Effect of Potential on Crevice Corrosion Kinetics of Alloy 22

CORROSION SCIENCE SECTION

015007-1 CORROSION—JANUARY 2010

Submitted for publication April 2009; in revised form, July 2009. ‡ Corresponding author. E-mail: [email protected]. * Dept. Materiales, Comisión Nacional de Energía Atómica, Instituto

Sabato, Universidad Nacional de General San Martín, Av. Gral. Paz 1499, B1650KNA San Martín, Buenos Aires, Argentina.

** GE Global Research, 1 Research Circle, CEB2505, Schenectady, NY 12309.

(1) UNS numbers are listed in Metals and Alloys in the Unifi ed Num-bering System, published by the Society of Automotive Engineers (SAE International) and cosponsored by ASTM International.

Effect of Potential on Crevice Corrosion Kinetics of Alloy 22

M.A. Rodríguez,‡,* R.M. Carranza,* and R.B. Rebak**

ABSTRACT

Alloy 22 (UNS N06022) is a Ni-Cr-Mo alloy, highly resistant to localized corrosion. It has a wide range of industrial applica-tions. Because of its versatility, Alloy 22 has been proposed as a corrosion-resistant barrier for high-level nuclear waste containers. Crevice corrosion is expected to occur in hot chlo-ride solutions if the corrosion potential is equal to or greater than the crevice corrosion repassivation potential. For less-aggressive environments, transpassive dissolution limits the alloy performance at high potentials. The goal of the present work was to establish the effect of potential on the kinetics of chloride-induced crevice corrosion of Alloy 22 at 90°C. Electrochemical tests carried out included cyclic potentiody-namic polarization (CPP) curves, polarizations at constant applied potentials followed by repassivation, and electrochem-ical impedance spectroscopy (EIS) measurements at anodic applied potentials. The attack profi les were measured using a roughness checker. Crevice corrosion susceptibility of Alloy 22 was independent of bulk solution pH, but it was a strong function of potential. Likelihood of transpassive dissolution was found to increase for high pH solutions. Two different regions were distinguished in polarization tests at a constant potential. A fi rst region was associated with crevice corrosion incubation and a second one was associated with crevice cor-rosion initiation and propagation. The crevice corrosion current

density was estimated as a function of potential. It increased with potential up to a maximum value of approximately 20 mA/cm2. Absence of pitting corrosion was explained in terms of such a small current density, which was insuffi cient to satisfy a critical value of the product x·i, necessary for pit-ting corrosion to occur. A diagram of crevice corrosion suscepti-bility and transpassive dissolution for Alloy 22 in 1 M chloride solutions at 90°C was presented. If the corrosion potential exceeds the crevice corrosion repassivation potential, crevice corrosion will initiate, provided that the diffusion path associ-ated with the crevice is long enough.

KEY WORDS: chloride, crevice corrosion, repassivation poten-tial, transpassivity, UNS N06022

INTRODUCTION

Alloy 22 (UNS N06022)(1) is one of the most versa-tile members of the Ni-Cr-Mo system. It was designed to withstand the most aggressive industrial applica-tions, including both reducing and oxidizing acids. Its balanced content of chromium and molybdenum provides protection against oxidizing and reducing conditions, respectively. Alloy 22 presents excel-lent resistances to pitting corrosion, crevice corro-sion, and environmentally assisted cracking in hot, concentrated chloride solutions.1-3 This alloy found different applications in chemical processing, pick-ling and metal fi nishing, pollution control, nuclear waste treatment, the pulp and paper industry, and fl ue-gas desulfurization plants.2-3 Because of its excel-lent corrosion resistance in a wide variety of environ-ments, Alloy 22 has been selected for the fabrication of the corrosion-resistant outer shell of the high-level

ISSN 0010-9312 (print), 1938-159X (online)10/000013/$5.00+$0.50/0 © 2010, NACE International


CORROSION—Vol. 66, No. 1 015007-2

nuclear waste container for the proposed Yucca Mountain repository.4

It is generally assumed that localized corrosion will only occur when the corrosion potential (Ecorr) is equal to or greater than a critical potential (Ecrit). This critical potential is usually set as the crevice cor-rosion repassivation potential.4-5 This parameter can be obtained from cyclic potentiodynamic polarization (CPP) curves or the Tsujikawa-Hisamatsu electro-chemical (THE) method using artifi cially creviced specimens.6-8 Alternative techniques are also pro-posed.9-10 In environments not able to promote local-ized corrosion, Ecrit is set as the potential for transpas-sive dissolution.5 This parameter can be obtained as a passivity breakdown potential from potentiodynamic polarization curves.

