AN ELECTROCHEMICAL STUDY ON PASSIVATION OF 

ALLOY 22 IN CHLORIDE-CONTAINING SOLUTIONS 

WITH AND WITHOUT SULFUR 

Prepared for 

U.S. Nuclear Regulatory Commission 

Contract NRC–02–07–006 

Prepared by 

H. Jung1

L. Yang1

T. Ahn2

1

Center for Nuclear Waste Regulatory Analyses 

San Antonio, Texas 

2

U.S. Nuclear Regulatory Commission 

Washington, DC 

September 2011
ii

ABSTRACT 

This report documents the results of a previously unpublished set of experiments that evaluated 

the effects of sulfur on the passivation behavior of Alloy 22 (Ni-22Cr-13Mo-4Fe-3W) in 

chloride-containing solutions with and without sulfur species at low temperatures {e.g., less than 

100 °C [212 °F]}. In earlier studies, an increase in the anodic current had been observed for 

Alloy 22 exposed to chloride solutions containing sulfur. The purpose of the additional 

experiments reported here was to use electrochemical methods to evaluate the hypothesis that 

the observed increase in anodic current was due to the oxidation of sulfur species dissolved in 

the test solution. 

The results in this study showed more than an order of magnitude increase in the anodic current 

when 0.01 M Na2S was added to either 0.5 M NaCl solution or simulated concentrated water at 

20, 50 and 95 °C [72, 140, and 203 °F]. The use of an electrode made of platinum, an inert 

metal, as a reference indicated that the observed increase was mainly due to the oxidation of 

sulfur species in solution and did not result from the active dissolution of Alloy 22. The Alloy 22 

specimens passivated in the presence of the sulfur-containing solutions, and the passive film on 

Alloy 22 was not compromised due to the presence of sulfur. There was no observable 

evidence of enhanced passive dissolution (i.e., general corrosion rate) of Alloy 22 due to sulfur. 

If there was a mechanism to enhance passive dissolution, the effect should be minimal. 

The information obtained in this study has resulted in an improved understanding of the 

corrosion behavior of Alloy 22 as an engineered material in various low-temperature 

geochemical environments characterized by the presence of aqueous species of sulfur and 

chloride. High concentrations of alloying elements (i.e., more than 22 wt% of chromium and 13 

wt% of molybdenum) in Alloy 22 keep the accumulation of sulfur on the alloy surface at a low 

enough concentration to maintain the passive film stability. 
iii

CONTENTS 

Section Page 

ABSTRACT ................................................................................................................................... ii 

FIGURES ..................................................................................................................................... iv 

TABLES ........................................................................................................................................ v 

ACKNOWLEDGMENTS .............................................................................................................. vi 

1 INTRODUCTION ............................................................................................................ 1-1

 1.1 Background Information ..................................................................................... 1-1 

 1.2 Objective and Scope .......................................................................................... 1-2 

2 EXPERIMENTAL DETAILS ........................................................................................... 2-1 

 2.1 Materials ............................................................................................................. 2-1 

 2.2 Test Solutions ..................................................................................................... 2-1 

 2.3 Test Procedures ................................................................................................. 2-1 

3 RESULTS AND DISCUSSION ....................................................................................... 3-1 

3.1 Passivation Behavior of Alloy 22 in 0.5 M NaCl Solution With and 

 Without Sulfur Addition ....................................................................................... 3-1 

3.2 Effect of Sulfide Oxidation on Anodic Current of Platinum ................................. 3-3 

3.3 Comparison of Polarization Curves of Alloy 22 and Platinum ........................... 3-5 

3.4 Passivation Behavior of Alloy 22 in Simulated Concentrated Water With 

and Without Sulfur Addition ................................................................................ 3-6 

3.5 Thermodynamic Stability of Sulfur-Metal (Ni, Cr, Mo)-Water Systems 

Below 100 °C [203 °F] ....................................................................................... 4-1 

4 SUMMARY ..................................................................................................................... 5-1 

5 CONCLUSIONS ............................................................................................................. 6-1

6 REFERENCES ................................................................................................................ 14 

APPENDIX A — IMPEDANCE SPECTRA OF ALLOY 22 MEASURED AT 

OPEN CIRCUIT POTENTIAL AT 60 AND 95 °C 

 [140 AND 103 °F] 

APPENDIX B — POTENTIAL-PH DIAGRAMS FOR METALS (Ni, Cr, 

 and Mo)-S-H2O SYSTEM AT 25 AND 90 °C 

 [77 AND 194 °F] 
iv 

FIGURES 

Figure Page 

1-1 Potentiostatic Polarization Curves at Various Potentials in Deaerated 

 0.5 M NaCl Solution at 22 °C [72 °F] (a) Without Sulfur and (b) With 

 0.01 M Na2S ................................................................................................................. 1-3 

3-1 Polarization Curves of the Rotating Alloy 22 Electrodes Tested in Aerated 

 0.5 M NaCl Solution With and Without 0.01 M Na2S Addition at 60 and 95 °C [140 and 

203 °F] (Scan Rate of 0.2 mV/s) .................................................................................... 3-2 

3-2 Cyclic Polarization Curve of the Alloy 22 Electrode Without Rotation in 0.5 M NaCl 

Solution Containing 0.01 M Na2S at 60 °C [140 °F] (Scan Rate of 

 0.5 mV/s) ........................................................................................................................ 3-2 

3-3 Optical Micrograph of the Alloy 22 Surface After Cyclic Polarization Test in 

 0.5 M NaCl Solution Containing 0.01 M Na2S at 60 °C [140 °F] .................................... 3-3 

3-4 Polarization Behaviors of Platinum Electrode in 0.5 M NaCl Solution With and 

 Without 0.01 M Na2S Addition at 60 and 95 °C [140 and 203 °F] (Scan Rate of 

 0.2 mV/s) ........................................................................................................................ 3-4 

