Abstract
Cyclic Corrosion Test (CCT) and Salt Spray Test (SST) for the paint adhesion at cross script were compared using painted Zn and Zn-Fe electroplated steel sheets with various contents of Fe. The changes of paint adhesion with the composition of Zn-Fe electrodeposits largely depended on the testing procedures, CCT and SST. When SST was employed, paint adhesion at cross script was improved with an increase of Fe content of Zn-Fe electrodeposits. On the contrary, in CCT paint adhesion was decreased with an increase of Fe content. In a range of Fe content of 70-90%, paint adhesion was worst. The paint adhesion of Zn-Fe electroplated steel sheets containing a small amount of Fe was superior to pure Zn plated steel sheet. Appearances of cross section of blisters were different in CCT and SST. In CCT, a lot of corrosion products accumulated under blister of specimen showing poor paint adhesion. On the contrary, in SST corrosion products did not appear. Corrosion potentials of blister and script were different in those corrosion tests. pH of the solution in blister in SST showed 10-12 and that of script showed 4-6. In the case of CCT, pH of the solution in blister was 4-6 and that of script showed 10-12. The corrosion behavior in CCT was similar with that of filiform corrosion. It was suggested that paint deadhesion in CCT was caused by lifting up the paint by corrosion products.
