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Quantitative Characterization of Alloy 22 Crevice Corrosion Damage Profiles (Paper) 

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NRC FORM 390A (12-2003) PRINTED ON RECYCLED PAPER 
1

QUANTITATIVE CHARACTERIZATION OF ALLOY 22 CREVICE CORROSION 

DAMAGE PROFILES 

Xihua He and Todd Mintz 

Center for Nuclear Waste Regulatory Analyses 

Southwest Research Institute®

6220 Culebra Road 

San Antonio, Texas 78238 

E-mail: xhe@swri.org 

Telephone: (210) 522-5194 

ABSTRACT 

Three- and two-dimensional (3-D and 2-D) laser profilometry scans were performed on two sets 

of Alloy 22 crevice corrosion specimens to accurately and quantitatively characterize the 

corrosion damage profiles. One set of specimens had multiple crevice contacts, and the other 

set had single crevice contact. Both sets of specimens showed that laser scans are more 

sensitive to variations in penetration sites, leading to larger maximum crevice corrosion 

penetration depths measured than those previously obtained by optical microscope. The laser 

scan measurements also captured the depth variation around grain boundaries. For Alloy 22, 

crevice corrosion attack may have initially followed metallurgical features such as grain 

boundaries. The deeper corrosion sites were about 200−500 μm [7.87−20.0 mil] away from the 

edge of the crevice. The central crevice region was uncorroded or less severely corroded, 

which can be explained by the potential drop in such a confined distance in the crevice region 

due to the tight crevice and resulting high resistivity along the current path. Most deep corrosion 

sites were steep and as small as 10 μm [0.39 mil] comparable to Alloy 22 grain sizes. 

Keywords: Alloy 22, crevice corrosion, corrosion profile, penetration depth 

INTRODUCTION 

Localized corrosion is one of the most insidious forms of metal failure, because when it occurs, 

it penetrates at a faster rate than general corrosion. This is especially true for nickel-based 

alloys such as Alloy 22 (Ni-22Cr-13Mo-4Fe-3W [UNS N06022]) that are highly resistant to 

uniform corrosion as a result of passive oxide film formation on the metal surface. Localized 

corrosion may also result in initiation sites for possible mechanical damage or failure. For 

Alloy 22, localized corrosion typically occurs in the form of crevice corrosion. Many studies 

have described Alloy 22 crevice corrosion initiation for various environmental conditions 

(Evans, et al., 2005; Ilevbare, et al., 2005; Dunn, et al., 2005, 2006); however, Alloy 22 crevice 

corrosion propagation studies are limited. 

Previously He and Dunn (2007a) used single and multiple crevice assemblies to investigate 

crevice corrosion propagation behavior of Alloy 22 by coupling to Alloy 22 in 5 M NaCl solution 

at 95 °C [203 °F]. Under the experimental conditions, Alloy 22 showed a strong tendency to 

repassivate, although crevice corrosion of Alloy 22 occurred after the addition of CuCl2 to the 

solution as an oxidant and the penetration rates were several orders of magnitude greater than 

the general corrosion rates. Later crevice corrosion studies by He, et al. (2007a,b) and He and 

Dunn (2007b) showed that crevice corrosion was initiated in 4 M MgCl2 solution using Alloy 22 

or Titanium Grade 7 crevice former by coupling to Alloy 22 or Titanium Grade 7 and the 
2

corrosion propagated without repassivation. In these studies, the corrosion features were 

examined primarily by optical microscope and the depths of crevice corrosion penetrations were 

determined using the microscopic method described in ASTM G46 (ASTM International, 2004). 

However, with the resolution and field of view limitations of optical microscopes, corrosion 

damage profiles such as the distribution and size of penetration sites were not well 

characterized. A wealth of information on these Alloy 22 specimens related to crevice corrosion 

propagation is now revealed. 

