At the same voltage between electrodes, the higher voltage drops occure near catode,

making impuct of electrochemical overpotential insignificant upon addition of NaCl into solution.

Effect would be even more prononced upon addition of HCl, you simply have higher concentration of H+ ions in the electrolite.

Dear Sebastián,

Thank you very much for your input. Linking the chloride concentration to the protectiveness of the corrosion product layer is an important key to understand the corrosion behavior as a whole (anodic kinetics + cathodic kinetics + corrosion products properties). But let me tell you this: the corrosion products thickness increased with the chloride concentration. And this somehow contradicts with the higher activity of the cathodic reduction with the higher chloride concentration. Please see attached this micrograph. It shows the thicker corrosion products in higher-chloride solutions at the end of the potentiodynamic scans.

I think during such an early stage of the cathodic reduction, one should exclude the possibility for significant interference from the corrosion products (regardless of their kinetics of precipitation in relation with the chloride concentration), what do you think?

Thank you Alexander, accounting for the potential drops could resolve this. But broadly speaking, Can we catalyze the reduction of hydrogen protons by adding higher concentrations of chloride (in form of NaCl, not HCl)?

Thank you again.

Thank you Sebastián,

Yes, I have applied a potentiostatic polarization at -2 VSCE for about 1000 seconds right before the potentiodynamic scan, so the potentiodynamic polarization has been applied on a fresh, clean surface.

Now what appears here is that we need to resolve two issues:

1. How increasing the chloride concentration increases the rate of hydrogen generation on a clean surface.

2. How increasing the chloride concentration catalyzes dissolution and in turn makes the corrosion products thicker.

If you have further comments please share.

Many thanks

Faysal

Looking at your polarization curves there is not a monotonic increase in current density with chloride concentration, so my first question is whether this is just random variation - is the effect reproducible?

Bob

Thank you Prof. Cottis,

Yes, in this figure the effect of chloride does not sound monotonic (nor reproducible, therefore, I think). I carried out similar tests for different steels in CO2-saturated brines and I found that in general the cathodic currents tend to increase with the chloride concentration. I was thinking that the chloride ions might have sort of catalyzing effect on the reduction process of H2O and hydrogen protons? How would this be probably the case? And would this necessarily change the mechanisms of the reduction or just increase its rate?

Would be so pleased to know your opinion.

Faysal

Dear Faysal, i have some questions regarding your findings:

1- Can you show your pH curve for teh different Cl- concentrations ?

2- Did you observe any modification regarding the morphology of the corrosion products in your different mediums ?

3- Can you check the CO2 solubility and its relation with chloride concentration variation ?

Thanks

Abdelkader's question's make a lot of sense - hydrogen evolution in CO2 solutions is complex owing to the relatively high concentration of undissociated H2CO3, the CO2/H2CO3 hydrolysis reaction and the consequent buffering of pH. If Cl- concentration influences CO2 solubility it could explain what you are seeing. Note that your polarization curve appears to show two cathodic reactions, one of which is diffusion controlled (current density independent of potential). Have you thought about what the two reactions are? My guess is that the oxygen content of the solution should be quite low, so the current density for the diffusion controlled reaction is too high for it to be oxygen reduction. However, this will depend a lot on exactly how the solution was saturated with CO2 and what the stirring conditions are.

Bob

Thank you Abdelkader and Prof. Cottis.

To answer Abdelkader questions:

1) The pH did not show significant variation with the chloride concentration.

2) Yes, I did. Interestingly, the corrosion products were MONOTONICALLY thicker with the chloride concentration at the end of the potentiodynamic polarization scans.

3) This is actually at the heart of my concern: Does increasing the chloride concentration decrease the CO2 solubility in a way that it results in lower H2CO3 and H+ concentrations that in turn drive both the cathodic reduction and corrosion reactions at lower rates.

To answer Prof. Cottis questions:

Yes, I did. Below -1 V vs. SCE, the cathodic reduction is driven by the charge-transfer-controlled reduction of water, and above -1 V vs. SCE the cathodic reduction is driven by the mass-transfer-controlled reduction of H2CO3 and hydrogen protons.

We need to solve three dilemmas here in regard to the significance of the chloride concentration with the cathodic reduction:

1) Does increasing chloride concentration significantly affect the CO2 hydration in a way that it would result in lower H2CO3 concentrations that drive the cathodic reduction at lower rates?

2) Does the chloride adsorb at the surface in a way that it impedes the reduction process?

3) Does the chloride have interfacial effects by which it enhances the transport of H2CO3 to the surface or catalyzes its reduction?

If you have further comments or inputs please share, I think a good consensus is starting to emerge.

Faysal

Dear Faysal

Based on your figure, the mass-transport controlled reaction is not much affected by chloride concentration, so you seem to be asking the wrong questions if the activation-controlled reaction is water reduction. Also, was your experiment performed with controlled mass transport? If not you can't rely on the exact values of the diffusion-controlled reaction.

Bob

Geochemists tell us that CO2 solubility decrease as TDS increase. by increasing your Chlorides content you are decreasing CO2 solubility= you should have (less) HER since your H+ source is decreasing in the solution. Other aspect is the chloride species participation in the cathodic process that you need to check in conjunction with Ecorr shift if it is in the positive direction. Another point is the involvement of iron_chloride intermediate adsorbent that you can (check) study by EIS at cathodic potential(s). Understanding mechanisms requires more investigations and results. Good Luck!!

What about a simple phenomena that chloride could damage the surface films making the bare surface available for cathodic reaction reaction. Raja

Did you carried out an experiment in an electrochemical cell with a divided or undivided electrode space? If the electrode space are not divided, you will probably see a recovery effect on the cathode active chlorine or chlorate which is formed at the anode. Describe the conditions of the experiment