Sacrificial anode

[ligertigon]

Ok, to those who don't know, this is real chemistry where one metal "sacrifices" itself to be corroded before another......... Steel, that is connected to it.

Any thoughts on a control group here wiring Magnesium bars to our chassis / bodywork, and logging rust progress for the year?

[boyzdad]

Didnt this used to be all the rage in the 50's/60's? As far as I remember (from articles I read, I not that old!) they (authors of the articles) seemed to think it was a good way of making money for the seller, with no evidence to sujest it worked.

I don't personally think I would like to go down this route, I need to spend my time on it trying to get it to work without breaking down

Sorry I can't be more positive.

[jimi]

Sacrificial anodes work well on ships/oil rigs etc very common, usually Zinc. No idea how well they would work on a car.

[Karl_T]

I doubt this would work very well at all, to be effective you need there to be a good electrical path through the metal from the sacrificial annode to the bit you're trying to protect, and also through the water. This is OK on ships and oilrigs that are welded to gether and immersed in water, but is unlikely to work on a car. I think you'd need multiple annodes very close to the bits you need to protect to ensure that they're both wetted at the same time.

karl_T

[jimi]

I doubt this would work very well at all, to be effective you need there to be a good electrical path through the metal from the sacrificial annode to the bit you're trying to protect, and also through the water. This is OK on ships and oilrigs that are welded to gether and immersed in water, but is unlikely to work on a car. I think you'd need multiple annodes very close to the bits you need to protect to ensure that they're both wetted at the same time.

karl_T

I vaguely remember reading something to that effect before

Jimi

[b2hbm]

The principle does work in cars, but not in the way you're looking at. When manufacturers use zinc sprayed panels or galvanised components, then that's the same process/chemistry they're using. As stated, it also works well on ships for the reasons that Karl cited, but with a car it's more of a problem.

Consider a flat panel with a lump of zinc bolted to it. The area protected around any electrode is dependant on the current generated from the bi-metallic couple, which boils down the the difference in the metals on the electrochenical series and the relative sizes of the anode/cathodes. And of course current will only flow when there's an electrolyte present - your salty water - but that's ok, because you only want it to work when corrosion is active.

So it's technically possible. The trouble is that even in a zinc/steel couple, the area protected away from the zinc is quite small. At a pure guess I'd say it's only a cm or so. Ok, that depends to some extent on the size of the zinc, but you get the picture - you'd need a lump bolting on every few cms. It gets even more complex when you start to get folds, creases and crevices caused by flange/lap welds - like the sills for example. You could design a system to work on the sills, but I think it would look like a strip of zinc along the length, in fact probably 2 or 3 strips would be needed per side.

That's why the makers go for galvanised or sprayed panels - the zinc coverage is 100% from new, so it's perfect. Any scratches or deterioration in the coating will leave a massive zinc/steel surface area ratio and thus the steel will be protected even though it's bare metal.

So yes, you could work out a system and certainly bolting some zinc in critical areas will do no harm and potentially some good, but personally I'd have more faith with internally applied waxoyl and preventative maintenance outside.

[ligertigon]

The principle does work in cars, but not in the way you're looking at. When manufacturers use zinc sprayed panels or galvanised components, then that's the same process/chemistry they're using. As stated, it also works well on ships for the reasons that Karl cited, but with a car it's more of a problem.

Consider a flat panel with a lump of zinc bolted to it. The area protected around any electrode is dependant on the current generated from the bi-metallic couple, which boils down the the difference in the metals on the electrochenical series and the relative sizes of the anode/cathodes. And of course current will only flow when there's an electrolyte present - your salty water - but that's ok, because you only want it to work when corrosion is active.

So it's technically possible. The trouble is that even in a zinc/steel couple, the area protected away from the zinc is quite small. At a pure guess I'd say it's only a cm or so. Ok, that depends to some extent on the size of the zinc, but you get the picture - you'd need a lump bolting on every few cms. It gets even more complex when you start to get folds, creases and crevices caused by flange/lap welds - like the sills for example. You could design a system to work on the sills, but I think it would look like a strip of zinc along the length, in fact probably 2 or 3 strips would be needed per side.

That's why the makers go for galvanised or sprayed panels - the zinc coverage is 100% from new, so it's perfect. Any scratches or deterioration in the coating will leave a massive zinc/steel surface area ratio and thus the steel will be protected even though it's bare metal.

So yes, you could work out a system and certainly bolting some zinc in critical areas will do no harm and potentially some good, but personally I'd have more faith with internally applied waxoyl and preventative maintenance outside.

Every little edge helps, if we are to keep these cars from rusting to the scrappy.
What are peoples thoughts on the zinc spray aerosols? better than paint?/waxoyl?

[anna]

how about a steel exhaust?

Many people change to stainless, and don't realise that it will actually promote corrosion on the other parts of the underbody. It's better to have a sacrificial part that's easily replaced rather than the chassis or something more problematic to replace.

It's one of the reasons why a titanium exhaust (whilst stupidly expensive anyway) is a poor idea.

[Jim-SR]

Every little edge helps, if we are to keep these cars from rusting to the scrappy.
What are peoples thoughts on the zinc spray aerosols? better than paint?/waxoyl?

incredibly expensive and probably wont work anywhere near as effectively as a proper zinc coating. waxoyl is cheaper and equally as effective if applied properly and regularly maintained.

[b2hbm]

incredibly expensive and probably wont work anywhere near as effectively as a proper zinc coating.

I've got to agree with that comment, when I mentioned sprayed panels earlier I was meaning metal spray or similar, not a paint spray.

With zinc-rich paints (which are sometimes used in industry to touch up coatings) then it's down to how much zinc is present in the paint layer, and usually it's not that much. And to be fair, if you had a 100% zinc content then it would probably have poor adhesion because the zinc is only there as a filler component of the paint layer.

And an aerosol spray is probably the worst scenario - thin paint, thin nozzle to atomise it all point to a low Zn content. A brush painted coat would be a lot better, but personally I go for any paint at all on the bare metal, followed by waxoyl and maintenance checks on the areas you can see.