3-5 Comparison of Polarization Curves on Alloy 22 and Platinum Electrodes in 

 0.5 M NaCl Solution Containing 0.01 M Na2S at 60 and 95 °C [140 and 

 203 °F] ............................................................................................................................ 3-5 

3-6 Polarization Curves of the Rotating Alloy 22 Electrodes in Simulated Concentrated 

Water (a) Without and (b) With 0.01 M Na2S Addition at 22, 60, and 95 °C [72, 140, and 

203 °F] (Scan Rate of 0.2 mV/s) .................................................................................... 3-7 
v 

TABLES

Table Page 

2-1 Chemical Composition of Mill-Annealed Alloy 22 (in Weight Percent) ........................... 2-1 

2-2 Chemical Composition of Simulated Concentrated Water in This Study ....................... 2-1 
vi 

ACKNOWLEDGMENTS

This report describes work performed by the Center for Nuclear Waste Regulatory 

Analyses (CNWRA) and its contractors for the U.S. Nuclear Regulatory Commission (USNRC) 

under Contract No. NRC–02–07–006. The activities reported here were performed on behalf of 

the NRC Office of Nuclear Material Safety and Safeguards, Division of High-Level Waste 

Repository Safety. This report is an independent product of CNWRA and does not necessarily 

reflect the view or regulatory position of USNRC. The NRC staff views expressed herein are 

preliminary and do not constitute a final judgment or determination of the matters addressed or 

of the acceptability of any licensing action that may be under consideration at USNRC. 

The authors gratefully acknowledge T. Mintz for his technical review, P. Mackin for 

his programmatic review, L. Mulverhill for her editorial review, and L. Naukam for her 

administrative support. 

QUALITY OF DATA, ANALYSES, AND CODE DEVELOPMENT 

DATA: All CNWRA-generated original data contained in this report meet the quality assurance 

requirements described in the Geosciences and Engineering Division Quality Assurance 

Manual. Sources for other data should be consulted for determining the level of quality for 

those data. Computational calculations have been recorded in CNWRA Scientific Notebook 900 

(Jung and Ellis, 2008). 

Reference 

Jung, H. and C. Ellis. "Effects of Sulfur on Passivation and Repassivation of Alloy 22 Using 

Scratch Test." Scientific Notebook No. 900. San Antonio, Texas: CNWRA. pp. 48–53 

and 56–71. 2008. 
1-1

1 INTRODUCTION 

This report is part of the knowledge management activities for the U.S. Nuclear Regulatory 

Commission (NRC) high-level waste repository safety program. The report provides previously 

undocumented results from recent corrosion tests that used electrochemical methods and 

thermodynamic computations to evaluate the potential for enhanced passive dissolution 

(i.e., general corrosion rate) of Alloy 22 (Ni-22Cr-13Mo-4Fe-3W) in the presence of sulfur. 

1.1 Background Information 

The U.S. Department of Energy (DOE) identified the long lifetime of Alloy 22 waste packages as 

an important attribute for isolating nuclear waste in a potential high-level waste repository at 

Yucca Mountain, Nevada, because corrosion of the waste package is considered to be an 

important potential degradation process under nominal repository conditions (DOE, 2002). The 

corrosion behavior of Alloy 22 has been the topic of many NRC/CNWRA investigations during 

the pre-licensing and licensing phases of the Yucca Mountain repository program (e.g., Jung, et 

al., 2009, 2007; Ahn, et al., 2008; Dunn, et al., 2005, 1998; Pensado, et al., 2002; Cragnolino 

and Sridhar, 1991). 

In the absence of environments leading to localized corrosion, Alloy 22 is expected to corrode 

uniformly (i.e., general corrosion) in an aqueous environment with ow corrosion rates under the 

potential repository conditions (Jung, et al., 2009, 2007; Ahn et al., 2008; Dunn, et al., 2005; 

Pensado, et al., 2002). The low corrosion rates are attributed to the presence of protective 

passive oxide film on the alloy surface. 

General corrosion rates could increase substantially, however, if the passive film became 

unstable, which would depend on material alteration states, such as alloy chemistry, structure, 

and exposure conditions during the disposal period. Such unstable passive film on Alloy 22 

could come from sulfur-induced corrosion (Ahn, et al., 2008; Yang, et al., 2004). Sulfur species 

present either in the alloy or in the environment could have a detrimental effect on the corrosion 

resistance of Alloy 22 by enhancing localized corrosion (i.e., pitting corrosion) susceptibility or 

general corrosion rate. 

Previously, Jung, et al. (2009) evaluated localized corrosion susceptibility of Alloy 22 due to 

sulfur-induced corrosion. Specifically, sulfur segregation by anodic dissolution of Alloy 22 and 

the resultant breakdown of the passive film were evaluated using two techniques: the scratch 

repassivation technique and the electrochemically accelerated dissolution process. For scratch 

repassivation tests, the passive film on Alloy 22 was mechanically removed by scratching the 

alloy surface in sulfur-containing solutions, and the resultant repassivation rates of Alloy 22 

were measured by recording current changes with time. The results showed that Alloy 22 

repassivated within a few seconds and the anodic current measured after scratching returned to 

its prescratch level. The results from the accelerated dissolution tests indicated that sulfur 

segregation could not affect the long-term stability of the passive film on Alloy 22. Based on test 

observations, Jung et al. (2009) concluded that sulfur segregation might not have any 

significant detrimental effect on the long-term passive film stability of Alloy 22. 

