XPS investigation on AISI 420 stainless steel corrosion in oil and gas well environments

The corrosion behaviour of 13Cr-martensitic stainless steel (AISI 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 AISI 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.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 AISI 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 content;
–	the presence of H2S favours the formation of less protective Fe-sulphide and Fe-oxide rich layers.

Furthermore from XPS results an index of protectivenessI _p = Cr⁺³/ (Cr⁺³ + Fe ^OX was defined and related to the environmental parameter and to the corrosion rates.