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Introduction to Corrosion

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Contents

´Introduction to corrosion
´Definitions
´How it happens / Mechanisms
´Thermodynamics of corrosion

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Introduction to Corrosion

´Corrosion is an undesirable process
...


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Sub topics of introduction part
´Causes of Corrosion
´Classification or theories of Corrosion
´Consequences(Effects) of Corrosion
´Corrosion Control (Protection against Corrosion)

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Causes
of
Corrosion
´ In nature, metals occur in two different forms
...


ØExcept noble metals, all other metals
are highly reactive in nature
...

environment to form stable compounds
called ores and minerals
...

corrosion resistance
...
g
...


E
...
Fe 2 O3 , ZnO, PbS, CaCO3 , etc
...
Dry or Chemical Corrosion
(II) Wet or Electrochemical Corrosion
This type of corrosion is due to Electrochemical corrosion involves:
the direct chemical attack of
•The formation of anodic and cathodic
metal surfaces by the
areas or parts in contact with each other
...

•Presence of a conducting medium
...

•Silver materials undergo
•Formation
of
corrosion
product
chemical corrosion by
somewhere between anodic and cathodic
Atmospheric H2 S gas
...

•Iron metal undergo chemical
This involves flow of electron-current
corrosion by HCl gas
...


Difference between Dry and Wet corrosion
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Table 3: Difference Between Dry and Wet Corrosion

Dry Corrosion
Corrosion occurs in the absence of
moisture
...


It involves direct attack of
chemicals on the metal surface
...


The process is slow
...


Corrosion products are produced
at the site of corrosion
...


The process of corrosion is
uniform
...


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Consequences (Effects) Of Corrosion
The economic and social consequences of corrosion include
I
...

II
...

III
...

IV
...

V
...

VI
...
g
...


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Definitions and Types
´Corrosion is the deterioration or destruction of metals
and alloys in the presence of an environment by chemical
or electrochemical means
...
”

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Corrosion Types
´General Attack Corrosion
´Localized Corrosion
´Galvanic Corrosion
´Environmental Cracking
´Flow-Assisted Corrosion (FAC)
´Intergranular corrosion
´De-Alloying
´Fretting corrosion
´High-Temperature Corrosion

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How Corrosion Happens
Types of Dry or Chemical Corrosion:
1
...
Corrosion by Hydrogen
3
...
Galvanic (or Bimetallic) Corrosion
II
...

• Liberation of iron

Liquid metal
corrosion

This is due to chemical
action of flowing liquid
metal at high
temperatures on solid
2) Oxygen is converted to
metal or alloy
...

pressure, causing cracking or Both these modes of
oxidation corrosion process
...

corrosion cause
Metal + Oxygen → Metal oxide
weakening of the solid
(corrosion product)
metal
...
Formation of anodic and cathodic areas
II
...

Consider the example of iron
...
If enough oxygen is present, ferrous
+
2 H (aq) + 2 e → H2 ↑(g)
hydroxide is easily oxidized to ferric
The overall reaction is:
hydroxide
...
H2 O)
(II)
...

2+

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Thermodynamics of corrosion
´ What is Thermodynamic?
Thermodynamics is the study of the effects of work, heat, and energy on a
system
...

´ The relationship between the change in Gibb's free energy and the
Electromotive force (E
...
F) of an electrochemical cell is given by
ΔG= -nFE
ΔG° = −nFE°
´ The Nernst Equation
Ece ll = E0 ce ll - (RT/ nF)lnQ

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Measuring the corrosion potential

´The potential of a corroding metal, often termed Ecorr , is probably
the single most useful variable measured in corrosion studies as
well as during the corrosion monitoring of complex field
situations
...


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Figure 1: Experimental set-up to measure the corrosion potential of a specimen

´The reference electrode is contained in a Luggin capillary to
prevent any contamination of the reference electrode by the
environment or the opposite
...

´The effects of aqueous environments on metals became known as
potential-pH (E-pH) diagrams, also called predominance or
Pourbaix diagrams
...
However, since the concentrations of [H+]
and [OH-] ions are related by the dissociation constant of water, these
equations can be summarized in a Nernst equation
...

+

EH

/ H2

°

= E H+/ H2 - 0
...
059 pH
´ The chemical behavior of water across all possible values of
potential and p H is divided into three regions
...

´ Water is therefore only thermodynamically stable between lines

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Reference

http:/ / www
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edu/ 23429909/ UNIT_II_ CORROSION_AND_CONTROL
08/ 04/ 2019
http:/ / pdfs
...
org 07/ 04/ 2019
http:/ / scib
...
live/ CORROSION_PRINCIPLE II_MODIGIED

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