CorrosionScience, Vol. 33,No. 2, pp. 295-306,1992 Printedin GreatBritain.
OOlO-938X/92 $5.00+ 0.00 0 1991PergamonPressplc
THE EFFECT OF H2S ON THE CREVICE CORROSION OF AISI 410 AND CA6NM STAINLESS STEELS IN 3.5% NaCl SOLUTIONS U. M.
The Ohio State University,
of Materials Science and Engineering, U.S.A.
Abstract-Crevice corrosion of two martensitic stainless steels of different composition were studied in sodium chloride solution with and without H$ at pH 7.0 and 4.2. In the presence of I$ S, crevice corrosion of both studied steels has been found to occur in the active region of potentials. In the nitrogen de-aerated solutions the corrosive attack occurred inside the crevice while in &S-containing solutions the corrosion occurred outside crevice mouth.
sulfide, which often occurs in natural and industrial environments, has a detrimental effect on the performance of metallic materials because it increases the rate of both general and localized corrosion and in many cases, enhances stress corrosion cracking and hydrogen embrittlement. The influence of H2S on different types of corrosion is a matter of considerable interest. In this respect, studies of the corrosion behavior of different metals and alloys in aqueous electrolytes containing H2S are of particular importance. According to many reports,le5 H2S increases the general corrosion rate of iron and nickel in electrolytic solutions. In acid solutions, H2S is adsorbed on the metal surface as either H2S or HS- and causes the depolarization of both the cathodic and anodic reactions to intensify and the exchange current density for the hydrogen evolution reaction to increase.6 The accelerating effect of sulfides on the anodic dissolution of iron is explained by the formation of a surface catalyst, namely either FeSH- 2 or Fe(H2S)ads.7 Extensive studies on the general corrosion of carbon steel in H2S containing solutions were performed by Morris et ~1.~Their experimental work included long term potentiostatic polarization measurements made on a rotating electrode of 1020 steel in de-aerated aqueous solutions containing 0.2 M Cl- and H2S at 2530°C. Many other researchers have found that the corrosion potential was more negative in the presence of H2S in the system. This shift of the corrosion potential was attributed to a negative shift of the reversible electrode potential of iron in the presence of H2S. In pure water solutions, the potential of iron is a function of the concentration of Fe2+ .8
*Present address: King Abdulaziz Manuscript received 4 June 1990.
-0.44 + 0.030 log [Fe2’]. University,
of Saudi Arabia.
U. M. DAWOUD. S. F. VANWEELEand Z. SZKLARSKA-SMIALOWSKA
However, in solutions containing sulfide ions, the activity of Fe*+ ions is affected by the ionization constant of H2S, the solubility product of FeS, and the Henry law constant for the H$ solution. At WC, the reversible potential of iron in H2S-containing solution is given by the equation:3 E” = -0.39 - 0.06 pH - 0.031 (PHZS)
where P&s is the partial pressure of hydrogen sulfide in contact with the given electrolyte. Morris et ~1.~ did not find the anodic and cathodic Tafel slopes to change in the presence of H2S. Their X-ray diffraction analyses of the corrosion products indicated that the only iron sulfide present was mackinawite (Fe,_,S). This compound does not form protective films on an iron surface. According to the authors point of view,3 the presence of a scale of mackinawite might, in certain conditions, enhance the rate of general corrosion of iron. Several research works were devoted to the effect of H2S on pitting. Even small concentrations of H2S in aqueous solutions were shown to decrease substantially the critical pitting potential of iron.9~“’ Newman et ~1.~found a black sulfide deposit in pits and assumed that the formation of such a porous sulfide deposit could retain the pit electrolyte within the pits and propagate pit growth. However, it appears that because of active metal dissolution in pits, the presence of H2S within pits might enhance localized dissolution in the same way as it enhances general corrosion. Yoshimo and Ikegaya” studied pitting of a 12 Cr-Ni-Mo martensitic stainless steel in chloride and sulfide environments. They found that even small amounts of H2S have a significant detrimental effect on the resistance of the steel to pitting. Saturation of a 1% NaCl solution with H$ at a partial pressure of 0.005 atm caused the pitting potential to decrease by 335 mV. At concentrations corresponding to H2S partial pressures situated between 0.005 and 0.5 atm, the pitting potential was proportional to log (P&. Pitting occurred only in solutions containing NaCl. Because the mechanisms of pitting and crevice corrosion of passive metals in NaCl solutions are alike, the effects of H2S are expected to be similar, and the research reported by Azuma et al. I2 bears this out. In their work, the crevice corrosion behavior of a 22% Cr duplex stainless steel and type 316 stainless steel in 2.5% NaCl solution was studied. Crevice corrosion was accelerated by increasing the partial pressure of H2S, increasing temperature, decreasing pH, and by the addition of elemental sulfur to the solution. The 22 Cr duplex stainless steel had higher resistance to crevice corrosion than 316 SS had. The mode of crevice corrosion was found to differ depending upon the partial pressure of H$. At low H2S pressures, corrosion took place inside the crevice, while at higher H2S pressures corrosion occurred at the mouth of the crevice. The following equation was derived to estimate whether H2S was stable in the crevice solution at a given pH and p&s: log&s
= -0.9 pH + 2.3 - 2.4.
