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Kwame Nkrumah University of

Science & Technology, Kumasi, Ghana

MSE 453/METE 457 CORROSION OF ENGINEERING MATERIALS/

CORROSION AND CORROSION CONTROL

Ing. Anthony Andrews (PhD)Department of Materials Engineering

Faculty of Mechanical and Chemical Engineering

College of Engineering

Website: www.anthonydrews.wordpress.comwww.knust.edu.gh

Course Objectives1. The nature of interactions between materials and environments.

2. Fundamental understanding of electrochemistry relevant to

corrosion phenomena.

3. Identify various forms of corrosion.

4. Provide methodologies for predicting, measuring, and analysing

corrosion performance of materials.

5. Identify practices for the prevention and remediation of corrosion

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Prerequisite

• Knowledge of electrochemistry/physical chemistry

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Forms of Assessment

Quizzes 10

Mid-Semester Exam 20

End Semester Exam 70

Total 100

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Course OutlineWeek Topic

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Introduction:

Reactivity types, corrosion definition, forms, effects,

costs, risk management, control measures

1-3

Corrosion thermodynamics:

Corrosion reactions, cell requirements, free

energy change, electrochemical potential, Eh-pH

(Pourbaix) diagrams, Nernst equation, reference

electrodes

4-6

Corrosion kinetics:

Electrical double layer, exchange current density,

activation and mass transport control, mixed potential

theory, polarization diagrams, passivity

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Course Outline

Week Topic

8-9 Forms of corrosion:

Uniform corrosion, pitting corrosion, crevice

corrosion, environmentally induced cracking,

dealloying, intergranular corrosion, erosion corrosion

10-12 Corrosion Control:

Materials selection, thermodynamic control

(cathodic protection), kinetic control (anodic

protection and corrosion inhibitors), protective

coatings

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Recommended Books• No specific text book.

• M. G. Fontana: Corrosion Engineering, McGraw Hill, 1998.

• D. A. Jones: Principles and Prevention of Corrosion,

Macmillan Publ. Co., 1996.

• W. Revie (ed.): Corrosion Handbook, Electrochemical Society

Series, John Wiley and Sons, 2000.

• M. Kutz (ed): Handbook of Environmental Degradation of

Materials, W. Andrew Publ., 2005, NY.

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Reactivity of Engineering Materials• Sublimation

– M (s) → M (g)

• Dissolution– MA (s) → M+ (aq) + A- (aq)

• Oxidation (Dry corrosion)– xM (s) + ½yO2 (g) → MxOy (s)

• Aqueous (Wet) corrosion– M (s) + 2 HA (aq) → MA2 (aq) + H2 (g)

• Erosion-corrosion– Wear influenced corrosion (chemical-mechanical)

• Environmentally assisted cracking– Brittle fracture below yield strength (material-stress-

environment)

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Types of Degradation

• Degradation can be classified into three main categories:

1. Corrosion and oxidation

2. Wear

3. Radiation damage

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Introduction - What is Corrosion?

• Undesirable deterioration of a material due to a reaction with its environment.

– Deterioration => physical deterioration of the material and its properties and thus its performance.

– Generally refers to metals because• highly conductive

• electrochemical reaction

• Materials that can corrode – ceramics, metals, polymers, composites.

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Why Do Metals Corrode?

1. Metals corrode due to chemical pollution/contamination of the environment.

2. Chemically unstable in the environment.

– Metallic elements occur abundantly in nature in their ores (oxides)

– Extractive metallurgy in reverse.

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The Corrosion Cycle of Steel

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Metals and their oresMetal Ore

Aluminium Bauxite

Copper Copper pyrite

Iron Haematite, Magnetite

Lead Galena

Magnesium Magnesite, Dolomite

Mercury Cinnabar

Nickel Penllandite

Silver Argentite

Tin Cassiterite

Titanium Rutile, Ilmenite

Zinc Zinc blend, Calamine

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Corrosion resistant metals

• Precious metals (gold, silver, platinum) are found in nature in their metallic state.

• What do these metals have in common?

– Aluminium

– Stainless steel

– Chromium

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Corrosion resistant metals

Anodized titanium ear rings and stainless steel eye glasses.

Anodized Ti

SS

frame

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Degrading Environments

Oxidation Aqueous corrosion

Atmosphere

(rural, marine, industrial)

Atmosphere

(rural, marine, industrial)

Chemical – Gases

(H, O, C, S, Cl, N, etc.)

