You will have a large volume stress corrosion cracking test!
The passive film of stainless can't take that level of chloride.
Can anyone, like EdStainless, tell us whether even AL6XN or 29-4C could tolerate this stuff at any temperature above ambient?

Mcguire;
Took your advice and purchased Sedrick's book "Corrosion of Stainless Steels". Per this reference, for boiling 42% MgCL2, the following materials had passed (no cracks);

Type 439
Type 444
E-Brite
29-4
29-4C

AL6XN - failed

metengr
  That's a good way to research this question. But while these won't crack by SCC in this medium, they will pit like crazy.

Well, this looks fun.
Do you know off hand what the pH of this solution is?
First, this is a lot harder than it looks.  If this is being transported there are all sorts of transport rules about containers.  don't get me started in the rail transport tules.  You must use designs and materials that they have approved.

1. You need a material in which the as welded fabrication welds will have a critical pitting temperature well above any anticipated ambiant temp.  Unless of course these vessles are small enough to be post-fabrication annealed.
2. An alloy with sigificant CSCC resistance will be needed.

The welding makes using a ferritic problematic.
439 and 444 don't have enough pitting resistance.
2205 does, but the welds would be questionable.
AL-6XN would work.  You could use alloy 59 weld filler and you should be OK.  You are well below the SCC temperatures.
A superduplex would also be an option.
I would have to look in order to see if Ti is an option.  Even if it would work right now Ti prices are high and lead times are very long (a year).

I would look seriously at lined carbon steel options.  These are going to need some serious QA, and they will need to be washed after each use and inpsected.  

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Corrosion never sleeps, but it can be managed.
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