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# Assessment of Stress Corrosion Cracking Susceptibility for Austenitic Stainless Steels Exposed to Atmospheric Chloride and Non-Chloride Salts (NUREG/CR-7170)

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## Publication Information

**Manuscript Completed: **** Date Published: **
**Prepared by:** Xihua He1 Todd S. Mintz1 Roberto Pabalan1 Larry Miller1 Greg Oberson2

1Center for Nuclear Waste Regulatory Analyses Southwest Research Institute 6220 Culebra Road San Antonio, TX 78238-5166

2U.S. Nuclear Regulatory Commission

Greg Oberson, NRC Project Manager

**Prepared for:** Office of Nuclear Regulatory Research U.S. Nuclear Regulatory Commission ****Washington, DC 20555-0001

NRC Job Code V6288

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## Abstract

Deliquescence and efflorescence tests of simulated sea salt indicated that the DRH of sea salt is close to that of calcium chloride (CaCl2) and magnesium chloride (MgCl2), in the range of about 20 to 30 percent relative humidity (RH) at temperature less than 80 °C [176 °F]. Type 304 stainless steel U-bend specimens exposed to cyclic AH between 10 and 30 g/m3 at temperatures from 35 to 60 °C [95 to 140 °F] exhibited SCC initiation at a surface salt concentration as low as 0.1 g/m2. For the elevated temperature testing in constant humidity conditions, SCC initiation was observed up to 80 °C [176 °F]. In both the cyclic and constant humidity testing, material in the metallurgically sensitized condition showed greater SCC susceptibility than as-received material or welded specimens. For the high-humidity test condition at 30 °C [86 °F] and 90 percent RH, the deliquescent solution was relatively dilute in chlorides, but SCC initiation was still observed for U-bend specimens. Finally, testing with Cring specimens strained to 0.4 and 1.5 percent, compared to about 15 percent for U-bend specimens, showed SCC initiation at both levels. The stress level at 0.4 percent strain is approximately the material yield stress. Based on this series of test results, it was concluded that austenitic-stainless steel is susceptible to chloride-induced SCC caused by salt deliquescence at RH above 20 to 30 percent even at relatively low salt concentrations and stress levels.

Concerning non-chloride salt atmospheric species, a literature review indicated that sulfate (SO42−), nitrate (NO3−), and ammonium (NH4+) are the most abundant soluble ions in atmospheric particulate matter at locations near industrial, commercial, or agricultural activities. Chloride (Cl-) and bisulfate (HSO4-) ions are also present in smaller amounts. Type 304 stainless steel U-bend specimens did not exhibit SCC when exposed to sulfate, bisulfate, and nitrate salts, but SCC was observed when ammonium nitrate (NH4NO3) was mixed with NaCl with nitrate-to-chloride molar concentration ratios of 3.0 and 6.0. The extent of SCC seemed to increase with increasing chloride concentration. Minor pitting corrosion was observed for Ubend specimens exposed to fly ash possibly because of the part per million level of chloride in the fly ash leachate.

Page Last Reviewed/Updated Wednesday, March 24, 2021