Factors affecting crevice corrosion susceptibil-ity of Alloy 22 in chloride solutions has been classifi ed into environmental and metallurgical.5 Crevice cor-rosion susceptibility increases with temperature and chloride concentration.6-7,9-10 Presence of species other than chlorides is benefi cial and reduces crevice corro-sion susceptibility. Nitrate, sulfate, carbonate, bicar-bonate, fl uoride, and some organic acids have proven to be inhibitors.5-6,8,11-13 The ratio R = [Inhibitor]/[Cl–] is reported to be a relevant parameter.6,11 Studies on the effects of crevice former materials indicate that the type of material and tightness of the crevice affect crevice corrosion susceptibility.14-16 Effects of the met-allurgical condition of the alloy includes the presence of a weld seam, type of annealing, oxide fi lm formed, surface fi nishing, etc. Studies of these factors resulted in different conclusions. Some authors report a detri-mental effect on corrosion resistance,11,17 while others indicate a neglecting effect.7,18

Studies of crevice corrosion kinetics of Alloy 22 in chloride solutions have been carried out. Kehler, et al.,19 studied the propagation of crevice corrosion in 5 M lithium chloride (LiCl) plus sodium nitrate (NaNO3) and sodium sulfate (Na2SO4) additions using a potential step-down technique. They informed the current-time behavior at each potential step followed a power law model. They indicated that crevice corrosion propagation is ohmically controlled below –0.080 V vs. silver/silver chloride (Ag/AgCl), and above this potential propagation is mass-transport-limited. He, et al.,15,20 performed electrochemical tests at the corrosion potential by the addition of copper(II) chloride (CuCl2) as an oxidant. They reported that propagation of crevice corrosion resulted in signifi -cant decreases of the corrosion potential. Penetration rate was observed to decrease in time as a result of stifl ing and repassivation of crevice corrosion. Kehler and Scully21 reported that crevice corrosion stabilizes at the sites where a row of metastable pits form at a critical distance from the crevice mouth. Metasta-ble corrosion events increase with potential and tem-perature. Yilmaz, et al.,22 applied a constant potential

above the crevice corrosion repassivation potential of Alloy 22 in chloride plus nitrate solutions. They report three characteristic domains or regions:

—nucleation—growth—stifl ing and arrest of crevice corrosionThe goal of the present work was to establish the

effect of potential on the kinetics of chloride-induced crevice corrosion of Alloy 22 at 90°C.

EXPERIMENTAL PROCEDURES

Alloy 22 (UNS N06022) specimens were prepared from wrought mill-annealed plate stock. The chemical composition of the alloy in weight percent was 59.56% Ni, 20.38% Cr, 13.82% Mo, 2.64% W, 2.85% Fe, 0.17% V, 0.16% Mn, 0.008% P, 0.0002% S, 0.05% Si, and 0.005% C (Heat 059902LL1). Two different types of specimens were used (Figure 1):

—Prismatic specimens: a variation of the ASTM G523 specimen, which contained an artifi cial crevice formed by a polytetrafl uoroethylene (PTFE) compression gasket

—Prism crevice assemblies (PCA): fabricated based on ASTM G48,24 which contained 24 artifi cially creviced spots formed by a ceramic washer (crevice former) wrapped with a thick PTFE tape.

The applied torque was 7.92 N-m (70 in-lb). The PCA specimen has been described before.7 The tested sur-face areas were approximately 10 cm2 for the pris-matic specimens and 14 cm2 for the PCA specimens. The specimens had a fi nished grinding with abrasive paper number 600 and were degreased in acetone (CH3COCH3) and washed in distilled water within the hour prior to testing.

FIGURE 1. Scheme of PCA and prismatic specimens.



Electrochemical tests were performed in deaer-ated 1 M sodium chloride (NaCl) at 90°C. Nitrogen (N2) was purged through the solution 1 h prior to testing and was continued throughout the entire test. The electrochemical tests were conducted in a 1-L, three-electrode vessel (ASTM G5).23 A water-cooled condenser combined with a water trap was used to avoid evaporation of the solution and to prevent the ingress of air (oxygen), respectively. The temperature of the solution was controlled by immersing the cell in a thermostatized bath, which was kept at a con-stant temperature. All the tests were performed at ambient pressure. The reference electrode was a satu-rated calomel electrode (SCE), which has a potential of 0.242 V more positive than the standard hydrogen electrode (SHE). The reference electrode was con-nected to the solution through a water-cooled Luggin probe. The counter electrode consisted of a fl ag of platinum foil (total area: 100 cm2) spot-welded to a

platinum wire. All the potentials in this paper are reported in the SCE scale.

Cyclic potentiodynamic polarization (CPP) tests (ASTM G61)25 were performed. The potential scan was started 0.15 V below the Ecorr in the anodic direction at a scan rate of 0.167 mV/s. The scan direction was reversed when the current density reached a current value in the range from 1 mA/cm2 to 10 mA/cm2. The exact current density value depended on the particu-lar test. A potentiostatic hold was performed for 5 min at the onset potential (0.15 V below Ecorr) before start-ing the scan.

Electrochemical tests were performed to study the crevice corrosion initiation, propagation, and repassiv-ation kinetics of Alloy 22 in a pH 2, 1-M NaCl solution at 90°C. They consisted in two stages described below.

Stage 1 (Crevice Corrosion Incubation and Propa-gation) — A fi xed anodic potential in the range from –0.2 VSCE to 0.7 VSCE was applied for a period of time long enough to produce propagation of crevice cor-rosion. An electrochemical impedance spectroscopy (EIS) measurement was performed at the correspond-ing applied potential, after crevice corrosion propaga-tion was initiated. Application of the corresponding potential was continued after EIS measurement.

