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Scratches in chromate conversion coated aluminum
We have recently started to subcontract machined aluminum that is chromated; the surfaces appear to have scratches that expose the bare aluminum but our supplier claims these are "normal"-is there a standard in the industry for what is acceptable? We have done spot tests which indicate that there is some level of exposed aluminum. The parts are used in the aviation industry but are not cosmetic in nature. It seems unlikely that the small imperfections we are seeing will cause any degradation to form/fit/function but I'm stuck in a quagmire with our Quality dept. Help!Beverly Rechkoff
Are these "scratches" actually rack marks from holding fixtures?
Until the legendary "sky hook" is actually invented, contact points are a necessary evil of rack finishing. Bulk processing may be possible, but aluminum parts are often to fragile for this kind of handling. The widely used specification for Chemical Conversion Coatings on Aluminum and Aluminum Alloys, Mil-C-5541 [link is to free spec at Defense Logistics Agency, dla.mil]E recognizes this fact in paragraph 3.5 Appearance. It states "The size and number of contact marks shall be at a minimum, consistent with good practice." This still leave a lot of room for interpretation. As a fallback the spec states, "If specified, contact marks shall be touched up with Mil-DTL-81706 [link is to free spec at Defense Logistics Agency, dla.mil] material ... to prevent localized corrosion." Obviously, finishers are going to avoid this added cost unless directed to by their customers.
Luke Engineering & Mfg. Co. Inc.
It sounds like your finisher is chromating your parts in a bulk method. In our shop we do chromating in bulk & rack methods. Basically, we look for certain things when deciding if a part can be done in bulk:
1) It must have no flat surfaces, where the parts will stick together or SCRATCH the coating off one another.
2) It must have no blind holes which would trap chemicals.
3) It must be hi volume.David A. Kraft
- Long Island City, New York