Modeling of Aqueous Corrosion in presence of Hydrogen Peroxide
Sent: Monday, April 01, 2013 11:51 PM
Question: This is my personal question about H2O2 (inhibitive or aggressive species) effect on anodic process in simulation model. Answer: Since passive current density is not affected by H2O2, equation 127 does not play an important role and its effect on anodic process is expressed by equation 116 and 117 in attached your paper, right?
Question: How is the effect of H2O2 handled in these equation?
I guess there must be further model equations which takes into account the effect of chemical species in these equations other than passive current density.
Answer: Eqs. 116 and 117 are always used. Eq. 127 is used for the passive current density, which enters eq. 116 and also for the critical current density (i.e., the current density that corresponds to the nose of the polarization curve). Eq. 127 actually does play an important role because it influences not only the passive current density but also the critical current density. As you correctly observed, I have not explained it in sufficient detail in my papers. This is my fault. In fact, the effect of eq. 127 on the critical current density is important only for carbon steel (and other alloys only in special cases). It is not important for corrosion–resistant alloys, which have been the focus of the overwhelming majority of our work in the past several years. This is why I missed this description.
The way it works is really simple. Eq. 127 applies to both the passive current density and the critical current density (the latter is really a reflection of the pre-passive state because it quantifies the active-passive transition). When eq. 127 is used for the critical current density, we have icrit(pH) instead of ip(pH) in eq. 127. As a default, exactly the same parameters K, l, c, and a are used for the critical current density as for the passive current density. However, there are exceptions for inhibitive species. Such exceptions are common for carbon steel. When there is an exception, the parameter K in eq. 127 is multiplied by a constant k for the critical current density. This is just one temperature-independent constant per inhibitive species. Such a constant can be quite significant and it is, indeed, significant for H2O2. The physical significance of it is simple – sometimes an inhibitor does not affect passive dissolution because the metal is already sufficiently passive at a given pH. But it does affect the prepassive behavior, which is reflected by the critical current density.
By the way, the parameters i20 and alpha2 in eq. 117 are evaluated numerically at each condition so that the whole anodic current density matches both the Flade potential and the critical current density.
Question: In equation 117, Flade potential is used as if it is a constant. However if Flade potential varies with H2O2 concentration as your explanation below, there must be another model equation which can correlate Ef and H2O2 concentration, I guess.
Answer: No, the Flade potential is only a function of pH. At least, this is a reasonable approximation because cannot complicate the model too much. It is the critical current density that varies with H2O2 concentration. So, when the H2O2 concentration sufficiently increases, the nose of the polarization curve recedes, becomes smaller and smaller but it is always terminated at the Flade potential. Sometimes you cannot see the Flade potential when the critical current density approaches the passive current density (which is very common for corrosion resistant alloys but can be also observed for carbon steel with sufficiently concentrated H2O2.
Sent: Friday, February 22, 2013 2:54 AM
Corrosion doc3
Question: I have completed the final project meeting at JAEA on Wednesday and haven't forwarded the revised results to them, yet.
Please let me confirm several points before proposing them the revised output. Can I calculate equilibrium potential by hand with Analyzer output from the following equation? H2O2(aq) + 2H(+) + 2e(-) -> 2H2O E0 = [ 2G0(H2O) - G0(H2O2) - 2G0(H+) ]/F/2
Answer: Yes, the standard-state Gibbs energies of formation of all species can be obtained from the Analyzer output. Then, the standard-state equilibrium potential can be obtained by hand (or in Excel) as you described above.
Question: Will it be useful if we can see equilibrium potential of specific reaction in Analyzer output
just like equilibrium constants, K?
Answer: Yes, such a facility could be created. The values would need to be transferred from the code to the report, which would require a fair amount of interface work because they are buried inside the code.
Question: In your revised model for temperature dependency, activation energy in exchange current density term is now 120,000 for H2O2. The value for O2 is 76,000. I know you mentioned that it was arbitrary but still I need to know how did you came to the value 120,000.
Answer: The idea is really simple. The current density for the reduction of H2O2 (the activation-controlled part, not the diffusion-related limiting current density) was decreasing fairly rapidly with temperature when the activation energy was 76000 (like for O2 reduction). As we discussed it before, this was because of a much stronger temperature dependence of the equilibrium potential for H2O2 reduction than for O2 reduction. Although I had no way to verify it, I did not like it because such a rapid decrease is not observed in common reduction reactions. Typically, the current density should either slowly increase or slowly decrease. So, I made it decrease slowly by raising the activation energy to 120,000. This is all that we can do in the absence of data at higher temperatures – we just need to seek the most reasonable behavior.
