Relation between wet adhesion of coated steel and stress changes in coating film has been investigated. Formation mechanism of ZnO under coating film has also been examined.
Zn3(PO4)24H2O under coating film dehydrates to Zn3(PO4)22H2O with cathodic electrodeposition (C-ED) baking. In the next place, Zn(OH)2 and Zn(H2PO4)2 are produced as a result of hydrolysis of partial Zn3(PO4)2H2O in water immersion. And then Zn(OH)2 dehydrates easily to ZnO. Formed ZnO weakens remarkably adhesion force between coating film and phosphate film. In the meantime, in C-ED coated steel, very large remaining stress is produced in coating film. The bonding between coating film and phosphate film weakened by the formation of ZnO is easily destroyed by this stress. Futhermore, stress due to expansion of phosphate film is introduced as a result of restoration from Zn(H2PO4)2 to Zn3-(PO4)24H2O in water immersion and then promotes the destruction. Accordingly wet adhesion of C-ED coated steel becomes poor.
The formation of ZnO and progress of hydrolysis reaction are controlled by Ni in phosphate film, and as the result, phosphate film is stabilized. And then stress due to expansion of phosphate film is not introduced because of the formation of amorphous phosphate film. Consequently phosphate film formed in high Ni++ concentration solution makes good wet adhesion.
