I would avoid stainless if salting is common. It is highly likely to pit in the crevice areas and will be quite expensive relative to other options.
Aluminum may not be much better in some cases. I've seen 10-15 yr. old corroded aluminum sliding door sills mounted on concrete foundations, and that is in "mild" California weather so your situation would probably be worse unless you search out and use one of the more corrosion resistant grades. ASM Metals Handbook indicates that aluminum in contact with fresh concrete may undergo some superficial etching from the high pH, but that the long term effect is oftem minimal. It does also say that aluminum components (such as highway poles and railings)in contact with concrete in crevice areas where salts may accumulate are often coated witha sealing compound to minimize salt getting into the crevices. It further says that aluminum alloys of 2XXX series and 7xxx series are less resistant to chloride pitting than alloys of the 3xxx, 5xxx and 6xxx series, with the 6xxx being the least preferred of the three choices. Typical seawater penetration rates are reportedly less than 0.001" in 10 years for the 3xxx and 5xxx alloys, but it is hard to relate sea water corrosion rates to what might happen if salt solutions get into your crevices and start to concentrate through evaporation.
Sounds like someone needs to invent and distribute some type of nonmetallic flashing if you want relaible 100 yr protection.
