Electrochemical Studies on the Corrosion Behavior1

of Conventional and High Alloy Steels inGulf Seawater

Anees. U. Malik and Shahreer AhmadResearch And Development Center,

Saline Water Conversion CorporationP. O. Box # 8328, Al-Jubail 31951, Kingdom of Saudi Arabia

SUMMARY

During the last 10-15 years, a considerable number of highly alloyed corrosion

resistance stainless steels, so called super stainless steels have been developed. More

emphasis was put on to increase the localized corrosion resistance, since conventional

stainless steels have rather limited resistance to localized corrosion. Typical examples of

new super stainless steels are the nitrogen-alloyed 6Mo austenitic and 25Cr duplex

steels. The largest application area for these steels has been seawater systems of various

kinds.

An attempt has been made to determine the corrosion resistance of some typical high

alloy stainless steels viz. 316L, 317L, 904L, duplex 2205, 3127hMO, 1925hMO,

254SMO, 654SMO and Remanit 4565, in chlorinated and unchlorinated Arabian Gulf

Seawater at ambient and 500C. The stainless steels 316L and 317L have been used as

reference alloys. Electrochemical Potentiodynamic cyclic polarization method have

been used to determine the Eb - passive film break down potential, Eprot - protection

potential and Imax - maximum current attained on scan reversal.

It was found that at 250C in chlorinated and unchlorinated seawater and at 500C in

unchlorinated seawater, the stainless steels 316L and 317L had poor resistance to

corrosion, Stainless steels 904L and duplex 2205 at 250C in chlorinated and

unchlorinated seawater showed good resistance to corrosion but at 500C these steels

failed to resist. Other alloys such as 3127hMO, 1925hMO, 254SMO, 654SMO and

1 Issued as Technical Report No. TR3804/APP96003, September, 1998.

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Remanit 4565 showed better corrosion resistance in all the test conditions. The alloys

having a PREN value of above or equal to 30 at 250C with and without chlorine and

PREN value of 35 at 500C or the sum of Cr+Mo above 25 can be used safely for

seawater applications

1. BACKGROUND

About 5 years back, the Corrosion Department of SWCC, RDC initiated a major project

entitled Crevice Corrosion in Stainless Steels (SWCC R&D Task 37). Most of the aims and

objectives of this project were completed by the middle of 1995. The results of the studies

appeared from time to time in the form of technical reports [ SWCC (RDC) 20, 22, 26 and

28] and papers in corrosion and desalination journals.

Last but not the least in importance, is the remaining part of the project dealing with the

influence of certain parameters such as temperature and residual chlorine and dominant alloy

addition on the localized corrosion behavior of steels as studied by electrochemical means.

Under the new guidelines of RDC management, a project proposal was submitted in

November, 1995 containing the above-mentioned tasks. The project entitled

Electrochemical Studies on the Corrosion Behavior of Conventional and High Alloy steels

in Gulf Seawater is of one year duration and was started in January, 1996.

2. OBJECTIVES

1. To study the effect of temperature and residual chlorine on the localized corrosion

characteristic of conventional and high alloy stainless steels.

2. To study the localized corrosion behavior of conventional and high alloy stainless steels

by electrochemical techniques.

3. To determine the influence of dominant alloy additions : Cr, Mo and Ni on the general

and localized corrosion of high alloy stainless steels in Gulf seawater environment and

chlorinated seawater.

3. INTRODUCTION

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Since the discovery of corrosion resistance properties of 12% Cr steels in the beginning of this

century, stainless steels have been most widely used structural materials after mild steel.

Besides convenient availability and moderate cost, stainless steels are easy to produce, cast or

fabricate, have versatile mechanical properties and excellent weldability. Over the years there

has been continuous innovations in the development of stainless steel technology. Although

austenitic stainless steels like 304, 316 or similar alloy have been formidable structural

materials since long, development of high alloy stainless steels containing 6% Mo in sixties and

the following decades, made possible the applications of stainless steels in most aggressive

environments such as seawater handling systems in place of much costiler nickel-base alloys.

The applications of conventional and high alloy stainless steels in marine environments

particularly pertinent to desalination have been dealt in number of review articles [Hassan and

Malik, 1989; Bardal et al., 1993; Malik et al., 1994; Tuthills et al., 1995; Oldfield and Todd,

1995, 1996].

Amongst the family of austenitic stainless steels, 316 L has been the most widely used

structural material in industry. It has been the most widely used material for marine installations

including desalination plant components which include flash chambers, distiller pipes, pumps

and condensers due to its excellent corrosion resistance to chloride attack. In spite of its

virtue as an outstanding corrosion and erosion resistance material, 316 L has its shortcoming

as vulnerable to local corrosion such as pitting or crevice attack in presence of chloride under

static or stagnant conditions [Sedricks, 1979; Smialowska, 1987; Stackle, 1974].

The pitting of stainless steels is the single most frequently occurring corrosion phenomenon in

seawater processing plants and has been the subject of numerous studies [Todd 1977,

Hodgkeiss, et al. 1987, Moriz. et al. 1986]. It is generally agreed that pit initiation occurs as a

result of breaking down of chromia film in presence of chloride, the propagation step in pitting

involves an autocatalytic process in which corrosion products are hydrolyzed to provide local

encrichment in chloride and acid concentrations. The pits also provide active crevices for a

formidable corrosion attack [Fontana, 1986].

