Please see below two microstructures (as an example for the argument above) I have studied in a number of environments (bicarbonate, CO2, and carbonate solutions)

Hey,

could you please name the material. I could imagine, that the rsistance of the material could be somehow related to the corrosion rate of the matrial. With respect to the fact, that due to the cooling rate bainite could still contain a higher amount of carbon atoms in solid solution than alpha-iron in a perlite grain, the resistance of the alpha iron should be smaller, which would allow for a higher reaction rate.

Within a pearlitic grain the current flow should take place just at the alpha-iron sites.

But that's just an assumption.

I hope, i was able to help out!

Kindly

Michael

Have you analyzed the samples after the corrosion test, is there any specific pattern on where the corrosion product is concentrated at?

Michael,

I referred to the material, it is steel. My concern is on linking between the steel microstructure and the rate of cathodic reactions. I thank you for the input you made.

Arun,

Yes, I have analyzed the samples. My concern is about what happens BEFORE the growth of the corrosion products. Thank you.

Check out " Correlation of Microstructural Parameters and Hydrogen Permeation in Carbon Steel", Johnson, Krauss, Wu and Tang, Met Trans A Vol 18A, April 1987.

Thank you Prof. Johnson.

To give general description it would be very difficult. You can do permeation test to see how hydrogen gets trapped in various types of steel. So cathodic reaction would be connected to diffusivity in the microstructure. H diffusion in steel is very complex and not fully understood. Permeation tests show various shapes of curves depending on filling of traps (grain boundaries, dislocations, vacancies). Lately a lot of simulations using DFT, TB, MD, kMC were performed to try to link H to various traps and its depth. However, when you have other elements beside Fe, things start to became very complex.

Thank you David. Yes, the rate of the cathodic reactions could be linked to both the microstructure and hydrogen permeation. My problem however seems more interfacial rather than to be relevant to what might happen underneath the surface (after diffusion). I am concerned about what governs the rate of the cathodic reduction in relation to specific superfacial microstructural features. Thank you for your comment.

It is well known that microstructure affects several corrosion mechanisms, namely localized mechanisms such as SCC, pitting, IGC, and then depending on our implemented modifications of alloy chemistry, plastic deformation, heat treatment, service environment and maintenance, we can have more or less stabilization of the FCC phases, easy or difficult grain deformation, smaller or greater ductility and toughness values, etc. See AJ Sedriks, Corrosion of stainless steels, 2nd ed, New York,, Wiley,1996.Hydrogen is usually involved in the cathodic half-reaction of corrosion, eg in SCC, and so any of the factors referred which affect hydrugen permeation and absorption would affect and correlate microstructure and the kinetics of corrosion, namely as far as the kinetics of the HER is concerned.

In corrosion, anodic and cathodic reactions has to take place in a same rate, then i think it is easier to answer your question by explaining the effect of microstructre on anodic reactions which is more familiar and easily can be found in papers.

...I was interested in the carbon content...

You concern is probably with the hydrogen adsorption and hydrogen recombination reactions occurring on the steel surface. One way to distinguish the effects of microstructure on these reactions is to find the surface hydrogen concentration for different microstructures under the same hydrogen charging situations. This can be obtained from hydrogen permeation experiments, tracer (isotope) experiments, or insitu hydrogen charging under SIMS. Of these, permeation is the easiest to perform. In addition to the flux and trapping characteristics, the surface hydrogen conc. can also be determined. The role of different microstructures is in the adsorption coefficient difference. Atomistic calculations (Embedded atom models) can also give the adsorption values, theoretically, but then those are really computationally involved.

Hydrogen formation and hydrogen recombination reactions occurring on the steel surface. These reactions are form prefferetially on grain boundaries, twins, and other defects in the surface microstrocture. By that way formation of hydrogen the free energy of the sample is lowered. Kinetics of hydrogen formations and recombibation are dependent on the surface microstructure.

Attached my published article for yours interesting.

If you are interested only in surface reaction on the cathode contact it would be governed by surface quality. By that I mean state of oxides, average Ra (polished, grinded or untouched). Also important would be current you are applying.

Hope this would help you with your work.

Thank you Prof. Rozenak and Aria for your comments and links.

Dear Faysal and other members of the ResearchGate, for more info please check Hydrogen Embrittlement group on LinkedIn. Group discuss matters related to hydrogen embrittlement mechanisms of metallic materials.

http://www.linkedin.com/groups/Hydrogen-Embrittlement-4991909/about

.

Paul and Aria thanks.

Thanks Milos, a lot of quite interesting discussions going on there.

Faysal, You are welcome. Please, fell free to join us.

My old question -discussion about hydrogen embrittlement mechanisms in steels:

https://www.researchgate.net/post/Is_there_any_definite_rule_to_determine_which_hydrogen_embrittlement_mechanisms_HEDE_HELP_or_some_other_is_dominant_for_steels

Just joined it. I am actually going to work on a hydrogen embrittlement project in the next few months and this will be of tremendous benefit for sure, thank you again Milos.

Faysal, I'm glad to hear that, enjoy. Please do not hesitate to post any question. The purpose of this professional group is to share knowledge and current "state-of-the-art" in hydrogen degradation phenomena.

Porosity helps to the bubble formation and thus reduce overvoltage if there is gasous product

The suggestion of Dr Milos Djukic should be followed. The subject is very complex and it is appropriate do not advance answers because many issues are in play being important to hear the real experts on subjects as hydrogen embrittlement.

César I agree with You.

The complexity of the issues I pointed out in Hydrogen Embrittlement Group Profile description.

Hydrogen damages is one of the most complex phenomena of degradation of metallic materials, due to a number of new unknowns in the kinetics of process, compared to the degradation in the absence of hydrogen in the metal.

Most often, a multidisciplinary approach based on the use of different experimental-laboratory methods, theoretical models, modern micro-and nano-research, micro-fracture mechanics and solid-state physics, provides the conditions for a much better understanding of extremely complex, multiple and simultaneously active mechanisms of Hydrogen Embrittlement (HE)..