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i

NASA/TM—2015–218206

Stress Corrosion Evaluation of Various Metallic

Materials for the International Space Station 

Water Recycling System

P.D. Torres

Marshall Space Flight Center, Huntsville, Alabama

March 2015

National Aeronautics and

Space Administration

Marshall Space Flight Center • Huntsville, Alabama 35812
ii

Available from:

NASA STI Information Desk

Mail Stop 148

NASA Langley Research Center

Hampton, VA 23681–2199, USA

757–864–9658

This report is also available in electronic form at

<http://www.sti.nasa.gov>

TRADEMARKS

Trade names and trademarks are used in this report for identification only. This usage does not constitute an official

endorsement, either expressed or implied, by the National Aeronautics and Space Administration.

Acknowledgments

The author wishes to acknowledge the following supporters:

Marshall Space Flight Center

EM32 (Metal Joining & Processes Branch): C. Russell, R. Hickman, H. Gill, R. Lee (ESSSA), 

and S. Hudson (retired)

EM30 (Metals Engineering Division): T. Vaughn

EM31 (Failure Analysis and Metallurgy Branch): B. Panda, G. Jerman, W. DeWeese, and J. Hastings

EM60 (Materials Selection & Control and Small Projects Branch): L. Foreman, D. Griffin, T. Wegrich, T. Miller, 

T. Malone, J. Davis, and E. Ordoñez (in ES13 now)

ES62 (ECLSS Development Branch): D. Carter, D. Long, D. Terrell, and J. Pruitt

EE04 (Flight Programs & Partnerships Chief Engineers Office): R. Mize and B. Mitchell

FP10 (ISS Office): K. Presson and W. Spearman

EM01 (Materials and Processes Laboratory): S. Singhal and M. Prince

EM02 (Resource Management Team): D. Mahieux, J. Smith, T. Bili, and C. Cunningham

EM10 (Materials Test Branch): A. Girgis, L. Sharff, W. Tilson, C. Harris, D. Wingard, G. Gordon, 

J. Perkins, E. Davis, and T. Malone

ES23 (Mechanical Fabrication Branch): J. Clounch

AS10 (Environmental Engineering & Occupational Health Office): J. Troy, S. Adkins, P. Buckelew, 

and W. Carter

AS42 (Transportation & Logistics Engineering Office): A. Richardson

ES11 (Project Engineering Branch): J. Gaines and D. Crump

ES60 (Systems Development, Integration & Test Division): R. Graves

ES21 (Structural & Mechanical Design Branch): A Howard (ESSSA) and M. Perry (ESSSA)

Scientific and Technical Information Group: Susan Burrer, Vicki Hocutt, and Mary Vaughn

Johnson Space Center

JSC-OB311: L. Shaw

JSC-EC311: A. Chambers
iii

TABLE OF CONTENTS

1. INTRODUCTION ............................................................................................................. 1

2. DEFINITION .................................................................................................................... 2

2.1 Stress Corrosion ............................................................................................................ 2

3. EXPERIMENTAL PROCEDURE .................................................................................... 3

3.1 Materials Evaluated ...................................................................................................... 3

3.2 Test Matrix ................................................................................................................... 4

3.3 Specimen Configurations .............................................................................................. 5

3.4 Tensile Tests of Specimens As Received ........................................................................ 5

3.5 Stressing of the Specimens ............................................................................................ 6

3.6 Test Environments and Equipment ............................................................................... 8

4. RESULTS AND DISCUSSION ......................................................................................... 10

5. CONCLUSIONS ................................................................................................................ 27

APPENDIX A—MATERIALS CERTIFICATIONS ............................................................. 28

APPENDIX B—TENSILE DATA FOR NONEXPOSED SPECIMENS .............................. 34

APPENDIX C—FORMULAS FOR PREPARING PRETREATED URINE ...................... 40

C.1 International Space Station Baseline—Current Pretreatment ....................................... 40

C.2 Proposed Alternate Pretreatment ................................................................................. 40

APPENDIX D—TENSILE DATA FOR EXPOSED SPECIMENS ...................................... 41

REFERENCES ....................................................................................................................... 51
iv

LIST OF FIGURES

 1. Round tensile specimen configurations ...................................................................... 6

 2. Stressing device and accessories: (a) Stressing device and (b) constant strain 

round specimen and frame assembly ......................................................................... 7

 3. Representative assembled and loaded stress corrosion specimens before test ............. 7

 4. Testing methods: (a) Total immersion in the ECLSS fluids to determine

acceptability, (b) total immersion in DI water, (c) 3.5% NaCl alternate 

immersion tester (not the actual specimens—for illustrative purpose only), 

and (d) 5% salt spray chamber ................................................................................... 9

 5. Representative specimens before and during exposure to pretreat ............................. 13

 6. Representative specimens before and during exposure to brine ................................. 14

 7. Representative specimens during exposure to DI water ............................................. 15

 8. Specimens after exposure to pretreat for 365 days ..................................................... 16

 9. Specimens after exposure to brine for 365 days ......................................................... 16

10. Cronidur 30 specimens during and after exposure to various complementary 

environments: (a) C30-24, C30-27, and C30-30 after 139 days in various

environments, (b) after 313 days of exposure to various environments, 

and (c) 359-day exposure described in table 5 ............................................................ 17

11. Photomicrographs of Inconel 625 initially stressed to 75% YS (58.3 ksi) 

and exposed for 365 days in (a) pretreat (sample 625-6) and (b) brine 

(sample 625-10) ......................................................................................................... 18

12. Photomicrographs of Hastelloy C276 initially stressed to 75% YS (50.3 ksi) 

and exposed for 365 days in (a) pretreat (sample C276-6) and (b) brine 

(sample C276-10) ...................................................................................................... 19

13. Photomicrographs of TiCP initially stressed to 75% YS (68.6 ksi) and exposed 

for 365 days in (a) pretreat (sample TiCP-6) and (b) brine (sample TiCP-10) ............ 20

14. Photomicrographs of Ti-6A-4V initially stressed to 75% YS (103.9 ksi) 

and exposed for 365 days in (a) pretreat (sample Ti-64-6) and (b) brine 

(sample Ti-64-10) ...................................................................................................... 21
v

LIST OF FIGURES (Continued)

15. Photomicrographs of Ti-6-4 ELI initially stressed to 75% YS (104 ksi) 

and exposed for 365 days in (a) pretreat (sample ELI-6) and (b) brine 

(sample ELI-10) ........................................................................................................ 22

16. Photomicrographs of Cronidur 30 initially stressed to 20% YS (50 ksi) 

and exposed for 341 days in (a) pretreat (sample C30-6) and (b) brine 

(sample C30-16) ........................................................................................................ 23

17. Stress corrosion specimens exposed to (a) phosphoric acid-based pretreat, 

(b) brine, and (c) DI water without failing and subsequently tensile tested. 

