Engineering Failure Analysis

Module 5

Environmentally Induced
Temperature Failure
 Environmental Degradation of Materials
(Corrosion)
• Materials are “attacked” by their operating environment.
• We will focus on the degradation of metals. This is called
corrosion.

• In metals, corrosion is produced by the loss of actual

material, which leaves the piece as an ion in solution, and
is carried away by an electrolyte.
• Rust is a symptom of this problem in steel, but there can
be corrosion without it.
 What is Corrosion??

• Electrochemical reaction involving an anode and a cathode.

• Deterioration of a material because of reaction with the
environment.

• Combines many elements of engineering and impacts ALL

engineering disciplines: Chemical Engineering, Mechanical
Engineering, Material Engineering, Electrical Engineering
and Civil Engineering.
What is Corrosion??

• Corrosion involves the interaction (reaction) between

a metal or alloy and its environment. Corrosion is
affected by the properties of both the metal or alloy
and the environment. The environmental variables
include:

• pH (acidity)
• Oxidizing power (potential)
• Temperature (heat transfer)
• Velocity (fluid flow)
• Concentration (solution constituents)
 What is Corrosion??

• Combination of the material and it’s environment -

Examples:

• No Problem:
• Lead in Water
• Aluminum in atmosphere
• Nickel in hydraulic fluid

• BAD:
• Steel in marine environment
• Cu in Ammonia
• Stainless Steel(SS) in chloride (Sea water)
• Lead in wine
Please note the presence of
stainless steel:
• Yes, under certain circumstances, stainless
becomes active.

• Factors: (These are bad for any metal!)

1. Low aeration in water
2. Low velocity water
3. Presence of Cl-. Chlorine is one of the worst offenders
in promoting corrosion.
UNIVERSALITY OF CORROSION

• Not only metals, but non-metals like plastics, rubber,

ceramics are also subject to environmental degradation
• Even living tissues in the human body are prone to
environmental damage by free radicals-Oxidative stress-
leading to degenerative diseases like cancer and
diabetes.
 CORROSION DAMAGE
• Disfiguration or loss of appearance
• Loss of material
• Maintenance cost
• Extractive metallurgy in reverse- Loss of precious minerals, power, water and man-power
• Loss in reliability & safety
• Plant shutdown, contamination of product etc

The consequences of corrosion are many and varied and the effects of these
on the safe, reliable and efficient operation of equipment or structures are
often more serious than the simple loss of a mass of metal. Failures of
various kinds and the need for expensive replacements may occur even
though the amount of metal destroyed is quite small.
COST OF CORROSION

• Direct & Indirect losses:

• Direct loss: Material cost, maintenance cost,

over-design, use of costly material
• Indirect losses: Plant shutdown & loss of
production, contamination of products, loss of
valuable products due to leakage etc, liability
in accidents
THE COST OF CORROSION
• Corrosion:
-- the destructive electrochemical attack of a material.
-- Al Capone's
ship, Sapona, Corrosion
off the coast costs
of Bimini. India Rs 2
lakh crore
Photos courtesy L.M. Maestas, Sandia
National Labs. loss a year
• Cost:
-- 4 to 5% of the Gross National Product (GNP)*
-- this amounts to just over $400 billion/yr**

* H.H. Uhlig and W.R. Revie, Corrosion and Corrosion Control: An Introduction to
Corrosion Science and Engineering, 3rd ed., John Wiley and Sons, Inc., 1985.
**Economic Report of the President (1998).
Types of Aqueous Corrosion Cells

• General Corrosion
• Localized Corrosion
• Pitting
• Crevice Corrosion
• Under-deposit Corrosion
• MIC
• Tuberculation
• Galvanic Corrosion
General Corrosion
 General Corrosion
O • Random Creation and Destruction of
O e- Anodes and Cathodes
e- M+
e- • Movement of Anodes and Cathodes
M+
e- O • Near Uniform Thinning
O
• Weight Loss is a Useful Measure
OH-
-
OH e-
e- M+
M+ e-
e- O
O e-
M+

Source: Corrosion, ASM Handbook, Volume 13, 1987

General Corrosion
Original Surface

Penetration due to Corrosion

Localized Corrosion
Carbon Steel
Localized Corrosion
Carbon Steel
Localized Corrosion
• Stationary Electrodes
M(OH)n M(OH)n
OH- OH- • All of the dissolution occurs in one
CI- location
Mn+ • Weight loss measurement – not
Volts: Saturated Ca lom el Ha lf-Cell Referenc e Elec trod e

