Corrosion Stream 1
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- Опубликовано: 12 июл 2024
- For 14 October 2020, I'll describe what PDFs and videos are available for remote learning on this CET 2A Corrosion course. I'll answer answer topic-related questions, if people post them below, in a future stream
Post any questions about the October 2020 course in the comments below here.
Not sure why the chat doesn't display (or it doesn't for me) - a mystery.
Hello! Thank you for the lectures; I thoroughly enjoyed them -- especially your analysis of the metallurgy of Star Trek.
I have a couple of brief questions I'd like to clear up:
- What is the Galvanic potential and how does one measure it? Is it simply the potential of a metal surface as measured relative to a S.H.E in a specific environment? E.g. the galvanic potential of iron in seawater could be measured by attaching this iron electrode to a standard hydrogen electrode (considered as the anode) and measuring the potential difference?
- Assuming my reasoning above is true, would it be accurate to say that the corrosion cell potential, E, could be directly deduced from the difference between the anode and cathode galvanic potentials?
- Could the E in the definition of η = E - E_0 (as discussed in relation to the Tafel equation) include any applied potentials (for example, in the corrosion protection systems discussed in lecture 8?)? How do we determine the sign of these applied potentials?
Live long and prosper 🖖
Luis
Let me see if I can answer these.
1. What is the Galvanic potential and how does one measure it? Is it simply the potential of a metal surface as measured relative to a S.H.E in a specific environment? E.g. the galvanic potential of iron in seawater could be measured by attaching this iron electrode to a standard hydrogen electrode (considered as the anode) and measuring the potential difference?
A: Yes. This is correct.
One additional comment from me: galvanic potential is measured on real surfaces, which usually have several electrochemical half-reactions taking place simultaneously. The galvanic potential is somewhere in the middle of equilibrium potentials of these half-reactions. The galvanic potential of the surface settles at a value for which the sum of all the anode and cathode currents on the surface add up to zero. (If there is a current imbalance, electrical charge accumulates on the surface, shifting the galvanic potential to oppose the current imbalance until it becomes zero - by LeChatelier.)
2. Assuming my reasoning above is true, would it be accurate to say that the corrosion cell potential, E, could be directly deduced from the difference between the anode and cathode galvanic potentials?
A: I wouldn't explain it like this. Because "anode" and "cathode" imply one half-reaction taking place, and galvanic potential is only a necessary concept when there are several.
The "reversible cell potential" in the notes is defined for an ideal model cell, in which only one anode reaction and one cathode reaction take place.
For the ideal model cell (e.g. page 14 of handout 1) we have "E_reversible_cell = E_0_cathode - E_0_anode" or, in words, the potential difference across the cell acting reversibly (losslessly, e.g. no loss to internal resistance etc. -- in practice, this is achieved when there is very low current) is equal to the difference between the cathode and anode equilibrium potentials.
Now, in the special case that you could build a physical device [say a zinc-copper cell, (Zn|Zn++||Cu++|Cu), with aqueous electrolyte of unit activity] in which the only thing happening at the anode was zinc dissolving (no hydrogen evolution) and the only this happening at the cathode was copper plating (probably true in practice), both reversibly, then the galvanic potential of each metal plate (which each host only one reaction) would be the same as the equilibrium potential of the metal's half-reaction. In that case, you could measure the galvanic potentials, and the difference would be the same as the reversible cell potential from the algebra.
But, in general, "E_reversible_cell" is a concept from the algebra of a one-anode + one-cathode system, and galvanic potential is measured when you have multiple reactions on a surface.
3. Could the E in the definition of η = E - E_0 (as discussed in relation to the Tafel equation) include any applied potentials (for example, in the corrosion protection systems discussed in lecture 8?)? How do we determine the sign of these applied potentials?
A: Yes. If you have an electrical power source, and a counter electrode (e.g. a driven anode or driven cathode), then you can force the surface potential of an object to take any value you want (within reason). You just need to supply the net current that flows through the object's surface chemistry at that potential.
In this case, I would call the "E" "E_applied" or "E_ICCP" where ICCP means impressed current corrosion protection.
You would simply set E_applied to be whatever you want. E.g. if you want your iron to be immune from corrosion, you could set E_applied to be just below the iron immunity/activity line on the pH-potential or Pourbaix diagram. (Or you could set the potential to ensure the net current of the object's surface is just slightly negative).
If you want to "find E" for a case such as a specified anode current density, just evaluate the anodic polarisation (η_anode) from the Tafel equation (I'm assuming you know E_0, i_0, et cetera), note that anodic η values are positive, and solve η = E - E_0 for E.
Hope that all helps.
Eric
@@FaranWhyde Many thanks for your reply!