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>

Joerg.