In recent years, pitting of metals particularly stainless steel has been the subject of extensive

and in depth studies [Oldfield et al., 1980; Smialowska, 1986; Malik et al., 1992]. The

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studies are mainly directed toward mechanistic aspects which subsequently provide

information useful for the development of pitting resistance materials. Electrochemical

techniques have been the principal tool for characterizing pitting corrosion. Pitting potential,

Epit, protection potential Epp and repassivating potential, Er are the important electrochemical

parameters for studying the pitting behavior of materials. Pitting potential, Epit is the potential

above which passive alloys are susceptible to pitting corrosion in halide solutions, but below

which pits can not be formed, although existing pits can grow if the potential is greater than

protection potential. The protection potential (Epp) is the potential value such that at potential

greater or equal to this potential, it can propagate, but below this potential, metal remains

passive. Er is called the repassivating potential and represents the potential at which the

current falls to the small values recorded during the forward scan. Pitting potential depends on

the composition of the bulk solution, and on the surface condition of metal, while protection

and repassivating potentials depend upon the composition of the solution contained in pits. Up

till recently, determining the pitting potential, Epit was fundamental in evaluating susceptibility of

different materials, in different environment to localized corrosion. However, it is accepted

widely that due to similarities between crevice and pitting corrosion mechanisms it would be

more appropriate to refer localized corrosion potential by a new parameter so called

breakdown potential, Eb. The latter is defined as a potential where pitting, crevice corrosion

or both will initiate or propagate. Logically it is more appropriate to represent localized

corrosion by Eb rather than referring to Epit or crevice potential separately [Neville et al.,

1996]. The use of Eb instead of Epit by several authors emphasizes that mechanisms other than

pitting can induce the loss of protectivity. However the importance of Epit determined from

potentiodynamic polarization curves cannot be undermined. It is a very useful parameter

particularly, in comparing the pitting resistance of different materials.

In a recent paper, Salvago and Funalalli [1996] determined the distribution of breakdown

potential (Eb) of stainless steels in 0.1 NHCl solutions. Representation of Eb distribution on the

graph of potential (E) vs log [commutative breakdown frequency, F] was found to be

particularly informative about localized corrosion resistance. The effect of annealing

temperature on the behavior of 25% Cr duplex UNS S32550 in HCl solutions was

investigated by determining critical pitting temperature (CPT) and pitting potential, Epit after

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annealing at different temperatures between 1020 and 1140 oC [Garfias, Sykes and Tuck,

1996]. It was found that the pitting in chloride solution took place preferentially in the ferrite

phase rather than in the austenitic phase. Higher annealing temperature (above 1060 oC)

increases the ferrite content and dilute the key alloying elements in the ferrite content thus

lowering the corrosion resistance of ferrite. Neville and Hodgkiess [1996] investigated the

effect of elevated temperature (up to 60 oC) and high velocity impinging flow on the corrosion

of stainless steels, Ni- and Co- base alloys in seawater. DC electrochemical experiments

demonstrated the clear effect of increased temperature in facilitating premature breakdown of

passivating film on of all materials. The effect of high velocity impinging flow was to further

shift the passivity breakdown potential to more active values but not necessarily to result in

greater depth of attack. With respect to materials comparison, the study has illustrated that

under the combined effect of high temperature and high velocity flow, the materials which resist

significant attack on a localized level are the Ni- based Inconel 625, the Co- based ultimate

and superaustenitic alloys.

A substantial amount of work has been carried out on localized corrosion behavior of steels in

seawater, however, most of the studies are mainly limited to artificial seawater which is

chemically very close to seawater but may not have similar characteristics mainly because of

the absence of biological activities. The composition of seawater varies with in wide limits. For

example, the dissolved salt content in Baltic Sea amount only 7 g/Kg (about 7,000 ppm) it is

about 43 g / Kg (about 43,000 ppm) in the Arabian Gulf Water. An approximately 35 g/Kg

salt content is present in Pacific and Atlantic lie somewhere in the middle [ Homig, 1978].

Seawater temperature also can vary widely from 20C in Arctic to 300C in tropics to 450C in

Arabian Gulf.

In a study [Agarwal et al 1991] concerning with the performance of platform pumping system,

a 2 test spool of alloy 1925hMo was exposed to chlorinated Norwegian seawater (salinity :

20,000 ppm) for period of 4 weeks. The test results showed that in chlorinated seawater at

300C alloy 1925 hMo was free of attack at chlorine level of 0.5 ppm. High chlorine levels and

/ or elevated temperatures may lead to corrosion attack preferably on the flange and under

gasket. Electrochemical testing of some duplex steels in Portland Harbor seawater (about

20,000 ppm Cl-, 2400 ppm SO4--) was carried out to assess the resistance to crevice

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corrosion [Carpenter et al 1986]. The results obtained could be correlated with service

experience and provide useful information for material selection in seawater handling systems.

Lee et al [1984] studied the effect of environmental variables e.g., pH, Cl- and DO on crevice

corrosion of stainless steels in Atlantic seawater (18,800 ppm Cl- and 2,500 ppm SO4--) at

Wrghtville Beach, N.C. The uniqueness of natural seawater has been emphasized by the

results of propagation studies at elevated temperatures. Substantial reduction in the extent of

propagation were noticed on increase in seawater temperature. It appears that reduction at

cathode surfaces may be rate controlling. Effect of surface finishing on the crevice corrosion

resistance of SS316 in Atlantic seawater (Salinity 31,400 - 35,100 ppm, Cl- 17,400 - 19,600

ppm) was studied [Kain, 1991]. Resistance of SS316 in natural seawater appeared to be

dependent on crevice geometry. Surface grinding was determined for both anodic dissolution

and cathodic polarization of stainless steel. Exposure to natural seawater can result in

considerable ennoblement of the corrosion potential over the artificial seawater of comparable

salinity.

Crevice corrosion propagation studies were carried out on Ni -base alloys 625 and C276 in

seawater of Key West, Florida with an avereage slainity of 37.3 ppt and a temperature of

26.40C [McCafferty et at, 1997]. Anodic polarization technique was used. Both the alloys

underwent crevice corrosion in seawater medium with active crevices were found to contain

concentrated amounts of dissolved Ni2+, Cr3+, Mo3+ and F2+ ions. In another study [Steismo

et al, 1997], eleven high alloyed stainless steels were tested for applications in chlorinated

Norwegian seawater (Salinity 20,000 ppm). Critical crevice temperature (CCT) was

determined using a potentiostatic test method. Results were evaluated in terms of Critical

Crevice Index, CCI (CCI = % Cr + 4.1% Mo + 27%N) value of the alloys and compared

the results of duplicate specimens in other tests. CCT was found to be in the same range for

the 6 austenitic and 5 duplex stainless steels. Repassivation of crevice corrosion was found to

occur at far lower temperatures than initiations of crevice corrosion.