Another aspect of sulfur-induced corrosion could be a potential enhancement of the general 

corrosion rate of Alloy 22. In the presence of sulfur during the scratch repassivation tests, a 

relatively high anodic current was observed compared to in the absence of sulfur during the 

potentiostatic polarization before and after a scratch. An example is shown in Figure 1-1. 

Based on the literature information on inert materials' electrochemical reactions to platinum 

(Al-Kharafi, et al., 2010), gold (Zhang and Nicol, 2003), and graphite (Ateya and Al-Kharafi, 

2002), the increase of anodic current was considered to be caused by oxidation of sulfur 

species (mainly hydrogen sulfide) dissolved in the solution. In Al-Kharafi, et al. (2010), the 
1-2

anodic current measured on the platinum electrode in the absence of sulfide ions was negligible 

compared to that measured in the presence of sulfide ions (0.15 M HS-

) in 3.5 percent NaCl 

solutions at 25 °C [77 °F]. Al-Kharafi, et al. (2010) concluded that an increase in anodic current 

resulted from sulfide ion oxidation. Therefore, it is possible that the increase in the anodic 

current observed in the scratch repassivation tests in Alloy 22 could be due to oxidation of sulfur 

species (i.e., mainly hydrogen sulfide) present in the solution. 

Sulfate reducing bacteria (SRB) are also a potential source for corrosive sulfur species 

(i.e., hydrogen sulfide), as metabolites in the proposed Yucca Mountain, Nevada, repository 

environment that could contact the surface of Alloy 22 waste packages (Yang, et al., 2004; 

Yang and Cragnolino, 2004). Yang, et al. (2004) evaluated microbially influenced corrosion—

specifically, the potential detrimental effect of SRB to induce localized corrosion for Alloy 22. 

Yang et al. (2004) also observed an increase in anodic current on the polarization curves for 

platinum and stainless steel when SRB or sulfide was present in the solution. The increase was 

believed to be caused by the oxidation of adsorbed sulfur species on the surfaces of platinum 

and stainless steel. However, the potential effect of sulfur on the anodic current of Alloy 22 was 

not confirmed by experiments. 

1.2 Objective and Scope 

The objective of the present work is to evaluate whether sulfur species oxidation affects the 

anodic current of Alloy 22 at low temperatures {e.g., below 100 °C [212 °F]}. In addition, the 

thermodynamic stability of metal sulfides is examined by utilizing the potential-pH diagrams for 

nickel-, chromium-, and molybdenum-sulfur-water systems at 25 and 90 °C [77 and 140 °F]. 

Finally, the effect of sulfur on the general corrosion rate of Alloy 22 is assessed. 

To evaluate the effects of oxidation of sulfur species on the anodic current of Alloy 22, corrosion 

tests were conducted using electrochemical methods in the chloride-containing solutions with 

and without sulfur at 22, 60, and 95 °C [72, 140, and 203 °F]. Platinum was used as a 

reference for comparison of polarization curves for Alloy 22. 
1-3

(a) 

(b) 

Figure 1-1. Potentiostatic Polarization Curves at Various Potentials in Deaerated 

0.5 M NaCl Solution at 22 °C [72 °F] (a) Without Sulfur and (b) With 0.01 M Na2S 

(Jung, et al., 2009) 
2-1 

2 EXPERIMENTAL DETAILS 

2.1 Materials 

The working electrodes for the tests were either cylindrical Alloy 22 or platinum flag samples 

(purity of 99.95 percent). Alloy 22 samples were machined from a thick plate with a diameter of 

12.075 mm [0.48 in] and a height of 10.16 mm [0.4 in]. Alloy 22 samples were polished to a 

600 grit finish, cleaned with ethanol, and dried. The chemical composition of mill-annealed 

Alloy 22 (HT 2277–3–3266) is shown in Table 2-1. 

2.2 Test Solutions 

Two test solutions (0.5 M NaCl and simulated concentrated water, the composition of which is 

shown in Table 2-2) containing 0 and 0.01 M Na2S were prepared from analytical-grade 

chemicals and deionized water. The initial pH of 0.5 M NaCl solutions without sulfur addition 

was approximately 6.7, while the initial pH of 0.5 M NaCl solutions with sulfur addition was 

approximately 10. The 0.5 M NaCl solutions containing 0.01M Na2S were adjusted to an initial 

pH of 7 with dilute HCl solution. The test solution was aerated by opening it to air. The test was 

conducted at the solution temperatures of 22, 60, and 95 °C [72, 140, and 203 °F]. 

2.3 Test Procedures 

Tests were conducted in a 1-liter [0.22 gal] glass cell. The saturated calomel, used as a 

reference electrode, was connected to the solution through a Lugging probe with a porous silica 

tip. A platinum mesh was used as a counter electrode. The exposed surface areas of the 

working electrodes for Alloy 22 and platinum flag samples were 3.851 and 10 cm2

 [0.597 and 

1.55 in2

], respectively. Polarization tests were made using an electrochemical interface and 

frequency response analyzer controlled by a personal computer. Unless otherwise indicated, 

potentiodynamic measurements were performed at the potential scan rate of 0.2 mV/s. During 

the tests, the test samples were either rotated at a speed of 60 rpm or not rotated at all. 

