Stress corrosion cracking in newer stainless steel grades

The primary objective of the project was to conduct a systematic evaluation of the occurrence of chloride-induced stress corrosion cracking in stainless steels, including newer steel grades for which there is less prior service experience on which to base material selection. The secondary objective was to evaluate and further develop laboratory test conditions to facilitate assessment of future materials. Primary attention has been paid to four core alloys: the austenitic S30403 and N08904 plus the duplex steels S32304 and S31803, in order to allow evaluation of the effect of both alloying level and alloy type (austentic or duplex). Other steel grades, particularly superaustenitic and superduplex steels, have been included in some investigations. The technical approach is divided into three parts: I Experimental evaluation of cracking resistance in a wide range of chloride-containing environments, with emphasis on those of relevance to service conditions. The main approach has been to define the limiting conditions for the occurrence of cracking, rather than quantifying the extent of cracking under given conditions as has been done in the past. The environments investigated includes the concentrated chloride environments, which are commonly used for laboratory testing of stainless steels, simulated condensed seawater, autoclave testing in dilute chlorides and caustic chlorides, and the highly oxidising conditions which occur in bleaching media in the pulp and paper industry. For evaluation purposes a common theme for all the environments investigated is the use of constant deflection specimens (U-bend) which permit simultaneous evaluation of several specimens. Supplementary studies are performed using constant load or slow strain rate testing. II Systematic investigation of critical experimental variables selected on the basis of the above work. These include primarily the effect of chloride concentration and temperature, also pH, redox potential or applied electrochemical potential and oxygen level. III Elucidation of the mechanisms for stress corrosion cracking to shed light on the reasons for the observed ranking effects. The primary tool is ex-situ chemical surface analysis using Glow Discharge Optical Emission Spectroscopy (GDOES) and X-ray Photoelectron Spectroscopy (XPS). In addition, the Contact Electric Resistance (CER) technique is used to evaluate the in-situ properties of the passive films. In the case of duplex stainless steels, cracking is governed by the complex mechanical and electrochemical interaction between the austenite and ferrite phases. This is elucidated by x-ray diffraction investigations of the residual and applied stresses in the two phases.