Stage 2 (Crevice Corrosion Repassivation) — Potential scanning was performed at 0.167 mV/s in the cathodic direction until repassivation was reached.

The EIS measurements were performed by apply-ing a 10-mV amplitude sinusoidal potential signal. This signal was superimposed to the corresponding applied potential. The frequency scan was started at 10 kHz and ended at 1 mHz.

Specimens were examined after testing in an optical microscope (OM) to establish the mode, loca-tion, and depth of the attack. Some specimens were observed with scanning electron microscopy (SEM). The attack profi le was measured using a roughness checker. Electrochemical tests were carried out using a potentiostat coupled to an impedance/gain-phase analyzer for EIS measurements.

RESULTS

CPP curves using prismatic and PCA specimens resulted in completely different behaviors (Figure 2). In the former case, a wide range of passivity was observed followed by a current increase due to trans-passive dissolution. In the latter, crevice corrosion initiation was observed at lower potentials than trans-passivity, and repassivation occurred in the reverse scan. Effect of pH was assessed for the two types of specimens. Bulk solution pH affected the potential for transpassive dissolution initiation (Figure 3). On the other hand, solution pH did not affect the potential corresponding to crevice corrosion repassivation in the range from pH 2 to pH 12 (Figure 4). Crevice cor-rosion repassivation potential is usually determined

FIGURE 2. Cyclic potentiodynamic polarization curve of prismatic and PCA Alloy 22 specimens in deaerated pH 2, 1-M NaCl solutions at 90°C.

FIGURE 3. Cyclic potentiodynamic polarization curve of prismatic Alloy 22 specimens in deaerated 1-M NaCl solutions at 90°C.



as the value corresponding to a current of 1 µA/cm2 in the reverse scan (ER1).

1,12 In the present work, ER1 was obtained from CPP curves using PCA specimens. ER1 was associated with crevice corrosion susceptibil-ity of Alloy 22. The potential corresponding to trans-passive dissolution initiation can be determined from CPP curves using prismatic specimens. This poten-tial was defi ned as the value corresponding to a cur-rent of 20 µA/cm2 in the forward scan (E20). Figure 5 shows the critical potentials, ER1 from PCA specimens and E20 from prismatic specimens, as a function of bulk solution pH in 1-M NaCl solutions at 90°C. ER1 was independent of bulk solution pH in a wide range of pH values. Crevice corrosion was observed in all cases with the only exception of the pH 13 solution. ER1 value obtained in 1 M HCl (pH 0) cannot be taken as a true repassivation potential because the alloy regained its initial active state at the end of the poten-tial scan. E20 as a function of pH was obtained from CPP tests using prismatic specimens in 1 M hydro-chloric acid (HCl) and in 1 M NaCl at pH values from 1 to 9.26 A signifi cant scatter of values was observed in solutions with pH ≥ 4. For CPP tests in these solu-tions, the bulk solution pH showed a change of more than 1 unit after measurements. A risk of crevice cor-rosion exists for potentials higher than ER1, while transpassive dissolution is expected for potentials higher than E20. To study the crevice corrosion kinet-ics, it is desirable to rule out transpassive dissolu-tion and oxygen evolution. A 1-M NaCl solution of pH 2 was selected for this purpose because there is a ~0.9-V range of crevice corrosion susceptibility with-out the infl uence of the other processes (Figure 5).

Polarization tests at different anodic potentials in the range of crevice corrosion susceptibility were per-formed using PCA specimens in pH 2, 1 M NaCl at 90°C. Figure 6 shows Stage 1 of these tests in which polarization was started up to crevice corrosion ini-tiation. Crevice corrosion was observed in the range

from –0.17 VSCE to 0.7 VSCE. Two regions or domains can be distinguished in Figure 6. In Region 1, the current decreased with time due to passive fi lm for-mation (crevice corrosion incubation). In Region 2, a current increase was observed due to crevice corro-sion initiation and propagation. The current in Region 2 increased for increasing potentials. For polarizations at –0.18 VSCE and –0.2 VSCE no crevice corrosion was detected. An incubation time (tinc) for crevice corrosion initiation can be defi ned as shown in Figure 7. Incu-bation time decreased with potential in the range from –0.17 VSCE to 0.2 VSCE (Figure 8). For potentials higher than 0.2 VSCE, tinc was constant and crevice corrosion initiated after a few seconds of polarization.

EIS measurements were performed at the applied potentials after crevice corrosion initiation (Region 2). The complex plane impedance plots (Figure 9) showed

FIGURE 4. Cyclic potentiodynamic polarization curve of PCA Alloy 22 specimens in deaerated 1-M NaCl solutions at 90°C.

FIGURE 5. Crevice corrosion repassivation potential (ER1) and transpassivity breakdown potential (E20) as a function of pH for Alloy 22 in 1-M Cl– solutions at 90°C. Data obtained from CPP curves in PCA and prismatic specimens.

