Machinery's Handbook, 31st Edition
SHEET METAL MILL FINISHES Table 18. Types of Surface Treatment Table 18. (Continued) Types of Surface Treatment
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Deposits a hard, natural, or colored oxide layer up to 0.0009842 in. or 25 µ m. Mainly for decoration, resists pitting and corrosion, while adding wear resistance and electrical insulation. Adds a gray to brown hard oxide layer of 0.0029528 to 0.0049213 in. or 75 to 125 µ m. Mainly used for engineering purposes to resist abrasion and wear. Provides various degrees of protective and decorative finishes that can withstand heavy chemical attack (if surface coating remains unbroken). Creates a smooth surface with high reflectivity. Used for printing text, decorative patterns, and so on. Added for decoration, protection, or other properties. Used in capital-intensive operations for coil coating. Effective, efficient, and common in the building industry.
Decorative anodizing
Electrochemical
Hard anodizing
Electrolytic polishing
Powder coating Screen printing
Organic surface coating
Coating with protective foils
Wet coating
Mechanical Finishes: Mechanical finishing is done before chemical finishing or anod - izing to remove surface defects and irregularities and/or add desired texture. While the resulting surface appearance can vary, most mechanical processes result in a directional or nondirectional sanded finish, uniform matte finish, or a specular (mirror-like) finish. The mechanical finish will show through anodic oxides, because uncolored anodic oxides are transparent or translucent; anodized finishes also conform to surface textures. Grinding requires a rotary grinder with a bonded adhesive cup wheel or canvas wheel, faced with suitable abrasives. Low-speed grinding at 59 in/min (1,500 mm/min) using aluminum oxide is preferred to avoid overheating the surface. Machine polishing involves wheels or belts with abrasives either bonded to them or sprayed on at regular intervals. Methods include both contact and mush polishing (where only the polishing compound touches the metal surface), the latter being more common. Polishing can include final buffing to remove emery marks. Chemical Finishes: Many types of chemical treatment are used on aluminum sheets as finishing processes, but few are final finishes. Most are intermediate steps in an overall finishing process, requiring subsequent application of some form of protective coating. There are numerous methods of providing intermediate process finishes on sheet metals that involve washing or dipping fabricated product in chemical solutions; many are propri- etary processes. Such treatments have various purposes but in general are used to: a) clean the surface, without otherwise affecting it; b) create a clean, matte, textured surface for applying a smooth, bright finish; or c) chemically add a film to provide a substrate for organic coatings. Conversion Coatings: Although chemical conversion coatings generally are used on alu- minum to prepare surfaces for painting, some can be used as a final finish. Since the natural oxide film present on aluminum surfaces does not always provide a good bond for paints, other organic coatings, and laminates, the chemical nature of the film is often converted to improve adhesion and undercoating protection. Such conversion films or coatings usually are applied using phosphate or chromate solutions, many of which are proprietary in nature. One of the simplest methods for improving adhesion of organic coatings is etching sur- faces with a phosphoric acid solution to provide a good mechanical bond. Finishes of this type, such as acid-chromate fluoride treatments, usually are governed by ASTM B449, “Specifications for Chromates on Aluminum.” Anodic Coatings: These economical, electrolytically formed oxides in sulfuric or phos- phoric acid coatings impart excellent, comparatively thick 0.0003 to 0.002 in. (8 to 50 µ m), transparent, durable, protective coatings to aluminum surfaces. Color anodic coatings are formed by: impregnating the coating with organic dyes or pigments; using alloys and pro- cesses that produce integral color; or electrolytically depositing pigments in the anodic film. Quality control tests after anodizing are performed on representative samples taken at random from rack loads to determine whether the coating meets the specified thickness and weight and is properly sealed. Test methods for checking thickness and weight are specified in: 1) ASTM B244, “Test Method for Measurement of Thickness of Anodic
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