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Anodizing
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Anodized Films: Aluminum anodization is a significant commercial process. Anodic films on aluminum are electrically insulating and tend to minimize galvanic corrosion where aluminum is in contact with other metals. The anodic film formed by anodizing also provides an excellent paint base and helps insure adhesion of organic coatings such as zinc chromate primer, paints, and lubricants. Aluminum anodization is typically considered in terms of the anodizing processes classified in Military Specification Mil-A-8625, as listed below. Type I – Chromic Acid Anodize Type II – Sulfuric Acid Anodize Type III – Hard Anodize Type I Chromic acid coatings are frequently preferred if there is a possibility of solution entrapment in joints, recesses, and weldments because the chromic acid leaves no corrosive residue. The chromic acid is inhibitive to aluminum and, thus, is especially good for coating aluminum castings. The characteristics of the film vary somewhat with the type of electrolyte used. Chromic acid-produced films are more dense, very thin (0.02 to 0.05 mil), and darker in color. The overall thickness of the coating formed is 50 percent penetration in the substrate and 50 percent growth over the original dimension of the part. Chromic acid films are typically harder than sulfuric acid films of the same thickness. Type II The sulfuric acid process is the most common method for anodization. The sulfuric acid process produces films of greater thickness (0.1 to 1.0 mil) and is particularly suited for applications where hardness and resistance to abrasion is required. However, where parts are subjected to considerable stress, such as aircraft parts, the possible presence of the corrosive acid residue is undesirable. The overall thickness of the coating formed is 67 percent penetration in the substrate and 33 percent growth over the original dimension of the part. The porous nature of sulfuric acid films prior to sealing is used to particular advantage in the production of colored surface finishes on aluminum and its alloys. The porous aluminum oxide absorbs dyes well, and subsequent sealing helps to prevent color loss in service. Although dyed anodized films are reasonably color-fast, they have a tendency to bleach under prolonged direct sunlight. Some of the colors produced at Hohman Plating are:
Type III The hard anodizing process also uses a sulfuric acid electrolyte, but the bath is maintained at higher sulfuric acid concentration, lower temperature, and higher current density, which increase film crystallinity. Nominal thickness of 1 to 2 mils is common for most applications. The overall thickness of the coating formed is 50 percent penetration in the substrate and 50 percent growth over the original dimension of the part. Parts processed by this method have a hardness of 65 Rockwell “C”. Hard anodic coatings generally have increased surface roughness as well as having the property of being softer on the top surface than down in the core of the coating. Such coatings may be processed oversized and then lapped down to final dimensions. Lapping of a hard anodized surface results in an exceedingly smooth, hard finish resembling polished stone. Hard anodizing is an excellent electrical insulator, and base for paint, dry film lubricants and other coatings. Sealing of hard coating is not recommended unless corrosion resistance is a factor. Wear resistance is sometimes reduced by sealing. Hard coatings may also reduce fatigue strength. These factors should be considered in proposed use of parts subjected to cyclic loads. Generally, hard coatings should not be used on parts, which during rework would normally require restoring of dimensional tolerances because of wear of hard coated surfaces. When the hard anodize is impregnated with PTFE, the coating not only has great wear and abrasion resistance, but also now has improved lubricating properties. Some applications for the impregnated coating are; valves, sliding parts, hinges, cams, pistons, and gears. |
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