- Email: [email protected]
Microscopic behavior of single corrosion pits: the effect of thiosulfate on corrosion of stainless steel in NaCl
PII:
Electrochimica Acta. Vol. 42, Nos 20-22, pp. 3281-3291, 1991 0 1997 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain s001348q97)00179-5 0013486/97 317.00 + 0.00
Microscopic behavior of single corrosion pits: the effect of thiosulfate on corrosion of stainless steel in NaCl J. 0. Park,* M. Verhoff and R. Alkire Department
of Chemical Engineering and Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
(Received 13 November 1996; in revised form 9 January 1997) Abstract-Pitting corrosion of Type 304 and 316 stainless steels was investigated in neutral chloride solutions containing various concentrations of thiosulfate (0.01 to 500 mM) and chloride (1 to 1000 mM). The effect of thiosulfate on pitting corrosion of stainless steel 304 was quantitatively studied at the microscopic level by initiating single pits on small wires in neutral chloride solutions containing thiosulfate. Analysis of single pit data indicated that thiosulfate may promote stability through either a cathodic side-reaction or through the existence of a salt film. In galvanostatic experiments, the potential response exhibited six types of behavior that depended on the concentrations of chloride and thiosulfate. Thiosulfate was found to promote the sustained growth, or stability, of pits within a concentration range that depended upon the concentration of chloride ion. An excess of thiosulfate prevented pit formation. Type 316 stainless steel required higher concentrations of thiosulfate to produce stable pits, as compared to Type 304. The effect of surface adsorption of thiosulfate on pitting of 316 stainless steel was investigated by an in situ radiotracer method. It was found that the extent of thiosulfate surface adsorption differed for the three levels of chloride in 0.1 mM NazSzOs, and that an increase in the surface concentration of thiosulfate from the small pit region caused formation of large pits. 0 1997 Published by Elsevier Science Ltd
INTRODUCTION Initiation of pitting corrosion often occurs owing to the presence of microscopic inclusions on the metal surface [14]. The local chemical environment which arises in the vicinity of such inclusions is different from that of the external bulk solution and is critical for determining whether or not a corrosion pit propagates or repassivates [5]. In the present work, the microscopic behavior of single pits was investigated to establish the effect of thiosulfate and chloride ions on the pitting corrosion of 304 and 316 stainless steel. First recognized in the paper industry [6, 71, thiosulfate has since been identified as a corrosion agent in the nuclear and mining industries. The *Author to whom correspondence should be addressed J.O. Park.
mechanism by which thiosulfate influences the corrosion of stainless steel is therefore of interest. MnS particles in stainless steel are known to be sites for the initiation of localized corrosion [8, 91 Lott demonstrated that thiosulfate, produced upon dissolution of manganese sulfide (MnS) inclusions [lo, 111, caused the breakdown of the passive film adjacent to the pit when present above a critical concentration in the presence of chloride ions. Horowitz also note that thiosulfate was an aggressive species which chemisorbs on the stainless steel passive film [ 121. Ke and Alkire demonstrated that significant dissolution of the steel matrix occurred adjacent to MnS inclusions in 304 stainless steel [13]. Oxide inclusions containing no sulfur showed either the formation of a microcrevice near the periphery of the inclusion or no evidence of corrosion whatsoever. In addition, they found that pits did not initiate adjacent to MnS inclusions that were below a cer-
3281
3282
J. 0. Park et al.
tain critical size, which they found to be approximately 0.7 pm. The authors proposed that only at MnS inclusions of sufficient size does the concentration of the sulfur species that is produced by the MnS dissolution increase above the critical concentration. Their findings supported the conclusion of Williams et al. [14] who proposed that the thiosulfate produced by inclusions influences the propagation of pits in stainless steel. Newman et al. [15] and Sury [16] proposed that thiosulfate was reduced to sulfur on the surface of the dissolving metal. The layer of sulfur was proposed to increase the dissolution rate and prevent repassivation of the metal surface. Newman and Franz [17] noted that the addition of thiosulfate, corrosion pits grown on 304 stainless steel in chloride solutions showed no tendency toward sponrepassivation. potentiostatic taneous From experiments on type 1018 carbon steel, Horowitz [12] determined that the ratio of metal atoms dissolved to thiosulfate ions consumed was between 100-400 to 1, and suggested that thiosulfate ions catalyzed the anodic dissolution of the metal. Well et al. [18] tested intergranular stress corrosion cracking in dilute thiosulfate solutions and found that the microcrack nucleation frequency dropped by a hundred-fold when the thiosulfate concentration was decreased from 10 to 1 ppm. The authors suggested that there existed a critical thiosulfate concentration that enhanced the corrosion of the steel. Most studies of the influence of thiosulfate on pitting corrosion of stainless steel have used surfaces which either had many pits of different ages, or which had very large dissolution sites produced by mechanical abrasion of the metal surface. In contrast, techniques for providing single corrosion sites have been found to be advantageous for quantitative micrometer scale studies on the effect of thiosulfate on the size, shape evolution, and growth rate of single corrosion pits on stainless steel. Wong [19] created single pits on 304 stainless steel using masked specimens which exposed an area of diameter 100 pm to a corrosion solution. Harb [20] created single pits on stainless steel by using small wires. Attempts to initiate single pits on stainless steel by using a laser initiation technique had, prior to the present work, been unsuccessful although the method has been found to work well on aluminium and iron [21, 221. The in situ radiotracer technique combines electrochemical and surface analytic techniques and provides information on kinetics of adsorption/desorption, surface concentration, and surface diffusion [23, 241. Thomas et al. used the radiotracer method to investigate adsorption of thiosulfate on stainless steel 316 in neutral perchlorate and sulfate electrolyte solution [25]. The purpose of the present study [26, 271 was to characterize the effect of thiosulfate on pitting cor-
rosion of 304 and 316 stainless steel. Galvanostic experiments were conducted to investigate the pitting behavior of stainless steel in solutions containing a wide range of concentrations of thiosulfate and chloride. The study of single pits was pursued to characterize quantitatively the effect of thiosulfate on pit growth at the microscopic scale. The radiotracer technique was used to establish the relationship of surface adsorption of thiosulfate and pitting corrosion. EXPERIMENTAL A series of galvanostatic, potentiostatic and radiotracer experiments was conducted. All solutions were prepared with 18 MS2 water and reagent grade NaCl and Na&03. For galvanostatic and radiotracer experiments, solutions were deaerated with N2 before the start of the experiments. Galvanostatic experiments
Cylindrical working electrodes (304 or 316 SS) were machined from bar stock (0.94 cm dia. by 0.96 cm height). A saturated calomel electrode (see) was used as reference electrode, and platinum gauze was used as counter electrode. Working electrodes were polished (Buehler, Ecomet 4) to a 5 pm finish (Buehler, Alpha Micropolish @ 1). The two compartment cell designed by Lott [lo] was used. One compartment was open and contained a Pt counter electrode, and the other was closed and contained working and reference electrodes. A potentiostatic power supply (PAR 273A) was used to apply a constant current of 25 pA/cm2, a value which corresponded to the passive current density of the polarization curve obtained at 0.1 V/s. The passive current value varied between 1 PA/cm2 at a scan rate of 0.0002 V/set and 60 PA/cm2 at 0.2 V/set. Potentiostatic experiments
The working electrode consisted of the exposed end of a 304 stainless steel wire (0.010 cm dia., A.D. McKay Inc., Darien, CT) which had been annealed, cast into epoxy (Resin, 20-8130-032, Hardener 20-8132-008, Buehler, Lake Bluff, IL), and polished to a 5 pm finish (Buehler, Alpha Micropolish @ 1). Fabricat io n of the electrodes employed procedures previously described by Harb [20]. The surface of the working electrode was mounted facing upward in an electrochemical cell, details of which are available elsewhere [27]. The reference electrode was a standard calomel electrode (see), and the counter electrode was Pt. Specimen potential was controlled with a potentiostat/galvanosat (Model 173/176, PAR). Current vs time data were recorded on an X-Y recorder (Houston 2000). After each experiment the electrode was examined under an optical microscope (EPIPHOT-TME, Nikon) to measure pit depth (+l pm); the diameter of the pit mouth opening was measured by using an
Effect of thiosulfate on pitting corrosion optical microscope (LABOPHOT, brated eye piece.
Nikon) with cali-
Radiotracer experiments The radiotracer cell designed by Wieckowski and co-workers [25] was used. Working electrodes were
machined from 316 SS bar stock (1.15 cm dia. and 0.5 cm thickness) and polished with wet emery paper (##4000) to a 6 pm finish. The 35S isotope was used to label $0: - ions (prepared at Amersham Co. as sodium (outer-35S) thiosulfate with a specific activity of 20 mCi/mmol and radiochemical purity of greater than 95%). Activities in solution and on the surface of the working electrode were detected with a scintillator. Because of unsatisfactory background to the signal ratio, concentrations of S20: higher than 0.1 mM was not used. Current was applied by a potentiostat (PAR, 362); the reference electrode (Ag/AgCl) was separated from the solution by a bridge system.
