Zinc and its alloys are more electronegative than steel and offer excellent sacrificial protection to underlying steel in corroding media. However, due to the very high electronegativity of these deposits, the rate of dissolution is very high. Chromate conversion coatings have been used extensively to prevent rapid Zn dissolution in aerospace, food processing and automobile industries. Chromium is favored due to its barrier and corrosion resistance properties. Chromium deposition is obtained from hexavalent Cr salt baths, which are subject to stringent regulations. Due to the toxicity of the metal and its salts, use of alternate baths is also undesirable. Effluent treatment in general is a considerable problem. Hence, there exists a need to develop an environmentally friendly coating that is also capable of preventing Zn dissolution. Several studies have been reported for developing chromium-free alternatives. However, these coatings suffer from poor stablity and/or high cost. Elisha Technologies Co., L.L.C. has discovered ways to grow high performance thin silicate based minerals on metallic substrates at low temperatures and ambient pressures. The mineralization process is done using a bath which has no pollutants. Deposition of silica films has been accomplished previously either by sol-gel deposition or through anodic deposition at high voltages. Cheng et al. report the development of an aluminosilicate coating through a sol-gel process in an autoclave at 175°C. While the deposit yields superior corrosion characteristics as compared to Cr conversion coatings, the entire synthesis procedure is laborious, time consuming and is not suited for commercial applications. Speers and Cahoon report the deposition of Si from alkaline silicate electrolytes by anodizing of Al at 350 V. However, this process is limited to Al (or similar metals which have stable anodic oxide films) and also involves application of large potentials. Elisha's process is general in nature and can be applied for a wide range of metals. Further, it is inexpensive and the entire process is environmentally friendly. In this paper we report on Elisha's mineralization procedure as applied to Zn coatings. The corrosion characteristics of the deposit have also been studied as a function of bath parameters. The bath properties have been optimized based on the corrosion data obtained.
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