Post on 14-Apr-2018
7/28/2019 GWarren Corrosion 411
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Department of Metallurgical and Materials Engineering
Basic Corros ion:
Recu rr ing Quest ions & Answers
Pro f. Garry W. Warren
November 2007
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Department of Metallurgical and Materials Engineering
Rationale
This presentation provides common examples of recurring
questions students pose in developing their proficiency in
electrochemistry & corrosion.
Typically these questions recur every year, year after year. The vast majority of such questions relate to critical basic
information covered in the first few weeks, i.e. the
foundation upon which the rest of the course depends.
Computer software is a practical way to expose students to
these questions.
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Department of Metallurgical and Materials Engineering
Recu rr ing Quest ions
Most recurring questions asked fall into one of thefollowing areas:
Terminology (knowing new terms, e.g. cathode vs. anode)
Thermodynamics (e.g. using the Nernst equation)
Sign conventions (e.g. G = +nFE vs. G =nFE)
Reference electrodes
Understanding the significance of the cathodic reaction
Understanding corrosion potentials
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Department of Metallurgical and Materials Engineering
Critical Terminology (cont)
On the first day of class students are provided a
handout entitled Important Corrosion Concepts to
Remember defining most of these terms.
That handout is available here:http://bama.ua.edu/~gwarren/
The explanation of electrolytic cells vs. galvanic cells
is best covered after some exposure to the EMF
series and Nernst equation (also found at the end ofthis presentation).
http://bama.ua.edu/~gwarren/http://bama.ua.edu/~gwarren/7/28/2019 GWarren Corrosion 411
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Department of Metallurgical and Materials Engineering
What are the most important th ings to know
about the EMF Series?
Emphasize the title Standard Reduction Potentials, E orStandard Oxidation Potentials, E
Each potential is tied to a half cell reaction.
Electrochemical reactions (corrosion) must involve two halfcells: one oxidation and one reduction.
The half cell assigned a voltage of zero is the reference halfcell.
Always include V vs. SHE or V vs. SCE (only then is choiceof reference half cell clear).
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Department of Metallurgical and Materials Engineering
Is there a connect ion between G & V?
The connection between Gibbs energy (G) and potential
or voltage (V) is given by either:
G =nFE OR G = +nFE
The choice is a convention, either is correct.
Persistent repetition of the texts choice ofnFE or+nFE
is worthwhile for two reasons:
To emphasize that this is the texts convention
To emphasize the importance of identifying the chosen
convention when consulting other texts or references
I prefer using G =nFE, until students have some
experience with the EMF series & the Nernst equation.
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Department of Metallurgical and Materials Engineering
Can I connect a sign convent ion to EMF Series?
Imagine three EMF series (no others are possible!).
The words oxidation or reduction with respect to cell potentials
also indicates selection ofnFE convention.
When you reverse the
reactions, change sign of E.Sign of E does NOT change if
reactions are reversed, hence the
title omits oxidation or reduction
Values of E in these 2 lists are identical.
Standard
Potentials(G = +nFE)E (V)
Cu2+/Cu + 0.342
H+/H2 0.0
Ni2+/Ni - 0.250
Standard Oxidation
Potentials(G =nFE)E (V)
Cu = Cu2+ +2e 0.342
H2 = 2H+ + 2e 0.0
Ni = Ni2+ +2e + 0.250
Standard Reduction
Potentials(G =nFE)E (V)
Cu2+ +2e = Cu + 0.342
2H+ + 2e = H2 0.0
Ni2+ +2e = Ni 0.250
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Department of Metallurgical and Materials Engineering
Why doesnt the sign of E change for +nFE?
Imagine three EMF series (no others are possible).
Standard Oxidation
Potentials
(G =nFE)E (V)
Cu = Cu2+ +2e 0.342
H2 = 2H+ + 2e 0.0
Ni = Ni2+ +2e + 0.250
Standard Reduction
Potentials
(G =nFE)E (V)
Cu2+ +2e = Cu + 0.342
2H+ + 2e = H2 0.0
Ni2+ +2e = Ni 0.250
Standard
Potentials
(G = +nFE)E (V)
Cu2+/Cu + 0.342
H+/H2 0.0
Ni2+/Ni 0.250
Values of E in these 2 lists are identical.
FornFE, sign of E + or
is chosen to agree with the
thermodynamic tendency.
For+nFE, sign of E is the experimentally
observed value of selected cell when
connected with H+/H2 half cell, so only one
value is ever observed.
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Department of Metallurgical and Materials Engineering
Ecorr Software
Computer Aided Instruction (CAI) is a practical way to
expose students to recurring questions.
Permits students to work outside class at any time
Allows more class time for other topics
Ecorrsoftware An introduction to corrosion, electrode potentials & electro-
chemical thermodynamics.
Focuses on many recurring corrosion questions via examples
and practice problems.
Ecorris available at MaterialsTechnology@TMS:
http://materialstechnology.tms.org/educ/educdigital.asp
http://materialstechnology.tms.org/educ/educdigital.asphttp://materialstechnology.tms.org/educ/educdigital.asp7/28/2019 GWarren Corrosion 411
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Department of Metallurgical and Materials Engineering
What does Ecorr do?
The following screens give a number of examples.
Some previous exposure to thermodynamics is useful
The user interacts with the program in various ways:
Answers to questions or calculations are entered bytyping in boxes or by clicking buttons
Clicking on red hot text opens popup windows with
more information on that term, concept or calculation.
