JOURNAL OF MATERIALS SCIENCE 25 (1990) 1407-1415 XPS investigation on AISI 420 stainless steel corrosion in oil and gas well environments G. FIERRO, G. M. INGO*, F. MANCIA, L. SCOPPIO, N. ZACCHETTI Centro Sviluppo Materiafi SpA, P. O. Box 10747 00100 ROMA-EUR (Italy) The corrosion behaviour of 1 3Cr-martensitic stainless steel (AISI 420) was investigated in CO2-H2S-CI- environments typical of oil and gas wells under different CO 2 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-CI- systems inhibited general cor- rosion 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/p = Cr+3/(Cr +3 + Fe ~ was defined and related to the environmental parameter EH2s,co 2 = pCO2/PH2S + pCO2 and to the corrosion rates. 1. Introduction The use limits of 13Cr-martensitic stainless steel (AISI 420) in gas and oil wells (CO2-H2S-C1- in aqueous solution, T = 80-120~ are difficult to determine due to the presence of different failure phenomena depending on the environmental variables (partial pressures, temperatures, pH, chloride content) [1-5]. The technological characterizations carried out so far, in autoclaves simulating operating conditions, have revealed [6-9] a high suspectibility of 13Cr stain- less steel to cracking induced by H2S aqueous system, Sulphide Stress Corrosion Cracking, (S.S.C.C.), Hydrogen Induced Cracking (H.I.C), and a good cor- rosion resistance in CO2 aqueous system due to the inhibiting effect of CO2 on the corrosion rate [10]. Moreover Stress Corrosion Cracking (S.C.C.) tests carried out in CO2-H2S aqueous systems showed that the Oth (stress-threshold for SSC onset) values obtained are higher than the typical values obtained in H2S systems. These corrosion phenomena have been explained by the growth of protective films [5, 7, 11, 12], but only in a few cases has the nature of these films been investigated and chromium carbonates, complex mix- ture of iron - chromium sulphide and carbonates have been determined [12, 13]. Moreover published data on iron-25Cr alloy exposed to H2S atmospheres (up to 300 ~ C) indicate the growth of FeS and Fe-Cr oxide layers [14, 15]. The aim of this work is to establish by means of X-ray Photoelectron Spectroscopy (XPS) the chemical composition of the films grown on AISI 420 in H2S-CO2-CI- aqueous environments in a wide range ofpH2S and pCO2 partial pressures, with a NaC1 content of 50gl -I, pH = 2.7 and 4.8, T = 80~ to relate the corrosion data with the nature of these complex films. 2. Experimental details The photoelectron spectra were performed with a Leybold & Heraeus LHS10 spectrometer equipped with an EAll electron analyser, using an unmono- chromatized A1K0q, 2 (hv = 1 486.6 eV) radiation. The electron analyser operated in FAT mode select- ing a constant pass energy of 50eV. Under such conditions the Full Width at Half Maximum (FWHM) of Ag3ds/2 peak of an Ar + sputtered silver metal surface was 1.01 eV. All measurements were per- formed at pressures lower than 10 10 mbar in the analysis chamber. The samples were in Ohmic contact with the electron spectrometer. The Binding Energies (B.E.) were referenced to the *Present address: CNR, Istituto di Teoria e Struttura Elettronica, P.O. Box I0, 00016 Monterotondo ~ Stazione, Roma. 0022-2461/90 $03.00 § .12 1990 Chapman and Hall Ltd. 1 407