CORROSION STUDIES ON SS 316 L INLOW pH HIGH CHLORIDE PRODUCT

WATER MEDIUM1

A.U. Malik, M. Kutty, Nadeem Ahmad Siddiqi,Ismaeel N. Andijani, and Shahreer Ahmad

ABSTRACT

A detailed laboratory study has been carried out to determine the pittingbehavior of 316L in chloride - containing aqueous solutions. The effects of Cl-concentration, pH, dissolved oxygen and temperature on the pitting have beeninvestigated under dynamic and static conditions. Weight loss, metallographyand electrochemical techniques have been employed during the investigation.

The weight loss measurement indicate extremely low losses during 4 monthsimmersion tests, however, weight losses recorded were highest at pH = 4 understatic conditions, and were lowest at pH = 7 under dynamic conditions.

Metallographic studies indicate that the number and depth of pits increasedwith increasing Cl- concentration and immersion time and maximum number ofpits were found on specimens immersed in solutions of pH 4. A parabolicrelationship exists between pit depth and Cl- concentration.

Polarization resistance, potentiodynamic polarization and cyclic polarizationwere the electrochemical techniques used during pitting investigations. Corrosionrate values were computed from polarization resistance plots and it was foundthat in general, corrosion rate increased linearly with increased Cl- concentrationin the range of 100 to 5000 ppm. With increasing pH, the corrosion rate decreasesbeing highest at pH=4 and lowest at pH=9. Potentiodynamic polarization studiesindicate shifting of E corr to more negative values with increasing Cl- concen-tration and temperature. From the results of cyclic polarization studies it isinferred that pitting potential, E pit is shifted to more negative (or active) valuewith increasing Cl- concentration and temperature. The electrochemicallymeasured pit potential, E pit and repassivating potential, Er are found to be linearjunctions of the logarithm of Cl- concentration. At a particular temperature andCl- concentration, the value of E pit shifts to more noble potential with increasein pH.

1 Issued in September, 1990.

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The results of this study exemplify the otent roles of various parametersin influencing the pitting mode of 316L in CI- containing medium. It has beenestablished that low pH, high Cl- content and stagnancy are the conditions mostsuitable for initiation and propagation of pitting in 316L.

1. BACKGROUND INFORMATION

Corrosion in the distillate header pipe line (WJ 11) at SWCC Azizyadesalination plant was reported in early August 1989 and SWCC Research andDevelopment Center (RDC) was asked to inspect the line and was subsequentlyassigned to investigate the problem. The RDC carried out extensive field andlaboratory investigations for determining the cause of failure and subsequently,submitted a brief report ( 1) containing the main results of investigations, possiblecauses of failure and remedial actions to be taken. Considering the seriousnessof the problem on corrosion and materials point of view in general and SWCCwater transmission systems in particular, a project proposal on ‘Corrosion Studiesof SS316L in low pH, High Chloride Product Water Medium’ was submitted toSWCC, Riyadh on August 19, 1989 and was accepted on August 29, 1989. Inthe original proposals a time period of 2 months was suggested for completionof the work. However,. when the project was initiated in September 1989, itwas the considered opinion of all the investigators that long term tests some upto one year durations are necessary under simulating conditions existing in thepipe lines. Moreover, electrochemical measurements which are an importantpart of the corrosion study could only be started when the EG and G corrosionsystem model 342-2 was installed in the middle of June 1990.

In August 1989, RDC inspected 50” diam SS 316L pipe line (WJ11) whichserved to carry product distillates from a train of MSF units to the CO2 andline dosing facilities. On inspection of the pipe line it was found that the innersurface of the pipe line was not uniformly or severely corroded albeit it hasrandomly scattered corrosion products at the walls and at the bottom. The bottomsurface (5 to 7 Clock position) was found to be pitted with pits vary from pinhole to large worm hole sizes, Each pit was either enveloped or encircled withbrownish red corrosion products. The section of the pipe between the twoinjection points of CO2 and lime was found to be particularly more corrodedthan the section up stream of the CO2 dosing point. During later visits inOctober - November, the condition of the pipe line (WJ 11) was deteriorated,many new pits were formed and the old pits became deeper and some to theextent of the whole thickness (1.6 mm) of the pipe line.

Recently, the pipe line (WJ 11) was repaired and put in operation and thealternative/parallel pipe line (WJ12) was drained off for inspection. WJ12 pipeline was examined in July 1990 and it was also found pitted but the conditionof the pipes was much better. On some locations on the wall of the pipes (about8 Clock position), green or brownish green deposits were found. On scratchingthe corrosion products with a thin sharp edge, the pits could be seen clearly.Figures 1 to 6 show some photographs of the pitted sections of the pipe lines.

