Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 428 of 707

MIL-W-6858—Welding, Resistance: Aluminum, Magnesium, etc.; Spot and Seam Silver QQ-S-365—Silver Plating, Electrodeposited, General Requirements for Zinc AMS 2402—Zinc Plating ASTM B 201—Testing Chromate Coatings on Zinc and Cadmium Surfaces ASTM D 2092—Preparation of Zinc-Coated Steel Surfaces for Painting MIL-A-81801—Anodic Coatings for Zinc and Zinc Alloys MIL-C-17711—Coatings, Chromate, for Zinc Alloy Castings and Hot-Dip Galvanized Surface MIL-T-12879—Treatments, Chemical, Prepaint and Corrosion Inhibitive, for Zinc Surfaces MIL-Z-17871—Zinc, Hot-Dip Galvanizing QQ-Z-325—Zinc Coating, Electrodeposited, Requirements for SPECIAL TREATMENTS Solutions containing chromium compounds are used in some processes where disagreement exists as to whether these form "true" chromate conversion coat- ings or combination coatings, or act as passivating processes. Electrolytic Processes Although early chromate conversion coatings for zinc were electrolytically applied, this method has been largely replaced by immersion processes. More recently, the use of electric current has reappeared with solutions containing mix- tures of chromates, phosphates, fluorides, etc., to produce "anodic coatings." The coatings, however, are not similar to anodic coatings such as those produced on aluminum. The coatings on zinc surfaces are complex combinations of chromates, phosphates, oxides, etc. They are formed with 100-200 V AC or DC, and the frit- ted coating will withstand more than 1,000 hr of salt spray. The process is used where outstanding corrosion resistance is needed. The coating also exhibits superior hardness, heat resistance, thickness, and dielectric strength when com- pared with normal chromate conversion coatings. Colors range from dark green to charcoal for different processes. Electrolytic treatments using chromium compounds are also applied to steel strip, where chromium along with oxides, etc., are deposited in a very thin, dis- continuous film. These processes, which promote lacquer and paint adhesion, may be more chromium plate than chromate coating. Coatings on Beryllium It has been reported that chromate conversion coatings can be applied to berylli- um to retard high-temperature oxidation in humid air. Chromate-Phosphate Treatments Chromate-phosphate treatments are based on chromate-phosphate mixtures and form a combination conversion coating on aluminum. The coating can appear prac- tically colorless to a light-green hue. These treatments have been used to impact col- or for decorative purposes or to provide an imposed base for subsequent lacquer or paint operations. 427

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