What is the corrosion mechanism in the HCN-H₂S-H₂O system?

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This system represents a typical low-temperature corrosion environment in refinery catalytic cracking units. Its corrosion mechanism can be divided into three core processes: fundamental electrochemical corrosion – when H₂S dissolves in water, it ionizes to produce H⁺ and HS⁻; anodic and cathodic reactions occur on the surface of carbon steel, resulting in the formation of FeS corrosion products. Initially, a protective layer of ferrous sulfide is formed. The destructive effect of CN⁻: HCN in the system ionizes to form CN⁻, which dissolves and destroys the FeS protective layer, exposing the metal substrate directly to the corrosive medium and significantly accelerating the rate of uniform corrosion. During the hydrogen-induced damage process, CN⁻ also inhibits the recombination of hydrogen atoms into hydrogen molecules, facilitating the penetration of hydrogen into the steel structure; this ultimately leads to hydrogen embrittlement, hydrogen-induced cracking, and sulfide stress corrosion cracking.