Anodising Problem: Grainy and Uneven Color
Q. We are a machine shop that has made some parts for a local company -- made from 6082 & Almac tool plate. We have used Almac for years without any anodising problems. Our customer came in today with a batch of parts that they had anodised. All the parts that we made from 6082 looked fine as where the Almac tool plate parts looked very "grainy" with an uneven colour - as though the acid had eaten away into the aluminium. I think this is what has happened as there are several fixing holes on the back of the component that are quite over size since anodising (accepts no go thread gauge) The colour is a "mid grey" but I am not sure this has much to do with the problem.
I apologise for my terminology and have only just come across this site, and I am amazed with the wealth of knowledge on here! If any one can point me in the right direction why this has happened I would be truly grateful.
- London, UK
A. If you've been doing this before on the same alloy and customer, there is a possibility that error occurred on the part of the aluminum alloy supplier. The aluminum batch that was prepared might not be homogeneously mixed, or simply the wrong alloy had been supplied. I based this opinion on your observation that it produced a grayish finish which may be indicative of richness in silicon (or magnesium?)Dado Macapagal
- Toronto, Ontario
A. What about the acid in the color bath ? It might be a good idea to check the concentration of acid in the color bath.
- SYDNEY, Australia
December 15, 2011
Q. Hi I have had a similar problem with some bike levers. I can anodise new alloy fine but when it came to doing my levers on my bike they went a very dark gray and I was unable to colour them. If anyone could help it would be lovely :)Kieren Rogers
- Bristol, U.K.
December 15, 2011
A. Hi, Kieren.
There are many different aluminum alloys -- Which contain different amounts of copper, silicon, magnesium, zinc, etc. And of those metals, only the aluminum portion can be anodized. That means that when an alloy is high copper for example, which might be the case on bike levers, it tends to anodize very dark. Is it possible that your successes were with a different alloy?
Sometimes some improvement is possible by de-smutting (dissolving the copper and silicon on the surface) between cleaning and anodizing, but this may or may not be enough (some castings alloys are not attractively anodizable).The problem is not solely age, but yes, age can cause segregation along grain boundaries and exacerbate the problem of poor looks and poor corrosion resistance. Good luck.
Ted Mooney, P.E.
Pine Beach, New Jersey
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