General Corrosion Resistance
Stainless steels are characterized by a very good aqueous corrosion resistance and by a very good resistance to oxidation at high temperatures. All stainless steels contain at least 11% Cr. Many contain nickel as well. For the aqueous corrosion resistance, the steels must contain a minimum of 11.5% chromium, which makes them passive in oxidizing solutions. Even more chromium is required for passivity in nonoxidizing solutions. Unless the chromium content is sufficient for passivity, the corrosion resistance of stainless steels is similar to steels without any chromium. Table 19.1 is a galvanic series of alloys. It shows that stainless steels may occupy two positions corresponding to the active and passive conditions.
There are five major types of stainless steels: ferritic, martensitic, austenitic, duplex, and precipitation hardenable.