Materials Science
TEGS 3591
Corrosion and degradation
of Materials
Prof. O. T. Johnson
(Department of Mechanical and Metallurgical Engineering)
CORROSION AND DEGRADATION
ISSUES TO ADDRESS...
• Why does corrosion occur?
• What metals are most likely to corrode?
• How do temperature and environment affect
corrosion rate?
• How do we suppress corrosion?
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Definitions
❑ Corrosion is the surface wastage that occurs when
metals are exposed to reactive environments.
❑ Corrosion is the result of interaction between a metal
and environments which results in its gradual
destruction.
❑ Corrosion is an aspect of the decay of materials by
chemical or biological agents.
❑ Corrosion is an extractive metallurgy in reverse.
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Definitions
❑ Corrosion is the deterioration of materials as a result of
reaction with its environment.
❑ Corrosion is the destructive attack of a metal by
chemical or electrochemical reaction with the
environment.
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Corrosion
Corrosive Environment
❑ Air and humidity. ❑ Hydrogen sulphide
❑ Fresh, distilled, salt and marine
❑ Sulphur dioxide and oxides of
water.
nitrogen.
❑ Natural, urban, marine and
❑ Fuel gases.
industrial atmospheres.
❑ Acids.
❑ Steam and gases, like chlorine.
❑ Alkalies
❑ Ammonia
❑ Soils 9
Consequences of Corrosion
❑ Plant shutdowns – Shutdown of nuclear plants, process
plants, power plants and refineries may cause severe
problems to industry and consumers.
❑ Loss of products, leaking containers, storage tanks,
water and oil transportation lines and fuel tanks cause
significant loss of product and may generate severe
accidents and hazards. It is well-known that at least
25% of water is lost to leakage.
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Consequences of Corrosion
❑ Loss of efficiency – Insulation of heat exchanger tubings
and pipelines by corrosion products reduces heat
transfer and piping capacity.
❑ Contamination – Corrosion products may
contaminate chemicals, pharmaceuticals, dyes,
packaged goods, etc. with dire consequences to the
consumers.
❑ Nuclear hazard – The Chenobyl disaster is a continuing
example of transport of radioactive corrosive products
in water, fatal to human, animal and biological life.
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THE COST OF CORROSION
• Corrosion:
--the destructive electrochemical attack of a material.
--Al Capone's
ship, Sapona,
off the coast
of Bimini.
• Cost:
--4 to 5% of the Gross National Product (GNP)*
--this amounts to just over $400 billion/yr**
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Five good reasons to study
Corrosion
❑ Materials are precious resources of a country – Our
material resources are dwindling fast. To preserve these valuable
resources, we need to understand how these valuable resources
are destroyed by corrosion and how they must be preserved by
applying corrosion protection technology.
❑ Engineering knowledge is incomplete without an
understanding of corrosion – Aeroplanes, ships, automobiles
and other transport carriers cannot be designed without any
recourse to the corrosion behaviour of materials used in these
structures.
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Five good reasons to study
Corrosion
❑ Causes several engineering disasters– crashing of civil and
military aircraft, naval and passenger ships, explosion of oil
pipelines and oil storage tanks, collapse of bridges and decks
and failure of drilling platforms and tanker trucks have been
witnessed in recent years. Applying the knowledge of corrosion
protection can minimize such disasters.
❑ The designing of artificial implants for human body–
requires a complete understanding of the corrosion science and
engineering. Surgical impants must be very corrosion-resistant
because of corrosive nature of human blood.
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Five good reasons to study
Corrosion
❑ Corrosion is a threat to the environment– For example,
water can become contaminated by corrosion products and
unsuitable for consumption. Corrosion prevention is integral to
stop contamination of air, water and soil.
