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Quality Control for Decorative Anodized Coatings

Above and beyond their protective function, decorative anodized coatings must also meet certain design requirements: The exact coloring plays an important role. But even small differences in the aluminum alloy can significantly affect the final hue. Therefore, an accurate review of the raw materials during incoming goods inspection is necessary to achieve consistent coloration and to prevent production wastage.

Not every aluminum alloy can be anodized for decorative purposes; therefore, mainly AL99, AIMg or AlMgSi alloys are used. For best results, these aluminum alloys must also be of highly pure anodizing quality. But even with supposedly the same alloys, slight differences in composition may occur from manufacturer to manufacturer and from batch to batch, which can then lead to significant color deviations during the anodizing process.

For decorative anodized finishes, the exact hue is actually an important aspect of the quality; discrepancies in coloration can drive manufacturing costs up, because they can require extensive reworking to correct problems or even a new production run. In order to avoid such issues, it is necessary to know whether one is indeed working with the exact same aluminum alloy โ€“ or not. The best way to check this is to determine the electrical conductivity of the raw material or semi-finished products before the anodizing process, as this parameter is sensitive to even small variances in composition.

As an example, four sheets each of the alloys AIMg3 and AlMgSi0.5 were color-anodized all together, resulting in two different shades of blue. The electrical conductivity of the two base materials showed significant differences, as seen in Table 1.

Conductivity measurements on color-anodized plates from the same anodizing process which resulted in two different shades of blue. The plates that turned out in color variant A were made of AIMg3 whereas those with color variant B were of AlMgSi0.5.
Color variant A part 1A part 2A part 3A part 4A
Mean value [MS/m] 31.33 31.26 31.20 31.27
Standard deviation [MS/m] 0.01 0.02 0.01 0.01
No. of measurements 10 10 10 10
Color variant B part 1B part 2B part 3B part 4B
Mean value [MS/m] 21.58 21.57 21.57 21.59
Standard deviation [MS/m] 0.01 0.01 0.01 0.01
No. of measurements 10 10 10 10

The conductivity of the plates was measured with the SIGMASCOPEยฎ SMP350 and the FS40 probe, both from FISCHER. Using the non-destructive phase-sensitive eddy current method, this instrument quickly and accurately determines the electrical conductivity of non-ferrous metals. The FS40 probe can handle a wide range of measuring frequencies (60-480 kHz), making it suitable for a variety of material thicknesses. Since the electrical conductivity is strongly temperature dependent, it also has an integrated temperature sensor. Furthermore, in order to allow for calibration on the supplied flat calibration standards when measuring rounded parts, curvature corrections starting at ร˜ 6 mm ensure consistently high precision: the diameter of the sampleโ€™s curvature is simply entered into the instrument.

To avoid differences in the final color of decorative anodized coatings, checking the conductivity of the raw material before anodizing is an appropriate quality control step. The combination of SIGMASCOPEยฎ SMP350 and FS40 probe is ideal for measuring the electrical conductivity of non-ferrous metals like aluminum alloys. For more information please contact your local FISCHER representative.

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