Metal Finishing Guide Book


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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 (Na2PO4. 12H2O), 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 dissolved into part of the potassium hydroxide in a separate tank and then decanted (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 plastic tank) divide the load into approximately equal areas and make each half a separate electrode. MOLYBDENUM After cleaning, to provide a water-break-free surface, the paint coatings can be applied. 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 treatments 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. TITANIUM Untreated titanium exists in the atmosphere as a passive material, and as such, it is paintable; however, adhesion is only moderate. A conversion coating or an anodize coating is recommended. A phosphate–fluoride conversion coating is applied by immersion for 2 to 3 minutes in the following solution: Sodium phosphate (Na2PO4), 6–7 oz/gal 132

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