it is just a steel tube of 5 mtr wide, and will be partly penetrated in the seabed, the rest is always under water. and it's not just steel but some special steel, they will have to last for at least 25-30 years. But after the installation of the monopile a transition piece (the one where the actual tower will be bolted onto, and with a platform for acces, and boatlandings with ladders etc...) will be grouted onto the monopile, and that one is well-painted, because that's the one that can rust because of seawater and oxygen. So all of the monopile is covered by mud, water or grout...