Specimens shown were exposed for 365 days, except Cronidur 30 specimens, 

which were exposed for 341 days ............................................................................... 26

18. Inconel 625 material certification .............................................................................. 28

19. Hastelloy C276 material certification ......................................................................... 29

20. TiCP material certification ........................................................................................ 30

21. Ti-6-4 material certification ....................................................................................... 31

22. Ti-6-4 ELI material certification ................................................................................ 32

23. Cronidur 30 material certification ............................................................................. 33

24. Inconel 625 stress-strain curves and tensile data for as-received specimens ................ 34

25. Hastelloy C276 stress-strain curves and tensile data for as-received specimens .......... 35

26. TiCP stress-strain curves and tensile data for as-received specimens .......................... 36

27. Ti-6-4 stress-strain curves and tensile data for as-received specimens ........................ 37

28. Ti-6-4 ELI stress-strain curves and tensile data for as-received 

specimens .................................................................................................................. 38

29. Cronidur 30 steel stress-strain curves and tensile data for as-received 

specimens .................................................................................................................. 39

30. Inconel 625 stress-strain curves and tensile data after a 365-day exposure 

to ECLSS pretreat and brine ..................................................................................... 42
vi

LIST OF FIGURES (Continued)

31. Hastelloy C276 stress-strain curves and tensile data after a 365-day exposure 

to ECLSS pretreat and brine ..................................................................................... 43

32. TiCP stress-strain curves and tensile data after a 365-day exposure to ECLSS 

pretreat and brine ...................................................................................................... 44

33. Ti-6-4 stress-strain curves and tensile data after a 365-day exposure 

to ECLSS pretreat and brine ..................................................................................... 45

34. Ti-6-4 ELI stress-strain curves and tensile data after a 365-day exposure 

to ECLSS pretreat and brine ..................................................................................... 46

35. Cronidur 30 steel stress-strain curves and tensile data after a 341-day 

exposure to ECLSS pretreat and brine ...................................................................... 47

36. Cronidur 30 steel stress-strain curves and tensile data after a 359-day exposure 

to mixed environments of 5% salt spray, high humidity, and 3.5% NaCl 

alternate immersion ................................................................................................... 50
vii

LIST OF TABLES

 1. Description of the test materials ............................................................................... 3

 2. Stress corrosion test matrix ....................................................................................... 4

 3. Averaged tensile data for the test materials ................................................................ 5

 4. Stress corrosion test results in various environments ................................................. 11

 5. Supplementary SCC test results for hardened Cronidur 30 steel in mixed 

environments and residual tensile strength ................................................................ 12

 6. Posttest evaluations for metals exposed to phosphoric acid-based ECLSS 

pretreat, brine, and DI water ..................................................................................... 24

 7. Test conditions for exposed Cronidur 30 steel specimens............................................ 48

 8. Average tensile data for nonexposed Cronidur 30 steel specimens ............................. 48

 9. SSC test conditions for Cronidur 30 steel specimens .................................................. 49
viii

LIST OF ABBREVIATIONS, ACRONYMS, AND SYMBOLS

√ surface finish (arithmetic average roughness value)

A/C air cool

Al aluminum

AMS Aerospace Material Specification

ASTM American Society for Testing and Materials

ATI Allegheny Technologies Incorporated

Bi bismuth

C carbon

Cb columbium

Co cobalt 

Cr chromium 

Cu copper

DI deionized (water)

E modulus of elasticity (Young’s Modulus)

ECLSS Environmental Control and Life Support System

EL elongation

ELI extra low interstitial

Fe iron

Gr grade

H hydrogen

HRC Rockwell C hardness

ISS International Space Station

Mg magnesium
ix

LIST OF ABBREVIATIONS, ACRONYMS, AND SYMBOLS (Continued)

MIL military

Mn manganese

Mo molybdenum

MSFC Marshall Space Flight Center

Msi megapounds per square inch

N nitrogen

NaCl sodium chloride

Nb niobium

Ni nickel

O oxygen

P phosphorus

Pb lead

pretreat pretreatment

R radius

RA reduction in area

S sulfur

SCC stress corrosion cracking

Si silicon

Sn tin

Ta tantalum

Ti titanium

Ti-6-4 Ti-6Al-4V

TiCP titanium commercially pure

UPA Urine Processing Assembly

UTS ultimate tensile strength
x

LIST OF ABBREVIATIONS, ACRONYMS, AND SYMBOLS (Continued)

UTSf final ultimate tensile strength (after exposure)

UTSi initial ultimate tensile strength (average for nonexposed specimens)

V vanadium

VAR vacuum arc remelting

W tungsten

Y yttrium 

YS yield strength (0.2% offset)

Zr zirconium
1

TECHNICAL MEMORANDUM

STRESS CORROSION EVALUATION OF VARIOUS METALLIC MATERIALS FOR THE 

INTERNATIONAL SPACE STATION WATER RECYCLING SYSTEM 

1. INTRODUCTION

The high cost of carrying water into space to be used at the International Space Station 

(ISS) generated the need to manufacture a water-recycling system to process urine and make it 

suitable for human consumption. For years, the Environmental Control and Life Support System 

(ECLSS) Urine Processing Assembly (UPA) has used sulfuric acid as one of the ingredients to 

pretreat the urine. However, the formation of crystals, believed to be associated with the use of this 

acid, was causing some problems. Phosphoric acid was then considered as a candidate to replace 

sulfuric acid. 

Because of this change in formulation of the pretreatment for processing the urine, a test 

program was undertaken at the Marshall Space Flight Center (MSFC) Materials and Processes 

Laboratory to perform various types of materials evaluations, including a stress corrosion characterization of six metallic materials: Inconel® 625, Hastelloy® C276, titanium commercially pure 

(TiCP), titanium 6Al-4V (Ti-6-4), Ti-6-4 extra low interstitial (ELI), and Cronidur® 30. The goal 

of this work was to determine if these materials have adequate stress corrosion resistance when 

exposed to the ECLSS UPA fluids. 

The first five listed were found resistant to stress corrosion based on no failures, metallography, and no reduction in load-carrying ability. As for Cronidur 30, though no failures occurred 

and metallography did not indicate stress corrosion cracking (SCC), several specimens experienced 

reduction in load-carrying ability (up to 42% in the brine). This deficiency is not expected to have 

an influence on the performance of this alloy as long as it is used in compression, but there are 

some limitations on its use for applications where sustained tensile stresses are involved. 
2

2. DEFINITION

2.1 Stress Corrosion

Stress corrosion may be defined as the combined action of sustained tensile stress and corrosion to cause premature failure of a susceptible material. Certain metallic materials are more susceptible than others. If a susceptible material is placed in service in a corrosive environment under 

tension of sufficient magnitude, and the duration of service is sufficient to permit the initiation and 

growth of cracks, failures can occur at a stress lower than the material would normally be expected 

to withstand. 
3

3. EXPERIMENTAL PROCEDURE

3.1 Materials Evaluated

A description of the alloys evaluated in this test program is presented in table 1. This information was obtained from the materials certifications (shown in app. A) and includes applicable 

specifications for the materials, form, heat treatment, alloy producer, heat number, hardness, and 

heat chemistry. 

Table 1. Description of the test materials.

