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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 chromate affected the adhesion of laminate films. The highest adhesive strength was obtained for the pretreatment films formed in a pretreatment solution with SiO 2 /Cr ratio of 4.0 and dry-type silica with a particle diameter of 7 nm. The good adhesion of chromate 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.

Improved corrosion resistance of steel sheets plated with zinciferous metal, and particularly 55% Al-Zn alloy coated steel sheets, has become necessary in recent years. However, 55% Al-Zn alloy coated steel sheets are susceptible to surface cracks during forming due to the hardness of the metallic coating. This research examined various non-chromate corrosion-preventive compounds for organic-inorganic composite coatings and found that a composite coating containing a mixture of Zn 3 (PO 4 ) 2 and CaSiO 3 provided the highest corrosion resistance among all the additives tested. The corrosion mechanism was investigated by post-test analysis of the specimens by SEM, AFM, TEM and EDX, revealing that an amorphous surface layer, which was approximately 30 nm thick and contained Ca, Cr, P, Si, C, and O, had newly formed on the steel surface exposed after deformation of the coating. It is presumed that Ca 2þ and HPO 4 2À were formed by dissolution of the CaO component in CaSiO 3 and hydrolysis of ZnO 3 (PO 4 ) 2 , respectively, and reacted to form an insoluble Ca-phosphoric acid compound which inhibited corrosion of the metal exposed by cracks.