Galvanic Corrosion: nickel-aluminum interface

A. Hi Zeeshan. I suspect that this "white appearance" is corrosion of the underlying aluminum, so I doubt that removal, if successful, would extend the life very much.

Although tin plating would be better than nickel plating in terms of galvanic compatibility with the aluminum, and some acids might be less destructive, I believe that the real issue is that the plating is much too thin. 0.8 µm (0.000032") is less than 10% of the nickel plating thickness recommended for even mild service. In all probability, the nickel plating is porous and pin-holed at that thickness and you are exposing the aluminum to the acid and the copper passivation chemical.

I would suggest experimenting with significantly thicker nickel plating as your first step.
Luck & Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Hi again. I suspect that trying to find pinholes via SEM/EDX may be a situation where "you can't see the forest for the trees" might apply, but I have no expertise in SEM/EDX. As previously noted, trying to get by with 8% of the suggested thickness for mild exposure would seem problematic even if you weren't acid-dipping the component :-)

Luck & Regards,

A. Hi Xiaoling. If the plating is thick enough and high quality enough to suffer no porosity or pinholes, galvanic corrosion is not an issue because the aluminum is completely covered and not exposed to the environment.

However, poor nickel plating would mean that both aluminum and nickel are exposed, and the plating would not prevent galvanic corrosion, so if the environment encourages it you would get it.

Luck & Regards,

A. Steve,

I hope I can offer some words of advice.

For the galvanic series, just try looking at aluminium for the metal to see the voltage difference.

Alodine is conductive but does add a small amount of resistance, however clamped down like you describe should not be a problem -- What I am trying to say is looks like tin to Alodine coated aluminium is sensible and should work but you better do some tests to be sure.

Regarding if you should or should not put on the Alodine coating I would come down on the side of should as without you will get aluminium oxide forming which is non-conductive and for sure this will mess your joint up.

A. Hi Michael. Tin would be more compatible with aluminum, and bare steel more compatible still. But many electronic devices do not see environmental conditions that would make galvanic corrosion an issue. It requires moisture and the presence of an ionic salt capable of transporting ions. I'd probably go with tin plating if no automotive or aerospace specification is involved.

Luck & Regards,

A. Hi Alan. The chem-film doesn't hurt, but (at least according to MIL-DTL-14072) it doesn't help either: see Mark Whittum's posting on thread 19266, which I just re-checked and found to be current. So, you are attaching nickel to aluminum, and these are considered galvanically incompatible with an EMF differential of about 0.6 volts. If you are not obligated to conform to a particular spec the issue becomes less clear, of course, as you will see from many threads here about "copper aluminum galvanic" and "nickel aluminum galvanic". Nickel is slightly worse than copper.

We don't know your situation, but if you can tin plate the copper instead of nickel plating it, you'll have a more galvanically-compatible joint. Good luck.

Regards,

A. Hi again. I still don't know quite what you are talking about (pipe flange? what is in the pipe?), inside a climate-controlled electronics cabinet, or out on an offshore oil platform?

The short answer is that the materials are highly incompatible and very subject to galvanic corrosion. So, unless it's a dry application in an air conditioned environment, it sounds like trouble. Once again, are you sure you can't change that nickel plating to tin? You could nickel plate the aluminum, but the plating must be heavy (pore-free) and unlikely to be scratched through. Good luck.

Regards,

A. Hi Alan. If the two flanges are plated with the same metal then there would be no galvanic corrosion because no other metal would be exposed. I think you may be misunderstanding what you were told about this, or the person who told you misunderstanding what he was told and leaving out a critical part of the statement.

For a galvanic current to flow and corrode something requires:
a. Two different metals with different positions on the galvanic series (the further apart they are, the more powerful the battery they create).
b. The two metals must be in metallic contact with each other so electrons can flow from one metal to the other. If insulators prevent metal to metal contact, electrons can't flow
c. A wet conductive situation that allows metal to ionize into solution.

If conditions "a" and "b" exist, electrons will want to flow from the more active metal (aluminum) to the more noble metal (nickel), but as long as condition "c" is not met, the circuit is not complete, so they can't. If condition "c" becomes met, the circuit becomes complete because electrons can flow from the aluminum to the nickel through the metal, and the aluminum atoms, now stripped of an electron, dissolve into solution as positively charged ions and migrate over to the nickel to recapture those electrons. So the aluminum corrodes.

As mentioned, aluminum and nickel are far from each other, so unless freedom from moisture is assured, this is a galvanic problem. Sorry, I don't know enough about RF flanges and aircraft fitting to know whether this is a problem or not.

Regards,

A. Hi Sepanta. If I am understanding your description, you have a galvanic compatibility issue no matter what conductive material you make the sheet metal shielding of because you are providing a metallic path from the nickel plating to the aluminum. But I'd probably tin plate it. Tin falls in between aluminum and nickel in the seawater series.

Luck & Regards,

A. Hi again. It depends on the smoothness of the sheetmetal (which helps reduce porosity and pinholes) and the thickness of the plating, as well as the environmental factors, but tin plating is very widely used in electronics work of all sorts; it's not uncommon to finish up with a chromate or other tarnish inhibitor.

Luck & Regards,