The Effect of Heat Treatment on the Degree of Sensitization of Type 304 Stainless Steel

Abstract

Two electrochemical techniques, anodic polarization and potentiostatic etching, were used in detecting the degree of sensitization (DOS) of austenitic stainless steel in an electrolyte which contained perchloric acid with NaCl as an additive. The DOS of the specimen could be quantitatively measured by the current density at 290 mVH during the polarization test, or by the total charge during the potentiostatic etching test. The DOS was measured for specimens in various sensitized conditions. Brief heating of a specimen at 800 C prior to a low temperature sensitization heat treatment (550 C) resulted in a much higher DOS than that of a specimen sensitized at the low temperature only. Data obtained by the electrochemical techniques were compared with the Huey test (boiling nitric acid test) and with the depth and width of attack in the grain boundaries. In depth Auger analyses was made of the oxide films formed in air on sensitized and solution annealed specimens. It was found that sensitized specimens were coated with a thicker but less compact film.