Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 419 of 707

tance, depending upon the basis metal, the treatment used, and the film thick- ness. Protection is due both to the corrosion-inhibiting effect of hexavalent chromium contained in the film and to the physical barrier presented by the film itself. Even scratched or abraded films retain a greatdeal of their protective val- ue because the hexavalent chromium content is slowly leachable in contact with moisture, providing a self-healing effect. The degree of protection normally is proportional to film thickness; there- fore, thin, clear coatings provide the least corrosion protection, the light iridescent coatings form an intermediate group, and the heavy olive drab to brown coatings result in maximum corrosion protection. The coatings are particularly useful in pro- tecting metal against oxidation that is due to highly humid storage conditions, expo- sure to marine atmospheres, handling or fingerprint marking, and other conditions that normally cause corrosion of metal. Bonding of Organic Finishes The bonding of paint, lacquer, and organic finishes to chromate conversion coat- ings is excellent. In addition to promoting good initial adhesion, their protective nature prevents subsequent loss of adhesion that is due to underfilm corrosion. This protection continues even thought he finish has been scratched through to the bare metal. It is necessary that the organic finishes used have good adhesive properties, because bonding must take place on a smooth, chemically clean surface; this is not necessary with phosphate-type conversion coatings, which supply mechanical adhesion that is due to the crystal structure of the coating. Chemical Polishing Certain chromate treatments are designed to remove enough basis metal during the film-forming process to produce a chemical polishing, or brightening, action. Generally used for decorative work, most of these treatments produce very thin, almost colorless films. Being thin, the coatings have little optical covering power to hide irregularities. In fact, they may accentuate large surface imperfections. In some instances, a leaching or "bleaching" step subsequent to chromating is used to remove traces of color from the film. If chemical-polishing chromates are to be used on electroplated articles, con- sideration must be given to the thickness of the metal deposit. Sufficient thickness is necessary to allow for metal removal during the polishing operation. Absorbency and Dyeing When initially formed, many films are capable of absorbing dyes, thus providing a convenient and economical method of color coding. These colors supplement those that can be produced during the chromating operation, and a great variety of dyes is available for this purpose. Dyeing operations must be conducted on fresh- ly formed coatings. Once the coating is dried, it becomes nonabsorbent and hydrophobic and cannot be dyed. The color obtained with dyes is related to the char- acter and type of chromate film. Pastels are produced with the thinner coatings, and the darker colors are produced with the heavier chromates. Some decorative use of dyed finishes has been possible when finished with a clear lacquer topcoat, though caution is required because the dyes may not be lightfast. In a few cases, film colors can be modified by incorporation of other ions or dyes added to the treatment solution. Although most coatings are soft and easily damaged while wet, they become rea- sonably hard and will withstand considerable handling, stamping, and cold form- Hardness 418

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