Impedance measurements were carried out in the frequency range of 100 kHz to 10 mHz with 

an alternating current voltage amplitude of 10 mV at the open circuit potential. The potentials, 

except the potential-pH diagrams, were measured; potentials are reported in this report against 

a saturated calomel reference electrode (SCE) via the conversion 

E (VSCE) = E (VSHE) -0.242V (2-1)

Table 2-1. Chemical Composition of Mill-Annealed Alloy 22 (in Weight Percent) 

Ni* Cr* Mo* W* Fe* Co* Si* Mn* V* P* S* C* 

Bal† 22.15 12.90 2.81 3.82 1.37 0.01 0.30 0.15 0.012 0.002 0.004 

*Ni—nitrogen; Cr—chromium; Mo—molybdenum; W—tungsten; Fe—iron; Co—cobalt; Si—silicon; 

Mn—manganese; V—vanadium; P—phosphorous; S—sulfur; C—carbon 

†Bal—balance

Table 2-2. Chemical Composition of Simulated Concentrated Water in This Study 

Ion K+ Na+ Mg2+ Ca2+ F- Cl− NO3

−

 SO4

2−

 HCO3

−

mg/L 3,400 40,900 <1 <1 1,400 6,700 6,400 16,700 70,000 
3-1 

3 RESULTS AND DISCUSSION 

This section summarizes and discusses the Alloy 22 experiment results. The polarization 

behaviors of Alloy 22 both in the presence and absence of sulfur in the test solutions 

(i.e., 0.5 M NaCl or simulated concentrated water) are evaluated, and the results are compared 

to those of platinum. Finally, the potential effect of sulfur-induced corrosion on the general 

corrosion rate of Alloy 22 is assessed. 

High anodic currents for Alloy 22 (Jung, et al., 2009) and stainless steel (Yang, et al., 2004) were 

observed when sulfur species were present in the solution. This increase was considered to be 

caused by the oxidation of the sulfur species present in the solution, rather than active dissolution 

of Alloy 22 or stainless steel. To confirm this hypothesis, experiments were conducted on Alloy 

22 using electrochemical methods. 

3.1 Passivation Behavior of Alloy 22 in 0.5 M NaCl Solution With and 

Without Sulfur Addition 

Figure 3-1 shows the polarization behavior of the rotating Alloy 22 samples in aerated 0.5 NaCl 

solution with and without 0.01 M Na2S addition at 60 and 95 °C [140 and 203 °F]. In the 

absence of Na2S, Alloy 22 showed a passive behavior at both 60 and 95 °C [140 and 203 °F] up 

to about 0.4 VSCE. The corrosion current at 95 °C [203 °F] appears slightly higher than at 60 °C 

[140 °F], which was confirmed from the impedance spectra measured at the open circuit 

potential (Appendix A, Figure A–1). The polarization resistances of Alloy 22 samples were 241 

and 144 kohms at 60 and 95 °C [140 and 203 °F], respectively. The impedance spectra were 

fitted using a simple Randle’s circuit. 

When 0.01 M Na2S was added, the polarization curves shifted to a high anodic current range 

and Alloy 22 appeared to be depassivated. The anodic currents increased more than an order 

of magnitude from the results in the solution without sulfur. This increase could be due to the 

enhanced anodic dissolution of Alloy 22 with the addition of Na2S and/or the anodic oxidation of 

the sulfur species dissolved in the solution, which could have contributed to the resultant anodic 

current observed. 

Because it was not clear whether the passivation of Alloy 22 was preserved when Na2S was 

added (as shown in Figure 3-1), a cyclic polarization scan was conducted to check for the 

presence of passivation. Figure 3-2 shows the cyclic polarization behavior of Alloy 22 without 

rotation in aerated 0.5 NaCl solution with 0.01 M Na2S addition at 60 °C [140 °F]. The anodic 

current obtained during the reversed scan was lower than that obtained during the forward scan, 

indicating that Alloy 22 was passivated and not actively corroded by the high anodic current 

during the forward scan up to 0.75 VSCE. If Alloy 22 had been actively dissolved, the anodic 

current obtained during the reversed scan should have been higher than the current obtained 

during the forward scan. Other investigators (Cragnolino, 2008) reported similar behavior 

showing higher current. When active dissolution or pitting occurs during the forward scan, the 

anodic current obtained during the reversed scan is usually higher than the value obtained 

during the forward scan at potentials above the repassivation potential. Figure 3-3 shows an 
3-2 

Figure 3-1. Polarization Curves of the Rotating Alloy 22 Electrodes 

Tested in Aerated 0.5 M NaCl Solution With and Without 0.01 M Na2S Addition at 

60 and 95 °C [140 and 203 °F] (Scan Rate of 0.2 mV/s) 

Figure 3-2. Cyclic Polarization Curve of the Alloy 22 Electrode Without Rotation in 

0.5 M NaCl Solution Containing 0.01 M Na2S at 60 °C [140 °F] (Scan Rate of 0.5 mV/s) 
3-3 

Figure 3-3. Optical Micrograph of the Alloy 22 Surface After Cyclic Polarization Test 

in 0.5 M NaCl Solution Containing 0.01 M Na2S at 60 °C [140 °F] 

optical micrograph of the general surface of Alloy 22 after a cyclic polarization scan. As seen in 

the figure, the surface is bright and polishing marks from a 600-grit finish are still present. If 

there had been active dissolution, the surface would have been rough and polishing marks 

would have been much less prominent or eliminated. No active dissolution or pitting corrosion 

on Alloy 22 was evident. 

