In Situ Raman Spectroscopy for Corrosion Products of Zinc in Humidified Atmosphere in the Presence of Sodium Chloride Precipitate

The corrosion products of zinc in an atmospheric environment in the presence of sodium chloride (NaCl) particles have been studied by in situ detection using Raman spectroscopy. Zinc plate with the NaCl precipitate at a density of 0.4 mg/cm² was exposed for 3 days or longer in air with controlled relative humidity (RH). The corrosion products of zinc were easily identified by comparison of the in situ Raman spectra measured during the corrosion process with standard spectra of various zinc compounds. Zinc corrosion largely depended on the RH of the atmosphere in the presence of NaCl precipitate. When the RH was <80%, the main corrosion product was zinc oxide (ZnO). When the RH was >80%, however, the corrosion products changed to aqueous zinc chloride (ZnCl₂) in the initial stage, and zinc hydroxy chloride or simonkollite (Zn[Cl]₂[Zn(OH)₂]₄) was formed in the latter stage. From the transient of the corrosion products measured by in situ Raman spectroscopy, the corrosion process of zinc in the higher RH environment was assumed as the following. The water layer adsorbed from the humidified air with the relatively high RH dissolved NaCl precipitate on the zinc surface. The water layer dissolving NaCl then corroded zinc by electrochemically coupled reactions: Zn → Zn²⁺ + 2e^– and O₂ + 2H₂O + 4e^– → 4OH^–. As the concentration of ZnCl₂ thus formed in the water layer increased with the proceeding corrosion and reached saturation, simonkollite began to precipitate from the concentrated ZnCl₂ solution according to the reaction: 4Zn²⁺ + 4OH^– + ZnCl₂(aq) → ZnCl₂(Zn[OH]₂)₄.