7.
The corrosion behaviour of 13Cr-martensitic stainless steel (AISI 420) was investigated in CO
2-H
2S-Cl
– environments typical of oil and gas wells under different CO
2 and H
2S partial pressures. The corrosion tests indicated that the AISI 420 steel was highly corrosion resistant to CO
2-induced phenomena (general corrosion and carbonate S.C.C.), while in the H
2S environment a high S.S.C.C. (Sulphide Stress Corrosion Cracking) susceptibility and high corrosion rates were found. Moreover, CO
2 in CO
2-H
2S-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 protectiveness
I
p = Cr
+3/ (Cr
+3 + Fe
OX
was defined and related to the environmental parameter
and to the corrosion rates.
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