3.2 Effect of Sulfide Oxidation on Anodic Current of Platinum 

Platinum was used as a reference to compare the polarization behaviors of Alloy 22 and to 

evaluate the net effect of sulfur species on the anodic current. Because platinum is a noble 

metal and inert to corrosion, it is reasonable to assume that the current measured during the 

polarization of platinum is from electrochemical reactions of dissolved sulfur species on the 

surface of platinum, not from the dissolution of platinum. Figure 3-4 shows the polarization 

curves on platinum without rotation obtained with and without 0.01 M Na2S addition to 

0.5 M NaCl solution. Similar to Alloy 22 in the same solutions, the anodic current obtained in 

the presence of Na2S is approximately two orders of magnitude higher than the anodic current 

obtained in the absence of Na2S. Because platinum is expected to be electrochemically inert 

in the NaCl and Na2S test solutions, the enhanced anodic current should be a result of 

sulfide oxidation. 

Al-Kharafi, et al. (2010) also observed an increase in the anodic current of platinum 

when sulfide was present in the solution at 25 °C [77 °F]. Al-Kharafi, et al. (2010) showed that 

the anodic current of platinum was low when it was polarized in a 3.5 wt% NaCl solution at room 

temperature, but the anodic current increased when 0.15 M Na2S was added. The current 
3-4 

Figure 3-4. Polarization Behaviors of Platinum Electrode in 0.5 M NaCl Solution With 

and Without 0.01 M Na2S Addition at 60 and 95 °C [140 and 203 °F] 

(Scan Rate of 0.2 mV/s) 

density near 0.0 VAg/AgCl

1

 was approximately 4 × 10−1 A/cm2 [2.6 A/ft2] with Na2S addition at the 

potential scan rate of 10 mV/s. Al-Kharafi et al. (2010) concluded that the high anodic current in 

the presence of Na2S was due to the oxidation of the sulfide ions. In addition, three anodic 

current peaks during the forward scan were observed at the potentials of -0.1, 0.475, and 

1.0 VAg/AgCl. These three peaks were attributed to (i) the formation of platinum sulfide and 

polysulfide; (ii) the deposition of elemental sulfur; and (iii) the oxidation of the deposited sulfur to 

form soluble sulfate ions (SO4

2-). 

In the oil and gas industry, hydrogen sulfide and associated sulfide ions have been shown to 

contaminate geothermal brines due to their toxicity and corrosiveness to the metal structures 

(Tuttle and Kane, 1981). One way hydrogen sulfide can be removed from the aqueous stream 

is by an electrochemical oxidation method: inert materials such as platinum, graphite, or gold 

are used as the working electrode to convert sulfide to elemental sulfur or sulfate, which is 

environmentally benign. Several electrochemical reactions can occur during sulfur species 

oxidation (Al-Kharafi, et al., 2010; Ateya and Al-Kharafi, 2002), including 

HS-

 + OH- = S + H2O + 2e- (3-1)

S2- = S + 2e-

 (3-2)

2HS-

 + 2OH- = S2

2- + 2H2O + 2e-

 (3-3)

HS-

 + 9OH- = SO4

2- + 5H2O + 8e-

 (3-4)

 

1

 Note: E (V, SCE) = E (V, Ag/AgCl) + 0.045V 
3-5 

In a neutral range of the test solutions for this study (about pH 7), hydrogen sulfide (HS-) ion 

could be a thermodynamically stable species (Langmuir, 1997). These ions have been 

recognized as the reducing agents and can be readily oxidized by O2, Cl2, and H2O2 (Weil and 

Sandler, 1991). Therefore, upon anodic oxidation, hydrogen sulfide can oxidize to elemental 

sulfur (S), polysulfide (S2

2−

), or sulfate (SO4

2−

) depending on the applied potential and pH, as 

seen in Appendix B, Figures B–1 through B–3. 

3.3 Comparison of Polarization Curves of Alloy 22 and Platinum 

Figure 3-5 shows the comparison of the polarization curves of the platinum electrodes shown in 

Figure 3-4 and the Alloy 22 electrodes shown in Figure 3-1 when 0.01 M Na2S was added to the 

0.5 M NaCl solution at 60 and 95 °C [140 and 203 °F]. The anodic polarization curves for these 

two types of electrodes have approximately the same shape and absolute value of anodic 

current between 0 and 0.6 VSCE. Because Alloy 22 was rotated at 60 rpm, while the platinum 

electrode was not rotated (i.e., 0 rpm), the anodic currents of Alloy 22 obtained with rotation 

should be higher than those obtained without rotation. This rotation-speed-base difference 

could be attributed to a different degree of diffusion-controlled process, resulting in different 

limiting currents. Similarly, the anodic current of platinum would be expected to be higher with 

rotation. Ateya and Al-Kharafi (2002) reported that the oxidation of sulfide on the surface of a 

graphite electrode was diffusion controlled in 0.58 M NaCl with 0.001 or 0.005 M Na2S. When 

the solution was stirred, the anodic-limiting current increased at least one order of magnitude. 

Again, because platinum is a noble metal and inert to corrosion, it is reasonable to assume that 

the current measured during platinum polarization should be due to the results of electrochemical 

reactions of dissolved sulfur species (i.e., sulfide oxidation) on the surface of platinum, not from 

platinum dissolution. Any contribution from platinum dissolution should be negligible, as 

Al-Kharafi, et al., (2010) observed. 

Figure 3-5. Comparison of Polarization Curves on Alloy 22 and Platinum Electrodes 

in 0.5 M NaCl Solution Containing 0.01 M Na2S at 60 and 95 °C [140 and 203 °F] 
3-6 

Note that the corrosion potential measured for the Alloy 22 electrode was similar to that 

measured for the platinum electrode, indicating that Alloy 22 was not actively corroding in the 

presence of 0.01 M Na2S. 

