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Relationship between wet adhesion of paint coating film (paint film) on coated steel (wet adhesion) and stress changes in paint film has been investigated. At the same time, the creation mechanism of ZnO under paint film has been examined.Zn3(PO4)2•4H2O under paint film dehydrates to Zn3(PO4) 2•2H2O with cathodic electrodeposition (C-ED) baking. In the next place, Zn(OH)2 and Zn(H2PO4)2 evolve as a result of hydrolysis of partial Zn3(PO4)2•2H2O in water immersion. Then, Zn(OH)2 dehydrates easily to ZnO. Formation of ZnO remarkably weakens the adhesion force between paint film and phosphate film. Meantime, in a C-ED coated steel, very large residual stress developes in paint fun. Bonding condition between paint film and phosphate film weakened by ZnO is easily destroyed by them. Furthermore, stress due to expansion of phosphate film builds up as a result of restoration from Zn(H2PO4)2 to Zn3(PO4)2•4H2O in water immersion and then promotes the destruction. Thus, wet adhesion in a C-ED coated steel becomes poor.Formation of ZnO and progress of hydrolysis reaction are controlled by Ni addition in phosphate film, and as the result, it becomes possible to stabilize the phosphate film. As a consequence, formation of amorphous phosphate film in high Ni++ concentration solution produces good wet adhesion, suppressing expansion of a phosphate film.

Relation between Wet Adhesion of Coated Steel and Stress Changes in Paint Film