The properties of electrodeposited Zn-Co coatings
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The Properties of Electrodeposited Zn-Co Coatings J. Mahieu, K. De Wit, A. De Boeck, and B.C. De Cooman (Submitted 26 January 1999; in revised form 10 May 1999) The possibility of increasing the corrosion resistance of automotive sheet steel by electrodepositing with Zn-Co alloy coatings was investigated. Process variables during electrodeposition such as current density, electrolyte flow rate, and pH were varied in order to examine their influence on the electroplating process. Cobalt contents varying from 0.2 to 7 wt% were easily obtained. The influence of these process parameters on the characteristics of the coating could be related to the hydroxide suppression mechanism for anomalous codeposition. The structure and the morphology of the coatings were determined using SEM and XRD analysis. Application properties important for coating systems used in the automotive industry, such as friction behavior, adhesion, and corrosion behavior, were investigated on coatings with varying cobalt content. The corrosion resistance of the Zn-Co alloy layers was found to be better than that of pure zinc coatings.
Keywords
corrosion resistance, electrodeposited coatings, Zn-Al coatings, Zn-Co coatings
1. Introduction Zinc and zinc alloy coatings are increasingly being used in the automotive industry to protect the car body from both perforation corrosion and cosmetic corrosion. In recent years there has been a growing interest in replacing pure zinc coatings with thinner, more corrosion-resistant zinc-alloy layers (Ref 1-3). Zinc-iron alloy layers, both electroplated and galvannealed, and electroplated zinc-nickel coatings are currently widely used in the automotive industry (Ref 4, 5). Zinc-cobalt alloys, however, are by far the most widely used zinc alloys for plating in North America to meet the increased demands for corrosion resistance (Ref 6). However, these layers are not currently used to plate sheet steel, but there is a widespread use of zinc-cobalt alloy coatings for nuts and bolts. As a consequence, a review of literature on this subject shows that little information is available on the use of zinc-cobalt alloy coatings on sheet steel. Moreover, the information is limited to a small range of cobalt in the coating, varying from 0 to 2 wt% (Ref 3, 7-9). This article focuses on the influence of cobalt content on the application properties of zinc-cobalt alloy electroplated sheet steel and evaluates the potential of zinc-cobalt electroplatings as coatings for automotive applications. First, the influence of process parameters such as bath composition, pH, current density, and flow rate of the electrolyte on the morphology and the cobalt content of the coatings is examined. Second, this study evaluates the influence of coating composition on the roughness, friction, powdering, and galling behavior as well as on the shear strength. The corrosion rate of the deposits was examined by electrochemical measurements. J. Mahieu, K. De Wit, and B.C. De Cooman, Laboratory for Iron and Steel Making, Univers
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