The effect of temperature on the corrosion of high alloy stainless steel in seawater (North Sea)

has been studied [Francis et al 1996]. Super duplex alloys are more resistant to crevice

corrosion at elevated temperatures as compared with 6Mo steels. Wallen and Bergqvist

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[1997] tested piping system, composed of real components made of 2 superaustenitic SS in

continuously chlorinated (2 ppm) seawater (Norwegian, Cl- 19,000 ppm)

A system consisting of pipes made of 6Mo steel, 254 SMO and flanges made of 7Mo steel

654 SMO was resistant to corrosion provided severe weld defects were absent. Immersion

tests in natural and chlorinated North Atlantic seawater show that super duplex and super

austenitic 6Mo steels have about the same resistance to initiation of crevice and pitting

corrosion [Wallen, 1998]. It has been shown that using superaustenitic 6Mo, superduplex or

7Mo flanges is a possible way to overcome most of crevice corrosion problems.

Corrosion behavior of selected materials in Arabian Gulf seawater was studied by investigating

the effect of salinity and temperature using electrochemical techniques [Al-Ghamdi et al,

1996]. The results indicate that high salinity alone does not contribute to corrosion trends in

seawater, the temperature also plays an important role. There is a steep rise in corrosion rate

by a 20 oC increase in temperature.

The pitting behavior of AISI 316 L in Gulf seawater was studied electrochemically [Malik and

Al-Fozan, 1994]. An increase in temperature caused a decrease in pitting potential and

increase in corrosion rate and pitting tendency. Pitting potential and corrosion rates were

found to be highly sensitive to surface treatment. Pitting potential was more noble for

specimens treated with HNO3 and lowest for sand blasted specimens.

The pitting behavior of some conventional and high alloy austenitic, ferritic and duplex stainless

steels has been studied at 50 oC in Gulf seawater [ Malik et al. 1994; 1995]. The pitting

potential appeared to be a logarithmic function of Cl- concentration. The conventional

austenitic stainless steels (AISI 304, 316 or 317) showed a relatively small shift in pitting

potential with variation in Cl- concentration. An increase in Cl- concentration resulted in a shift

to more negative (or active) Epit. Pitting potential, Epit was found to be a liner function of Cr,

Cr + Ni and PREN of steels as well as the induction time, t. Although conventional and super

stainless steels behave similarly, two separate liner relationship with different slopes are

observed.

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In chloride containing environments where conventional stainless steels are subjected to

localized corrosion under certain conditions, the addition of relatively large amounts of Mo (

6) and chromium provide excellent corrosion resistance against localized corrosion resistance.

The synergic effect of chromium and molybdenum in resisting pitting was first shown by Lorenz

and Medawar [1969]. The relative effect of Cr, Mo and nitrogen on pitting or crevice

corrosion can be assumed qualitatively by pitting resistance equivalent, PREN which is

represented by the empirical equation [Herbsleb, 1982].

PREN = % Cr + 3.3 x % Mo + 16 X % N

A high nitrogen multiple of 30 has been used for selected alloys instead of 16 [Henbner et. al.,

1989]. A PREn above 38 is supposed to provide good resistance to marine corrosion

[Glover, 1988].

Crevice corrosion is another form of localized corrosion which is quite frequented in marine

structures, pipe lines, pumps or shafts or any location where there is an occurrence of crevice

or cavity along with accumulation of stagnant liquid (water) or deposit formation and it is not

surprising that attack is localized to shielded areas and rest of the surface remains unaffected.

In most cases pitting is precursor of crevice corrosion and there are obvious similarities

between crevice corrosion and pitting corrosion mechanisms.

The origin of crevice attack stems from the compositional differences developing in the

solution, inside and outside of the crevice. These difference lead to the formation of macro

corrosion cells with the inner crevice acting as an anode resulting in rapid dissolution of the

metal [Oldfield and Sutton, 1978]. The differences between the outside and inside solution

can be manifold : differential aeration cell, metal ion concentration, pH, chloride ion

concentration, inhibitor concentration, etc. [Ijesseling, 1980]. The effect of crevice corrosion

can range from pitting to uniform corrosion of the metal surface within the crevice. Several of

the critical factors which affect the crevice corrosion of stainless steel in seawater include the

salinity, temperature, dissolved oxygen level, velocity of seawater, the chromium and

molybdenum contents of the stainless steel, the electrochemical behavior of the passive film,

chemical reaction, transport mechanism and crevice type [Lee and Kain, 1983].

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The crevice corrosion manifests itself in many cases [Beavers, 1986] : bolt head to washer,

washer to base plate, tube to tube sheet, sleeve to pump shaft, gasket to flange face, teflon to

metal, barnacle to metal, valve stem packing, pump shaft packing, etc. Immersion and

electrochemical tests were utilized by many authors to study the mechanism of localized

corrosion.

The behavior of crevice corrosion was investigated electrochemically by many authors for

determining parameters such as critical crevice solution, passive current, critical crevice

potential, critical crevice temperature and variation of potential with time. The propagation of

crevice corrosion has been studied by using different electrochemical techniques such as

potentiodynamic, potenitostatic, galvanostatic, remote crevice assembly, compartmentalized

cells, etc. [Oldfield et al., 1980, Hack, 1983, Davies Smith et al. 1987, Yashiro et al. 1990

and Kain et al. 1985]. The effect of crevice former and crevice geometry on the crevice

corrosion of 316 L SS in Arabian Gulf seawater was investigated by immersion tests and

electrochemical techniques [Malik and Al-Fozan, 1994]. The corrosion potential during

induction period was found sensitive to the geometry and type of crevice former used. The

effect of dominant alloy additions on the crevice corrosion behavior of some conventional and

high alloy stainless steels was studied in seawater at 50 oC [Malik et al., 1995]. The results of

the study indicates : (i) Besides superstainless steels, conventional stainless steels containing

more than 6% Mo can be used with little risk of corrosion for seawater applications involving

crevice forming systems, (ii) Crevice corrosion may initiate in high alloy stainless steel but

usually the subsequent propagation step is not followed (iii) CCS pH appears to be a liner

function of PREN showing a strong but negative dependence of critical solution aggresivity on

Cr, Mo and N contents of the steels and (iv) At a constant torque, the surface finish (from

rough to smoother) and increasing temperature enhance the possibility of corrosion in crevice

forming systems. The pitting and crevice corrosion tendencies of stainless pipings in

chlorinated seawater were tested electrochemically and by crevice corrosion exposures [Boah

and Frazer, 1996]. It has been shown that besides 254 SMO which did not corrode in

chlorinated seawater (25,000 to 50,000 ppm Cl- and 0-10 ppm residual chlorine) all other

alloys developed crevices and suffered some degree of corrosion.

