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    Testing of Anodic Coatings on Aluminum Alloys: A Review

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    The test methods will be presented and reviewed for evaluating specific properties and parameters of anodic coatings on aluminum alloys: (1) thickness, (2) hardness, (3) dielectric properties, (4) mechanical continuity, (5) sealing effectiveness, (6) color matching, and (7) color stability. The thickness of the anodic coating is probably considered the most important propety to be determined, because most of the other properties are dependent upon it. The gravimetric, optical, and eddy current methods will be described and discussed. Hardness of çoatings may vary widely; and for many applications involving wear, knowledge of hardness is very important. Hardness tests will be reviewed. Electrical dielectric properties of the anodizing and the determinations of a breakdown voltage of the coating are essential for safety and to insure the proper service conditions and limitations. The testing methods, capable also of determining the leakage existent for elevated voltages, will be discussed.

    Where the corrosion protection afforded by the anodic coating is essential (as in anodized cans, and so forth) the mechanical continuity of the coating is of paramount importance because any discontinuity will involve very high corrosion current densities, causing extremely rapid failures. The detection of discontinuities is an essential test that will be reviewed. Also essential to optimize the corrosion protection by the anodizing is the effectiveness of the seal extended by the anodic coating. A simple test will be described applicable to finished items and to periodic control of the continuing effectiveness of the sealing. The use of anodizing for a large variety of esthetic applications, from architecture to costume jewelry, dictates that two important parameters must be controlled: color matching and color stability. Appropriate test methods for these exist and will be described and discussed.


    Aluminum alloys, anodic coatings, hardness, thickness, wear resistance, dielectric strength, sealing, eddy currents, color fastness, optical microscopy, conductivity, tristimulus, chromaticity

    Author Information:

    Lacey, EA
    Senior engineer and staff scientist, Emerson Electric Co., St. Louis, MO

    Sonnino, C
    Senior engineer and staff scientist, Emerson Electric Co., St. Louis, MO

    Committee/Subcommittee: B08.01

    DOI: 10.1520/STP20035S