Inconel 625 (Pyromet 625):

Applicable specifications: AMS 5666 Rev F, ASTM B446-03 (2008) (grade 1)

Form: 0.375-in-diameter bar 

Heat treat: Hot isostatic temperature alloy annealed ground

Company: Carpenter 

Heat No.: 600213

Hardness as shipped: 29 HRC

Heat chemistry (wt%): 0.04C, 0.06Mn, 0.09Si, 0.004P, 0.001S, 22.34Cr, 60.69Ni, 8.81Mo, 0.06Co, 

0.15Al, 0.28Ti, 3.58Cb, 0.01Ta, 3.59Cb, 3.78Fe

Hastelloy C276 (Nickelvac C-276):

Applicable specifications: AMS 5750C, ASTM B574-2006-1

Form: 0.375-in-diameter round bar, centerless ground surface

Heat treat: 2050 °F, 30 minutes, water quenched (solution annealed) (heat treat code 2050A)

Company: ATI Allvac

Heat No.: 91KF

Hardness as shipped: 23 HRC

Heat chemistry (wt%): 0.004C, <0.0003S, 0.51Mn, 0.02Si, 15.07Cr, 15.22Mo, 0.02Co, <0.01Ti, 0.18Al, 

<0.01Zr, 6.32Fe, 0.02Cu, 59.14Ni, <0.004P, 0.03Cb, <0.01Ta, 3.24W, 0.20V, 0.04Cb, 0.19(Ti+Al), 

59.16(Ni+Co) Traces: <0.00001Bi, <0.0001Pb, <0.0005Sn, <0.001O, 0.0047N

TiCP (Titanium Commercially Pure):

Applicable specifications: AMS 4921 Rev L, ASTM B348, Gr4, Rev: 09, MIL-T-9047 Gr4 Rev: G 

Form: Centerless ground 0.375-in-diameter round bar (melt method was 2-VAR)

Heat treat: Annealed 1300 °F 1 hour A/C (per MIL-H-81200)—as shipped 

Company: Dynamet (A Carpenter company); melt source was Timet 

Heat No.: H16512

Hardness: 24 HRC

Heat average chemistry (top and bottom values were averaged) (wt%): 0.048C, 0.135Fe, 0.375O, 

0.008N, <0.05 others each, <0.20 others total, balance Ti

Ti-6-4 (Titanium 6Al-4V):

Applicable specifications: AMS 4928 (2007), ASTM F 136 Rev 12, AMS 4930 Rev G, ASTM B348 

Gr 23 Rev 11, AMS 6932 Rev A, MIL-T-9047 Rev G, AMS 6931 Rev B, ASTM F 1472 Rev 08

Form: 0.375-in centerless ground round bar (melt method was 3-VAR)

Heat treat: Material was annealed in accordance with MIL-H-81200B 

Company: Dynamet (A Carpenter Company); melt source was Timet

Heat No.: H18003

Hardness: 34 HRC

Heat average chemistry (top and bottom values were averaged) (wt%): 6.035Al, 0.030C, 0.185Fe, 

0.0095N, 0.115O, 4.08V, <0.0004Y, Others each <0.10, Others total <0.30, Balance Ti
4

Table 1. Description of the test materials (Continued).

Ti-6-4 ELI (Ti-6-4 extra low interstitial):

Applicable specifications: AMS 6932 Rev A, ASTM F136 Rev 12, ASTM B348 Gr23 Rev 11, AMS 4930 

Rev G, MIL-T-9047 Rev G 

Form: 0.375-in-diameter centerless ground bar (melting method was 3-VAR)

Heat treat: Material was annealed in accordance with MIL-H-81200B

Company: Dynamet (A Carpenter company); melt source was Timet

Heat No.: H18826

Hardness: 35 HRC

Heat average chemistry (top and bottom values were averaged) (wt%): 6.07Al, 0.0285C, 0.18Fe, 0.0065N, 

0.13O, 4.13V, <0.0004Y, 0.0022H, Others each <0.10, Others total <0.30, Balance Ti 

Cronidur 30:

Applicable specifications: AMS 5898

Form: 0.7165-in-diameter rod

Heat treatment (per AMS 5898A, sections 3.4.6 to 3.4.6.1): Harden at 1925 ± 25 °F/30 ± 3 min 

in a neutral atmosphere, quench in oil to room temperature, temperature at 350 ± 10 oF/60 min ± 5, 

subzero cool to –100 ± 20 °F/2 ± 0.25 hours, warm in air to room temperature

Heat No.: 32012

Hardness: >58 HRC

Chemistry per AMS 5898A (wt%): 0.28–0.34C, 0.30–0.60Mn, 0.30–0.80Si, 0.020PMax, 0.010SMax, 

14.5–16.0Cr, 0.95–1.10Mo, 0.35–0.44N, 0.30NiMax

Note: Alternate name shown in parentheses.

3.2 Test Matrix

A stress corrosion test matrix showing number of specimens and how they were allocated 

to various types of tests is presented in table 2. 

Table 2. Stress corrosion test matrix.

Material No. of Samples Remarks

Inconel 625, Hastelloy C276,

TiCP, Ti 6-4, and Ti-6-4 ELI

60 (12 samples/alloy 

per figure 1(a))

For each alloy:

3 for tensile data

4 in pretreat (3 @ 75% YS, 1 @ 0% YS)

4 in brine (3 @ 75% YS, 1 @ 0% YS) 

1 in DI water

Cronidur 30 

(hardened to Rockwell C 58 

minimum, as for bearings)

33 samples per figure 1(b) 3 for tensile data

10 in pretreat (3 @ 50 ksi, 3 @ 25 ksi, 3 @ 15 ksi, and 1 w/o stress)

10 in brine (3 @ 50 ksi, 3 @ 25 ksi, 3 @ 15 ksi, and 1 w/o stress)

3 for salt fog test @ 25 ksi (for comparison)

3 for 3.5% NaCl alternate immersion test at 25 ksi (for comparison)

3 for high humidity test at 25 ksi (for comparison)

1 in DI water

Total number of round tensile specimens: 93
5

3.3 Specimen Configurations

For the first five alloys listed in table 2, 12 specimens per alloy were fabricated per figure 1(a).

Since Cronidur 30 was expected to be more susceptible than the rest of the materials, 33 specimens 

were fabricated for a more thorough evaluation. Configuration shown in figure 1(b) was used for 

machining the Cronidur 30 specimens. This configuration has a bigger size of threads than the 

configuration used for the other materials, which can prevent failures in the threaded portion 

of the specimen. The shoulder is also larger to prevent failures out of the gauge length. 

3.4 Tensile Tests of Specimens As Received

Three specimens from each alloy were tensile tested as received and the results obtained 

are summarized in table 3. Detailed results of the tensile tests are presented in appendix B. If the 

results of elongation and reduction in area are used to sort the alloys from most ductile to least 

ductile, the metals can be arranged in the following order: Hastelloy C276, Inconel 625, TiCP, 

Ti-6-4 ELI, Ti-6-4, and Cronidur 30. Notice the low ductility values of Cronidur 30, 2.2% elongation and 1.6% reduction in area, compared to the values for other alloys. Averaged Cronidur 30’s 

ultimate tensile strength and yield strength (YS) values of 300.4 ksi and 252.7, respectively, are 

significantly higher than the values for the rest of the materials included in this evaluation. The 

Rockwell C hardness (HRC) for this material after hardening was more than 58. 

Table 3. Averaged tensile data for the test materials.

Alloy

Designation

UTS

(ksi)

YS

(ksi)

EL

(%)

RA

(%)

E

(Msi)

Inconel 625 136.0 77.7 61.3 48.5 28.1

Hastelloy C276 120.7 67.0 69.1 60.4 31.8

TiCP 110.9 91.4 37.4 42.8 15.1

Ti-6-4 156.3 138.5 17.3 33.4 15.1

Ti-6-4 ELI 157.2 138.7 19.6 36.2 16.0

Cronidur 30 300.4 252.7 2.2 1.6 29.3

Notes:

• Values shown are the averages of three samples. 

• Tensile data were obtained at MSFC (EM10) and presented in appendix B for individual 

 samples.