RESULTS
AND DISCUSSION
Galvanostatic experiments
Lott and Alkire [1 1] had previously characterized “active” and “passive” surfaces by observing the potential response. In the present investigation, it was found that, by using the same basis of judgment, “active” surfaces produced pits that grew until termination of the experiments while “passive” surfaces had many small pits which repassivated during the course of the experiments. That is, the “passive” region defined by Ke and Alkire [9] actually exhibited early stages of pit growth which, however, was not stable and which evidently termi-
z
u
nated spontaneously by repassivation. A series of galvanostatic experiments at various combinations of [SzO: -1 and (Cl-] was therefore conducted to obtain a more precise compilation of the effect of these ions on stainless steel pitting. 304 vs 316. From a series of galvanostatic experiments on 304 SS, Lott and Alkire [1 1] determined the concentration range of Cl- and $0: - which separated the “active” and “passive” regions; this separation is shown by the solid line in Fig. 1. 11 also shows results from a similar series of experiments on 316 SS. Concentrations which exhibited “active” behavior are indicated by solid circles, while conditions that gave “passive” behavior are indicated by hollow circles. The dashed line separates the “active” and “passive” regions for 3 16 SS. 11 indicates that higher [S20: -1 was required to produce an active surface on 316 SS. It was observed that the 304 SS surfaces produced fewer pits, but that they were often adjacent to each other in clusters, while 316 SS resulted in more pits, but that they were more uniformly distributed over the surface. Categories of pitting behavior. For 316 SS, it was found upon microscopic observation of pit morphology that the general categories of “active” and “passive” could be further sub-divided into six categories of behavior. These categories are indicated by the symbols in Fig. 2. The six categories were (a) small pits, (b) large pits, (c) active pits, (d) few pits, (e) small pits with rings and (f) no pits. Each of these categories were designated based on pit morphology, and will be discussed in the paragraphs below. Potential responses, which were found to be unique to each category, will also be discussed. It
4:
%
3 i: 2 E G
3283
‘y
0.1
I
so CONCENTRATION
100
OF CHLORIDE,
200 mM
Fig. 1. Concentration of thiosulfate and chloride that correspond to active (0) and passive (0) regions of 316 SS corrosion. The dashed line indicates the boundary between these regions. The solid line indicates the boundary reported perviously for 304 SS.
3284
J. 0. Park et al.
CONCRNTRATION
OF CHLORIDE,
around 400 mV and oscillated. At high concentration ratios of chloride to thiosulfate, such as 1OOO:lor SOO:l, the potential response was difficult to differentiate from those of pure chloride (ie in the absence of thiosulfate) [26] where the oscillations were very frequent, and the amplitude was small. Observation of specimens after such experiments showed that the surfaces were covered with small pits with diameters of several microns as shown in Fig. 4(a). As [Cl-J increased, the potential at which the pit initiation occurred was found to decrease. At [Cl-] of 10 mM, the oscillations occurred between 0.2 V to 0.25 V vs see; when [Cl-] was further increased to 500 mM, oscillations took place between 0.05 V to 0.1 V vs see. As [S20:-] increased, the micropits grew bigger before their death; such conditions also showed larger and less frequent oscillations. As can be seen from Fig. 3(a), although the oscillations were all relatively small and frequent, they were not uniform. It is suggested that the oscillations result from initiation and death of individual pits.
mM
Fig. 2. Concentration of thiosulfate and chloride for six categories of potential responses resulting from galvanostatic experiments using 316 SS, 25 pA/cm2. 0, small pits; A, large pits; 0, active pits; W, few pits; 0, small pits with ring; 0, no pits.
can be Seen in Fig. 2 that the combinations with the same ratio of [Cl-]/&O:-] (such as 10/l, lOO/lO and lOOO/lOOmM/mM), exhibited different pitting behavior. Thus it was concluded that not only the ratio of the ions but also the concentrations of the ions influenced the pitting mechanism. (a) Small Pits Figure 3(a) shows a typical potential response in the small pit formation region, indicated by open circles in Fig. 2. The potential response was characterized by oscillations of small amplitude and by slowly decreasing overall potential. At the moment current was applied, the potential increased to
(b) Large Pits In low concentrations of chloride (100 mM or less), certain concentrations of thiosulfate produced large but non-propagating pits on the surface (indicated by the filled triangle in Fig. 2 and shown in Fig. 4(b)) and gave the characteristic potential response shown in Fig. 3(b). Compared to the potential responses in the “small pit” region, the frequencies of the potential oscillations were much less, and the amplitude magnitude was consistently about 400 mV. Figure 2 shows that when [ClJ was less than 10 mM, then a solution containing [SzO: -1 of higher than 1 mM tended to hinder formation of pits rather than to produce active pits.
L
z-o,-
d------
(a) --
(b)
(d) --
(6) --
-1
J
(Cl -
2 3 c
-0.4 0.60.4 -
0 P OS?-
oo-0.4)) 0
0.4
0.8
1.2
I.6
0
0.4
0.6
TIME
(;I:,
1.6
0
w 0.4
0.6
1.2
1.6
Fig. 3. Potential responses of the six pitting categories, 316 SS, 25 PA/cm*. (a) small pits, 500, I mM Na2S203; (b) large pits, 1 mM NaCl, 0.1 mM Na&Os; (c) active pits, 500 mM NaCI, 0. I mM Na&Os; (d) few pits, 500 mM NaCI, 500 mM Na&OJ; (e) small pits with rings, 5 mM NaCI, 10 mM Na2S203; (f) no pits, IO mM NaCI, 20 mM Na$$Os.
Effect of thiosulfate on pitting corrosion
3285
Fig. 4. Micrographs of surfaces of the six pitting categories, 316 SS, 25 yA/cm2. (a) 100 mM NaCl and 1 mM Na2S203, 2 h; (b) 1 mM NaCl and 0.1 mM Na2S203, 2 h and 40 min; (c) 100 mM NaCl and 10 mM Na2S203, 2 h; (d) 500 mM NaCl and 500 mM Na2S.203, 2 h; (e) 5 mM NaCl and 10 mM Na$203, 2 h; (f) 10 mM NaCl and 50 mM Na&03, 2 h. Between 10 and 100 mM [Cl-], the “large pit” region occurred in a narrow range of [SzO: -1, and a further increase in [SzO: - ] produced active pits. Above 100 mM [Cl-], “large pit” behavior was not observed. It was therefore seen that a sufficient amount of thiosulfate was needed to be present to sustain the growth of a pit. In the “large pit” region, it was found that [SzO: -1 was evidently not sufficient to sustain pit growth, and that pits died after they grew to 15-35 pm. It was suggested in the discussion of the “small pit” region that the potential oscillations were caused by the initiation and death of pits. In the “large pit” regions it was observed that the number of large pits corresponded to the number of potential oscillations. Thus it was found that the size and the number of pits were predicted by the number of potential oscillations. Furthermore, an analysis of the frequency of pit formation and size of pits gave an indication of how chloride and thiosulfate influence pit growth. Table 1 shows the frequency of pit formation, measured by counting the ,number of oscillations in one hour, and the average diameters of the pits, obtained by measuring 2 to 4 representative pits on the surface. The data in Table 1 show that [Cl-] did not influence the size of the pits because an increase in [Cl]/[S,O: -1 from l/O.1 to 2/0.1 resulted in the same average pit diameter. On the other hand, an increase in [Cl-]/[SzO: -1 from 2/0.1 to 2/0.3 resulted in significantly larger pits. Finally, in the case of the same concentration ratio of the two ions, 5/0.5 and l/0.1, the greater [SzO: -1 resulted in less frequent pit formation and larger pits. From these data it was concluded that
thiosulfate enhanced pit growth, consistent with the observations of Horowitz [12] and Well et al. [18]. (c) Active Pits It was found that active pits were formed on 316 SS (indicated by filled circles in Fig. 2) for 20 <[Cl-] < 1000 mM, and when [S20: -1 was approximately 10 mM. Figure 3(c) shows that the applied current initiated pits producing potential oscillations similar to that in the small pit regions. When one of the pits became active, the potential decreased. It was found that in the galvanostatic experiments, only one pit propagated. The growth of the pit produced a smooth potential-time trace which decreased slowly over the course of one hour. The time it took to produce an active pit was reproducible; an increase in [SzOs ’ -1 was found to decrease the time it took to form an active pit. In the range of 20 <[Cl-] < 300 mM, the largest pits were formed at [SzO: -1 right above the small pit Table 1. Number of pits produced and their average diameters for different ratios of chloride to thiosulfate concentrations in “large pit region” Wl/KW: (mWmM) 5013 10/l 1012 510.5 2/o. 1 210.3 l/0.1
No. of pits/hr
Avg. diameter of pits (Pm)
5.8 9.4 8.3 9.4 11 1.2 12
35 N/A 30 23 18 30 18
1
3286
J. 0. Park ef al.
or large pits region, producing pits in diameter of 200-300 pm and 30-35 pm deep after a 2 h experiment as shown in Fig. 4(c). When [SzO: -1 was increased further, the size of the active pits decreased to 100 pm or less, and yellow rings formed around the pits. Also, the depth of the pits was reduced to about 20 pm, and in certain cases, the oxide film could be seen around the edge of the pits. (d) Few Pits
A further increase of [S20:-] from the active pit region for [Cl-j > 10 mM resulted in few pits on the surface (indicated by filled squares in Fig. 2 and shown in Fig. 4(d)). Such behavior was rarely observed when [Cl-] was lower than 10 mM; for the region 500 < [SzO: - ] < 1000 mM, such behavior occurred over a much wider range of [S20: -1 than was found for the active pit response. The potential response of the “few pit” region, shown in Fig. 3(d), was similar to that of the “active pit” region except that the potential drop occurred right after the current was applied without many potential oscillations. The lack of oscillations suggest that pit formation was not taking place and that another process was causing the potential drop. Instead of pitting corrosion, general corrosion might be taking place. Among the few pits that were found on the surfaces, some were crystalline shaped; most being rectangles or triangles. (e) SmaN Pits with Rings The 316 SS surface in the “small pits with rings” region, indicated by cross-filled squares in Fig. 2, was characterized by pits of only a few pm in diameter and possessing a ring of discoloration surrounding the pit mouth opening which was observed when the 316 SS surface was viewed, ex situ, with an optical microscope as shown in Fig. 4(e). A potential response typical of the “small pits with rings” region is shown in Fig. 3(e). In this region, the initiation of pits did not cause oscillations in the potential response as they did in the “small pits”, “large pits”, and “active pits” regions. (f) No Pits When, for 10 mM < [Cl] < 20 mM, the [S*O: -1 was sufficiently high, the 316 SS surface was found to be completely free of pits as shown in Fig. 4(f). This “no pit” region is indicated by open squares in Fig. 2. On application of current, the potential moved gradually, over a period of 5-10 min, to the oxygen evolution potential as shown in the potential response in Fig. 3(f). For [Cl-j > 20 mM, experiments with [SzO: -1 sufficiently high to yield a response in the “no pits” region were not conducted.
