Corrosion resistance of Stainless Steel

Corrosion resistance

Chromium (Cr) content sets stainless steels apart from other steels.
The unique self-repairing 'passive' surface layer on the steel is due to the chromium.
Commercially available grades have around 11% chromium as a minimum. These can be either ferritic or martensitic, depending on carbon range control.

Increasing chromium enhances corrosion and oxidation resistance, so a 17% Cr 430 (1.4016) ferritic would be expected to be an improvement over the '410S' (1.4000) types.
Similarly martensitic 431 (1.4057) at 15% Cr can be expected to have better corrosion resistance than the 12% Cr 420 (1.4021 / 1.4028) types.
Chromium levels over 20% provide improved 'aqueous' corrosion resistance for the duplex and higher alloyed austenitics and also forms the basis of the good elevated temperature oxidation resistance of ferritic and austenitic heat resisting grades, such as the quite rare ferritic 446 (25% Cr) or the more widely used 25 % Cr, 20% nickel (Ni) austenitic 310 (1.4845) grade.

In addition to this basic 'rule', nickel (Ni) widens the scope of environments that stainless steels can 'handle'.
The 2% Ni addition to the 431 (1.4057) martensitic type improves corrosion resistance marginally but its main purpose is to improve the impact toughness of the steel.  Additions of between about 4.5% and 6.5% Ni are made in forming the duplex types. The austenitics have ranges from about 7% to over 20%.
The corrosion resistance is not simply related to nickel level however. It would be wrong to assume that a 304 (1.4301) with its 8% Ni therefore has better corrosion resistance that a 1.4462 duplex with only 5% Ni.

More specific alloy additions are also made with the specific aim of enhancing corrosion resistance.
These include molybdenum (Mo) and nitrogen (N) for pitting and crevice corrosion resistance. The 316 types are the main Mo bearing austenitics. Many of the currently available duplex grades contain additions of both Mo and N.

Copper is also used to enhance corrosion resistance in some 'common', but hazardous, environments such as 'intermediate' concentration ranges of sulphuric acid. Grades containing copper include the austenitic 904L (1.4539) type and some 25% Cr "superduplex" steels such as 1.4501 and 1.4507.