Galvanized vs painted Carbon Life cycle

[ironmon]

I am a structural engineer designed a replacement steel members for a corroded (painted carbon steel) platform, outside of the spray chamber in the casting area of a steel mill.

The steam created from the spray chamber gets trapped under this platform, the decking is diamond plate, which causes the rising vapor to get trapped and condensate on the steel.

Generally the horizontal surfaces, bottom flange of the I beams gets the worst attack, also webs of the beams in areas where the framing creates corners have substantial loss.

The client wants a piece by piece replacement approach.

I am proposing replacing the I beam joists with Galvanized Tee's
1. eliminate bottom horizontal surface
2. thicker webs (generally 2x thicker)

here are my guestions.....

Will the addition of these galvanized members help the surronding corroded members?
Or will the surronding corroded member severly shorten the life of the new galvanized members?

What is the expected life cycle of galvanized steel compared to plain steel? .....in a test tube

The worst corroded member (total loss thru a w14x22 web)
is this the corrosion rate .023 in / 14 years = 0.016 in/year?

Would a member twice as thick last twice as long? Or in other words is corrosion rate of carbon steel linear?

Thanks

 

[ash9144]

Hot dip galvanized will hold up better than paint for structural steel. The thicker webbing will hold up longer. The rate is going to depend on the chemicals involved and type of corrosion. If the first blast of steam removes the galvanizing then it will corrode just as fast as carbon. Most likely the galvanizing will last several years before significant corrosion begins.

 

[ironmon]

Assuming the environment and the shape is the same and painted carbon steel corroded at a rate of 0.23 in/14 years.

How long would galvanized steel be expected to last?

Is there a ballpark answer to this?

 

[strider6]

you can find corrosion rate and a lot of tech info on galvanized carbon steel at this address:

http://www.galvanizeit.org/

there is also a life cycle cost calculator of different option for corrosion control like painting or galvanized cs.
i think it could give you the right answers.

regarding the other question the corrosion rate is usually not linear in atmospheric exposure, it tend to decrease and reach a plateau after sometime that depends on the aggressiveness of the atmosphere. but this is a general rule.

S.

http://www.corrosionist.com

 

[gr2vessels]

The hot dip galvanised surfaces are regularly painted also with an organic paint to extend the life of the galvanised members. Your application seems to be suitable for additional coating.
cheers,
gr2vessels

 

[ironmon]

Thanks for the info.
I'm learning of the fly, and before I read the last post I asked our field guy for Ph measurments of the water and was reading Strider6's link to the AGA.

And looking at the painted HDG info.

The link was very helpful, thanks Strider.

 

[moltenmetal]

Is there a particular reason you wouldn't use grating instead of diamond plate in this case? Wouldn't trap the steam, and is available hot dip galvanized...