The objective of this work is to accurately characterize the crevice corrosion damage profiles of 

crevice-corroded Alloy 22 specimens from previous work (He and Dunn, 2007a; 

He, et al., 2007b) using a 3- and 2-D laser profiling to better understand Alloy 22 crevice 

corrosion propagation. This study aims at providing supplementary and new insights into the 

Alloy 22 crevice corrosion process. 

EXPERIMENTAL METHODS 

The crevice corrosion damage profiles of two sets of crevice corrosion posttest specimens were 

characterized. The experimental details associated with the first set of specimens were 

reported in He, et al. (2007b). In this set of tests, the Alloy 22 crevice specimens were in 

contact with Alloy 22 or Titanium Grade 7 crevice former to form 24 crevice contacts. The 

crevice assembly was galvanically coupled to a large Alloy 22 plate if the crevice former was 

Alloy 22. If the crevice former was Titanium Grade 7, then the crevice assembly was 

galvanically coupled to a large Titanium Grade 7 plate. Both of these galvanic couplings 

occurred through a potentiostat functioning as a zero-resistance ammeter. The area ratio of the 

crevice specimen to the coupling plate was 1:14. The tests were conducted in 4 M MgCl2

solution at 95 °C [203 °F] for different durations to observe the crevice corrosion initiation and 

propagation. The experimental setup, the galvanic coupling current and potential, and the 

optical photos of the crevice specimens are in Figures 2-2, 3-16, and 3-17 reported in He, et al. 

(2007b). 

The experimental details associated with the second set of crevice specimens were reported in 

He and Dunn (2007a). In these tests, the front circular surface of an Alloy 22 cylindrical 

specimen measuring 6.2 mm [0.24 in] in diameter and 48.6 mm [1.91 in] in length was in contact 

with polytetrafluoroethylene to form a single crevice. The crevice was galvanically coupled to an 

Alloy 22 plate. The tests were performed in 5 M NaCl solution at 95 °C [203 °F] for different 

durations. The optical photos of the posttest crevice specimens (Figure 8) were reported in 

He and Dunn (2007a). 

The 3- and 2-D scans of the crevice corrosion specimens to characterize the crevice corrosion 

damage profile were performed with a video coordinate measuring machine, which is equipped 

with an optical probe and a laser probe. Laser profilometry provides fast, accurate noncontact 

assessments of corrosion. Its detailed, 3-D topographic profiles support analysis and 

interpretation of complex corrosion processes. The highest depth resolution (z axis of the 

machine) is 0.5 μm [0.02 mil], and the lateral surface resolution is 1 μm [0.04 mil] (x and y 

axes). For the crevice specimens in the current work, the machine was programmed to conduct 

multiple, evenly spaced line scans covering the corroded area of interest. The resolution used 

in this work was 10 μm [0.4 mil] in the x axis and 50 μm [2 mil] in the y axis. The multiple line 

scans are plotted as a 3-D image of the corroded area, and each line scan is displayed as a 2-D 

profile. The machine was calibrated, and the measured depth was found to be in perfect 

agreement with depths measured by mechanical tools. 
3

The 3-D laser scans were assembled together with multiple line scans. Each line scan is 

initiated and completed in an uncorroded area of the sample material. The total mass loss from 

the crevice corrosion process in a particular area can also be calculated by subtracting the 

average height from the uncorroded area from all the data points to get a total depth loss. This 

is accomplished by averaging the height of the base measurements collected from each side of 

the crevice region to create an average height for the uncorroded base region. The difference 

between this uncorroded average height value and each data point for that line scan was used 

to calculate the depth loss. The total volume loss was calculated by adding all the volume units 

(depth loss multiplied by 10 μm [0.4 mil] and 50 μm [2 mil]). The mass loss was calculated from 

the volume loss by multiplying by the material density. 

