Critical pitting temperature for Type 254 SMO stainless steel in chloride solutions

Corrosion Science 49 (2007) 263–275

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Critical pitting temperature for Type 254 SMOstainless steel in chloride solutions

E.A. Abd El Meguid *, A.A. Abd El Latif

Department of Electrochemistry and Corrosion, National Research Centre, Dokki, Cairo, Egypt

Received 12 October 2004; accepted 6 June 2006Available online 10 August 2006

Abstract

The variation with time of the open circuit potential of high molybdenum containing stainlesssteel (Type 254 SMO) was measured in 4% sodium chloride solution in the temperatures range30–100 �C. The plot of steady state potentials as function of temperature showed an inflection at50 �C, attributed to the decrease of oxygen solubility in test solution above 50 �C. Potentiodynamiccycling anodic polarization technique was used to determine the critical pitting potential (Epit) andthe critical protection potential (Eprot) of the steel in 4–30% NaCl solutions at temperatures between30 and 100 �C. By plotting the two values versus solution temperature, the corresponding critical pit-ting (CPT) and the critical protection (CPrT) temperatures were determined. Both parametersdecreased with increasing chloride content. Above the CPT, Epit and Eprot decreased linearly withlog[Cl�]. The addition of bromide ions to the solution shifted both Epit and Eprot towards positivevalues. In 4% NaCl, Epit increased linearly with pH in the range 1–10. The combined effect of chlo-ride ion concentration and pH on the morphology of the pits was examined by scanning electronmicroscopy (SEM) following potentiodynamic cycling anodic polarization.� 2006 Elsevier Ltd. All rights reserved.

Keywords: A. Stainless steel; B. Polarization; C. Pitting corrosion

1. Introduction

High strength molybdenum containing stainless steels exhibit excellent corrosion resis-tance and have been recently used in marine applications [1,2]. Type 254 SMO stainless

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doi:10.1016/j.corsci.2006.06.011

* Corresponding author. Tel.: +202 3335969; fax: +202 3370931.E-mail address: [email protected] (E.A. Abd El Meguid).

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264 E.A. Abd El Meguid, A.A. Abd El Latif / Corrosion Science 49 (2007) 263–275

steel (6% Mo) is a high strength molybdenum containing stainless steel and containshigher quantities of the alloying elements than conventional stainless steels such as 304,316 and 316L. The excellent corrosion resistance of a Type 254 SMO stainless steel in chlo-ride solution is due to the synergistic action of its alloying elements [3]. This steel is envi-ronmentally acceptable and fulfills all the required qualification for use in thermaldesalination plants. However, there have been few studies on its performance [4,5]. TheCPT as a pitting criterion was introduced by Brigham [6]. His method involved controllingpotentiostatically the sample in chloride solution and raising the temperature of the solu-tion at a rate of 0.6 �C min�1. The criterion for pitting was the temperature at which thecurrent density increased sharply to a value of 10 lA cm�2. In another method [7] to deter-mine CPT a weak galvanostatic anodic polarization was applied to the steel sample with asmall current and the electrode potential was recorded while the solution temperature wascontinuously increased by a programmed heating. The CPT was the temperature at whicha sudden potential drop was recorded due to stable pit formation. Salinas-Bravo and New-man [8] proposed a method to determine the CPT using zero resistance ammeter. Qvarfort[9] determined the CPT of various stainless steels in chloride solution by measuring poten-tiodynamic polarization curves at different temperatures and plotting the breakdownpotential versus temperature; the abrupt transition with increasing temperature fromtranspassive corrosion to pitting corrosion indicates the CPT.

The objective of the present work is to determine the critical pitting (CPT) and thecritical protection (CPrT) temperatures in 4%, 10% and 30% NaCl. The influence of scanrate, pH and addition of bromide ions to chloride ions on both Epit and Eprot wereinvestigated. The surface morphology of the pits obtained on Type 254 SMO steel afterpotentiodynamic anodic polarization in 4% NaCl was also studied.

