Dear Pabitra Maji

many potentiostats come with corrosion software which allows you to calculate corrosion current density and simultaneously corrosion rate mm/year for the given metal or alloy. But if you have an Origin graph, first you have to determine corrosion current density extrapolating Tafel lines to the corrosion potentials, using a linear fit function (see attached graph).

Knowing the corrosion current density you can calculate corrosion per year (mm/year) by:

K = jcor*8760*M*10/(ro*n*F)

where jcorr in A/cm2, ro, g cm-3, is density, metal or alloy, t = 8760 h in year, 10 conversions factor cm u mm, n exchanged number of electrons, F = 26.8 Ah/mol.

You can use Elchemviewer (http://www.lchem.cz/elchemviewer.htm).

Branimir N. Grgur Thank you for your suggestion. That is really helpful. But, how to determine the number of exchanged electrons denoted by 'n' in the equation?

Dear Pabitra Maji

The number of exchanged electrons has to be known (depending on investigated metals or alloy), for example for iron dissolution is 2 (Fe2+).

In the case of alloys n, M (molar mass) and ro can be calculated by

n(average) = Sum(ni)xXi(metals) in alloy

Mav = Sum(Mi)xXi(metals) in alloy

Ro(average) = Sum(Roi)xXi(metals) in alloy

where Xi(metals) is weight ratio of metal in alloy. For example Al3Cu

n(average) = 3*0.75 +2*0.25 = 2.75

One more suggestions,

all above is valid only for the homogenous - uniform corrosion, in the case of non-uniform corrosion (pitting, crevice, intergranular..) it's cannot be applied.

Greetings

In my opinion, the ASTMG 102 "Standard Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements" is the best choice to follow.

Best regards

I take the chance of this thread of answers to ask for another related question, in particular addressed to Branimir N. Grgur. Since Tafel approximation applies for a given redox couple can we really make us of it for the determination of the exchange current density (or corrosion current) in corrosion studies since the redox couples involved in the current traces are different for the anodic and cathodic branches? Most of the electrochemical investigations do not include the ionic species in the electrolyte that constitues the metal (e.g. Fe, Cr, Ni etc.), so the most common cathodic reaction would be HER or ORR. Thus, in principle, extrapolation of the Tafel lines should not meet at the corrosion potential.

Dear Sylvain Brimaud

Very complex question on which researchers usually do not thinks

But, the corrosion usually does not include a redox couple. Corrosion potential is between reversible potentials (Er) for metal dissolution (anodic reaction) and reversible potentials of the oxidizing species reduction (cathodic). Traces of the metal ions in the solution influences the reversible potentials of the metal (alloy) electrode, and below Er , could be in principle electrodeposited but usually under very low limiting diffusion current, much smaller than oxygen reduction or hydrogen evolution reaction. Complex forms of the Butler-Volmer equation for corrosion, include four parts anodic and cathodic part for metal dissolution-deposition, and anodic and cathodic part of reducing species. But is a very complex and cathodic part of anodic and anodic part of the cathodic reactions that are neglected. You are right " Thus, in principle, extrapolation of the Tafel lines should not meet at the corrosion potential. ", especially when oxygen is under mixed activation-diffusion control. In that case, anodic Tafel line could be used to estimate corrosion current density (like in the case of determining exchange current density for hydrogen evolution on nonnoble metals which do not oxidize H2 to H+ or water.).

Dear Branimir N. Grgur

Thanks for your answer. I rencently started to work on corrosion on stainless steels (SS), and these things puzzle me. I fully agree that dissolved metal traces will result in strong diffusional limitations for the current, and that the presence of, e.g., dissolved O2 would overcome it. In that case, and in my understanding, the corrosion potential should be seen as a mixed potential where |ia| = |ic| (ia for the dissolution of the metal and ic for the ORR). Thus, extrapolation of Tafel line for the anodic branch to the corrosion potential to get the corrosion current (i0) would not be at the thermodynamic value (eta = 0) of the redox couple generating the dissolved ionic species from the metal. This might lead to large misestimation of i0 (especially if different SS present signifcant differences in reactivity for the ORR or HER). This is somehow different than the case of the HER on non-noble metals since for the latter we know the thermodynamics value of the potential at which the Tafel line should be extrapolated.

Well, maybe one should simply accept the we define the corrosion current (i0) like this (extrapolation of the Tafel line at the corrosion potenital), eventhough it 'streches' the definition and applicability of the Butler-Volmer equation.

Do you have by chance some references that discusses these points?

Best wishes

Dear Sylvain Brimaud

You are dealing with the most complicated system. SS usually exhibit neither anodic nor cathodic Tafel slope (except in acidic media, posses cathodic, from which you can estimate jcorr). On the anodic side is in the passive range, and if you are working in a neutral saline solution both cathodic reaction is under diffusion or activation-diffusion control, and anodic in the passive state. So Tafel does not apply.

Please find two papers dealing with the most usual cases of jcorr determination for different cases.

Gretings

Branimir

Many thanks for the refs Branimir.

Best wishes,

Sylvain

Dear Sylvain Brimaud

It is my pleasure

Best regards

Branimir

Dear Mr. Maji. I work with Corrview software to analysis the Tafel curves that obtained from polarization test. This software could calculate the Icor. Then by related equation, corrosion rate could be obtained.