corrosion lec. 2

7/28/2019 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

10/04/2013 Corrosion Engineering - Lec 2 2


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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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Environment is the key factor in any

corrosion situation.

Practically, Environment refers to

microenvironment condition or local

environment.



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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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Metal ions go into solution at

anodic areas by the following

anodic reaction:

o Rapid reaction

o Anode employing external

current

o Iron corrodes


eFeFe 22


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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.


222 HeH


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If dissolved oxygen presents in

the solution an oxygen reduction

reaction will occur:


OHeOH 22 244


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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.


OHHOH2

222 )(222 OHFeOOHFe

2

)(OHFe


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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.


3222 )(42)(4 OHFeOOHOHFe

3)(OHFe


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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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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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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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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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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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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


pHFeLog 24.13][ 2


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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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In the presence of

hydrogen or

oxygen:

o Does Au corrode?o Does Ag corrode?

o Does Zn corrode?


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 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).


eCuCu 2

CueCu 2


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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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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!!


D

ieR 13.0


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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


corrosion lec. 2

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