Dichromate Sealing and Fatigue Life


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Q. We are looking for some references that show that dichromate sealing restores the loss in fatigue life caused by hard anodizing aluminum alloys. R.R.


A. To my knowledge, there is no such thing as complete restoration of fatigue life by dichromate sealing of anodized aluminum. It is a fact that dichromate sealing does somewhat reduce loss in fatigue strength. The effect is different for different alloys, and individual situations and conditions vary somewhat. For example, a hard, thick anodic coating on 6061 lowers the endurance or fatigue strength appreciably. On 7075 alloy, the loss of fatigue strength is somewhat less than on 6061. When dichromate sealing hard anodic coatings, the seal can be shortened somewhat to minimize loss of hardness caused by sealing. However, care must be used in sealing if superior corrosion resistance is also required. Coating thickness has little affect on loss of strength. Loss of fatigue strength is usually only a significant factor when high stresses are induced. Anodizing affects fatigue strength very little at low levels of stress.

If loss of fatigue strength is a critical factor in a project, I would recommend comparative testing to see just what the difference in performance is for your parts. You can read more about it in The Surface Treatment and Finishing of Aluminum and its Alloys, by Wernick, Pinner and Sheasby, Fifth edition, Vol 2, pp 718–720. I believe the latest printing of this reference is Edition Six. You can also do an on-line search for various papers on the subject. They may, or may not, be applicable to your situation. 

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