Underpaint corrosion of zinc-coated steel sheet studied by in situ raman spectroscopy

The corrosion of zinc-coated steel sheets was studied by Raman spectroscopy. In aerated 0.03 M NaCl solutions, a large variety of corrosion products were identified in situ on unpainted sheets after 7 days of immersion. Zinc oxide, zinc hydroxychloride and zinc hydroxide were observed. For painted specimens, two main products, zinc oxide and zinc hydroxychloride, have been identified under paint blisters. It appeared that a different type of corrosion process is occurring under a paint film. It is hypothesized that the barrier properties of the paint are capable of modifying the mechanisms and thus the nature of the corrosion products. However, for both unpainted and painted specimens the nature of the corrosion products seems to depend on the local pH values in relation to chloride concentrations.