Metal Finishing Guide Book


Issue link:

Contents of this Issue


Page 142 of 903

3. Acid deoxidize (optional) 4. Rinse 5. Conversion coat 6. Rinse 7. DI/Acidulated/Seal rinse (optional) 8. Dry. In the basic five-stage operation, the deoxidize and seal rinse stages are often omitted. The chromate bath initially functions as a deoxidizer before reacting to produce the desired conversion coating. Many installations easily meet this design criteria, but others, utilizing alkaline cleaners loaded with dissolved aluminum, generally produce poor quality and powdery films. Cleaning, on the other hand, is very critical. General dirt, surfaces oils, grease, debris, and natural oxides must be removed in the cleaning process to achieve uniform coating and paint adhesion. Nonetch and etch-type cleaners may be applied with satisfactory results. Slight etching is desirable. Timely rinsing prevents soil dry-on and redeposition. Rinse stages require fresh water input to neutralize, dilute and prevent crosscontamination between the stages. Water quality requirements for the chromate and final rinse stages are even more critical. Excessive hard water salts and soluble contaminants are detrimental to coating performance. Very often, these contaminants have been known to precipitate on the chromated film and cause paint blistering or corrosion problems when moisture penetrates the paint film in the field. The water supply should not exceed the following impurity limits. 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 recommended for the chromate and final rinse stages if the incoming water quality 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 recommendations on equipment and bath maintenance must be observed to produce acceptable 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. 139

Articles in this issue

view archives of Metal Finishing Guide Book - 2012-2013