Using Pourbaix diagrams to calculate corrosion in water

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Discussion Overview

The discussion revolves around the use of Pourbaix diagrams to analyze the corrosion behavior of copper in water at a specific pH level (pH = 8). Participants explore the relationship between electrochemical potential and the stability of copper, as well as the practical aspects of measuring potential in experimental setups.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant inquires about the appropriate electrochemical potential to use for determining the stability of copper at pH = 8 and whether to consider all values of E between certain lines on the Pourbaix diagram.
  • Another participant explains that the Pourbaix diagram indicates the stability of different forms of a metal over various potentials and pH levels, and mentions that potential can be measured using a potentiometer or applied voltage.
  • A participant suggests connecting copper to a standard hydrogen electrode (SHE) to measure the voltage, noting that without a specified E value, a unique answer cannot be provided.
  • There is a query about the factors that contribute to metal corrosion, with a suggestion that oxygen might be a key element.
  • Another participant questions whether copper would corrode in a water solution devoid of oxygen, indicating uncertainty about the implications of oxygen's presence on corrosion rates.
  • One participant states that oxidation occurs above 0.1V, while copper remains stable below this potential.
  • A later reply emphasizes that the discussion is not an assignment but rather an exploration of the topic, clarifying the process of using the Pourbaix diagram to determine stability based on pH and potential.

Areas of Agreement / Disagreement

Participants express differing views on the role of oxygen in corrosion and the conditions under which copper remains stable. There is no consensus on the implications of oxygen's absence for corrosion rates, and the discussion remains unresolved regarding the specific potential range for corrosion at pH = 8.

Contextual Notes

Participants acknowledge the need for clarity in the question regarding the potential range for copper corrosion, and there are references to the limitations of the Pourbaix diagram in capturing all relevant factors affecting corrosion.

wnvl2
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Homework Statement
Which of the metals Al, Cu, Fe, Ag, Pb and Au can (thermodynamically) corrode in water that is Slightly acidic (pH = 6) (AI, Cu, Fe, Ag, Pb) Slightly basic (pH = 8) (AI, Cu, Fe, Pb) Very acidic (pH = 1) (AI, Cu, Fe, Ag, Pb) Very basic (pH = 14) (AI, (Cu), Fe, Pb) . .
Relevant Equations
Use Pourbaix diagram
koperpb.png


Let us solve it for Cu at pH =8. I found this Pourbaix diagram. What is the electrochemical potential I should use to check that Cu is there the stabel form. Should I check for all values of E between the blue lines at pH = 8? How can I measure the E value in reality?
 
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In the case of pH 8, your task is pretty easy. But in general, the Pourbaix diagram will give you the potentials/pH’s over which different forms of a metal are stable in water (usually).

As for what the potential is in an experimental situation, it’s either chosen by you when you apply a voltage or it can be measured straightforwardly by a potentiometer.
 
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If I am right, I can connect the copper to a SHE, measure the voltage and that is the E I have to use in the Pourbaix diagram. As the E is not specified in the original question, I can not give a unique answer.

oefPourbaix.png
 
wnvl2 said:
As the E is not specified in the original question, I can not give a unique answer.
You should try to answer the question nonetheless.

I'll try to frame the question somewhat more clearly. For your example, copper at pH=8, over what range of potential will it corrode?
 
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What it is that is present everywhere and makes metals corrode?
 
Oxygen?
 
Combined with pH it should give you some idea about the potentials involved.

Not that they are ignored on the diagram.
 
Does that mean that if the copper is in a water solution in abscence of oxygen that the copper will not corrode?
 
  • #10
wnvl2 said:
Does that mean that if the copper is in a water solution in abscence of oxygen that the copper will not corrode?
I’m not sure what borek is getting at, but this is definitely not true. Did you try answering my question in post 5?
 
  • #11
Above 0.1V there is oxidation. Below 0.1V cupper is stable.
 
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  • #12
Great! Can you figure out the rest of the question now?
 
  • #13
In fact it is not an assignment I have to submit, but I think the policy is that I put this kind of questions here. I am even not a student in chemistry, I just try to understand it. The solution of the exercice is clear. You look up the pH, you draw a vertical line. You see for which E the metal is stable. The E can be measured with an SHE connected via a salt bridge with the water around the metal.
 
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