Metal Finishing Guide Book


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Table II. Typical Salt Spray Data for Electroplated Zinc Treatment Hours to White Corrosion Untreated <8 Clear chromate 24-100 Iridescent yellow 100-200 Olive drab 100-500 more soluble hexavalent chromium compounds from a freshly formed coating, resulting in a decrease in protective value. If a hot rinse is used to aid drying, avoid temperatures over about 150OF (66OC) for more than a few seconds. This leaching effect sometimes is used to advantage. In instances in which a highly colored or iridescent coating may be objectionable, a prolonged rinse in hot water can be used as a "bleaching" step to bring the color to an acceptable level. Instead of hot water leaching, some systems incorporate dilute acids and alkalis to accelerate this step. Solution Control Because most chromate processes are proprietary, it is suggested that the suppliers' instructions be followed for solution make-up and control. Even though specific formulations will not be discussed, certain general principles can be outlined, which apply generally to chromate solutions. The combination of hexavalent chromium concentration, activator type and concentration, and pH, i.e., the "chemistry" of the solution, largely determines the type of coating that will be obtained, or whether a coating can be obtained at all, at given temperatures and immersion times. It is important that these factors making up the "chemistry" of the solution be properly controlled. As the solution is depleted through use, it is replenished by maintenance additions, as indicated by control tests or the appearance of the work. Fortunately, analysis for each separate ingredient in a chromate bath is not necessary for proper control. A very effective control method uses pH and hexavalent chromium analysis. The pH is determined with a pH meter and the chromium is determined by a simple titration. Indicators and pH papers are not recommended because of discoloration by the chromate solution. Additions are made to the solution to keep these two factors within operating limits. The amount of control Table III. Typical Salt Spray Data for Copper and Brass Treatment Hours to Green Corrosion Copper, untreated <24 Copper, bright chromate 24 Copper, heavy chromate 50 Brass, untreated 24 Brass, bright chromate 100 Brass, heavy chromate 150 Table IV. Typical Salt Spray Data for Aluminum Hours to White Corrosion Alloy No Treatment Clear Yellow-Brown 3003 24 60-120 250-800 2024a <24 40-80 150-600 413.0 <24 12-24 50-250 a Heat treatment will affect the final results. 432

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