I always appreciate when my readers contact me for more in-depth information regarding a specific issue. This time the question was regarding galvanic corrosion between aluminum and stainless steel.
The question was:
I´m still a bit confused on the effect anodizing has to this corrosion problem. You stated that it can be superior choice but also make it worse. How will I know?
Below you will find my answer:
Aluminum is a reactive (un-noble) metal compared to most of the metals used. Aluminum will therefore almost always be the anode, the part which corrodes, in contact with other metals, but because of the natural formed oxide layer Aluminum can be a called a passive metal. So Aluminum behaves as a very stable metal, especially in oxidizing media such as air, water, etc.. This natural formed oxide layer differs in density compare to the underlying aluminum, which makes the aluminum oxide less likely to crack when deformed. The dissolution rate of aluminum oxide depends on the pH value, see figure below.
The corrosion rate (dissolution rate of the aluminum oxide) is not solely dependent on the pH but also what kind of acid or alkaline solution we are talking about. Sodium hydroxide at 0.1 g/l is 25 times higher than in an ammonia solution at 500 g/l. For the acids solutions of hydrochloric acid or hydrofluoric acid are much more aggressive than solutions of acetic acid. It is though very important to recognize the different slopes of the curve depending on which side on the pH scale the aluminum is exposed too. High pH has much higher corrosion rate than for a low pH.
So what I am saying is, take a careful look at your environment, if you are out of the pH range 4.5 - 8.5 you should immediately be aware of a possible corrosion issue. This is the same whether you have an anodized surface or not. The protective oxide film will not be protective anymore, leaving a part smaller or bigger part of the aluminum unprotected.
If a very small area of the protective film has been destroyed, exposing a small anode area, as shown it the picture in the post, due to cracks or a scratch or something else then you will know that you have a corrosion problem.
This will lead to a small anodic area (un-protective aluminum) relative to the cathode area (the stainless steel) and this should be avoided. The larger the relative anode area, the lower the galvanic current density on the anode, the lesser the attack.
A practical illustration of what I am saying is on anodized aluminum frames in windows, especially in salty environments. Some have stainless steel clips riveted to them destroying the anodic oxide film, in other places there are normal carbon steel bolts through the frames with no destruction. There is zero evidence of corrosion between the steel bolts and the anodized aluminum frame. In the areas of the rivets the anodized aluminum frame is completely and totally eaten away underneath the stainless steel clips. Outside this region there is no corrosion.
If you have a specific problem, I would happy to help you as a consultant . Take a look at one of my products and let me know if you would be interested.
Troubleshooting Aluminum surface issues!
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