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how is corrosion controlled with all that steel in seawater?
by njh on Sun Jan 24th, 2010 at 04:28:18 PM EST
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.


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 ]
same as ship hulls - with chunks of lesser metals covering the surface of the steel (Im hopeless, I never know if it's anodic or cathodic protection, maybe the chemists on board can help).

In the long run, we're all dead. John Maynard Keynes
by Jerome a Paris (etg@eurotrib.com) on Sun Jan 24th, 2010 at 05:17:30 PM EST
[ Parent ]
It is called "sacrificial anode":
<a href="http://en.wikipedia.org/wiki/Sacrificial_anode">http://en.wikipedia.org/wiki/Sacrificial_anode</a>
It uses the destructive effect of electrochemical corrosion to protect the steel.

The german article
<a href="http://de.wikipedia.org/wiki/Opferanode">http://de.wikipedia.org/wiki/Opferanode</a>
mentions a way to use cheap material if an electrical current is forced between the steel and the sacrified material. Zinc is more expensive than steel and electricity should not be a problem, so this variant may be used in offshore windpower. But I don't know for shure.

The picture with the tripod looks like the turbine at Hooksiel (a single near-shore installation for testing and exercising). In 2008 I was on vacation there and saw some preparations for the erection going on.
<a href="http://www.bard-offshore.de/en/projects/nearshore/hooksiel">http://www.bard-offshore.de/en/projects/nearshore/hooksiel</a>


by josch222 on Mon Jan 25th, 2010 at 03:53:16 AM EST
[ Parent ]
yes, it is the first prototype installed in water of the BARD 5MW turbine. I put up more pictures of their factories here: http://www.eurotrib.com/story/2009/3/12/114958/542

In the long run, we're all dead. John Maynard Keynes
by Jerome a Paris (etg@eurotrib.com) on Tue Jan 26th, 2010 at 03:33:06 PM EST
[ Parent ]
Right on both counts: it's cathodic protection through the use of a sacrificial anode.

One of the most common installations for my photovoltaic systems, back when I was a PV designer in the 1990s, was cathodic protection of oil pipelines.  (others included powering hazard warning lights on oil rigs, and microwave telecom relays for PDO Oman). Renewables helping out the fossil fuel industry.  Strange world, huh?

by LondonAnalytics (Andrew Smith) on Mon Jan 25th, 2010 at 09:34:49 AM EST
[ Parent ]
I don't think it is strange. I believe the fossil fuel industry is not so much ideological about renewables.
It is simply about maximizing profit. They really like it when (and are pushing for) their propaganda departments and politicians take an ideological approach to prolong the use of their existing infrastructure and monopolies in this field.
Especially as the stuff gets more and more expensive over time they try to squeeze as much as possible out of it. But in fact they are already heavily investing (and profiting) from renewables. They simply don't talk so much about it but of course they want to control the inevitable transition.
From my experience they use every technology available and have no reservation with green technology. I'm developing customer specific electronics and software for Li-battery management systems which are used in all kind of stuff that moves around: cars, buses, bikes, planes, inspection robots and AUVs. Although this is not exclusively "green" technology, it is somewhat related because of the hype of electric and hybrid cars and other projects (like Solar Impulse etc.). But of course the raw material industry uses the advantages of Li-Batteries for offshore prospection, pipeline inspection and the like.
I think they will use wind and solar energy to get the last drops of oil (and other resources) in a few decades, not for burning it up in cars or power plants but maybe for expensive drugs or otherwise difficult to produce plastics or other needed products.

I would use the old offshore oil and gas platforms to set up high voltage dc converters and the pipelines as protection pipes (or as the ground wire too) for the cables to the shore. Surrounding the platforms big offshore wind farms could be built then.
I'm pretty shure there are studies about that in the drawers of the fossil industry already.


by josch222 on Tue Jan 26th, 2010 at 06:14:42 AM EST
[ Parent ]
They really like it when (and are pushing for) their propaganda departments and politicians take an ideological approach to prolong the use of their existing infrastructure and monopolies in this field.

That's the key: they hate losing market share. However, the presently most widespread renewables -- on-shore wind, rooftop solar --, especially when paired with a feed-in law, are structurally predisposed to bring in many new small owners and thus break monopolies. The energy giants will (do) like off-shore wind much more (not to mention centralised solar power, hence the support for the IMO pie-in-the-sky Desertec). But, even when existing energy giants build renewables, they will be less enthusiastic about an accelerated replacement of their existing plants, though.

*Lunatic*, n.
One whose delusions are out of fashion.

by DoDo on Wed Jan 27th, 2010 at 10:03:44 AM EST
[ Parent ]


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