Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 51 of 331

Total dissolved solids should not exceed 150 ppm, chlorides 15 ppm, sulfates (as SO4) 25 ppm, and total hardness (as CaCO3) 200 ppm. Deionized water is rec- ommended for the chromate and final rinse stages if the incoming water qual- ity falls below these limits. TESTING AND QUALITY CONTROL Testing and quality control are important parts of any manufacturing process. A large majority of the prepaint processes are proprietary. Supplier recommen- dations on equipment and bath maintenance must be observed to produce ac- ceptable results consistently. Frequent bath analysis or on-line process control greatly reduces rejects and lowers cost. COATING WEIGHT PROCEDURE Coating weight and occasional salt spray determinations on the unpainted parts are important tools in evaluating work quality. The coating weight procedure is as follows: 1. Solvent degrease and blow dry a 3- x 3-inch panel. Weigh accurately to ± 0.2 mg. 2.Immerse for 2 to 5 minutes in a molten salt bath (temperature 620 to 670°F) consisting of reagent grade sodium nitrite (NaNO2). Aged panels may require up to 15 minutes for stripping. 3.Remove from the salt bath and carefully rinse in cold water. 4.Dip for 30 seconds in equal parts by volume of concentrated nitric acid and water at room temperature. 5.Rinse thoroughly in cold water and blow dry. 6.Reweigh. 7. Repeat steps 2 through 6 until the weight loss is less than 0.6mg. 8.Calculate the costing weight as follows: Coating weight = total weight loss/total area Verify that the observed coating weight meets the required specifications. Manufacturing specifications for coating weights differ for various applications. 50

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