Why does hand sweat cause metal to rust?

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Hand sweat can cause metal to rust, as the water, salts (such as sodium chloride) and organic acids (such as lactic acid) in sweat undergo electrochemical corrosion reactions with the metal surface. Sweat forms an electrolyte solution on the skin surface, which promotes the oxidation of metals; this leads to the formation of rust (Fe₂O₃·xH₂O) or patina (Cu₂(OH)₂CO₃), especially on metals such as iron and copper. The specific mechanism is as follows: Electrolyte effect: Sweat contains ions such as Na⁺ and Cl⁻, which form a conductive liquid film that facilitates the dissolution of the anodic metal (e.g., Fe → Fe²⁺ + 2e⁻) as well as the reduction of oxygen at the cathode (O₂ + 2H₂O + 4e⁻ → 4OH⁻). Acidic environment: The pH value of sweat on the hands is around 5-6, indicating a weakly acidic condition; this can dissolve the oxide layer on metal surfaces and accelerate corrosion. Formation of oxidation products: The resulting Fe²⁺ is further oxidized to Fe³⁺, ultimately forming loose rust that loses its protective properties, allowing corrosion to progress further.