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

Scanning Kelvin Probe (SKP) and FTIR microscopy were applied to study the atmospheric corrosion of galvanized steel coated by electrophoretic epoxy resin (ED) at a defect.The SKP was useful to determine the spatial separation of the electrochemical reactions at a defect and surrounding metal/paint interface and to evaluate the formation of the galvanic cells. FT-IR microscopy was helpful to identify the composition and distribution of the corrosion products in the galvanic cells.It was shown that the cathodic delamination of coating takes place after deposition of a thick water electrolyte film in the defect. The anodic undermining of the coating is favoured in case of atmospheric corrosion under thin electrolyte films. The anodic de-adhesion starting from defect reaching the zinc layer and from the non protected cut edge in case of exposure in the salt spray conditions was also determined.The role of the formation of confined volume underneath the delaminated paint on the rate of anodic undermining is discussed.

► Spreading of corrosion from the defect underneath the polymeric coating.
► Scanning Kelvin Probe and FTIR microscopy were applied.
► HDG steel with electrophoretic epoxy resin was studied.
► Mechanism of coating de-adhesion depending from corrosion conditions is evaluated.