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The average analysis results of chloride in distillate before and aftertreatment for the months : September 1988 to July 1989 are given in table 1.Table 2 provides chemical analysis of the distillate (before treatment) collectedfrom distillate header (OWJ-12) on August 9, 1989. It represents typical datafor a daily analysis. The distillate is normally slightly acidic (pH - 6.2) andhaving Cl- contents varying from 15 to 150 ppm. The pipe line was understagnant conditions for some months during the service period of approximately6 years.

This report presents the results of a detailed study on the effects of tem-perature, dissolved oxygen, pH, Cl- concentration and total dissolved solids(TDS) in water on the pitting behaviour of SS 316L under simulated dynamicand stagnant conditions. The main objective of this study is to assess theperformanance of SS 316L as structural material under various conditionsoperative in desalination plants. The major investigative procedures employedare : weight loss, electrochemical measurements, metallography and micro-structural determinations.

2. INTRODUCTION

Pitting is a localized corrosion attack on metals and alloys in aqueousenvironments. It is a major cause of failure of chemical process and desalinationplants, water storage tanks and pipe lines, pumps and valves, petroleum refineriesetc. Due to localized nature of pitting corrosion, formation of pits is confinedto much smaller areas compared to overall exposed surface. Broadly speaking,the initiation of pitting is the result of the breakdown of the passive film onthe metal due to the presence of some anions such as Cl- and subsequentestablishment of an electrochemical cell in which damaged site acts as an anodeand the passive site acts as a cathode. The building up of corrosion productson the mouths of the pits may result in the formation of crevices thus producingmore aggressive corrosion attack. Iron - base alloys including carbon steels andstainless steels are most prone to pitting in dissolved CO2 - and Cl- containingenvironments as observed in brine recycle and blow down pumps, flash chambersand demisters of desalination plant, and steam boiler, feed water heater tubesand steam turbine blades of power plants, (2-6).

Austenitic stainless steels are the most commonly used materials due to theirhigh strength and superior corrosion resistance in moderately severe environment.AISI 304L (Cr : 18, Ni : 9, C : .02) provide acceptable corrosion resistance inmost of the technological applications, however, in Cl- containing aqueousenvironments 304L SS invariably undergoes pitting and Mo- containing austeniticsteels of similar compostion have been found more resistant to pitting.(7-10).

Austenitic AISI 316L (Cr : 16.5, Ni : 10, Mo : 2, C :.02) is considered tobe one of the most resistant of common stainless steels under marine environmentsand therefore, is an important structural material for desalination plants. Eventhis alloy may fail under conditions of high chloride content and stagnations(11-13). Figure 7 to 10 show some photomicrographs of cross- sections from a316 pipe which failed while stored in a marine environment. A range of highalloy austenitic steels such as 254 SMO, AL-6XN, 20 Cb3, Hastelloy C, cronifer1925 LC etc. have been recommended for applications under more severe

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environmental conditions. In these alloys, the stability of passive film withrespect to pitting initiation is controlled primarily by Cr, Mo and N. The relativeeffect of these elements on pitting/crevice corrosion can be assessed from widelyused PREN, pitting resistance equivalent.

PREN = Cr% + 3.3 Mo% + 16 N%Table 3 provides composition and PR EN of some important high corrosionresistance structural steels.

Numerous papers have appeared in recent years regarding the pittingbehaviour of stainless steels in aqueous solutions containing single or combinationof anions (7- 10,14,15). Particular interest has been shown for the Cl- due toits presence in the seawater as the major constituent and its role as an activepitting agent. Besides the influence of temperature, flow/velocity, pH and Cl-concentration on pitting, the synergic effect of anion such as SO4--, S2 O3--,Cl O4- etc. have also been studied. The main objective of these studies is todetermine the exact role of Cl- in initiation of the pits and to determine themechanism. Electrochemical techniques have been the main tool of theseinvestigations. It will be revealed from the survey of the recent literature onpitting, as given in the following section, that the exact role of differentparameters in pitting of stainless steels is yet to be fully understood and morework is needed particularly with reference to pitting and crevice of stainlesssteels in industrial environments.

