On Jun 19, 6:59 pm, The Natural Philosopher wrote:
On Jun 19, 10:29 am, Fred wrote:

I know you can get a black "paint" to paint over rust, but how does it
work and is it any better than using normal gloss paint? Presumably
forming a barrier to stop water getting in is half the battle?

The one I have seen is bizarrely water based, I would have thought
water is the last thing you want to apply to rust!

Neither water nor air cause rust. Its both together. Or water plus some
other oxidant like whatever seawater has in it.

The oxidant in sea-water is just dissolved or entrained air. The
reason sea-water is so powerful at rusting is that it contains
chloride ions which either make the rust layer more permeable, or
catalyze the process (I can't remember which now - it's more than a
quarter century since I did Metallurgy!).


Eliminate either one and you are on a winner.

Quite - which is why the Titanic hasn't turned into a pile of brown
sludge - the air can't get to it, so it doesn't rust.