FIGURE 6. Anodic current density as a function of time for different applied potentials (Stage 1) for PCA Alloy 22 specimens in deaerated pH 2, 1-M NaCl solutions at 90°C.



ation curves was similar for the different initial applied potentials. A close inspection of the zero current potential zone of each curve reveals certain differences (zoom in Figure 10). The potential corresponding to zero current was higher as the applied potential in Stage 1 increased. Repassivation potentials (ER1) obtained from these curves were plotted as a function of the anodic accumulated charge after crevice corro-sion initiation (Figure 11). ER1 increased for higher accumulated charges.

A set of polarization tests was performed at a con-stant potential of 0.0 VSCE. Polarization was fi nished at different times after crevice corrosion initiation (Region 2), Figure 12. Incubation time was similar for the tests.

Figures 13 through 19 show pictures of differ-ent specimens after testing. Contours of the attacked areas are also shown for some specimens. They were obtained by using an image processing software. Localized attacked areas were calculated from these contours. Crevice corrosion appeared as a localized

FIGURE 7. Determination of the incubation time (tinc) for crevice corrosion initiation in Stage 1 of polarization tests.

FIGURE 9. Complex plane impedance plot corresponding to EIS measurements at different applied potentials in PCA Alloy 22 specimens after crevice corrosion initiation in deaerated pH 2, 1-M NaCl solutions at 90°C.

FIGURE 8. Incubation time as a function of applied potential in Stage 1 of polarization tests.

FIGURE 10. Repassivation curves (Stage 2) for PCA Alloy 22 specimens in deaerated pH 2, 1-M NaCl solutions at 90°C.

a depressed semicircle aspect for all the potentials, which is a typical behavior of charge-transfer-con-trolled processes. The size of the semicircles decreased as applied potential increased, indicating an increas-ing rate of the charge-transfer process. No indication of a diffusion-controlled process was observed. How-ever, the non-steady state of crevice corrosion propa-gation could have masked a diffusion-controlled process in the low-frequency part of diagrams.

Figure 10 shows Stage 2 of the polarization tests, which consisted of a potentiodynamic polarization scan in the cathodic direction until crevice corrosion repassivation was reached. The shape of the repassiv-



attack under the ceramic crevice formers. For applied potentials higher than 0.3 VSCE, crevice corrosion was also found under the top PTFE compression gas-ket. An increase of the attacked area was observed as polarization time increased, for a constant poten-tial of 0.0 VSCE (Figures 13 through 15). Figure 20 cor-responds to a specimen after testing at 0.4 VSCE. The attacked profi le was measured along line 1 and line 2. These lines included creviced and uncreviced areas. Profi les are shown in Figures 21 and 22. An uneven profi le of crevice corrosion attack was observed. Pen-etration was shown to be a function of position inside the crevice, indicating that there is a distribution of current densities in the crevice. Nevertheless, when penetration and distance are drawn in a one-to-one scale (Figure 23), the profi le is not as steep as could be inferred from Figures 21 and 22. The attacked area calculated by using the contours is smaller than the real one, but it is a good approximation. Table 1 pres-ents a list of the estimated attacked areas along with other relevant information of the tests. The total area occluded by the crevice formers was 1.6 cm2.

Microscopic observations (Figures 24 through 27) indicated that grains of the alloy were discernible. This morphology of attack was described as crystal-line.5 This attack follows the higher energy planes in the crystal structure of the grains; therefore, the low-energy planes are left visible on the corroded surface. Lines of attack described elsewhere as trenches8 were observed at high applied potentials (Figure 27). A dark precipitate was found around the localized attacked areas and sometimes covering them. Energy-disper-sive x-ray (EDX) analyses were performed using SEM to detect the elements present in these precipitates. Results are listed in Table 2. Besides the constituents of Alloy 22 (60% Ni, 22% Cr, 13% Mo, 3% W, 3% Fe, and 0.16% Mn), Cl and Si were detected. Cl was pres-ent in the solution (as chloride) and Si could have pro-ceeded from the glass cell. The elements W and Mo were highly enriched in the precipitate, Fe was present in the same amount as in the alloy, Cr was slightly depleted, and Ni was highly depleted in the precipitate.

DISCUSSION

Kehler, et al.,6 reported that crevice corro-sion repassivation potential is independent of pH in the range from pH 2.75 to 7.75 for Alloys 625 (UNS N06625) and 22. Results from the present work con-fi rm this observation (Figure 5). These authors indi-cate that repassivation potential is independent of anodic accumulated charge associated with crevice attack in a wide range (10–1 C/cm2 to 103 C/cm2 for Alloy 625 and 10–2 C/cm2 to 102 C/cm2 for Alloy 22). Sridhar and Cragnolino27 report that pitting repas-sivation potential for Alloy 825 (UNS N08825) and Type 316L (UNS S31603) stainless steel decreases for increasing anodic accumulated charge, but it remains

constant above a certain threshold value. In the pres-ent work, ER1 showed a small increase for increasing accumulated charges (Figure 11).

Here, ER1 ranged from –0.2 VSCE to –0.14 VSCE; and in constant potential tests (Stage 1), the lowest poten-tial at which crevice corrosion initiated was –0.17 VSCE. This suggests that crevice corrosion initiation and repassivation potentials would coincide if potential scans were performed at a low enough scan rate.