The effect of Cl- on the pitting susceptibility of various metals and alloysand particularly stainless steels has been extensively investigated by numerousresearchers and is reviewed in a number of books, reviews and articles (16-22).Smialowska (23) found that a majority of construction materials suffer pittingonly in solutions containing Cl- or other halogen ions. Three main reasons aregiven for the specific effects of chloride and its ability to produce pitting. Firstlyforming complex with cation and hydroxide, secondly, increasing the activityof hydrogen ions in the pit electrolyte and thirdly, forming a salt layer at thebottom of pits. The third factor appears to explain the specific role of halidesin pitting attack. It is suggested that transmission from passivity to pittingcondition can be explained by competitive adsorption mechanism (24,25) inwhich chloride ions move into the double layer (oxide/liquid interface) of theelectrode surface, eventually reaching at a critical potential (E pit), correspondingto the Cl- concentration required to displace adsorbed oxygen species. Thepresence of adsorbed Cl- increases the potential difference across the passivefilm thereby enhancing the rate of Fett diffusion from the metal/film interfaceto film/solution interface. This leads to the formation of cation vacancies atthe metal/film interface which normally disappear into the bulk of the metal.When the Cl- concentration is such that the rate of cation diffusion and thusthe formation of cation vacancies is greater than the rate of disappearance ofcation vacancies, voids develop at the metal/film interface. Continued growthof a void results in the localized collapse of the passive film, which will sub-sequently dissolve faster than other regions of the passive film leading to pitgrowth and ultimately substrate alloy dissolution. Anions other than Cl- suchas BO3--, SO4-- and ClO4-- also adsorb on the surface, displacing Cl-. Thusin presence of these anions if the competitive adsorption mechanism is applicable;

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it will take longer for the Cl- to achieve critical concentration required for pitformation (26,27). This would result in a shift of E pit to move noble potentials.Perchlorate ions have been reported to adsorb least of all the common ions (26).The would explain the inferior inhibiting ability of C1O4- compared to that ofSO4--. In neutral aqueous solutions containing chromate and nitrate as inhibitors,with Cl-, Br- and SO4-- as pitting corrosion agent, the order of decrease ofcorrosivity of the aggressive ions is SO4-- > Cl- > Br- and aggressiveness ofhalides towards passivated steels was found to decrease in the order : Cl- > Br-> I- (28). Nitrogen has been found to encourage the retention of Cr in thepassive films and near surface layers during corrosion of stainless steels, whichimprove pitting resistance (29). In newspaper machines, pitting is observeddespite low chloride concentration in the process water. This is now known tobe caused by thiosulfate ions, which can induce pitting on 304 SS withoutassistance from chloride if sulfate thiosulfate ratio is within a certain range. For316L, this sulfate pitting did not occur unless the Cl- concentration is fairlyhigh (l0-2 M or 350 ppm) and unless the molar concentration of chloride exceededthat of sulfate (30). This explains the previously unexpected pitting of 316Lequipments in splash zones, where a concentrated chloride/sulfate solution candevelop by evaporation.

While studying pitting corrosion of 304 and 316 austenitic steels coveredwith anodic oxide film it was found that pitting current, ip constits of 3 periods: pit incubation ( O < t < ti) in which ip scarcely flows, pit nucleation (ti < t < T)and pit growth (T < t ) (31). The incubation time, ti is the time at which theoutermost layer of a film is dented by Cl- ions, depends up on the nature ofthe bound water and the stability of the film, but not the film thickness. Theinduction time, T is the time at which the film is completely perforated, itincreases with increasing film thickness and increasing potentials. Nishimuraetal (32,33) found that pit initiation is trongly related to 2 different types ofbound water in a film. It was found that the pitting behaviour of alloys coveredwith the passive film is largely dependent on the film thickness and ion selectivity.From investigations on pitting susceptibility of 304 SS it was concluded thatpitting is related to semiconductive property of the oxide film (34). Duringpitting of 304 SS in Cl- containing aqueous solutions, it was found that a linearrelationship exists between pit nucleation time and log chloride concentration,and between pitting potential and log induction time (35). The impedancemeasurements showed that the charge transfer resistance decreased withincreasing temperature. Higher the Cl- concentration, lower the charge transferresistance, hence less the protective oxide film.

Kineties of pit generation of stainless steel in presence of Cl- was studiedby measuring pit initiation under potentiostatic and pitting potential underpotentiokinetic conditions (36, 37). Pitting generation rate, g is expressed by g= K. Cn exp (naqE/KBT). For short holding times and for high scanning rates,g does not depend upon scanning rate. For longer tests, the passive film propertiesvary during the test and g decreases with time, and therefore, the measuredpitting potential must be considered vary cautiously.

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Electrochemical techniques have been frequently used for determining thecorrosion behaviour of materials in general, and pitting performance in particular.A large number of publications appeared in recent years on practical applicationsof electrochemical polarization methods in pitting corrosion (38-45).Potentio-dynamic polarization techniques were used to determine the charac-teristic pitting potential for alloy 800 in Cl- and Cl- + SO4-- media (46).Decreasing the Cl- concentration resulted in noble shifts of the critical nucleationpotential whereas increasing sulfate concentration resulted in improved chloridepitting resistance. Immunity to pitting corrosion was evident at Cl- below 300ppm. The critical nucleation potential varies linearity with the logarithm of Cl-activity. The occurance of pit initiation at transpassive potentials can berationalized in terms of localized breakdown of a surface film that is somewhatless protective than that prevalent in the passive region. Measurements of pitinitiation and pit propagation for Fe-Cr-Ni alloys in Cl- environments byelectrochemical techniques were reported (47). Correlations based on the valuesof the pitting potential, E Pit, Crossover or protective potential Ex, corrosionpotential, E corr and Slope ∆E/∆i were used to predict pit initiation and pitpropagation. For example, if Ecorr > Ep + 0.1 V, then good resistance to pitting,if ∆E/∆i < 0.18 V/decade then there is good resistance of pitting. Systemsexhibiting poor resistance to pit propagation should have E Corr values betweenEp and Ep + 0.lV.3. EXPERIMENTAL