RESULTS 

Specimens with 24 Crevice Contacts Tested in 4 M MgCl2 at 95 °C [203 °F] 

Figure 1 shows the optical photo of one posttest Alloy 22 crevice specimen. Before the test of 

galvanic coupling to Titanium Grade 7 in 4 M MgCl2 solution, the specimen weighed 23.29 g 

[0.052 lb]. Only 1 out of the 24 crevice contacts was corroded under the Titanium Grade 7 

crevice former. Figures 1(b) and 1(c) show the 3-D laser profile viewing from the top and 

bottom, respectively. Figure 1(d) shows the corrosion depth profile passing the deepest site in 

the crevice from left to right in Figure 2(b). The deepest penetration was ~180 μm [7.09 mil], 

which is higher than the 155 μm [6.10 mil] measured previously by optical microscope. The 

deeper penetrations were near the edge of the crevice, but not exactly at the edge. For the 

penetration profile shown in Figure 1(d), the corrosion along the crevice was about 2,500 μm 

[98 mil] wide and the deepest penetration sites were 439 μm [17.3 mil] and 321 μm [12.6 mil] 

from the two edges of the crevice at the right and left. Using the method developed in this 

paper and using the Alloy 22 density value of 8.69 g/cm3

 [543 lb/ft3], the weight loss from the 

metal dissolution was calculated to be 3.78 mg, 0.016% of the original weight, which is slightly 

lower than the 5.12 mg weighed by the balance. In this test, the charge that passed through the 

external circuit measured by the potentiostat was 5.24 coulombs, which is equivalent to metal 

loss of 1.27 mg using 23.28 g/equivalent [0.051 lb/equivalent] (ASTM International, 2008). The 

weight loss calculated from the charge was only one-third of that measured by the quantitative 

method developed in this paper, suggesting that most of the crevice corrosion is supported by 

the cathodic reaction on Titanium Grade 7 which directly contacts the Alloy 22 specimen. 

Figure 2 shows more optical photos and 3-D laser scan images of other Alloy 22 crevice 

specimens from the same set of tests. From the shallow corroded area in Figures 2(a) and 2(c), 

the profile shows that corrosion started around the edge of the crevice and appeared to start at 

discrete sites. Later on, the corrosion propagated further into the crevice region as shown in the 

deep site in Figures 2(a) and 2(b); however, the deepest penetration was mostly at roughly 

500 μm [19.7 mil] away from the edge as observed in Figure 1. Table 1 lists the penetration 

depths currently measured from the laser scan and previously measured by the optical 

microscope. It appears that the laser scan measured deeper penetration than the 

optical microscope. 

The images shown in Figures 1 and 2 are the Alloy 22 specimens after crevice corrosion by 

coupling to Titanium Grade 7. He, et al. (2007b) showed that for the Alloy 22 specimens 

coupled to Alloy 22, crevice corrosion occurred at most of the 24 possible crevice contacts, 

whereas by coupling to Titanium Grade 7, crevice corrosion was limited to 1 or 2 crevice 

contacts out of 24 crevice contacts. A laser scan was also performed on some crevices of the 
4

Alloy 22 specimens to examine the corrosion penetration profile. The 3-D scan images are 

shown in Figure 3 along with the optical photos in He, et al. (2007b). The crevice corrosion 

areas examined were much shallower compared to the areas in Figures 1 and 2, but the profile 

still shows that the crevice corrosion started around the edge. The optical image in Figure 3(c) 

appears to show that the entire site n was evenly corroded. However, the 3-D laser profile 

shows that corrosion was not uniform across the crevice site and that the corrosion was deeper 

near the edge while there was less corrosion at the central region. The 2-D profile that includes 

crossing the deepest penetration area in Figure 3(c) shows that the deepest penetration was 

288 μm [11.3 mil] deep and was 291 μm [11.5 mil] from the edge of the crevice region at the left 

in the figure. The shallowest penetration in the central region was about 126 μm [4.96 mil] deep 

and was about 1,611 μm [63.4 mil] from the edge at the left. 

