2015 Volume 64 Issue 10 Pages 466-474
The mechanism of corrosion inhibition of Mannich-modified phenolic resin coating which is one of the alternatives to chromate conversion coating on the electrogalvanized steel sheet was investigated by cross-section analysis and electrochemical measurements of the coating film. The 200ºC cured coating film which has good corrosion inhibition has the higher charge transfer resistance (Rct) and the higher film resistance (Rf) calculated by electrochemical impedance measurement than the 80 ºC cured coating film. The increase of these resistances is due to decrease of water absorption property of the film caused by crosslinking reaction. Namely the film resistance is enhanced by reduction of ionic conductivity of the film, and the charge transfer resistance is enhanced by reduction of the supply of the water containing dissolved oxygen to interface between the film and substrate. The phosphoric acid added to coating solution forms an insoluble zinc phosphate coating layer at the coating / material interface. The zinc phosphate coating layer, which is formed in close contact with the complex surface shape of the substrate by chemical interaction with the substrate surface by chemical interaction with the material surface, enhances the barrier effect to obstruct the contact of substrate with the water containing dissolved oxygen. Consequently the corrosion inhibition of the coating film is excellent. On the other hand, the 80ºC cured coating film without phosphoric acid, which has no chemical interaction with the substrate, has some voids (defects) at film /substrate interface and its corrosion inhibition is inferior.