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The effect of chlorination on some high alloy grade stainless steels (254 SMO, Monit and

Sandvik Sanicro 28) in North Seawater was studied [Wallen and Henrikson, 1989]. The

investigations showed that continuously chlorinated seawater was considerably more

aggressive than unchlorinated seawater or intermittently chlorinated seawater and that a high

temperature increased the risk for localized corrosion at the same chlorine concentration. The

highest alloyed steel grades were very resistant to crevice corrosion even in continuously

chlorinated seawater- Contrary to crevice corrosion, the risk of galvanic corrosion decreased

considerably if the seawater was chlorinated. Hack [1983] studied the crevice corrosion

behavior of austenitic and ferritic stainless steels at 30 oC and showed that some high alloy

steels are not only more resistant than AISI 316 but also exhibit resistance equivalent to more

costly Ni-base alloys. Austenitic stainless steel required 8% Mo to prevent crevice corrosion

whereas ferritic steels required 25% Cr and about 3.5% Mo. Using an accelerated technique,

Oldfield [1990] studied the influence of alloying element addition on the crevice corrosion

behavior of a number of commercial steels in a marine environment. Amongst the commercial

steels, 254 SMO provided the best resistance. Corrosion resistance increases if the Mn level

is less than 0.5% or there is a decrease in S level. Oldfield [1988] reported the effect of

temperature and surface roughness` on the seawater crevice corrosion resistance of

commercial and model stainless steels including 316 and 20 Cr-6Mo types containing 2

ranges of nickel : 10 to 30% and 20 to 40%. The following conclusions were drawn from the

study regarding resistance to crevice corrosion initiation : (i) better resistance with rough

surfaces in comparison to ground to polished surfaces (ii) resistance increases as the

temperature decreases from 70 to 50 oC and (iii) little influence of variation in nickel contents.

For resistance to corrosion propagation : (i) Ni has been found extremely beneficial in

improving the resistance of 316 type steels and (ii) Ni above 20% did not have any significant

effect.

4. EXPERIMENTAL

4.1 Materials

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Nine different commercial grade stainless steels, 316L, 317L, 904L, 3127hMo, 1925hMO,

254SMO, Duplex 2205, Remanit 4565 and 654SMO were used for this study. Table 1 lists

the composition of these alloys.

The seawater and chlorinated (0.2 ppm residual chlorine) seawater mentioned in this report is

considered as Arabian Gulf raw seawater unless otherwise mentioned. The composition of the

seawater is given in Table 2.

4.2 Sample Preparation

For potentiodynamic cyclic polarization (PCP) experiments, round samples of ~15mm dia.

and 3-5 mm thick were machined from the sheets and were abraded sequentially on 120, 180

and 320 grit SiC paper to simulate service condition. The abraded samples were cleaned in

ethanol by ultrasonic cleaner. All the specimens were abraded only one hour before each

experiment.

Rectangular coupons of the size 100x20x3-5 mm were cut from the sheets and were grinned

by surface grinder for open circuit potential measurements (OCP). The surface finish was ~

0.12 mm rms. The grinned samples were cleaned with ethanol in ultrasonic cleaner. The

samples were prepared few days before the experiment and were kept in a dessicator.

4.3 Open Circuit Potential Measurements

Open circuit potential measurements of nine different alloys (listed in Table 1) were carried out

in chlorinated (0.2-0.65 ppm residual chlorine) seawater at 250C and raw seawater at 500C.

In the experiments with chlorine dosing the residual chlorine was monitored every day except

weekends. A special test cell was designed and fabricated in the lab with the help of

instrumentation section and workshop of the RDC (Figure 1). Two multipen chart recorders of

6 and 3 pens were electrically connected to the specimens. The change in voltage, against the

SCE, used as reference electrode, was plotted Vs time. Everyday the data were transferred in

to the computer and a combined graph was drawn to study the change in potential for each of

the alloys.

4.4 Potentiodynamic Cyclic Polarization Experiments (PCP)

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These experiments were carried out in seawater at room temperature (250C) with and without

dosing of sodium hypochlorite (NaOCl) solution and in raw seawater at 500C. The residual

chlorine was measured by HACH DREL/1C portable kit.

All potentiodynamic cyclic polarization (PCP) experiments were carried out using the

corrosion cell from EG&G model K0047 (Figure 2) with saturated calomel electrode (SCE) as

reference and graphite rods as counter electrode. Flat sample holder from EG&G model

K0105 (Figure 3) which provides 1 cm2 exposed surface area of the sample was used for all the

PCP experiments.

The flat sample holder (K0105) made up of high density polyethylene (HDPE) supplied with

the instrument could not be used for samples having a thickness of more than 2 -2.5 mm.

Since the thickness of outer wall adjacent to the washer was about 1.0 mm, on applying force

by tightening the screw, the outer wall was used to bulge out. As a result of bulging, the

electrolyte use to reach inside the holder during the experiments and consequently, the

experiment had to be aborted. Sometimes its affects had been noticed only after finishing the

experiment. Therefore, a new sample holder, which could be capable of holding samples up

to 4.0 mm thick and with an increased outer wall thickness was designed and fabricated in the

workshop (Figure 3). The new holder overcome all the shortcomings observed in the old one.

