The Impact of Foreign Materials on Aluminum Parts before Anodizing
Although simple in principle, there is a lot of confusion over how aluminum parts need to arrive at the metal finisher so that they can be anodized. Any time a component part comes from a completed assembly that has been taken apart, there is a high risk that some other material will come along and interfere with the coating process.7 Sources of Contamination on Parts before Anodizing Aluminum
We’ve put together these guidelines on some common materials to watch out for, and when present, to remove before sending the parts to your finisher.
Only bare aluminum will form an anodic coating in the anodizing process. All other materials either act to inhibit the anodic coating from forming, or will burn. In some situations, the foreign material will cause a surface defect. In others, the part can be destroyed.
Starting from simple to more complicated (and less common), below we discuss some situations where different types of foreign materials get in the way of producing a good quality anodic coating.
Foreign Materials that Matter
- Heavy machining oils and lubricants—Generally, these can be removed during the pre-treatment process before anodizing, but not always. To the extent that they cannot be removed by soaping the part, they act like a mask and inhibit the formation of the coating and usually cause an un-uniform and streaked coating. The best bet is to make sure the parts arrive clean and free from oils.
- Adhesives—Adhesives, whether intentionally on the part or not, act like a mask. No coating can form below the adhesive. It can be very difficult to see the adhesive, especially when it is a clear adhesive. However, after processing, the areas with adhesive jump out, as there will be no coating. Sometimes this can appear as “dots” or can cover sections of the part (see Figure 1).
- Tape—Although recommendations to the contrary, clear tape sometimes is used to affix a label directly to a part. However, tape or residue from the tape adhesive can be left on the part. Both need to be removed thoroughly so they don’t act like a mask (see Figure 2).
- Paint—Paints will act like a mask and will need to be removed.
- Other metals—Sometimes parts come back that have been disassembled without all screws and rivets being removed. Basically all other metals will burn in the anodic coating process, and therefore, have to be removed entirely before anodizing can take place.
- Anodic coatings—As long as the metal finishers know that the part was previously anodized (and the coating is still on the part—and this might not be obvious if it is a clear type II decorative anodic coating), there is no problem in running the part again and putting a new anodic coating on it. (See our recent blog post on fixing cosmetic damage on aluminum medical devices.) However, as a general rule, the prior anodic coating must be removed first before a new coating can form, as the old coating will inhibit a good contact with the rack and can cause other problems when forming a new coating.
- Intermetallic particles—this is also called “inclusion.” Aluminum alloys for commercial applications contain numerous intermetallic inclusions such as Cu, Zn, Fe, Mg, Si, and so on. If these inclusions exist with visible size on the surface to be anodized, they form a second-phase intermetallic surface during anodization. This results in white spots, flaking, pitting, staining, void spots, shiny dots, and so on. These are due to different oxidation rates among the metals with pure aluminum (i.e., aluminum has a relatively faster oxidation rate than any other metal compounds).
If you are unsure about the condition of the part, the best bet is to do a thorough analysis first and communicate any uncertainty to the metal finisher before sending out the part for anodizing.
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