Question: During the meeting, I got one question that I couldn't answer properly.
The effect of H2O2 on corrosion as cathodic reaction is well described by the following reaction.
i_H2O2 = (i*_H2O2)*(H2O2^q)*(H+^r)*EXP(-alpha_H2O2*F*(E-E0)/RT) + mass transfer limitation
The problem was the effect on anodic side.
Through below discussion, it was confirmed that the model did not consider H2O2 effect on passive current density.
Then does it affect to active-passive transition behavior only?
i.e. anodic process changes by H2O2 only for transition region and not in active region or passive region.
Answer: Yes, this is the key. This goes back to the nature of carbon steel, which is a weekly passivating metal. This means that it does not become passive unless the pH is moderately high, above ~10. Below this pH, we have a wide active-passive transition (which show on the polarization curve as a “nose” region). At pH of about 7, we will still get a passive current density at higher potentials but at lower potentials the “nose” is so wide that large corrosion current densities are possible (if there is a suitably strong cathodic process to drive the corrosion). So, what happens if there is an inhibitor? The inhibitor suppresses the nose of the polarization curve. Instead of the anodic current density going up and down through the nose and approaching the passive current density at the Flade potential, the anodic current density goes simply up at some point and the nose vanishes. This is the same what happens in moderately alkaline solutions or for stainless steels or Ni-base alloys in wide ranges of pH (except acidic solutions). The inhibitor sometimes often the passive current density if it is strong but it does not have to do it. H2O2 is a rather weak inhibitor but it is efficient enough to suppress the nose of the polarization curve.
By the way, you can imagine what would happen if the nose was not suppressed – then H2O2 would strongly increase corrosion because the cathodic process of H2O2 reduction is very strong. This is by the way the same kind of behavior that is observed with oxidizing inhibitors such as nitrates or chromates – they have a dual effect.
Question: Could you elaborate a bit more on the mechanism about how H2O2 and its complex solid on meal surface are affecting to anodic side? I guess some parameters in attached model equations were developed. The H2O2 surface complex solid is neither passive film nor scales such as FeCO3. Therefore its physical model image is unclear to me, yet.
Answer: Clearly, there is no scale. But there is a promotion of passivity. If the nose of the polarization curve disappears, it means physically that passivity becomes well-developed at lower potentials. This is the same mechanism as with other oxidizing inhibitors. Physically, it starts with adsorption on the metal surface. Since the adsorbed molecule is an oxidizer and is not very stable in the solution (H2O2 and NO2- are completely metastable and CrO42- can be easily reduced) it can act as a donor of oxygen. This oxygen-donor action contributes to the formation of an oxide film even though the pH is not high enough to make the oxide stable on a thermodynamic basis. The key is that the oxygen donor stabilizes the oxide on the surface at lower potentials (i.e., within the original potential range of the nose). Another way to look at it is that the effective Flade potential is shifted towards lower values because it easier to stabilize the oxide when an aggressive oxygen donor is available.
Sent: Monday, February 18, 2013 6:07 AM
Question: I have looked at corrat.dat file. Attached are the excerpt from the file. For the H2O2 reduction reaction, I can not understand why the stoicheometric coefficient for electron is -1 and not -2. For the O2 reduction reaction, it is -4 and consistent with the reaction equation. Could you confirm all these data given for H2O2 are correct? Reference condition and activation energy are same but exchange currents differ a lot and reaction orders for H+ are also different.
Answer: The number of electrons is indeed the most controversial assumption that I made in this project but I made it for a good reason. Mechanistically, reactions on a metal surface may proceed with a different number of electrons than the number that is written in the formal equation. One well-known example is the reduction of oxygen. It may be a four-electron reaction (like in the formal reaction) or a two-electron reaction. On ferrous metals, it is usually a four-electron reaction and this is what we have in the databank. Another well-known example is the oxidation of iron. According to the Bockris-Drazic-Despic mechanism, it is a one-electron reaction and according to the Hausler mechanism, it is a two-electron reaction. The number of electrons that I am mentioning here is the number of electrons that are transferred in the rate-limiting step.
For H2O2 reduction, the number of electrons has never been investigated in the literature so we do not know what it should be in reality. However, I have determined that the limiting current densities are more reasonable when we assume a one-electron reduction. This is easily visible because the limiting current density is proportional to the number of electrons in the reaction.
Regarding the order with respect to H+, it is an empirical parameters and it has been determined with reasonable certainty because we have corrosion potential data at various pH values.