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CORROSION OF ZINC IN ACID
• Two reactions are necessary:
2+ −
-- oxidation reaction: Zn → Zn + 2e
+ −
-- reduction reaction: 2H + 2e → H2 (gas )
• Other reduction reactions:
-- in an acid solution -- in a neutral or base solution
O2 + 4H+ + 4e − → 2H2 O O2 + 2H2 O + 4e− → 4(OH) −
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STANDARD HYDROGEN (EMF) TEST
• Two outcomes:
--Metal sample mass --Metal sample mass
--Metal is the anode (-) --Metal is the cathode (+)
o o
Vmetal 0 (relative to Pt) Vmetal 0 (relative to Pt)
Standard Electrode Potential
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STANDARD EMF SERIES
• EMF series • Metal with smaller
Vo
metal metal o
Vmetal corrodes.
Au +1.420 V
+0.340 • Ex: Cd-Ni cell
Cu
Pb - 0.126
Sn - 0.136
Ni - 0.250
o
Co - 0.277 DV =
Cd - 0.403 0.153V
Fe - 0.440
Cr - 0.744
Zn - 0.763
Al - 1.662
Mg - 2.262
Na - 2.714
K - 2.924
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GALVANIC SERIES
• Ranks the reactivity of metals/alloys in seawater
Platinum
Gold
Graphite
Titanium
Silver
316 Stainless Steel
Nickel (passive)
Copper
Nickel (active)
Tin
Lead
Iron/Steel
Aluminum Alloys
Cadmium
Zinc
Magnesium
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Example
A thick steel sheet of area 400 cm2 is exposed to air near the
ocean. After a one-year period it was found to experience a
weight loss of 375 g due to corrosion. To what rate of
corrosion, in both mpy and mm/yr, does this correspond?
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CLASSWORK
A piece of corroded steel plate was found in a submerged
ocean vessel. It was estimated that the original area of the
plate was 10 in.2 and that approximately 2.6 kg had
corroded away during the submersion. Assuming a
corrosion penetration rate of 200 mpy for this alloy in
seawater, estimate the time of submersion in years. The
density of steel is 7.9 g/cm3.
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FORMS OF CORROSION
• Stress corrosion
Stress & corrosion
• Uniform Attack work together • Erosion-corrosion
Oxidation & reduction at crack tips. Break down of passivating
occur uniformly over layer by erosion (pipe
surface. elbows).
• Selective Leaching • Pitting
Preferred corrosion of Downward propagation
one element/constituent of small pits & holes.
(e.g., Zn from brass (Cu-Zn)).
• Intergranular
Corrosion along
grain boundaries, • Galvanic
often where special Dissimilar metals are
• Crevice Between two
phases exist. pieces of the same metal.
physically joined. The
Rivet holes
more anodic one
corrodes.(see Table
17.2) Zn & Mg
very anodic.
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Uniform Corrosion: Rust!
Prevention:
• Paint
• Plate
• Sacrificial anode
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Galvanic Corrosion
Causes:
Dissimilar metals
Electrolyte
Current Path
Described by Galvanic Series
Solutions:
Choose metals close in galvanic series
Have large anode/cathode ratios
Insulate dissimilar metals
Use “Cathodic protection”
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Pitting and Creviced Corrosion
Causes: concentration gradients in
Prevention:
electrolyte cause some areas high in ion
concentrations that accelerate oxidation Weld – don’t rivet
Use non-absorbing gaskets
Polish surfaces
Add drains – avoid stagnant water
Adjust composition; e.g., add Mo to SS
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Intergranular Corrosion
Occurs in specific alloys – precipitation of corrosive
specimens along grain boundaries and in particular
environments
e.g. : Chromium carbide forming in SS, leaving adjacent areas depleted in Cr
Solutions: High temp heat treat to redissolve carbides
Lower carbon content (in SS) to minimize carbide
formation
Alloy with a material that has stronger carbide
formation (e.g., Ti or Nb) 44
Erosion Corrosion
Causes: abrasive fluids impinging on
surfaces
Commonly found in piping, propellers,
turbine blades, valves and pumps
Solutions:
•Change design to minimize or eliminate fluid
turbulence and impingement effects.
