Passivity of Sanicro28 (UNS N-08028) stainless steel in polluted phosphoric acid at different temperatures studied by electrochemical impedance spectroscopy and Mott-Schottky analysis

Corrosion resistance of a highly alloyed austenitic stainless steel (Sanicro28) in 50 wt.% H3PO4 industrial medium containing impurities at temperatures from 20 °C to 80 °C was evaluated after different immersion times. Electrochemical measurements (polarization curves, OCP, EIS and Mott-Schottky) demonstrated that Sanicro28 passivates spontaneously. From impedance results, film thicknesses of about 1.6-4.5 nm were obtained. At low temperature, the resistance to corrosion increases with immersion time due to the formation of iron phosphate and/or chromium phosphate. At higher temperature, phosphate formed a porous polyphosphate film identified by μ-Raman. No pits are initiated on surface whatever the temperature.