Kinetics of Iron Carbonate and Iron Sulfide Scale Formation in CO₂/H₂S Corrosion

Kinetics of iron carbonate and iron sulfide scale formation in CO₂/H₂S corrosion was investigated by individually studying iron carbonate formation in pure CO₂corrosion, iron sulfide formation in N₂/H₂S corrosion, and the mixed iron carbonate/sulfide formation in CO₂/H₂S corrosion.

The first part of the project was to investigate kinetics of iron carbonate scale formation in CO₂ corrosion. A unified iron carbonate solubility expression which accounts for both temperature and ionic strength effects was proposed based on the literature data. The weight change method was developed to more accurately define kinetics of scale formation in CO₂ corrosion and demonstrated that the old data from literature are one to two orders of magnitude too high. Based on the experimental data, a reliable iron carbonate formation equation was developed to describe iron carbonate scale growth on the steel surface in CO₂ corrosion.

The second part of the project was to investigate the mechanism and kinetics of iron sulfide formation in N₂/H₂S environment. The solubility limits of hydrogen sulfide and iron sulfides were clarified based on the literature data. Using weight change method, both the corrosion rate of the steel and the retention rate of the scale were found. It was also concluded that mackinawite is the predominant iron sulfide formed on the steel surface under the test conditions studied, most likely by a direct reaction of H₂S with the underlying steel. Based on the experimental results, a mechanistic model of uniform H2S corrosion of mild steel was presented that was able to predict corrosion rate with time.

Finally, kinetics experiments conducted in CO₂/H₂S solution proved that the makeup of the surface scale not only depends on the water chemistry and the respective solubility of iron carbonate and iron sulfide, but also on the competitiveness of the two scale formation mechanisms. Based on the experimental data it was found that mackinawite was the predominant scale formed on the steel surface which protected the steel from corroding in CO₂/H₂S corrosion. The mechanistic model for H₂S corrosion was extended to predict the CO₂/H₂S corrosion process by considering the effect of the presence of CO₂.