Standard potentials are available in a pull down menu Menu allows user to navigate to other parts of program
Any screen can be printed.
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Department of Metallurgical and Materials Engineering
Can you use potent ials to p redict react ions?
Below is one of several examples addressing this question for standardconditions. Potentials are hot text and remind the user how each was
obtained.
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Department of Metallurgical and Materials Engineering
What if act iv i t ies are no t un ity?
First the relation of G to E yields the Nernst equation. Activity, activitycoefficient and concentration are defined via hot text popup windows.
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Department of Metallurgical and Materials Engineering
What if act iv i t ies are no t un ity?After obtaining Es 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.
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Department of Metallurgical and Materials Engineering
Whats the significance of the reference electrode?
In principle any half cell can be selected as a reference, but only some areexperimentally convenient. When selected as a reference it is assigned a
value of zero volts, e.g. hydrogen or SCE shown below.
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Department of Metallurgical and Materials Engineering
How do I convert a po tent ial vs . SCE to
another reference electro de?Such conversions are simply adjusting the zero point on the potential scale
using the E value of the current reference electrode on the new scale.
Two more examples
involving different
reference electrodes
are given.
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Department of Metallurgical and Materials Engineering
Whats the difference between a half cell
po tent ia l and a co rros ion po tent ia l?
The diagram shows that a corrosion potential is a combination of
two half cells, the oxidation of Fe and the reduction of O2?
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Department of Metallurgical and Materials Engineering
Why is the cathod ic react ion impo rtant?
Several possible cathodic reactions exist. Knowing which one occursoffers different choices for limiting corrosion.
Red numbersreveal popup
windows that
show how the
value was
calculated.
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Department of Metallurgical and Materials Engineering
How can I determ ine the cathodic react ion?
The decision is a thermodynamic one. Through Nernst eqn calculationsthe user determines Sum A and Sum B, then selects an answer.
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Department of Metallurgical and Materials Engineering
Whats the difference between the Galvanic
Series & EMF Series?
After giving a
definition of each
series, the user
measures the
corrosion potentialfor each metal by
clicking & dragging
each one into the
white box.
This shows that
Ecorrs are notsingle half cells.
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Department of Metallurgical and Materials Engineering
Whats the difference between galvanic
corros ion and regular corros ion?
The difference is demonstrated with a movie that places the reduction
half cell on the surface of the more noble metal for galvanic corrosion.
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Department of Metallurgical and Materials Engineering
Jo ining diss im i lar metals is of ten necessary,
how is galvanic corros ion m inimized?
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Department of Metallurgical and Materials Engineering
Whats an example of a poor choice of two
diss im i lar metals?
Shown is one
example, for Fe
and brass.
User must enter
answers to
questions in
boxes.
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Department of Metallurgical and Materials Engineering
Whats an example of a poor choice of
relat ive areas?
Combining stainless
and Al is rarely a
good choice, but if
necessary oneoption is better than
the other.
The user must click
on the appropriate
image to answer.
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Department of Metallurgical and Materials Engineering
Sign convent ions are real ly con fusing, what
are my cho ices?
This section of
Ecorrcan be
omitted if desired.
It is probably
most useful foradvanced study.
nFE = non-IUPAC
+nFE = IUPAC
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Department of Metallurgical and Materials Engineering
How many possibi l i t ies are there?
The user can click
on each button,
work with the
same example for
each case and
compare them.
ONLY 4
permutations are
possible!
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Department of Metallurgical and Materials Engineering
How can I ever remember this?
Practice, practice,
practice!
Using the buttons
on this summary
screen the usercan review any of
the four possible
permutations.
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Department of Metallurgical and Materials Engineering
Galvanic Cell
Reactions occurspontaneously
when connected by a conductoror electrolyte.
Chemical energy is converted to
electrical energy.
Examples: AA battery, car battery
(when it is being discharged), i.e.
nearly every corrosion reaction
Electrolytic Cell
Reactions do not occur without
applying an external potential suchthat Eexternal > Ecell.
Electrical energy is used to cause
the desired chemical reaction.
Examples: electroplating of Cu,
Au, Ag, car battery (when it is
being charged)
This question is best answered by comparing one with the other.
Whats the difference between
an electrolyt ic cel l and a galvanic cel l?
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Department of Metallurgical and Materials Engineering
Isnt the anode always negative?
Zn Cu
Zn+2
soln Cu+2
soln
e-
ANODE CATHODE
- +
Zn = Zn+2+2e- Cu+2+2e- = Cu
oxidation reduction
Pt Pt
e-
ANODE CATHODE
-+
-
PS+
2H2O = O2+2H++2e- 2H++2e- = H2
oxidation reduction
O2 H2
Galvanic Cell Electrolytic Cell(electrolysis of water)
Absolutely not! See the two examples below.Neverassociate the sign of E with anode or cathode.
What is always true is anode = oxidation & cathode = reduction.
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Department of Metallurgical and Materials Engineering
Remember
Corrosion is inevitable. Only under impractical conditions
can it be 100% eliminated, but we can reduce or minimize it.
Ecorrprogram downloads are available through
MaterialsTechnology@TMS
http://materialstechnology.tms.org/educ/educdigital.asp
Contact information:
Comments are welcome, please reply via
MaterialsTechnology@TMS
http://materialstechnology.tms.org/educ/educdigital.asphttp://materialstechnology.tms.org/educ/educdigital.asp