Role of Thiosu[fate and Chloride
Thiosulfate ions are thought to influence anodic dissolution of metal, and/or hinder repassivation of the metal. To summarize the results presented in Figs l-4, it was found that thiosulfate alone did not initiate pits but was influential in propagation of pits. The addition of !$_O:- to Cl- solutions caused different types of pitting behavior which we designated by six categories based on microscopic examination of pit morphology. The pitting behavior depended on both the ratio of the concentration of the two ions, and also on the magnitude of the concentrations. In the presence of chloride, a sufficiently high value of [SzO:-] caused a pit to grow until termination of the experiment, while excess of [SzO: - ] prevented any formation of pits. Furthermore, in solutions of a sufficiently low concentration of [SzO: - ] the potential responses exhibited oscillations. As the number of oscillations in potential could be correlated with the number of pits, it was concluded that the chloride was more influential in the initiation of pits. Potentiostatic
experiments
The galvanostatic experiments summarized above demonstrated that thiosulfate influenced the growth of corrosion pits on 316 SS in neutral chloride solutions. In what follows, results of potentiostatic experiments on 304 SS are discussed. In the first two subsections, qualitative results are discussed and compared with galvanostatic results. In the remaining subsections, single pit experiments are quantitatively analyzed in order to ascertain the mechanism by which thiosulfate affects pit growth. Eflect of Thiosulfate
on Pit Initiation
Electrodes on which potentiostatic experiments were performed at potentials between +450 mV (see) and +950 mV (see) in a solution of 0.03 M Na2S203 did not initiate pitting. However, potentiostatic pitting experiments that were performed in 0.5 M NaCl containing either 0.03 or 0.06 M Na&Os at +450 mV produced pitted surfaces. Also, electrodes pitted in solutions containing 0.5 M NaCl. The results of these experiments agreed with the results of Newman [7] and Lott [lo] and the galvanostatic results of this paper, which suggested that the surface of stainless steel remained passive in solutions of thiosulfate only and that chloride was required for breakdown of the passive film. Thus, it was concluded that chloride is required for breakdown of the passive film and that thiosulfate is not [12]. Effect of Thiosulfate
on Pit Growth
Pits that were initiated in 0.5 M NaCl at a potential of +450 mV (see) grew to have openings of 5 pm and had lifetimes of up to 2 s. All of the pits that were initiated in 0.5 M NaCl spontaneously
Effect of thiosulfate on pitting corrosion
repassivated; and, the pits were round and smooth when viewed from above. The rounded shape suggested transport control such as that expected owing to the presence of a resistive surface layer of salt film [5, 281. The bahavior and appearance of pits grown in 0.5 M NaCl solutions were also reported by Frankel et al. [29]. They found that the pits grew to sizes of between 1 pm and 10 pm, grew for times as long as 15 s, and spontaneously passivated. From our observations it was concluded that a solution containing only 0.5 M NaCl is not sufficiently aggressive to sustain putting on Type 304 stainless steel. Some of the pits that initiated in 0.5 M NaCl + 0.03 M Na2SZ03 at +450 mV (see) spontaneously repassivated. These pits grew for up to 5 s at which time they had mouth openings of up to 10 pm. The openings were round when viewed from above and the inner surfaces were smooth. Other pits that initiated in 0.5 M NaCl + 0.03 M Na2S203 at +450 mV (see) stopped growing only after the applied potential was turned off; such pits were round and smooth when viewed from above and grew to have mouth openings of up to 60 pm and for up to 70 s. Thus, in some situations, the conditions for which are not known, pit growth in 0.5 M NaCl in the presence of 0.03 M NazSz03 was sustained, or stabilized. From the difference in behavior of pits grown in electrolytes with and without thiosulfate, it was concluded that thiosulfate, when present at a “critical” concentration level, changes the growth behavior of pits. Finally, all pits that initiated in 0.5 M NaCl + 0.06 M Na2S203 at +450 mV (see) repassivated spontaneously. The pits grew to have mouth openings of up to 10 pm and for up to 5 s and were round and smooth when viewed from above. That is, whereas a Na2S203 concentration of 0.03 M apparently stabilized pit growth, a concentration of 0.06 M apparently did not. The galvanostatic results presented earlier also showed that, for a given concentration of NaCl, there is a value of Na&03 concentration above which pit propagation was hindered . To summarize, thiosulfate was observed to have a significant effect on pit growth but not on pit initiation. At a concentration of 0.03 M NazSzOJ in 0.5 M NaCI, pits initiate and grow, and some apparently stabilize in that they continue to grow until the applied potential is turned off. However, at a concentration of 0.06 M Na2S203 pits initiate and grow but repassivate after about 5 s. The conditions required for stable pit growth have been the subject of numerous studies, as described earlier. In the next section, a quantitative analysis of the corrosion data for the determination of the mechanism by which thiosulfate stabilizes pit growth is described.
3287
Surface Area and Volume of Single Pits
In previous analyses of data on smooth, hemispherical pits, the shape was approximated as a spherical segment. Thus, Buzza [21], Harb [20], and Wong [19] were able to estimate values for the active area of a corroding pit and for the volume of material removed from the pit by dissolution using the following formulas: Spherical segment volume = i nh(3rz -I- h’), Spherical segment surface area = 2nrh, where r is the radius of the pit mouth and h is the pit depth.
(1) (2)
opening
Single Pit Analysis (a) Growth rate of pits. In Fig. 5 the measured pit radii for single pits that repassivated spontaneously in three different solutions (0.50 M NaCI, and 0.50 M 0.50 M NaCl + 0.03 M Na&03, NaCl + 0.06 M Na2S203) at an applied potential of +450 mV are plotted as a function of time. Also shown in Fig. 5 are lines that represent a least squares fit of the three sets of data, from which it may be recognized that the growth rates for pits grown in electrolytes with and without thiosulfate are not significantly different. Several studies have suggested [15, 14, 301 that the stability of pit growth on stainless steel in solutions of thiosulfate, as compared to the lack of stability of pit growth on stainless steel in solutions without thiosulfate, is due to the catalytic action of thiosulfate. If the action of thiosulfate were catalytic, it would be expected that the growth rate of the pits grown in solutions with and without thiosulfate would be different. However, since Fig. 5 shows that the difference in growth rate was not evident, it was therefore concluded that thiosulfate does not catalytically stabilize the growth of pits.
Fig. 5. Pit radius at the time the pit growth terminated. Lines represent least squares fit of the data. d500mM NaCI; - - - A - - - 500 mM NaCl + 30mM Na&03;-- n --500 mM NaCl + 60 mM Na&Os.
J. 0. Park et al.
3288
130 120 .
A
A
110
AA A
loo
so 80 10 .
A
A
80
'A A A
A
A
rol/o
A
80
loo
10' Time (8)
Time (8)
Fig. 6. Summary of the current efficiency for single pits grown to various times. Each point represents one pit. All pits were grown in 0.5 M NaCl + 0.03 M Na&Or at +450 mV see and repassivated only after the applied potential was terminated.
(6) Current efficiency. For each pit, the average current efficiency was calculated by comparing the time-integrated current with the current calculated by using Faraday’s law in conjunction with the
volume of the single pit measured after completion of an experiment. On the basis of the assumption that iron, chronium, and nickel go into solution of Fe(U), Cr(III), and Ni(II) [31], respectively, the dissolution of 1 mm3 of steel (specific gravity 7.89) corresponds to 30 coulombs of electricity. Figure 6 is a plot of the current efficiency vs time for pits grown in 0.50 M NaCl + 0.03 M Na2Sz03 at +450 mV and that did not repassivate spontaneously. In Fig. 6 it may be seen that the current efficiency decreased with time. The current efficiency of less than 100% would suggest that a cathodic side-reaction may be occurring on the surface of the pit, concurrent with the dissolution of the metal. The reduction of thiosulfate, as has been proposed by several investigators [15, 161 could be occurring on the surface of the pit to produce a layer of sulfur that prevents the reformation of a passive film. The data in Fig. 6 are consistent with such an explanation. (c) Radius of single pits. Figure 7 summarizes measurements of the radius of single pits that did not repassivate spontaneously at +450 mV and which were grown for various periods of time in a solution of 0.5 M NaCl + 0.03 M Na2S203. A linear regression of the data gave r =4 . lt0.60.
(3)
The variation of pit radius with time is very close to a square root dependence and thus suggests a growth mechanism that is transport limited. For pit growth limited by diffusion controlled dissolution of a precipitated salt film in the absence of convection, Beck and Alkire [28] estimated that the pit radius would vary with time according to
Id
Fig. 7. Summary of the radius of the pit opening and depth of single pits grown for various times. Each point represents one pit. All pits were grown in 0.5 M NaCI + 0.03 M Na2S203 at +450 mV SCE and repassivated only after the applied potential was terminated. Solid line shows the theoretical result calculated with Eq. (4).
the following relationship r=
[rf+rF)]“‘.
t4)
In order to test whether it is reasonable to assume that a saturated salt film exists on the pit surface, a value of C, was calculated from equation (4). By using the fact that, at 4 seconds, a pit had a radius of 9 pm and by using D = 5 x lo-O6 cm’/s, M = 63 gm/gmol, and p = 9 gm/crn3, equation (4) gives a saturation concentration of about 3 M. This is a reasonable value for a saturation concentration for typical metal salts and thus supports the view that a salt film may be present on the surface of the pit. It has been suggested [28] that the existence of a salt film promotes pit stability. If a salt film controls the stability of pitting corrosion, however, it would be expected that the salt would contain thiosulfate since thiosulfate plays a role in enhancing pit stability. If it is further assumed that an iron salt precipitates, since iron is most likely to be the cation of highest concentration in the pit, it would then be expected that an iron chloride salt would precipitate, not iron thiosulfate, because iron chloride is less soluble than iron thiosulfate. Thus, it is difficult to explain how thiosulfate increases pit stability via the formation of a salt film. (d) Current density of single pits. For pits that did not spontaneously repassivate in a solution of 0.5 M NaCl + 0.03 M NazS203 and at a potential of +450 mV, the anodic current density was calculated at the time of completion of each experiment by dividing the current measured at the end of the experiment by the pit surface area estimated with equation (1). Figure 8 summarizes the anodic current density of each pit indicated in Fig. 7 at the
Effect of thiosulfate on pitting corrosion
3289
cate that a salt film may be present on the pit surface. Radiotracer experiments
nms(s) Fig. 8. Summary of the anodic current density of single pits grown for various times. Each point represents one pit. All pits were grown in 0.5 M NaCl + 0.03 M Na&Os at +450 mV see and repassivated only after the applied potential was terminated. Solid line shows the theoretical result calculated with equation (6). time of pit death. A linear regression of the data in
Fig. 8 gives i = 11.9t-0.60.
