Corrosion Resistance of Cold Worked S.S.304

[givonz]

I know that when Non-magentic Austenitc Stainless Steel 304 is cold worked it transforms into a magnetic Martensite phase. But I want to know if this with effect the corrosion resistance of the Stainless Steel.

Any help will be appreciated.

givonz

 

[Kenneth]

See thread330-35718...

 

[mcguire]

Givonz
The corrosion resistance of stainless steel is dependent almost only on the chemistry of the exposed surface. Transformation does not alter that. Cold work can abrade the surface and change corrosion reistance, usually negatively. It can result in foreign materials to be embedded which is also hurt resistance.
But merely transforming without diffusion as with a martensitic transformation doesn't do a thing.

 

[rustbuster1]

It depends on the environment. The hardened martensitic zone is more susceptible to hydrogen cracking in acidic exposure.

 

[mcguire]

Rustbuster1 is quite correct about the hydrogen embrittlement risk for the martensite phase. So, if there were corrosion pitting, you could get stress corrosion cracking under conditions where it would not exist for fully austenitic 304.

 

[itdiy]

Does pittng could cause SCC in fully austenitic? how much stress(yield strength?) is needed for SCC in this case?

 

[mcguire]

Fully annealed austenitic stainless with yield strength as low as 35,000psi can fail by SCC. It can happen at temperatures as low as 30C in slight chloride environments, although 50-60C and higher chloride is more likely.
Pitting seems to be a necessary pre-condition for SCC. There has been endless debate over whether the mechanism is micro corrosion or hydrogen embrittlement. If austenite is more stable, SCC is less likely. If you can choose the alloy to avoid pitting you have done all you can unless you switch to the less SCC-prone duplex stainlesses.

 

[pickuw]

Dear givnoz
There are two factors governing the corrosion in SS:
1. Environment
2. Chemical composition.
If marteniste is present somehow in Austenitis SS, the corrosion resistance has to decrease in this area. Also this phase is brittle. As long as the protective oxide layer is in tact and at low pressures no effect takes place. If for any reason the protective oxide layer is removed from the martensitic region then corrosion can be rapid. Also if pressures are high the unequal elasticities of the two phase can cause break in the oxide layer and cause corrosion. ssp
pic, kuw.