At 1 atm of H2S, NiS is stable both inside and outside of the crevice and activity of H2S within the crevice decreases. According to these authors, the inside of the crevice is less influenced by H2S and has a lower depassivation pH than the outside surface. At the crevice edge, both the H2S activity and depassivation pH are higher than within the crevice. As a result, corrosion occurs at the crevice mouth.
of AISI 410 and CA6NM
Crevice corrosion has been observed to occur in the oil industry on certain steels which were not in the passive state in air-free brine containing H2S, hence on those steels with assumed electrode potentials situated within the active range. In such a case, no breakdown of passivity is needed to initiate crevice corrosion or pitting. The purpose of the present work is to compare the susceptibility to crevice corrosion of two stainless steels used in the oil industry, one with only Cr and another with Cr + Ni + MO, on exposure to H$free and H-$-containing 3.5% NaCl solutions.
Two martensitic stainless steels (SS) of the following nominal compositions were studied: AISI 410 SS with C 0.14, Mn 0.40, Si 0.49, Cr 12.1, Ni 0.28, P 0.020, S 0.011, and AISI CA6NM cast SS with C 0.06, Mn1.0,Si1.5,Cr11.5-14.0,Ni4.0,Mo0.7,allinwt%.Thesesteelsweredeliveredintheformofathick plate and a casting, respectively. Two types of samples were prepared by machining: one in the form of coupons with a flat exposed area of 1 cm2, and another in the form of cylindrical rods, 1.2 cm in diameter. The coupons were used for polarization and galvanic current measurements, and the rods were used for crevice corrosion experiments. The unexposed surfaces of the coupons were protected by a layer of varnish, and their exposed surfaces were polished to 600 grit and degreased. The rod samples were soldered to steel holders which served to maintain the rod axis in vertical position. The assembly was mounted into epoxy resin and the flat surface of the rod sample, 1.13 cm2 in area, was polished to 600 grit and degreased. The experiments were performed at 25°C in 3.5% NaCl solutions which differed in pH value and content of dissolved gas. Solution A was de-aerated by bubbling pure nitrogen and had a pH of 7.0. Solution B was de-aerated in the same way as A but was acidified with HCl to pH 4.2. Solution C was saturated with H2S by continuously sparging hydrogen sulfide into it, and it also showed a pH of 4.2. The polarization curve measurements were made using a potential scan rate of 360 mV h-‘. Susceptibility to crevice corrosion was determined at different applied anodic potentials by measuring the lapse of time (induction time) after which the current density began to increase rapidly. The anodic potential was applied after steady state corrosion was reached. All potential values given below are expressed in the SCE (saturated calomel electrode) scale. The galvanic corrosion experiments were made by immersing two identical coupon samples into the two compartments of an electrolytic cell, separated by a glass filter; one compartment contained solution B, another contained solution C. The two electrodes were connected with a zero resistance ammeter. The resulting galvanic current was monitored constantly, and from time to time, potentials of both electrodes were measured against a reference electrode. For the crevice corrosion measurements, a technique described in detail elsewhereI was used. This technique consists of pressing a Plexiglas rod, 0.7 cm in diameter, with its flat bottom against the central part of the exposed rod specimen surface.
