Pitting Liability of Thermally Oxidised Austenitic Steel

Abstract Effect of thermal oxidation on the behaviour of stainless steel (SS) in an ferrichlorid solution was investigated. Sheet specimens of austenitic AISI 316L with the observed area S = 90 cm 2 were immersed into the solution in the initial state, after heating in air isothermally at 200, 400, 600, 800 and 1000°C respectively or in temperature gradient, and after heating and subsequent cleaning of specimens in an nitric acid/hydrofluoride acid mixture. Spot potential profiles of specimens were taken up using a simple device with graphite counter-electrode i.e. by measuring local potential differences between specimen and graphite. The presence of thermal oxides enhances the non-uniformity of profile, which decreases after the removal of oxides by cleaning in the nitric acid/hydrofluoride acid mixture. However, the initial degree of uniformity is not re-established. Prolonged exposure to ferrichlorid solution provokes pitting corrosion of specimens heated isothermally at 400° C and above it or heated in temperature gradient. On such specimens pits number n p , their depths h p and orifice areas S p .