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Spot potential profiles and pitting liability of thermally oxidised stainless steel

Effect of thermal oxidation on the behavior of stainless steel (SS) in FeCl3-solution was investigated. Rectangular sheet specimens of SS AISI 316L with observed area S=90 cm2 were immersed into the solution (a) in the initial state, (b) after heating in air isothermally at 200, 400, 600, 800 and 1000 °C respectively, or in temperature gradient between 1100 and 100 °C, and (c) after heating and subsequent removal of thermal oxides in HNO3/HF-mixture. Spot potential profiles of specimens were taken up using a device with graphite counter-electrode by measuring potential differences between specimen and graphite, Δ E. The presence of thermal oxides enhances profile non-uniformity that corresponds to increased electrochemical heterogeneity of SS surface. The removal of thermal oxides decreases profile non-uniformity, but it does not restore the initial degree of homogeneity. The prolonged exposure to FeCl3-solution provokes pitting corrosion of specimens heated isothermally at 400 °C and above it or heated in temperature gradient. On such specimens pits number, np, their depths, hp, and orifice areas, Sp, were determined. Maximum pits density, np/S, is observed after heating at 600 °C. Average pit depths, hp, and orifice areas, Sp, are affected by heating conditions, too. The removal of thermal oxides influences the values of np/S, hp, and Sp, considerably. Possible causes of the observed phenomena are discussed.

Stainless steel; thermal oxidation; spot potentials; pitting

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