Transcript Slide 1

Department of Metallurgical and Materials Engineering
Basic Corrosion:
Recurring Questions & Answers
Prof. Garry W. Warren
November 2007
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.
Department of Metallurgical and Materials Engineering
Recurring Questions

Most recurring questions asked fall into one of the
following 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”
Department of Metallurgical and Materials Engineering
What are the critical terms to know?

Electrochemistry (corrosion) is loaded with interrelated terms
somewhat unique to the discipline, for example:
Anode
Anodic
Active
Oxidation
Oxidation potential



Cathode
Cathodic
Noble
Reduction
Reduction potential
EMF series
Galvanic series
Electrolytic cell
Galvanic cell
Terms on left & right are related, but are NOT equal.
It is best to clearly define all of these from day one!
Department of Metallurgical and Materials Engineering
Critical Terminology (con’t)



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 of
this presentation).
Department of Metallurgical and Materials Engineering
What are the most important things to know
about the EMF Series?





Emphasize the title “Standard Reduction Potentials, Eº” or
“Standard Oxidation Potentials, Eº”
Each potential is tied to a half cell reaction.
Electrochemical reactions (corrosion) must involve two half
cells: one oxidation and one reduction.
The half cell assigned a voltage of zero is the “reference” half
cell.
Always include V vs. SHE or V vs. SCE (only then is choice
of reference half cell clear).
Department of Metallurgical and Materials Engineering
Is there a connection 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 text’s choice of –nFE or +nFE
is worthwhile for two reasons:
– To emphasize that this is the text’s 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.
Department of Metallurgical and Materials Engineering
Can I connect a sign convention to EMF Series?


Imagine three EMF series (no others are possible!).
The words “oxidation” or “reduction” with respect to ½ cell potentials
also indicates selection of –nFE convention.
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– = H2 0.0
Ni2+ +2e– = Ni – 0.250
Standard
Potentials
(G = +nFE)
Eº (V)
Cu2+/Cu + 0.342
H+/ H2
0.0
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”
Department of Metallurgical and Materials Engineering
Why doesn’t the sign of Eº change for +nFE?

Imagine three EMF series (no others are possible).
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– = H2 0.0
Ni2+ +2e– = Ni – 0.250
Standard
Potentials
(G = +nFE)
Eº (V)
Cu2+/Cu + 0.342
H+/H2
0.0
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.
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

Ecorr software
– An introduction to corrosion, electrode potentials & electrochemical thermodynamics.
– Focuses on many recurring corrosion questions via examples
and practice problems.

Ecorr is available at MaterialsTechnology@TMS:
http://materialstechnology.tms.org/educ/educdigital.asp
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 by
typing 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.
Department of Metallurgical and Materials Engineering
Can you use potentials to predict reactions?
Below is one of several examples addressing this question for standard
conditions. Potentials are hot text and remind the user how each was
obtained.
Department of Metallurgical and Materials Engineering
What if activities are not unity?
First the relation of G to E yields the Nernst equation. Activity, activity
coefficient and concentration are defined via hot text popup windows.
Department of Metallurgical and Materials Engineering
What if activities are not unity?
After applying the Nernst equation to half cells, several examples for overall
reactions are given. Standard potentials are obtained first as shown below.
Department of Metallurgical and Materials Engineering
What if activities are not unity?
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.
Department of Metallurgical and Materials Engineering
What’s the significance of the reference electrode?
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.
Department of Metallurgical and Materials Engineering
How do I convert a potential vs. SCE to
another reference electrode?
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.
Department of Metallurgical and Materials Engineering
What’s the difference between a half cell
potential and a corrosion potential?
The diagram shows that a corrosion potential is a combination of
two half cells, the oxidation of Fe and the reduction of O2?
Department of Metallurgical and Materials Engineering
Why is the cathodic reaction important?
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.
Department of Metallurgical and Materials Engineering
How can I determine the cathodic reaction?
The decision is a thermodynamic one. Through Nernst eqn calculations
the user determines Sum A and Sum B, then selects an answer.
Department of Metallurgical and Materials Engineering
What’s the difference between the Galvanic
Series & EMF Series?
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.
Department of Metallurgical and Materials Engineering
What’s the difference between galvanic
corrosion and regular corrosion?
The difference is demonstrated with a “movie” that places the reduction
half cell on the surface of the more noble metal for galvanic corrosion.
Department of Metallurgical and Materials Engineering
Joining dissimilar metals is often necessary,
how is galvanic corrosion minimized?
Department of Metallurgical and Materials Engineering
What’s an example of a poor choice of two
dissimilar metals?
Shown is one
example, for Fe
and brass.
User must enter
answers to
questions in
boxes.
Department of Metallurgical and Materials Engineering
What’s an example of a poor choice of
relative areas?
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.
Department of Metallurgical and Materials Engineering
Sign conventions are really confusing, what
are my choices?
This section of
Ecorr can be
omitted if desired.
It is probably
most useful for
advanced study.
–nFE = non-IUPAC
+nFE = IUPAC
Department of Metallurgical and Materials Engineering
How many possibilities 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!
Department of Metallurgical and Materials Engineering
How can I ever remember this?
Practice, practice,
practice!
Using the buttons
on this summary
screen the user
can review any of
the four possible
permutations.
Department of Metallurgical and Materials Engineering
What’s the difference between
an electrolytic cell and a galvanic cell?
This question is best answered by comparing one with the other.
Galvanic Cell
Electrolytic Cell
Reactions occur spontaneously
when connected by a conductor
or electrolyte.
Reactions do not occur without
applying an external potential such
that Eexternal > Ecell.
Chemical energy is converted to
electrical energy.
Electrical energy is used to cause
the desired chemical reaction.
Examples: AA battery, car battery
(when it is being discharged), i.e.
nearly every corrosion reaction
Examples: electroplating of Cu,
Au, Ag, car battery (when it is
being charged)
Department of Metallurgical and Materials Engineering
Isn’t the anode always negative?
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.
Electrolytic Cell
Galvanic Cell
(electrolysis of water)
e-
+
e-
-
Zn
Cu
+
Zn+2 sol’n
Cu+2 sol’n
ANODE
CATHODE
Zn = Zn+2+2eoxidation
Cu+2+2e- = Cu
reduction
+
PS
-
O2
Pt
ANODE
2H2O = O2+2H++2eoxidation
H2
Pt
-
CATHODE
2H++2e- = H2
reduction
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.
Ecorr program downloads are available through
MaterialsTechnology@TMS
http://materialstechnology.tms.org/educ/educdigital.asp
Contact information:
– Comments are welcome, please reply via
MaterialsTechnology@TMS