Originally posted by FordMonkey:
Some have higher nickel content which should make it more resistant.


It is not Nickel but Chromium that makes stainless steel corrosion resistant. The Chromium Oxide layer that forms on the surfaces is continuous, adherent and insoluble. Aluminized steel also resists corrosion becaise an aluminum oxide layer forms that stabilizes the surface.

Iron or mild steel forms various iron oxides on the surface. None are adherent so they fall off exposing new surface which rapidly corrodes. The addition of Vanadium, Tungsten or Molybdenum to various steels also results in the formation of a corrosion resistant surface layer. These are not as resistant to corrosion as chromium oxide, however. Nickel improves the mechanical properties of steel, but the adherence and continuity of Nickel Oxide is marginal and the resultant corrosion resistance is limited.

There are three things to watch out for with chrome based 3xx and 4xx series stainless steels:

1. Depletion of the chromium can occur in welds reducing corrosion resistance. How many Ford (and other manufacturers) stainless steel exhausts have we seen fail in the welds? Poor welds can be brittle and crack.

2. Stainless steel is not completely resistant to chloride (salt) induced corrosion. Stainless will pit over time. However, good stainless will not rust through in 10-20 years (or longer) unless exposed to severe conditions.

3. Polishing a tarnished stainless surface will remove chrome oxide and increase the rate of material loss. You should wash stainless steel surfaces when chloride, sulfate, etc. are present, but polishing (other than for cosmetic reasons) does not help.

Hope this helps...