Single-Crevice Specimens Tested in 5 M NaCl at 95 °C [203 °F] 

3-D laser scans were performed on 6 single-crevice specimens after corrosion in 5 M NaCl 

solution with the addition of 2 × 10−4

 M CuCl2 at 95 °C [203 °F]. For this set of tests, crevice 

corrosion was initiated with the addition of CuCl2. The details on crevice corrosion propagation 

were discussed in He and Dunn (2007a). Figure 4 shows the 3-D laser profile of the corroded 

circular region and the 2-D line scan profile passing the deepest penetration site. The dotted 

lines in the 3-D image are the approximate locations of the 2-D line scan. Consistent with the 

optical images shown in He and Dunn (2007a) and inserted in Figure 4 for comparison, the 

deepest penetration is near the edge of the crevice, leaving most of the central region less 

corroded (note that the orientation of the 3-D image in the figure may not be the same as that of 

the optical image). The 2-D profiles in Figures 4(b), 4(c), and 4(e) show that the deepest 

penetration sites were less than 200 μm [7.87 mil] from the edge and the sites were sharp and 

small compared to other shallower penetration sites. Figure 5 shows the blow-up plots of the 

deepest penetration sites in Figures 4(a), 4(b), and 4(c). The expanded plots show that the 

deepest penetration sites were within the size range of 10−50 μm [0.39−2.0 mil] where 10 μm 

[0.39 mil] is the laser scan step interval. The penetration depths of the deepest sites measured 

from the laser scan are summarized in Table 2 along with the maximum penetration depth 

measured previously using an optical microscope. Consistent with the penetration depths 

shown in Table 1, the laser scan resulted in greater depth than measurement by optical 

microscope. 

DISCUSSION 

Maximum Penetration Depths Measured by Laser Scan and Optical Microscope 

Tables 1 and 2 consistently show that laser scans measured greater maximum crevice 

corrosion depths than those measured by optical microscope. Figure 5 shows that some deep 

sites could be as small as 10 μm [0.39 mil], which is the highest resolution used in the current 

work. It is possible that deeper sites could be smaller than 10 μm [0.39 mil] if higher resolution 

laser scan is performed; these sites were not captured by the optical microscope due to its 

limitation in resolution. To understand whether these newly measured deeper penetration 

depths would affect the penetration rates of Alloy 22 crevice corrosion as discussed in He and 

Dunn (2007a), these depths were plotted in the same figure for comparison in Figure 6. It 

shows that at earlier times, crevice corrosion of Alloy 22 actually propagated faster than 

calculated previously; however, at a later period, the penetration depth fell into the upper 

fit bound. 
5

Correlation of Crevice Corrosion Attack Profile with Metallurgical Features 

The 2-D crevice corrosion profiles in Figures 1(d), 3(c), 4, and 5 show that most of the deep 

sites were in the range of 10−50 μm [0.39−2.0 mil], which is similar to the grain size of Alloy 22 

(Dunn, et al., 2003). The profiles also show that the corroded area is not always uniform; there 

were peaks and dips in the corroded region. The correlation of depth variation to 2-D profile 

and grain sizes seems to suggest that crevice corrosion attack follows metallurgical features 

such as grain boundaries. Although there is no in-situ data to support this speculation, the 

scanning electron micrographs with lower and higher magnification in Figure 7 of two corroded 

cylindrical specimens in Figures 4(a) and 4(f) show that corrosion occurred along grain 

boundaries. Especially for the specimen shown in Figure 7(a), the test was stopped after 

0.5 days. The higher magnification clearly shows corrosion at grain boundaries. The top-left 

part of the specimen in Figure 7(b) is not as severely corroded as the lower part; however, 

Figure 7(b) shows that the grain boundaries of this part were etched by the acid produced in the 

crevice corrosion process. The posttest examination supports the possibility that crevice 

corrosion attack may have initially followed metallurgical features such as grain boundaries. 

