Letter 1119

Long term corrosion resistance of aluminum

---

I have been unable to locate any hard data on the long term resistance of certain aluminum alloys in a salt fog environment. We need a low cost finish such as Iridite, but able to withstand up to 5 years in a warm, salty environment. Painting is expensive. Anodize is OK, but is there a cheaper alternative?

The alloys I am interested in are 2014-T6, 6061-T6, and 7075-T6. I would like to see evidence of the corrosion resistance of these alloys after 5 years or more in a salt environment when coated with Iridite, or anodize, or any other low-cost suggestion. I have an immediate need.

Bruce Chisholm
Lockheed Martin

Bruce:

One possibility is plating the aluminum alloy with high purity aluminum. This plating acts as a sacrificial anode to any Al alloy because there are no galvanic sites to initiate corrosion - similar to the ALCOA Alclad material used in aircraft skins. The plating takes Alodine or Irridite or other aluminum conversion coatings and a bolt manufacturer in MI has tested aluminum plated and chromated steel bolts for over 6,300 hrs in B-117 [link is to spec at TechStreet] salt spray. Price may be competitive or not depending on the size/geometry of the parts and the thickness required, but I thought you should know that plating With aluminum is available.

F.A. Sandy Donaldson
- Minnesota

I would be reluctant to use either 7075 or 2014 in a seacoast environment even with a protective coating due to the risk of exfoliation corrosion. 6061 is not susceptible to this type of attack, but may sustain some minor pitting and surface deposits even with anodizing.

Look in the literature for references to experiments at Kure Beach. There has been tons of work there on long-term exposure of steels with various compositions and coatings to sea coast environment. There may also be experiments there with aluminum.

Larry Hanke materials testing laboratory Minneapolis, Minnesota

ASM International has a good handbook which addresses corrosion behavior of aluminum. It is the ASM Specialty Handbook [an error occurred while processing this directive]

  and they are located at Materials Park, OH. The ISBN is 0-87170-496-X.

Lee C. Branch

Thank you all so much for your valuable information. I appreciate the time you took to respond to my needs.

Of particular help was the ASM International Handbook on Aluminum. This book has literally everything you need on aluminum, including the long-term data I was looking for. I highly recommend the book to anyone with a question about aluminum.

Thank you again.

Bruce Chisholm
Lockheed Martin