Effect of alloying elements on the corrosion behavior of 0Cr12Ni3Co12Mo4W ultra high strength stainless steel

The effect of alloying elements on the corrosion behavior of 0Cr12Ni3Co12Mo4W ultra high strength stainless steel (UHSSS) in 0.1M Na₂SO₄ solutions with Cl^- at pH=3 was studied by XPS, AES, SEM, EDS and a series of electrochemical measurements. The passive film with a thickness of about 7nm, formed on the UHSSS surface in 0.1M Na₂SO₄ solution at pH=3, contained an outer layer as n-type semiconductor with Fe₃O₄ and NiO, and an inner layer as p-type semiconductor with Cr₂O₃. The electrochemical measurements results showed that two kinds of pitting corrosion, outer pitting and inner pitting, occurred on the surface of UHSSS specimens in 0.1M Na₂SO₄ + 1wt.% NaCl solution at pH=3. The outer corrosion products were mainly composed of the oxides of Fe, Cr, Mo and Co, while the inner corrosion products were mainly composed of the oxides of Cr and Mo. During the inner pitting corrosion, the alloying elements Fe and Co were dissolved and diffused out of the pits, while Cr and Mo were dissolved and then deposited in the pits.