This holder was used for carrying out all electrochemical experiments in chlorinated and

unchlorinated seawater.

The PCP experiments were carried out on flat round ~15mm dia. specimens in seawater with

and without chlorine (residual chlorine 0.2 ppm) at room temperature. The samples were

immersed in the test solution 15 minutes before starting the experiment. The potentiodynamic

tests were carried out using a computer controlled EG&G model 273 potentiostat . The

experiment was programmed to change the specimen potential relative to reference (SCE)

electrode from its free corrosion potential or open circuit potential (OCP) in both the

directions i.e. cathodic and anodic. The current corresponding to each potential was recorded

and displayed on the monitor continuously. The experiment was programmed to start the

reverse scan after attaining a current density of 100 mA/cm2 to zero Volts Vs SCE. Each

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experiment was repeated 4-6 times and average of nearest three experiments test data has

been reported in this report.

4.5 Residual Chlorine Degradation Measurements

The degradation of different level of residual chlorine (0.2-0.7 ppm) with time was monitored

at 250C in open and closed systems. Initially the residual chlorine level was raised to a

predetermined value, then the degradation with time was monitored at short intervals during

the first few hours and later on at an increased intervals. Figure 4 shows the change in the

concentration of residual chlorine in an open and closed system with time. It is evident that

initially the degradation of chlorine was very fast and later on it gets slowed down.

Some trial experiments were carried out to find out the degradation of chlorine in presence of

some alloys. Figure 5 shows the variation in OCP with time in presence of SS316L and 0.23

and 0.74 ppm of residual chlorine at room temperature. It was found that the maximum effect

of chlorine dosing on open circuit potential of the alloy was observed after 15 minutes of

chlorine dosing. The corrosion potential becomes more noble and reaches to a maximum value

after ~15 minutes of chlorine dosing. After 20-25 minute of chlorine addition the potential

starts decreasing and reaches to same potential as was before chlorine dosing after more than

10 hours.

The total potential scanning range for alloys under test was 1.5-2.5 Volts. For SS316L the

total scan range is about 1.5 Volts while for other alloys is about 2.5 Volts. Therefore, the

duration required with conventional scan speed (20mV/min.) for an average scan range of 2.5

Volts will be about 2.0 hours. However it has been observed that the maximum effect of

chlorine dosing appears after 15 minutes of dosing which persist up to ~1.0 (Figure 5) and then

hour starts decreasing very rapidly. Therefore, on the basis of trial experiments of chlorine

degradation with time in presence and absence of alloy the potentiodynamic cyclic polarization

experiments were set up in such a way so that the effect of chlorine on alloys could be studied.

Therefore a scan rate of 60 mV/min. after 15 minutes of sample immersion was selected for all

the experiments. Although this scan rate is 3 times higher than the conventional scan rate.

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Recently Latha and Rageswari [1997] reported the results for stainless steels at same scan

rate.

For the materials showing passive behavior in static seawater on increasing the potential from

the free corrosion potential value Ecorr in anodic direction, low currents were observed until a

potential, Eb is reached. Eb represents the potential at which the current starts to rise

significantly indicating a loss of passivity or film breakdown. The anodic polarization scan was

reversed once the current reached 100 mA/cm2. The current Imax represents the maximum

current attained should it not begin to fall immediately after scan reversal. Er or Eprotis called

repassivation potential at which the current falls to the small values recorded during the

forward scan [Neville and Hodgkiess 1996]. Figure 6 represents the various points on a

Potentiodynamic cyclic polarization curve showing Eb, Er or Eprot and Imax.

5. RESULTS AND DISCUSSION

When stainless steel is exposed to seawater its corrosion potential quickly rises in the noble

direction. At the start of the experiment, low potential is observed but reaches seemingly to a

constant level after few weeks of exposure [Mollica and Tervis 1976, Holth et. al., 1988;

Gallagher and Malpas, 1989].

The natural seawater contains living organisms, which form a slime layer on the metal surfaces

immersed in natural seawater. The biofilm formed on the surface has a strong catalytic effect

on the cathodic reaction of corrosion process, i.e. reduction of oxygen [Mollica and Tervis

1976 and Johsen 1983]. This results in more noble corrosion potential than in sterile chloride

solutions and an increased cathodic efficiency at cathodic polarization. For this reason a

natural seawater is more corrosive than artificial seawater or sodium chloride solutions. The

noble potential shown by the stainless steels in natural seawater increases the risk of galvanic

corrosion on less noble materials.

Amaya and Miykui [1994] also reported noble corrosion potential of stainless steel type 316L

and 304L in natural seawater. The cathodic polarization behavior indicated the existence of

some higher redox potential substance than oxygen. They identified this substance as hydrogen

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peroxide, which could be generated by reduction of oxygen in presence of enzyme (Oxidaze),

in the natural biofilm.

The biological activity in seawater may cause practical problems leading to costly production

disturbances. For example, a high degree of fouling creates the friction losses in a pipeline and

may induce erosion in certain metals. Furthermore, even slight fouling lowers the thermal

efficiency of heat exchanger surface. The conventional way to prevent biofouling or to reduce

its effects is to chlorinate the water. This is normally done by adding a strong hypochlorite

solution or by electrolyzing the water, continuously or intermittently [Wallen, 1990].

Chlorine is a strong oxidant which displaces the corrosion potential of stainless steels to more

noble values [Malpas et. al., 1986]. In a continuos chlorination system the rate of increase in

potential increases with increase in chlorination but always faster than in unchorinated water

[Gundersen, 1988]. The final potentials are also considerably more positive than those

measured in unchlorinated water. Therefore increasing the risk of pitting and crevice corrosion.

The activity of the biofilm is considerably reduced if the seawater is heated to a temperature

above ambient temperature, normally to 30-350C. The other factor that might have an effect

on biofilm is the flow rate of seawater. Mollica and Tervis [1976], reported that no active

biofilm develops if the flow rate exceeds 2m/Sec. While Johsen and Bradar [1986] founded

that even 4.5 m/Sec. does not reduce its activity.

