Stainless steel is a corrosion resistant steel alloy with a minimum of 10.5% chromium. There are different grades and surface finishes of stainless steel produced to accommodate the environment in which the material will be implemented. Unlike carbon steel, stainless steel does not rust when exposed to air and moisture due to the sufficient amount of chromium present. The chromium creates an invisible passive film of chromium oxide that will not let oxygen attack the surface and prevents rusting of the iron base.   

Stainless steels can generally be categorized into 7 categories:

 Ferritic Stainless Steels

Chromium is their major alloying element

Ductility and formability less than austenitic grades

Magnetic

Highly corrosion resistant, less durable than austenitic

Cannot be hardened by heat treatment

Contain between 10.5%-27% chromium and very little nickel

Common grades: part of the 400 series like 409, 410S, 430

 Austenitic Stainless Steels

Comprise of 70% of total stainless steel production

Hardenable by cold working

Usually non-magnetic

High hardness and yield strength

Contain a maximum of 0.015% carbon, minimum of 16% chromium and sufficient nickel and/or manganese

Superaustenitic stainless steels (AL-6XN and 254 SMO)  have great resistance to chloride pitting and crevice corrosion

Common grades: 300 series like 304, 316, 320, 321, 347,309

 Martensitic Stainless Steels

 Highly machinable

Very strong and tough

Can be hardened by heat treatment

Magnetic

Contains chromium (12-14%), molybdenum (0.2-1%), nickel, (0 - < 2%), carbon (0.1 - < 1%)

Common grades: 410, 420, 440

 Duplex Stainless Steels

often divided into 3 sub classes

Lean Duplex

Standard Duplex

SuperDuplex

Mixed microstructure of austenite and ferrite about a 50/50 mix

Higher strength and better stress corrosion cracking resistance than most austenitic alloys

Greater toughness than ferritic alloys, especially at low temperatures

Good resistance to localized corrosion, particularly pitting, crevice corrosion and stress corrosion cracking

Common grades: 2205 and 2507

 Precipitation-Hardening Martensitic Stainless Steels        

Designed to be formable in the solution annealed condition

Can be hardened by heat treatment

Contain chromium and nickel as major alloying elements

Corrosion resistance is usually better than that of straight chromium ferritics

Most common grade is 17-4PH,17-7PH,

 Superaustenitic Stainless Steels

Same structure as austenitic alloys

Enhanced levels of elements like chromium, nickel, molybdenum, copper and nitrogen

Superior strength and corrosion resistance

Common Grades: AL-6XN and 254 SMO

 Superferritic

Structure and properties similar to ferritic alloys

Enhanced levels of chromium and molybdenum

Increased resistance to high temperatures and corrosive environments like seawater

19-9DL is non-magnetic stainless steel and to be widely used for aerospace industry, automotive industry,construction industry