Stainless Steel Grid Deck on Galvanized Stringers

[bridgebuster]

Somehow I've gotten sucked into a construction problem:

A walkway over a salt water river. The deck is 1-3/4" thick stainless steel open grating supported on galvanized stringers. The question of galvanic corrosion came up after the project was bid (the project was bid based on 50% plans; details are worked out as the job progresses).

All of the literature I read suggests this is a poor combination of materials. It's too late to order galvanized decking. One solution is to paint the top of the the stringers to create a barrier. However, the contractor's proposed cost is astronomical (even by Federal bailout standards). My concern is that the paint will deteriorate over time and there will be no way to reapply a coating without dismantling the deck, railings, and light poles.

One alternative was to install neoprene sheeting in lieu of painting but the architect said no way.

My question: Is it possible to install some type of cathodic protection system? Is the concern about corrosion overblown? Should we just put the stainless in contact with the galvanizing?

Thanks for your opinions.

 

[gr2vessels]

For your application, the concern is a bit exagerated. Obviously, in time you will have some degree of corrosion, but most likely will be the loss of galvanizing, rather than attack on Stainless Steel, The salt ladden atmosphere won't be a salt bath, you'll see shiny days and then the galvanic corrosion will stop (you need the brine for conductivity and without water...no corrosion). However, bear in mind that once the zinc is gone, there is nothing to protect the carbon steel structure, with the corrosion made worst by the presence of the stainless steel as a cathode. Out of curiosity, do you intend to use galvanised fasteners? because if you do, they'll be the first to go corroded, very quick. However, the stainless steel bolts will do similar, but localised damage. Use galvanised bolts and insulating collars under the head and nuts, that might help.
Cheers,
gr2vessels

 

[TVP]

As gr2 mentioned, it is the specifics of the joint that will determine the degree of corrosion. If you can use some type of insulator between the galvanized steel stringers and the stainless steel grating, that will significantly reduce the corrosion potential. Can you post some sketches, drawings, etc. that would further clarify the proposed joint geometry?

Separate from the design details, rinsing the walkway periodically with tap water may be helpful to keep any salt from accumulating in the crevices, etc. Some times a small amount of maintenance like this can improve an otherwise undesirable situation.

 

[corrosionman]

Bridgebuster Why not just rubber pads to separate the SS and Galvanised materials and plastic spacers under the bolts. So easy at construction stage but so difficult afterwards
CM

 

[bridgebuster]

Thanks for the responses. I attached a detail (see Page 2 of the file).

I got sucked into this when someone in the office said "...we got this with bridge with metal decking and the contractor want to know how to put it in over the splice plates, any ideas?" The plans showed a typical splice for a stringer - top and bottom plate on each flange. Anyway, we changed the top splice plate to a longer thicker plate with countersunk holes, that's when I open big mouth about dissimilar metals.

We proposed sheet neoprene - the landscape architect didn't want it - "unsightly!" We suggested painiting the top flange. I'm not crazy about the idea or painting and galvanizing. Do you think it would be worth it to consult a metallurgist to determine a corrosion rate?

 

[EdStainless]

If he doesn't like rubber try a thermo plastic. In a nice off-white color it should not be too visible. His option is a lot of rust streaking.
How much does it rain in this location? A lot of rain would help.
You also need to make sure that the joints can drain. I know that people tend to design stuff square and flat, but any trapped water will be a problem.
isolation and drainage should give you a long service life.

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Plymouth Tube

 

[robsalv]

Bridgebuster, would you expect the walkway to be relatively dry? What are the expect local atmospheric conditions? Is this walkway out in the open, exposed to prevailing salt laden winds??

If corrosion is a concern, wouldn't the best cost option be to go fully galvanised? That'd remove the dissimilar metal problem.

I'm also a bit uncomfortable about the stainless in a salt environment. Any high residual stress areas (welding, cold working etc) could become susceptible to cracking if chlorides concentrate up.

The talk of crevices and saltwater doesn't bode well either - even for a galvanised design.

Any rate, eventually the stringers will corrode in isolated locations and that will be unsightly. If the stainless' Chromium oxide layer stays intact, then it will drive CS corrosion if an electrolyte exists.

I think the CP/IP system you'd need would be fairly expensive and complicated (and unsighly) to ensure decent cover.


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"Life! No one get's out of it alive."
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[bridgebuster]

robsalv - the walkway will be out in the open; the deck will be about 2 feet above high water. It's too late to go all galvanized; SS deck has been fabricated.

 

[EdStainless]

But it will be open above to rain, correct?
If so then use plastic spacer blocks and make sure that it can drain. NO TRAPPED WATER is the key.

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Plymouth Tube

 

[bridgebuster]

yes open to the rain