5.1 Potentiodynamic Cyclic Polarization Studies (PCP)

Figures 7-33 show the potnetiodynamic scan plot for all nine alloys under study. The values of

breakdown potential (Eb), protection potential (Eprot) and maximum current density attained

after the start of reverse scan ( at 100 mA/cm2) at 250C with and without chlorination and at

500C in seawater are given in tables 3-5. The breakdown potential of different alloys at 250C

in seawater follow the sequence:

316L

The Eb for all the alloys except 316L and 317L are in the range of 900-1000 mV SCE. A

similar trend was observed in chlorinated seawater also. In chlorinated seawater the values of

Eb were slightly lower as compared to raw seawater. At 500C, the Eb of all alloys follow the

following sequence:

316L

250C, and 904L and Duplex 2205 at 500C can suffer from general as well as crevice and

pitting corrosion due to low Eb and high Imax values.

Figures 37-42 show graph of Eb Vs PREN value (Table 1) and sum of Cr+Mo concentrations

for all the nine alloys at 250C with and without chlorine (0.2 ppm) and at 500C. At 250C with

and without chlorine an increase in Eb value is observed above a PREN value of 30 and 500C

above 35 an increase in PREN value can be seen (Figure 39). The sum of Cr+Mo concentrations

in the alloys above 25 (Figures 40-42)show an increase in the Eb value for all the alloys under all

test conditions. Therefore it can be concluded that the alloys having a PREN value of above or

equal to 30 at 250C with and without chlorine and PREN value of 35 at 500C or the sum of

Cr+Mo above 25 can be used safely for seawater applications.

The results of potentiostatic studies indicate excellent corrosion resistance by 6Mo steels in

natural seawater (250C and 500C) and chlorinated seawater (250C). The findings are in

confirmetry with the results reported by Agarwal et al [1991], Francis et al [1996], Wallen

and Bergqvist [1997] and Steismo et al [1997] from similar studies on these alloys in Atlantic

and Norwegian seawaters.

5.2 Open Circuit Potential (OCP) Studies

The variation in open circuit potential (OCP) of all the alloys at 250C in chlorinated and at

500C in unchlorinated seawater can be seen in figures 43-50 and 52-60 respectively. A

significant enoblement in OCP at different concentrations of residual chlorine was observed in

all the alloys (Figures 43-50). The OCP increases with increase in residual chlorine at 250C

(Figure 51). At 500C, OCP of 1925HMO, 254SMO, 654SMO, 3127HMO and Remanit 4565

reached to zero value with in 10 hours after the immersion. While 316L, 317L and 904L take

about 15 hours to reach this value. In case of Duplex 2205 , a frequent change in potential

from anodic to cathodic and vice versa was observed (Figure 58). This indicates that some

corrosion phenomenon is occurring at the metal/solution interface. On examination under

scanning electron microscope (SEM) Duplex 2205 showed some pits (Figures 61-62) at the

water/air interface. None of the other alloy showed any pit formation during the 1600 hours

immersion.

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The use of stainless steels 316L, 317L at 250C in seawater applications is unsafe. Because of

their very low Eb and high Imax values, a further decrease in Eb values in chlorinated seawater

increases the risk of their use. Because the OCP at high level of residual chlorine is very near

to the Eb, especially for 316L and 317L at 250C and duplex 2205 at 500C in seawater (Table

4-5), a sharp decrease in Eb value from 1095 mV SCE at 180C to 417 at 300C was reported

[Neville and Hodgkiess, 1996]. Frequent change in OCP, pit formation and sharp decrease in

Eb value provide strong evidence that the duplex 2205 steel is not a suitable alloys for high

temperature seawater applications.

6. CONCLUSIONS

6.1 Specific Conclusions

Following conclusions can be drawn from the studies :

1. All the stainless steels under test show an increase in open circuit potential in presenceof residual chlorine.

2. The enoblement in open circuit potential increases with increase in the quantity ofresidual chlorine.

3. The Eb values of stainless steels 316L, 317L and 904L at 250C in seawater follow thesequence : 250C > 250C (Chlorinated) > 500C.

4. Duplex 2205 steel shows a drastic reduction in Eb from 1010 mV at 250C to 325 mV at500C therefore its use should be restricted to low temperature applications.

5. The Eb values of 3127HMO, 9125HMO, 254SMO, 654SMO and Remanit 4565under all the test conditions except duplex 2205 steel at 500C does not show anysignificant variations.

6. A remarkably large difference in Eb and Eprot values of stainless steels 904L and duplex2205 at 500C was observed, which show the susceptibility to general and crevice orpitting corrosion for these alloys at this temperature.

7. At 500C all the alloys other than 316L, 317L, 904L and duplex 2205 can be usedsafely.

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8. Alloys having PREN value grater than 35 showed Eb values in a close range e.g., 950-1016 (at 250C), 95-1003 (at 2500C, residual chlorine 0.2-0.25 ppm) and 879-923 (at500C).

6.2 General Conclusions

Electrochemical studies carried out on high alloy stainless steels provide useful information

regarding the general and localized corrosion behavior of 316L, 317L, 904L, duplex 2205,

3127HMO, 1925HMO, 254SMO, 654SMO and Remanit 4565 alloys in normal and

chlorinated seawater. Chlorination level should be maintained to such values so as to prevent

the formation of biofilm but not enough to increase the open circuit potential. High alloy

stainless steels such as 3127hMo, 1925 hMo, 254 SMO, 654 SMO and Remanit 4565 have

very little tendency to crevice and pitting corrosion in normal seawater at 250C and 500C and

chlorinated seawater at 250C.

7. RECOMMENDATIONS

The electrochemical data generated during this study provide useful information about the

corrosion resistance of conventional and exotic construction materials in chlorinated and

normal seawater. This information together with the information available about the corrosion

behavior of materials from previous studies (SWCC (RDC) Reports 20, 22, 26 and 28 )

could be complied and may be utilized for the material selection in existing or future

desalination or seawater handling system.

The corrosion data of the materials in Gulf Seawater computed from aforementioned studies

should be compared with data, if available from similar studies carried out in waters of

different oceans.

