BeginnerTopic 23 of 100Materials & Chemical
Stainless steels
Chromium forms a passive oxide layer providing corrosion resistance.
In depth
Stainless steels contain ≥10.5% Cr which forms a passive Cr₂O₃ film. Types: ferritic (430), austenitic (304, 316: Ni, non-magnetic, excellent corrosion), martensitic (410, 420: hardenable), duplex (high strength and SCC resistance), precipitation hardening. Sensitisation causes weld decay.
Key points
- Passivation
- PREN=Cr+3.3Mo+16N
- Chloride pitting
Example316 stainless with Mo is used in marine and chemical environments.
What is Stainless steels?
Chromium forms a passive oxide layer providing corrosion resistance.
Why is it important to understand stainless steels?
Stainless steels resist corrosion through a thin passive chromium oxide layer, so they are the default choice for hygienic and corrosive settings.
Why does stainless steel need chromium?
At about 11 percent or more chromium, a self-healing oxide film forms on the surface and protects the steel from corrosion.
Can stainless steel still corrode?
Yes. Chloride ions, crevices and high temperatures can break the passive layer, causing pitting or crevice corrosion.
How is Stainless steels used in practice?
Typical applications include making food processing equipment from austenitic stainless steel, choosing a grade for a chemical tank that holds acid and specifying stainless fasteners for a marine structure.
What should I learn before Stainless steels?
Study "Cast irons" first (topic 22 of 100 in Materials & Chemical). It covers the groundwork this topic builds on.
What are the key points of Stainless steels?
Passivation. PREN=Cr+3.3Mo+16N. Chloride pitting.
Can you give an example of Stainless steels?
316 stainless with Mo is used in marine and chemical environments.
What comes after Stainless steels?
The next topic is "Aluminium alloys", which builds directly on this one.
How difficult is Stainless steels?
It is classed as Beginner in Materials & Chemical (topic 23 of 100). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.