In Pure metals the presence of grain boundaries will decrease the corrosion resistance. Therefore smaller the grains more will be grain boundaries and less corrosion resistance (in terms of intergranular corrosion).

Whereas in case of alloys, the segregates are formed in the grain boundaries. These segregates at grain boundaries can act as cathodic or anodic region compared to material in the grain. Then the corrosion rate depends on active anodic area (i.e preferential corrosion or etching effect of either grains or grain boundary). And hence in some alloys corrosion rate is higher when having large grains and lower in others.

(In case of pure metals i believe more the grain boundaries higher will be corrosion rates- in terms of intergranular corrosion)

Dear @Abhishek Ramakrishna Shetty

Even in the case of Pure Metals, majority of the cases have reported the grain refinement decreased the corrosion rate. They said that that a uniform coherent hydroxide layer would be formed on the surface and protect the substrate. Please see the following review papers:

http://corrosionjournal.com/doi/10.5006/1.3462912?code=nace-prem-site

https://link.springer.com/content/pdf/10.3103/S1068375517050039.pdf

I agree with both the previous answers, However, the answer for the last part of the question " So why dislocations inside the grains accelerate the corrosion rate while dislocations in GBs decreases the corrosion rate?? " is not convincing. For pure metals, I guess, analyzing the energy aspects could answer this part of the question.

Dear Khausik Narasimhan ,

In the following paper, HS Kim, mentioned that decrease in grain size reduces the corrosion rate,due to uniform MgO/Mg(OH)2 formation on the surface, while dislocations in the matrices increase the corrosion rate.

However researches who reported the beneficial side of grain boundaries on corrosion resistance enhancement believe that dislocations in the grain boundaries may decrease the mismatch energy between surface layer (Mg(OH)2) and substrate and so enhance the coherency of these two materials. So my question is that why dislocations in the matrix (inside the grains) can not enhance the coherency of surface layer with matrix?

Article Effect of thermal treatment on the bio-corrosion and mechani...

Dear Ahmad and Khausik ,

two types of grain boundaries (GB) dislocations can be distinguished: which belong to GB structure and which don’t belong to GB structure

[M. Grabski. MOBILITY OF DISLOCATIONS IN GRAIN BOUNDARIES. Journal de Physique Colloques, 1988, 49 (C5), pp.C5-497-C5-506. . HAL Id: jpa-00228058 https://hal.archives-ouvertes.fr/jpa-00228058].

Different energies and therefore property can be assigned to them. The misorientation angle, type and position of the GB is also important. So, determination of the GB and GB dislocation effect is difficult.

The effect of crystallographic grain orientation and deformation was investigated in the work:

[Influence of the crystallographic orientation of grains and plastic deformation on the electrochemical behavior of pure aluminum in sodium chloride solution / Zbigniew SZKLARZ, Halina KRAWIEC, Mirosław WRÓBEL // Diffusion and Defect Data – Solid State Data. Part B, Solid State Phenomena ; ISSN 1012-0394. — 2015 vol. 227, s. 19–22. — Bibliogr. s. 22, Abstr.. — Corrosion 2014 : international scientific conference : Gliwice, 18–21 November, 2014. — tekst: http://www.scientific.net/SSP.227.19.pdf]

Yours,

Mirek

Dear @M_Wrobel

Thanks for the reply. Let's suppose a pure Mg with given orientation (like 001). I am trying to narrow down my question to the effect of dislocaitons and grain boundaries regardless to the impuries, orientaion and so on. What is your opinion about the Grain Boundaries and Dislocations (in GB or in Grains) effects on dislocations?

@Ahmad Bahmani:

To start, I have a question/remark regarding your initial question. You say initially:

Theoretically grain boundaries are defects zones and corrosion can initiates from these zones (like etching process where GBs appears). so corrosion rate should increases with increasing grain size.

The former is in my opinion correct (i.e. GB are full of defects and hence normally ease corrosion), but as a consequence corrosion rate should decrease with increasing grain size.

From a general point of view: dislocations are crystal defects and hence are potential energy sources. This intrinsically means that they will increase corrosion rate (as shown by the influence of dislocations on the corrosion rate inside the grains). Nevertheless, this intrinsic effect might be masked and even overcome by secondary effects linked to the presence of dislocations or grain boundaries (e.g. the building up of a more homogeneous passivation layer).

It is important to notice that grain boundaries are not synonym to dislocations: grain boundaries are basically the litter bin of the material were all the misfits try to fit in, including dislocations. So trying to explain differences in corrosion rate inside the grain and in the boundaries by solely looking at dislocations is not correct, as grain boundaries might give rise to very different mechanisms (e.g the formation of a very corrosion resistant phase/passivation layer on the grain boundary). This also explains why different results are reported in literature for the corrosion rate at grain boundaries: the net effect of all the 'misfits' present in the grain boundaries might be to increase the corrosion rate, or decrease the corrosion rate, depending on the specific case.

