There is (offshore wind powered) light at the end of the tunnel! | 30 comments (30 topical, editorial, 0 hidden)

Re: rust (none / 0)

Oh yes, we did sacrificial anodes and galvanic protection (where you provide the electrons directly from a wind turbine, and make the anode either something that doesn't corrode (perhaps carbon?) or something you don't care about (old cars and tractors, apparently!)) at school, I was just wondering whether they even attempted to paint them, or use special steel or whatever.

So my question for you is: why do zinc coated things last a lot longer than steel? If zinc is more reactive why doesn't it all just disappear quickly and revert to iron?

by njh on Mon Jan 25th, 2010 at 06:01:50 AM EST
[ Parent ]

Re: rust (none / 0)

The foundation pipes will be rammed into the ground, I don't think any coating will survive this procedure. Special (sea water resistant) steel would be too expensive, given the large quantities necessary.
Zinc coating is a bit different: The corrosion protection results from the coating of the entire surface. Within air the zinc forms a dense coating of zinc carbonate that protects the zinc from further corrosion.
Of course if the zinc layer is destroyed at some place the electrochemical process begins and its speed will depend on the area affected and the amount of humidity and ions present.
But even without holes in the coating the zinc layer degrades at free air, I believe it is roughly about 1µm/a.

Joerg.

by josch222 on Mon Jan 25th, 2010 at 06:55:40 AM EST
[ Parent ]

Re: rust (none / 0)

So zinc is both a cathodic protector and a 'paint'. That's very clever. Does water attack the zinc carbonate, or is it also ok in fresh water? sea water?

I guess zincalum is better because aluminium oxide is so hard and inert.

Thanks for the chemistry lesson!

by njh on Mon Jan 25th, 2010 at 05:10:10 PM EST
[ Parent ]

Re: rust (none / 0)

Yes, as long as it is a complete coating it works as a kind of paint, if it is pierced it begins to work as a sacrificial anode, at least at this area.
As far as I know zinc carbonate is soluble in fresh water only in very small amounts, but I don't know about sea water. I think this depends on the kind and amount of other ions (esp. chlorine) present, and as one can deduce from the fact that sacrificial anodes work, it will be destroyed when the zinc is connected electrically to a chemical nobler metal.
Don't know about Zincalum.

by josch222 on Tue Jan 26th, 2010 at 04:37:44 AM EST
[ Parent ]

Re: rust (none / 0)

Maybe Cl- ions will have the same effect as for alumina, ie, hole piercing!

by Xavier in Paris on Tue Feb 9th, 2010 at 12:56:42 PM EST
[ Parent ]

There is (offshore wind powered) light at the end of the tunnel! | 30 comments (30 topical, 0 editorial, 0 hidden)

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