Passive State Behavior of Special Austenitic and Ferritic Stainless Steels in Phosphoric Acid Polluted by Sulfide Ions Available to Purchase

The passive state behavior of the ferritic Z1 CD 29-4 stainless steel (SS) in industrial phosphoric acid (40 wt% H₃PO₄ with 1,000 ppm chloride [Cl^–] and 330 ppm sulfate [SO₄^2–]) polluted with 20 ppm sulfide ions (S^2–) was compared to that of the austenitic Z2 CNDU 25-25 SS, which is known for its high corrosion resistance. Comparison was made using electrochemical techniques, electron spectroscopy for chemical analysis (ESCA), and secondary ion mass spectroscopy (SIMS). The presence of sulfide ions (S^2–) increased the dissolution rate of the austenitic SS to near that of the ferritic SS. Degradation of the protective passive film on the austenitic SS was attributed to formation of compounds of nickel and copper at the expense of chromium and molybdenum. Behavior of the two SS in the passive state was not very different.