Polarization curves recorded for 410 and CA6NM steels are illustrated in Figs 1 and 2 respectively. The polarization curve for 410 in H$-saturated 3.5% NaCl solution C shows a monotonic increase in current with increasing applied potential, hence no passivation. In contrast, on exposure to solutions A and B (pHs 7 and 4.2 respectively) the passivation curves for steel 410 show an active-passive behavior. This behavior was also reported by Martin, 6 who found marginal passivation of 410 stainless steel in sweet brines, and no passivation in brines containing 50 ppm sulfide. The results obtained in the present work for CA6NM steel showed a greater ability to passivate in H,?S-free solution than did the 410 steel, but showed no passivation in H2S saturated solution. A similar behavior was reported by Yoshimo and Ikegaya’ for F6NM stainless steel in 0.5% acetic acid solution.
U. M. DAWOUD,S. F. VANWEELEand Z. SZKLARSKA-SMIALOWSKA
Current Density, pA/cm2 FIG. 1. Polarization curves for 410 type stainless steel in solutions A (3.5% NaCl bubbling with N2, pH 7); B (3.5% NaCl bubbling with N2, acidified to pH 4.2); and C (3.5% NaCl bubbling with H2S, pH 4.2).
Crevice corrosion measurements
Some examples of current vs time curves obtained at different constant potentials for 410 SS in solutions A and C are given in Figs 3 and 4 respectively. At high applied anodic potentials, the increase in the current occurs in a short time; for example, in approximately 1 min in solution A at a potential of -300 mV(SCE). At lower potentials, longer induction times are observed, for example, over 500 min at -575 mV(SCE). The same trend is observed for solution B. In solution C (saturated with H2S) the behavior of 410 SS is slightly different. The induction time increases as the potential is lowered from -550 to -625 mV(SCE). Between -625 and -650mV(SCE) the induction time drops back to a low value. Below -650 mV(SCE), the induction time again rises as the potential is lowered. In the case of CA6NM steel, the induction time decreases in all the studied solutions when the applied anodic potential is decreased.
g 0 z i -200 E d 0 -400 a -600 -800 100
Current Density, FIG. 2.
Polarization curves for CA6NM SS in solutions A, B and C.
Crevice corrosion of AISI 410 and CA6NM steels
Time, min. FIG. 3.
Current density vs time for 410 SS in solution C at constant potentials of -650 and -625 mV(SCE)
Figures 5 and 6 show curves representing the dependence of initiation time on the applied potential for 410 and CA6NM steels in the three studied environments. Potentiostatic determination of the characteristic crevice corrosion potential showed that for both the 410 and CA6NM steels the critical crevice potential was more negative in the H2S containing solution, followed by that in de-aerated solution of pH 7, and was the least negative in de-aerated solution of pH 4.2. Compared with 410 steel, CA6NM steel showed in all solutions higher critical crevice corrosion potentials, indicating a higher resistance to crevice corrosion. Table 1 lists the critical crevice corrosion potentials (E,), corrosion potentials (I&,,,) and crevice overpotential (E,,,) for both steels. The critical crevice corrosion potentials were estimated from the potential vs crevice initiation time curves (Figs 5 and 6); corrosion potentials were taken from polarization plots (Figs 1 and 2). The crevice corrosion overpotential which was defined by the difference between the critical crevice
Time, min. Current density vs time for 410 SS in solution A at constant potentials of -560 and -575 mV(SCE).
U. M. DAWOUD, S. F. VANWEELE and Z. SZKLARSKA-SMIALOWSKA
700 500 300 .:
:; E i= c
.o ‘ii s 40+
30 20 -
: I :
10 cl I
-300 -400 -500
-600 -700 -800
Potential, mV, FIG. 5.
time for crevice corrosion potentials.
of 410 SS on the applied
Potential, mVscE FIG. 6.
of initiation time for crevice applied potentials.
SS on the
Crevice corrosion of AISI 410 and CA6NM steels TABLE 1.