(5)
The variation of pit radius with time is close to the square root dependence that would characterize a growth mechanism that is transport limited. For diffusion limited growth, Beck and Alkire [28] estimated the current density to be i=
zFDC, [r: + (2DCJ4t/p)]“2
’
By using the parameter values cited previously along with the data in Fig. 8, it was found that equation (6) indicated a saturation concentration of 4.5 M, a value that is comparable to the value (3 M) obtained from Fig. 7. Both sets of data indi-
The adsorption of thiosulfate onto the surface of a 316 SS electrode during galvanostatic experiments was investigated by using the radiotracer method. A bulk concentration of 0.1 mM thiosulfate was studied under three concentrations of chloride (1, 10 and 50 mM). As shown in Fig. 9, these compositions corresponded to one in the “large pit” region (1 mM NaCI) and two in the “small pit” region (10 and 50 mM NaCl). Figure 9 shows that when the current was applied (t = 0), the surface concentration of S20: - began to increase at a rate which depended on chloride concentration. At 1 mM chloride solution, the initial rate of increase of thiosulfate surface concentration was higher as compared to the adsorption containing both 10 and 50 mM NaCl. Furthermore, at 1 mM NaCI, the surface concentration reached a plateau after one hour of the experiment, perhaps indicating that the surface became saturated. For a chloride concentration of 10 mM chloride, there was initially a significant rate of adsorption of thiosulfate, indicated by a steeper slope; after 30 min, the rate of adsorption was much slower. Finally, for a chloride concentration of 50 mM, the large initial rate of increase of surface adsorption was not observed. Rather, a more gradual increase in surface concentration was observed and was comparable to the rate of adsorption observed after 30 min in the solution containing 10 mM chloride. The results of the radiotracer experiments can be rationalized if it is assumed that chloride is influential in pit initiation and that thiosulfate is influential in pit propagation, as proposed in this manuscript.
A
A
3
A A
AA
0 oo”
A
4r
2-
'A AA0
o
0 0
Time (hr) Fig. 9. Surface concentration of thiosulfate in three chloride concentrations, 316 SS, 25 pA/cm*: A I mM NaCI, 0.1 mM Na&03; 0 IO mM NaCI. 0. I mM Na2S201; 0 50 mM NaCI, 0. I mM Na&Os.
J. 0. Park et al.
3290
At 1 mM NaCl, the rate of pit initiation, as indicated by potential traces in the galvanostatic experiments, was less when compared to the two other chloride concentrations. Instead, large but non-propagating pits formed. It is proposed that the higher rate of thiosulfate adsorption prevented the interaction of chloride with the passive film, thereby pit decreasing the amount of initiation. Furthermore, because the amount of pit initiation was reduced, the surface roughness of the 316 SS surface did not increase significantly as it would were there more small pits forming, allowing for the apparent saturation of the surface concentration of thiosulfate. However, for the chloride concentrations of 10 and 50mM NaCl which fall in the “small pits” region, the influence of the chloride on the 316 SS surface appeared to dominate. Higher concentrations of chloride reduced the surface concentration of thiosulfate. Also, with the larger amount of pit initiation, there was, perhaps, an increase in the roughness of the surface that allowed for the gradual increase of thiosulfate adsorption observed for both 10 and 50 mM NaCl. CONCLUSIONS The effect of thiosulfate and chloride ions on pitting corrosion of stainless steel was investigated by conducting a series of galvanostatic and potentiostatic experiments in conjunction with adsorption measurements made by a radiotracer method. It was found that the critical concentration of thiosulfate required for 316 SS to become active was about four-fold higher than the value reported previously for 304 SS (Fig. 1). A series of galvanostatic experiments on 316 SS produced six categories of pitting (Fig. 2) that had uniquely different potential responses (Fig. 3). Two of the responses corresponded to limiting potential behavior obtained from pure chloride and pure thiosulfate solutions. The pure chloride solution produced many pits that died out very quickly, while thiosulfate solution produced no pits. The remaining four potential responses resulted from the various combinations of the two ions which led to differences in the formation and propagation of pits. By comparing the growth of pits grown at +450 mV (see) in 0.5 M NaCl containing various concentrations of thiosulfate (0, 0.03, 0.06 M) it was concluded that thiosulfate changes the growth behavior of pits. Moreover, it was found that the rates of growth in 0.5 M NaCl are not different from the rate of growth in solutions containing 0.03 M or 0.06 M NazSzOs (Fig. 5). It was thus concluded that thiosulfate does not induce pit stability through a catalytic mechanism. For pits that grow in 0.5 M NaCl + 0.03 M Na&Os and that do not repassivate spontaneously, it was found that the current efficiency decreased with time (Fig. 6). These data are consistent with
the suggestion that a reduction reaction occurs on the surface of the pit, such as reduction of thiosulfate to produce a layer of sulfur. Linear regression of the points in Figs 7 and 8 corresponding to pits that did not repassivate spontaneously in 0.5 M NaCl + 0.03 M Na2Sz03 showed that the radius (Fig. 7) and current density (Fig. 8) were consistent with the hypothesis that the pit growth rate was limited by the diffusion controlled dissolution of a precipitated metal salt film. In several concentrations of chloride for a given thiosulfate concentration, the surface concentrations of thiosulfate were measured by combining an in situ radiotracer method with galvanostatic experiments. Three different types of thiosulfate adsorption behavior were observed for three different chloride concentrations in 0.1 mM Na&Os solution (Fig. 9). It was observed from the radiotracer experiments that the extent of pit initiation and propagation was influenced by the adsorption behavior of thiosulfate. ACKNOWLEDGEMENTS This investigation was supported by the U.S. Department of Grant No. Energy DEFG0291ER45439, through the Materials Research Laboratory, University of Illinois, Urbana-Champaign. M.V. received support in the form of a National Science Foundation Fellowship. The authors acknowledge with appreciation the use of laboratory facilities as well as many useful discussions with Professor Andrzej Wieckowski of the Department of Chemistry. REFERENCES 1. G. S. Eklund, J. Electrochem. Sot. 123, 170 (1976). 2. G. Wranglen, Corrosion Science 14, 331 (1974). M. Rychcik 3. M. Smialowski, Z. Szklarska-Smialowska, A. Szummer, Corrosion Science 9, 123 (1969). 4. J. Stewart and D. E. Williams, Corrosion Science 33, 457 (1992). Localized Corrosion, (Edited 5. Z. Szklarska-Smialowska, by R. W. Staehle, B. F. Brown, J. Kruger, A. Agrawal) p. 312, NACE-3, Houston (1974). 6. S. W. Dhawale, Chem. Education 70. 12 (1993). 7. R. C. Newman; Corrosion-NACE 41; 456 (1985). 8. Z. Szklarska-Smialowska, Pitting Corrosion of Metals, NACE, Houston (1986). 9. R. Ke and R. C. Alkire, J. Electrochem. Sot. 139, 1573 (1992). Ph.D. Thesis, University of Illinois, 10. S. E. Lott, Urbana (1987). Il. S. E. Lott and R. C. Alkire, J. Elecrrochem. Sot. 136, 973 (1989). 12. H. H. Horowitz, J. Elecrrochem. Sot. 132, 2064 (1985). Sot. 142, 13. R. Ke and R. Alkire, J. Electrochem. 4056 (1995). J. Stewart, in Crificaf Facfors in 14. D. E. Williams, Localized Corrosion (Edited bv G. S. Frankel. R. C. Newman) p. 36, Ekctrochemical Society, Pennington, New Jersey (1991). H. Isaacs and B. Alman, Corrosion15. R. Newman, NACE 38, 261 (1985).
Effect of thiosulfate 16. P. Sury, Corros. Sci. 16, 879 (1976). 17. R. Newman and E. Franz, Corrosion-NACE
40,
325 (1984). 18. D. B. Well, J. Stewart, R. Davidson, P. Scott M. and
D. E. Williams, Corrosion Science 33, 39 (1992). 19. K. P. Wong, M.S. Thesis, University of Illinois, Urbana, IL (1985). 20. J. N. Harb, MS. Thesis, University of Illinois, Urbana, IL (1986). 21. D. W. Buzza, Ph.D. Thesis, University of Illinois, Urbana, IL (1992). 22. R. K. Ulrich and R. C. Alkire, Corrosion Science 23, 1153 (1983). 23. R. E. White, J. O’M. Bockris, B. E. Conway, Modern Aspects of Electrochemistry No. 21, Plenum Press, New York (1990). 24. J. Lipkowski, P. Ross, Adsorption of Molecules at Meral Electrodes. VCH Publisher. Inc., New York (1992).
on pitting corrosion
3291
25. A. E. Thomas, Y-E. Young, M. Gamboa-Aldeco, K. Franaszczuk and A. Wieckowski, J. Electrochem. Sot. 142,476
(1995).
26. J.
0. Park, M.S. Thesis, University of Illinois, Urbana, IL (1993). 27. M. L. Verhoff, M.S. Thesis, University of Illinois, Urbana, IL (1994). 28. T. R. Beck and R. C. Alkire, J. Electrochem. Sot. 126, 1662 (1979). 29. G. S. Frankel, L. Stockert and H. Bohni, CorrosionNACE 38,406 (1992). 30. G. Cragnohno and D. D. Macdonald, CorrosionNACE 38,406 (1982). 31. Y. Hisamatsu, T. Yoshii, Y. Matsumara, in Localized Corrosion (Edited by R. W. Staehle, B. Brown, J.
Kruger, (1974).
