Metal Finishing Guide Book

2012-2013

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Table I. Materials for Sheetmetal Constructions Steel: Soft, unalloyed High strength, micro alloyed, dual phase P-alloyed Steel coated with molten metals: Zinc + 0.1%-0.2% aluminum (galvanized) Zinc + 0.1%-0.2% brushed (1/2 galvanized) Zinc + 0.1%-0.2% aluminum + app. 10% iron (Galvanneal) Zinc + app. 5% aluminum + app. 1% mix metal (Galfan) Zinc + app. 30% aluminum + app. 0.2% magnesium (Lavegal) Zinc + app. 55% aluminum + app. 1.5% silicon (Galvalume) Steel electrolytically Zinc Zinc + app. 10% nickel Zinc + app. 16% iron Zinc + app. 16% iron (as base coat) [zinc + app. 83% iron (as a top coat)] Aluminum: Aluminum���magnesium Aluminum���magnesium���silicon Aluminum���magnesium���manganese coatings are developed to meet the increasing demands placed on the total finish of the parts; therefore, a close working relationship has been formed with the paint industry to create processes requested by the market. Also, a more widespread use of zinc coated steel and aluminum substrates has forced the manufacturer of zinc phosphate processes to develop processes that can treat a wide variety of metal mixes through one and the same process. Some of the commonly used substrates in the prepaint operations are discussed below. A properly performed zinc phosphate coating prior to paint application will create the following: good bonding of the zinc phosphate coating with the metal; good bonding of the paint film in the microporous zinc phosphate coating; reduction of electrical corrosion currents underneath the paint; chemical resistance of the zinc phosphate coating; reduction of electrical corrosion currents underneath the paint; chemical resistance of the zinc phosphate coating; and a diffusion barrier against water and oxygen. All of these create retardation of corrosion underneath the paint coating starting at a paint chip. SUBSTRATES The principal material to which zinc phosphate is applied is steel as well as, in varying proportions according to the particular part, precoating steel and aluminum. Precoated steel is used to improve the corrosion protection of the part, whereas aluminum and thin-walled, high-strength steels are employed to save weight. Table I surveys the industrial materials employed 122 Fig. 2. Percent paint loss related to surface carb on content on steel after gravelometer test and 480 hours of salt spray.

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