Distance of the Deepest Penetration Sites from the Edges of the Crevice 

Figures 1 through 4 show that the deeper corrosion sites were about 200−500 μm 

[7.87−20.0 mil] away from the edge and the central crevice region was uncorroded or less 

severely corroded. This phenomenon can be explained using the ohmic potential (IR) drop 

mechanism developed by Pickering (1989) and Xu and Pickering (1993) (I is the anodic current 

from metal dissolution in the crevice and R is the solution resistivity along the current path). The 

IR drop mechanism relates the crevice corrosion propagation profile to the electrode potential 

profile in the crevice. For the posttest crevice specimen in Figure 4(a), Figure 8 plots the 

relationship between crevice current and crevice potential demonstrating the passive to active 

transition after crevice corrosion initiation. Crevice corrosion led to about a 400 mV potential 

drop in the crevice. This potential drop shifted the crevice to an active state leading to a current 

peak. According to the 2-D crevice corrosion profile, for Alloy 22 the potential drop is only 

confined to a small distance (hundreds of microns) in the crevice region. Pickering (2001) 

showed that for a T-2205 duplex stainless steel, the potential drop extended for over 1 cm 

[0.4 in] into the crevice region for a gap opening of 0.30 mm [12 mil] in acidified NaCl solution. 

The much smaller distance in potential drop observed here could be due to the much tighter 

crevice, which results in higher resistivity along the current path. This potential drop is also 

coupled with chemical conditions, especially pH changes in the crevice as can be seen from the 

acid etching of surrounding crevice regions in Figure 2. 

SUMMARY AND CONCLUSIONS 

3-D and 2-D laser scans were performed on Alloy 22 crevice corrosion specimens to 

characterize the corrosion damage profiles. Laser scans are more sensitive for capturing deep 

and narrow penetration sites leading to greater measured maximum crevice corrosion 

penetration depths than those measured by optical microscope. Most deep corrosion sites were 

steep and as small as 10 μm [0.39 mil], comparable to Alloy 22 grain sizes. Laser scan can 

capture the depth variation around grain boundaries. For Alloy 22, crevice corrosion attack may 

have initially followed metallurgical features such as grain boundaries. The deeper corrosion 

sites were about 200−500 μm [7.87−20.0 mil] away from the edge, and the central crevice 

region was uncorroded or less severely corroded. The initiation of crevice corrosion 

experiments led to several hundreds of millivolts voltage drop in a small distance (hundreds of 
6

microns) in the crevice region due to the tight crevice and resulting high resistivity along the 

current path. 

In the future, higher resolution scans may further reveal penetration profile details. In addition, 

chemically cleaning the specimen could further remove corrosion product from the surface for 

better profiling. 

ACKNOWLEDGMENTS 

The authors gratefully acknowledge the reviews of Drs. O. Pensado and J. Winterle and the 

editorial review of L. Mulverhill. Appreciation is due to J. Gonzalez for assistance in the 

preparation of this paper. 

This paper describes work performed by the Center for Nuclear Waste Regulatory Analyses 

(CNWRA®) and its contractors for the U.S. Nuclear Regulatory Commission (U.S. NRC) under 

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

U.S. NRC Office of Nuclear Material Safety and Safeguards, Division of High-Level Waste 

Repository Safety. This paper is an independent product of the CNWRA and does not 

necessarily reflect the view or regulatory position of the U.S. NRC. 

REFERENCES 

ASTM International. “Metals Test Methods and Analytical Procedures.” ASTM G46-94 (2004): 

Standard Guide for Examination and Evaluation of Pitting Corrosion. Vol. 3.02: Wear and 

Erosion—Metal Corrosion. Published on CD-ROM. West Conshohocken, Pennsylvania: 

American Society for Testing and Materials International. 2004. 

ASTM International. “Standard Practice for Calculation of Corrosion Rates and Related 

Information from Electrochemical Measurements.” ASTM G102-89 (2008): Standard Guide for 

Examination and Evaluation of Pitting Corrosion. Vol. 3.02: Wear and Erosion—Metal 

Corrosion. Published on CD-ROM. West Conshohocken, Pennsylvania: American Society for 

Testing and Materials International. 2008. 

