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: March 2013
Date Published: February 2014

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

Most spent nuclear fuel dry storage canisters used in the United States are fabricated from austenitic stainless steel. Canisters in externally vented shielding structures may be exposed to airborne chemical species during service. Species may include chloride-rich salts from marine environments, de-icing salts, or condensed water from cooling towers, as well as a range of other non-chloride-rich salts originating from industrial, agricultural, and commercial activities. This report documents the results of a systematic study on the stress corrosion cracking (SCC) susceptibility of austenitic stainless steel exposed to these species. It is postulated that with sufficient ambient humidity, SCC could initiate by deliquescence of species on the canister surface, particularly at locations such as welds. A series of tests was performed to gain further insight into the conditions where SCC could occur, investigating such parameters as temperature, humidity, salt concentration, material metallurgical condition, and stress level. The tests included:

	(i)	Measurement of deliquescence relative humidity (DRH) and efflorescence relative humidity of sea salt and its pure salt constituents;
	(ii)	SCC testing of austenitic stainless steel specimens exposed to sea salt within the range of absolute humidity expected in natural conditions;
	(iii)	SCC testing of austenitic stainless steel specimens exposed to sea salt at elevated temperatures;
	(iv)	SCC testing of austenitic stainless steel specimens exposed to sea salt at high-humidity conditions;
	(v)	SCC testing of austenitic stainless steel specimens exposed to sea salt with various applied strains and stresses; and
	(vi)	Deliquescence and efflorescence measurements of non-chloride-rich atmospheric species and limited SCC testing of austenitic stainless steel specimens exposed to these species.

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.