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Interior protection of steel automotive coolant pipes

I am a restorer of '60s era Mercedes mechanical systems and engines. These systems use several pipes and tubes that carry coolant, oil, air (fresh and crankcase ventilation), and fuel. When new, all these pipes were yellow zinc plated, at least on the outside. I assume that they were also zinc plated on the inside (or, possibly, galvanized), but don't know that for a fact.

The problem is that any protection on the inside of the pipes is stripped out by the acid solution used to clean the pipes prior to re-plating, and zinc re-plating will not replace that inside protection very far inside the pipe (more than about one times the I.D. of the pipe).

I need some way to achieve the correct cosmetic appearance on the outside of the pipe while providing good corrosion protection on the inside of the pipe. If I have to sacrifice one for the other, then the corrosion protection has the priority.

One high-end plating shop suggested plating the parts with electroless nickel to obtain the protection on the inside, and then finish with zinc plating to obtain the appearance on the outside.

Would this actually work? Also, is there a limit to how small the pipe I.D. can be and still get the protection all the way through the length of the pipe? The original pipes do not corrode easily so there is some practical means available to obtain the protection and appearance, but we have had no luck determining what that method was. It has occurred to me that most pipes could be hot galvanized to get some protection on the inside, but I think that would create many problems trying to get the correct appearance on the outside.

Chris Johnson
Automotive restorer - Creede, Colorado, USA
June 24, 2009

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July 1, 2009

The electroless nickel should give you the corrosion resistance you need, but I don't know about putting zinc plating on top of the nickel. I have put EN on brass, but that's the closest I have to experience with zinc on nickel. A test panel would answer the question fairly quickly.

If you could set up your plating tank so that you circulated a hot EN solution through the interior of the pipe or if you masked the outside of the pipe (the plastic dip for tool handles works well), you could EN the interior and zinc plate the exterior.

Good luck.

Michael Costello
DOD Primary Contractor - Grand Junction, Colorado, USA