+ 0.4

+ 0.3
+ 0.2
+ 0.1

0
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8

-0.9
-1.0
-1.1
-1.2
-1.3
-1.4
-1.5
-1.6
-1.7
-1.8
Magnesium

Alum inum
Manganese

Cadm ium
Zinc

High Purity Iron

Alloy 255 (Ferralium )
Lead

Alloy PE 62
Copper

Carbon Steel
Alloy 26-1, 26-1 1/4

Alloy 18-18-2
Tin
MONEL alloy 451

Alloy 3RE60

INCOLOY alloy 840, 50 Ni - 50 Cr

Alloy 17-4pH

Nic kel Silver

Brass Alloys

90-10 Copper-Nic kel

70-30 Copper-Nic kel

Bronze Alloys

Alloy 70Cb3
Alloy 230 (Corronel)

Cast Iron
Alloy 20
Stainless Steel 304, 316, 316L, 317

Ni-Resist 2
Alloy 254 SLX

Alloy 904L

INCOLOY alloy 825

INCONEL alloys 600, 601, 690, 702, 748, X750

Alloy G
Alloy B, P, PD (Illium )

Alloy 6X (HA)
MONEL alloys 400, 404, 405R, K500

Alloy 700 (Jessop)

Nic kel 200, 270

Alloy G
Silver
Alloy 6X

INCOLOY alloy 800

Alum inum Alloy 5052

INCONEL alloys 617, 618E, 625

Titanium
Stainless Steel 430

+ 0.4-0.48V Platinum

Ga lva nic Series - Conc entra ted Hyd roc hloric Ac id a t 25°C
92345r1

[Crum and Sc a rb erry, Corrosion of Nic kel Base Alloys Conferenc e Proc eed ing s - ASM 1985]

useful
e e-
• Local Penetration
• Sometimes local weakening
• May or may not jeopardize
Source: Corrosion, ASM Handbook, Volume 13, 1987 structural integrity
• Determines “failure”
 Corrosion Samples
in
Different Applications
Underground corrosion

Buried gas or water supply pipes can suffer severe corrosion

which is not detected until an actual leakage occurs, by which
time considerable damage may be done.
 Electronic components

In electronic equipment it is very important that there should be no

raised resistance at low current connections. Corrosion products can
cause such damage and can also have sufficient conductance to cause
short circuits. These resistors form part of a radar installation.
 Corrosion influenced by flow-1

The cast iron pump impeller shown here

suffered attack when acid accidentally entered
the water that was being pumped. The high
velocities in the pump accentuated the corrosion
damage.
 Safety of aircraft

The lower edge of this aircraft skin panel has

suffered corrosion. Any failure of a structural
component of an aircraft can lead to the most
serious results.
Motor vehicle corrosion and safety

The safety problems associated with corrosion of motor vehicles is

illustrated by the holes around the filler pipe of this petrol
tank. The danger of petrol leakage is obvious. Mud and dirt
thrown up from the road can retain salt and water for prolonged
periods, forming a corrosive “poultice”.
 Corrosion at sea

Sea water is a highly corrosive electrolyte towards mild

steel. This ship has suffered severe damage in the areas which
are most buffeted by waves, where the protective coating of
paint has been largely removed by mechanical action.
 Aluminium Corrosion
The current trend for
aluminium vehicles is not
without problems. This
aluminium alloy chassis
member shows very
advanced corrosion due to
contact with road salt from
gritting operations or use in
coastal / beach regions.
Damage due to pressure of expanding rust

The iron reinforcing rods in this

garden fence post have been set
too close to the surface of the
concrete. A small amount of
corrosion leads to bulky rust
formation which exerts a pressure
and causes the concrete to
crack. For structural engineering
applications all reinforcing metal
should be covered by 50 to 75 mm
of concrete.
 “Corrosion” of plastics

Not only metals suffer

“corrosion”
effects. This dish is
made of glass fibre
reinforced PVC. Due
to internal stresses and
an aggressive
environment it has
suffered
“environmental stress
cracking”.
 Galvanic corrosion