• UTS: ultimate tensile strength, YS: yield strength, EL: % elongation, RA: % reduction in area, 

 E: modulus of elasticity (Young’s Modulus)

• Heat treatment is shown in table 1.
6

0.75 in

(a)

(b)

0.125 in ± 0.001

0.125 in R

F1_1511

0.06 in R 0.156 in

16√

1/4-20 UNC Thread 

Size Both Ends 

(Class 2A)

2 in

0.375 in 0.25 in 0.25 in 0.375 in

30°

0.75 in

0.125 in ± 0.001

1/8 R

0.281 in 0.06 R

16√

3/8-16 UNC Thread 

Size Both Ends 

(Class 2A)

2 in

0.375 in 0.25 in 0.25 in 0.375 in

30°

Notes:

(1) Tolerances: ±0.005 in, except otherwise specified.

(2) Surface finish (arithmetic average roughness value): 16 μin for the reduced section, 32 μin for the rest. 

(3) Thread dimensions must be as specified. Measurement by fabricator is mandatory.

(4) No undercutting of radii permitted.

(5) Gauge section to be concentric with axis within 0.002 in total indicator reading (gauge section of the tensile

 cannot have more than 0.002 in total run-out) and parallel.

(6) No file marks or nicks permitted within gauge section.

(7) Drawing not to scale.

(8) Configuration (a) was used for five of the six materials tested. Configuration (b) was used for Cronidur 30

 in order to obtain a larger size of threads and shoulder. 

Figure 1. Round tensile specimen configurations.

3.5 Stressing of the Specimens

After obtaining the tensile data, the strain corresponding to the desired stress levels shown 

in table 2 were obtained and the specimens were stressed by using the stressing device shown 

in figure 2(a). The various components of the stressing fixtures are identified in the schematic 
7

diagram shown in figure 2(b). The document associated with this testing technique is the American 

Society for Testing and Materials (ASTM) G49 (Standard Practice for Preparation and Use of 

Direct Tension Stress-Corrosion Test Specimens).1 In this method, the specimen is assembled into 

the stressing fixtures and an extensometer component is attached on the specimen reduced section. 

Two sidebars are then pushed toward the center by means of the device (see fig. 2(b)). The strain 

is measured by obtaining the difference between the initial and final readings. Representative stress 

corrosion specimens after they were loaded and before exposure are presented in figure 3. 

(a) (b)

F2_1511

Nut

Crosshead

Round Specimen

Sidebar

Figure 2. Stressing device and accessories: (a) Stressing device and (b) constant strain 

round specimen and frame assembly.

F3_1511

Figure 3. Representative assembled and loaded stress corrosion specimens before test.
8

3.6 Test Environments and Equipment

The stress corrosion testing was conducted using both phosphoric acid-based ECLSS pretreatment (pretreat) and brine solutions. The pretreat, which contains phosphoric acid and small 

amounts of chromic acid, has a pH in the vicinity of 2. Appendix C shows the formulas used for 

preparation of the reformulated pretreat and the baseline pretreat. The brine was obtained by distillation of the pretreat, which removed part of the water resulting in a more concentrated solution. 

One control sample from each alloy was exposed to deionized (DI) water. Exposure to the three 

environments—pretreat, brine, and DI—was carried out by completely immersing the specimens 

in the liquids contained in plastic containers and exposing them up to 1 year. 

Cronidur 30, a bearing material, was expected to exhibit some degree of susceptibility 

because of its high hardness (HRC 58), therefore this material was further evaluated in three 

additional environments: 

 (1) High humidity in a cabinet that maintained a relative humidity within an approximate 

range of 85% to 95% at 95±3 oF (Filter & Pump Manufacturing Co. Corrosion Test Cabinet Type 

411.1 ACD, serial No. S-6310, NASA property No. 1535092).

 (2) 5% salt spray at 95±3 oF per ASTM B1172 (by using Filter & Pump Manufacturing Co. 

Corrosion Test Cabinet type 411.1 ACD, serial No. 6198, NASA property No. G80024).

 (3) 3.5% NaCl alternate immersion per ASTM G443 by using an in-house-built Ferris 

wheel type apparatus. The Ferris wheel apparatus allows the specimens to stay in the salt solution 

for 10 minutes of every hour, followed by a 50-minute drying cycle. These cycles are repeated for 

the duration of the test. 

The alternate immersion method is the preferred method to evaluate metallic materials 

for stress corrosion cracking to eventually rate them in MSFC-STD-3029,4 which is the baseline 

document used by NASA and contractors. For additional details about the 5% salt spray and 

3.5% NaCl alternate immersion methods, see ASTM B117 and G44, respectively. Photographs 

related to the test methods described are represented in figure 4. 
9

(a) (b)

(c) (d)

F4_1511

For Comparison

Figure 4. Testing methods: (a) Total immersion in the ECLSS fluids to determine

acceptability, (b) total immersion in DI water, (c) 3.5% NaCl alternate 

immersion tester (not the actual specimens—for illustrative purpose only), 

and (d) 5% salt spray chamber.
10

4. RESULTS AND DISCUSSION

The stress corrosion results are presented in table 4. As shown in the table, no failures 

occurred in the pretreat, brine, or any of the supplementary tests performed. The maximum length 

of exposure for the materials evaluated was 1 year. 

The Cronidur 30 specimens that survived 313 days of exposure to 3.5% NaCl alternate 

immersion, salt spray, and high humidity at 25 ksi (10% YS) stress level (last three rows in table 4) 

were unloaded and reloaded to a higher stress, as shown in table 5. 

The Cronidur 30 specimens were not stressed initially to high percentages of the yield 

strength because this material, as hard as it was tested, was expected to show some susceptibility 

to stress corrosion. Surprisingly, they did not fail, even after increasing the stress as seen in table 5. 

Appearance of the specimens before and during exposure is presented in figures 5 through 

10. Note that Cronidur 30 specimens are not shown in figure 9 as they were removed from test 

at a later date.

At the end of the exposures, the specimens were cleaned and unloaded. Representative 

specimens were subjected to metallography and the results are presented in figures 11 through 16. 

As seen, metallography did not indicate any corrosion or stress corrosion attack on the materials. 

The remaining stress corrosion specimens were tensile tested to determine any reduction 

in tensile strength and the results are presented in table 6. Detailed postexposure tensile tests data 

are presented in appendix D. Appendix D also shows initial average tensile data for comparison, 

as well as the stress corrosion test conditions.

Inconel 625, Hastelloy C276, TiCP, Ti-6-4, and Ti-6-4 ELI did not experience reduction in 

tensile strength. One Cronidur 30 specimen out of 10 exposed to pretreat experienced an 8% reduction in tensile strength (specimen No. C30-4). That sample was originally stressed to 50 ksi (20% of 

YS). Three of 10 specimens exposed to brine experienced reduction in tensile strength: (1) Specimen 

C30-19, 40% reduction (originally stressed to 25 ksi (10% YS)), (2) specimen C30-20, 42% reduction (originally stressed to 15 ksi (6% YS)), and (3) specimen C30-23, 16% reduction (tested with no 

applied stress). Of the six alloys tested, Cronidur 30 has the highest tensile strength (300.4 ksi) and 

hardness (HRC 58), and it was expected to be more susceptible than the rest of the materials tested. 

It must be mentioned that this material is expected to be used in bearing applications in the water 

recovery system. For this application, the material will be mostly in compression. 
11

Table 4. Stress corrosion test results in various environments.

