The alloy studied, U50, is a ferritic-austenitic duplex stainless steel. In addition to the two major phases (that is, ferrite and austenite), several additional phases form during low and intermediate temperature aging, namely, M7C3, M23C6, σ, χ, R, γ2, and α′. This study details the precipitation of these phases and their influence on the mechanical and localized corrosion properties. The sigma phase has the most deleterious influence on the impact properties. The α′ produces the largest increase in the yield and ultimate tensile strength. The precipitation of M23C6 and σ adversely affects the pitting resistance of U50 in 0.1NHC1 because of the accompanying chromium and molybdenum depletion of the matrix. The formation of α′ also severely reduces the chromium content of the ferrite phase and thereby inhibits its passivation. The large volume percent of ferrite in the alloy prevents intergranular corrosion that typically accompanies grain boundary precipitation of M23C6 in single-phase stainless steels.

The apparatus and technique employed in slow strain-rate testing of stainless steels in high temperature high purity water are described and discussed. Typical results from work in progress on the influence of heat treatment and surface treatment on the intergranular stress corrosion cracking of Types 304 and 308 stainless steel and Alloy XM-19 are described. The correspondence between strain-rate tests varies from alloy to alloy. The minimum strain rate required to cause intergranular cracking varied significantly with the alloy and with the surface treatment. It is suggested that reduction of area and observation of the fracture mode provide better measures of environmental interactions than measures of tensile parameters obtained from the test records.

In situ surface enhanced Raman spectroscopy (SERS) was used to investigate the surface films that form on 304 and 316L stainless steels in a number of acidic solutions that either cause stress corrosion cracking (SCC) at room temperature (O.75M HCl) or do not cause SCC at room temperature (0.75M NaCl (pH3), 0.74M H2SO4, 0.87M HC1O4 and 0.75M HBr). The results indicate the same film forms on the steels in all solutions except 1M NaCl (pH3). Hence, while a specific surface film may be necessary for SCC, it is not sufficient to cause SCC of tensile stressed stainless steels. It was also determined that adsorption of chloride ions on stainless steel in 0.75M HCl does not occur in the range of potentials in which SCC occurs. Hence, the role of chloride in causing SCC of stainless steels in acidic solutions at room temperature is not associated with either the formation of specific surface films or adsorption of chloride ions.

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