2. Experimental details

The electrodes were supplied by Avesta AB (Sweden) in the form of sheets. They werecut from 1 mm thick sheet of a UNS S 31254 (Type 254 SMO) stainless steel, with the fol-lowing chemical composition (wt.%) 0.017 C, 17.9 Ni, 19.7 Cr, 6 Mo, 0.195 N, 0.7 Cu andbalance Fe. A 1 mm thick sheet was cut into flags of 11 mm · 11 mm for electrochemicalmeasurements. The electrodes were polished with emery paper down to 1000 grit and theiredges and backs were coated with a high strength epoxy resin (Devcon), leaving10 mm · 10 mm surface area to avoid crevice effects. The stem of the flag was embeddedin the Devcon and used for the electrical connection. The polished electrodes were rinsedwith acetone and distilled water before exposure to the test solution in a vertical orienta-tion, while electrodes to be examined metallographically were further polished with0.05 lm alumina paste, ultrasonically cleaned and finally rinsed with acetone and distilledwater before potentiostatic treatment. All solutions were prepared from analytical gradereagents and double distilled water. The experiments were conducted under thermostatedconditions at a desired temperature ranging between 30 and 100 �C. A conventional500 ml electrolytic cell was attached to a condenser to avoid evaporation of the electrolyteat higher temperatures. A saturated calomel electrode (SCE) and a graphite rod were usedas reference and counter electrodes, respectively. Open circuit potential and anodic poten-tiodynamic cycling measurements were made using a Potentiostat/Galvanostat Model263A EG&G PARC connected to a fully compatible IBM computer and controlled byEG&G model 352 Corrosion Measurement and Analysis Software. Potentiodynamic

E.A. Abd El Meguid, A.A. Abd El Latif / Corrosion Science 49 (2007) 263–275 265

cyclic anodic polarization tests were made in different media at a scanning rate of 5 mV s�1

from �100 mV to +1500 mV with respect to the free corrosion potential (Ecorr) of thesteel. Then the scan direction was reversed at 1500 mV. Both of Epit and Eprot were deter-mined from potentiodynamic polarization curves as the potentials at which the currentdensity exceeds 100 lA cm�2 in the forward scan and goes below to 100 lA cm�2 in thereverse scan, respectively. The value of 100 lA cm�2 was chosen because it correspondsto a marked deviation from the passive current density (10 lA cm�2). At the end of eachexperiment, the electrode was examined microscopically. In most cases, crevice corrosionwas successfully prevented; if evidence of crevice corrosion was observed, the sample wasnot included for determining pitting potentials. The critical pitting (CPT) and the criticalprotection temperatures were determined at the abrupt transition from transpassive corro-sion to pitting corrosion when breakdown potential and Eprot are plotted versus temper-ature, respectively. The temperature figures given refer to the temperature of the solution.

3. Results and discussion

3.1. Open circuit potential behaviour

Fig. 1a presents the open circuit potential transients for Type 254 SMO steel in 4%NaCl at different temperatures between 30 and 100 �C. At all temperatures the potentialdrifted towards less negative values immediately on immersion. This ennoblement of thepotential is attributable to healing of the pre-immersion air formed oxide film and furtherthickening of the oxide film as a result of the interaction between the electrolyte and themetal surface [10]. The growth of oxide film continues until the film acquires a thicknessthat is stable in the electrolyte. During this step the alloy oxidation is under anodic controland the reduction of oxygen is the cathodic reaction [11]. Fig. 1b shows the variation of thesteady state potential (Est) of the alloy in 4% NaCl with temperatures ranging between 30and 100 �C. Est increases with increasing temperature from 30 to 50 �C then decreases withrising temperature. Shams El Din et al. [2] demonstrated that the surface of high molyb-denum containing stainless steel immersed in seawater at high temperatures became richwith molybdenum ions due to outward diffusion of molybdenum ions through the oxidefilm as confirmed by X-ray surface analysis. Therefore, the behaviour indicated inFig. 1b could be attributed to the dissolution of the passive film on the steel in the temper-ature ranging from 30 to 50 �C was under mixed control (anodic and cathodic). Whileabove 50 �C, the oxygen solubility in the solution decreases with increasing temperature,so the passivation process becomes under cathodic control.

3.2. Critical pitting and protection potentials

The potentiodynamic cyclic anodic polarization technique was used to investigate thebehaviour of Type 254 SMO steel in 4%, 10% and 30% NaCl at different temperatures.As example, the curves of Fig. 2 show the anodic potentiodynamic cycling polarizationcurves of the alloy recorded at a scan rate of 5 mV s�1 in an aerated 4% NaCl (pH 6.5)at 32 and 90 �C. Two types of behaviour were recorded depending on the solution temper-ature. (i) At temperatures lower than 89 �C a well defined passive region was observed witha passivation current density of �10 lA cm�2 which extended up to the onset of transpas-sive corrosion at �1000 mV (SCE). Upon the reverse anodic scan, the current dropped to

