Corrosion Protection for Submerged and Exposed Structures 1

[1337engie]

Hi everyone,

I've been looking at installing a steel set of steps that will be bolted to a concrete wall. These steps will be exposed or submerged depending on the tide.

With regard to the corrosion protection of the steps I've been receiving conflicting information from several sources. One group believe hot dipped galvanising will do the trick, the other group believe zinc primer with coats of two pack epoxy and two pack polyurathane is more suitable.

Wondering if there were any thoughts on this?

Cheers,
Tim.

 

[TVP]

Both coatings are used for this type of environment. I would probably lean toward the organic coatings over galvanizing as it would likely be more resistant to damage.

 

[SJones]

Or, forget the coatings and go for an appropriate alloy that won't need coating and all the maintenance difficulties that will go with it. Failing that, thermal spray aluminium finds some favour in marine applications.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[LittleInch]

If all it was doing was just sitting there you might get away with either, but if you have steps then this will be subject to kicks, scrapes, bending forces and simply will not last. Moreover, steps are used by people to climb up and down - Failure of a single step could easily lead to a fall, presumably onto something rather hard. The only thing that really works is the right material - super duplex or something like that. It will always look good and providing you don't contaminate the surface with some steel or use low grade bolts, will last a significant period longer.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[berkshire]

1337engie (Mechanical)
One thing you have to consider is traffic up and down these steps. The hot dipped galvanizing will work at first, but being a sacrificial coating will eventually erode away exposing the base metal. Foot traffic tends to erode a small area on each step which can be touched up by a maintenance crew using cold galvanizing paint.
The biggest problem with these types of structures, is that in most places they never get maintained.
Which brings you to your next question, how long do they have to last?
If that answer is more than a few years , look at Steve Jones's solution.
B.E.

You are judged not by what you know, but by what you can do.

 

[1337engie]

Hey guys,

thanks for the advice, really appreciate it.

 

[weldstan]

Have you considered FRP?

 

[1gibson]

Yeah, why metal? How about those composite "deck boards." Or heavy wire mesh, whole assembly coated in rubber/plastic (like park benches or playground equipment.)

 

[stevenal]

The salt water will get under any coating you apply, whether rubber, plastic, paint, epoxy or aluminum. The coating will then function to keep the water in during the low tides. HDG is not a simple coating though, it penetrates and mixes into the surface layers of the steel. Being metal, it will respond to physical wear better than most of the alternatives offered above. It is galvanically sacrificial, but even when the underlying steel is exposed the remaining zinc continues to protect. Just the same, you might want to consider bolting an anode to the steps. And when the steel is exposed, there is no entry point for the salt water. Don't be too quick to reject HDG.

 

[TomDOT]

I like the anode comment. May as well use a magnesium anode, to slow the zinc loss as well.