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It appears they are using a more reactive metal, such as zinc, electrically attached to the pipe. It would "soak up" excess electrons generated by stray currents in the structure. Or is that just my hyperactive imagination, Jerome?

"It is not necessary to have hope in order to persevere."
by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Sun Jan 24th, 2010 at 05:13:26 PM EST
[ Parent ]
no, that's exactly it.

In the long run, we're all dead. John Maynard Keynes
by Jerome a Paris (etg@eurotrib.com) on Sun Jan 24th, 2010 at 05:18:04 PM EST
[ Parent ]
I'm back for a few days, and cannot resist a temptation such as a scientific question/answer.

For more details, the corrosion process in work here is galvanic corrosion. All metals can be put on a scale of corrosion-readiness, with gold on top. When two metallic parts are electrically connected (as when both of them are inmersed in water), one of them will concentrate all the corrosion, while the other will remain protected.

The corrosion process itself consists in electrons flowing from the corroded part to the protected part, leaving ionized atoms getting away from the metal.

In marine applications, Zinc is used as easy-to-corrode metal, because it protects all metals usually employed in metallic contructions (steel alloys included).

In the image below, Copper is more corrosion-resistant than steel, so steel is corroded. But Steel is mechanically more resistant than copper, so the structure may weaken. The solution is to use sacrificail Zinc connection between both metals. It will protect both pipes, but will require periodic maintenance (easy: to replace the sacrificial Zinc part).
schematical representation of galvanic corrosion process

an anodic Zinc sacrificial part

by Xavier in Paris on Mon Jan 25th, 2010 at 03:51:39 AM EST
[ Parent ]
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 ]
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 ]
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 ]
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 ]
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 ]

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