Corrosion mechanism of low-alloy steel used for flexible pipe in vapor-saturated H2S/CO2 and H2S/CO2-saturated brine conditions

Immersion experiment was used to elucidate the corrosion mechanism of low-alloy pipeline steel exposed to vapor-saturated H2S/CO2 and H2S/CO2-saturated brine conditions in terms of microstructure, corrosion kinetics, corrosion products, surface/cross-sectional morphology, and elemental distribution. The experimental results demonstrate that the microstructure of designed steel is tempered martensite, and the corrosion rate in H2S/CO2-saturated brine condition is larger than that in vapor-saturated H2S/CO2 condition. The main corrosion product in two corrosion conditions is FeS, which shows various crystal structures. The size of corrosion products formed in vapor-saturated H2S/CO2 condition is smaller than that of corrosion products formed in H2S/CO2-saturated brine condition, which contributes to the difference of water chemistry in two corrosion conditions. Cr-rich compounds are primarily formed on the coupons surface of two corrosion conditions. The schematic models of corrosion mechanism in vapor-saturated H2S/CO2 condition and H2S/CO2-saturated brine condition are proposed based on experimental results.