The Effects of Trace H2S Levels on the Structure of Surface Scales in CO2 Corrosion

Abstract

The CO2 corrosion of carbon steel is very strongly influenced by the formation of surface scales of corrosion products. Hence, the structure and composition of those scales has been the subject of much research. For oxygen-free brines in contact with gaseous CO2 at partial pressures up to ~50 bar, and temperatures up to ~120 °C, the formed scales are generally found to consist of a mixture of Fe3C (cementite) and FeCO3 (siderite). In simulated oil and gas production brines, the siderite incorporates Ca and Mg, while DeMarco et al [1] identified the scales that form over short times at ambient temperature as a mixture of Fe2(OH2)CO3 and Fe2O2CO3.Williams and colleagues [2-7] carried out detailed studies of the scales formed in solutions of moderate chloride content (0.5 – 1 M), saturated with CO2 at ~ 1 bar, pH adjusted to ~ 6.5 and at temperatures of ~ 80 °C. For the solutions containing only sodium chloride, siderite (FeCO3) was the major phase in the scale, though chukanovite (Fe2(OH)2CO3) was also present in some cases. With the addition of magnesium chloride, both siderite and chukanovite were always found in the scale, while there was no evidence of magnesium incorporation until the bulk concentration of MgCl2 was ~ 0.1 M. Considering the effect of low-level Cr and Mo alloying, it was found in the base sodium chloride solutions that mixed siderite/chukanovite scales were formed, but the scale became thinner and the chukanovite-to-siderite ratio decreased with alloy content. Even for alloys with 3.5 wt% Cr there was no evidence of discrete Cr (hydr)oxide layers within the scale. However, the addition of trace levels of H2S (∼ 0.5 μM) produced more significant changes. First, only chukanovite and FeS (mackinawite) were now found in the scale, with no siderite. Second, for the unalloyed steel and for alloys with 1 wt% Cr and up to 0.7% Mo, the scales were highly porous. Finally, for the 3.5wt% Cr alloy, a non-porous, protective layer was formed, about 200–600 nm thick.In this paper, we utilize ab-initio techniques to investigate the effects of very low level H2S on carbonate scale formation. In the past DFT has been used to understand the various aspects of corrosion and the design of corrosion inhibitors for various conditions [9-11]. We have used similar approach for studying the H2S adsorption on siderite and chukanovite surfaces, considering the experimental observation by Hassan et al [2], that trace amounts of H2S result in chukanovite over siderite. Initial studies show that there is a 0.7 eV higher interaction energy (more thermodynamically favorable) for H2S adsorption on chukanovite compared to siderite. This may imply the possibility that H2S could encourage chukanovite formation over siderite formation in aqueous environments.We further investigate this possibility using DFT to understand the mechanism that leads to formation of chukanovite. We aim to use implicit solvation in order to bring the effect of water in the vicinity. We further use the cluster approach within DFT, to explain the effect that alloying has on the scale formation. We take a bare Fe cluster and replace one of its atoms with an alloying element and study the electronic structure in the presence of H2S and CO2. The potential energy surface gets modified for a H2S—Fe interaction in the presence of Ni. We expect that this work would help us to have a deeper understanding of how alloying can impact or rather initiate certain kind of scale formation over others.