Soil

(pipelines, storage tanks)

Fresh Water

(potable, heat transfer, steam

generation)

Sea Water

Chemical – Liquids

(acids, bases, solvents)

Biological (biofilms, human body)

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Atmospheric Corrosion

• Corrosion of materials exposed to air and its pollutants,

rather than immersed in liquid

• Classification:

– Dry – very low humidity

– Dump – created at certain critical humidity level (adsorption of

water molecule)

– Wet – associated with dew, ocean spray, rainwater and other

forms of water splashing

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Types of atmospheric corrosion

1. Rural

– Least corrosive

– Does not contain chemical pollutants

– Contain organic and inorganic particulates

– Principal corrodents are moisture, oxygen and carbon dioxide

2. Urban

– Similar to rural type with little industrial activity

– Contaminants include SOx and NOx variety from motor

vehicles and domestic fuel emissions

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Types of atmospheric corrosion

3. Industrial

– Associated with heavy industrial processing facilities

– Contain concentration of SO2, chlorides, phosphates, nitrates

4. Marine

– Windswept chloride particles gets deposited on surfaces

– Highly corrosive and strongly depends on wind direction, wind

speed, and distance from coast

– Equivalent environment is created by use of deicing salt on the

road

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Theory of atmospheric corrosion

• Condition: Presence of electrolyte

• Proceeds by balancing anodic and cathodic reactions

– Anodic reaction – dissolution of the metal

• For Fe: 2Fe = 2Fe2+ + 4e

– Cathodic reaction – oxygen reduction reaction

• O2 + 2H2O + 4e = 4OH-

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Practical variables in atmospheric corrosion

1. Time of wetness (TOW)

– Directly determines the duration of the electrochemical

corrosion processes

– It depends on relative humidity

2. Sulphur dioxide

– Produce from combustion of sulphur containing fossil fuels

– It has high solubility in water

– Forms H2SO4 in surface moisture films

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Practical variables in atmospheric corrosion

3. Chlorides

– Enhance surface electrolyte formation by hygroscopic salts such as NaCl and MgCl2

– Increases atmospheric corrosion rates

4. Other atmospheric contaminants

– Hydrogen sulphide, hydrogen chloride, and chloride present in the atmosphere intensifies corrosion rate

5. Temperature

– Effect of temperature on atmospheric corrosion rate is complex

– Corrosion rate can increase or decrease www.knust.edu.gh

Classification of Corrosion

• Three major classification system:

– Nature of corrodant

• “Wet” corrosion versus “Dry” corrosion

– Mechanism of corrosion

• Chemical versus Electrochemical

– Appearance of corrosion

• Uniform versus Localised

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Why Worry?

• Waste of resources – energy, manpower and materials

– 1 ton of steel turns to rust every 90 seconds

– 380 kg CO2 emission for every tonne of steel

– 50% of every ton of steel is used to replace corroded steel

• Financial losses

– insurance companies paid out US$91 billion in the 1990s for natural disasters

– direct cost of corrosion in the USA in 2001 was US$276 billion (~4% GDP)

– 25% of corrosion cost can be saved using existing technology and knowledge

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Co$t of Corrosion

• Direct cost:

– Replacement of damaged/corroded equipment.

– Use of corrosion resistant alloys.

– Use of coatings and inhibitors.

– Electrochemical protection measures.

• Indirect cost

– Loss of production during plant downtime.

– Loss of product due to leakage.

– Loss of efficiency due to corrosion.

– Overdesign.

– Loss of human lives due to explosion, fire, etc.

Corrosion in Action

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Corrosion in Action

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Corrosion in Action

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Degradation of some common

materialsWood

• The environmental factors that affect degradation in

wood include:

• Biological organisms – fungi and insects

• Risk of wetting or permanent contact with water

– Wood is susceptible to attack when the moisture content

exceeds 20%

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Degradation of some common

materials

• Physical and mechanical effects of degradation in wood include:• Change in cross-sectional dimensions, swelling and shrinkage

• Strength and stiffness decrease as moisture content increases

• Durability is affected

• Coatings can be compromised

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Degradation of some common

materials

Plastics

• UV light will weaken certain plastics and produce a chalky faded

appearance on the exposed surface

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Degradation of some common

materialsMetals

• Most metals corrode because they react with oxygen in the

atmosphere, particularly under moist conditions – this is called

oxidation

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Degradation of some common

materialsMetals

• Ferrous metals are susceptible to oxidation and require ongoing

maintenance or they will suffer inevitable structural failure

• Choice of metal, environmental location and design features must

all be considered carefully

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Corrosion resistant metals

Metals

• Some non-ferrous metals are particularly resistant to corrosion

Copper Cladding Zinc Cladding

Corrosion Risk Management

Risk = Probability x Consequence ($)

What needs to be minimised?

Include in Design stage.

Include in Operation stage (maintenance).

Co

nse

qu

ence

Probability

4

3

2

1

A B C D

Extreme Risk –

Extensive risk controls

must be applied

High Consequence Risk –

Risk controls required

Moderate Risk – Some

risk controls required

Low Risk – Risk

controls may be justified

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Controlling Corrosion

• Materials selection

• Proper design

• Electrochemical protection (cathodic and anodic)

• Inhibitors (organic and inorganic)

• Paints and coatings (metallic and non-metallic)

• Environmental control