3.4 Passivation Behavior of Alloy 22 in Simulated Concentrated 

Water With and Without Sulfur Addition 

Figure 3-6 shows the polarization curves of the rotating Alloy 22 electrodes in the simulated 

concentrated water with and without 0.01 M Na2S addition tested at 22, 60, and 95 °C [72, 140, 

and 203 °F]. In the absence of Na2S, both anodic and corrosion currents increased gradually as 

the temperature increased from 22 to 60 and 95 °C [72 to 140 and 203 °F], which is consistent 

with the previous polarization test results on Alloy 22 in the literature (Dunn, et al., 2005; 

Pensado, et al., 2002; LIoyd, et al., 2003). 

Similar to the polarization curves obtained in 0.5 M NaCl solution, the anodic currents in 

simulated concentrated water containing 0.01 M Na2S were more than an order of magnitude 

higher than the anodic current obtained in the absence of Na2S. Even if there is a difference in 

the solution chemistry between the 0.5 M NaCl solution and simulated concentrated water 

(e.g., pH of 7 for 0.5 M NaCl and pH of ~10 for simulated concentrated water), the higher anodic 

current is most likely due to the anodic oxidation of sulfur species, rather than from the 

dissolution of Alloy 22, according to the discussions of and findings for the 0.5 M NaCl solution 

in the previous sections. 

3.5 Thermodynamic Stability of Sulfur-Metal (Ni, Cr, Mo)-Water 

Systems Below 100 °C [203 °F] 

In the previous report (Jung, et al., 2007), several metal sulfides, including nickel and 

molybdenum sulfides, were identified as the stable phases at 25 and 90 °C [77 and 194 °F]. As 

seen in the thermodynamic stability diagrams for the nickel-sulfur-water system (Appendix B, 

Figure B–1), nickel can oxidize to form different phases of nickel sulfides (e.g., Ni3S2, NiS, and 

Ni3S4) in a wide range of potential and pH regions at both temperatures. These nickel sulfides 

can inhibit formation of a protective passive oxide film on Alloy 22. 

To the contrary, there are no stable phases of chromium sulfides, as seen in Appendix B, 

Figure B–2. Chromium can form chromium oxide (Cr2O3) rather than sulfides in a wide range of 

potentials and pHs. The superior stability of chromium oxide may be one reason Alloy 22 could 

repassivate quickly after scratching and could also maintain passivation when the polarization 

scan was reverse tested in the sulfur-containing solutions, as seen in Figure 3-3. 

In the case of molybdenum, the calculated stability diagrams at 25 and 90 °C [77 and 194 °F] 

(Appendix B, Figure B–3) indicate that there is a strong tendency to form metal molybdenum 

sulfides such as MoS2 and MoS3. However, molybdenum plays a beneficial role in passivation 

of nickel-molybdenum alloys (Marcus and Moscatelli, 1989) and stainless steel (Elbiache and 

Marcus, 1992) by reducing sulfur content on the alloy surface. Several mechanisms were 

proposed to reduce sulfur content, including removing sulfur as the dissolved forms of 

compounds (e.g., molybdenum sulfides or clusters) (Marcus and Moscatelli, 1989) or reducing 

the residence time of the adsorbed sulfur on the alloy surface by desorbing sulfur occupying the 

active sites present on the metal surface (Betts and Newman, 1993). 
3-7 

(a) 

(b) 

Figure 3-6. Polarization Curves of the Rotating Alloy 22 Electrodes in 

Simulated Concentrated Water (a) Without and (b) With 0.01 M Na2S Addition 

at 22, 60, and 95 °C [72, 140, and 203 °F] (Scan Rate of 0.2 mV/s) 
4-1 

4 SUMMARY 

To evaluate a potential effect of sulfur on the passivation and general corrosion rate of Alloy 22, 

electrochemical testing was conducted in the solutions containing sulfur species at low 

temperatures {e.g., below 100 °C [212 °F]}. Thermodynamic stability of metal sulfides was also 

examined utilizing the potential-pH diagrams for nickel-, chromium- and molybdenum-sulfurwater systems. The electrochemical testing results indicate that the increase in the anodic 

current on Alloy 22 was mainly due to oxidation of sulfur species, not the active dissolution of 

Alloy 22. The passive film of the Alloy 22 was not compromised by the presence of sulfur. 

Thermodynamic calculations predicted the formation of a stable chromium oxide (Cr2O3) on 

the alloy surface over a wide range of potentials and pH at low temperatures (Appendix B, 

Figure B–2). High concentrations of alloying elements (>22 wt% of chromium and 13 wt% of 

molybdenum) in Alloy 22 are believed to keep accumulated sulfur content at a low enough 

concentration on the Alloy 22 surface to maintain stability of the passive film on the alloy 

surface. As long as the beneficial roles of these alloying elements in Alloy 22 passivation were 

maintained, potential deleterious effects of sulfur present in the environment, if in existence, 

would be minimal. To further enhance our understanding and quantify any potential 

enhancement in the general corrosion rate of Alloy 22 from sulfur, weight loss measurements 

could be used to separate the net contribution of metal dissolution from the measured anodic 

current due to oxidation of sulfur species dissolved in the solution. 

The corrosion tests and thermodynamic calculations in the present study provide no observable 

evidence of sulfur affecting the passivation of Alloy 22 and increasing its general corrosion rate. 
5-1 

5 CONCLUSIONS 

In this report, the potential effect of sulfur presence on passivation of Alloy 22 at low 

temperatures {e.g., below 100 °C [212 °F]} was evaluated through scoping corrosion tests and 

thermodynamic computation. There are three main conclusions of the study. 

• Polarization test results showed that there was an increase greater than an order of 

magnitude in the anodic current on Alloy 22 when 0.01 M Na2S was added to 

either 0.5 M NaCl solution or simulated concentrated water at 22, 60, and 95 °C 

[72, 140, and 203 °F]. By utilizing platinum, it was found that this increase was mainly 

due to oxidation of sulfur species dissolved in the solution. The increase was not from 

the active dissolution of Alloy 22. 

