Oilfield corrosion scaling : sour environments

Acid gas corrosion of carbon and low alloy steel equipment utilised for hydrocarbon transportation is a major concern for the oil and gas industry. This can often result in the formation of adhered corrosion products i.e. scales. Such scales have been known to either reduce corrosion or exacerbate it, depending on the environmental conditions. The objective of this research to improve the understanding of scale characteristics and associated corrosion behaviour in H2S-N2 and CO2-H2S solutions. The temporal evolution of scale characteristics and complementary corrosion behaviour was explored by developing scales on Fe substrates in H2S 1%/N2 99% saturated solution at 80°C and pHs of 5.3 and 6.8 over 144 h. Mackinawite (FeS) was developed in both conditions, key details about the structural growth and disorder are also provided. Mackinawite developed in both conditions exhibited porosity. Interestingly, STEM imaging revealed nanoscale porosity in mackinawite developed in both conditions, sufficiently large enough to allow the H2S 1%/N2 99% saturated solution to access the Fe surface and cause corrosion. EIS, however, revealed mackinawite developed in pH of 6.8 to be protective against corrosion while mackinawite developed on pH of 5.3 was not. This finding highlights the lack of correlation between scale morphology and corrosion behaviour, questioning the scale characteristics which actually govern these corrosion traits. Subsequently, the scale development in H2S 1%/CO2 99% saturated solution at 80°C was investigated. The effect of pH was explored by immersing Fe substrates in three different bulk solution pHs i.e. 4.5, 5.3 and 6.8 for 144 h. Fe immersed in pH 4.5 exhibited the highest resistance to iron dissolution, despite no scale detection through GIXRD and SEM. Conversely, Fe immersed in bulk solution pHs of 5.3 and 6.8 exhibited scale formation, coupled with much lower resistances to iron dissolution. Interestingly, Fe immersed in pHs of 4.5 and 5.3 also exhibited pitting corrosion, with much deeper pits observed in the more acidic condition. Thereafter, a temporal evolution study into scale characteristics in H2S 1%/CO2 99% saturated solution, pH of 6.8 over 144 h revealed the scale to be a mixture of mackinawite and chukanovite (Fe2(OH)2CO3) , which is not protective against general corrosion. Additionally, chukanovite developed in the presence of H2S exhibited platy characteristics, albeit on a much smaller length scale than observed in the literature.