SKP and FT-IR microscopy study of the paint corrosion deadhesion from the surface of galvanized steel

摘要

Scanning Kelvin Probe (SKP) and FT-IR microscopy (FT-IR) were used to study the atmospheric corrosion of zinc, galvanized steel (HDG) and HDG coated by electrodeposited epoxy resin (ED). NaCI was locally deposited on the bare metal surface or inside the defect of the coating. The SKP was useful to determine the spatial separation of the electrochemical reactions across corroding surface to evaluate the formation of the galvanic cells. FT-IR microscopy was helpful to define the composition and distribution of corrosion products in galvanic cells. The main aim of this study was to evaluate the mechanisms of atmospheric corrosion and spatial separation of the corrosion reactions leading to corrosion de-adhesion of the paint. At the beginning, deposition of the droplet of a NaCI water electrolyte on bare zinc surface led to a spatial separation of cathodic and anodic reactions. The centre of deposition corresponds to the potential of active zinc dissolution that led to spreading of the cathodic reaction to the surrounding "passive" metal surface. Similar separation takes place across defect in ED coated HDG contaminated by NaCI. The cathodic spreading led to cathodic delamination of the coating.