Commercial grade SS 316L (17.1 Wt% Cr, 11.3 wt%, Ni 2.1 wt% Mo, 0.02wt%C and balance - iron) in sheet and rod forms was used for the studies.

For immersion tests, coupons of about 5 cm2 area were cut from the sheetand abraded sequentially with 180,320,400 and 600 grit SiC papers. The abradedcoupons were cleaned in an ultrasonic cleaner followed by drying. The driedspecimens were weighted.

For electrochemical measurements, circular flat test specimens as well ascylindrical test specimens were used. The circular flat test specimens of 1.5 -1.6 cm diameter were punched or cut. The exposed area of the test specimenswhich was screwed in the sample holder was 1 cm2. Figure 11 shows a drawingof a flat specimen holder.

Potentiodynamic polarization tests were carried out on an EG & G model342-2 soft corr measurement system. The system was consisted of Model 273polentiostat/galvanostat, Model 342 soft Corr Soft ware and Model 301 BM PS/2.All the experiments were carried out using a corrosion cell with saturated calomelas reference and graphite as counter electrodes (EG&G Model K 0047). Severalseries of experiments were carried out in order to study the effect of Cl-concentration, pH, TDS and stagnancy on the behaviour of 316L steel using testsolutions with varying compositions and conditions as shown below:

PH 4, 7, 9Cl- concentration (ppm) 0, 10, 100, 150, 300, 500, 1000

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TDS (ppm) 100, 200, 300 (see Table 4A for details)

Artificial sea water See Table 4B

Immersion time (weeks) 4, 8, 12, 16

Condition Static and Dynamic

Temperature 25 + 2oC

Dissolved oxygen (ppm) 6 + 0.5

Previously weighed coupons were immersed in test solutions for varioustime intervals of 1,2 and 4 months. At the end of the test periods the couponswere taken out, washed in distilled water, dried and their weights were deter-mined.

Following the immersion test, the microstructural examination of all thetest specimens was carried out to assess the extent of the localized attack andmorphology of the corrosion products. The shape, size and density (distribution)of the pits were determined metallographicaly using an optical microscope (ASTMG46-76).

4. ELECTROCHEMICAL MEASUREMENTS

4.1 Open Circuit Corrosion Potential (OCP)

OCP of 316L immersed in chloride solutions were recorded under staticand dynamic conditions. For these measurements, a separate cell with 316Lcoupon as W.E. and S.C.E as a reference electrode was used. It took 24 - 48hrs. to achieve a constants potential corresponding to open circuit potential(OCP). Figures 12 to 15 show time vs OCP plots for 316L steel immersed insolutions of 300 and 1000 ppm Cl- at pH 4,7 and 9 at different temperaturesdeaerated conditions.4.2 Potentiodynamic Polarization

Potentiodynamic Polarization measurements were carried out using a scanrate of 0.1 mv/s commencing at a potential about 250 mv more active than thestable OCP and terminating at a potential about 500 mv more positive than OCP.Before starting the polarization scan, the specimen in the sample holder (W.E)was left in the cell for about 1 hr for attaining a steady state which is shownby a constant potential and current at the commencement of the experiment.All potentials were measured vs SCE.

4.3 Cyclic Polarization

During a potentiodynamic run, upon attaining a current density of 25uA/cm2, the scan direction was reversed, potentials were scanned back to thestarting potential.

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4.4 Polarization Resistance

Polarization resistance measurements were conducted at a scan rate of 0.1m v/s with starting and final potentials corresponding to -20m V and + 20 mVvs OCP, respectively. The maximum current range was 0.1 uA.

5. RESULTS

5.1 Weight Loss Studies

Figures 16 and 17 show weight loss vs Cl- concentration plots at a fixedimmersion time (4 months) and at different pH’s. As seen from the plots, noregular trend was observed under static or dynamic conditions. However, thefollowing generalizations can be made regarding the behaviour of 3 16L undervarying parameters e.g., pH, time interval and Cl- concentrations.

(i) No perceptible weight loss was observed during immersion periods of upto6 weeks irrespective of Cl- concentration, pH and dynamic or static con-ditions.

(ii) Extremely low weight losses (10 - 50 ug/cm2) were observed duringimmersion time exceeding 6 to 8 weeks.

