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Graphite Corrosion on Anodized Aluminum
Q. We make a part out of 6061-T6 aluminum that we then hard coat anodize. The assembly into which this part goes is lubricated with natural graphite. The assembly could be stored in a somewhat moist environment. We haven't noticed any corrosion problems with this part; however, I have not been able to find anything specifically relating to the corrosiveness on aluminum in this type of environment. The question comes up every now and then about whether the graphite is corrosive and I would like to be able to cite something other than our observations.
John Braden- Tucker, Georgia, US
2001
A. Hello John!
I'm assuming that you're concerned with galvanic corrosion -- the graphite making the aluminum corrode faster by being cathodic to it. If so, don't worry- the hard anodize serves as a sufficient barrier to be a proper fix. Check out MIL-STD-889 on "Dissimilar Metals", which lists hard anodizing as the premier mitigation step when you have a concern about galvanic corrosion of aluminum.
Hope this helps!
Lee Gearhart
metallurgist - E. Aurora, New York
2001
A. While the anodizing does indeed act as a good barrier, in the long term the absence of corrosion would depend on the absence of corrosive ions. Graphite is not corrosive in itself towards aluminum, but in the presence of chloride ions (such as from sea-water or marine atmosphere) it can cause galvanic (bimetallic) corrosion to occur. The anodizing will however prevent the chloride ions from reaching the metal substrate for a long time - perhaps several years. So there should be no concern with moisture alone, but if chloride is present, there might be long-term concern.
John E. LeitchHulett Aluminium - Pietermaritzburg, South Africa
2001
A. Graphite is electrically conductive. One form of corrosion is a result of vibration (fretting corrosion). Solid Film Lubricant Coatings use primarily Molybdenum Disulfide for lubrication. Every military spec for solid film lubricants must be certified as graphite free. Graphite is also soft and does not provide heavy load lubrication.
Eddie Andrews- Monrovia, California, USA
2001
A. Anodizing is about .001 to .002 thousands inch thick. So if the parts are fretting it is likely that the anodizing in that area is worn off. So normal corrosion like oxidizing will occur. Also petroleum products will trap moisture and air next to the metal so you will see it as long as there is no protection like anodizing. Lubricants don't solve everything. Sometimes they are part of the problem.
Ronald Higgins- Sumter, South Carolina
2005
Q. In reference to graphite being used on aluminum. Can I safely spray powdered graphite aluminum into an aluminum rifle magazine to help with lubrication? I have been told it might chemically react and ruin the magazine.
Jerry Miller- Oakville, Missouri
December 2, 2013
Q. We have aluminum forgings that are 2000 series alloy. From what I understood about the process from our customer was they used graphite while forging and then these parts were baked with the graphite still upon the parts, so the parts look really dirty, but none of it wipes of by hand.
During our cleaning process, doing a standard soap, etch, deox there was still a residue of the graphite on the parts that we were unable to remove chemically, short of wiping all the parts by hand.
Has anyone experienced this issue, is there a way to solve this issue?
- Dexter, Michigan, U.S.A.
January 22, 2014
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