Topic: Anodize, Alodine and and Fatigue Life

There is an article about Anodizing and Fatigue Life (EAA SportAviation, Feb. 09). The article is about an anodized control tube, under a relatively low load, failed in a helicopter, causing a crash. I was wondering what the differences were between Anodizing and applying Alodine to a part (pros and cons)? Would you get the same results?

   Picture 003.jpg


Re: Anodize, Alodine and and Fatigue Life

Alodine is simply a "trade" name for a Military spec (Mil-C-5541) conversion coating. It provides good corrosion protection even when scratched, as it has “self healing” properties. It can be used as sacrificial coating for corrosion protection.Alodined 2024 aluminum can withstand salt spray for anywhere from 150-600 hours before forming white corrosion. Untreated 2024 corrodes in less than 24 hours. It also provides an excellent electrically conductive surface, which helps provide good electrical bonding in an airframe. And finally paint sticks to alodine extremely well. In some cases, it can substitute for primer.

Anodizing is a whole other animal. It’s an electrolytic bath for producing hard anti-corrosive, abrasion resistant oxide surface layer on aluminum alloys. To me this accident immediately brings around the question of if anodizing can have an effect on the fatigue strength of process parts. And because load carrying members are anodized a great deal in the aerospace industry – this topic has been studied by many groups in the past.

One study relevant to the EAA article had the following results on cast Al alloys: in a 3% chromatic acid anodizing process with a 0.0025 - 0.005 mm thick coating applied, produced little or no effect on the fatigue strength. A 15% sulfuric bath with a coating 0.0125-0.005mm thick also had little effect initially regarding fatigue strength, but did suffer a slight reduction in the life at higher stress levels. Subsequently it was then found that, thicker anodized coatings, 0-1 mm produced by the chromic acid process, and 0.025 mm thick produced by the sulfuric process resulted in a 35% reduction in fatigue strength.

There are many types of Al alloys, so remember the results of the above tests only apply to certain alloys. The results can be different based on the alloy and the type of anodize bath, but the fact of the matter is that anodizing can cause a reduction in fatigue strength. There are many other known techniques to both monitor these issues and correct, and even repair Al strength in this process such as the fact that much of the lost strength can be restored with a final processing step of sealing the coating in a boiling dichromate bath. At the end of the day - it all boils down to the shop, and the personnel processing the parts. This is why I always say you can't have too much ".

Thanks for asking that great “real world” question Mike! I hope this helps.


Re: Anodize, Alodine and and Fatigue Life

Hi Mike

I can see that you already got an answer, saw your Q at Facebook.

Sunny regards




Re: Anodize, Alodine and and Fatigue Life

Regarding the fatigue failure of the helicopter component, it would be important to know the alloy and temper as well as the form of stress in application.

Bill Corcoran