The electrochemical corrosion process of hydrogen sulfide
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In oil and gas extraction, compared to CO2 and O2, H2S has a higher solubility in water; once it dissolves in water, it immediately ionizes, making the water acidic. H2S = HS- + H+; HS- = S2- + H+. The hydrogen ions produced by this ionization are strong depolarizers that readily take electrons at the cathode, thereby promoting the dissolution of iron at the anode and leading to overall corrosion of steel. Its electrochemical corrosion process is as follows: Anodic reaction: Fe – 2e⁻ = Fe²⁺. Cathodic reaction: 2H⁺ + 2e⁻ = H₂. The product of the anodic reaction is Fe²⁺ + S²⁻ = FeS. The FeS formed as a result of the anodic reaction is usually a protective layer with a defective structure; it has poor adhesion to the steel surface, tends to peel off easily, is susceptible to oxidation, and can be penetrated by chloride ions. Moreover, it has a relatively positive potential, so it acts as a cathode together with the steel substrate, forming an active microcell that continues to corrode the substrate.
Hydrogen sulfide (H₂S) easily dissolves in water during oil and gas extraction, and once dissolved it ionizes to release hydrogen ions (H⁺), causing the water to become acidic. These hydrogen ions act as \"depolarizers\" in electrochemical corrosion; they readily steal electrons from the steel surface (the cathode), accelerating the dissolution of the steel (the anode). The specific corrosion process is as follows: – Anode (steel): Iron loses electrons and turns into iron ions (Fe²⁺). - Cathode: Hydrogen ions gain electrons and turn into hydrogen gas (H₂). - Corrosion products: Iron ions combine with sulfur ions (S²⁻) to form ferrous sulfide (FeS). The problem is that the resulting FeS protective layer has poor quality—it has an incomplete structure, does not adhere well, tends to fall off, can be oxidized, and chloride ions can easily penetrate it. What’s more troublesome is that the potential of FeS is higher than that of steel, so it becomes the cathode itself, forming a microcell with the steel matrix and thus continuing to corrode the steel, worsening the situation further. In short, H₂S not only causes corrosion but also enables the corrosion products to further accelerate damage; its corrosive effect is more problematic than that of CO₂ and O₂. .