Zink coating underwater?

[Nick.K]

There seem to be two schools of thought about underwater zinc coatings for steel; some say that if the coating is damaged the underlying zinc will act as an annode leading to greater damage as it is erroded and others say that it provides good protection. What is the forums opinion?

[Stumble]

Zinc under water is just a bad idea. Even the best prepared piece will have some nicks, scratches or otherwise, and once that happens galvanic corrosion becomes impossible to control.

The problem is that those things like screws that might be galvanized, would then be mild steel underneath, and the localized current between the zinc and the steel will corrode both away in no time. The option is to either use a suitable material from the get go, like brass, or bronze, go to a high tech like titanium (not nearly as expensive as many think) or use another building material.

That being said, there may be a specific use where I could see the advantage, but not in general.

[Nick.K]

What about zinc based primers and other coatings?

[Stumble]

Nick,

I thought you were talking about galvanized parts and the like. I have no experience with hull coatings. So take this with a grain of salt... From what I understand zinc coating have no positive effect of the inhibition of rust or corrosion on steel hulls. Current best practice is to apply a significant coat of epoxy barrier coat over fresh sandblasted steel, then apply the anti-fouling of your choice over the epoxy.

Again, I have limited knowledge of this area however.

[CDK]

The magical properties of zinc are greatly exaggerated.
It corrodes easily and rapidly and unlike aluminum it does not form a sealing layer that protects the underlying metal. The only reason it is used to combat corrosion is the negative voltage potential it generates in a wet environment. A sacrificial zinc electrode can prevent or reduce corrosion of steel, but the key word here is 'sacrificial'.

In a less aggressive environment like air, a thin layer of zinc, obtained by galvanizing, will protect steel in a similar manner until it is fully oxidized. Hot dipped zinc protects longer because the layer thickness is at least 10 times that of galvanized steel, but when submerged and part of a moving object, the corrosion process accelerates enormously.

It is true that a zinc layer protects steel even when damaged (because of the voltage potential) but the remaining zinc oxidizes faster.

For a zinc coating to act in the same manner, it must be electrically conductive and in contact with the underlying steel. And that is where things go wrong....
A zinc coating that has mechanical strength and firmly adheres to steel has metal particles embedded in a resin and does not conduct electricity, a coating with a very high zinc content doesn't adhere well and can be damaged by a fingernail.

As a sacrificial corrosion protection system underwater all zinc coatings are useless, simply because there is not enough zinc to sacrifice.