Principle of advanced corrosion and corrosion control

1. Principle of advanced corrosion
and corrosion control

2. Exams and distribution of grades
midterm exam: 45 %
Final exam: 45%
Quizes 10%
Midterm exam date: first of khordad or May 22
References:
1corrrosion and corrosion control by H.H.
Uhlig.
2Corrosion Engineering by fantana and
Greene

3. Course contents
 1-Introduction, definition of
corrosion and its importance 2-
Electrochemical mechanism of
corrosion
 3-corrosion thermodynamics
 4-pourbaix diagrams
 5-Reference electrodes
 6-Kinetics of corrosion
 7- passivity of metals
 8-types of corrosion
 9-Methods of corrosion control

4. What is corrosion?
Importance of corrosion
conservation Safety Economics

5. Economics
Losses of metals, pipelines, tanks, machinery parts, ships,
briges, and so on
Safety
Performance of equipments like Tanks under pressure, boilers,
Metalic containers having radioactive materials, Turbine
blades, aireplanes and so on.
Conservation
Preserving of strategic metals sources like
chromium, Nickel and so on.

6. Economics
Direct losses
1- shutdown of the plants
2-losses of products
3Losses of efficiency
4Contamination of products
5-Overdesign
Indirect losses

7. 2-Electrochemical Mechanisms:
Dry cell
based on Faraday laws
K=M/nF ،W=KIt
W is the weight of corroded metel, in
gram.ٌ
L is current in ampere,
T is the time in seconds,
K is electrochemical equivalent constant, in
coulomb,
M is Molecular weight of substance

8. Definition of anode and cathode:
Anodic Reactions:
Zn=zn2++2e
Al=Al3++3e
Fe2+=Fe3++e
Cathodic Reaction:
H+ +2e=H2
Cu2+ +2e=Cu
Fe3+ +e=Fe2+

9. Types of cells:
1- Galvanic cells
2-concentration cells

10. 3-cells with different temperature
Corrosion thermodynamics
Tendency to Corroion and electrode potential
Free energy change
ΔGo=-142600 cal.
ΔGo=-28600 cal.
ΔGo=+15700 cal.
ّ
Mg+H2O (L)+1/2O2=Mg(OH)2 (S)
Cu+H2O (L)+1/2O2 (g) =Cu(OH)2 (s)
Au+3/2H2O (L) +3/2O2 (g) = Au(OH)3 (s)
ΔG=-nFE 1Cal=4.184 joules
F=96500 C
T1
T2

11. Nernst equation and calculation of half cell
lL+ mM +......=qQ + rR +…….
ΔG- ΔGo =[(qGQ-qGQ )+(rGR-rGR )+….]-[(lGL-lGL )+(mGM-mGM )+…..]
o o o o
،In equilibrium state
ΔG =0
K is equilibrium constant

12. When activity is 1
‫ﺎﯾ‬ ΔG – ΔGo =0
E=Eo
………………………………………………….
ΔG=-nEF
ΔGo =-nEoF
This is a Nernst equation and aL the activity of dissolved substance L can be written as
aL= γ x C
Concentration should be in molarity which is mole per 1000 gram of water.
γ is activity coeficientand it is a function of temperature and concentration
If L is a gas its activity is equal to fugacity
At low pressures, Fugacity=Pressure if pressure is in atomphere
1

14. ‫ﺲ‬ ‫ﭘ‬
For calculating the cell potential it is convenient to calculate the potentialof
each half cell by Nernst equation and then sum them algebriacally.
For example for half cell of zn =zn2++2e
Ezn=Eo - RT/2F ln (a 2+ )/a
zn zn zn
when temperature is at 25 degree celcius, we have, T=25+273=298oK then,
RT/2F ln =(2.303)x 8.314x298/2x96500 log=0.0592/2 log
Ezn = Eo -0.0592/2 log a 2+/a
zn zn zn
..................................................................................................................
Determination of corrosion tendency of a cell by Nernst
.equation:
First, you assume cu is anode and is corroded

15. =+
If Ecell =+ then our assumption is correct otherwise is not correct.
.........................................................................................
:
Ecell=
Therefore,

16. pH measurements
In water we have
aH
+=aOH
-
Therefore, at 25 0C , we have,
Kw= aH
+ x aOH
- =1x10-14
and pH can be calculated as:
pH=-log √KH2O = √1x10 =7.0
-14

17. Oxygen electrode and differential aeration cell
Oxygen electrode
Oxygen differential cell

18. Pourbaix diagram:
Inclined line includes electron , OH- andH+
For example,
2Fe2++ 3H2O=Fe2O3 +6H+ +6e
Oxygen evolution occurs above line b not under that lineas:
H2O=1/2O2 + 2H+ + 2e
Hyrogen evolution occurs under line a not above thatas:
H+=1/2H2 -e
pourbaix
A reaction which is not included OH- H+, the activity of ion in that reactionis 1x10-6
Foe example for reaction,
Fe=Fe2+ + 2e
Ф=-.44 +.0592/2 log (1x10-6) =-.62 v ،

19. Electromotive Force (EMF) and Galvanic series

20. Limitation on Electromotive Force series( EMF)
1Activity of 1 needs a certain concentration of metal salt in solution which
some times because of lack of enough solubility of that salt in solution is
impossible to to place that metal in EMF series.
2In practice, real activities of some ions in equilibrium state for a given metal
will change with the solution
3 EMF series does not consider the passive state of metal,
.4-alloys are not existed in this series
Galvanic series
In this series, the real potential of metal or alloys are measured in solution with
respect to reference electrode.