I think that grain size reduction and refinement exposes more metallic atoms to take part in the electrochemical reaction of corrosion. For a metal which can form protective oxide layers like Al, Cr, etc, this initially high rate of the reaction can also lead to more rapid formation of the oxide layer with very easier linkage of the oxide nuclei into a highly continuous and protective corrosion product layer due to the reduced interparticle distance. Such phenomenon can therefore decrease the corrosion rate (increase the corrosion resistance of the metal with time), and the continuous supply of metallic atoms from the interior of the material through the grain boundaries to the reaction from sustains the protective layer. If, however, the oxide layer is non protective then a rapid dissolution of the formed oxide nuclei ensues and increases the dissolution (higher corrosion rate) of the material during the corrosion. Again, the non protective layer will give way to greater effect of localized corrosion due to abundance of grain boundary triple junctions in the microstructure of the metallic material with reduced grain size.

Dear Ahmad Bachmari,

I think that the effect of GB disorientation can not to be escaped (orientation is the plane and the direction that lies in it e.g. (ukl)[uvw]). The low angle GB (with a low disorientation) has dislocation models, the large angle ones have not. In addition, there is a relationship of the GB disorientation and chemistry.

Yours,

Mirek Wrobel

Dear @Mirek Wrobel

I didn't get what you mean by dislocation model where you said LAGB has dislocation model. would you please explain it more.

Thanks

Dear Ahmad Bahmani,

maybe the video lesson

https://www.coursera.org/learn/material-behavior/lecture/OzUBH/5-15-dislocation-boundaries-low-angle-boundaries

will help you understand what I meant

Yours

Mirek

I agree with the answer Mr. Onyeachu Ikenna

The grain boundaries usually contain the secondary phase in alloy such as Al-Si alloy. the lattice structure in the grain boundaries are   less than in primary phase, therefore have a high energy result in increased ability to corrosion.

Reducing the size of the secondary phase and spreading of this phase along of high fraction of grain boundaries through severe plastic deformation may enhance the corrosion resistance as Mr. Onyeachu Ikenna mentioned.

I observed a significant reduction in corrosion rate that after SPD. My alloy system contained long network kind precipitates which broke down in small particle after SPD and distributed nearly uniformly in the matrix. After seeing the microstructre of both as cast and wrought alloy I concluded that uniform distribution of second phase increase the corrosion resistance.@Ahamad Bahmani

Sourav Dutta Mohamed Abdelgawad Gebril

Dear Sourav,

GBs may act as abarrier for corrosion propagation [Han, 2012] or they may prepare dislocations to reveal the mismatch of surface layer and matrix [birbilis, 2010]. o we need to conclude the GBs and dislocations influence the corrosion rate.

GBs may act as a barrier for corrosion propagation [Han, 2012] or they may prepare dislocations to reveal the mismatch of surface layer and matrix [birbilis, 2010].

GBs also may enhance the corrosion rate by increasing dislocations number [song, 2010]. It means that the is no single opinion about how corrosion rate is affected by GBs.

[1] G. Han, J.-Y. Lee, Y.-C. Kim, J.H. Park, D.-I. Kim, H.-S. Han, S.-J. Yang, H.-K. Seok, Preferred crystallographic pitting corrosion of pure magnesium in Hanks’ solution, Corrosion Science 63 (2012) 316-322.

[2] N. Birbilis, K.D. Ralston, S. Virtanen, H.L. Fraser, C.H.J. Davies, Grain character influences on corrosion of ECAPed pure magnesium, Corrosion Engineering, Science and Technology 45(3) (2010) 224-230.

[3] D. Song, A. Ma, J. Jiang, P. Lin, D. Yang, J. Fan, Corrosion behavior of equal-channel-angular-pressed pure magnesium in NaCl aqueous solution, Corrosion Science 52(2) (2010) 481-490.

I have summarized and concluded everything I learnt from my studies as well as responses from this question in this article:

Article Formulation of corrosion rate of magnesium alloys using micr...

Hi Mr. Bahmani. I recently investigate the effect of grain size on the corrosion rate of low carbon steel, I hope it would be helpful!

Maryam Soleimani

Please read the question completely.

It is about the different effect of dislocation in grain interior and grain boundary.

The question was asked few years ago; and now the related work has published in an article formulating all effects. A new formula has been developed.

DOI: 10.1016/j.jma.2019.12.001

Hi Ahmad,

Both your statements are correct.

In active corrosion environment, such as iron, the impurities from the GB influences the corrosion rate. Therefore, larger grains size with less GB is better.

But in passivating corrosion environment, such as Aluminium, the high density of GB will initiate the corrosion and forms a film which will act as barrier. In this case fine grain size with higher GB is better in long run.

I hope this will be useful.

Hamed Mirzadeh Dear Dr Mirzadeh,

The question is for a long time ago. Please read the question carefully. I don't know why the answer has got couple of recommendation.

Please read my papers in the last year (especially JArticle Formulation of corrosion rate of magnesium alloys using micr...

Article Controlling the microstructure and texture using multi-direc...

It is for 2010 that the researchers have found the Hall-Petch type equation for corrosion rate. It can not be satisfying at all.