INFLUENCE OF THE NITRIDING REGIME ON THE CORROSION RESISTANCE OF DUPLEX STAINLESS STEELS IN 48% PHOSPHORIC ACID

Abstract

This study investigates the influence of nitriding regime and method on the corrosion resistance of duplex stainless steels in 48% phosphoric acid. Ion and gas nitrided specimens treated under two temperature–time conditions (420 °C / 20 h and 560 °C / 6 h) were analyzed. Corrosion behavior was evaluated using a gravimetric method by monitoring the relative mass change Δm/S and the instantaneous corrosion rate V(t). A non-stationary corrosion process was identified, characterized by successive stages of surface activation, passivation, and cyclic breakdown and re-formation of the protective layer, particularly pronounced for high-temperature nitriding. Ion nitriding at 420 °C / 20 h provided the most stable corrosion behavior and earlier establishment of a quasi-stationary regime. The results highlight the importance of a kinetic approach for assessing corrosion resistance in highly aggressive acidic environments.