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Fig. 1. (a) The variation of the open circuit potential with time for 254 SMO stainless steel in 4% NaCl at: (1)32 �C, (2) 40 �C, (3) 50 �C, (4) 60 �C, (5) 70 �C, (6) 80 �C, (7) 90 �C, (8) 100 �C. (b) Effect of temperature on Est of254 SMO stainless steel in 4% NaCl.


its passive value at �950 mV (SCE), indicating the absence of pitting corrosion. (ii) At andabove 89 �C the current density increased greatly at 550 mV (SCE) to reach more than50 mA cm�2 before potential reversal. A hysteresis loop was recorded indicating the occur-rence of pitting corrosion. Eprot was the potential below which the alloy came back into theimmune region. It has been also reported [12] that DE = Epit � Eprot is a susceptible pittingfactor for stainless steels. It was also observed that the passivation current densityincreases with increasing the solution temperature up to 15 lA cm�2. This trend is becausemost of the electrochemical and chemical reactions proceed more rapidly at highertemperatures.

3.3. The critical pitting and protection temperatures

Plots of breakdown potential and Eprot for the steel in different concentrations ofNaCl versus temperature are shown in Figs. 3–5. All curves have S-shape. Denoting thatbelow a certain temperature there occurs an abrupt decrease in both potentials. Thetransition from transpassive corrosion to pitting corrosion is defined as CPT, whereas

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Fig. 2. Potentiodynamic cyclic anodic polarization for 254 SMO stainless steel in 4% NaCl with potential scanrate 5 mV s�1 at (a) 32 �C, (b) 90 �C.


the transition from the most noble values of Eprot to the active potential value has thesame trend as CPT and is so-called the critical protection temperature. However, theCPrT occurs at a lower temperature level than CPT, indicating that stable pits formedat CPT can grow at lower temperature. The values of CPT for Type 254 SMO steel in4%, 10% and 30% NaCl solutions are 89, 67 and 57 �C, respectively. On the other hand,the corresponding CPrT are 64, 45 and 37 �C. In this respect our results differ from theseof Brigham and Tozer [13]; they found that CPT of 304L and 317 L stainless steels wereindependent of FeCl3 concentration used in immersion tests. Similarly, Qvarfort [9]claimed that CPT for Type 254 SMO steel is independent of chloride concentration inthe range of 1–5 M NaCl.

In order to verify the values of CPT for a Type 254 SMO steel in chloride solutionspotentiostatic polarization were carried out in 4%, 10% and 30% NaCl solutions at the pit-ting potentials and at 89, 67 and 57 �C, respectively. The results are represented in Fig. 6.The curves of this figure show that in 4% and 10% NaCl solutions, the pitting current den-sity of the steel increased with time to reach a maximum value of 30 and 40 mA cm�2 after6 and 10 min, respectively. The current then decreased to a steady value of 12 mA cm�2

after 100 min at 57 �C, on the other hand, the current density in 30% NaCl increased stea-dily with the time. These results indicate that stable pits were formed, in all test solutionsand confirmed in the same meantime, dependence of CPT on the chloride concentration.The temperature difference between CPT and the CPrT also depends on the chloride ioncontent of the solution and vary between 20 and 25 �C. Steinsmo et al. [1] reported a dif-ference of 10 �C between the CPT of rolled and cast stainless steel UNS S31254. Accordingto Salinas-Bravo and Newman [8] the CPT of metals and alloys could be understood interms of passivation of the alloy occurring at a finite critical current density even in themost aggressive local environment that can be generated in the pit. Laycock et al. [14]

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Fig. 3. Effect of temperature on breakdown potential (j) and Eprot (d) for 254 SMO stainless steel in 4% NaCl.


modified the Salinas model and suggested that the CPT corresponding to anodicpassivation current occurring under a freshly precipitated FeCl2 salt film. The transitionfrom transpassive corrosion to pitting corrosion upon increasing the temperature indicatesthat steel resistance to pitting corrosion depends on the solution temperature. Wang et al.[15] attributed the decreased in pitting resistance with increasing the solution temperatureto two different reasons. First, the porosity of the passive film increased with increasingtemperature; this idea was supported by the finding that chloride ions are found to beincorporated into the passive films at high temperatures. Second, the passive film under-went an intrinsic modification in chemical composition and/or physical structure resultingin a variation in the density of vacancies in the oxide film. In concordance with this con-cept, Manning and Duquette [16] found that the oxide film on stainless steel change fromp-type at room temperature to n-type at higher temperatures, there are However, noenough data on the effect of temperature on the composition and structure of the passivefilm to decide which one of these alternatives is correct. Szklarska-Smialowska [17] is ofthe opinion that at elevated temperatures, the chemisorption of the chloride ions on themetal surface becomes stronger. This idea would entail that the pitting potential movetowards less positive values as the temperature increases. This would explain the resultsof Fig. 2 in that a decrease from transpassive corrosion potential to pitting corrosionpotential occurs at or higher than CPT.

