Pitting of 7075 and 2024 Al Alloys

Q. I would like to know more about the pitting problems in 7075 and 2024 aluminum alloys. We currently make anodized parts from both alloys and on occasion we see pitting after cleaning. I have recommended RO water in the tanks and DI water in the seal tanks.

I have done tests on how we apply the voltage in the anodizing tank, slowly 5 V, wait 5 min, until it reaches 18 volts for 35 min. 

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My questions are:
1) Why do we see the pitting in the same lot, sometimes 5% sometimes 50%?
2) What precautions can we take to pinpoint the cause of this pitting, beside the materials? 
3) What causes discoloration of the parts?

Thanks. E.I.

A. It is unclear to me if you are seeing the pitting after cleaning, anodizing or following both processes. A few references I checked indicated specifically that 2000 and 7000 series aluminum alloys not be subjected to uninhibited, high-pH alkaline cleaning solutions due to the risk of intergranular attack.

Although it seems like a foregone conclusion, the first thing you should do is obtain a copy of the certificates of analysis that would provide chemical composition of the materials you are working with. It is possible that a different grade of aluminum could react differently. The second item that you should check for on the certificate would be the heat treatment given to the parts (T4, T6, etc.). This would indicate the times and temperatures the metal saw and provide some insight as to the state of some of the alloying ingredients.

For instance, 7075 nominally contains 4.4% copper for strengthening. The material would first have to be quenched to “freeze” the copper in solid solution. Tempering to increase strength would then just start to precipitate some of this copper from solid solution, particularly since this is such a high amount and is near or at the solubility limit.

If a significant amount of copper had come out of solid solution, it is likely that it would set up an electrochemical potential difference with the aluminum, because the copper would be considerably noble or cathodic to the aluminum. This potential difference would drive pitting of the adjacent aluminum in almost any aqueous environment, but would be accelerated by aggressive cleaning solutions that would drive that reaction more quickly.

My initial suggestions would be to 1) examine and confirm the chemistry of the materials you are supposed to be using and 2) understand and verify the heat treatment of the parts.

If composition and temper are correct and necessary for the application (high strength), then you may need to examine your cleaning process and move to a less aggressive process that does not require a deoxidation step. ?