A. Agrawal)
p. 427, NACE-3,
Houston
Electrochimica Acta. Vol. 42, Nos 20-22, pp. 3281-3291, 1991 0 1997 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain s001348q97)00179-5 0013486/97 317.00 + 0.00
Microscopic behavior of single corrosion pits: the effect of thiosulfate on corrosion of stainless steel in NaCl J. 0. Park,* M. Verhoff and R. Alkire Department
of Chemical Engineering and Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
(Received 13 November 1996; in revised form 9 January 1997) Abstract-Pitting corrosion of Type 304 and 316 stainless steels was investigated in neutral chloride solutions containing various concentrations of thiosulfate (0.01 to 500 mM) and chloride (1 to 1000 mM). The effect of thiosulfate on pitting corrosion of stainless steel 304 was quantitatively studied at the microscopic level by initiating single pits on small wires in neutral chloride solutions containing thiosulfate. Analysis of single pit data indicated that thiosulfate may promote stability through either a cathodic side-reaction or through the existence of a salt film. In galvanostatic experiments, the potential response exhibited six types of behavior that depended on the concentrations of chloride and thiosulfate. Thiosulfate was found to promote the sustained growth, or stability, of pits within a concentration range that depended upon the concentration of chloride ion. An excess of thiosulfate prevented pit formation. Type 316 stainless steel required higher concentrations of thiosulfate to produce stable pits, as compared to Type 304. The effect of surface adsorption of thiosulfate on pitting of 316 stainless steel was investigated by an in situ radiotracer method. It was found that the extent of thiosulfate surface adsorption differed for the three levels of chloride in 0.1 mM NazSzOs, and that an increase in the surface concentration of thiosulfate from the small pit region caused formation of large pits. 0 1997 Published by Elsevier Science Ltd
INTRODUCTION Initiation of pitting corrosion often occurs owing to the presence of microscopic inclusions on the metal surface [14]. The local chemical environment which arises in the vicinity of such inclusions is different from that of the external bulk solution and is critical for determining whether or not a corrosion pit propagates or repassivates [5]. In the present work, the microscopic behavior of single pits was investigated to establish the effect of thiosulfate and chloride ions on the pitting corrosion of 304 and 316 stainless steel. First recognized in the paper industry [6, 71, thiosulfate has since been identified as a corrosion agent in the nuclear and mining industries. The *Author to whom correspondence should be addressed J.O. Park.
mechanism by which thiosulfate influences the corrosion of stainless steel is therefore of interest. MnS particles in stainless steel are known to be sites for the initiation of localized corrosion [8, 91 Lott demonstrated that thiosulfate, produced upon dissolution of manganese sulfide (MnS) inclusions [lo, 111, caused the breakdown of the passive film adjacent to the pit when present above a critical concentration in the presence of chloride ions. Horowitz also note that thiosulfate was an aggressive species which chemisorbs on the stainless steel passive film [ 121. Ke and Alkire demonstrated that significant dissolution of the steel matrix occurred adjacent to MnS inclusions in 304 stainless steel [13]. Oxide inclusions containing no sulfur showed either the formation of a microcrevice near the periphery of the inclusion or no evidence of corrosion whatsoever. In addition, they found that pits did not initiate adjacent to MnS inclusions that were below a cer-
3281
3282
J. 0. Park et al.
tain critical size, which they found to be approximately 0.7 pm. The authors proposed that only at MnS inclusions of sufficient size does the concentration of the sulfur species that is produced by the MnS dissolution increase above the critical concentration. Their findings supported the conclusion of Williams et al. [14] who proposed that the thiosulfate produced by inclusions influences the propagation of pits in stainless steel. Newman et al. [15] and Sury [16] proposed that thiosulfate was reduced to sulfur on the surface of the dissolving metal. The layer of sulfur was proposed to increase the dissolution rate and prevent repassivation of the metal surface. Newman and Franz [17] noted that the addition of thiosulfate, corrosion pits grown on 304 stainless steel in chloride solutions showed no tendency toward sponrepassivation. potentiostatic taneous From experiments on type 1018 carbon steel, Horowitz [12] determined that the ratio of metal atoms dissolved to thiosulfate ions consumed was between 100-400 to 1, and suggested that thiosulfate ions catalyzed the anodic dissolution of the metal. Well et al. [18] tested intergranular stress corrosion cracking in dilute thiosulfate solutions and found that the microcrack nucleation frequency dropped by a hundred-fold when the thiosulfate concentration was decreased from 10 to 1 ppm. The authors suggested that there existed a critical thiosulfate concentration that enhanced the corrosion of the steel. Most studies of the influence of thiosulfate on pitting corrosion of stainless steel have used surfaces which either had many pits of different ages, or which had very large dissolution sites produced by mechanical abrasion of the metal surface. In contrast, techniques for providing single corrosion sites have been found to be advantageous for quantitative micrometer scale studies on the effect of thiosulfate on the size, shape evolution, and growth rate of single corrosion pits on stainless steel. Wong [19] created single pits on 304 stainless steel using masked specimens which exposed an area of diameter 100 pm to a corrosion solution. Harb [20] created single pits on stainless steel by using small wires. Attempts to initiate single pits on stainless steel by using a laser initiation technique had, prior to the present work, been unsuccessful although the method has been found to work well on aluminium and iron [21, 221. The in situ radiotracer technique combines electrochemical and surface analytic techniques and provides information on kinetics of adsorption/desorption, surface concentration, and surface diffusion [23, 241. Thomas et al. used the radiotracer method to investigate adsorption of thiosulfate on stainless steel 316 in neutral perchlorate and sulfate electrolyte solution [25]. The purpose of the present study [26, 271 was to characterize the effect of thiosulfate on pitting cor-
rosion of 304 and 316 stainless steel. Galvanostic experiments were conducted to investigate the pitting behavior of stainless steel in solutions containing a wide range of concentrations of thiosulfate and chloride. The study of single pits was pursued to characterize quantitatively the effect of thiosulfate on pit growth at the microscopic scale. The radiotracer technique was used to establish the relationship of surface adsorption of thiosulfate and pitting corrosion. EXPERIMENTAL A series of galvanostatic, potentiostatic and radiotracer experiments was conducted. All solutions were prepared with 18 MS2 water and reagent grade NaCl and Na&03. For galvanostatic and radiotracer experiments, solutions were deaerated with N2 before the start of the experiments. Galvanostatic experiments
Cylindrical working electrodes (304 or 316 SS) were machined from bar stock (0.94 cm dia. by 0.96 cm height). A saturated calomel electrode (see) was used as reference electrode, and platinum gauze was used as counter electrode. Working electrodes were polished (Buehler, Ecomet 4) to a 5 pm finish (Buehler, Alpha Micropolish @ 1). The two compartment cell designed by Lott [lo] was used. One compartment was open and contained a Pt counter electrode, and the other was closed and contained working and reference electrodes. A potentiostatic power supply (PAR 273A) was used to apply a constant current of 25 pA/cm2, a value which corresponded to the passive current density of the polarization curve obtained at 0.1 V/s. The passive current value varied between 1 PA/cm2 at a scan rate of 0.0002 V/set and 60 PA/cm2 at 0.2 V/set. Potentiostatic experiments
The working electrode consisted of the exposed end of a 304 stainless steel wire (0.010 cm dia., A.D. McKay Inc., Darien, CT) which had been annealed, cast into epoxy (Resin, 20-8130-032, Hardener 20-8132-008, Buehler, Lake Bluff, IL), and polished to a 5 pm finish (Buehler, Alpha Micropolish @ 1). Fabricat io n of the electrodes employed procedures previously described by Harb [20]. The surface of the working electrode was mounted facing upward in an electrochemical cell, details of which are available elsewhere [27]. The reference electrode was a standard calomel electrode (see), and the counter electrode was Pt. Specimen potential was controlled with a potentiostat/galvanosat (Model 173/176, PAR). Current vs time data were recorded on an X-Y recorder (Houston 2000). After each experiment the electrode was examined under an optical microscope (EPIPHOT-TME, Nikon) to measure pit depth (+l pm); the diameter of the pit mouth opening was measured by using an
Effect of thiosulfate on pitting corrosion optical microscope (LABOPHOT, brated eye piece.
Nikon) with cali-
Radiotracer experiments The radiotracer cell designed by Wieckowski and co-workers [25] was used. Working electrodes were
machined from 316 SS bar stock (1.15 cm dia. and 0.5 cm thickness) and polished with wet emery paper (##4000) to a 6 pm finish. The 35S isotope was used to label $0: - ions (prepared at Amersham Co. as sodium (outer-35S) thiosulfate with a specific activity of 20 mCi/mmol and radiochemical purity of greater than 95%). Activities in solution and on the surface of the working electrode were detected with a scintillator. Because of unsatisfactory background to the signal ratio, concentrations of S20: higher than 0.1 mM was not used. Current was applied by a potentiostat (PAR, 362); the reference electrode (Ag/AgCl) was separated from the solution by a bridge system.