•Use other materials that resist erosion
•Remove particulates from fluids
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Selective Leaching
• Occurs in alloys in which one element is
preferentially removed – e.g., in Brass,
Zinc is electrically active and is removed,
leaving behind porous Copper
• Occurs in other metals, such as Al, Fe,
Co, Cr
Solutions:
• Use protective coating to protect surfaces
• Use alternative materials
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Stress Corrosion
Aka: stress corrosion cracking
Cracks grow along grain
boundaries as a result of residual or
applied stress or trapped gas or
solid corrosion products
e.g., brasses are sensitive to
ammonia
Stress levels may be very low
Solutions: Reduce stress levels
Heat treatment
Atmosphere control
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Hydrogen Embrittlement
• Metals loose strength when Hydrogen is absorbed
through surface, especially along grain boundaries
and dislocations
• Often occurs as a result of decorative plating
• High strength steels particularly susceptible
• Can be removed by “baking” the alloy
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CONTROLLING CORROSION
• Self-protecting metals!
--Metal ions combine with O
to form a thin, adhering oxide layer that slows corrosion.
• Reduce T (slows kinetics of oxidation and reduction)
• Add inhibitors
--Slow oxidation/reduction reactions by removing reactants
(e.g., remove O2 gas by reacting it w/an inhibitor).
--Slow oxidation reaction by attaching species to
the surface (e.g., paint it!).
• Cathodic (or sacrificial) protection
--Attach a more anodic material to the one to be protected.
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Corrosion prevention
Sacrificial Anode Applied Voltage
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Surface coatings &
Passivation
Some materials, such as
Aluminum or Stainless
Steel, form oxide
barrier coatings that
prevent oxidation at
active surface – this is
called “passivation”
Surface can be coated with protective layers:
painted, anodized, plated (Caution!!! Cracks in
plating or paint can lead to crevice corrosion!)
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Schematic of the interface of
a passivating alloy surface in
contact with a biological
environment
Modular junction taper connection of a
total hip arthroplasty showing corrosion
of the taper connections. Macrograph of
deposits of CrPO4 corrosion particle
products on the rim of a modular Co-Cr
femoral head.
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Metal Degradative concerns
• High release of ionic metallic debris
• Toxicity: Metal-on-metal bearings are not recommended for
patients with poorly functioning kidneys because metal ions
excreted through the kidneys can build up in the blood.
• osteolysis and implant loosening in total hip patients with metal-on-
metal bearings may be associated with hypersensitivity to metallic
debris
• Surface replacement with metal on metal is a new technology that
has gained a great deal of recent interest. Hip surface replacement
preserves more bone in the patient than conventional hip
replacement. This has the potential of being a first-line treatment
of end-stage arthritis in younger, active patients.
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Metal on Polyethylene Bearings
• The adverse effects of oxidation during radiation
sterilization
• Polyethylene components, like most medical devices,
are sterilized by exposure to gamma radiation. The
radiation, while penetrating through the component,
has sufficient energy to break the chains that form the
molecular backbone of the polymer. If the radiation
exposure is performed while the component is exposed
to air, the broken ends can react with oxygen, causing
harmful changes, including a decrease in molecular
weight, a dramatic loss of ductility, and a decrease in
strength. The combined effect may make the
polyethylene markedly more susceptible to wear.
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Approaches to minimize
degradation of PE
• Placing polyethylene joint replacement components into
sealed packages that contain either a vacuum or an inert gas,
such as nitrogen or argon, instead of air.
• Replacing radiation altogether, instead exposing polyethylene
components to ethylene oxide or gas plasma, neither of which
imparts sufficient energy to cause oxidation.
• Increasing dose of radiation to promote crosslinking of
polymer chains
• early results show a dramatic decrease in wear of between 30 and 96
percent in total hip replacements over that seen with conventional
polyethylene.
• Disadvantage: increased crosslinking makes material more brittle
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SUMMARY
• Corrosion occurs due to:
--the natural tendency of metals to give up electrons.
--electrons are given up by an oxidation reaction.
--these electrons then are part of a reduction reaction.
• Metals with a more negative Standard Electrode
Potential are more likely to corrode relative to
other metals.
• The Galvanic Series ranks the reactivity of metals in
seawater.
• Increasing T speeds up oxidation/reduction reactions.
• Corrosion may be controlled by:
-- using metals which form -- adding inhibitors
a protective oxide layer -- painting
-- reducing T --using cathodic protection.
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