Dear,
I'm trying to see if there is a rough (even very rough) statistical link with stainless steels chemical composition + microstructure + ... and the pitting potential.
It would be interesting to get a better indicator than the PREN to evaluate the corrosion resistance of stainless steels.
The main drawback of the PREN is that it does not take in account the Carbon amount (and if we want to get further, neither the Sulfure or Phosphore, that are very important inpurities). So that is only usable to roughly compare Stainless Steels with the same amount of Carbon.
I had a look at some results from saline fog tests I made. As a first approach I tried to divide it by log(%C² + 1), it gives a better idea but not at the point that there would be some rough proportionality with the saline fog results I know. It is clearly lacking a consideration for microstructure for example.
One other problem is that salin fogs results are not so stable. Using one of the normalized pitting potential tests would be better.
Is there some currently established relations or attempts that could prove to be usefull at the first steps of material selection ? The corrosion is too complex to have more than a very rough law, however that could still have some uses.
If not, I would still be interested in a datasheet with the measurement of different stainless steels pitting potential in some standardized conditions to see how it is distributed.
Regards
I think, most of the research on pitting of stainless steels aim at developing a correlation between composition, microstructure, and pitting resistance. However, as mentioned, the corrosion process is very complex because the pitting is a stochastic process and influenced by pitting inducing anions (mostly Cl-, but also SO42-), pH, temperature, dissolved oxygen, concentration of other oxidizers and time. Pitting depends on the stability of the surface passive layer. The passive film stability is affected by small perturbations in the service conditions including stress level. With these many parameters affecting the process it is really difficult to propose a meaningful mathematical function to describe a pitting potential.
For materials selection, PREN is a good indicator. There are studies to correlate the pitting resistance using artificial intelligence, for example: Journal of Applied Logic 10 (2012) 291–297.
I think, most of the research on pitting of stainless steels aim at developing a correlation between composition, microstructure, and pitting resistance. However, as mentioned, the corrosion process is very complex because the pitting is a stochastic process and influenced by pitting inducing anions (mostly Cl-, but also SO42-), pH, temperature, dissolved oxygen, concentration of other oxidizers and time. Pitting depends on the stability of the surface passive layer. The passive film stability is affected by small perturbations in the service conditions including stress level. With these many parameters affecting the process it is really difficult to propose a meaningful mathematical function to describe a pitting potential.
For materials selection, PREN is a good indicator. There are studies to correlate the pitting resistance using artificial intelligence, for example: Journal of Applied Logic 10 (2012) 291–297.
Dear Dr. Thibaut Deveraux,
I would like to emphasize that, beyond the parameters that you have correctly mentioned, surface finishing plays a pivotal role in determining both aesthetics and intrinsic localized corrosion resistance of artefacts as well as finished steel products. Generally speaking, the higher the steel surface polishing – namely the lower its surface roughness – the greater its corrosion resistance. There are many finishing techniques for stainless steel depending on the technological cycle steel has undergone. European Regulations – EN 10088 part 2 and 3 – and US Regulations – ASTM A-380 and ASTM A-967– provide a comprehensive list of standard surface finishings for stainless steel, although the same type of finish may take different names. Surface finishings seem to be particularly significant for austenitic steels (e.g. AISI 304, AISI 316L, etc.) whose resulting finish is deemed even more important than their chemical composition.
Good work and my best regards, Pierluigi Traverso
Dear Pierluigi Traverso
Yes of course, I did not mention it but the tests shall be conducted in the exact same conditions regarding the parts shape, surface and the environnement.
Such corrosion results can only be used as indicative comparaison between materials and are not meant to give the exact corrosion resistance of a part. This vary extremly fast with the above parameters. I had stopped most Duplex parts at 6000 H of ISO 9227 saline fog with no corrosion but I have also seen corrosion à 350 H (!!!) on parts with bad design.
Regards
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