Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 50 of 331

ments such as wire brushing and sandblasting. Wash primers are primarily pig- mented polymeric organic chromate compounds similar to paint. They have ex- cellent corrosion protection and adhesion properties. Humidity resistance is poor. Environmental restrictions of the last decade have generated exciting interest in chromium-free products. A substantial number of these have met acceptable requirements for the beverage can industry. Testing is in progress in coil coating and architectural segments. There is a strong indication that chromium-free products will eventually meet the stringent requirements of the architectural and transportation industries. Some of the shortcomings of current products are being gradually eliminated. New high performance top coat paints such as sili- cones, fluoropolymers, and powder coats can minimize performance require- ments for these prepaint treatments. Advances in application technology of elec- trocoat systems are also proving beneficial. PROCESS GUIDELINES Conversion coating processes are essentially multistep operations. Precondi- tioning stages, such as cleaning, rinsing, and postrinse treatments are significant factors in assuring critical final results. Spray and immersion systems usually show the same order of operation. Stages consist of the following: 1. Clean 2. Rinse 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 de- sign criteria, but others, utilizing alkaline cleaners loaded with dissolved alu- minum, 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 uni- form 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 cross- contamination between the stages. Water quality requirements for the chromate and final rinse stages are even more critical. Excessive hard water salts and sol- uble 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. 49

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