Alloy

Test 

Environment

Stress Level

(% YS)

Stress Level

(ksi) Failure Ratio Days to Fail

Inconel 625 Pretreat 75 58.3 0/3 NF in 365 days

0 0 0/1 NF in 365 days

Brine 75 58.3 0/3 NF in 365 days

0 0 0/1 NF in 365 days

DI water 75 58.3 0/1 NF in 316 days

Hastelloy C276 Pretreat 75 50.3 0/3 NF in 365 days

0 0 0/1 NF in 365 days

Brine 75 50.3 0/3 NF in 365 days

0 0 0/1 NF in 365 days

DI water 75 50.3 0/1 NF in 316 days

TiCP Pretreat 75 68.6 0/3 NF in 365 days

0 0 0/1 NF in 365 days

Brine 75 68.6 0/3 NF in 365 days

0 0 0/1 NF in 365 days

DI water 75 68.6 0/1 NF in 316 days

Ti-6-4 Pretreat 75 103.9 0/3 NF in 365 days

0 0 0/1 NF in 365 days

Brine 75 103.9 0/3 NF in 365 days

0 0 0/1 NF in 365 days

DI water 75 103.9 0/1 NF in 316 days

Ti-6-4 ELI Pretreat 75 104.0 0/3 NF in 365 days

0 0 0/1 NF in 365 days

Brine 75 104.0 0/3 NF in 365 days

0 0 0/1 NF in 365 days

DI water 75 104.0 0/1 NF in 316 days

Cronidur 30

(hardened,

HRC 58 or

higher)

Pretreat 6 15.0 0/3 NF in 341 days

10 25.0 0/3 NF in 341 days

20 50.0 0/3 NF in 341 days

0 0 0/1 NF in 341 days

Brine 6 15.0 0/3 NF in 341 days

10 25.0 0/3 NF in 341 days

20 50.0 0/3 NF in 341 days

0 0 0/1 NF in 341 days

DI water 10 25.0 0/1 NF in 316 days

5% salt spray 10 25.0 0/3 NF in 313 days

High humidity 10 25.0 0/3 NF in 313 days

3.5% NaCl AI 10 25.0 0/3 NF in 313 days

*NF = no failures
12

Table 5. Supplementary SCC test results for hardened Cronidur 30 steel

in mixed environments and residual tensile strength.

Specimen 

No.

Initially Tested 

at 25 ksi in the 

Environments 

Shown Below 

Without Failing

Removed, 

Unloaded, 

Reloaded, and 

Subjected to 

the Following 

Environment

Stress Level 

for the Final 

Exposure

(% YS)

Stress Level 

for the Final 

Exposure

(ksi) Result

Residual 

Tensile 

Strength 

(UTSf)

(ksi)

UTSf 

/

UTSi

*

C30-24 5% salt spray 

at 95 oF for 

313 days

3.5% NaCl 

alternate 

immersion 

for 46 days

20 50 Survived 

359 days 

total exposure

304.9 1.01

C30-25 5% salt spray 

at 95 oF for 

313 days

3.5% NaCl 

alternate 

immersion 

for 46 days

20 50 Survived 

359 days 

total exposure

298.6 0.99

C30-26 5% salt spray 

at 95 oF for 

313 days

3.5% NaCl 

alternate 

immersion 

for 46 days

30 75 Survived 

359 days 

total exposure

308.5 1.03

C30-27 High humidity 

at 95 oF 

for 313 days

3.5% NaCl 

alternate 

immersion 

for 46 days

30 75 Survived 

359 days 

total exposure

299.7 1.00

C30-28 High humidity 

at 95 oF 

for 313 days

3.5% NaCl 

alternate 

immersion 

for 46 days

40 100 Survived 

359 days 

total exposure

319.7 1.06

C30-29 High humidity

at 95 oF 

for 313 days

3.5% NaCl 

alternate 

immersion 

for 46 days

40 100 Survived 

359 days 

total exposure

290.8 0.97

C30-30 3.5% NaCl 

alternate 

immersion 

for 313 days

3.5% NaCl 

alternate 

immersion 

for 46 days

50 126 Survived 

359 days 

total exposure

303.6 1.01

C30-31 3.5% NaCl 

alternate 

immersion 

for 313 days

3.5% NaCl 

alternate 

immersion 

for 46 days

50 126 Survived 

359 days 

total exposure

295.3 0.98

C30-32 3.5% NaCl 

alternate 

immersion 

for 313 days

3.5% NaCl 

alternate 

immersion 

for 46 days

66 166 Survived 

359 days

total exposure

287.5 0.96

*UTSi=300.4 ksi (averaged initial ultimate tensile strength obtained from nonexposed specimens).
13

F5_1511

625-4 Before Exposure

276-4 Before Exposure

TiCP-4 Before Exposure

64-4 Before Exposure

ELI-4 Before Exposure

C30-4 Before Exposure

625-4 After 181 Days in Pretreat

276-4 After 181 Days in Pretreat

TiCP-4 After 181 Days in Pretreat

64-4 After 181 Days in Pretreat

ELI-4 After 181 Days in Pretreat

C30-4 After 139 Days in Pretreat

Figure 5. Representative specimens before and during exposure to pretreat.
14

625-8 Before Exposure

276-8 Before Exposure

TiCP-8 Before Exposure

64-8 Before Exposure

ELI-8 Before Exposure

C30-14 Before Exposure

625-8 After 181 Days in Brine

276-8 After 181 Days in Brine

TiCP-8 After 181 Days in Brine

64-8 After 181 Days in Brine

ELI-8 After 181 Days in Brine

C30-14 After 139 Days in Pretreat

F6_1511

Figure 6. Representative specimens before and during exposure to brine.
15

625-12 After 114 Days in DI Water

276-12 After 114 Days in DI Water

TiCP-12 After 114 Days in DI Water

64-12 After 114 Days in DI Water

ELI-12 After 114 Days in DI Water

C30-33 After 114 Days in DI Water

F7_1511

Figure 7. Representative specimens during exposure to DI water.
16

F8_1511

Figure 8. Specimens after exposure to pretreat for 365 days.

F9_1511

Figure 9. Specimens after exposure to brine for 365 days.
17

Typical Appearance of a Nonexposed Cronidur 30 Specimen

5% Salt Spray

High Humidity

3.5% NaCl Alternate 

Immersion

(a)

(b)

(c)

C30-24 After 139 Days in 5% Salt Fog

C30-27 After 139 Days in High Humidity

C30-30 After 139 Days in 3.5% NaCl Alternate Immersion

F10_1511

Figure 10. Cronidur 30 specimens during and after exposure to various complementary 

environments: (a) C30-24, C30-27, and C30-30 after 139 days in various

environments, (b) after 313 days of exposure to various environments, 

and (c) 359-day exposure described in table 5.
18

(a)

(b)

× 20 Magnification, Unetched × 100 Magnification, Etched 

(15 mL HCl, 10 mL Acetic Acid, 10 mL HNO3)

× 20 Magnification, Unetched × 100 Magnification, Etched 

(15 mL HCl, 10 mL Acetic Acid, 10 mL HNO3)

F11_1511

Figure 11. Photomicrographs of Inconel 625 initially stressed to 75% YS (58.3 ksi) 

and exposed for 365 days in (a) pretreat (sample 625-6) and (b) brine 

(sample 625-10).
19

(a)

(b)

× 20 Magnification, Unetched × 100 Magnification, Etched 

(30 mL HCl, 5 mL HF, 10 mL HNO3)