RESULTS
AND DISCUSSION
Galvanostatic experiments
Lott and Alkire [1 1] had previously characterized “active” and “passive” surfaces by observing the potential response. In the present investigation, it was found that, by using the same basis of judgment, “active” surfaces produced pits that grew until termination of the experiments while “passive” surfaces had many small pits which repassivated during the course of the experiments. That is, the “passive” region defined by Ke and Alkire [9] actually exhibited early stages of pit growth which, however, was not stable and which evidently termi-
z
u
nated spontaneously by repassivation. A series of galvanostatic experiments at various combinations of [SzO: -1 and (Cl-] was therefore conducted to obtain a more precise compilation of the effect of these ions on stainless steel pitting. 304 vs 316. From a series of galvanostatic experiments on 304 SS, Lott and Alkire [1 1] determined the concentration range of Cl- and $0: - which separated the “active” and “passive” regions; this separation is shown by the solid line in Fig. 1. 11 also shows results from a similar series of experiments on 316 SS. Concentrations which exhibited “active” behavior are indicated by solid circles, while conditions that gave “passive” behavior are indicated by hollow circles. The dashed line separates the “active” and “passive” regions for 3 16 SS. 11 indicates that higher [S20: -1 was required to produce an active surface on 316 SS. It was observed that the 304 SS surfaces produced fewer pits, but that they were often adjacent to each other in clusters, while 316 SS resulted in more pits, but that they were more uniformly distributed over the surface. Categories of pitting behavior. For 316 SS, it was found upon microscopic observation of pit morphology that the general categories of “active” and “passive” could be further sub-divided into six categories of behavior. These categories are indicated by the symbols in Fig. 2. The six categories were (a) small pits, (b) large pits, (c) active pits, (d) few pits, (e) small pits with rings and (f) no pits. Each of these categories were designated based on pit morphology, and will be discussed in the paragraphs below. Potential responses, which were found to be unique to each category, will also be discussed. It
4:
%
3 i: 2 E G
3283
‘y
0.1
I
so CONCENTRATION
100
OF CHLORIDE,
200 mM
Fig. 1. Concentration of thiosulfate and chloride that correspond to active (0) and passive (0) regions of 316 SS corrosion. The dashed line indicates the boundary between these regions. The solid line indicates the boundary reported perviously for 304 SS.
3284
J. 0. Park et al.
CONCRNTRATION
OF CHLORIDE,
around 400 mV and oscillated. At high concentration ratios of chloride to thiosulfate, such as 1OOO:lor SOO:l, the potential response was difficult to differentiate from those of pure chloride (ie in the absence of thiosulfate) [26] where the oscillations were very frequent, and the amplitude was small. Observation of specimens after such experiments showed that the surfaces were covered with small pits with diameters of several microns as shown in Fig. 4(a). As [Cl-J increased, the potential at which the pit initiation occurred was found to decrease. At [Cl-] of 10 mM, the oscillations occurred between 0.2 V to 0.25 V vs see; when [Cl-] was further increased to 500 mM, oscillations took place between 0.05 V to 0.1 V vs see. As [S20:-] increased, the micropits grew bigger before their death; such conditions also showed larger and less frequent oscillations. As can be seen from Fig. 3(a), although the oscillations were all relatively small and frequent, they were not uniform. It is suggested that the oscillations result from initiation and death of individual pits.
mM
Fig. 2. Concentration of thiosulfate and chloride for six categories of potential responses resulting from galvanostatic experiments using 316 SS, 25 pA/cm2. 0, small pits; A, large pits; 0, active pits; W, few pits; 0, small pits with ring; 0, no pits.
can be Seen in Fig. 2 that the combinations with the same ratio of [Cl-]/&O:-] (such as 10/l, lOO/lO and lOOO/lOOmM/mM), exhibited different pitting behavior. Thus it was concluded that not only the ratio of the ions but also the concentrations of the ions influenced the pitting mechanism. (a) Small Pits Figure 3(a) shows a typical potential response in the small pit formation region, indicated by open circles in Fig. 2. The potential response was characterized by oscillations of small amplitude and by slowly decreasing overall potential. At the moment current was applied, the potential increased to
(b) Large Pits In low concentrations of chloride (100 mM or less), certain concentrations of thiosulfate produced large but non-propagating pits on the surface (indicated by the filled triangle in Fig. 2 and shown in Fig. 4(b)) and gave the characteristic potential response shown in Fig. 3(b). Compared to the potential responses in the “small pit” region, the frequencies of the potential oscillations were much less, and the amplitude magnitude was consistently about 400 mV. Figure 2 shows that when [ClJ was less than 10 mM, then a solution containing [SzO: -1 of higher than 1 mM tended to hinder formation of pits rather than to produce active pits.
L
z-o,-
d------
(a) --
(b)
(d) --
(6) --
-1
J
(Cl -
2 3 c
-0.4 0.60.4 -
0 P OS?-
oo-0.4)) 0
0.4
0.8
1.2
I.6
0
0.4
0.6
TIME
(;I:,
1.6
0
w 0.4
0.6
1.2
1.6
Fig. 3. Potential responses of the six pitting categories, 316 SS, 25 PA/cm*. (a) small pits, 500, I mM Na2S203; (b) large pits, 1 mM NaCl, 0.1 mM Na&Os; (c) active pits, 500 mM NaCI, 0. I mM Na&Os; (d) few pits, 500 mM NaCI, 500 mM Na&OJ; (e) small pits with rings, 5 mM NaCI, 10 mM Na2S203; (f) no pits, IO mM NaCI, 20 mM Na$$Os.
Effect of thiosulfate on pitting corrosion
3285
Fig. 4. Micrographs of surfaces of the six pitting categories, 316 SS, 25 yA/cm2. (a) 100 mM NaCl and 1 mM Na2S203, 2 h; (b) 1 mM NaCl and 0.1 mM Na2S203, 2 h and 40 min; (c) 100 mM NaCl and 10 mM Na2S203, 2 h; (d) 500 mM NaCl and 500 mM Na2S.203, 2 h; (e) 5 mM NaCl and 10 mM Na$203, 2 h; (f) 10 mM NaCl and 50 mM Na&03, 2 h. Between 10 and 100 mM [Cl-], the “large pit” region occurred in a narrow range of [SzO: -1, and a further increase in [SzO: - ] produced active pits. Above 100 mM [Cl-], “large pit” behavior was not observed. It was therefore seen that a sufficient amount of thiosulfate was needed to be present to sustain the growth of a pit. In the “large pit” region, it was found that [SzO: -1 was evidently not sufficient to sustain pit growth, and that pits died after they grew to 15-35 pm. It was suggested in the discussion of the “small pit” region that the potential oscillations were caused by the initiation and death of pits. In the “large pit” regions it was observed that the number of large pits corresponded to the number of potential oscillations. Thus it was found that the size and the number of pits were predicted by the number of potential oscillations. Furthermore, an analysis of the frequency of pit formation and size of pits gave an indication of how chloride and thiosulfate influence pit growth. Table 1 shows the frequency of pit formation, measured by counting the ,number of oscillations in one hour, and the average diameters of the pits, obtained by measuring 2 to 4 representative pits on the surface. The data in Table 1 show that [Cl-] did not influence the size of the pits because an increase in [Cl]/[S,O: -1 from l/O.1 to 2/0.1 resulted in the same average pit diameter. On the other hand, an increase in [Cl-]/[SzO: -1 from 2/0.1 to 2/0.3 resulted in significantly larger pits. Finally, in the case of the same concentration ratio of the two ions, 5/0.5 and l/0.1, the greater [SzO: -1 resulted in less frequent pit formation and larger pits. From these data it was concluded that
thiosulfate enhanced pit growth, consistent with the observations of Horowitz [12] and Well et al. [18]. (c) Active Pits It was found that active pits were formed on 316 SS (indicated by filled circles in Fig. 2) for 20 <[Cl-] < 1000 mM, and when [S20: -1 was approximately 10 mM. Figure 3(c) shows that the applied current initiated pits producing potential oscillations similar to that in the small pit regions. When one of the pits became active, the potential decreased. It was found that in the galvanostatic experiments, only one pit propagated. The growth of the pit produced a smooth potential-time trace which decreased slowly over the course of one hour. The time it took to produce an active pit was reproducible; an increase in [SzOs ’ -1 was found to decrease the time it took to form an active pit. In the range of 20 <[Cl-] < 300 mM, the largest pits were formed at [SzO: -1 right above the small pit Table 1. Number of pits produced and their average diameters for different ratios of chloride to thiosulfate concentrations in “large pit region” Wl/KW: (mWmM) 5013 10/l 1012 510.5 2/o. 1 210.3 l/0.1
No. of pits/hr
Avg. diameter of pits (Pm)
5.8 9.4 8.3 9.4 11 1.2 12
35 N/A 30 23 18 30 18
1
3286
J. 0. Park ef al.
or large pits region, producing pits in diameter of 200-300 pm and 30-35 pm deep after a 2 h experiment as shown in Fig. 4(c). When [SzO: -1 was increased further, the size of the active pits decreased to 100 pm or less, and yellow rings formed around the pits. Also, the depth of the pits was reduced to about 20 pm, and in certain cases, the oxide film could be seen around the edge of the pits. (d) Few Pits
A further increase of [S20:-] from the active pit region for [Cl-j > 10 mM resulted in few pits on the surface (indicated by filled squares in Fig. 2 and shown in Fig. 4(d)). Such behavior was rarely observed when [Cl-] was lower than 10 mM; for the region 500 < [SzO: - ] < 1000 mM, such behavior occurred over a much wider range of [S20: -1 than was found for the active pit response. The potential response of the “few pit” region, shown in Fig. 3(d), was similar to that of the “active pit” region except that the potential drop occurred right after the current was applied without many potential oscillations. The lack of oscillations suggest that pit formation was not taking place and that another process was causing the potential drop. Instead of pitting corrosion, general corrosion might be taking place. Among the few pits that were found on the surfaces, some were crystalline shaped; most being rectangles or triangles. (e) SmaN Pits with Rings The 316 SS surface in the “small pits with rings” region, indicated by cross-filled squares in Fig. 2, was characterized by pits of only a few pm in diameter and possessing a ring of discoloration surrounding the pit mouth opening which was observed when the 316 SS surface was viewed, ex situ, with an optical microscope as shown in Fig. 4(e). A potential response typical of the “small pits with rings” region is shown in Fig. 3(e). In this region, the initiation of pits did not cause oscillations in the potential response as they did in the “small pits”, “large pits”, and “active pits” regions. (f) No Pits When, for 10 mM < [Cl] < 20 mM, the [S*O: -1 was sufficiently high, the 316 SS surface was found to be completely free of pits as shown in Fig. 4(f). This “no pit” region is indicated by open squares in Fig. 2. On application of current, the potential moved gradually, over a period of 5-10 min, to the oxygen evolution potential as shown in the potential response in Fig. 3(f). For [Cl-j > 20 mM, experiments with [SzO: -1 sufficiently high to yield a response in the “no pits” region were not conducted.