• Even if a high concentration of sulfide (0.01 M Na2S) was present in the solution, 

Alloy 22 could passivate. The passive film stability was not compromised due to sulfur in 

the solution. No evidence was observed that indicated enhancement of passive 

dissolution (i.e., general corrosion rate) of Alloy 22 due to sulfur presence in the 

environment. If there was a mechanism to enhance passive dissolution, the effect 

should be minimal. 

• High concentrations of alloying elements (i.e., >22 wt% of chromium and 13 wt% of 

molybdenum) could contribute to keeping the accumulated sulfur content on the alloy 

surface low enough to maintain passive film stability of Alloy 22. As long as the 

beneficial roles of alloying elements are maintained, the passive film on Alloy 22 will not 

likely deteriorate due to the presence of sulfur. 
6-1 

6 REFERENCES 

Ahn, T., H. Jung, X. He, and O. Pensado. “Understanding Long-Term Corrosion of Alloy 22 

Container in the Potential Yucca Mountain Repository for High-Level Nuclear Waste Disposal.” 

Journal of Nuclear Materials. Vol. 379. pp. 33–41. 2008. 

Al-Kharafi, F.M., A.Y. Saad, B.G. Ateya, and I.M. Ghayad. “Electrochemical Oxidation of Sulfide 

Ions on Platinum Electrode.” Modern Applied Science. Vol. 4, No. 3. pp. 2–11. 2010. 

Ateya, B.G. and F.M. Al-Kharafi. “Anodic Oxidation of Sulfide Ions From Chloride Brines.” 

Electrochemistry Communications. Vol. 4. pp. 231–238. 2002. 

Betts, A.J. and R.C. Newman. “The Effects of Alloyed Molybdenum on the Activation of Anodic 

Dissolution by Reduced Sulfur Compounds.” Corrosion Science. Vol. 34. pp. 1,551–1,555. 

1993. 

Cragnolino, G. Corrosion Fundamentals and Characterization Techniques in Techniques for 

Corrosion Monitoring. L. Yang, ed. Abington, Cambridge, United Kingdom: Woodhead 

Publishing. 2008. 

Cragnolino, G. and N. Sridhar. “A Review of Localized Corrosion of High-Level Nuclear Waste 

Container Materials-I.” CNWRA 91-004. San Antonio, Texas: CNWRA. 1991. 

DOE. DOE/RW–0539–1, “Yucca Mountain Science and Engineering Report: Technical 

Information Supporting Site Recommendation Consideration.” Rev. 1. Las Vegas, Nevada: 

DOE, Office of Civilian Radioactive Waste Management. 2002. 

Dunn, D.S., O. Pensado, Y.-M. Pan, R.T. Pabalan, L. Yang, X. He, and K.T. Chiang. “Passive 

and Localized Corrosion of Alloy 22—Modeling and Experiments.” CNWRA 2005-02. Rev. 1. 

San Antonio, Texas: CNWRA. 2005. 

Dunn, D.S., Y. Pan, and G. Cragnolino. “Effects of Environmental Factors on the Aqueous 

Corrosion of High-Level Radioactive Waste Containers—Experimental Results and Models.” 

CNWRA 99-004. San Antonio, Texas: CNWRA. 1998. 

Elbiache, A. and P. Marcus. “The Role of Molybdenum in the Dissolution and the Passivation of 

Stainless Steels With Adsorbed Sulphur.” Corrosion Science. Vol. 33, No. 2. pp. 261–269. 

1992. 

Jung, H., T. Mintz, L. Yang, and T. Ahn. “Long-Term Persistence of the Passive Film on 

Alloy 22 at Elevated Temperature in Potential Yucca Mountain Repository Environments.” 

ML090220175 and ML090220186. Washington, DC: NRC. 2009. <www.nrc.gov/readingrm/ 

adams.html> (09 February 2009). 

Jung, H., T. Mintz, D.S. Dunn, O. Pensado, and T. Ahn. “A Review of the Long-Term 

Persistence of the Passive Film on Alloy 22 in Potential Yucca Mountain Repository 

Environments.” ML072880595. Washington, DC: NRC. 2007. <www.nrc.gov/readingrm/adams.html> (05 March 2008). 
6-2 

Langmuir, D. Aqueous Environmental Geochemistry. Upper Saddle River, New Jersey: 

Prentice-Hall, Inc. pp. 449. 1997. 

Lloyd, A.C., D.W. Shoesmith, N.S. McIntyre, and J.J. Noël. “Effects of Temperature and 

Potential on the Passive Corrosion Properties of Alloys C22 and C276.” Journal of the 

Electrochemical Society. Vol. 150. pp. B120–B130. 2003. 

Marcus, P. and M. Moscatelli. “The Role of Alloyed Molybdenum in the Dissolution and the 

Passivation of Nickel-Molybdenum Alloys in the Presence of Adsorbed Sulfur.” Journal of the 

Electrochemical Society. Vol. 136, No. 6. pp. 1,634–1,637. 1989. 

Pensado, O., D.S. Dunn, G.A. Cragnolino, and V. Jain. “Passive Dissolution of Container 

Materials—Modeling and Experiments.” CNWRA 2003-01. San Antonio, Texas: CNWRA. 

2002. 

Tuttle, R.N. and R.D. Kane, eds. “H2S Corrosion in Oil and Gas Production: A Compilation of 

Classic Papers.” Houston, Texas: National Association of Corrosion Engineers. 1981. 

