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The effect of pretreatment on the corrosion resistance of zinc-rich painted steel sheets was evaluated by salt spray tests and the corrosion behavior was investigated electrochemically and by ESCA. The results obtained were as follows, (1) Steel sheet with 6-7μm composite coating containing about 70wt% zinc powder showed excellent formability. (2) Cold-rolled steel sheet with composite coating had poor corrosion resistance, indicating the importance of the pretreatment. (3) Pretreatment by Zn-Co-Mo electrodeposition produced superior corrosion resistance to that produced by Zn plating. (4) The excellent corrosion resistance of composite coated sheet pretreated by Zn-Co- Mo electrodepositi on depended on two factors, a) the corrosion products of Co and/or Mo controlled the increase in the anodic and cathodic reaction rates, which decreased the dissolution of Zn in the plated layer and prevented the growth of blisters; and b) most of the Zn in the composite coating and in the plated layer remained as a corrosion product on the surface, protecting the steel sheet substrate.
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