GWarren Corrosion 411

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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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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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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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/


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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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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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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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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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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.asp


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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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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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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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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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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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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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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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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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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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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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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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Jo ining diss im i lar metals is of ten necessary,

how is galvanic corros ion m inimized?


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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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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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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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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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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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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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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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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

GWarren Corrosion 411

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