Dunn, D.S., D. Daruwalla, and Y.-M. Pan. “Effect of Fabrication Processes on Material 

Stability⎯Characterization and Corrosion.” San Antonio, Texas: CNWRA. 2003. 

Dunn, D.S., Y.-M. Pan, L. Yang, and G.A. Cragnolino. “Localized Corrosion Susceptibility of 

Alloy 22 in Chloride Solutions: Part I—Mill-Annealed Condition.” Corrosion. Vol. 61. 

pp. 1,078-1,085. 2005. 

Dunn, D.S., Y.-M. Pan, L. Yang, and G.A. Cragnolino. “Localized Corrosion Susceptibility of 

Alloy 22 in Chloride Solutions: Part II—Effect of Fabrication Processes.” Corrosion. Vol. 62. 

pp. 3–12. 2006. 

Evans, K.J., A. Yilmaz, S.D. Day, L.L. Wong, J.C. Estill, and R.B. Rebak. “Using 

Electrochemical Methods to Determine Alloy 22 Crevice Corrosion Repassivation Potential.” 

Journal of Metals. Vol. 57. pp. 56–61. 2005. 
7

He, X., D.S. Dunn, and C.S. Csontos. “Crevice Corrosion Initiation and Propagation of Similar 

and Dissimilar Metal Couples in the Engineered Barrier System of the Potential Yucca Mountain 

Repository.” San Antonio, Texas: CNWRA. 2007a. 

He, X., D.S. Dunn, and A.A. Csontos. “Corrosion of Similar and Dissimilar Metal Crevices in the 

Engineered Barrier System of a Potential Nuclear Waste Repository.” Electrochimica Acta. 

Vol. 52. pp. 7,556–7,569. 2007b. 

He, X., D.S. Dunn. “Crevice Corrosion Penetration Rates of Alloy 22 in Chloride-Containing 

Waters.” Corrosion. Vol. 63. pp.145–158. 2007a. 

He, X., D.S. Dunn. “Crevice Corrosion Propagation Behavior of Alloy 22 in Extreme 

Environments.” Proceedings of the CORROSION ’07 Conference. Paper No. 578. Houston, 

Texas: NACE International. 2007b. 

Ilevbare, G.O., K.J. King, S.R. Gordon, H.A. Elayat, G.E. Gdowski, and T.S.E. Gdowski. “Effect 

of Nitrate on the Repassivation Potential of Alloy 22 in Chloride-Containing Environments.” 

Journal of the Electrochemical Society. Vol. 152. pp. B547–B554. 2005. 

Pickering, H.W. “Status of the IR Mechanism for Describing Crevice Corrosion.” Proceeding of 

NACE Research Topical Symposium on Localized Corrosion. G.S. Frankel, and J.R. Scully, 

eds. Houston, Texas: NACE International. pp. 30–64. 2001. 

Pickering, H.W. “The Significance of the Local Electrode Potential within Pits, Crevices, and 

Cracks.” Corrosion Science. Vol. 29. pp. 325−341. 1989. 

Xu, Y. and H.W. Pickering. “The Initial Potential and Current Distributions of the Crevice 

Corrosion Process.” Journal of the Electrochemical Society. Vol. 140. pp. 658−668. 1993. 
8

Table 1. Alloy 22 Crevice Specimen Penetration Depth and Weight Loss After Crevice 

Corrosion Tests in 4 M MgCl2 Solution at 95 °C [203 °F] Using Titanium Grade 7 as 

Crevice Former by Coupling to Titanium Grade 7 

 22 Days 28 Days 34 Days 76 Days 

By laser scan 180 180 155 400 Maximum Penetration 

Depth (μm) By optical 

microscope 

130 150 77 277 

Original Weight of Specimen (g) 23.6 23.3 23.8 23.5 

By charge 3.00 1.26 1.26 2.72 

Weight Loss (mg) By balance 5.54 5.12 2.18 6.75 

By integration 4.62 3.78 2.24 7.25 

Table 2. Maximum Penetration Depths of Alloy 22 Crevice Specimen Shown in Figure 4 

