The protective nature of passivation films on zinc: wetting and surface energy

The influence of passivation film surface chemistry on the atmospheric corrosion of zinc was investigated. Crystalline oxide compounds replicating natural zinc corrosion products were deposited from aqueous solution, and grown as coatings on rolled zinc plates. The energetic properties of the oxide surfaces were characterised by water contact angle determination and inverse gas chromatography. Water contact angles were determined by liquid penetration and dynamic Wilhelmy techniques, and a good correlation was observed between the two methods. In comparison with wetting approaches, inverse gas chromatography was found to overestimate values for the dispersive contribution to the solid surface energy. Zinc hydroxychlorides possessed the lowest contact angles whilst naturally formed zinc oxide-hydroxide coatings were the most hydrophobic. The ability of zinc oxidation products to retain moisture was investigated by correlating wetting data with the criteria for droplet retention on planar surfaces. The response of the various oxides to rain washing events and water adsorption processes was discussed. Comparison of the corrosion rates of thin liquid films versus droplets was explored experimentally by adjustment of droplet contact angles through the addition of a non-ionic surfactant. Mass gain data indicated that corrosion rates decreased with decreasing contact angle, suggesting that hydrophobic surfaces are not necessarily advantageous for improving corrosion performance. Crown Copyright (C) 2004 Published by Elsevier Ltd. All rights reserved.