XPS INVESTIGATION ON AISI-420 STAINLESS STEEL CORROSION IN OIL AND GAS WELLS ENVIRONMENTS

The corrosion behaviour of 13Cr-martensitic stainless steel (AlSl 420) was investigated in CO2-H2S-Cl- environments typical of oil and gas wells under different CO2, and H2S partial pressures. The corrosion tests indicated that the AlSl 420 steel was highly corrosion resistant to CO2-induced phenomena (general corrosion and carbonate S.C.C.), while in the H2S environment a high S.S.C.C. (Sulphide Stress Corrosion Cracking) susceptibility and high corrosion rates were found. Moreover, CO2 in CO2-H2S-Cl- systems inhibited general corrosion and S.S.C.. phenomena by favouring the formation of a protective film. By means of X-ray photoelectron spectroscopy (XPS) the chemical nature of the films grown on AlSl 420 in different environmental conditions was investigated and the following statements were drawn out: - CO2 favours the growth of a hydrated Cr-oxide rich protective film with a low Fe-oxide and sulphide conten; - the presence of H2S favours the formation of less protective Fe-sulphide and Fe-oxide rich layers. Furthermore from XPS results an index of protectíveness Ip: Cr+3/(Cr+3 + Feox,) was defined and related to the environmental parameter E#H2S,CO2 = pCO2/pH2S + pCO2 and to the corrosion rates.