× 20 Magnification, Unetched × 100 Magnification, Etched 

(30 mL HCl, 5 mL HF, 10 mL HNO3)

F12_1511

Figure 12. Photomicrographs of Hastelloy C276 initially stressed to 75% YS (50.3 ksi) 

and exposed for 365 days in (a) pretreat (sample C276-6) and (b) brine 

(sample C276-10).
20

(a)

(b)

× 20 Magnification, Unetched × 100 Magnification, Etched 

(Kroll’s Reagent)

× 20 Magnification, Unetched × 100 Magnification, Etched 

(Kroll’s Reagent)

F13_1511

Figure 13. Photomicrographs of TiCP initially stressed to 75% YS (68.6 ksi) and exposed 

for 365 days in (a) pretreat (sample TiCP-6) and (b) brine (sample TiCP-10).
21

(a)

(b)

× 20 Magnification, Unetched × 100 Magnification, Etched 

(Kroll’s Reagent)

× 20 Magnification, Unetched × 100 Magnification, Etched 

(Kroll’s Reagent)

F14_1511

Figure 14. Photomicrographs of Ti-6A-4V initially stressed to 75% YS (103.9 ksi) 

and exposed for 365 days in (a) pretreat (sample Ti-64-6) and (b) brine 

(sample Ti-64-10).
22

(a)

(b)

× 20 Magnification, Unetched × 100 Magnification, Etched 

(Kroll’s Reagent)

× 20 Magnification, Unetched × 100 Magnification, Etched 

(Kroll’s Reagent)

F15_1511

Figure 15. Photomicrographs of Ti-6-4 ELI initially stressed to 75% YS (104 ksi) 

and exposed for 365 days in (a) pretreat (sample ELI-6) and (b) brine 

(sample ELI-10).
23

(a)

(b)

× 20 Magnification, Unetched × 100 Magnification, Etched 

(Kalling’s Reagent)

× 20 Magnification, Unetched × 100 Magnification, Etched 

(Kalling’s Reagent)

F16_1511

Figure 16. Photomicrographs of Cronidur 30 initially stressed to 20% YS (50 ksi) 

and exposed for 341 days in (a) pretreat (sample C30-6) and (b) brine 

(sample C30-16).
24

Table 6. Posttest evaluation for metals exposed to phosphoric acid-based 

ECLSS pretreat, brine, and DI water.

Specimen 

No.(1) Alloy

Test 

Environment

Stress Level

(% YS)

Stress Level

(ksi)

Test

Duration

UTSi

(2)

(ksi)

UTSf

(3)

(ksi) UTSf/UTSi

Evaluation 

Performed

625-4 In 625 Pretreat 75 58.3 365 136.0 134.8 0.99 Tensile

625-5 In 625 Pretreat 75 58.3 365 136.0 137.6 1.01 Tensile

625-6 In 625 Pretreat 75 58.3 365 136.0 – – Metall.

625-7 In 625 Pretreat 0 0 365 136.0 137.2 1.01 Tensile

625-8 In 625 Brine 75 58.3 365 136.0 135.9 1.00 Tensile

625-9 In 625 Brine 75 58.3 365 136.0 137.3 1.01 Tensile

625-10 In 625 Brine 75 58.3 365 136.0 – – Metall.

625-11 In 625 Brine 0 0 365 136.0 136.0 1.00 Tensile

625-12 In 625 DI water 75 58.3 316 136.0 137.4 1.01 Tensile

276-4 C276 Pretreat 75 50.3 365 120.7 122.5 1.01 Tensile

276-5 C276 Pretreat 75 50.3 365 120.7 123.1 1.02 Tensile

276-6 C276 Pretreat 75 50.3 365 120.7 – – Metall.

276-7 C276 Pretreat 0 0 365 120.7 122.0 1.01 Tensile

276-8 C276 Brine 75 50.3 365 120.7 120.0 0.99 Tensile

276-9 C276 Brine 75 50.3 365 120.7 122.4 1.01 Tensile

276-10 C276 Brine 75 50.3 365 120.7 – – Metall.

276-11 C276 Brine 0 0 365 120.7 123.6 1.02 Tensile

276-12 C276 DI water 75 50.3 316 120.7 123.4 1.02 Tensile

TiCP-4 TiCP Pretreat 75 68.6 365 110.9 114.2 1.03 Tensile

TiCP-5 TiCP Pretreat 75 68.6 365 110.9 112.8 1.02 Tensile

TiCP-6 TiCP Pretreat 75 68.6 365 110.9 – – Metall.

TiCP-7 TiCP Pretreat 0 0 365 110.9 112.8 1.02 Tensile

TiCP-8 TiCP Brine 75 68.6 365 110.9 112.4 1.01 Tensile

TiCP-9 TiCP Brine 75 68.6 365 110.9 111.9 1.01 Tensile

TiCP-10 TiCP Brine 75 68.6 365 110.9 – – Metall.

TiCP-11 TiCP Brine 0 0 365 110.9 111.7 1.01 Tensile

TiCP-12 TiCP DI water 75 68.6 316 110.9 114.6 1.03 Tensile

Ti-64-4 Ti-6-4 Pretreat 75 103.9 365 156.3 159.5 1.02 Tensile

Ti-64-5 Ti-6-4 Pretreat 75 103.9 365 156.3 157.8 1.01 Tensile

Ti-64-6 Ti-6-4 Pretreat 75 103.9 365 156.3 – – Metall.

Ti-64-7 Ti-6-4 Pretreat 0 0 365 156.3 158.2 1.01 Tensile

Ti-64-8 Ti-6-4 Brine 75 103.9 365 156.3 157.1 1.01 Tensile

Ti-64-9 Ti-6-4 Brine 75 103.9 365 156.3 154.5 0.99 Tensile

Ti-64-10 Ti-6-4 Brine 75 103.9 365 156.3 – – Metall.

Ti-64-11 Ti-6-4 Brine 0 0 365 156.3 156.7 1.00 Tensile

Ti-64-12 Ti-6-4 DI water 75 103.9 316 156.3 159.9 1.02 Tensile

ELI-4 Ti-6-4 ELI Pretreat 75 104.0 365 157.2 161.7 1.03 Tensile

ELI-5 Ti-6-4 ELI Pretreat 75 104.0 365 157.2 162.2 1.03 Tensile

ELI-6 Ti-6-4 ELI Pretreat 75 104.0 365 157.2 – – Metall.

ELI-7 Ti-6-4 ELI Pretreat 0 0 365 157.2 163.5 1.04 Tensile

ELI-8 Ti-6-4 ELI Brine 75 104.0 365 157.2 163.6 1.04 Tensile

ELI-9 Ti-6-4 ELI Brine 75 104.0 365 157.2 162.6 1.03 Tensile
25

Table 6. Posttest evaluation for metals exposed to phosphoric acid-based 

ECLSS pretreat, brine, and DI water (Continued).

Specimen 

No.(1) Alloy

Test 

Environment

Stress Level

(% YS)

Stress Level

(ksi)

Test

Duration

UTSi

(2)

(ksi)

UTSf

(3)

(ksi) UTSf/UTSi

Evaluation 

Performed

ELI-10 Ti-6-4 ELI Brine 75 104.0 365 157.2 – – Metall.