(iii) Weight losses recorded were highest at pH = 4 under static conditions, andwere lowest at pH = 7 and under dynamic conditions.

5.2 Metallographic Examinations

Figures 18 to 23 show some surface microstructures of SS 316L steelspecimens immersed in Cl- containing solutions at pH 4,7 and 9 under staticand dynamic conditions. Pitting of 316L occurs under varying Cl- concentrationand pH. Under similar conditions, maximum number of pits were found onspecimens immersed in solutions of pH 4 and minimum on specimens immersedin solutions of pH 7. Under dynamic conditions (when the solutions were agitatedcontinuously during the entire periods of immersion test) the number of pitsobserved on the surface of the specimen was much smaller than under static orstagnant condition. In general, the number and depth of the pits increased withincreasing Cl- concentration and immersion time. For example, when theimmersion time was increased to 4 months, there was an over all increase in pitnumbers as well as their sizes. At pH 4 in static conditions, several big andsmall pits could be identified. At pH 7 and 9 pitting was comparatively lesssevere but a few deep pits could be seen under static as well as dynamic conditions.The studies also indicate that in presence of TDS, pitting occurs but the effectis much less aggressive.

5.3 Pit Depth Measurements

The depth of the pits on 316L specimens was measured microscopically.Minimum and maximum pit depths were measured for a particular specimensand also average depths were determined by considering the depth of all thepits present. Figure 24 shows plots of maximum pit depths vs Cl- concentration,

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for 316L immersed in solutions of 300 ppm Cl- concentration and differing pHfor an immersion period of 4 months. The plots provide following interestinginformations.

(i) The pit depth increases with increasing Cl- concentration. A parabolicrelationship appears to exist between pit depth and Cl- concentration asindicated by the linear nature of pit depth vs concentration of chlorideplots (fig. 25).

(ii) Pit depths of the specimens immersed in solutions of pH4 was much greaterthan those immersed in solutions of higher pH e.g., 7 and 9.

Figures 26 to 31 show the optical micrographs of pits produced ‘bypotentiodynamic anodic polarization or cyclic polarization of 316L in differentchloride concentrations. Most of the pits were generally of small diameter (i.eless than 50 um). Table 5 describes the morphology of pits generated in chloridemedia under different conditions.

5.4 Electrochemical Measurements.

5.4.1. Polarization Resistance: Polarization resistance measurements using 316Lcoupons immersed in Cl- containing solutions were carried at 25, 50 and80 C under varying pH, Cl- and aerated and deaerated conditions. Typicalpolarization resistance plots are shown in Fig. 32 to 38. Table 6 lists thecorrosion rate values computed from the plots. The polarization dataprovide following information regarding the behaviour of 316L in Cl-containing solutions.

i) At pH = 4 the corrosion rate increases linearly with increasing Cl-concentration in range of 100 ppm to 5000 ppm although highercorrosion rates are observed at 500 ppm.

ii) With increasing pH, the corrosion rate decreases being highest at pH= 4 and lowest at pH = 9.

iii) Corrosion rate increases with increasing temperature being highest at 80 C and lowest at 30 C.

iv) Corrosion rates under deaeration are much lower than aerated con-ditions.

v) Corrosion rates of 316L in seawater is 0.064 MPY which is about thesame as that in a solution containing 1000 ppm Cl-.

5.4.2. Potentiodynamic Polarization: Typical potentiodynamic anodic polarizationplots for 316L in Cl- containing solutions are shown in Fig.39 to 45. Thevalues of Corrosion current (I corr), corrosion potential (E corr), Tafelconstants ( Ba and Bc) and corrosion rates computed from potentiodynamicplots are given in Table 7. In general, E Corr shifts to more negative

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values with increasing Cl- concentration and temperature. The corrosionrate increases with increasing Cl- concentration and is maximum at pH= 4.

5.4.3. Cyclic Polarization: Some representative cyclic polarization curves for 316Lin Cl- containing solutions are shown in Fig. 46 to 54. A hysteresis loopis traced during reverse scan indicating the the possibility of pitting. Table 8 lists the values of pitting potential (E pit) and protection potential (EProt), which is defined as the potential where forward and reverse scanscross, repassivating pitting polential (Er) is also considered, it is definedas the most active potential at which the nucleation of unstable (i.e.repassivating) pits can occur and is characterized by a reversible increasein current density (CD). The stable nucleation potential (E Pit) is assessedas that at which a stable increase in CD occurs., indicating the initiationof non-repassivating pits. Eprot is the most active potential at which pitpropagation can occur. In general,. pitting potential, E Pit is shifted tomore negative (or active) value with increasing Cl- concentration andtemperature. At a particular temperature and Cl- concentration, the valueof E Pit shifts to more noble potential with increase in pH. The elec-trochemically measured pit potential, Epis found to be a linear functionof the logarithm of Cl- concentration (Fig.55).

