 mol% V2O5 fusedsalt there was no evidence of internal sulfidation at 24 h,however the significant amount of sulfur in the oxide-meltinterface, suppose the formation of metallic sulphides,especially in the form of NiS. Therefore, it would be possi-ble the presence of this gradient at 24 h, nevertheless, dueto the experiments were conducted in air, the salt aciditycould not be replenished, thus the self-sustained corrosionfor a long time could not be supported. But, the basic dis-solution in this case could have a significant influence dueto the expected high concentration of the Na2O species inthe melt, which is the basic component. Also, it is impor-tant to take account that this criterion was made for a purenickel oxide and for pure Na2SO4, and certainly vanadiumpentoxide has an important interaction in the chemistry ofthe melt high in sulfate, but a lower acidic dissolution couldcarry out in the high sulfate melt with respect to that inhigh vanadium molten salt.

It is unquestionable that a knowledge of the chemistryof the melt during the experiments is necessary to establishthe corrosion mechanism in the determination of an alloyresistance. On the other hand, the application of the elec-trochemical techniques have been an important tool inthe determination of the corrosion rates and corrosionmechanism, therefore, both measurements, chemical andelectrochemical would aid the interpretation of high tem-perature corrosion by molten salts and development ofnew protective materials. In the present work, the electro-chemical noise time series was analyzed to determine thecorrosion rate and the type of corrosion for HK-40m underboth experimental conditions. The high sulfate case pre-sented current noise signals exhibiting low amplitude tran-sients accompanied with random oscillations, althoughsome greater transients were observed, evidencing thenucleation of some localized corrosion sites or the ruptureand recovery of the passive film. The electrochemical cur-rent noise for HK-40m exposed to the high vanadium mol-ten salt showed higher values of current and higheramplitude transients. These results suggested that the cor-rosion process could be somewhat different at both expo-sures, the HK-40m suffering a more intensive corrosionprocess when exposed to the high vanadium molten salt.According to the localization index, HK-40m suffered a

mixed corrosion process during its exposure to the high sul-fate molten salt, whereas localized corrosion occurred dur-ing its exposure to the high vanadium molten salt. Fromthe optical images, the localized corrosion was in the formof inter-granular attack.

5. Conclusions

A study of corrosion performance from electrochemicalnoise technique and polarization curves, together withSEM and XRD analysis was obtained for the HK-40malloy exposed during 24 h to high sulfate (80 mol%Na2SO4–20 mol%V2O5) and high vanadate (80 mol%V2O5–20 mol%Na2SO4) molten salts at 700 �C. The resultsindicated that the exposure to high vanadium presented amajor dissolution of the initially formed chromium oxidelayer, after which a medium concentrated nickel oxide layerwas observed under chromium oxide that behaved as theprotective layer in subsequent stages. The corrosion ratesobtained from the electrochemical noise were highercompared to that at the high sulfate molten salt. TheHK-40m suffered inter-granular attack, in contrast to theexposure to high sulfate, where the corrosion mechanismwas through a mixed corrosion process. The optical imagesof HK-40m freed from the corrosion products togetherwith the SEM mappings of the main elements of the corro-sive system supported these conclusions. It can be inferredthat inter-granular corrosion and higher corrosion ratesobtained for HK-40m alloy after the exposure to the highvanadium molten salt was due to that 80 mol% V2O5–20 mol% Na2SO4 mixture is very acidic, extremely corro-sive, potentially oxidant, and it has a great capability forabsorbing oxygen, which transport to the metal surface isthrough counter-diffusion of ions V5+ and V4+, being thiskind of transference faster and typical of melts high invanadium pentoxide that retain essentially semi-conductingcharacteristics. On the other hand, the chemical acidic orbasic dissolution that the oxide scale suffers by the moltensalts must be especially taken into account to determine thecorrosion activity. This dissolution depends on the basicityof the melts, which increases with the increase of vanadiumpentoxide in pure Na2SO4 melt. Considering that the solu-bilities of all the metallic oxides by acidic dissolution ismuch higher than in pure sodium sulfate or molten saltswith lower concentration of vanadium. Certainly, theaggressiveness of the high vanadium molten salt togetherwith the presence of chromium carbides in the grainboundaries of the HK-40m provoked an intergranularlocalized corrosion.

Acknowledgements

The author gratefully thanks the Promep Program fromthe Public Education Secretary of Mexico for the supportto carry out the doctoral project, under the ReferenceNumber UEMOR-01.


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Documents

Corrosion study of HK-40m alloy exposed to molten sulfate or vanadate