After Crevice Corrosion Tests in 5 M NaCl Solution at 95 °C [203 °F] by Coupling to 

Alloy 22 

Test Duration (days) 0.5 3.1 4.9 6.9 11.9 70.9 

Maximum By laser scan 135 251 307 163 313 404 

Penetration Depth 

(μm) By optical 

microscope 

90 143 187 107 220 313 
9

-200

-150

-100

-50

0

0 1000 2000 3000 4000

x axis (μm)

z axis (μm)

deepest site

 

(a) Optical Image (b) 3-D Laser Scan Image (Top View) 

(c) 3-D Laser Scan Image (Bottom View)

(d) 2-D Laser Scan Profile 

Figure 1. Optical Photograph, 3-D Laser Scan, and 2-D Profile of Alloy 22 Corroded Area 

After Crevice Corrosion Tests in 4 M MgCl2 Solution at 95 °C [203 °F] Using Titanium 

Grade 7 as Crevice Former by Galvanic Coupling to Titanium Grade 7 

28 days 
10

(a) Alloy 22 Coupled to Titanium Grade 7 for 22 Days 

(b) Alloy 22 Coupled to Titanium Grade 7 for 34 Days 

 

(c) Alloy 22 Coupled to Titanium Grade 7 for 76 Days

Figure 2. Optical Photograph and 3-D Laser Scan of Alloy 22 After Crevice Corrosion 

Tests in 4 M MgCl2 Solution at 95 °C [203 °F] Using Titanium Grade 7 as Crevice Former 

by Coupling to Titanium Grade 7

22 days 

34 days 

76 days 

m

n

Site m Site n 
11

-350

-300

-250

-200

-150

-100

-50

0

50

0 2000 4000 6000 8000 10000 12000

x axis (μm)

z axis (μm)

deepest site

(a) 5.4 Days 

(b) 36 Days 

2-D Profile Along Line in Site n

(c) 79 Days 

Figure 3. Optical Photos and 3-D Laser Scan of Alloy 22 After Crevice Corrosion Tests in 

4 M MgCl2 Solution at 95 °C [203 °F] Using Alloy 22 as Crevice Former by Coupling to 

Alloy 22 

m Site m

n 

Site n 
12

-140

-120

-100

-80

-60

-40

-20

0

20

40

0 1000 2000 3000 4000 5000

x axis (μm)

z axis (μm)

deepest site

-250.00

-200.00

-150.00

-100.00

-50.00

0.00

50.00

100.00

0 1000 2000 3000 4000 5000

x axis (μm)

z axis (μm)

175 μm

deepest site

(a) 0.5 Days 

(b) 3.1 Days 

Figure 4. 3-D Laser Scan and 2-D Profile of Alloy 22 After Crevice Corrosion Tests in 5 M 

NaCl Solution at 95 °C [203 °F] With the Addition of 2 × 10-4 M CuCl2 Using 

Polytetrafluoroethylene as Crevice Former by Coupling to Alloy 22 With Test Duration of 

(a) 0.5 Day, (b) 3.1 Days. The Diameters of All the Cylindrical Specimens Are 6.2 mm 

[0.24 in], and the Unit of the Scale in the 3-D Image is μm. 
13

-400.00

-350.00

-300.00

-250.00

-200.00

-150.00

-100.00

-50.00

0.00

0 1000 2000 3000 4000

x axis (μm)

z axis (μm)

deepest site

-180.00

-160.00

-140.00

-120.00

-100.00

-80.00

-60.00

-40.00

-20.00

0.00

20.00

40.00

0 1000 2000 3000 4000

x axis (μm)

z axis (μm)

deepest site

(c) 4.9 Days

(d) 6.9 Days 

Figure 4 (continued). 3-D Laser Scan and 2-D Profile of Alloy 22 After Crevice Corrosion 