8. FUTURE WORK PLAN

1. Long term crevice corrosion studies of these alloys under flowing seawater conditionand at different level of intermittent and continuos dosing of chlorine and temperatureshould be studied in a test loop or pilot plant to ascertain their resistance.

2. To prevent the formation of biofilm but not enough to increase the open circuit potential,threshold level of residual chlorine should be determined.

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REFERENCES

1. Agrwal, D.C., Jesner, M.R. and Rockel, M.B., (1991), 6% Mo Austenitic SS selection forOffshore Applications, 23rd Offshore Technology Conference (OTC), Houston, Texas.

2. Al-Ghamdi, A.M. Nusair, Cocks, F.H. (1996) Corrosion Behavior of Selected Metals inArabian Gulf Seawater" Proceeding of the 7th Middle East Corrosion Conference,Bahrain 475-486.

3. Amaya, H and Miyuki, H,, (1994), Proceedings International Symposium, Techno-Ocean 94,1, 493-98.

4. Beavers, J.A. Kpock, and Berry, W.E., (1986), Corrosion of Metals in Marine Environment,Metals and Ceramic Information Center Report, MCIC 86-50.

5. Boah, J.K. and Frazer, P., (1996), Localized Corrosion Tendencies of Piping Materials Usedin Chlorinated Seawater, Proceeding of the 7th Middle East Corrosion Conference,Bahrain, 409.

6. Bordal, E., Drougli, J.M., and Cartland, P.L., (1993) Corrosion Science 35, 257-267.

7. Carpenter, M.D., (1986), British Corrosion J., 21(1), 54-48.

8. Davies- Smith, L.R., Lane, J.D. and Riley, T., (1987), Br. Corrosion J., 22(2), 90-94.

9. Fontana, M.G., (1986), Corrosion Engineering McGraw Hill, New York.

10. Francis, R., Hughes, S. and Byrne, G., (1991), The Effect of Temperature on the Corrosionof Higher Alloy SS in Seawater, Proc. VII Middle East Conference, NACE, Bahrain,340.

11. Gallagher, P. and Malpas R.E., (1989), Corrosion/89, paper no.113.

12. Garfias - Mesias, L.F., Sykes, J.M. and Tuck., C.D.S, (1996), Materials Performance 27,53.

13. Glover, T.J., (1988), Materials Performance, 27(1), 53.

14. Gundersen ,R., (1988), Proceedings UK Corrosion/88, Brighten, UK, Oct.3-5, 125-36.

15. Hack H.P., (1983), Materials Performance 22, 29.

16. Hassan, M.A., and Malik, A.U., (1989), Desalination, 74, 157-170.

17. Henbner, U., Rockel, M. and Wallis, E., (1989), Eerstoffe Korrosion, 40, 418.

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18. Herbsleb, G., (1982), Werkstoffe and Korrosion, 33, 334.19. Hodgkeiss T., and Ary, N.G., (1985), Desalination 55, 229.

20. Hodgkeiss, T., Maciver, A., and Chong. P.Y., (1987), Desalination 66, 147.Holthe R, Gartland, P.O. and Bardal, E., (1988), Proceedings 7th International Congress onMarine Corrosion and Fouling, Valencia, Spain.

21. Hmig, H.E., Seawater and Sewater Distillation, (1978), Vulkan - Verlag, Essen.

22. Ijerseling, F.P., (1980), Br. Corrosion J., 15, 51.

23. Johnsen R, Bardal, E. and Durgli M., (1983), Proceedings 9th Scandinavian CorrosionCongress, Copenhagen, Denmark, Sept.12-14, 1, p.371-82.

24. Johnsen, R. and Olsson, (1986), Proceedings 10th Scandinavian Corrosion Congress,Stockholm, Sweden, June 2-4, 135-42.

25. Kain, R.M., and Sleep, T., (1985), ASTM, STP 866, 299-323.

26. Kain, RM., (1991), Effect of Surface Finish on the Crevice Corrosion Resistance of SS inseawater and Related Environment, Corrosion 91, paper 508.

27. Lee. T.S., and Kain, R.M., (1983), Corrosion '83, NACE, paper 69.

28. Lee. T.S., Kain, R.M. and Oldfield, J.W., (1984), Materials Performance, 23(7), 9-15.

29. Latha, G. and Rageswari, S., (1997), Corrosion protection & control, Feb., 22-29.

30. Lornz, K., and Medawar, G., (1969), Thyssen Forschung, 1(3), 93.

31. Malik, A.U., and Al-Fozan, S., (1994), Desalination, 97, 199-212.

32. Malik, A.U., Kutty, P.C.M., Siddiqi, N.A., Andijani, N. and Ahamd, S., (1992), CorrosionScience 33(11), 1089-1827.

33. Malik, A.U., Kutty, P.C.M., Siddiqi, N.A., Andijani,I. N. and Al-Fozan, A., S. (1994),Desalination 97, 171-187.

34. Malik, A.U., Siddiqi, N.A., Ahamd, S. and Andijani, N., (1995), Corrosion Science, 37(10),1521-1535.

35. Malik, A.U., Siddiqui, N.A., and Andijani, I.N., (1994), Desalination, 97, 189-97.

36. Malpas R.E., Gallagher P. and Shone E.B., (1986), Proceedings Chlorination of Seawatersystems and its Effect on Corrosion, Birmingham, UK, Mar.4, Paper TRCP 2919 R,Soc. of Chem. Ind.

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37. McCarfferty, E., Bogar, F.D., Thomas II, E.D., Creegan, C.A., Lucas, K.E. and Kaznoff,A.I., (1997), Corrosion, 53(10), 755-761.

38. Mollica A, Tervis A., (1976), Proceedings 4th International Congress on Marine Corrosionand Fouling, Antibes, France, June 14-16, 351-65.

