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In the HCN-H₂S-H₂O corrosion system, the role of the FeS film has obvious duality and is not a single protective effect.: Protective effect under normal working conditions: When the pH value of the system is greater than 6 and there is no interference from high concentrations of CN⁻, the FeS film generated by the reaction of H₂S and Fe can cover the surface of the steel, blocking the further outward diffusion of iron ions, greatly reducing the corrosion rate of the metal matrix, and achieving a protective effect similar to that of a passivation film. Destruction and acceleration in the presence of HCN CN⁻ in the system will undergo a complex reaction with FeS, directly dissolving and destroying the protective film, exposing the metal matrix again ； At the same time, after losing the barrier of the FeS film, hydrogen atoms can more easily penetrate into the steel, greatly increasing the risk of hydrogen bubbling and sulfide stress corrosion cracking, which will in turn accelerate the overall corrosion process. Negative effects of membrane structural defects. If the FeS film produced has a loose and porous powdery structure, it will form a macro-corrosion battery with the steel matrix. FeS serves as the cathode and the steel serves as the anode. On the contrary, it will significantly accelerate the rate of local corrosion and cause deeper pitting and ulcer-like corrosion.