Tests in 5 M NaCl Solution at 95 °C [203 °F] With the Addition of 2 × 10-4 M CuCl2 Using 

Polytetrafluoroethylene as Crevice Former By Coupling to Alloy 22 With Test Duration of 

(c) 4.9 Days, (d) 6.9 Days. The Diameter Of All The Cylindrical Specimens Are 6.2 mm 

[0.24 in] and the Unit of the Scale in the 3-D Image is μm.
14

-400.00

-350.00

-300.00

-250.00

-200.00

-150.00

-100.00

-50.00

0.00

50.00

0 1000 2000 3000 4000

x axis (μm)

z axis (μm)

deepest site

-400.00

-300.00

-200.00

-100.00

0.00

100.00

200.00

0 1000 2000 3000 4000

x axis (μm)

z axis (μm)

deepest site

(e) 12 Days 

(f) 71 Days 

Figure 4 (continued). 3-D Laser Scan and 2-D Profile of Alloy 22 After Crevice Corrosion 

Tests in 5 M NaCl Solution at 95 °C [203 °F] With the Addition of 2 × 10-4 M CuCl2 Using 

Polytetrafluoroethylene as Crevice Former By Coupling to Alloy 22 With Test Duration of 

(e)12 Days, and (f) 71 Days. The Diameter Of All The Cylindrical Specimens Are 6.2 mm 

[0.24 in] and the Unit of the Scale in the 3-D Image is μm. 
15

-140

-120

-100

-80

-60

-40

-20

0

1100 1200 1300 1400 1500 1600 1700 1800

x axis (μm)

z axis (μm)

deepest site

-250.00

-200.00

-150.00

-100.00

-50.00

0.00

50.00

0 200 400 600 800

x axis (μm)

z axis (μm)

deepest site

-400.00

-350.00

-300.00

-250.00

-200.00

-150.00

-100.00

-50.00

0.00

3300 3400 3500 3600 3700 3800 3900 4000

x axis (μm)

z axis (μm)

deepest site

(a) 0.5 Days 

(b) 3.1 Days 

(c) 4.9 Days 

Figure 5. Expanded 2-D Plots of the Deepest Crevice Corrosion Sites in Figures 4(a), (b), 

and (c) 
16

0 20 40 60 80

Time, Days

100

200

300

400

500

Maximum penetration depth, μm

Depth by optical microscope

Depth by laser line scan

Exponential fit

Figure 6. Crevice Corrosion Penetration Depths Measured by Laser Scan Comparing 

With Those Measured Previously by Optical Microscope as a Function of Time For Tests 

on Alloy 22 at 95 °C [203 °F] in 5 M NaCl Solution With the Addition of 2 × 10–4 M CuCl2. 

The Continuous Line Is the Best Fit (Function of the Form ktn

) to the Depth Data (Plotted 

as Circles) Previously Measured by Optical Microscope. The Dotted Lines Are Upper and 

Lower Fit Bounds Enclosing a 95% Confidence Interval of the Previous Data. 
17

(a) 0.5 Day 

(b) 71 Days 

Figure 7. SEM Photographs of Two Alloy 22 Crevice Specimens in Figures 4(a) and 4(f) 

Showing Corrosion Along Grain Boundaries 
18

-0.2 0 0.2 0.4 0.6

Potential (VSCE)

2x10-3

4x10-3

6x10-3

8x10-3

Current density, A/cm2

5 M NaCl with 2X10-4 M CuCl2, 95°C, Coupled to Alloy 22 Plate

Figure 8. Relationships Between Crevice Current and Crevice Potential for Alloy 22 

Galvanically Coupled to Alloy 22 Plate Recorded in 5 M NaCl Solution With the Addition 

of CuCl2 at 95 °C [203 °F]