Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 33 of 331

coatings are developed to meet the increasing demands placed on the total fin- ish 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 cre- ate the following: good bonding of the zinc phosphate coating with the metal; good bonding of the paint film in the microporous zinc phosphate coating; re- duction of electrical corrosion currents underneath the paint; chemical resis- tance of the zinc phosphate coating; reduction of electrical corrosion currents un- derneath the paint; chemical resistance of the zinc phosphate coating; and a diffusion barrier against water and oxygen. All of these create retardation of cor- rosion underneath the paint coating starting at a paint chip. SUBSTRATES The principal material to which zinc phosphate is ap- plied is steel as well as, in vary- ing proportions according to the particular part, precoat- ing steel and aluminum. Pre- coated 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 in- dustrial materials employed 32 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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