Effects Of Elemental Sulfur And CO₂on P110Steel Corrosion Behavior

The oil and gas field has become one of the most serious corrosive environments due to the co-existing sulfur and CO2. Especially, elemental sulfur is hydrolyzed to produce a strong adsorption of H2S, HS-and S2-at high temperature (≥80℃). The result is the abnormal complex, highly corrosive environment. The present work has used the coupon immersion and electrochemical techniques to study the corrosion behavior of P110steel, and analysize CO2corrosion, sulfur corrosion and their corrosion correlation on P110steel in simulated corrosive medium at atmospheric pressure.The corrosion rate results show that with increasing temperature, the corrosion rates, including CO2corrosion, sulfur corrosion and co-existing sulfur and CO2conditions are exponentially increasing. CO2corrosion and elemental sulfur corrosion are mainly uniform corrosion and localized corrosion, respectively. In S/CO2co-existing conditions, uniform corrosion occurs at low temperatures (70℃), the localized corrosion (pitting corrosion) appears at high temperatures (90℃). Elemental sulfur promotes localized corrosion, and CO2inhibits elemental sulfur corrosion.With increasing temperature, the corrosion products increase. When calcium atoms replace part of the iron atom in iron carbonate, the complex salts (Ca, Fe2, Mg)CO3and Mg3Ca(CO3)4appears. At high temperatures (90℃), corrosion scales are consisted of FeS (Mackinawite), part of which will be changed into Fe3S4and Fe1-xS (Pyrrhotite). It is known that these forms of FexSy scales will be varied from protective to non-protective, and become the great impact on the corrosion. At90℃, under co-existing S and CO2conditions, as a result, the formation of a protective iron carbonate films and the corrosion scales on the matrix surface can protect the elemental sulfur and P110steel from direct contact, and hinder the mass transfer, resulting in inhibiting on further corrosion due to the corrosive medium not to reach the surface of substrates and the reaction of Fe2+and negative ions.In CO2or elemental sulfur condition, the corrosion potential plays a shift to more negative value, and the corrosion current density performs an increasing tendency. The controlling process transforms the cathode process into anodic process. The polarization resistance becomes small. All the above, the conditions are the temperature from70℃to90℃. The improvement of temperature will promote the corrosion. While corrosion potential shift is sharply positive value in S/CO2co-existing conditions. At70℃, an AC impedance spectrum shows that the low frequency can partly reflect the capacitive reactance arc caused by corrosion product film. At90℃, an AC impedance spectrum characteristic of low frequency part is of the diffusion Warburg impedance.At the low temperature (70℃), it is principal that the elemental sulfur corrosion mechanism is caused by the direct S/Fe reaction. At the high temperature (90℃), the corrosion rate of P110steel increases sharply, the direct and hydrolysis reaction corrosion mechanism will exist at the same time.