STRESS-CORROSION CRACKING OF STAINLESS-STEELS IN HIGH-TEMPERATURE CAUSTIC SOLUTIONS

Stress corrosion cracking (SCC) tests were conducted by the Slow Strai n Rate (SSR) technique at strain rates ranging from 5 x 10(-6) to 4 x 10(-8) s(-1) in 30% sodium hydroxide at 200 degrees C on three austeni tic (S31603, ASTM F138, N08904) and a duplex (S31803) stainless steels . The duplex steel exhibited the best performances while the N08904 st eel had the worst SCC resistance; it was found that the increase of ni ckel content from 11.5% of S31603 steel to 24.5% of N08904 steel did n ot improve the caustic SCC resistance. The SCC susceptibility is not r educed by increasing the purity of the steels: S31603 steel had practi cally the same behaviour of the ASTM F138 steel, produced by ESR, havi ng less inclusion content. The SCC susceptibility increased monotonica lly with decreasing strain rate and was dramatically affected by addit ions of sulphide ions: in this environment all the steels examined beh aved unsatisfactorily. These last two points, together with the fact t hat the free corrosion potential of the examined steels was situated i n the active range, are indicative of the important role played by hyd rogen in SCC in caustic solutions at least in the experimental conditi ons adopted. (C) 1997 Elsevier Science Ltd.