Two different regions were distinguished in Stage 1 of the polarization tests (Figures 6, 7, and 12). Region 1 was associated with crevice corrosion incu-bation and Region 2 was associated with crevice cor-rosion initiation and propagation. A third region associated with repassivation for longer polarization times is reported elsewhere.22 Total current (I) mea-sured in Region 2 of Stage 1 can be expressed as the sum of the contributions from passive dissolution and

FIGURE 11. Crevice corrosion repassivation potential obtained in Stage 2 as a function of anodic accumulated charge after crevice corrosion initiation in Region 2 of Stage 1.

FIGURE 12. Anodic current density as a function of time for an applied potential of 0.0 VSCE for PCA Alloy 22 specimens in deaerated pH 2, 1-M NaCl solutions at 90°C.



crevice corrosion. Equation (1) expresses the total cur-rent in terms of passive current density (ipas), passive area (Apas), crevice corrosion current density (icrev), and the active area that suffers crevice corrosion (Acrev).

I i A i Apas pA as crev crev×ipas + ×icrev (1)

Let us assume icrev remained approximately constant with time, being only a function of potential. A simi-lar hypothesis was proposed by Galvele, et al.,28 in the study of pitting of ferritic stainless steels in chlo-ride solutions. They calculate the pitting current den-sity as a function of potential by measuring the depth

FIGURE 13. Image of a PCA Alloy 22 specimen polarized for 2 h at 0.0 VSCE in deaerated pH 2, 1-M NaCl at 90°C.

FIGURE 14. Image of a PCA Alloy 22 specimen polarized for 5.2 h at 0.0 VSCE in deaerated pH 2, 1-M NaCl at 90°C.

FIGURE 15. Image of a PCA Alloy 22 specimen polarized for 24 h at 0.0 VSCE in deaerated pH 2, 1-M NaCl at 90°C.



FIGURE 17. Image of a PCA Alloy 22 specimen polarized for 5 h at 0.1 VSCE and repassivated in deaerated pH 2, 1-M NaCl at 90°C. Contours of the attacked areas are also presented.

FIGURE 16. Image of a PCA Alloy 22 specimen polarized for 24 h at –0.14 VSCE and repassivated in deaerated pH 2, 1-M NaCl at 90°C. Contours of the attacked areas are also presented.



and diameter of pits. Taking this assumption into account, the observed total current increase in Region 2 was fully ascribed to the increase of Acrev with time. The fi rst term in Equation (1) can be neglected as ipas decreases with time. These approximations allow us to calculate icrev as a function of potential according to Equation (2). The crevice corrosion current density can be obtained as the quotient of the total current at the end of Stage 1 and the attacked area calculated by using the contours (Figures 16 through 19).

i

IA

crevcrev

=

(2)

The calculated icrev as a function of potential is shown in Figure 28. This should be taken as an average value of the distribution of current densities in the active crevice. icrev increased with potential in the range from –0.17 VSCE to 0.2 VSCE showing a Tafel-like relationship. The Tafel slope was 0.2 V/decade. This

value is higher than the Tafel slope usually found for charge-transfer processes with a two-electron exchange (0.12 V/decade). Signifi cant ohmic drop caused by the presence of a salt fi lm in the crevice corroded area could explain this observation. From 0.3 VSCE to 0.6 VSCE, icrev displayed a constant value of 10 mA/cm2 to 20 mA/cm2, and a small drop was observed for 0.7 VSCE. This could be attributed to a diffusion-controlled process governing the anodic kinetics. Nevertheless, impedance diagrams did not show any indication of a diffusion control in the anodic kinetics (Figure 9).

Crevice and pitting corrosion are considered the same process from an electrochemical point of view, only differing in geometrical aspects.29 It is well rec-ognized that a critical value of the product x·i is nec-essary for pitting/crevice corrosion to occur (where x is the diffusion path and i is the current density).29-30 Galvele reviewed current densities inside pits for pure metals and alloys.30 They range from 0.1 A/cm2 to

FIGURE 18. Image of a PCA Alloy 22 specimen polarized for 3.5 h at 0.4 VSCE and repassivated in deaerated pH 2, 1-M NaCl at 90°C. Contours of the attacked areas are also presented.



70 A/cm2, most of them at room temperature. Current densities higher than 1 A/cm2 are reported for ferritic stainless steels in 1 M HCl.28 They increase monotoni-cally with potential. Current densities for crevice corrosion of Alloy 22 estimated in the present work can be considered very low, since they were limited to 20 mA/cm2. This could explain the fact that no pitting corrosion has been observed for Alloy 22 in similar conditions. Since current density cannot exceed 20 mA/cm2 and diffusion path provided by passive fi lm rupture is small, the critical conditions x·i > (x·i)crit cannot be satisfi ed at any potential.

