Passivation behaviour of stainless steel (UNS N-08028) in industrial or simplified phosphoric acid solutions at different temperatures

The corrosion behaviour of Sanicro28, in industrial phosphoric acid and simplified solution of pure H3PO4 containing sulphates and chlorides, is investigated during 48 h between 20 and 80 °C. Results show higher corrosion resistance in the industrial medium. Film thickness evaluated by EIS remains constant in industrial acid due to polyphosphate that promotes crystallisation of the Cr2O3 inner layer. Conversely, the passive film is partially destroyed at 80 °C in simplified acid. XPS and Raman analysis show that phosphate and molybdenum are both present in the passive film and in corrosion products. The role of Ca present in the industrial acid is discussed.