Modifying the alloying composition makes steels corrosion-resistant by passivation -- generally. Does this necessarily change the corrosion mechanisms?
Interesting question. Generally, passivity is considered a separate mechanism from active (aka. general) corrosion. Usually, pitting and other forms of localized attack are considered the result of local breakdown of passivity, but different mechanisms of corrosion. During active corrosion, different carbon steels alloys usually exhibit remarkable similar corrosion rates. There is a figure comparing corrosion rate data for 8 different alloys buried underground at 47 different locations in the US published in Journal of Research NIST, vol 115 No. 5, p373-392.
Interesting question. Generally, passivity is considered a separate mechanism from active (aka. general) corrosion. Usually, pitting and other forms of localized attack are considered the result of local breakdown of passivity, but different mechanisms of corrosion. During active corrosion, different carbon steels alloys usually exhibit remarkable similar corrosion rates. There is a figure comparing corrosion rate data for 8 different alloys buried underground at 47 different locations in the US published in Journal of Research NIST, vol 115 No. 5, p373-392.
If alloying the steel with elements exhibiting low overpotential for hydrogen evolution there might be hydrogen embrittlement as an additonal corrosion mechanism when measuring in aqueous solutions.
Alloy composition will change the material corrosion potential. The corrosion potential is directly related to the reactions occurring on the alloy surface. If you have unalloyed iron in deaerated brine, the mechanism will be
anodic Fe ---> Fe+2 + 2 e
cathodic 2 H++ 2e ----> H2
As you change the alloy, you change the potential of the anodic reaction.
anodic Fe-Cr ----> Fe+2 + Cr+? + ?e
Cathodic 2 H++ 2e ----> H2
I think it is pretty obvious the reaction mechanism will be different!
In carbon steel, Fe only participate in corrosion, whether by alloying composition or not.....
Again, essentially everyone is correct. The explanation is that corrosion is such a highly stochastic process that for low alloy steels that the sample-to-sample scatter is large compared to effects of changes in alloy composition. Scientifically, we know there has to be an effect, but we cannot measure it reliably due to the large Fe corrosion current and the natural statistical scatter in this current. From an engineering standpoint, if you cannot measure it, do you care? I have been told that scrap yard workers can identify different steels by the atmospheric corrosion products that form, but I doubt that this works between different low alloy steels.
I think it depends on the situations involved. In some cases, the composition can significantly affect the corrosion. One example is that addition of Cr to a level less than 1% can almost stop flow assisted corrosion of carbon steel feeder pipes in nuclear industry.
Corrosion depends as much on alloy metallurgy (chemistry and other associated issues) as on environment. Some environment could amplify and some don't. One has to look or establish possible anodic and cathodic reactions and how they are affected by alloy chemistry and the environment. For example, when oxygen reduction is rate determining reaction and any contribution from hydrogen evolution is insignificant, the alloy chemistry doesn't matter. The same can be said about anodic reactions in some media; minor alloying elements may not matter. But, when it comes to atmospheric corrosion, it does matter; as the corrosion product stays back on the metal surface even if Cr content is less and chemistry of the films can greatly differ even with marginal variation in alloy chemistry.
In essence, one should look at the mechanisms and the ability of the measuring technique (as Dr. Ricker said earlier) and of course you can add several others. Raja
Dear Omar Yepez,
Is this reaction possible/reported? I might have overlooked in the literature survey?
anodic Fe-Cr ----> Fe+2 + Cr+? + ?e
If you are interested in localized corrosion (esp. pitting corrosion or microbially influenced corrosion), the answer is definitely yes. One of my papers illustrates how the alloying elements are implicated (likely actively participated) in such corrosion processes:
Shi, X., Avci, R. and Lewandowski, Z. Comparative Study in Chemistry of Microbially and Electrochemically Initiated Pits of Type 316L Stainless Steel. Corrosion Science 2003, 45(11): 2577-2595.
Dear Xianming Shi,
May I, very humbly, submit below an observation where an altogether contrary inference had been reported when carbon steel is alloyed with Cr in a simple corrosion reaction.
Corrosion Science
Volume 16, Issue 12, 1976, Pages 909–914
X-ray photoelectron spectroscopy was applied to study the composition of the passive film formed on an extremely corrosion resistant amorphous Fe-10at.%Cr-13at.%P-7at.%C alloy in 1 N HCl. The passive film consists mainly of hydrated chromium oxyhydroxide which is a common major constituent of passive films on crystalline stainless steels. The extremely high corrosion resistance of the amorphous alloy can only in part be attributed to the formation of a protective hydrated chromium oxyhydroxide film.
Pls. consider it as an academic discussion otherwise I value your excellence in this field because I have never worked in this field. Of course,I had remained associated with this field during my teaching career. Regards.
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