having a quick scout on google - its an alloy with about 10% chromium in and sometimes nickel too.
400 Series Martensitic - Typical grade: 410
Straight chromium (12-18%), magnetic and can be hardened by heat treatment. Typical use: Fasteners, pump shafts
400 Series Ferritic - Typical grade: 430
Straight chromium (12-18%), "low" carbon, magnetic, but not heat treatable. Typical use: Appliance trim, cooking utensils
200/300 Series Austenitic - Typical grade: 304
Chromium (17-25%)/Nickel (8-25%), non-magnetic, not heat treatable. Can develop high strength by cold work. Additions of molybdenum (up to 7%) can increase the corrosion resistance. Typical use: Food equipment, chemical equipment, architectural applications
Precipitation Hardening - Typical grade: 17-4
Chromium (12-28%)/Nickel (3-9%), martensitic or austenitic. Develop strength by precipitation hardening reaction during heat treatment. Typical use: Valves, gears, petro-chemical equipment
Duplex - Typical grade: 2205
Chromium (18-25%)/Nickel (4-7%) and up to 4% molybdenum. More resistant to stress corrosion cracking than austenitic, yet tougher than fully ferritic alloys. Typical use: Pipelines, pressure vessels, shafting
(cut and paste from the very interesting 'stainless steel information center')
it seems its the chromium that forms an oxide on the surface that prevents corrosion - and presumably prevents preferential corrosion with aluminium krabs since they are not actually in contact.
So there you go (I must get out more
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