Could I settile for galvanized steel over stainless for salt corrosion resistance?

Question

Bear with me, I'll get to the chemistry soon.

I have a float tank that I want to attach a skimmer to. Unfortunately, the enclosure is weakened by cutting a hole out of the ABS plastic. The weight of the salt water (1.25SG) to somehow causes a bend in the plastic just enough that getting a good seal isn't possible without additional support.

The float tank consists primary of (pardon the units) 800lbs of $\ce{MgSO4}$ Heptahydrate raised to about 96 degrees Fahrenheit. The pH is roughly 7.2.

My concern with supporting the ABS plastic is that the Epsom salt water may corrode the metal, if it somehow isn't protected by a liner. Would I absolutely need to have stainless steel, or could I get by with galvanized (which is much cheaper).

In case it matters, I estimate the volume of water directly pressing on the skimmer opening to be about 9+ gallons (again at 1.25sg).

Thank you for answering non-chemist, Ben

Answer 1

I'm a chemist, but this seems more like a question for an engineer. Unfortunately I don't have some references to back up my answer.

If you use galvanized steel you have a protective layer on the outside of the steel. As soon as you handle it for attaching, welding... you destroy/scratch/bruise this top layer and the underlying steel gets in contact with the salt solution which is normally aggressive towards metal. This mostly is right for e.g. chloride salts in a mix with oxygen and I'm not quite sure how corrosive MgSO4 is. Here is a paper about corrosive currents of steel in Saltwater/cement solutions, but that`s not my speciality: http://www.jmst.org/fileup/PDF/pass543.pdf

Stainless steel is more robust, because it's an alloy of steel and chromium. Therefor it's not just covered with a protective layer but saver as a whole. An I guess it's therefor recommended.

(As a side note, some years ago I had a field trip to a big chemistry company with lots and lots of pipelines. We had some time there with the technical stuff in charge of teh pipelines. They explained, that the most annoying pipelines were the ones with salt water, since it was more aggressive towards the pipes than acids, mineral oils, gases etc.)

Answer 2

That’s true about the welded joint for two galvanized objects. The welded area needs to be galvanized after welding to provide the protective layer (anode). Depending on the solution in contact with the galvanized and time and temp you have varying degrees of protection. Galvanized pipes etc are ok in general weather but salt water for instance will consume the zinc or magnesium rather quickly and once that’s breached the chlorides will have a field day with the metal underneath.

Answer 3

I'm only a chemist by desire and curiosity but as a Practical Maintenance Engineer let me throw this out there.

Galvanized steel is a really bad idea. Just about any commercial sheet metal is going to breakdown relatively quickly in your environment. Brass is the best of a bad list but, it too, will ultimately corrode.

Find someone with a plastic welder. Harbor Freight carries one the would work for you.... Repair and Reinforce container with another piece of ABS. You could use a number of different plastic pieces of a number of different plastic materials to build a frame around your tub.

Or

Owens Corning sells buckets, tubes, bottles, and boxes of (in Ultra Hi-Tech terms) pookey* stuff and substrates to repair ABS tanks for consumer use (think Koi ponds / waterscapes). See what is available and check the SDS of products that look workable.

*Pookie, in this context; any repair product in any form that will stick well enough when cured to contain without seepage the chemicals that need to be contained, under the circumstances of the intended use without altering said chemicals.

Good luck!