9/15/2010 by Tim Kenney
Anodizing converts the surface of an aluminum alloy into aluminum oxide. An anodized surface offers a number of special benefits:
- Increased corrosion resistance
- Improved paint and adhesive bonding
- Increased electrical resistance
- Increased abrasion resistance
An anodized surface can also be produced in a wide range of decorative colors. Color anodizing can be accomplished by several methods:
Dying: The pores in the anodized surface can absorb organic dyes. The depth of dye absorption depends upon the thickness of the anodized layer and the number of pores in that layer. Color matching is easiest when only one dye is used. The process becomes more difficult when mixtures of two or more dyes are used.
Mineral Pigmentation: Mineral pigments can be precipitated within pores of the anodized layer. As an example, iron oxide precipitated from a ferric ammonium oxalate solution produces gold colored surfaces.
Integral Color Anodizing: In this process pigmentation is accomplished by the inclusion of very small particles in the coating in an anodic reaction between the aluminum and the electrolyte. Composition and temper have a significant effect on the color produced. Alloys containing manganese and silicon tend to color black or grey. Alloys containing copper and chromium tend to color yellow to green.
Electrolytic Color Anodizing: In electrolytic color anodizing, an anodizing process is followed by electrodeposition of metallic pigments from a dissolved metal salt in the pores of the anodized layer in a second operation. The resulting colors are very stable.
The past few months have brought us several clients experiencing color variations in anodized aluminum parts. In some cases, the uniformity of color was of primary importance and a root cause of the non-uniform appearance was required. In other cases the client was concerned that the lack of uniform color was the result of a material defect that would compromise the service life of the part.
Visual examination of submitted samples showed three types of color variation.
Single Random Non-Uniformity:
A single, random area of non-uniform color on a part is shown in Figure 1. Figure 2 is a metallographic section of the same area. The metallographic cross section shows the defect area to be an area of significant chemical segregation as seen by the visible banding. Chemical analyses of the dark bands and the light area by ICP spectroscopy showed the dark bands to be silicon rich and the light area to be silicon free. This variability in silicon content was a segregation defect in the original billet which was deformed during extrusion.
Figure 1: Single Area of Non-Uniform Color
Figure 2: Metallographic section of above area
Figure 3 shows one of four areas of non-uniform color, equally spaced every 90°, on the surface of a part with a round cross section. Figure 4 is a metallographic section of the same area. The metallographic cross section shows the defect areas to be a relatively thin bands. Chemical analyses of a band and the adjacent areas by energy dispersive spectroscopy showed the dark band to be oxygen rich.
The symmetrical oxide bands are extrusion defects. The extruded aluminum was allowed to oxidize prior to reforming after passing over the fins supporting the center mandrel.
Figure 3: Symmetrical Areas of Non-Uniform Color at 90° intervals
Figure 4: Metallographic section of above area
Random areas of non-uniform color:
Figure 5 shows multiple, random areas of non-uniform color on a black anodized surface. Figure 6 shows a metallographic section of the same surface. The areas of variable color correlate to different crystallographic orientations of the relatively large grains of the alloy.
Figure 5: Multiple Random Color Variations
Figure 6: Metallographic cross section of above