Stainless steel and corrosion Flashcards
How does metals corrode?
Metals corrode through a process known as electrochemical corrosion, which involves chemical reactions between the metal, an electrolyte (usually water or moisture), and oxygen from the air.
What is stainless steel?
Stainless steel = rostfritt stål
A type of steel that is very resistant to corrosion.
Contains more than 13 % of Cr since iron will form corrosion.
Very low carbon content 0.05% to avoid selective corrosion in austenitic and ferritic steels.
The outermost layer of the material are quite prone (benägen) to react with oxygen to form an oxide.
When Cr2O3 forms it becomes a stable layer that protects the underneath material form corrosion attack
What is important to think of when making and using stainless steel?
- Since Cr is also a carbide former, the C content of these steels must be very low to guarantee corrosion resistance.
- Once the Cr is consumed from the bulk material below 13wt% the material is no more self protective
When can stainless steel corrode either way?
- If production goes wrong
- If cooling down goes wrong
- If Cr for some reason reacts with C and forms Chromium Carbide
Which are the “real stainless steel grades”?
- Austenitic SS
Most known stainless steels, and can be more or less stable depending on the nickel content. To stabilize you have to increase the nickel and chromium content - Martensitic SS
These steel grades must have certain mechanical properties to be practically useful: such properties are provided by the carbon content of the martensite (matrix) and by precipitation hardening through Cr carbide precipitation. - Ferritic SS
Good with varios temperatures. Poor mechanical properties but potentially very high temperature corrosion resistance - Dual Phase SS
Which is the basic steel of Austenitic SS?
AISI 304 and it has two verisons:
- X5CrNi18-10; AISI 304
- X2CrNi18-11; AISI 304L: has low carbon content means less interstitial solid solution
Nowadays the carbon content can be even lower than 0.02wt%
Has a good weldability, even though the welding may locally increase the austenitic grain size, the material is still ductile.
What is PREN?
It’s a measurement of the resistance to localized corrosion. The higher is the PREN the higher is the corrosion resistance
PREN = %C + 3,3%Mo + 16% N
PRI = chromium + Mo
PREN added nickel to PRI
In what form does the Cr needs to be in in the SS?
Cr in stainless steels must remain in solid solution. in such a way, the oxide layer can be formed and corrosion resistance remains guaranteed.
How can we stabilize austenite?
Unstable austenite due to phase transformations → increase the nickel and chromium content to stabalize.
Tell me some important features about AISI 304
AISI 304 has good weldability, in fact, even though the welding may locally increase the austenitic grain size, the material is still ductile.
If we instead have AISI 304L:
L stands for LOW CARBON - due to the work against corrosion.
Describe the mechanical properties of the Austenitic SS
- Ductility can be partially restored using a recrystallization annealing heat treatment.
- Such heat treatment must be performed avoiding the following temperature range during heating and cooling steps: the sensitization range for austenitic SS falls between 450 and 850 °C). Forbidden range since chromium carbides will form at grain boundaries.
Senzitisation = Intergranular Corrosion
Tell me about Martensitic SS
- High carbon content -> low corrosion resistance but increased hardness
- Cr is low
- Dont use in aggressive environments
- Important to temper in right temperature, since it can lead to embrittlement. <200dC. 500-550dC for balance between hardness and toughness.
Tell me about Ferritic SS
- Cr never lower than 12%, but can vary
- One of the principal ferritic SS is the AISI 430 (0.08% C and 17% Cr) it has a
good corrosion resistance and a very high ductility.
- Hot formed
- Cold worked
- Recrystallization annealing