Weil, E.D. and S.R. Sandler. “Sulfur Compounds.” Kirk-Othmer Encyclopedia of 

Chemical Technology. 4th Edition. Vol. 23. M. Howe-Grant, ed. New York City, New York: 

John Wiley & Sons. 1991. 

Yang, L. and G. Cragnolino. “Studies on the Corrosion Behavior of Stainless Steels in 

Chloride Solutions in the Presence of Sulfate Reducing Bacteria.” CORROSION/2004. 

Paper No. 04598. Houston, Texas: NACE International. 2004. 

Yang, L., S. Birnbaum, and G. Cragnolino. “Microbially Influenced Corrosion Studies on 

Engineered Barrier System Materials.” CNWRA 2005-01. San Antonio, Texas: CNWRA. 

2004. 

Zhang, S. and M.J. Nicol. “An Electrochemical Study of the Dissolution of Gold in Thiosulfate 

Solutions.” Journal of the Applied Electrochemistry. Vol. 33. pp. 767–775. 2003. 
APPENDIX A 

IMPEDANCE SPECTRA OF ALLOY 22 MEASURED AT 

OPEN CIRCUIT POTENTIAL AT 60 AND 95 °C [140 AND 103 °F] 
A–1 

0 50000 100000 150000 200000

-150000

-100000

-50000

0

Z'

Z''

Nyquist Plot

0.5 M NaCl at 60 oC

0.5 M NaCl at 95 oC

0.5 M NaCl with 0.01 M Na2S at 60 oC

0.5 M NaCl with 0.01 M Na2S at 95 oC

(Ohms)

(Ohms)

(a) Nyquist Plot 

(Ohms)

10-2 10-1 100 101 102 103 104 105 103

104

105

106

Frequency (Hz)

|Z|

Bode Plot

0.5 M NaCl at 60 oC

0.5 M NaCl at 95 oC

0.5 M NaCl with 0.01 M Na2S at 60 oC

0.5 M NaCl with 0.01 M Na2S at 95 oC

10-2 10-1 100 101 102 103 104 105

-75

-50

-25

0

Frequency (Hz)

theta (Degree)

(b) Bode Plot 

Figure A–1. Impedance Spectra of Alloy 22 Measured at Open Circuit 

Potential in 0.5 M NaCl With and Without 0.01 M Na2S Addition at 

60 and 95 °C [140 and 203 °F]; (a) Nyquist Plot, (b) Bode Plot 

IMPEDANCE SPECTRA OF ALLOY 22 MEASURED AT 

OPEN CIRCUIT POTENTIAL AT 60 AND 95 °C [140 AND 103 °F]
APPENDIX B 

POTENTIAL-PH DIAGRAMS FOR METALS (NI, CR, AND MO)-S-H2O 

SYSTEM AT 25 AND 90 °C [77 AND 194 °F] 
B–1 

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 

E(VSHE)

pH

O2 Evolution

H2 Evolution

HSO4

-

SO4

2-

S(s)

H2S(a) HSS-2

Ni2+

Ni

Ni3S2

NiO Ni(OH)3

-

Ni3S4

NiS

(a) 

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 

E(VSHE)

pH

O2 Evolution

H2 Evolution

HSO4

- SO42-

S(s)

H2S(a)

HSS-2

Ni2+

Ni

Ni3S2

NiO

Ni(OH)3

-

Ni3S4

NiS

(b) 

Figure B–1. Potential-pH Diagram for Ni-S-H2O System at (a) 25 °C [77 °F] 

and (b) 90 °C [194 °F] for 10−6

 M of Ions (Jung, et al., 2007) 

POTENTIAL-PH DIAGRAMS FOR METALS (NI, CR, AND 

MO)-S-H2OSYSTEM AT 25 AND 90 °C [77 AND 194 °F]
B–2 

-2.0

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 

E(VSHE)

pH

O2 Evolution

H2 Evolution

HSO4

-

SO4

2-

S(s)

H2S(a) HSS2-

Cr3+

Cr

Cr2O3

CrO4

2-

Cr2+

HCrO4

-

(a) 

-2.0

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 

E(VSHE)

pH

O2 Evolution

H2 Evolution

HSO4

-

SO4

2-

S(s)

H2S(a) HSS2-

Cr3+

Cr

Cr2O3

CrO4

2-

Cr2+

HCrO4

-

(b) 

Figure B–2. Potential-pH Diagram for Cr-S-H2O System at (a) 25 °C [77 °F] 

and (b) 90 °C [194 °F] for 10−6

 M of Ions (Jung, et al., 2007) 
B–3 

-2.0

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 

E(VSHE)

pH

O2 Evolution

H2 Evolution

HSO4

-

SO4

2-

S(s)

H2S(a) HS- S-2

HMoO4

-

Mo

MoS2

MoO4

2-

MoS3

MoO2

(a) 

-2.0

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 

E(VSHE)

pH

O2 Evolution

H2 Evolution

HSO4

-

SO4

2-

S(s)

H2S(a) HSS-2

HMoO4

-

Mo

MoS2

MoO4

2-

MoS3

MoO2

(b) 

Figure B–3. Potential-pH Diagram for Mo-S-H2O System at (a) 25 °C [77 °F] 

and (b) 90 °C [194 °F] for 10−6

 M of Ions (Jung, et al., 2007) 
B–4 

Reference

Jung, H., T. Mintz, D.S. Dunn, O. Pensado, and T. Ahn. “A Review of the Long-Term 

Persistence of the Passive Film on Alloy 22 in Potential Yucca Mountain Repository 

Environments.” ML072880595. Washington, DC: NRC. 2007. <www.nrc.gov/readingrm/adams.html> (05 March 2008).