Figure 29 shows icrev and tinc as a function of potential—the lower the icrev, the higher the tinc. It is likely that the frequency and magnitude of metasta-ble events leading to crevice corrosion increased with potential, reducing incubation time.21 Two poten-tial regions were distinguished, by considering the effect of icrev in crevice corrosion susceptibility. From –0.17 VSCE to 0.25 VSCE the crevice corrosion suscepti-

bility can be considered low, as long diffusion paths are necessary to satisfy the critical condition x·i > (x·i)crit. From 0.25 VSCE to 0.7 VSCE the crevice corrosion susceptibility can be considered high, since shorter diffusion paths can stabilize crevice corrosion. Figure 30 shows a diagram of crevice corrosion susceptibility and transpassive dissolution for Alloy 22 in 1 M chlo-ride solutions at 90°C. This diagram was constructed with the data obtained in the present work. An extrap-olation of the regions of different crevice corrosion susceptibility was made, considering the lack of effect of bulk solution pH on crevice corrosion. Analysis of the diagram leads to interesting observations. High-pH solution increases the likelihood of transpassive dissolution. Alloy susceptibility to crevice corrosion increases with potential but is independent of bulk solution pH. If the corrosion potential exceeds the crevice corrosion repassivation potential, crevice cor-rosion will initiate provided that the diffusion path associated with the crevice is long enough.

FIGURE 19. Image of a PCA Alloy 22 specimen polarized for 3.5 h at 0.6 VSCE and repassivated in deaerated pH 2, 1-M NaCl at 90°C. Contours of the attacked areas are also presented.



CONCLUSIONS

❖ Crevice corrosion susceptibility of Alloy 22 was independent of bulk solution pH, but it was a strong function of potential. Likelihood of transpassive disso-lution was found to increase for high-pH solutions.

❖ Two different regions were distinguished in polar-ization tests at a constant potential. Region 1 was associated with crevice corrosion incubation and Region 2 was associated with crevice corrosion initia-tion and propagation. The crevice corrosion current density was estimated as a function of potential by calculating the quotient of the fi nal current at the end of Region 2 and the attacked area. Crevice corrosion current density increased with potential up to a maxi-mum value of approximately 20 mA/cm2. Absence of pitting corrosion of Alloy 22 in chloride solutions was explained in terms of such a small current density. It was insuffi cient to satisfy a critical value of the prod-uct x·i, necessary for pitting corrosion to occur.❖ A diagram of crevice corrosion susceptibility and transpassive dissolution for Alloy 22 in 1 M chlo-ride solutions at 90°C was presented. If the corrosion potential exceeds the crevice corrosion repassivation potential, crevice corrosion will initiate provided that the diffusion path associated with the crevice is long enough.

FIGURE 20. Image of one face of a PCA Alloy 22 specimen polarized at 0.4 VSCE. Contours of the attacked areas are presented along with lines 1 and 2 corresponding to measurements of the attacked profi le.

FIGURE 21. Attacked profi le corresponding to Line 1 in Figure 20.

FIGURE 23. Attacked profi le drawn in a one-to-one scale.

FIGURE 22. Attacked profi le corresponding to Line 2 in Figure 20.



FIGURE 24. SEM image of a PCA Alloy 22 specimen polarized for 24 h at –0.14 VSCE and repassivated in deaerated pH 2, 1-M NaCl at 90°C.

FIGURE 26. SEM image of a PCA Alloy 22 specimen polarized for 3.5 h at 0.3 VSCE and repassivated in deaerated pH 2, 1-M NaCl at 90°C.



TABLE 1Relevant Data from Electrochemical Tests for Alloy 22 Prism Crevice Assemblies Specimens in Deaerated pH 2, 1-M NaCl at 90°C

Corroded Polarization EIS Starting Maximum Depth E (VSCE) Area (cm2) Hold Time (s) Time (s) of Attack (µm)

–0.2 — 36,000 — — –0.18 — 94,000 — — –0.17 0.30 82,700 77,400 <5 –0.14 0.30 87,100 88,900 <5 –0.1 0.43 17,500 15,500 <5 0.0 0.53 18,700 16,100 <5 0.1 0.45 17,800 15,700 20 0.2 0.60 15,900 14,700 70 0.3 0.67 12,400 8,900 200 0.4 0.53 12,400 8,900 260 0.5 0.64 12,400 8,900 250 0.6 0.75 12,400 8,900 180 0.7 0.69 12,600 9,000 170



TABLE 2Atomic Percent of Elements Detected by Energy-Dispersive X-Ray Analysis in Precipitates Found on Localized Attacked

Areas in Prism Crevice Assemblies Specimens in Deaerated pH 2, 1-M NaCl at 90°C

E (VSCE) Mo Si Cl Cr Mn Fe Ni W

–0.17 33.56 — 5.61 25.75 1.50 3.06 23.28 7.25 –0.14 38.67 — 8.96 22.47 1.49 — 17.07 11.34 –0.1 38.60 — 6.10 20.95 1.58 2.71 20.75 9.31 0.0 43.06 — 6.56 15.60 1.93 2.69 17.00 13.17 0.1 29.54 11.23 6.62 19.57 1.42 2.65 20.64 8.33 0.2 44.97 — 7.66 14.74 2.19 3.05 15.27 12.12 0.3 24.80 7.00 3.99 19.28 1.36 3.15 35.84 4.59 0.4 19.80 7.50 5.00 23.82 1.24 3.12 34.92 4.62 0.5 25.72 9.84 5.21 20.33 1.30 2.89 28.57 6.16 0.6 32.34 — 4.96 24.68 1.11 2.72 29.19 5.01 0.7 22.40 7.41 3.11 20.28 1.21 3.01 37.86 4.71

ACKNOWLEDGMENTS

R.M. Carranza acknowledges fi nancial support from the Agencia Nacional de Promoción Científi ca y Tecnológica of the Ministerio de Educación, Ciencia y Tecnología from Argentina, and from the Universidad Nacional de San Martín.