39. Moniz, B.J. and Pollocks, W.J., eds., (1986), 'Process Industries Corrosion' NACE,Houston.

40. Neville, A. and Hodkiess, T., (1996), Corrosion Science, 38, 927-956.

41. Oldfield, J.W., (1990), Corrosion 46, 574.

42. Oldfield, J.W., and Sutton, W.H., (1980), Br. Corrosion, J., 15, 31.

43. Oldfield, J.W., and Todd, B., (1995), A Review of Materials and Corrosion Desalination -Key Factors for Plant Reliability - Award Winning Treatise. IDA World Congress onDesalination and Water Science, Abu Dhabi, November 18-24.

44. Oldfield, J.W., and Todd, B., (1996), Desalination 108, 27-36.

45. Salvago, G., and Fumagalli, (1996), Corrosion, 52, 760-767.

46. Sedricks, A.J., (1979), 'Corrosion of Stainless Steels' Wiley, New York.

47. Smialowska, Z.S., (1986), 'Pitting Corrosion of Metals' NACE, Houston.

48. Smialowska, Z.S., (1987), 'Industrial Problems Treatment and Control Techniques' PergamonPress, Oxford.

49. Stackle, R.W., Brown, B., Kruger, J. and Agrawal, A., (eds.), (1974), 'Localized Corrosion'3rd Edition, NACE Houston.

50. Steismo, U. and Drugli, J.M., (1997), Corrosion, 53(11), 26-32.

51. Todd, B., (1977), 'Materials for Seawater System' Chemistry and Industry, July 2, 14-22.

52. Tuthills, A.H., Moller, G.E., Todd, B. and Hornburg, D., (1995), 'The suitability of 6% MoAustenitic SS for Desalination Plant Services', Proc. IDA World Conference onDesalination and Water Sciences, Abu Dhabi, November 18-24. VII p. 412.

53. Wallen, B, (1990), acom, No.4-90.

54. Wallen, B., and Henrikson, S., (1989), Werkstoffe and Korrosion 40, 602-615.

55. Wallen, B. and Bergqvist, A., (1997), The seawater Resistance of a Superaustenitic 7MoStainless Steel, Acom, 4.

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56. Wallen, B., (1998), Corrosion of Duplex Stainless Steels in Seawater, Acom, 1.

57. Yashiro, H.Y. Tarnno, K., Hanayama, H. and Mirura, A., (1990), Corrosion 46, 727.

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Table 1. Composition of different alloys (wt.%), balance in all the alloys is Iron and

PREN values.

S.No Alloy Cr Mo Ni C Cu Mn N PREN 1. SS 316-L 16 3 11 0.02 - 1.0 25 2. SS 317-L 18 3 13 0.02 - 2.0 30 3. SS 904-L 20 4.74 24.48 0.017 1.4 1.5 35 4. 3127 hMo 27 6.5 32 0.016 1.2 1.5 48.5 5. 1925 hMo 21 6 25 0.016 0.9 0.9 0.2N 41 6. 254 SMO 20 6 18 0.02 0.7 - 0.2N 43.5 7. Duplex -2205 22 3 6 0.02 - 1.5 0.15N 34 8. Remanit - 4565 22-25 4-6 16-18 0.02 - 5.0 0.5N 40 9. 654 SMO 25 7 22 0.015 0.4 - 0.5N 57

Table 2. Composition of Arabian Gulf seawater at AL-Jubail.

Constituents Arabian Gulf Seawater,Al-Jubal

Cations (ppm)

Sodium, Na+

Potassium, K+

Calcium, Ca2+

Magnesium, Mg2+

Copper, Cu2+

Iron, Fe3+

Strontium, Sr2+

Boron, B3+

13440

483

508

1618

0.004

0.008

1

3

Anions (ppm)

Chloride, Cl-

Sulfate, SO4=

Bicarbonate, HCO3-

Carbonate, CO3=

Bromide, Br-

Fluoride, F-

Silica, SiO2

24090

3384

160

-

83

1

0.09

Other Parameters

Conductivity, (mS)

pH

Dissolved Oxygen (ppm)

Carbon di Oxide (ppm)

Total suspended solids (ppm)

Total Dissolved Solids (ppm)

Temperature Range (0C)

62800

8.1

7

2.1

20

43800

18-33

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Table 3. Breakdown Potential (Eb), and protection Potential (Eprot) versesSaturated Calomel Electrode (SCE) and maximum current after scanreversal (Imax) of nine different alloys in Arabian Gulf water at 250C.

S.No Alloys Eb(mV)

Eprot(mV

Imax(mA/cm2)

1. 316-L 228 -209 5412. 317-L 524 -173 1803. 904-L 1016 1002 1304. 3127hMO 950 849 2045. 1925hMO 968 950 1356. 254SMO 918 909 1417. Duplex-2205 1010 968 1248. Remanit 4565 1039 929 1279. 654 SMO 1001 908 155

Table 4. Breakdown Potential (Eb), and protection Potential (Eprot) versesSaturated Calomel Electrode (SCE) and maximum current after scanreversal (Imax) of nine different alloys in chlorinated (0.2-0.25 ppm) inArabian Gulf water at 250C.

S.No Alloys Eb(mV)

Eprot(mV

Imax(mA/cm2)

1. 316-L 197 -220 4792. 317-L 300 -165 4233. 904-L 905 822 1224. 3127hMO 905 852 1465. 1925hMO 989 884 1466. 254SMO 911 866 1427. Duplex-2205 1003 957 1458. Remanit 4565 952 914 11409. 654 SMO 948 878 144

Table 5. Breakdown Potential (Eb), and protection Potential (Eprot) versesSaturated Calomel Electrode (SCE) and maximum current after scanreversal (Imax) of nine different alloys in natural seawater at 500C.

S.No Alloys Eb(mV)

Eprot(mV

Imax(mA/cm2)

1. 316-L 43 -102 16072. 317-L 159 -90 8163. 904-L 647 -114 12304. 3127hMO 879 798 4245. 1925hMO 907 778 5156. 254SMO 923 923 1287. Duplex-2205 354 -203 1328. Remanit 4565 921 921 1269. 654 SMO 870 870 130

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Figure 1. Photographs showing the Open Circuit Potential measurement system

Figure 2. Photograph of K0047 corrosion measurement cell kit measurement

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