huangyhg

weld damage on stainless steel
i have a shallow, trench drain that was fabricated out of stainless steel. its intended purpose is to run across an entrance way to a parking garage. the pan is about 4 inches deep, and is embedded into the concrete and slopes to one end.
the drain is then covered with a galvanized bar grating that supports wheel loads from cars driving over it.
the problem i have is that we had specified some small headed anchors to tie the stainless steel drain into the concrete. these anchors were located along the bottom of the drain in pairs and now there are tell-tale burn marks from the welding of the headed studs to the bottom of the drain.
the owner is concerned about future rusting at these locations - we are in snow country where chlorides are used to melt the ice.
the burn marks are simply discolorations in the metal. there is no pitting or warping. looks almost like a bruise.
is this a concern? if so, what could be done to protect the "bruises" against future corrosion?

jae,
it depends on the alloy of stainless steel used. the "burn marks" are indicative of a microstructure change in the metallurgy of the stainless steel in the heat affected zone of the material. again, depending on the alloy used, this can lead to intergranular corrosion of the stainless steel. intergranular corrosion results from localized depletion of chromium from the welding process.
to counteract this potential to occur at weld locations, a low carbon content stainless steel should be specified. (use 316l instead of 316, for example).
another thing to be aware of is what the stud material is made of and what the weld rod used was. i believe all should have been stainless steel. while i agree with ron regarding the stainless steel pan, if the studs and weld rod were a36 (mild steel), they are very suseptable to rusting and that could then corrode the stainless steel pan at the stud locations.
also, i'm not sure of the compatability of the galvanized grating and the stainless steel. are they in contact with each other? why was stainless steel chosen in the first place over galvanized or cast iron trenches?
thanks for the replies:
ron: i don't know the exact grade of stainless - i'll check into that. since i'm in an "as-is" situation, i cannot go back and re-specify the steel, so is there anything to do about these spots (about the size of a quarter)?
jheidt2543: studs were stainless too. also, the entire trench is lapped over the top edge, and shelf with a traffic bearing waterproof membrane (neogard) that will separate the grating from the trench drain....no direct contact.
so what options do i have on these spots? can i cover them with something? leave them?
i wouldn't worry about stainless steel corroding in a parking garage application (even if it is grade 304 vs 316)....you and i will probably both be long dead and gone before you see anything worse than what you see now.
i would be more concerned about avoiding contact between dissimilar metals, which you appear to have addressed by separating them with a membrane.
jae...an asphaltic coating wouldn't hurt. gives a bit of extra protection for the spots. in the situation you've inherited, there isn't much you can do.
ron

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