STUDY OF INTERESTING BLISTERING PATTERN ON COIL-COATED GALVANIZED STEEL, USING KELVIN PROBE AND EIS TECHNIQUES

摘要

Field-testing of coil-coated galvanized steel after deformation resulted in an unusual corrosion pattern. Instead of corrosion beginning at the scribe and moving outwards in a more-or-less uniform front, blisters are scattered in the region of the scribe. The blisters are not connected; all seem to have approximately the same size and development, whether they are 1 mm or 40 mm from the scribe. The paint between blisters, and between blisters and scribe, appears to be well-adhering and intact. Higher levels of deformation result in a broader distribution and higher frequency of blisters. The scanning Kelvin probe and electrochemical impedance techniques have been used to study this behavior. Using the Kelvin probe on the unexposed, most-deformed sample, a large zone was observed with low Volta potential. This was not seen on the undeformed or less-deformed samples, indicating that the deformation caused defects in the primer or rapid delamination at the metal/primer interface. These defects or delamination zones are believed to be the sites where corrosion will occur. EIS measurements supported this conclusion. A theory is proposed wherein current is carried along the surface of the coating during wet periods. Higher levels of deformation are believed to cause more defects in the paint film, which means that more intermittent corrosion cells can be set up between the scribe and the defects. The initial zinc corrosion generates hydroxide ions; these may undergo degradative hydrolysis reactions with the polyester binder. The hydrolysis results in both reduced polymer crosslink density, and formation of hydrophilic ionic and polar products at the metal-polyester interface. Both can lead to de-adhesion between the organic coating and the zinc surface; the decreased crosslink density would also be expected to result in increased water and oxygen transport through the coating.