This rainwater guttering is made of aluminium

and would normally resist corrosion
well. Someone tied a copper aerial wire
around it, and the localised bimetallic cell led
to a “knife-cut” effect.
 Galvanic corrosion

The tubing, shown here was part of an aircraft’s hydraulic system. The
material is an aluminium alloy and to prevent bimetallic galvanic
corrosion due to contact with the copper alloy retaining nut this was
cadmium plated. The plating was not applied to an adequate thickness
and pitting corrosion resulted.
 Galvanic corrosion

This polished Aluminium

rim was left over
Christmas with road salt
and mud on the rim.
Galvanic corrosion has
started between the
chromium plated brass
spoke nipple and the
aluminium rim.
 Galvanic corrosion

Galvanic corrosion can

be even worse
underneath the tyre in
bicycles used all winter.
Here the corrosion is so
advanced it has
penetrated the rim
thickness.
 Corrosion fatigue

- Fatigue in a corrosive environment

- Mechanical degradation of a material under the joint action of

corrosion and cyclic loading

- Nearly all engineering structures experience some form of

alternating stress - exposed to harmful environments during their
service life
- The environment plays a significant role in the fatigue of high-strength
structural materials like steel, aluminum alloys and titanium alloys -
Materials with high specific strength are being developed to meet the
requirements of advancing technology - their usefulness depends to a
large extent on the degree to which they resist corrosion fatigue

- The phenomenon should not be confused with stress corrosion cracking,

where corrosion (such as pitting) leads to the development of brittle
cracks, growth and failure

- The only requirement for corrosion fatigue is that the sample be under
tensile stress.
To meet the needs of advancing technology, higher-
strength materials are developed through heat treatment
or alloying

Such high-strength materials generally exhibit higher

fatigue limits - can be used at higher service stress levels
even under fatigue loading

Presence of a corrosive environment during fatigue

loading eliminates this stress advantage, since the fatigue
limit becomes almost insensitive to the strength level for
a particular group of alloys

This effect is schematically shown for several steels in the

diagram on the left, which illustrates the debilitating
effect of a corrosive environment on the functionality of
high-strength materials under fatigue.
Corrosion fatigue in aqueous media is an electrochemical behaviour

Fractures are initiated either by pitting or persistent slip bands

Corrosion fatigue may be reduced by alloy additions, inhibition and

cathodic protection, all of which reduce pitting

Since corrosion-fatigue cracks initiate at a metal's surface, surface

treatments like plating, cladding, nitriding and shot peening were
found to improve the materials' resistance to this phenomenon
 High-temperature corrosion

Chemical deterioration of a material (typically a metal) as a result of

heating

This non-galvanic form of corrosion can occur when a metal is subjected to

a hot atmosphere containing oxygen, sulfur, or other compounds capable of
oxidizing (or assisting the oxidation of) the material concerned

For example, materials used in aerospace, power generation and even in

car engines have to resist sustained periods at high temperature in which
they may be exposed to an atmosphere containing potentially highly
corrosive products of combustion.
The products of high-temperature corrosion can potentially be turned to
the advantage of the engineer

The formation of oxides on stainless steels, for example, can provide a

protective layer preventing further atmospheric attack, allowing for a
material to be used for sustained periods at both room and high
temperatures in hostile conditions

Such high-temperature corrosion products, in the form of compacted

oxide layer glazes, prevent or reduce wear during high-temperature
sliding contact of metallic (or metallic and ceramic) surfaces.
 Sulfates

Two types of sulfate-induced hot corrosion are generally

distinguished

Type I takes place above the melting point of sodium

sulphate

Type II occurs below the melting point of sodium sulfate

but in the presence of small amounts of SO3
 STRESS CORROSION CRACKING

Stress corrosion cracking (SCC) continues to be an important problem

for several industries, as the number of alloys–environment
combinations that cause SCC has been steadily increasing.

An alloy is said to have failed by SCC, if the cracking has occurred due
to a conjoint action of tensile stress and corrosive environment.

The role of stress along with the environment is one of synergy and both
must act simultaneously.
What is interesting and is beneficial from engineering perspective -
Only certain alloy–environment combinations can cause SCC.

Actually, the problem associated with SCC is not just the tendency of an
alloy to fracture as much as the way it fractures and the stress at which it
becomes susceptible to failure.

