Study On The Semiconductor Properties Of Corrosion Products Of P110 Steel In CO / H ₂ S Environment

The corrosion film on the surface of P110tubing steel formed in saturated CO2/H2S plays a key role to corrosion rate, corrosion type and the effect of corrosion inhibitor, etc. That’s because the further corrosion of steel strongly depends onthe transport of aggressive ionic and the electronic conductivity, which directly relates to the semiconductor property of the film. But for the moment, people had not yet established a relationship of corrosion electrochemistry and semiconductor property. A preliminary exploration was done to explain the relationshipin this work.Thus, three typical corrosion films were prepared respectively in CO2or H2S dissolved distilled water as expected. One is pure FeCO3corrosion film. Another is FeS1-xfilmincluding mackinawite (FeSo.9) andcubic ferrous sulfide. The other is FeS1+xfilmincludingpyrrhotite (FeS), pyrrhotite (Fe1-xS) and pyrite (FeS2). The relation of semiconductor property and corrosion resistance of the three corrosion filmsin NaCl, Na2SO4and NaHCO3solutions was investigated by the methods of Mott-Schottky, capacitance measurement, potentiodynamic polarization, weight loss, X-ray Diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectrometer (XPS).The physicochemical mechanism for the semiconducting property of FeCO3film was explained by the Point Defect Model (PDM). The results showed that in the solution dissolved with NaCl or Na2SO4, the FeCO3film presented p-type semiconductor with high acceptor density for doped by VFe2-. The integrity of microstructure was damaged. The film had lower anti-corrosion ability. However, in the solution dissolved with NaHCO3, n-type semiconductor with low donor density was obtained for the FeCO3film. The compact microstructure was maintained. The film had higher anti-corrosion capacity.The different semi-conductor behaviors of FeS1-x film must be associated with well-defined chemical compositions. Thus, XPS was used. The results showed that p-type semiconducting property with high acceptor density was found in the NaCl and Na2SO4solutiondue to increasing amounts of FeS2and the reducing Fe/S atomic ratio. FeS1-x scale presented not well-arranged microstructure. The ability of resisting corrosion was decreased. However, FeS1-x scale in NaHCO3solution showed n-type semiconductor with low donor density for small amounts of FeS2product and the stable Fe/S atomic ratio. The compact microstructure was maintained and the higher anti-corrosion capacity was presented.It was similar to the FeS1-x film, the physicochemical mechanism for different semi-conductor behaviors of FeS1+x film was explained by XPS. The results indicated that the FeS1+x film had bipolar semiconductor property in the NaCl and Na2SO4solution, i.e., the excess of S2-in Fe1-xS and/or FeS2presented p-type semiconductor as inner layer and the excess of Fe2+in FeS2presented n-type semiconductor as outer layer. The integrity of microstructure was destroyed in a certain extent. In NaHCO3solution, n-type semiconductor was obtained as outer layer of FeS1+x film transform into FeS1-xfilm.The stable and dense microstructure was not damaged. The FeS1-xfilm inhibits the corrosion of steel effectively.