Dear Dr. Meeta Ashok Kamde ,

I suggest you to have a look at the interesting, following paper which will help you clarify many of your doubts:

-Fundamentals and advances in magnesium alloy corrosion

M.Esmaily, J.E.Svensson, S.Fajardo, N.Birbilis, G.S.Frankel, S.Virtanen, R.Arrabal, S.Thomas, L.G.Johansson

Progress in Materials Science, Vol. 89, Pages 92-193 (2017)

Available at: https://www.sciencedirect.com/science/article/pii/S0079642517300506

In general, the electrochemical tests are started when a stable potential is reached ... (e.g. 2h) in the cases of materials that suffer from localized corrosion and in aggressive solution this condition can also be reached but if our aim is to measure for example a corrosion rate through potentiodynamic polarizations and the Tafel lines, we certainly underestimate the situation that actually occurs in fact we do not take into account the pitting phenomenon, unless we study it with more accentuated anodic polarizations ... but this is another history.

Gravimetric weight loss measurements are certainly a coarser method, they require you to follow the samples for a long time (... and procedures to be carried out at the end of the exposure) but here we can verify a more real trend that also allows a verification of any attacks localized.

I don't want to be misunderstood; I do not want to say that the electrochemical tests are not reliable but must be conducted with a certain criterion depending on the case and, sometimes, criticalities may emerge which I can try to overcome by adopting appropriate methods. In fact, I have very precise electrochemical tests available but systems that behave in a complex way.

I apologize for the long answer, my best regards.

Pierluigi Traverso

P.s .: a reasonable answer; the chloride solutions are the most aggressive that there may be, even in low concentrations (even if in many tests an aqueous solution at 3.5% by weight is used) and capable of triggering localized phenomena by destroying the passivation layer that may be present on the substrate. Chloride ions are very aggressive for both magnesium and aluminium alloys.

Dear Dr. Meeta Ashok Kamde ,

I suggest you to have a look at the interesting, following paper which will help you clarify many of your doubts:

-Fundamentals and advances in magnesium alloy corrosion

M.Esmaily, J.E.Svensson, S.Fajardo, N.Birbilis, G.S.Frankel, S.Virtanen, R.Arrabal, S.Thomas, L.G.Johansson

Progress in Materials Science, Vol. 89, Pages 92-193 (2017)

Available at: https://www.sciencedirect.com/science/article/pii/S0079642517300506

In general, the electrochemical tests are started when a stable potential is reached ... (e.g. 2h) in the cases of materials that suffer from localized corrosion and in aggressive solution this condition can also be reached but if our aim is to measure for example a corrosion rate through potentiodynamic polarizations and the Tafel lines, we certainly underestimate the situation that actually occurs in fact we do not take into account the pitting phenomenon, unless we study it with more accentuated anodic polarizations ... but this is another history.

Gravimetric weight loss measurements are certainly a coarser method, they require you to follow the samples for a long time (... and procedures to be carried out at the end of the exposure) but here we can verify a more real trend that also allows a verification of any attacks localized.

I don't want to be misunderstood; I do not want to say that the electrochemical tests are not reliable but must be conducted with a certain criterion depending on the case and, sometimes, criticalities may emerge which I can try to overcome by adopting appropriate methods. In fact, I have very precise electrochemical tests available but systems that behave in a complex way.

I apologize for the long answer, my best regards.

Pierluigi Traverso

P.s .: a reasonable answer; the chloride solutions are the most aggressive that there may be, even in low concentrations (even if in many tests an aqueous solution at 3.5% by weight is used) and capable of triggering localized phenomena by destroying the passivation layer that may be present on the substrate. Chloride ions are very aggressive for both magnesium and aluminium alloys.

Dear Meeta,

Would you please share your electrochemical test data or graphs ? And also pictures of your samples after immersion? And the chemical composition and temperature@ of your test ?

I got a question for you : Did you investigate the surface of your samples by microscope or only weighted them ?

Yes dear Meeta Ashok Kamde you are absolutely right , for magnesium alloys, the corrosion rate of immersion tests is quite higher than the corrosion rate you obtained from the electrochemical polarization test.

Now, if you freeze all the experimental parameters for both test techniques you still have the factor of exposure time and its effect on corrosion rate as the immersion tests is long term test and the polarization test is very short term test and also the factor of exposure surface area of the specimen as for immersion testing this area is larger while in polarization tests only 1cm2 is used.