REFERENCES

1. R.B. Rebak, Corrosion and Environmental Degradation, vol. 2 (Weinheim, Germany: Wiley-VCH, 2000).

2. A.I. Asphahani, Arab. J. Sci. Eng. 14, 2 (1989): p. 317. 3. P.E. Manning, J.D. Smith, J.L. Nickerson, Mater. Perform. 27, 6

(1988): p. 67. 4. G.M. Gordon, Corrosion 58 (2002): p. 811. 5. R.B. Rebak, “Factors Affecting the Crevice Corrosion Susceptibil-

ity of Alloy 22,” CORROSION/2005, paper no. 05610 (Houston, TX: NACE, 2005).

6. B.A. Kehler, G.O. Ilevbare, J.C. Scully, Corrosion 57, 12 (2001): p. 1,042.

7. K.J. Evans, A. Yilmaz, S.D. Day, L.L. Wong, J.C. Estill, R.B. Rebak, J. Met. 57, 1 (2005): p. 56.

8. G.O. Ilevbare, Corrosion 62, 4 (2006): p. 340. 9. D.S. Dunn, Y.-M. Pan, L. Yang, G.A. Cragnolino, Corrosion 61, 11

(2005): p. 1,078. 10. A.K. Mishra, G.S. Frankel, Corrosion 64, 11 (2008): p. 863. 11. D.S. Dunn, Y.-M. Pan, K.T. Chiang, G.A. Cragnolino, X. He, J.

Met. 57, 1 (2005): p. 49.

12. R.M. Carranza, M.A. Rodríguez, R.B. Rebak, Corrosion 63, 5 (2007): p. 480.

13. R.M. Carranza, C.M. Giordano, M.A. Rodríguez, R.B. Rebak, “Ef-fect of the Addition of Organic Acids to the General and Crevice Corrosion Behavior of Alloy 22 in Chloride Solutions,” CORRO-SION/2008, paper no. 08578 (Houston, TX: NACE, 2008).

FIGURE 28. Crevice corrosion current density as a function of the applied potential for Alloy 22 in pH 2, 1-M NaCl at 90°C.

FIGURE 30. Diagram of crevice corrosion susceptibility and transpassive dissolution for Alloy 22 in 1-M chloride solutions at 90°C.

FIGURE 29. Crevice corrosion current density and incubation time as a function of the applied potential for Alloy 22 in pH 2, 1-M NaCl at 90°C.



14. X. Shan, J.H. Payer, “Comparison of Ceramic and Polymer Crev-ice Formers on the Crevice Corrosion Behavior of Ni-Cr-Mo Alloy C22,” CORROSION/2007, paper no. 07582 (Houston, TX: NACE, 2007).

15. X. He, D.S. Dunn, A.A. Csontos, Electrochim. Acta 52 (2007): p. 7,556.

16. K.J. Evans, R.B. Rebak, J. ASTM Int. 4, 9 (2007): paper ID JAI101230.

17. D.S. Dunn, Y.-M. Pan, L. Yang, G.A. Cragnolino, Corrosion 62, 1 (2006): p. 3.

18. R.M. Carranza, M.A. Rodríguez, R.B. Rebak, “Anodic and Ca-thodic Behavior of Mill-Annealed and Topologically Closed Packed Alloy 22, in Chloride Solutions,” CORROSION/2008, paper no. 08579 (Houston, TX: NACE, 2008).

19. B.A. Kehler, G.O. Ilevbare, J.R. Scully, “Crevice Corrosion Behav-ior of Ni-Cr-Mo Alloys: Comparison of Alloys 625 and 22,” in Proc. CORROSION/2001 Research Topical Symposium, Localized Corrosion, eds. G.S. Frankel, J.R. Scully (Houston, TX: NACE, 2001), p. 30.

20. X. He, D.S. Dunn, Corrosion 63, 1 (2007): p. 145. 21. B.A. Kehler, J.R. Scully, Corrosion 61, 7 (2005): p. 665. 22. A. Yilmaz, P. Pasupathi, R.B. Rebak, “Stifl ing of Crevice Corro-

sion in Alloy 22 During Constant Potential Tests,” paper

PVP2005-71174, ASME Pressure Vessels and Piping Conf., held July 17-21 (New York, NY: ASME International, 2005).

23. ASTM G5-94 (2004), “Standard Reference Test Method for Mak-ing Potentiostatic and Potentiodynamic Anodic Polarization Mea-surements” (West Conshohocken, PA: ASTM International, 2004).

