Effect of pretreatment film composition on adhesion of organic film on zinc coated steel sheet

Abstract

It is known that chromate pretreatment films with dry-type silica adhere well to organic films such as paint and laminate films on a zinc coated steel sheet. The role of surface polarity in the adhesion mechanism has been investigated for a long time, but remains unclear. We examined the correlation between the composition of chromate pretreatment films with dry-type silica and their adhesion to a laminate film on a zinc coated steel sheet, for both the wet silica and without silica types of chromate pretreatment film. We also investigated the adhesion mechanism by analyzing the pretreatment films using SEM, AFM, BET, TEM and EDX. The results were as follows. Not only the type and particle size of silica, but also the silica/chromate ratio and the coexistence of chromáte affected the adhesion of laminate films. The highest adhesive strength was obtained for the pretreatment films formed in a pretreatment solution with SIO2/: ratio of 4.0 and dry-type silica with a particle diameter of 7 nm. The good adhesion of chromáte pretreatment films with dry-type silica is due to an anchor effect by sub-micron surface roughness and an increased number of interface bonding sites by a nano-porous structure. The formation of an insoluble Cr(III) compound layer plays an important role in binding silica particles to the zinc coating surface. © 2010 The Japan Institute of Metals.