Now, for chloride solution consider the corrosion rate is the sum of the general corrosion and pitting corrosion rates, the general corrosion is evident in immersion test which tends to increase the sum of corrosion rate, but only specific pitting corrosion occurred in the polarization test while the general corrosion is not evident which tends to decrease the sum of corrosion rate.

I hope this explanation is useful and if you interesting for explanations on the effect of exposure surface area of the specimen on the corrosion rate of Mg alloys for both test techniques, I will do gratefully.

Best regards

I agree with what Mohammed Hussein j. H. Al'Atia .

The corrosion activity of Mg in both variants is reflected coyote. This difference is mainly due to the time and temperature differences between the medium and the studied sample.

In case of weight loss, the calculated corrosion rate is an average value over the exposure period, assuming uniform corrosion on the whole surface. Its accuracy depends on the duration of the corrosion test, adsorption of corrosion products and/or inhibitor on the alloy’s surface, and ease and completeness of their removal before weighing. Localized corrosion induced by the IMPs in Mg alloys may give some error in the calculated corrosion rate.

Electrochemical tests give you the corrosion rate at the exact particular time, and in case of Mg alloys even obtaining the reproducible results might be a problem due to mentioned above IMPs. Also important is surface preparation!

Dear Dear Meeta Ashok Kamde,

They differ in many important factors that affect the corrosion rate and mechanism. Accelerated hydrogen evolution rate is high in electrochemical results. Electrochemical results can measure the anomalous chemical attack occurring simultaneously with the normal electrochemical corrosion attack while in immersion only uniform attack they measure. Electrochemical signal during electrochemical measurements are anomalous. This anomalous electrochemical behaviour of the metallic surface was more significant for the commercially pure Mg alloy. The calculated corrosion rate is an average value over the exposure period, assuming uniform corrosion on the whole surface. Immersion test of Mg alloys are giving lower corrosion rate in sulphate solution than chloride solution but the electrochemical results are giving higher corrosion rate for sulphate solution than the chloride solution.

Ashish

Electrochemical measurements are the accelerated test methods to investigate the corrosion behavior of desired material in a corrosive environment. So, there are many factors which affect the results, for example one of the dominant factors is the quality of surface preparation as the other researchers mentioned and also the sweep rate of potential in DC measurements and also low frequency range in A.C measurements. The point you need to consider is the corrosion mechanism that you are recommended to investigate the surface morphology of working electrode under microscope, because sometimes localized corrosion is able to affect the electrochemical curves and corrosion current density in such a way that the corrosion current density would be more than the related corrosion current density attributed to the corrosion rate gained by weight loss method. So, you are recommended to do your investigation by electrochemical methods beside the microscopic assessments of the surface morphology after accomplishing your electrochemical tests.

I think the difference that you are observing in weight loss and electrochemical tests could be related to the occurrence of localized corrosion due to the presence of sulfide or chloride in the test silution.

Dear Meeta Ashok Kamde, Pure Mg does not form an oxide layer, so it is degradable in any solution with the varied rate of removal.

In the immersion test, (say for example, after 1st day or 15 days or more) the steady-state condition has achieved and the steady-state current (Iss) is a significant parameter that gives lower corrosion rate values. Whereas, the Potentiodynamic (PD- test) is an accelerated electrochemical test; usually conducted just after OCP (maximum of 2 hours or less ). So, it would give different corrosion rate.

If possible you can perform potentiostatic scan at OCP values that you have observed during PD- test, use that OCP values and hold your samples for hours (more than 10 hours) to analysis the steady-state current density (Iss) in your chloride/sulphate system. (Convert the graph in log/ln scale to see the actual steady-state current density Iss values)

Secondly, why corrosion rate (rate of removal of Mg ions) varies in a different solution, for that you need to conduct cyclic voltammetry (CV) for studying what electrochemical (redox) reaction has been occurring in the studied (chloride/sulphate) environment. The conjugate of the anodic and cathodic peaks will give you an idea that which compounds are responsible for different corrosion rate.

Thanks

Continue........ and compare corrosion current density (Icorr) and steady-state current density (Iss) values, so you will find your answer that why different corrosion rate are showing in the same system.