Hydrogen embrittlement problems in Type III Hard Anodize

A. I wouldn't be so sure that the problem is hydrogen embrittlement; anodizing exposes aluminum to oxygen rather than hydrogen. But hard anodizing is a very brittle coating, and fairly thick. If it was .002" on each side, it could have a serious effect on the strength of the material, expecially its fatigue strength. If the shop had tried to anodize the parts unsuccessfully, stripped the coating, and anodized again, the strength reduction could be even more serious.

First, I would suggest making sure that both vendors used the same anodizing thickness. Second, I'd make sure they didn't strip and re-anodize. And third, I'd talk to the engineer; when a part requires strength, hard anodizing is usually not a good coating option.

A. A .002" hardcoat anodize would consume .001" of material from each side or a total of .002" from a cross section. This would reduce your total tensile strength by .002/.100 or 2%. However, hardcoating significantly reduces fatigue strength.

A. Bending stresses are inversely proportional to the square of the thickness rather than the thickness, so it's about a 4% reduction. And if stripped and re-anodized, about 8% reduction, but I'd certainly agree that we're not anywhere close to accounting for the reported 70 percent drop in strength.

A. The coating is aluminum oxide, it is "hard", you squeeze a nut down on it, then it will certainly "crack". The tendency to crack under such pressure is a function of alloy (you did not say), thickness (you did not say), whether work hardened by forging or machining, free acid, current density, temperature. Please come back to us with some information.

A. Are there threaded holes that are being coated? If so, that could be the main problem. If the customer comes along and installs a screw into a coated hole there is a good chance of cracking. It's best to not coat threaded holes if the customer can allow it.

One thing to check is the condition of the bath and also the processing conditions. With the hot weather we had, it's possible that the parameters were changed unless your facility is tightly controlled in terms of a/c. If the bath is in bad shape and a lot of additives are being put in, perhaps even without the knowledge of a supervisor or manager, the coating could crack as a result. Another problem is if too high of a current is used or if poor contact is made. There are several additives that will tend to plate quickly if a high current is used and then later you can see cracking and other defects, particularly if the item in questions is subjected to a load. If you have poor or intermittent contact, that will tend to drive additives to plate more quickly.

I would plan on doing testing in-house before shipping any more parts. Find out exactly how the parts are failing and get everything you need to do the testing there at your facility. If the parts pass your tests but the customer still complains, it could be that the customer is using excessive force due to a new employee or for whatever reason.