CRITICALCREVICE CORROSION, CORROSION POTENTIALS
AND CREVICE OVERPOTENTIALS FOR AISI 410 AND CA6NM
mV(SCE) AISI 410
E,, = -580 mV E cnrr = -675mV
E con = -7OOmV
EC, = -540 mV
E,, = -400 mV
E cnrr = -625mV
&or,= -6OOmV E,,, = 200mV
EC,= -675mV E corr = -720 mV E,,, = 45 mV
E,, = -580mV E corr = -700 mV E “CC= 120 mV
corrosion potential and the corrosion potential was assumed driving force needed to produce crevice corrosion.
to be a measure
EDAX Analysis of corrosion products The results of EDAX (Energy Dispersive X-ray Analysis) of corrosion products formed on both studied steels in hydrogen sulfide saturated NaCl are listed in Table 2. The corrosion products formed on 410 steel in H$ saturated NaCl solution polarized for 3600 min at -525 mV(SCE) were analysed. The major elements present were sulfur and iron with an atomic iron to sulfur ratio of approximately 2 : 1. The corrosion products formed on CA6NM steel in H$ saturated solution after 3600 min of exposure at -560 mV(SCE) were enriched in nickel and iron. Corrosion morphology In experiments performed in solution A of pH 7 at the studied potentials, corrosion occurred inside the crevice and had the form of a corroded ring of a diameter slightly less than that of the crevice. In addition, pits situated on the remaining metal surface surrounded by the corroded ring were present. Experiments performed in solution B of pH 4.2 showed a slightly different corrosion mode from
STEELS FORMED IN H&SATURATEDNaCl
Si S P Cl Cr Fe Ni
1.38 28.67 1.12 2.84 2.82 63.17
11.22 35.37 22.55
those in solution A of pH 7, because at applied potentials less negative than -425 mV(SCE) both the corroded ring and pits were observed, but at lower anodic potentials only crevice corrosion occurred. The corrosive attack in the H2S-containing solution C was quite different from that in the nitrogen de-aerated solutions in which corrosion did not occur inside the crevice but only outside its mouth. Galvanic couple measurements
Figure 7 shows current density vs time curves for galvanic couples of 410 SS, 3.5% NaCl, pH 4.2, N,113.5% NaCl, pH 4.2, H$410 SS. The steel in hydrogen sulfide was anodic to the steel in the solution de-aerated with nitrogen during the entire experiment. Potentials of the electrodes in both compartments of the galvanic cell are shown in Fig. 8. At the end of the experiment, the steel in the HzS-saturated 100
Time, min. FIG. 7.
in current with time for the galvanic cell: 410 SS13.5% NaCl, N,113.5% NaCl, pH 4.2, H,S1410 SS.
Time, min. FIG. 8.
with time for the anode (A) and cathode cell as in Fig. 7.
(C) in the galvanic
of AISI 410 and CA6NM
in current with time for the galvanic cell: CA6NM SS13.5% NaCI, pH 4.2, N2113.5% NaCl, pH 4.2, H,SICA6NM SS.
_,,L. FIG. 10.
with time for the anode (A) and cathode cell as in Fig. 9.
(C) in the galvanic
solution was coated with a black precipitate, the sample in the de-aerated solution showed faint smudges of corrosion products. CA6NM steel showed the same behavior as 410 steel. Its potential in the presence of H2S in the solution was always anodic to that in solutions de-aerated with nitrogen. The current density vs time curve for a galvanic couple of CA6NM steel in H,S-saturated solution and CA6NM steel in de-aerated solution of pH 4.2 is shown in Fig. 9. The sample exposed to H,S solution was lightly coated with a black film at the end of the experiment; the sample in the de-aerated solution showed no signs of corrosion. Figure 10 shows changes in potential of both electrodes in the galvanic couple with time. During the experiment shown in Fig. 10, the potentials came closer together as time increased, but did not touch or cross.
U. M. DAWOUD, S. F. VANWEELEand Z. SZKLARSKA-SMIALOWSKA DISCUSSION
As seen in Table 1, the critical crevice corrosion potential is higher in acidified nitrogen de-aerated solution (B) than in neutral nitrogen de-aerated solution (A), but the crevice corrosion overpotential is lower in acidified solution which means that crevice corrosion occurs more easily in acidified than in neutral solution. The critical crevice corrosion potentials for CA6NM steel are several hundred milivolts more noble than those for AISI 410 steel. This is caused by the greater contents of Cr, MO and Ni in CABNM steel resulting in better passivation properties of the former than the latter material. The critical crevice corrosion potentials of 410 stainless steel in hydrogen sulfidesaturated solution C and de-aerated solution B of pH 4.2 are in the active region where a passive film does not exist on the metal surface. In de-aerated neutral solution A, the 410 steel is in an active/passive transition state where a passive film is not fully developed. CA6NM steel in de-aerated solution assumes a critical crevice corrosion potential in the passive region. However, in NaCl solution bubbled with H2S, crevice corrosion occurs in the region of active potentials and passivation does not occur. The EDAX analyses of corrosion products on AISI 410 and CA6NM stainless steels indicate that these products are mostly composed of iron sulfide and nickel sulfide respectively. As seen in Fig. 6, 410 SS exhibits a discontinuity of the time to initiate crevice corrosion between -625 and -650mV(SCE). At an applied potential of -625 mV(SCE), the time necessary to initiate crevice corrosion is 20 min; lowering the potential to -650 mV(SCE) reduces the time to initiate crevice corrosion to 2 min. This behavior is possibly connected with different corrosion products forming in the two different potential regions. Above approximately -625 mV(SCE) the stable phase is Fe& (pyrite), and below -625 mV(SCE) the stable phase is FeS (mackinawite). Under open circuit potential conditions, mackinawite formation occurs by the following processes: Anodic Fe + Fe’+ + 2e-
H2S + Hz0 + H+ + HS- + Hz0
HS- + HzO_, H+ + S*- + H20.
