Posted: Jun 24, 2011 10:55 am
Funny you should ask this. I've been working a lot with piling the last few weeks. And we were talking about this at work, specifically with regards to a cement poo-water tank at a local waste water treatment plant.
None of us are metallurgists here at work, but our hypothesis is that it's the free oxygen which causes the oxidation and you don't get a lot of that when metal is completely submerged or buried.
The i-beam shapes are actually h-piles, made of HP shapes (different than the traditional W, WF, S, or M shapes that are traditionally associated with i-beams). These HP shapes better approximate squares. The pipe piles are pipe shapes (with the standard nominal size = O.D., rather than a round tube shape generally has a larger O.D. than the nominal shape that describes it).
We're using a combination of HP and pipes on the dock facility I'm doing just north of New Orleans. Most of the river piling is pipe and a special grade of weathering steel (ASTM A606) that forms a sacrificial rust coating so as to slow or prevent complete corrosion over time. Typically though, it's a regular carbon steel, when the pipe or HP is completely buried or submerged.
Cathodic protection is common on ship hulls. When I used to go on drydock inspections, they'd have little blocks of zinc that would be strapped to the ship hull every so many feet. Many of these would be completely dissolved after the typical 5 yrs of service between drydockings.
Also though, on some bridges we're doing up here, we're using square concrete piling (14"x14"x35' long) under the bridge abutments and bent foundations. The contractor has asked us to approve a design for steel piling to use in place of the concrete because its easier/quicker for him to install (although the per foot cost is more expensive). It's also a fuck of a lot easier to field splice in case he fucks it up.