24. ASTM G48-03 (2009), “Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Al-loys by Use of Ferric Chloride Solution” (West Conshohocken, PA: ASTM International, 2009).

25. ASTM G61-86 (2009), “Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys” (West Conshohocken, PA: ASTM International, 2009).

26. M.A. Rodríguez, “Corrosion Kinetics of Ni-22Cr-13Mo-3W Alloy as a Candidate Material for High-Level Nuclear Waste Containers” (M.Sc. thesis, Instituto de Tecnología “Prof. J. A. Sabato,” Univer-sidad Nacional de General San Martín, Argentina, 2004).

27. N. Sridhar, G.A. Cragnolino, Corrosion 49, 11 (1993): p. 885. 28. J.R. Galvele, J.B. Lumsden, R.W. Staehle, J. Electrochem. Soc.

125, 8 (1978): p. 1,204. 29. Z. Szklarska-Smialowska, Pitting and Crevice Corrosion of Metals

(Houston, TX: NACE, 2005). 30. J.R. Galvele, J. Electrochem. Soc. 123, 4 (1976): p. 464.

CORROSION RESEARCH CALENDAR

CORROSION accepts notices of calls for papers and upcoming research grants, meetings, symposia, and conferences. All pertinent information, including the date, time, location, and sponsor of an event should be submitted as far in advance as possible to: Suzanne Moreno, CORROSION Editorial Assistant, fax: +1 281-228-6359 or e-mail: [email protected].

2010

* January 18-21—NACE NPP Coatings Symposium—Orlando, FL; Contact Calae McDermott, Phone: +1 281-228-6263; E-mail: [email protected].

* January 18-22—NACE International/NUCC/ASTM Joint Meeting—Orlando, FL; Contact Laura Herrera, Phone: +1 281-228-6242; E-mail: [email protected].

January 19-21—Northeast Corrosion Conference (NECC) 2010—Springfi eld, MA; Contact Greg Hansen, Phone; +1 518-766-3611, ext. 2029.

February 9-10—10,000 Lakes Pipeline Corrosion Control Seminar—Prior Lake, MN; Contact Marty Iozzo, Phone: +1 763-559-5059; E-mail: [email protected].

February 10-11—Liberty Bell Corrosion Control—Horsham, PA; Contact David Krause, Phone: +1 484-467-5015; E-mail: [email protected].

February 14-17—13th Middle East Corrosion Conference and Exhibition—Manama, Bahrain; Contact Conference Secretariat, Phone: +973 17810733; E-mail: [email protected]; Web site: www.mecconline.org.

* February 15-18—Northern Area West-ern Conference—Calgary, AB, Canada; Contact Colin McGovern, E-mail: [email protected]; Web site: www.nacecalgary.ca/conference.htm.

February 17-18—Marine Coatings—London, UK; Contact The Royal Institution of Naval Architects, Web site: www.rina.org.uk/marinecoatings2010.

* March 14-18—CORROSION 2010 Con-ference and Expo—San Antonio, TX; Contact FirstService, NACE International, Phone: +1 281-228-6223; E-mail: fi [email protected]; Web site: www.nace.org.

* March 18—Corrosion Management Conference: Pipeline Corrosion—San Antonio, TX; Contact Calae McDermott, Phone: +1 281-228-6263; E-mail: [email protected].

April 8-9—2010 FHWA Bridge Engi-neering Conference—Orlando, FL; Contact Atorod Azizinamini, E-mail: [email protected].

May 4-6—44th Annual Western States Corrosion Seminar—Pomona, CA; Contact Sylvia Hall, Phone: +1 323 564-6626; E-mail: [email protected].

* May 20—Corrosion Control by Coat-ings: The Series - Immersion Coatings—Houston, TX; Contact Laura Herrera, Phone: +1 281-228-6242; E-mail: [email protected].

July 24-25—Gordon Research Con-ference and Seminar on Aqueous Corrosion, 2010—New London, NH; Contact Harry Tsaprailis, Phone: +1 614-761-6919; E-mail: [email protected]; Web site: www.grc.org/programs.aspx?year=2010&program=grs_corraq.

September 13-17—EUROCORR 2010—Moscow, Russia; Web site: www.eurocorr.org.

* September 19-24—Corrosion Tech-nology Week 2010—Orlando, FL; Contact Calae McDermott, Phone: +1 281-228-6263; E-mail: [email protected].

* October 12-15—Eastern Area Confer-ence—Washington, DC; Contact Laura Herrera, Phone: +1 281-228-6242; E-mail: [email protected].

* October 17-21—MS&T (Materials Sci-ence and Technology) Conference—Houston, TX; Co-sponsored by ASM International, American Ceramic Society, Association for Iron & Steel Technology, and The Minerals, Metals & Materials Society; Web site: www.ihs.com/Events/index.htm.

* October 27-29—North American In-dustrial Coating Show—Indianapolis, IN; Contact Laura Herrera, Phone: +1 281-228-6242; E-mail: [email protected].* Sponsored or cosponsored by NACE

International.

Effect of Potential on Crevice Corrosion Kinetics of Alloy 22

Documents

Transcript of Effect of Potential on Crevice Corrosion Kinetics of Alloy 22