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The metastable pits that grow below the CPT can propagate at and above the CPT onone hand and never become stable below the CprT on the other hand. Ressell et al. [18]found that the CPT for a series of high nitrogen stainless steels as well as a Type 304and Type 316 SS are correlated with the pitting resistance equivalent (PRE) for PRE from20 to 45.

The relation between the difference between the pitting and protection potentials(DE = Epit � Eprot) and temperature for a Type 254 SMO steel in 4%, 10% and 30% NaClsolutions is presented by the curves of Fig. 7. The curves are characterized by a potentialpeak. The peaks should be attributed to the formation of metastable pits below the CPTthat can grow at higher anodic current density. Above the CPT, the values of DE tend tobe temperature and chloride concentration-independent. This behaviour suggests that sta-ble pits have their own pitting solution compositions which are independent of the bulkenvironment.

3.4. Effect of some factors on both Epit and Eprot

The effects of the chloride ion concentration, pH and bromide addition on the anodicpotentiodynamic cycling polarization curves of the test steel were investigated at 90 �C. Allexperiments are carried out in 4% NaCl solution. Both Epit and Eprot were found to beindependent of the polarization scan rate when varied. This behaviour suggests that above

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Fig. 6. Potentiostatic anodic polarization for 254 SMO stainless steel in (a) 4% NaCl at 89 �C and 500 mV,(b) 10% NaCl at 67 �C and 500 mV, (c) 30% NaCl at 57 �C and 250 mV.


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Fig. 7. Effect of temperature on DE values for 254 SMO stainless steel at different NaCl concentrations.


the CPT pits grow under a critical current density stabilizing the salt film [13]. Similarbehaviour was noted for Type 904L in bromide solution at low scan rates [19] and forType 304 SS in 0.1 M NaCl [20].

Fig. 8 depicts the variation of Epit and Eprot for Type 254 SMO stainless steel as func-tion of the logarithm of chloride concentration at 90 �C. The two parameters are relatedaccording to

E ¼ A� B log½Cl��where A and B are constants. It was found that B value is 126 ± 20 mV/decade for thevariation of both Epit and Eprot with logarithm chloride ion concentration. Eprot valuesare displaced from that for Epit by 485 mV in the negative direction. This suggests that pitsdeveloping on Type 254 SMO stainless steel can remain active if the potential shiftedwithin this amounts towards more negative value. It has been reported that the B values

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Fig. 8. Effect of chloride ion concentration on Epit and Eprot for 254 SMO stainless steel at 90 �C.

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Fig. 9. Effect of pH on Epit and Eprot for 254 SMO stainless steel in 4% NaCl at 90 �C.


for different steels are dependent on the steel type and the aggressiveness of the environ-ment [21]. However, Newman [22] reported that the slope B should be 60 mV for steelin chloride solution at 25 �C. He attributed the higher values of B founded for stainlesssteels to the formation of chloro-complexes of chromium.

The relation between the pitting parameters (Epit and Eprot) and the pH of 4% NaClsolution is presented in Fig. 9. The Epit decreases linearly with decreasing pH with a slopeof �30 mV/pH whereas Eprot is pH independent. The present results are in agreement withthat reported for 904L in chloride solution [12,23]. This behaviour could be ascribed to theenhancement of the local acidity inside the pit at low pH value while OH� inhibit thepitting attack to limited extent at higher pH values.

The dependence of both Epit and Eprot for 254 SMO steel on chloride ions to bro-mide ions ratio at a fixed halide content of 4% at 90 �C is shown in Fig. 10. The pitting

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Fig. 10. Effect of bromide ions to chloride ions ratio at a fixed halide ions concentration of 4% on Epit and Eprot

for 254 SMO stainless steel at 90 �C.


parameters (Epit and Eprot) increased slightly with increasing amount of bromide. Thisimprovement in the pitting resistance of the steel on the addition of bromide ions tochloride ions at 90 �C is attributable to competitive adsorption between bromide andchloride ions as well as to changes in the chemistry and electrochemistry in the pit.

