Basic Corrosion: Recurring Questions & Answers. Prof. Garry W. Warren. November 2007. Rationale. This presentation provides common examples of recurring questions students pose in developing their proficiency in electrochemistry & corrosion.
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Anode
Anodic
Active
Oxidation
Oxidation potential
EMF series
Electrolytic cell
Cathode
Cathodic
Noble
Reduction
Reduction potential
Galvanic series
Galvanic cell
http://bama.ua.edu/~gwarren/
G = –nFE OR G = +nFE
Standard Oxidation
Potentials
(G = –nFE)
Eº (V)
Cu = Cu2+ +2e– –0.342
H2 = 2H+ + 2e– 0.0
Ni = Ni2+ +2e– + 0.250
Standard Reduction
Potentials
(G = –nFE)
Eº (V)
Cu2+ +2e– = Cu + 0.342
2H+ + 2e– = H20.0
Ni2+ +2e– = Ni – 0.250
Standard
Potentials
(G = +nFE)
Eº (V)
Cu2+/Cu + 0.342
H+/H20.0
Ni2+/Ni  0.250
Values of Eº in these 2 lists are identical.
When you reverse the reactions, change sign of Eº.
Sign of Eº does NOT change if reactions are reversed, hence the title omits “oxidation” or “reduction”
Standard Oxidation
Potentials
(G = –nFE)
Eº (V)
Cu = Cu2+ +2e– – 0.342
H2 = 2H+ + 2e– 0.0
Ni = Ni2+ +2e– + 0.250
Standard Reduction
Potentials
(G = –nFE)
Eº (V)
Cu2+ +2e– = Cu + 0.342
2H+ + 2e– = H20.0
Ni2+ +2e– = Ni – 0.250
Standard
Potentials
(G = +nFE)
Eº (V)
Cu2+/Cu + 0.342
H+/H20.0
Ni2+/Ni –0.250
Values of Eº in these 2 lists are identical.
For –nFE, sign of Eº “+” or “–”
is chosen to agree with the thermodynamic tendency.
For +nFE, sign of Eº is the experimentally observed value of selected ½ cell when connected with H+/H2 half cell, so only one value is ever observed.
http://materialstechnology.tms.org/educ/educdigital.asp
Below is one of several examples addressing this question for standard conditions. Potentials are hot text and remind the user how each was obtained.
First the relation of G to E yields the Nernst equation. Activity, activity coefficient and concentration are defined via hot text popup windows.
After applying the Nernst equation to half cells, several examples for overall reactions are given. Standard potentials are obtained first as shown below.
After obtaining Eº’s the user is led term by term through the Nernst equation to calculate the overall reaction potential. Each box requires user input, and the final answer requires a calculation.
There are several other examples similar to the one shown here.
In principle any half cell can be selected as a reference, but only some are experimentally convenient. When selected as a reference it is assigned a value of zero volts, e.g. hydrogen or SCE shown below.
Such conversions are simply adjusting the zero point on the potential scale using the Eº value of the current reference electrode on the “new” scale.
Two more examples involving different reference electrodes are given.
The diagram shows that a corrosion potential is a combination of two half cells, the oxidation of Fe and the reduction of O2?
Several possible cathodic reactions exist. Knowing which one occurs offers different choices for limiting corrosion.
Red numbers reveal popup windows that show how the value was calculated.
The decision is a thermodynamic one. Through Nernst eqn calculations the user determines Sum A and Sum B, then selects an answer.
After giving a definition of each series, the user “measures” the corrosion potential for each metal by clicking & dragging each one into the white box.
This shows that Ecorr’s are not single half cells.
The difference is demonstrated with a “movie” that places the reduction half cell on the surface of the more noble metal for galvanic corrosion.
Shown is one example, for Fe and brass.
User must enter answers to questions in boxes.
Combining stainless and Al is rarely a good choice, but if necessary one option is better than the other.
The user must click on the appropriate image to answer.
This section of Ecorr can be omitted if desired.
It is probably most useful for advanced study.
–nFE = nonIUPAC
+nFE = IUPAC
The user can click on each button, work with the same example for each case and compare them.
ONLY 4 permutations are possible!
Practice, practice, practice!
Using the buttons on this summary screen the user can review any of the four possible permutations.
This question is best answered by comparing one with the other.
Absolutely not! See the two examples below.
Never associate the sign of E with “anode” or “cathode.”
What is always true is anode = oxidation & cathode = reduction.
e

PS
+
e
O2
H2

+
Zn
Cu
+

Pt
Pt
Zn+2 sol’n
Cu+2 sol’n
ANODE
CATHODE
ANODE
CATHODE
2H2O = O2+2H++2e 2H++2e = H2
oxidation reduction
Zn = Zn+2+2e Cu+2+2e = Cu
oxidation reduction
http://materialstechnology.tms.org/educ/educdigital.asp