Metal finishing Q&As since 1989

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Chrome bolts paired with zinc nuts?

February 22, 2011

Firstly I want to say thanks for all the information I've got from this website in the past. No matter what I search for on Google, I always end up here :)

Unfortunately though I can't seem to find an answer to this problem anywhere...

I've been searching far and wide for some high tensile bolts (12.9) that are plated bright silver and will not corrode if left out in the English weather. Stainless steel I've been told is brittle and only comes in 8.8 tensile form. Zinc plated has the strength, but the plating is prone to wear off 'relatively' quickly.

Then I came across a place that chrome plates HT 12.9 bolts. Perfect I thought. Only problem is I can't seem to find any chrome plated HT nuts to match with the bolts!

Now on to my question....

Will I suffer any galvanic corrosion or other issues if I mix a chrome bolt with a zinc nut? I'm not fussed about the zinc nut corroding eventually as it is out of sight.

Help!

Jim Nadier
Hobbyist - UK

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Hi Jim,

If there is any moisture exposure you will almost certainly suffer from galvanic corrosion.

To be honest it is not unusual to have bright zinc plated HT bolts, they are only going to be brittle if they have either not been heat treated correctly or, more likely, they have not been de-embrittled after plating correctly.

You can reduce galvanic corrosion between the nut and the bolt somewhat by using a grease or sealant around the nut to prevent moisture ingress.

Brian Terry
Aerospace - Yeovil, Somerset, UK
February 25, 2011

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February 26, 2011

Hi Brian,

Thanks for the response, even though I was secretly hoping the answer would be different :)

The issue I've got is that I'm refurbishing some 'split rim' car alloy wheels. A total of 20 bolts join the 'spokes' of the wheel to the outer rim, so I would assume that given the weight of the car, and other factors such as cornering/braking forces, the bolts would need to be as high tensile as possible.

This led to me choose HT 12.9 bolts which only seem to come in two aesthetically pleasing finishes: Chrome or Zinc.

Zinc appears to be the easy and cheap option, but as the wheels are going to be exposed to the elements I'm not sure if they will last very long before the zinc wears away and the underlying steel begins to rust?

Chrome on the other hand seems more durable from what I've read? The only problem of course being that I cannot find any chrome nuts of similar tensile strength. And if I mate it with a nut coated plated in another type of metal I'm likely to encounter galvanic corrosion.

I suppose using grease around the nut is a possibility, but due to the fact that these wheels will be constantly exposed to the elements and also cleaning by the owner, I'm not sure that the grease or sealant would stay on very long?

Jim Nadier
- United Kingdom

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Why not check with the wheel manufacturer and see whether grade 12.9 is necessary? I'm guessing that 18-8 stainless may be strong enough, can be polished mirror bright, will resist corrosion better than any plating, and of course stainless nuts are readily available.

Jeffrey Holmes, CEF
Spartanburg, South Carolina
First of two simultaneous responses -- February 28, 2011

Personally, I would not put chrome plated nuts on chrome plated bolts. Why not have the zinc nuts stripped and electroless nickel plated to an appropriate thickness and then baked at 400 F for a couple of hours to remove most of the hydrogen embrittlement before it weakens the nut.

James Watts
- Navarre, Florida
Second of two simultaneous responses -- February 28, 2011