3.5. Surface morphology of the pits

Pits formed on stainless steel in chloride solution are not hemispheres but grow underthe metal surface, emerging at numerous points to make a lacy pattern. The developmentof lacy metal covers over stable pits is now understood in terms of passivation andundercutting near the pit mouth [24,25]. Fig. 11 shows SEM micrographs of a Type 254SMO steel surface after anodic potentiodynamic cycling at 5 mV s�1 and at 90 �C, underthe influence of chloride concentrations and pH. A noncircular pit with collapse of thecentral part of the cover into the pit was obtained in 4% NaCl (pH 6.5), as shown inFig. 11a. As the concentration of NaCl is increased to 10% (pH 6.5) the number and sizeof the pits increased with complete removed of the pit cover (Fig. 11b). This structure canbe explained by considering the dependence of dissolution rate on pit solution chemistry,together with diffusion transport of corrosion products away from the pit [26]. Thenumber of lace-like pits increased at pH 3 whereas it decreased at pH 10 and a large areaof the pit became shallow, as shown in Fig. 11c and d, respectively. In the case of pits with

Fig. 11. SEM micrographs of 254 SMO steel surface after Potentiodynamic cyclic anodic polarization treatmentat 90 �C in (a) 4% NaCl at pH 6.5, (b) 10% NaCl at pH 6.5, (c) 4% NaCl at pH 3 and (d) 4% NaCl at pH 10.


collapsed covers, elongated pits ranging between 200 and 350 lm were observed. The restof the surface exhibited lace-like pits where the alloy cannot achieve the very high anodiccurrent density required to propagate an undercutting pit. Thus lace-like pits keep theconcentration of their own solution which sustains the pit growth under the surface.

4. Conclusions

The results allow the following conclusions to be drawn:

1. The OCP of a Type 254 SMO in 4% NaCl shifts towards less negative values withincreasing immersion time, indicative of repair of the passive film. However, the steadystate potential value increases with temperature up to 50 �C. This behaviour could beattributed to the change in the oxygen reduction reaction kinetics as a function of tem-perature where at temperature less than 50 �C, the passive film formation was undermixed potential but above 50 �C it become under cathodic control.

2. The critical pitting temperature (CPT) and the critical protection temperature (CPrT) ofType 254 SMO steel in 4% NaCl are 89 and 64 �C, respectively, these values decreasewith increasing the chloride concentration.

3. Above the CPT, the value of DE (Epit � Eprot) of a Type 254 SMO stainless steel is tem-perature and chloride concentration-independent.

4. Above the CPT, both Epit and Eprot of a Type 254 SMO are independent of scan rate,but decrease linearly with the logarithm chloride ions. Only Epit is pH dependent. Thevalues of Epit and Eprot slightly increase with increasing the bromide additions.

Acknowledgement

E.A. wishes to thank the Alexander von Humboldt Foundation for providing theequipment.

References

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[10] U.R. Evans, The Corrosion and Oxidation of Metals, Edward Arnold, London, 1960, p. 898.[11] J.M. Abd El Kader, A.M. Shams El Din, Br. Corros. J. 14 (1979) 40.[12] E.A. Abd El Meguid, N.A. Mahmoud, V.K. Gouda, Br. Corros. J. 32 (1997) 68.[13] R.J. Brigham, E.W. Tozer, Corrosion 28 (1972) 177.[14] N.J. Laycock, M.H. Moayed, R.C. Newman, J. Electrochem. Soc. 145 (1998) 2622.[15] J.H. Wang, C.C. Su, Szklarska-Smialowska, Corros. Sci. 44 (1988) 732.[16] P.E. Manning, D.J. Duquette, Corros. Sci. 20 (1980) 597.[17] Szklarska-Smialowska, Corrosion 30 (1974) 161.[18] J.H. Russell, B.S. Covino Jr., S.J. Bullard, Corrosion 57 (2001) 360.[19] E.A. Abd El Meguid, Corrosion 53 (1997) 623.


[20] H.P. Leckie, H.H. Uhlig, J. Electrochem. Soc. 113 (1966) 1262.[21] E.A. Abd El Meguid, N.A. Mahmoud, V.K. Gouda, Br. Corros. J. 33 (1998) 42.[22] R.C. Newman, Corrosion 57 (2001) 1030.[23] G. Ruijin, S.C. Srivastava, M.B. Ives, Corrosion 45 (1989) 874.[24] N.J. Laycock, S.P. White, J.S. Noh, P.T. Wilson, R.C. Newman, J. Electrochem. Soc. 145 (1998) 1101.[25] N.J. Laycock, S.P. White, J. Electrochem. Soc. 148 (2001) B264.[26] P. Ernst, N.J. Laycock, M.H. Moayed, R.C. Newman, Corros. Sci. 39 (1997) 1133.

Critical pitting temperature for Type 254 SMO stainless steel in chloride solutions

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