Galvanic corrosion; Copper eyelet (from CHT sender) bolted to an Aluminum Head

[vonsteimel]

Greetings,

We've switched our CHT (cylinder head temperature senders) to a better more reliable type.

However the connecting "eyelet" is solid copper (unknown grade/alloy), in contrast to our old ones that were zinc-plated steel (like the spark plug).

These copper eyelets just go around the spark plug and are held (i.e. bolted) to the aluminum cylinder head via the spark plug. This can and will be doused with high volumes of saltwater & spray during operation.

There is a notion here at my work that we need to zinc plate these copper eyelets but my opinion falls on the contrary. For one instance, just look at the statue of liberty. Shes been standing in salt for years and is doing fine! It may develop a green patina but should otherwise be okay.

There are a few concerns however. Since copper is more noble than both aluminium and zinc, will the copper still develop a protective patina or will it continually attack the zinc and the aluminum?

I assume that galvanic corrosion will take place but only until the copper is covered in patina, sandwich between a layer of aluminum oxide (from the head) and zinc oxide (from the spark plug).

Is my thinking correct?
Thanks,

VS

 

[CoryPad]

The galvanic corrosion will be more severe than you realize. Can you replace the copper with aluminium? If you need copper, then you need to isolate the copper-aluminium couple. A zinc coating may work, but there could be better options. Can you provide any more details about the design and environment?

 

[TVP]

Galvanic corrosion will almost certainly keep the copper and aluminum surfaces in an active corrosion state, meaning that neither one will develop a stable, passive oxide layer. This sounds like a severe limitation for the aluminum cylinder head.

FYI, Lady Liberty required a substantial renovation due to galvanic corrosion of the structure beneath the copper skin:

http://www.copper.org/education/liberty/liberty_reclothed2.html

 

[vonsteimel]

It is on an engine head obviously, that's used on an amphibious vehicle. Sometimes these vehicles operate in or near the oceans. And may be subjected to high volumes of saltwater & spray. It should be rinsed after use but that does not always happen. I'm not sure of the exact material types other than just "copper" and "aluminum" and "zinc".

It is just a simple "J-type" CHT sender, which may get up to about 250F degrees max. Usually run about 200F.

Our old ones seemed to get by. They were corrode after a year or two but not eaten away.

We have no specific requirement for the copper, it is just they way the new ones come. They are much easier to mount and have proven more reliable in testing. Our others were cheap china crap, these are made in the USA.

Why would the oxide layers not stabilize? These senders are meant to be bolted to cylinder heads, many of which are aluminum...?

It is possible to put thin washers of different material if it would help relieve the problem any.
As I mentioned, we were able to get a satisfactory performance out of the zinc senders, although they did corrode.

 

[EdStainless]

The oxides that form will be porous and allow water to saturate them keeping the galvanic cell active.
The easiest would be to Zn plate the Cu. You could also Terne plate them with 50/50 Zn/Sn (it used to be lead but not any more).
This might be the most technically correct method but it should prove serviceable.

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Plymouth Tube

 

[vonsteimel]

"The oxides that form will be porous and allow water to saturate them" -- this applies to which materials?

I was taught copper developed a good patina that provided fairly good corrosion protection. As seen with lady liberty & other architectural uses.

If we can get by 3 or 4 years without replacing, it would be an improvement.

 

[EdStainless]

Zn and Al will form porous oxides.
Using bare Cu could accelerate corrosion of the spark plug, perhaps leading to it rusting into the head.
And the bare Cu could lead to localized attack of the Al head as well.

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Plymouth Tube

 

[vonsteimel]

Ed,
So would zinc plating be any improvement? The only benefit I see is that the head would be the cathode instead of the anode. The engine is inverted so I don't think plug rusting into the head would be an issue. Especially since the plugs don't stay in that long without either being replaced or cleaned (2-stroke engine). Once the copper develops a patina, it would not longer create galvanic corrosion, correct?

Is it possible to accelerate the patina with some acid solution? Would this prevent, or significantly reduce, the galvanic corrosion versus an untreated copper eyelet?

 

[vonsteimel]

Would accelerating the Copper patina solve the problem?

 

[Nereth1]

Sorry for a potentially silly question, but would the copper develop a patina at all? Coupled with a much larger relative surface area of zinc/aly, which are far more active than the copper, would it not just be protected quite effectively from oxidation and thus never develop the patina?

 

[vonsteimel]

Once installed, I would think so.

However, I'm wondering if we used some type of acid to accelerate the patina before installing, if it would solve the problem. I think the copper oxide should be quite stable.

Does anyone have any input on doing this?

 

[metalman8357]

As EdStainless keeps mentioning, the copper oxide and/or patina will be pourous. While this may protect the copper by itself from further oxidation, it will undoubtably lend itself to galvanic corrosion when in contact with a more active metal. And as I'm sure you've seen before, aluminum is one of the worst metals that you can have in terms of galvanic corrosion with coppper (the literature is flooded with info on this).

Think of the aluminum and copper that are in contact being seperated by a thin porous material. As long as there is moisture at the joint, it will wick into the pores in the oxide layer via capillary action. This will complete the 'circuit' and allow corrosion of the aluminum to occur at an accelerated rate.

You mention using a passivating treatment to form the patina on the copper but this will not help. Consider stainless steel that is passivated - It will still undergo galvanic corrosion with a more active metal.

Your best bet is gasketing or changing the material.