Products Finishing

NOV 2013

Products Finishing magazine is the No. 1 industrial finishing publication in the world. We keep our readers informed about the latest news and trends in plating, painting, powder coating, anodizing, electrocoating, parts cleaning, and pretreatment.

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Page 45 of 59

CLINIC ALUMINUM ANODIZING Clear Anodize as a Paint Base Q. Our parts are generally Type III (hardcoat) anodized, sometimes duplex-sealed and sometimes unsealed. We just recently learned of the surfactant issue during sealing from one of your articles. Our end-products are subject to sea water, and we try to adhere paint and polyurethane potting compounds to some of the parts. Would paint adhesion or adhesive bonding be enhanced by abrasive blasting prior to anodizing, maybe at 30 to 60 grit? Some of our testing has shown that abrasive blasting may help, but we don't know what effect it may have on the corrosion protection generally provided by anodizing. Our current plan is to clean, abrasive blast, clean, Type III anodize, duplex seal (no surfactants) and try to topcoat before getting contaminated. —B.T. A. You are on the right path by eliminating surfactants in the sealing process. If Type II and Type III anodic coatings are sealed in this manner, it should provide an excellent base for the application of organic coatings (paint). Bear in mind that anodic coatings produced on a "roughened" substrate (blasted, sanded, etc.) may be less corrosion-resistant than anodic coatings produced on a "smooth" substrate. While normally a roughened surface may provide more "tooth" for an adherent coating, the anodic coating can be rough and may not provide a highquality, corrosion-resistant surface. Producing an anodic coating that is continuous and free of any voids is maximized when the substrate is smooth. An anodic coating produced on a "smooth" surface, when properly sealed, may provide enhanced corrosion resistance. Coating adhesion testing can be done on coupons that are rough and coupons that are smooth, and the results compared. It can then be determined if roughness of the substrate will be a significant factor. It sounds like you may have already done that. Corrosion testing of anodized, unpainted coupons with smooth and roughened surfaces may also be done, and the results compared. It is best to use aluminum oxide media if you are going to blast the parts. Do not use glass beads (silica), as the silica can be imbedded in the aluminum substrate and can be difficult to remove in the cleaning process prior to anodizing. This may leave a gray or brownish haze on the parts after anodizing. Since the parts are painted, that may not be a concern. However, the coating quality may not be as good as it would be after using aluminum oxide as the blasting medium, because there could be silica particles at the substrate/anodic coating interface and/ or trapped in the oxide itself. If you desire to blast with glass bead, I would recommend that the parts be etched after blasting in an acid etch bath containing fluoride in order to assure the removal of imbedded silica in the aluminum parts. This could be an ammonium bifluoride bath, or a mixture of nitric acid and hydrofluoric acid. Special precautions are required when using solutions that contain fluorides. 44 Protection from Laser Marking on Aerospace Parts alloy aerospace products that are finished using either chemical film (MIL-C-5541) or anodizing in accordance with MIL-A-8625. The question has arisen of whether or not we need to touch up the areas where laser markings have been placed in order to preserve the corrosion protection. Is this a valid concern or not? —B.J. A. I think your question regarding protection of the bare substrate LARRY CHESTERFIELD / Anodizing on aerospace parts Technologies Inc. is an important one. There is a very straightforward answer, but providing you with some additional information may be in order. The answer to your question is one that must be determined as part of the contract or purchase agreement between the finisher and the customer. This is needed even if the "customer" is an internal one within the same company. So the customer and the finisher must reach agreement as to whether or not it is acceptable to have the bare substrate exposed, if the laser markings have penetrated the coating. If this is not acceptable, then there must be agreement as to how this situation will be handled by the finisher. One scenario might be that the laser engraving could be done prior to the finishing operation. The situation to be addressed in that case is to make sure that the engraved area is, in fact, resistant to corrosion after the finishing operation has been completed. If required, salt spray testing could be done after anodizing or chem film. If anodized or chemfilmed parts are to be painted, the testing could be done after painting. It is the responsibility of the customer to determine this and pass this information along to the finisher. Another scenario could be that the bare metal resulting from the engraving process done after finishing is simply touched up with a suitable conversion coating. Again, the parts may then be painted or not. You get the idea. Not all laser engraving penetrates the anodic coating. This does not apply to parts that are chem film only. There are two types of lasers that are used for etching or scribing (laser engraving) metal surfaces. One is called YAG (yttrium, aluminum, garnet), a crystalline, solid-state laser that emits a beam at wavelength 1.064 micrometers (1064 nanometers). This type of laser is capable of scribing right through the anodic coating to expose the bare substrate. Turning the power to maximum will determine how deep NOVEMBER 2013 — Q. We have a number of aluminum

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