Role of Thiosu[fate and Chloride
Thiosulfate ions are thought to influence anodic dissolution of metal, and/or hinder repassivation of the metal. To summarize the results presented in Figs l-4, it was found that thiosulfate alone did not initiate pits but was influential in propagation of pits. The addition of !$_O:- to Cl- solutions caused different types of pitting behavior which we designated by six categories based on microscopic examination of pit morphology. The pitting behavior depended on both the ratio of the concentration of the two ions, and also on the magnitude of the concentrations. In the presence of chloride, a sufficiently high value of [SzO:-] caused a pit to grow until termination of the experiment, while excess of [SzO: - ] prevented any formation of pits. Furthermore, in solutions of a sufficiently low concentration of [SzO: - ] the potential responses exhibited oscillations. As the number of oscillations in potential could be correlated with the number of pits, it was concluded that the chloride was more influential in the initiation of pits. Potentiostatic
experiments
The galvanostatic experiments summarized above demonstrated that thiosulfate influenced the growth of corrosion pits on 316 SS in neutral chloride solutions. In what follows, results of potentiostatic experiments on 304 SS are discussed. In the first two subsections, qualitative results are discussed and compared with galvanostatic results. In the remaining subsections, single pit experiments are quantitatively analyzed in order to ascertain the mechanism by which thiosulfate affects pit growth. Eflect of Thiosulfate
on Pit Initiation
Electrodes on which potentiostatic experiments were performed at potentials between +450 mV (see) and +950 mV (see) in a solution of 0.03 M Na2S203 did not initiate pitting. However, potentiostatic pitting experiments that were performed in 0.5 M NaCl containing either 0.03 or 0.06 M Na&Os at +450 mV produced pitted surfaces. Also, electrodes pitted in solutions containing 0.5 M NaCl. The results of these experiments agreed with the results of Newman [7] and Lott [lo] and the galvanostatic results of this paper, which suggested that the surface of stainless steel remained passive in solutions of thiosulfate only and that chloride was required for breakdown of the passive film. Thus, it was concluded that chloride is required for breakdown of the passive film and that thiosulfate is not [12]. Effect of Thiosulfate
on Pit Growth
Pits that were initiated in 0.5 M NaCl at a potential of +450 mV (see) grew to have openings of 5 pm and had lifetimes of up to 2 s. All of the pits that were initiated in 0.5 M NaCl spontaneously
Effect of thiosulfate on pitting corrosion
repassivated; and, the pits were round and smooth when viewed from above. The rounded shape suggested transport control such as that expected owing to the presence of a resistive surface layer of salt film [5, 281. The bahavior and appearance of pits grown in 0.5 M NaCl solutions were also reported by Frankel et al. [29]. They found that the pits grew to sizes of between 1 pm and 10 pm, grew for times as long as 15 s, and spontaneously passivated. From our observations it was concluded that a solution containing only 0.5 M NaCl is not sufficiently aggressive to sustain putting on Type 304 stainless steel. Some of the pits that initiated in 0.5 M NaCl + 0.03 M Na2SZ03 at +450 mV (see) spontaneously repassivated. These pits grew for up to 5 s at which time they had mouth openings of up to 10 pm. The openings were round when viewed from above and the inner surfaces were smooth. Other pits that initiated in 0.5 M NaCl + 0.03 M Na2S203 at +450 mV (see) stopped growing only after the applied potential was turned off; such pits were round and smooth when viewed from above and grew to have mouth openings of up to 60 pm and for up to 70 s. Thus, in some situations, the conditions for which are not known, pit growth in 0.5 M NaCl in the presence of 0.03 M NazSz03 was sustained, or stabilized. From the difference in behavior of pits grown in electrolytes with and without thiosulfate, it was concluded that thiosulfate, when present at a “critical” concentration level, changes the growth behavior of pits. Finally, all pits that initiated in 0.5 M NaCl + 0.06 M Na2S203 at +450 mV (see) repassivated spontaneously. The pits grew to have mouth openings of up to 10 pm and for up to 5 s and were round and smooth when viewed from above. That is, whereas a Na2S203 concentration of 0.03 M apparently stabilized pit growth, a concentration of 0.06 M apparently did not. The galvanostatic results presented earlier also showed that, for a given concentration of NaCl, there is a value of Na&03 concentration above which pit propagation was hindered . To summarize, thiosulfate was observed to have a significant effect on pit growth but not on pit initiation. At a concentration of 0.03 M NazSzOJ in 0.5 M NaCI, pits initiate and grow, and some apparently stabilize in that they continue to grow until the applied potential is turned off. However, at a concentration of 0.06 M Na2S203 pits initiate and grow but repassivate after about 5 s. The conditions required for stable pit growth have been the subject of numerous studies, as described earlier. In the next section, a quantitative analysis of the corrosion data for the determination of the mechanism by which thiosulfate stabilizes pit growth is described.
3287
Surface Area and Volume of Single Pits
In previous analyses of data on smooth, hemispherical pits, the shape was approximated as a spherical segment. Thus, Buzza [21], Harb [20], and Wong [19] were able to estimate values for the active area of a corroding pit and for the volume of material removed from the pit by dissolution using the following formulas: Spherical segment volume = i nh(3rz -I- h’), Spherical segment surface area = 2nrh, where r is the radius of the pit mouth and h is the pit depth.
(1) (2)
opening
Single Pit Analysis (a) Growth rate of pits. In Fig. 5 the measured pit radii for single pits that repassivated spontaneously in three different solutions (0.50 M NaCI, and 0.50 M 0.50 M NaCl + 0.03 M Na&03, NaCl + 0.06 M Na2S203) at an applied potential of +450 mV are plotted as a function of time. Also shown in Fig. 5 are lines that represent a least squares fit of the three sets of data, from which it may be recognized that the growth rates for pits grown in electrolytes with and without thiosulfate are not significantly different. Several studies have suggested [15, 14, 301 that the stability of pit growth on stainless steel in solutions of thiosulfate, as compared to the lack of stability of pit growth on stainless steel in solutions without thiosulfate, is due to the catalytic action of thiosulfate. If the action of thiosulfate were catalytic, it would be expected that the growth rate of the pits grown in solutions with and without thiosulfate would be different. However, since Fig. 5 shows that the difference in growth rate was not evident, it was therefore concluded that thiosulfate does not catalytically stabilize the growth of pits.
Fig. 5. Pit radius at the time the pit growth terminated. Lines represent least squares fit of the data. d500mM NaCI; - - - A - - - 500 mM NaCl + 30mM Na&03;-- n --500 mM NaCl + 60 mM Na&Os.
J. 0. Park et al.
3288
130 120 .
A
A
110
AA A
loo
so 80 10 .
A
A
80
'A A A
A
A
rol/o
A
80
loo
10' Time (8)
Time (8)
Fig. 6. Summary of the current efficiency for single pits grown to various times. Each point represents one pit. All pits were grown in 0.5 M NaCl + 0.03 M Na&Or at +450 mV see and repassivated only after the applied potential was terminated.
(6) Current efficiency. For each pit, the average current efficiency was calculated by comparing the time-integrated current with the current calculated by using Faraday’s law in conjunction with the
volume of the single pit measured after completion of an experiment. On the basis of the assumption that iron, chronium, and nickel go into solution of Fe(U), Cr(III), and Ni(II) [31], respectively, the dissolution of 1 mm3 of steel (specific gravity 7.89) corresponds to 30 coulombs of electricity. Figure 6 is a plot of the current efficiency vs time for pits grown in 0.50 M NaCl + 0.03 M Na2Sz03 at +450 mV and that did not repassivate spontaneously. In Fig. 6 it may be seen that the current efficiency decreased with time. The current efficiency of less than 100% would suggest that a cathodic side-reaction may be occurring on the surface of the pit, concurrent with the dissolution of the metal. The reduction of thiosulfate, as has been proposed by several investigators [15, 161 could be occurring on the surface of the pit to produce a layer of sulfur that prevents the reformation of a passive film. The data in Fig. 6 are consistent with such an explanation. (c) Radius of single pits. Figure 7 summarizes measurements of the radius of single pits that did not repassivate spontaneously at +450 mV and which were grown for various periods of time in a solution of 0.5 M NaCl + 0.03 M Na2S203. A linear regression of the data gave r =4 . lt0.60.
(3)
The variation of pit radius with time is very close to a square root dependence and thus suggests a growth mechanism that is transport limited. For pit growth limited by diffusion controlled dissolution of a precipitated salt film in the absence of convection, Beck and Alkire [28] estimated that the pit radius would vary with time according to
Id
Fig. 7. Summary of the radius of the pit opening and depth of single pits grown for various times. Each point represents one pit. All pits were grown in 0.5 M NaCI + 0.03 M Na2S203 at +450 mV SCE and repassivated only after the applied potential was terminated. Solid line shows the theoretical result calculated with Eq. (4).
the following relationship r=
[rf+rF)]“‘.
t4)
In order to test whether it is reasonable to assume that a saturated salt film exists on the pit surface, a value of C, was calculated from equation (4). By using the fact that, at 4 seconds, a pit had a radius of 9 pm and by using D = 5 x lo-O6 cm’/s, M = 63 gm/gmol, and p = 9 gm/crn3, equation (4) gives a saturation concentration of about 3 M. This is a reasonable value for a saturation concentration for typical metal salts and thus supports the view that a salt film may be present on the surface of the pit. It has been suggested [28] that the existence of a salt film promotes pit stability. If a salt film controls the stability of pitting corrosion, however, it would be expected that the salt would contain thiosulfate since thiosulfate plays a role in enhancing pit stability. If it is further assumed that an iron salt precipitates, since iron is most likely to be the cation of highest concentration in the pit, it would then be expected that an iron chloride salt would precipitate, not iron thiosulfate, because iron chloride is less soluble than iron thiosulfate. Thus, it is difficult to explain how thiosulfate increases pit stability via the formation of a salt film. (d) Current density of single pits. For pits that did not spontaneously repassivate in a solution of 0.5 M NaCl + 0.03 M NazS203 and at a potential of +450 mV, the anodic current density was calculated at the time of completion of each experiment by dividing the current measured at the end of the experiment by the pit surface area estimated with equation (1). Figure 8 summarizes the anodic current density of each pit indicated in Fig. 7 at the
Effect of thiosulfate on pitting corrosion
3289
cate that a salt film may be present on the pit surface. Radiotracer experiments
nms(s) Fig. 8. Summary of the anodic current density of single pits grown for various times. Each point represents one pit. All pits were grown in 0.5 M NaCl + 0.03 M Na&Os at +450 mV see and repassivated only after the applied potential was terminated. Solid line shows the theoretical result calculated with equation (6). time of pit death. A linear regression of the data in
Fig. 8 gives i = 11.9t-0.60.
