Stress corrosion cracking of stainless steels in high temperature caustic solutions

Abstract

Stress corrosion cracking (SCC) tests were conducted by the Slow Strain Rate (SSR) technique at strain rates ranging from 5 × 10−6 to 4 × 10−8 s−1 in 30% sodium hydroxide at 200 °C on three austenitic (S31603, ASTM F138, N08904) and a duplex (S31803) stainless steels. The duplex steel exhibited the best performances while the N08904 steel had the worst SCC resistance; it was found that the increase of nickel content from 11.5% of S31603 steel to 24.5% of N08904 steel did not improve the caustic SCC resistance. The SCC susceptibility is not reduced by increasing the purity of the steels: S31603 steel had practically the same behaviour of the ASTM F138 steel, produced by ESR, having less inclusion content. The SCC susceptibility increased monotonically with decreasing strain rate and was dramatically affected by additions of sulphide ions: in this environment all the steels examined behaved unsatisfactorily. These last two points, together with the fact that the free corrosion potential of the examined steels was situated in the active range, are indicative of the important role played by hydrogen in SCC in caustic solutions at least in the experimental conditions adopted.