Explanation for initiation of fitting corrosion of stainless steels at sulfide inclusions

An explanation for the chemical changes which lead to high-rats dissolution of sulfide inclusions and hence to pit initiation is found in the composit ion changes induced in the sulfides as the steel cools from the melting tem perature of thr steel to the solidification temperature of the sulfide, In this temperature regime, on cooling, substantial chromium depiction of the steel may occur around the sulfide, and metallic phases rich in iron and de picted in chromium may precipitate within the inclusion. These chromium-dep leted zones can provide the high-rate dissolution which triggers pitting co rrosion. Experimental observations on artificial inclusions are supported b y thermodynamic calculations. (C) 2000 The Electrochemical Society. S0013-4 651(99)05-008-9. All rights reserved.