ELI-11 Ti-6-4 ELI Brine 0 0 365 157.2 161.7 1.03 Tensile

ELI-12 Ti-6-4 ELI DI Water 75 104.0 316 157.2 162.7 1.03 Tensile

C30-4 Cronid. 30 Pretreat 20 50.0 341 300.4 277.1 0.92 Tensile

C30-5 Cronid. 30 Pretreat 20 50.0 341 300.4 295.5 0.98 Tensile

C30-6 Cronid. 30 Pretreat 20 50.0 341 300.4 – – Metall.

C30-7 Cronid. 30 Pretreat 10 25.0 341 300.4 306.2 1.02 Tensile

C30-8 Cronid. 30 Pretreat 10 25.0 341 300.4 305.7 1.02 Tensile

C30-9 Cronid. 30 Pretreat 10 25.0 341 300.4 320.4 1.07 Tensile

C30-10 Cronid. 30 Pretreat 6 15.0 341 300.4 314.1 1.05 Tensile

C30-11 Cronid. 30 Pretreat 6 15.0 341 300.4 314.8 1.05 Tensile

C30-12 Cronid. 30 Pretreat 6 15.0 341 300.4 295.9 0.99 Tensile

C30-13 Cronid. 30 Pretreat 0 0 341 300.4 294.1 0.98 Tensile

C30-14 Cronid. 30 Brine 20 50.0 341 300.4 302.7 1.01 Tensile

C30-15 Cronid. 30 Brine 20 50.0 341 300.4 309.7 1.04 Tensile

C30-16 Cronid. 30 Brine 20 50.0 341 300.4 – – Metall.

C30-17 Cronid. 30 Brine 10 25.0 341 300.4 308.7 1.03 Tensile

C30-18 Cronid. 30 Brine 10 25.0 341 300.4 300.9 1.00 Tensile

C30-19 Cronid. 30 Brine 10 25.0 341 300.4 179.9 0.60 Tensile

C30-20 Cronid. 30 Brine 6 15.0 341 300.4 174.1 0.58 Tensile

C30-21 Cronid. 30 Brine 6 15.0 341 300.4 295.0 0.98 Tensile

C30-22 Cronid. 30 Brine 6 15.0 341 300.4 288.5 0.96 Tensile

C30-23 Cronid. 30 Brine 0 0 341 300.4 251.6 0.84 Tensile

C30-33 Cronid. 30 DI Water 10 25.0 316 300.4 312.8 1.04 Tensile

(1) Samples 1–3 of each metal were used for obtaining initial tensile data without exposure.

(2) UTSi

 = initial average ultimate tensile strength of nonexposed specimens.

(3) UTSf = final ultimate tensile strength of exposed specimens.

Representative samples that were tensile tested are shown in figure 17. A neck-down effect 

(localized area reduction of the specimen during plastic deformation) can be observed for all the 

specimens except for Cronidur 30. That can be explained by the low ductility of this material when 

it is in the hard condition.

The stress corrosion specimens that were exposed to mixed environments were also tensile 

tested and the final strength compared with the initial strength. Those results are presented in the 

last two columns of table 5. The ratio of final-to-average initial strength values ranged from 0.96 

to 1.06, and the average value was 1. These results are significantly better than expected for this 

material in NaCl and high humidity environments. 
26

625-5 276-5 TiCp-5 64-5 ELI-5 C30-5

625-8 276-8 TiCp-8 64-8 ELI-8 C30-15

625-12 276-12 TiCp-12 64-12 ELI-12 C30-33

F17_1511

(a)

(b)

(c)

Figure 17. Stress corrosion specimens exposed to (a) phosphoric acid-based pretreat, 

(b) brine, and (c) DI water without failing and subsequently tensile tested. 

Specimens shown were exposed for 365 days, except Cronidur 30 specimens, 

which were exposed for 341 days.
27

5. CONCLUSIONS

All the stress corrosion specimens exposed to the phosphoric acid-based ECLSS pretreat 

and brine solutions survived exposures of up to 1 year. Therefore, they passed the first stage of this 

evaluation, consisting of exposing the samples to the fluids under a sustained tensile stress. 

The following stage consisted of performing posttest evaluations that included metallography to examine the microstructure and tensile tests to determine residual strength. Inconel 625, 

Hastelloy C276, TiCP, Ti-6-4, and Ti-6-4 ELI did not show corrosion or stress corrosion cracks in 

the metallographic analysis and did not experience reduction in tensile strength as a result of the 

exposure to the ECLSS fluids. Therefore, these alloys, as tested, can be considered resistant to stress 

corrosion and corrosion in the brine and pretreat.

Cronidur 30 steel exposed to the ECLSS fluids did not show corrosion or stress corrosion cracks in the metallographic analysis, but several specimens suffered reductions in the tensile 

strength. That reduction was moderate in the pretreat (1 specimen out of 10 suffered an 8% reduction) and significant in brine (3 specimens out of 10 suffered reductions of 16%, 40%, and 42%). 

Even when reductions in strength were found, the results are still better than expected for this highhardness (HRC >58), high-strength (300.4 ksi UTS) material. This material could be used successfully under compression, such as for bearings; however, sustained tensile stresses on this material 

should be avoided, or at least maintain them as low as possible. 

Supplementary tests performed on Cronidur 30 in mixed environments of high humidity, 

salt spray, and salt water alternate immersion did not produce any failures, and tensile test results 

after exposure were also acceptable. 
28

APPENDIX A—MATERIALS CERTIFICATIONS

Materials certifications are shown for Inconel 625 (fig. 18), Hastelloy C276 (fig. 19), TiCP 

(fig. 20), Ti-6-4 (fig. 21), Ti-6-4 ELI (fig. 22), and Cronidur 30 (fig. 23).

Figure 18. Inconel 625 material certification.
29

Figure 19. Hastelloy C276 material certification.
30

Figure 20. TiCP material certification.
31

Figure 21. Ti-6-4 material certification.
32

Figure 22. Ti-6-4 ELI material certification.
33

Figure 23. Cronidur 30 material certification.
34

APPENDIX B—TENSILE DATA FOR NONEXPOSED SPECIMENS

Tensile data for nonexposed specimens are shown for Inconel 625 (fig. 24), Hastelloy C276 

(fig. 25), TiCP (fig. 26), Ti-6-4 (fig. 27), Ti-6-4 ELI (fig. 28), and Cronidur 30 (fig. 29).

Figure 24. Inconel 625 stress-strain curves and tensile data for as-received specimens.
35

Figure 25. Hastelloy C276 stress-strain curves and tensile data for as-received specimens.
36

Figure 26. TiCP stress-strain curves and tensile data for as-received specimens.
37

Figure 27. Ti-6-4 stress-strain curves and tensile data for as-received specimens.
38

Figure 28. Ti-6-4 ELI stress-strain curves and tensile data for as-received specimens.
39

Figure 29. Cronidur 30 steel stress-strain curves and tensile data for as-received specimens.
40

APPENDIX C—FORMULAS FOR PREPARING PRETREATED URINE

C.1 International Space Station Baseline—Current Pretreatment

Add 15.9 mL of pretreat stabilizer (54.5% DI water, 9% chromium trioxide, and 36.5% sulfuric acid) and 265 mL of DI water to 1 L of urine.

C.2 Proposed Alternate Pretreatment

Add 17.5 mL of pretreat stabilizer and 265 mL of DI water to 1 L of urine. The pretreat 

stabilizer will contain 2.36 mL of 85% phosphoric acid and 0.94 mL of the 30% CrO3 oxidizer 

solution. The formulas for preparing pretreated urine oxidizer solution is prepared by adding 

300 gm of chromium trioxide in 700 gm of ultrapure DI water. 