The fracture is mostly brittle in nature and an alloy can fail much below
its tensile strength. Hence, the failure can be rapid and can occur below
the stress levels specified by the codes.
 Characteristics of SCC

SCC process constitutes initiation and growth of cracks in an alloy - though

the final failure of a component always happens to be due to mere overload
mechanical failure.

Stress corrosion cracks are brittle in nature and they exhibit crack branching

Broadly speaking, grain boundary chemistry

and dislocation structure influence the nature of
cracking.
 KIc SCC KIscc
Alloy
MN/m3/2 environment MN/m3/2

13Cr steel 60 3% NaCl 12

18Cr-8Ni 200 42% MgCl2 10
Cu-30Zn 200 NH4OH, pH7 1
Aqueous
Al-3Mg-7Zn 25 5
halides
Ti-6Al-1V 60 0.6M KCl 20
 Effect of SCC on Mechanical Properties

Industrial components fail prematurely, if the conditions, viz., environment,

alloy metallurgy, temperature, and tensile stresses, favour SCC.

The tensile stress referred here would encompass both applied and residual
stress.

The residual stress can arise out of any of the following factors: fabrication
process, such as welding, grinding, machining, rolling, etc.
In addition, the accumulation of corrosion products within the wedges of a
component can also exert tensile stresses.

The presence of both of these stresses lowers the allowable operating stress levels.

SCC can severely affect allowable operating stresses and fracture toughness.

Notably, failure can occur even if the components experience stress levels much
below the yield strength of the alloy. Cases of components failure, even if they
were subjected to 20% of their yield strength, are known.

Stress corrosion cracking is a time-dependent process. For a given environmental

conditions the time to failure depends on the applied stress. The rise in stress
lowers the time to failure
 Factors Affecting SCC

The factors affecting SCC can be broadly classified into two types, those
concerning the metal/alloy and the others related to the environment.

However, the interaction of the environment with the metal produces

another factor, which is electrochemistry.
The following issues are the major concern for SCC resistance of
an alloy

(a) Nature of environment

(b) Passive film stability

(c) Metallurgy
i) Phases
ii) Grain boundaries
iii) Dislocations
iv) Grain size and orientation
 Controlling SCC
Several of the following methods can be judiciously applied to control SCC
1. Use alloys which are resistant to SCC
2. Eliminate species (such as chloride and oxygen) responsible for SCC
3. Lower the tensile stress. If it is residual, consider post-weld/work heat
treatment
4. Apply compressive stresses through methods such as shot-peening
5. Lower stress concentration in components
6. Apply coating
7. Apply cathodic protection
 DAMAGES DUE TO HYDROGEN

Hydrogen in metals/alloys such as steels, stainless steels and alloys of

aluminum, titanium, magnesium, zirconium causes premature cracking and
lowers ductility and toughness.

The mechanism of cracking is wide ranging and depends on material,

environment, loading conditions, temperature, etc.
Hydrogen damage is the generic name given to a large number of metal
degradation processes due to interaction with hydrogen.

Hydrogen is present practically everywhere, several kilometres above the

earth and inside the earth.

Engineering materials are exposed to hydrogen and they may interact

with it resulting in various kinds of structural damage.

“some remarkable changes produced in iron by the action of hydrogen

and acids”.
During the intervening years many similar effects have been observed in different
structural materials, such as steel, aluminium, titanium, and zirconium.

Because of the technological importance of hydrogen damage, many people

explored the nature, causes and control measures of hydrogen related degradation
of metals.

Hardening, embrittlement and internal damage are the main hydrogen damage
processes in metals.

Hydrogen may be picked up by metals during melting, casting, shaping and

fabrication. They are also exposed to hydrogen during their service life. Materials
susceptible to hydrogen damage have ample opportunities to be degraded during
all these stages.
Solid solution hardening

Metals like niobium and tantalum dissolve hydrogen and experience

hardening and embrittlement at concentrations much below their solid
solubility limit. The hardening and embrittlement are enhanced by increased
rate of straining.
Hydride embrittlement

In hydride forming metals like titanium, zirconium and vanadium, hydrogen

absorption causes severe embrittlement.

At low concentrations of hydrogen, below the solid solubility limit, stress-assisted

hydride formation causes the embrittlement which is enhanced by slow straining.

