INTRODUCTION TO CORROSION
SCIENCE AND ENGINEERING-UNIT-1
CHE-545-172
DR IME B.OBOT
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Corrosion Science and Engineering
Corrosion
Science Engineering
Thermodynamics: Kinetics: Predict Forms of
Predicts Corrosion Speed of Corrosion and
Tendency Corrosion Control Methods
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I-DEFINITION OF CORROSION
Corrosion is the destructive attack of a metal by chemical or
electrochemical reaction with its environments.
In some cases chemical attack accompanies physical
deterioration as described by the following terms: corrosion-
erosion; corrosive wear or fretting corrosion.
Non-metals are not included in this definition of corrosion
The term corrosion is restricted to the chemical attack on
metals.
Rusting applies to the corrosion of iron or iron based alloys
with formation of corrosion products consisting largely of
hydrous ferric oxides
Nonferrous metals, therefore, corrode, but do not rust.
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II-Corrosion Science VS Corrosion Engineering
The principles of Chemistry must be understood in order to
understand corrosion reactions. This is because corrosion processes
are mostly electrochemical in nature.
The “Corrosion Scientist” studies corrosion mechanisms to improve:
The understanding of the causes of corrosion.
The ways to prevent or atleast minimize damage caused by
corrosion.
Develops better criteria for cathodic protection.
Outlines the molecular structure of corrosion inhibitors
The “Corrosion Engineer” applies scientific knowledge to control
corrosion:
Cathodic protection to protect buried pipelines.
Test and develop new and better paints.
Prescribes proper dosages of corrosion Inhibitors.
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Relationship between Corrosion Science and
Engineering 5
Why do we study Corrosion
Human life and safety
The cost of corrosion is enormous
Conservation of Materials
Corrosion is Inherently a difficult
phenomenon to understand, and its
study in itself is challenging and
Interesting pursuit
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III-THE ECONOMICS OF CORROSION
Annual Global Cost of Cost of Corrosion:
Corrosion= $2.5 Trillion; Direct Cost
amounts to 3-4% of Capital Cost
global GDP (2016) NACE
Report. Control Cost
Annual Cost of corrosion Design Cost
in Saudi Arabia = 80 Indirect Cost
Billion SAR (2011) Shutdown
Report. Loss of products
Annual cost of Corrosion Contamination of
in USA= $276 B (2001) Products
Report.
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IV-RISK MANAGEMENT
Risk, R, is defined as the
probability ,P, of an
occurrence multiplied by
consequences, C, of the
occurrence.
R=P×C
The Risk of a corrosion
related failure equals the
probability that such a
failure will occur
multiplied by the
consequences of that
failure
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V-Challenges for Todays Corrosion
Scientist and Engineers
The development of protective surface treatment and corrosion
Inhibitors which are environmentally friendly.
An improved conservation of materials through the development of
corrosion resistant surface alloys which confine alloying elements to the
surface rather than employing conventional bulk alloying.
The formulation of new generation of stainless steels containing
replacement of chromium and other critical metals
An improve understanding of passivity
Understanding the mechanism of the breakdown of passive oxide films
by chloride ions and subsequent pitting of the underlying metals
Prediction of lifetime of metals and components from short-term
experimental corrosion rate
Development of smart self healing coatings
Development of high Temperature corrosion Inhibitors
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Brief Review of Corrosion Basics
PHYSICAL PROCESSES OF DEGRADATION
Fracture-Failure of a metal under an
applied stress.
Fatigue-Failure of a metal under an
applied repeated cyclic stress.
Wear-Rubbing or sliding of materials
on each other
Erosion or cativation erosion-
Mechanical damage caused by the
movement of a liquid or a collapse
of vapor bubbles against a metal
surface
Radiation damage-Interaction of
elementary particles (e.g. neutrons
or metal ions with a solid metal so as
to distort the metal lattice
Ship Near Waterline
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Electrochemical Reactions
Corrosion is an electrochemical process which operates through coupled
electrochemical half-cell reactions.
Half Cell Reaction: This consist of oxidation and reduction half cell
reactions. Or anodic and cathodic half cell reaction.
Anodic Reactions-Here electrons are produced on the right hand side of
the reaction. There is loss of electrons and an increase in oxidation
number. Also loss of the metals occurs on the anodic reaction.
Fe (s) → Fe2+(aq) + 2e-
Al(s) → Al3+(aq) + 3e-
Cathodic Reactions-Here electrons are produced on the left hand side of
the reaction. There is gain (consumption of electrons and a decrease in
oxidation number.
2H+(aq) + 2e- → H2(g)
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Coupled Electrochemical Reaction
On the corroding metal surface,
anodic and cathodic reactions
occur in a coupled manner at
different places on the metal
surface. The overall chemical
reaction is thus the sum of the
two half-cell reactions. Iron in acidic solution
Conditions necessary for
Corrosion to occur:
(1) An anodic reaction
(2) A cathodic reaction
(3) A metallic path of contact
between anodic and cathodic
sites
(4) The presence of an electrolyte
Iron in neutral or basic solution 12
Three simple properties of solution
pH is a measure of the acidity (or alkalinity) of a solution. It is defined as:
pH=-log[H+] = log 1/[H+]
For Neutral solution, pH =7
For Acidic solution, pH ˂ 7
For Alkaline or basic solution, pH ˃ 7
Note: Most solutions have pH between 0-14.
But lower or higher pH are possible (e.g. 12M HCl has a pH =-1.1)
Ionization Constant for water:
Kw=[H+][OH-] = 1.0 x 10-14
Solubility Products:
When an ionic solid (acid, base, or salt) dissociates into ions in solution;
AxBy(s) ↔ xAy+(aq) + yBx-(aq) If the ion product exceeds the
tabulated value of Ksp, then
The solubility product Ksp is defined as: precipitation of solid AxBy occurs.
Ksp = [Ay+]x × [Bx-]y Otherwise, precipitation does not
occur. 13
The Faraday and Faradays Law
One mole of electrons, contains Avogadro’s number
(6.023x1023) electrons
Charge on one electron is 1.6 ×10-19Coulomb
Hence the charge on one mole of electrons= 6.023x1023 ×
1.6x10-19= 96500 C or Faraday
The process of corrosion involves simultaneous charge
transfer and mass transfer across metal/solution
interface. This link between charge transfer and mass
transfer is known as Faraday (F):
F=96500 C/(mole of e) or F= 96,500/equivalent
Faradays law states that the mass (w) of metal corroded is
given by
W= ItA/nF
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