Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 56 of 331

Hot water rinse and dry and the part is ready for priming. This treatment provides less corrosion protection than the chromates used in the past, but when used with a corrosion-protective paint system, it is satisfactory. Anodizing should be done on products used in a severe environment or sub- ject to mechanical abuse that may damage the paint. The following solution is recommended: Potassium hydroxide, 22 oz/gal Aluminum hydroxide (see note 1), 4.5 oz/gal Potassium fluoride (anhydrous), 4.5 oz/gal Trisodium phosphate (Na2 PO4 . 12H2 O), 4.5 oz/gal Potassium manganate [or permanganate (see note 2)], 2.5 oz/gal Room temperature Current density, 18–20 A/ft2 Voltage, 0–60 VAC Time, ~8 min Note 1: Aluminum (1100 alloy to prevent impurities in the bath) can be dis- solved into part of the potassium hydroxide in a separate tank and then de- canted (or filtered) into the tank. Note 2: Dissolve permanganate in water prior to adding to the tank. Make the parts one electrode and the tank the other electrode, or (in a plas- tic tank) divide the load into approximately equal areas and make each half a sep- arate electrode. MOLYBDENUM After cleaning, to provide a water-break-free surface, the paint coatings can be ap- plied. Wash primer can be used but is not necessary. NICKEL Nickel will respond to the wash primer pretreatment before applying the topcoat. A light sandblasting to produce a matte surface, which can be primed with the wash primer or a regular primer, will produce satisfactory topcoat adhesion. SILVER The cleaned surface can be painted without further processing. Silver that has been chromate conversion coated will also paint satisfactorily. The conversion coating stabilizes the silver from sulfide tarnishing. Surfaces with sulfide tarnish will accept paint so long as the tarnish is not powdery. STAINLESS STEEL Stainless steel should be cleaned and can then be given one of the passivation treat- ments followed by wash primer, or it can be sandblasted and followed as soon as possible with either a wash primer or a regular primer. Heat-treated materials that have been put through an alkaline scale conditioner and have then either been acid cleaned or hydroblasted to remove scale residues can be painted without further processing. 55

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