6. DISCUSSION

The immersion tests carried out on 316L coupons at different Cl- con-centrations, pH and time intervals and under static and dynamic conditions showextremely low weight losses (10 - 50 ug in typical 4 months runs). At a particularchloride concentration and immersion time, the weight losses were more or lessindependent pH and static or dynamic conditions. The number and depth ofthe pits generally increased with increasing immersion time. Maximum numberof pits were found on specimens immersed in solutions of PH 4. At pH 7 and9, the number of pits were smaller but some of them were deep. Under dynamicconditions, the number of pits observed on the surface of the specimen wasmuch smaller than under stagnant condition.

Austenitic steels are virtually immune to general corrosion while in contactwith fresh or saline water at natural pH. However, in the presence of Cl- thesesteel are subjected to local attack in the form of pitting or crevice corrosion dueto break-down of protective Cr2 O3 film at random sites. The pitting on thepassive surface has been explained by the competitive adsorption mechanism inwhich chloride ion move into metal/oxide film interface at the metal surface.At a particular chloride concentration, a critical potential (E Pit) develops whichis sufficient to displace adsorbed oxygen species (protective oxide layer). Itappears that low pH and stagnancy provide most favourable conditions for pitgrowth. In a typical case, under conditions of 4-5 ppm dissolved oxygen and25 C, the pits grow maximum to 450 and 325 microns at pH 4 under static anddynamic conditions, respectively when 316L specimens were immersed in 300ppm chloride solutions for 4 months. At higher pH’s (7 and 10) the depth rarelyexceeded 70 microns. The pit depth appears to be a parabolic function of Cl-concentration and therefore with increasing Cl- concentration the rate of pittingappears to slowdown.

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The corrosion rates of 316L in 1000 ppm chloride solutions at differentpH’s as determined from polarization resistance technique were found to behighest at pH = 4 and the lowest at pH = 9. Similar information was obtainedfrom potentiodynamic polarization and Tafel plots. At a definite pH and a fixedchloride concentration, the corrosion rate of 316L increases with increasingtemperature (Fig. 56). The corrosion rates in deaerated conditions (1.4 ppm O2)were much lower than under ordinary conditions (6.8 ppm O2).

Considering the effect of pH on pitting potential, the potential was foundto be shifting to move positive values with increasing pH, however, no systematicvariation in protective polential values was found. The effect of chlorideconcentration on the stable pit nucleation potential (E Pit) is consistent with thatobserved with other systems whereby the potential varies with the logarithemicof Cl- activity. Lackie and Uhlig (24) reported a 88 mV shift in the criticalnucleation potential on a ten fold increase in Cl- activity in the concentrationrange of .01 to 1M (350 to 35,000 ppm). In this study a shift of about 200 mVwas observed on a similar increase in the concentration range of 100 ppm to5000 ppm at pH = 4. The repassivative potentials (Er) were also found to shiftto move moble values with decreasing Cl- content. Like pitting potential, Epit, the repassivating potential, Er also varies linearily with the logarithm of theconcentration of chloride (Fig. 57). At chloride level of 100 ppm or below norepassivating potential was observed. The corrosion rate of 316L in artificialseawater (Cl- 24153 ppm and pH = 7.3) was slightly lower than the corrosionrate is solution of pH 4 but is higher than in solutions of pH 7 and 10.

At chloride concentrations at which stable nucleation was evident (100 ppmto 30,000 ppm) extensive hysteresis was observed upon scan reversal withrepassivation occurring in the vicinity of OCP. The pitting potential (E Pit) wasinvariably more positive than the protective potential (E pro). The differencebetween pitting and protective potential decreases with increasing Cl- con-centration and is a linear function of logarithm of Cl- concentration (Fig.58).The occurrence of pit initiation at transpassive potentials can be rationalized interms of localized breakdown of a surface film on 316L that is some what lessprotective (i.e. higher rate of general dissolution) than that prevalent in thepassive region. The nucleation of repassivating pits at overpotentials far belowthat required for stable pitting indicates the enunciation of a dynamic processof pit initiation and repassivation prior to the development of propagation pits.

From the foregoing studies, it is evident that at a given temperature anddissolved oxygen concentration, pH, Cl- concentration and flow conditions areimportant parameters affecting the corrosion behaviour of 316L in water. Ithas been established that low pH, high Cl- content and stagnancy are theconditions most suitable for initiation and propagation of pitting in 316L. The316L pipe line has deep pits that were produced due to intermittent stagnancyof aerated distillate water for periods varying from a few weeks to one year.The Cl- content of the distillate varied from 30 - 150 ppm during this periodas shown by laboratory analytical data sheet and having pH approached to acidicrange. One important consequence of the pitting is the occurrence of crevicecorrosion inside the pits. Due to the blocking of the months of some of the pitswith the corrosion products, more crevice are formed resulting in spreading ofcorrosion throughout the length and breadth of the pipe line.

