Corrosion Lec. 2
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Transcript of Corrosion Lec. 2
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CORROSIONENGINEERING
Corrosion Principles
Lecture No. 2
Samah Yousif
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Lecture Content
Introduction
How corrosion occurs?
Thermodynamic principles of corrosion Kinetics principles of corrosion
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Introduction
Material selection is a critical decision in material fabrication.
The most important characteristics to be considered in materialselection are:
1. Material properties
2. Temperatures effect on the mechanical properties
3. Corrosion resistance
4. Required properties
5. Ease of fabrication
6. Availability7. Cost
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How corrosion occurs?
Environment is the key factor in any
corrosion situation.
Practically, Environment refers to
microenvironment condition or local
environment.
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How corrosion occurs?
Corrosion occurs as a result of electrochemical
reactions combined with an electrical current
flows between anodes and cathodes.
As water and steels or iron-based alloys are
commonly used industrially, they will be used
for corrosion illustration.
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How corrosion occurs?
Metal ions go into solution at
anodic areas by the following
anodic reaction:
o Rapid reaction
o Anode employing external
current
o Iron corrodes
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eFeFe 22
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How corrosion occurs?
Free electrons combine with
Hydrogen ions in the cathodic
reaction:
o Slower reaction
o Proceeds rapidly in acids, and
slowly in alkaline or neutral
media.
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222 HeH
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How corrosion occurs?
If dissolved oxygen presents in
the solution an oxygen reduction
reaction will occur:
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OHeOH 22 244
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How corrosion occurs?
Adding the anodic reaction to the oxygen
reduction reaction, using ( ), leads
to:
oWhere is the Ferrous oxide,
oWhite when pure, but green to greenish black after
air oxidation.
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OHHOH2
222 )(222 OHFeOOHFe
2
)(OHFe
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How corrosion occurs?
At the outer surface of the oxide film, access to
dissolved oxygen converts ferrous oxide to
hydrous ferric oxide:
oHydrous ferric oxide is orange to red-
brown in color and makes up most of ordinary rust.
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3222 )(42)(4 OHFeOOHOHFe
3)(OHFe
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How corrosion occurs?
In summary, corrosion is an
electrochemical process with
two directions;
o Ions dissolving in the solution(oxidation or corrosion)
o Ions precipitation in the metal
(reduction)
These two processescontinue until equilibrium
achieved.
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How corrosion occurs?
Corrosion can be divided into:
1. Aqueous corrosion
2. Atmospheric corrosion
3. Hot corrosion
The above mechanism is applicable only for aqueous andatmospheric corrosion where an electrolyte surrounds themetal.
In high temperature corrosion only oxygen or gases surroundsthe metal.
In common, all corrosion types involve electrochemicalreactions.
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How corrosion occurs?
For corrosion to occur,reactions must continue tomaintain electrical equilibrium.
The speed of reactiondetermines the speed andseverity of corrosion and viceversa.
The speed of reaction ismeasured by the electricalcurrent (Icorr), as shown inEvans Diagram.
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How corrosion occurs?
The necessary components to set up an electrochemical cell (Metalliccorrosion):
1. Anode the corroding electrode.
2. Cathode the passive, non-corroding electrode.
3. The conducting medium the electrolyte the corroding fluid.
4. Completion of the electrical circuit through the material.
Cathodic areas can arise in many ways:o Dissimilar metals.
o Corrosion products.
o Inclusions in the metal, such as slag.
o Less well-aerated areas.
o Areas of differential concentration.
o Differentially strained areas.
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Thermodynamic principles in corrosion
Thermodynamics is used extensively in corrosion
studies.
We will focus on:
o Free energy change (G)
opH effect
o Potential difference and metal corrosion
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Thermodynamic principles in corrosion
Free energy change (G):
It is a quantitative measurement for reaction potential.
o G0 stable --- no corrosion
Q: How G is calculated for chemical and
electrochemical reactions?
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Thermodynamic principles in corrosion
pH effect:
From thermodynamics: the tendency to form Fe2+ or
Fe(OH)2 is a function of the pH.
o Acidic solution (low pH), corrosion increases.
oAlkaline solution (high pH), corrosion occurs slowly
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pHFeLog 24.13][ 2
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Thermodynamic principles in corrosion
Potential difference and metal corrosion:
o For corrosion to occur, the potential of the anodic
reaction should be less than the cathodic reaction
potential.
o Therefore, corrosion can be predicted by using the
EMF series, which shows the standard potential
taking the hydrogen as 0.
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Thermodynamic principles in corrosion
In the presence of
hydrogen or
oxygen:
o Does Au corrode?o Does Ag corrode?
o Does Zn corrode?
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Element Electrode Potential (V)
Au 1.7
O2 1.23
Pt 1.2
Ag 0.8
Cu 0.34
H2 0.0
Pb -0.13
Sn -0.14
Ni -0.25
Fe -0.44
Cr -0.74
Zn -0.76
Al -1.66
Mg -2.36
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Thermodynamic principles in corrosion
Thermodynamic principles can help explain a
corrosion situation (stability of chemical species
and reactions associated with corrosion
processes). However, thermodynamic
calculations cannot be used to predict corrosion
rates.
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Kinetic principles in corrosion
Potential difference cannot indicate the speed of corrosionas kinetic factors might effect the corrosion rate.
Eg. consider a Cu electrode immerged in CuSo4 solution,
o The anodic reaction:
o The cathodic reaction:
The reaction speed is estimated by Exchange CurrentDensity (Io) = Ia - Ic.
At equilibrium the speed of anodic and cathodic reactionsare equal, thus Ia = Ic and Io = 0 (corrosion doesnt occur).
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eCuCu 2
CueCu 2
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Kinetic principles in corrosion
Corrosion occurs in thefollowing cases:
Case 1:
o
Zn electrode connectedto Pt electrode immergedin a solution containshydrogen atoms.
Case 2:
o Zn electrode connectedto Cu electrode in anelectrochemical cell.
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Kinetic principles in corrosion
The speed of chemical reactions (corrosion) can be estimated byFaradays first and second laws .
The rate of corrosion can be calculated by:
Where:
R is the rate in mpy = mil per year (1mil = 0.001 inch)
i = current density A/cm2
e = equivalent weightD = density of Metal, g/cm3
Prove!!
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D
ieR 13.0
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Kinetic principles in corrosion
Factors affecting corrosion rate:
1- Metal properties:o Purity, crystal structure, metal surface, internal stresses,
etc
2- Surrounding environment:o Solution concentration and components: pH, purity, O2
presence,
o
Solution flow speedo Solution temperature
o Corrosion prevention
o Start-up and shutdown
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END OF LECTURE 2
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