Stainless Steel & Corrosion
Stainless steel is a widely used expression. Stainless steel is an alloy of iron, chromium and nickel. This may generally be understood to mean that the metal does not rust. It would be more accurate to say that it is ‘stainless in the right environment’. ‘Stain-less’ is not 'completely stain-free’ in any environment.
The choice of stainless steel grade should be made based upon a combination of factors:
- Required mechanical properties
- Processing, manufacture and fabrication
- Welding processes
- Surface finish
- Corrosion performance optimised for the environmental conditions.
Mechanism of corrosion.
Stainless steel is ‘stainless’ because it has a coherent, stable, passive surface layer which remains corrosion free in a general environment of air and moisture. This layer is ‘rust free’ until the passive film is disrupted.
The passive layer is generated by the addition of Chromium(Cr) and Nickel (Ni) in steel.
In type 304 stainless this is produced by the % of Chromium (Cr) and Nickel (Ni) in steel. This develops an austenitic structure in the final alloy. Austenite is non magnetic.
In type 316 stainless steels the relative % of Nickel (Ni) is higher and a % of Molybdenum (Mo) is added. The structure is austenitic. The higher Ni content and the presence of Mo makes the final alloy more resistant in the presence of Chloride (Cl) ions (eg. marine environments).
Ferritic stainless (eg 3Cr12) have lower alloy requirements, approx. 12%Cr but no Ni. The alloy has a ferritic structure and is magnetic.
Duplex type stainless has a much higher Cr content, a reduced Ni content, a % Mo and other alloying elements. Produces a duplex structure of both Austenite and Ferrite.
Stainless Steel Grade Corrosion Performance & Uses
Grade 304 – general purpose stainless steel with good corrosion resistance for most applications. Uses – architecture, general and process engineering, food and domestic applications.
Grade 316 – used where a higher corrosion resistance is required eg. marine environments. Uses – architecture, general and process engineering, food and domestic applications.
3Cr12 – used in mildly corrosive environments where a better life cycle costing can be achieved versus carbon or galvanised steel. Uses - engineering and vehicle bodies.
2205 – Duplex - has superior corrosion resistance by comparison to 316 with enhanced resistance to stress corrosion cracking. Uses – swimming pools and process engineering.