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7. CONCLUSIONS

The pitting behavior of 316L in Cl- containing solutions is greatly influencedby the variation in Cl- concentration, pH, dissolved oxygen, temperature andflow conditions. It has been established that low pH, high Cl- and stagnancyare the conditions most favourable for initiation and propagation of pits in 316Lsteel. The pitting in 316L header pipe line was-primarily the-aftermath of highCl-, low pH and intermittent stagnancy of the distillate water in the subject pipe line

.

REFERENCES

1.

2.3.4.

5.6.7.8.9.10.11.12.13.14.15.16.

1 7 .

1 8 .

1 9 .

20.21.22.

An investigation on the condition of distillate header life line at Al-Khobardesalination plant- S.Szlarka - Smialowska,“Corrosion, RDC Report (1989).T.Hodgkiers,A.Maciver and P.Y.Chong, Desalination, 66, 147 (1987).T.Hodgkiers and N.G. Ary Desalination, 55, 229 (1985). E.H.Newton eta1 : R & D Progress Report No.278, U.S. office of SalineWater, Arthur D. Little Inc. Report, Cambridge M A,1967.J.W. Oldfield and B.Todd, Desalination, 55, 261 (1985).Desalination, 44, 209 (1983).D.J.Olsson and B.Wallen, Desalination, 44,241,(1983).E.H.Phelps, R.T.Jones and H.P. Leckie,J. Electrochem. Soc.,ll6,813 (1969).J.Oldfield and W.H.Sultor, Br.Corr.J. 15, 31 (1980).G.V.Akimov Corrosion 15,455 (1959).H.E.Deverall and J.R.Maurer, Materials Perforamnce, 17 ( 1978).H.P.Hack,Materials Performance, 22,24 (1983).A.J.Sedricks ‘Corrosion of Stainless Steels’ Hun Wiley & Sons, N.Y.(1979).J.Oldfield and B.Todd, Desalination 38,233 (1981).D.W.Black and R.M.Morris, Desalination 39, 229,(1981).Localized Corrosion 3rd edition, R.Stackle, B.Brown J.Kruger, A. Agrawal,Eds.,NACE,Houston, Texas P.252 (1974).J.R.Galvele, Passivity of Metals (Ed.), R.P.Frankenthal, J. Kruger,Elec-trochem Soc.,N.J. P.285 (1978).B.Baroux,“Parsivation and Localized Corrosion of Stainless Steels” in Pas-sivity of Metals and Semiconductors (Ed.M.Forment),531, Elsvier (1983).Z.Szklarska-Smialowska, Pitting, Corrosion of Metals, NACE Publication(1986).Z.Szklarska-Smialowska, Corrosion 27, 223 (1971).M.Janik-Gzachor,G.C.Wook and G.E.Thomson, Br. Corr.J.15,154 (1980).H.H.Strehlow, Werkst. Und Korrosion, 35, 437 (1984).

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23.

24.25.26.27.28.29.30.31.32.33.34.35.

36.37.38.39.40.41.42.43.44.45.

46.47.

Industrial Problems Treatment and Control Techniques Pergamon PressOxford (1987).H.P.Leckie, H.H.Uhlig, J.Electrochem. Soc.,ll3, 1262, 1967.J.Horvath, H.H.Uhlig, J.Electrochem.,l15, 791,(1968).H.E.H.Bird, B.R.Pearson and P.A.Brook, Corrosion Science,28, 81 (1988).H.C.Brookes and F.J.Graham, Corrosion, 45, 287 (1989).S.M.Sayed and H.A.El Shayeb, Corrosion Science, 28,153 (1988).S.J.Powell, E.E.Stansburry and C.D.Lundin, Corrosion, 45, 125 (1989).A.Garner, Corrosion, 45, 282 (1989).R.Nishimura and K.Kudo, Corrosion 44, 29 (1988).R.Nishimura, M. Araki, K.Kudo, Corrosion 40,465 (1984).R.Nishmura,Corrosion 43,486,( 1987).G.Bianchi, A.Cerguetli,F.Mazza, S.Torchio, Corrosion Science 10, 19 (1970).J.H.Wang, C.C.Su and Z.Szklarska-Smialowska, Corrosion Science, 44,732(1988).

