Is it Anodized or Not?

Q. What would be the best way to indicate whether or not an aluminum part has been electro-polished and anodized? We are currently making parts for boats out of 6061 alloy aluminum and they need an anodized coating on them to prevent them from oxidizing. Recently we have run into an issue with our parts not being anodized by the vendor. The parts are electropolished and then clear anodized, whereas previously our parts were bright dipped and anodized. The anodizer told us to use a continuity tester, which will show if there is a coating. And now they’re saying that the continuity tester is not a good source to prove if there is anodizing on them or not.  Do you have any suggestions? J.B.

A. There are at least two ways to do a quick check of the coating on your parts: 1) Use a continuity tester to see if there is coating enough to be an insulative barrier (the anodic coating) on the part.  Obviously, if there is continuity, there is no coating. 2) Use an “eddy current” instrument to actually measure the coating thickness.  There are several easy-to-use instruments on the market for measuring non-conductive coatings on non-ferrous substrates. Visit PFonline.com/suppliers to do a search.

I would suggest working closely with the anodizer and ask them to produce several sample parts that are electropolished and anodized. It would be wise to specify a coating thickness requirement with minimum and maximum values of thickness as well as different degrees of brightness. Ask the anodizer to produce samples that are within the range of those specifications. They could have varying degrees of brightness and anodic coating thickness, so you can select the appearance you like best and still have the corrosion protection required. Ask the anodizer to certify that a representative sample of production parts in each batch that is anodized falls within the range of coating thickness you have selected.

When aluminum parts are brightened, either by buffing, brightening or both, the anodic coating tends to dull the brightness somewhat. The thicker the coating, the less bright they appear.