Notes:

(1) Per these procedures, the chromium concentration in the pretreated urine is ≈800 mg/L 

and ≈5,000 mg/L in the brine. 

(2) In the baseline pretreated urine, the sulfate concentration is ≈8,000 mg/L and 

≈50,000 mg/L in the brine. 

(3) For the ‘alternate’ pretreated urine, the phosphate concentration is ≈20,000 mg/L 

and the sulfate is ≈1,600 mg/L. In the alternate brine, the phosphate is ≈130,000 mg/L 

and the sulfate is ≈10,000 mg/L. 

(Private Communication, E-mail from Donald L. Carter, MSFC-ES62, dated April 10, 2013.)
41

APPENDIX D—TENSILE DATA FOR EXPOSED SPECIMENS

Tensile data for exposed specimens are shown for Inconel 625 (fig. 30), Hastelloy C276 

(fig. 31), TiCP (fig. 32), Ti-6-4 (fig. 33), Ti-6-4 ELI (fig. 34), and Cronidur 30 (fig. 35). The stress 

corrosion test conditions are also shown, as well as the average tensile data for nonexposed specimens for comparison.
42

Figure 30. Inconel 625 stress-strain curves and tensile data after a 365-day exposure 

to ECLSS pretreat and brine.
43

Figure 31. Hastelloy C276 stress-strain curves and tensile data after a 365-day exposure 

to ECLSS pretreat and brine.
44

Figure 32. TiCP stress-strain curves and tensile data after a 365-day exposure to ECLSS 

pretreat and brine.
45

Figure 33. Ti-6-4 stress-strain curves and tensile data after a 365-day exposure 

to ECLSS pretreat and brine.
46

Figure 34. Ti-6-4 ELI stress-strain curves and tensile data after a 365-day 

exposure to ECLSS pretreat and brine.
47

Figure 35. Cronidur 30 steel stress-strain curves and tensile data after a 341-day 

exposure to ECLSS pretreat and brine.
48

Table 7 shows the SCC test conditions for Cronidur 30 steel specimens for figure 35. 

Table 7. Test conditions for exposed Cronidur 30 steel specimens.

Sample

Stress Level 

During 

SCC Test 

(%YS)

Stress Level 

During 

SCC Test 

(ksi)

Environment 

During 

SCC Test

C30-4 20 50 Pretreat

C30-5 20 50 Pretreat

C30-7 10 25 Pretreat

C30-8 10 25 Pretreat

C30-9 10 25 Pretreat

C30-10 6 15 Pretreat

C30-11 6 15 Pretreat

C30-12 6 15 Pretreat

C30-13 – – Pretreat

C30-14 20 50 Brine

C30-15 20 50 Brine

C30-16 20 50 Brine

C30-17 10 25 Brine

C30-18 10 25 Brine

C30-19 10 25 Brine

C30-20 6 15 Brine

C30-21 6 15 Brine

C30-22 6 15 Brine

C30-23 – – Brine

C30-33 10 25 Deionized water

Average tensile data for nonexposed Cronidur 30 steel specimens are shown in table 8.

Table 8. Average tensile data for nonexposed Cronidur 30 steel specimens.

UTS 

(ksi)

YS

(ksi)

E

(Msi)

EL

(%)

RA

(%)

300.44 252.65 29.33 2.17 1.61
49

Table 9 shows the SCC test conditions for Cronidur 30 steel specimens for figure 36 

(exposed to various environments).

Table 9. SSC test conditions for Cronidur 30 steel specimens.

Sample

Initially Tested 

at 25 ksi in the 

Environments Shown 

Below 

Without Failing

Removed, Unloaded, 

Reloaded, 

and Subjected 

to the Following 

Environment

Stress Level 

for the Final 

Exposure

(%YS)

Stress Level 

for the Final 

Exposure

(ksi)

C30-24 5% salt spray at 95 oF 

for 313 days

3.5% NaCl alternate 

immersion for 46 days

20 50

C30-25 5% salt spray at 95 oF 

for 313 days

3.5% NaCl alternate 

immersion for 46 days

20 50

C30-26 5% salt spray at 95 oF 

for 313 days

3.5% NaCl alternate 

immersion for 46 days

30 75

C30-27 High humidity at 95 oF 

for 313 days

3.5% NaCl alternate 

immersion for 46 days

30 75

C30-28 High humidity at 95 oF 

for 313 days

3.5% NaCl alternate 

immersion for 46 days

40 100

C30-29 High humidity

at 95 oF for 313 days

3.5% NaCl alternate 

immersion for 46 days

40 100

C30-30 3.5% NaCl alternate 

immersion for 313 days

3.5% NaCl alternate 

immersion for 46 days

50 126

C30-31 3.5% NaCl alternate 

immersion for 313 days

3.5% NaCl alternate 

immersion for 46 days

50 126

C30-32 3.5% NaCl alternate 

immersion for 313 days

3.5% NaCl alternate 

immersion for 46 days

66 166
50

Figure 36. Cronidur 30 steel stress-strain curves and tensile data after a 359-day exposure 

to mixed environments of 5% salt spray, high humidity, and 3.5% NaCl 

alternate immersion.
51

REFERENCES

1. ASTM G49-85 (Reapproved 2005), “Standard Practice for Preparation and Use of Direct 

Tension Stress-Corrosion Test Specimens,” ASTM International, West Conshohocken, PA, 

2005.

 

2. ASTM B117-11, “Standard Practice for Operating Salt Spray (Fog) Apparatus,” ASTM 

International, West Conshohocken, PA, 2011.

3. ASTM G44-99 (Reapproved 2005), “Standard Practice for Exposure of Metals and Alloys 

by Alternate Immersion in Neutral 3.5 % Sodium Chloride Solution,” ASTM International, 

West Conshohocken, PA, 2005.

4. MSFC-STD-3029, Rev A, “Guidelines for the Selection of Metallic Materials for Stress 

Corrosion Cracking Resistance in Sodium Chloride Environments,” NASA Marshall Space 

Flight Center, Huntsville, AL, 2005.
52

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Stress Corrosion Evaluation of Various Metallic Materials

for the International Space Station Water Recycling System

P.D. Torres

George C. Marshall Space Flight Center

Huntsville, AL 35812

National Aeronautics and Space Administration

Washington, DC 20546–0001

Unclassified-Unlimited

Subject Category 26

Availability: NASA STI Information Desk (757–864–9658)

Prepared by the Materials and Processes Laboratory, Engineering Directorate

M–1395

Technical Memorandum

NASA/TM—2015–218206

International Space Station, Environmental Control and Life Support System, Urine Processing Assembly, stress corrosion cracking, 

corrosion, metallic materials, urine pretreatment, urine brine, water recycling in the Space Station, water recovery in the Space Station

01–03–2015

UU 66

NASA

U U U

A stress corrosion evaluation was performed on Inconel 625, Hastelloy C276, titanium commercially pure 

(TiCP), Ti-6Al-4V, Ti-6Al-4V extra low interstitial, and Cronidur 30 steel as a consequence of a change in 

formulation of the pretreatment for processing the urine in the International Space Station Environmental Control and Life Support System Urine Processing Assembly from a sulfuric acid-based to a phosphoric acid-based solution. The first five listed were found resistant to stress corrosion in the pretreatment 

and brine. However, some of the Cronidur 30 specimens experienced reduction in load-carrying ability.

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