B.Baroux, Corrosion Science, 28, 969 (1988)K.E.Heusler and L.Fisher, Werkst Und Korrosion,27, 551 (1976).B.E.Wilde, J.S.Armijo,Corrosion 23, 208 (1967).W.Schwenk, Corrosion Science, 5,245,( 1965).H.P.Leckie,J.Electrochem. Soc,l17,1152 (1970).E.A.Lizlovs, A.P.Bond, Corrosion, 31, 219 (1975).P.E.Manning, Corrosion,36, 468 (1980).H.C.Man, D.R.Gabe, Corrosion Science, 21, 323 (1981).P.E.Morris, R.C.Scarberry, Corrosion, 28,444 (1972).J.G.Stoecker, O.W.Scebert, P.E.Morris, Practical Applications of Poten-tiodynamic Polarization curves in Material Selection, Materials Performance,22,13, (1983).G.Palumbo, P.J.King and K.T.Aust, Corrosion 43, 37 (1987).Measurement of Pet Initiation and Propogation for Fe-Cr-Ni alloys in acidenvironments by Electrochemical Techniques, P.E.Morris in ElectrochemicalTechniques for Corrosion Engineers, P.287, R.Baboian, Editor, NACE(1986).

141

Table- 1

Average Results of Chloride in Distillate Before and After Treatment

CHLORIDE IN

MONTH

SeptemberOctoberNovermberDecemberJanuaryFebruaryMarchAprilMayJuneJuly

YEAR DISTILLATE DISTILLATEAFTER BEFORETREATMENT TREATMENT

32.5 ppm21.4 ”46.5 ”80.0 ”163.7 ”123.7 ”46.7 ”62.6 ”58.5 ”57.0 ”30.6 ”

27.5 ppm16.0 ”41.5 ”75.0 ”158.7 ”118.0 ’41.0 ”57.0 *53.0 ”53.0 ”26.0 ”

* Distillate after treatment with CO2, Lime & Sodium hypochlorite generatedfrom seawater.

Note: Chlorination by treatment with sodium hypochlorite contribute 5 + 1 ppmchloride to the distillate.

1 4 2

143

144

145

146

147

148

149

Figure 1 : Photograph of the inner surface of the header pipe lineshowing pits at the bottom (6oClock position) covered withthe corrosion products (OJll)

Figure 2 : Photograph of the inner surface of the header pipe lineshowing pits at 4 or 8O Clock position ( OJll)

150

151

Figure 5: Photograph of the inner surface of the header pipe lineshowing pit at the wall (9O Clock) covered with the corrosionproducts (0512 pipe line)

Figure 6: Same as Fig.6 but after corrosion products are removed(0512)

216152

Figure 7: Photo micrograph, of a cross sectionof a 316L pipe showing a deep pit.

Figure 8: Same as Fig.7 but at a differentlocation.

217153

Figure 9 : Scanning electron micrograph of across section of a failed 316Lpipe showing a deep crack and thepresence of micro pits.

Figure 10: Scanning electron micrograph of across section of a failed 316Lpipe showing extensive cracking &pitting.

218154

155

156

157

158

159

160

161

Figure 18 : Photo micrograph of a cross section of 316Lspecimen showing a deep pit (Cl-: 300ppm,pH=4,immersion period: 4 months under staticcondition)

Figure 19: Photomicrograph of a cross section of a pitted316L specimen ( Cl-:300 ppm,pH=7, immersionperiod : 4 months under static condition)

226 162

_ *

Figure 20: Photo micrograph of a cross section of a pitted316L specimen (Cl-: 300 ppm, pH=9, immersionperiod : 2 months under static condition.)

Figure 21: Photomicrograph of a cross section of 316Lspecimen showing the presence of shallowpits(Cl-: 300ppm, pI-i=4,immersion period :4 months under dynamic condition)

227163

Figure 22 : Photomicrograph of a cross section of apitted 316L specimen (C1-:XQqm,pH=7,Immersion period: 4condition)

months under dynamic

Figure 23: Photomicrograph of a cross section ofa pitted 316L specimen (TDS:200ppm,pN=7 immersion period: 3 months undercondition)

228 164

165

166

Figure 26: Photo micrograph of a cross sectionof 316L specimen pitted potentio-dynamically(Cl-:lOOppm pH 4)

Figure 27: Photomicrograph of a cross sectionof 316L specimen pitted potentio-dynamically (Cl-: 300ppm pH-4)

231167

Figure 28: Photo micrograph of a cross sectionof 316L specimen pitted potentio-dynamically (Cl-:5OOOppm pH-4)

Figure29: Photo micrograph of a cross sectionof 316L specimen pitted potentlo-dynamically (Cl-: 1000 ppm pH-7)

232168

Figure 30: Photo micrograph of a cross sectionof 316L specimen pitted potentio-dynamically (Cl-:1000 ppm,pH-7,underdeaerated condition)

Figure 31: Photo micrograph of a cross sectionof 316L specimen pitted potentio-dynamically(Artificia1 sea water,pH 9.9)

233169

170

171

172

173

174

175

176

177

178

179

180

181

182

183

184

185

186

187

188

189

190

191

192

193

194

195

196