(5)
The variation of pit radius with time is close to the square root dependence that would characterize a growth mechanism that is transport limited. For diffusion limited growth, Beck and Alkire [28] estimated the current density to be i=
zFDC, [r: + (2DCJ4t/p)]“2
’
By using the parameter values cited previously along with the data in Fig. 8, it was found that equation (6) indicated a saturation concentration of 4.5 M, a value that is comparable to the value (3 M) obtained from Fig. 7. Both sets of data indi-
The adsorption of thiosulfate onto the surface of a 316 SS electrode during galvanostatic experiments was investigated by using the radiotracer method. A bulk concentration of 0.1 mM thiosulfate was studied under three concentrations of chloride (1, 10 and 50 mM). As shown in Fig. 9, these compositions corresponded to one in the “large pit” region (1 mM NaCI) and two in the “small pit” region (10 and 50 mM NaCl). Figure 9 shows that when the current was applied (t = 0), the surface concentration of S20: - began to increase at a rate which depended on chloride concentration. At 1 mM chloride solution, the initial rate of increase of thiosulfate surface concentration was higher as compared to the adsorption containing both 10 and 50 mM NaCl. Furthermore, at 1 mM NaCI, the surface concentration reached a plateau after one hour of the experiment, perhaps indicating that the surface became saturated. For a chloride concentration of 10 mM chloride, there was initially a significant rate of adsorption of thiosulfate, indicated by a steeper slope; after 30 min, the rate of adsorption was much slower. Finally, for a chloride concentration of 50 mM, the large initial rate of increase of surface adsorption was not observed. Rather, a more gradual increase in surface concentration was observed and was comparable to the rate of adsorption observed after 30 min in the solution containing 10 mM chloride. The results of the radiotracer experiments can be rationalized if it is assumed that chloride is influential in pit initiation and that thiosulfate is influential in pit propagation, as proposed in this manuscript.
A
A
3
A A
AA
0 oo”
A
4r
2-
'A AA0
o
0 0
Time (hr) Fig. 9. Surface concentration of thiosulfate in three chloride concentrations, 316 SS, 25 pA/cm*: A I mM NaCI, 0.1 mM Na&03; 0 IO mM NaCI. 0. I mM Na2S201; 0 50 mM NaCI, 0. I mM Na&Os.
J. 0. Park et al.
3290
At 1 mM NaCl, the rate of pit initiation, as indicated by potential traces in the galvanostatic experiments, was less when compared to the two other chloride concentrations. Instead, large but non-propagating pits formed. It is proposed that the higher rate of thiosulfate adsorption prevented the interaction of chloride with the passive film, thereby pit decreasing the amount of initiation. Furthermore, because the amount of pit initiation was reduced, the surface roughness of the 316 SS surface did not increase significantly as it would were there more small pits forming, allowing for the apparent saturation of the surface concentration of thiosulfate. However, for the chloride concentrations of 10 and 50mM NaCl which fall in the “small pits” region, the influence of the chloride on the 316 SS surface appeared to dominate. Higher concentrations of chloride reduced the surface concentration of thiosulfate. Also, with the larger amount of pit initiation, there was, perhaps, an increase in the roughness of the surface that allowed for the gradual increase of thiosulfate adsorption observed for both 10 and 50 mM NaCl. CONCLUSIONS The effect of thiosulfate and chloride ions on pitting corrosion of stainless steel was investigated by conducting a series of galvanostatic and potentiostatic experiments in conjunction with adsorption measurements made by a radiotracer method. It was found that the critical concentration of thiosulfate required for 316 SS to become active was about four-fold higher than the value reported previously for 304 SS (Fig. 1). A series of galvanostatic experiments on 316 SS produced six categories of pitting (Fig. 2) that had uniquely different potential responses (Fig. 3). Two of the responses corresponded to limiting potential behavior obtained from pure chloride and pure thiosulfate solutions. The pure chloride solution produced many pits that died out very quickly, while thiosulfate solution produced no pits. The remaining four potential responses resulted from the various combinations of the two ions which led to differences in the formation and propagation of pits. By comparing the growth of pits grown at +450 mV (see) in 0.5 M NaCl containing various concentrations of thiosulfate (0, 0.03, 0.06 M) it was concluded that thiosulfate changes the growth behavior of pits. Moreover, it was found that the rates of growth in 0.5 M NaCl are not different from the rate of growth in solutions containing 0.03 M or 0.06 M NazSzOs (Fig. 5). It was thus concluded that thiosulfate does not induce pit stability through a catalytic mechanism. For pits that grow in 0.5 M NaCl + 0.03 M Na&Os and that do not repassivate spontaneously, it was found that the current efficiency decreased with time (Fig. 6). These data are consistent with
the suggestion that a reduction reaction occurs on the surface of the pit, such as reduction of thiosulfate to produce a layer of sulfur. Linear regression of the points in Figs 7 and 8 corresponding to pits that did not repassivate spontaneously in 0.5 M NaCl + 0.03 M Na2Sz03 showed that the radius (Fig. 7) and current density (Fig. 8) were consistent with the hypothesis that the pit growth rate was limited by the diffusion controlled dissolution of a precipitated metal salt film. In several concentrations of chloride for a given thiosulfate concentration, the surface concentrations of thiosulfate were measured by combining an in situ radiotracer method with galvanostatic experiments. Three different types of thiosulfate adsorption behavior were observed for three different chloride concentrations in 0.1 mM Na&Os solution (Fig. 9). It was observed from the radiotracer experiments that the extent of pit initiation and propagation was influenced by the adsorption behavior of thiosulfate. ACKNOWLEDGEMENTS This investigation was supported by the U.S. Department of Grant No. Energy DEFG0291ER45439, through the Materials Research Laboratory, University of Illinois, Urbana-Champaign. M.V. received support in the form of a National Science Foundation Fellowship. The authors acknowledge with appreciation the use of laboratory facilities as well as many useful discussions with Professor Andrzej Wieckowski of the Department of Chemistry. REFERENCES 1. G. S. Eklund, J. Electrochem. Sot. 123, 170 (1976). 2. G. Wranglen, Corrosion Science 14, 331 (1974). M. Rychcik 3. M. Smialowski, Z. Szklarska-Smialowska, A. Szummer, Corrosion Science 9, 123 (1969). 4. J. Stewart and D. E. Williams, Corrosion Science 33, 457 (1992). Localized Corrosion, (Edited 5. Z. Szklarska-Smialowska, by R. W. Staehle, B. F. Brown, J. Kruger, A. Agrawal) p. 312, NACE-3, Houston (1974). 6. S. W. Dhawale, Chem. Education 70. 12 (1993). 7. R. C. Newman; Corrosion-NACE 41; 456 (1985). 8. Z. Szklarska-Smialowska, Pitting Corrosion of Metals, NACE, Houston (1986). 9. R. Ke and R. C. Alkire, J. Electrochem. Sot. 139, 1573 (1992). Ph.D. Thesis, University of Illinois, 10. S. E. Lott, Urbana (1987). Il. S. E. Lott and R. C. Alkire, J. Elecrrochem. Sot. 136, 973 (1989). 12. H. H. Horowitz, J. Elecrrochem. Sot. 132, 2064 (1985). Sot. 142, 13. R. Ke and R. Alkire, J. Electrochem. 4056 (1995). J. Stewart, in Crificaf Facfors in 14. D. E. Williams, Localized Corrosion (Edited bv G. S. Frankel. R. C. Newman) p. 36, Ekctrochemical Society, Pennington, New Jersey (1991). H. Isaacs and B. Alman, Corrosion15. R. Newman, NACE 38, 261 (1985).
Effect of thiosulfate 16. P. Sury, Corros. Sci. 16, 879 (1976). 17. R. Newman and E. Franz, Corrosion-NACE
40,
325 (1984). 18. D. B. Well, J. Stewart, R. Davidson, P. Scott M. and
D. E. Williams, Corrosion Science 33, 39 (1992). 19. K. P. Wong, M.S. Thesis, University of Illinois, Urbana, IL (1985). 20. J. N. Harb, MS. Thesis, University of Illinois, Urbana, IL (1986). 21. D. W. Buzza, Ph.D. Thesis, University of Illinois, Urbana, IL (1992). 22. R. K. Ulrich and R. C. Alkire, Corrosion Science 23, 1153 (1983). 23. R. E. White, J. O’M. Bockris, B. E. Conway, Modern Aspects of Electrochemistry No. 21, Plenum Press, New York (1990). 24. J. Lipkowski, P. Ross, Adsorption of Molecules at Meral Electrodes. VCH Publisher. Inc., New York (1992).
on pitting corrosion
3291
25. A. E. Thomas, Y-E. Young, M. Gamboa-Aldeco, K. Franaszczuk and A. Wieckowski, J. Electrochem. Sot. 142,476
(1995).
26. J.
0. Park, M.S. Thesis, University of Illinois, Urbana, IL (1993). 27. M. L. Verhoff, M.S. Thesis, University of Illinois, Urbana, IL (1994). 28. T. R. Beck and R. C. Alkire, J. Electrochem. Sot. 126, 1662 (1979). 29. G. S. Frankel, L. Stockert and H. Bohni, CorrosionNACE 38,406 (1992). 30. G. Cragnohno and D. D. Macdonald, CorrosionNACE 38,406 (1982). 31. Y. Hisamatsu, T. Yoshii, Y. Matsumara, in Localized Corrosion (Edited by R. W. Staehle, B. Brown, J.
Kruger, (1974).
A. Agrawal)
p. 427, NACE-3,
Houston