Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 425 of 707

Table II. Typical Salt Spray Data for Electroplated Zinc Treatment Untreated Clear chromate Iridescent yellow Olive drab Hours to White Corrosion <8 24-100 100-200 100-500 Although rinsing should be thorough, this step can also affect the final character of the chromate film and should be controlled with respect to time and tempera- ture, for consistent results. Prolonged rinsing or the use of very hot rinsewater can dissolve, or leach, the 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 150O F (66O C) for more than a few seconds. This leaching effect sometimes is used to advantage. In instances in which a highly col- ored 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 suppli- ers' instructions be followed for solution make-up and control. Even though spe- cific formulations will not be discussed, certain general principles can be out- lined, 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 appear- ance of the work. Table III. Typical Salt Spray Data for Copper and Brass Treatment Copper, untreated Copper, bright chromate Copper, heavy chromate Brass, untreated Brass, bright chromate Brass, heavy chromate Table IV. Typical Salt Spray Data for Aluminum Hours to White Corrosion Alloy 3003 2024a 413.0 No Treatment 24 <24 <24 aHeat treatment will affect the final results. 424 Hours to Green Corrosion <24 24 50 24 100 150 Clear 60-120 40-80 12-24 Yellow-Brown 250-800 150-600 50-250

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