The net reaction being u>o Fe + H2Sw
FeS + 2H”.
Pyrite has been postulated to form from mackinawite by the following reaction: FeS + So+ FeS2.
The formation of FeS2 has been reported to be a slow process in comparison to FeS formation.5 The shorter crevice corrosion initiation time at -650 mV(SCE) than at -625 mV(SCE) may be caused by the faster kinetics of FeS formation compared to those of FeS2.
of AISI 410 and CA6NM
Galvanic couples between steel exposed to de-aerated solution B of pH 4.2 and steel exposed to H2S saturated solution C were used to simulate conditions during crevice corrosion in natural, H2S saturated brines. It was found for both 410 and CA6NM that the steel exposed in the H2S saturated solution was always anodic to the same steel in the de-aerated solution. THE
NaCl SOLUTIONS OF H2S
In de-aerated sodium chloride solutions at applied anodic potentials, crevice corrosion of CA6NM stainless steel occurs in the passive potential region, while that of 410 steel occurs both in the passive region and at potentials where a passive film starts to form but does not fully develop. The difference in the crevice corrosion morphology observed in de-aerated sodium chloride solution and in hydrogen sulfide-saturated solution suggests two different mechanisms of crevice corrosion. The occurrence of crevice corrosion in pure NaCl solutions can be explained by the following generally accepted mechanism. Initially, uniform dissolution of the metal surface takes place inside and outside the crevice. As corrosion progresses in the crevice, the limited volume of solution becomes saturated with metal ions. Precipitation of hydroxides takes place when the solution in the crevice is saturated. A product of hydrolysis, H+ , builds up in the crevice. To neutralize the excess of positive charge in the crevice, chloride ions migrate into the crevice, increasing the acidity within the crevice. In the presence of H2S in NaCl solution, crevice corrosion of both the studied steels has been found to occur in the active region of potentials, hence under conditions where the steel surface was not covered with an oxide film. In this case crevice corrosion occurs because of galvanic couple formation between the metal being in contact with the solution situated inside the crevice where a low concentration of H2S occurs, and the outside metal surface where a high concentration of H2S is present. Because the steel exposed to a high concentration of hydrogen sulfide is anodic to the steel in the crevice, the metal is attacked outside and not inside the crevice. SUMMARY
(1) Both 410 and CA6NM steels exhibit an active-passive behavior in de-aerated 3.5% NaCl solutions of pH 7 and 4.2 but the critical current density for passivation of 410 SS is about one order of magnitude higher than that of CA6NM SS. (2) Crevice corrosion of both steels occurs in all three solutions but the critical potential for crevice corrosion is more negative in the H,S-containing solution. (3) CA6NM SS is more resistant to crevice corrosion in all three environments studied than is 410 SS. (4) The mechanism of crevice corrosion in H,S-saturated NaCl solution differs from the mechanism of crevice corrosion in H*S-free solution. In the former case, a galvanic couple operates between the metal surface being in contact with crevice solution which is depleted of the H,S (cathode) and the outside metal surface being in contact with the H2S saturated solution (anode). As a result, the mouth of the crevice suffers corrosion. Acknowledgement-This for helpful discussions.
work was supported
by a Shell Grant.
wish to thank Dr P. Rhodes
U. M. DAWOUD,S. F. VANWEELEand Z. SZKLARSKA-SMIALOWSKA
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