Co2 Corrosion of the Welded Joint of an X65 Steel: Analysis of Surface Film Formed

Abstract

Carbon dioxide (CO2) corrosion, also known as sweet corrosion of steels has increased in recent years due to the injection of CO2 into oil wells in order to increase oil recovery and production. Despite the fact that carbon steel has a low corrosion resistance, it is widely used in the petroleum industry due to its economic advantage. It has been reported that when ferrous steels are exposed to a corrosive medium, they form a protective surface film which helps to decrease the rate of corrosion that would have occurred otherwise. It has been observed that many factors including; differences in chemical composition, temperature, flow rate, pressure and pH can affect the properties of the protective layer formed on the steel surface. A significant variation in microstructure contributes to localized corrosion due to non-homogenous surface film formation. Researchers have therefore emphasized the importance of understanding the specific nature of the film produced under certain conditions. This research aims at characterizing the surface film which forms across the welded joint of an X65 steel exposed to brine and CO2 produced by sugar-fermenting yeast at 65oC. SEM results showed that the film formed across the regions of the weld varied in the degree of protectiveness, which was responsible for the corrosion rates observed.