XPS Investigation on the Corrosion Behavior of 13Cr-Martensitic Stainless Steel in CO2-H2S-Cl− Environments

The corrosion behavior of 13Cr-martensitic stainless steel was investigated in CO2-H2S-Cl− environments typical of oil and gas wells under different CO2 and H2S partial pressures, with an NaCl content of 50 g/L, pH = 2.7 and 4.8, T = 80°C. The 13Cr was highly corrosion resistant to CO2-induced phenomena (general corrosion and carbonate stress corrosion cracking [SCC]), while the presence of H2S resulted in high sulfide stress corrosion cracking (SSCC) susceptibility and high corrosion rates. Moreover, the CO2 in CO2-H2S-Cl− systems inhibited general corrosion and SSCC phenomena by favoring the formation of a protective film. X-ray photoelectron spectroscopy (XPS) analysis of the films grown in different environmental conditions revealed: CO2 favors the growth of a Cr oxide-rich protective film with a low Fe oxide-sulfide content;the presence of H2S favors the formation of less protective Fe sulfide- and Fe oxide-rich layers. An index of protectiveness Ip = Cr3+/(Cr3+ + Feox) was defined and related to the environmental parameter EH2S,CO2=pCO2/pH2S+pCO2 and to the actual corrosion rates.