At hydrogen concentrations above the solubility limit, brittle hydrides are

precipitated on slip planes and cause severe embrittlement. This latter kind of
embrittlement is encouraged by increased strain-rates, decreased temperature and
by the presence of notches in the material.
Creation of internal defects

Hydrogen present in metals can produce several kinds of internal defects like
blisters, shatter fracture, flakes, fish-eyes and porosity.

Carbon steels exposed to hydrogen at high temperatures experience hydrogen attack

which leads to internal decarburization and weakening.
Blistering

Atomic hydrogen diffusing through metals may collect at internal defects like inclusions and
laminations and form molecular hydrogen.

High pressures may be built up at such locations due to continued absorption of hydrogen
leading to blister formation, growth and eventual bursting of the blister.

Such hydrogen induced blister cracking has been observed in steels, aluminium alloys,
titanium alloys and nuclear structural materials.

Metals with low hydrogen solubility (such as tungsten) are more susceptible to blister
formation.

While in metals with high hydrogen solubility like vanadium, hydrogen prefer to induce
stable metal-hydrides instead of bubbles or blisters.
It is possible to broadly classify the hydrogen-related failures into the
categories as below.

• Hydrogen cracking occurring at low temperature aqueous/wet conditions

Stepwise cracking (SWC) or hydrogen-induced cracking
Hydrogen embrittlement

• Metal damage occurring at elevated temperatures, in presence of

hydrogen containing gases and/or steam.
 Process Variables affecting Hydrogen-related Cracking Phenomena.

• Presence of water is essential - hydrogen is produced from water through

cathodic part of the corrosion reaction.

• Generally, wet hydrogen attack does not occur in neutral environments.

The environment needs to be acidic (pH below 4) if only H2S is present
and even alkaline environments (pH above 8) can be effective, if dissolved
cyanide is also present.

• H2S levels above 50 ppm are normally required for the attack.
• Most of the wet hydrogen-related attack occurs at operational
temperatures between ambient and about 1500C.

• Presence of Cl− and CO2 along with H2S, such as that occurring in oil
wells, accelerates hydrogen cracking phenomena through reduction of pH
and removing protective corrosion product scale.

• In addition to the above factors, pickling of steels, use of wet welding

electrodes, and even simple corrosion phenomena can introduce hydrogen
into the materials.

• The species such as sulfides, cyanides, arsenic, antimony, and

phosphorus in the corrosive environments or on the surface of the
material can enhance hydrogen permeation into the material.
 Controlling Low Temperature Hydrogen-Induced Cracking
• Reduce the proportion of nonmetallic inclusions by lowering the S content
• Modify the morphology of S segregation by adding Ca which spheroidize the nonmetallic
inclusions
• Lower the tensile stresses
• Providing right tempering heat treatment in the case of steel to eliminate detrimental effect of
martensite in steels. In the case of aluminium alloys provide a overaging treatment.
• Prefer the presence of Cu (>0.2%) as an alloying element
• Prefer plate mill products over hot strip mill products for the steel
• Avoid hydrogen entry into steel during surface treatment and fabrication
• If the above is not possible, bake the components to remove the hydrogen.
 High Temperature Hydrogen Damage/Decarburization

At elevated temperatures, well above 150∘C, both molecular and atomic hydrogen,
if present on steel surface, can react with carbides leading to decarburization and
methane formation.

The steel can suffer two types of decarburization/hydrogen attack.

The presence of hydrogen environment over industrial components is sometimes

intentional and in some other times unintentional.

Hydrocarbon industries deal with large quantities of hydrogen at various

plants/units, which is intentional.

On the other hand, hydrogen generated as a result of steam/water reacting/corroding

with steel in a boiler is unintentional.
 Creep of metallic materials

Tendency of a solid material to move slowly or deform permanently under

the influence of mechanical stresses.

It can occur as a result of long-term exposure to high levels of stress that are
still below the yield strength of the material.

Creep is more severe in materials that are subjected to heat for long periods,
and generally increases as they near their melting point.
The rate of deformation is a function of the material's properties, exposure
time, exposure temperature and the applied structural load.

Creep is usually of concern to engineers and metallurgists when evaluating

components that operate under high stresses or high temperatures.

Unlike brittle fracture, creep deformation does not occur suddenly upon
the application of stress. Instead, strain accumulates as a result of long-
term stress. Therefore, creep is a "time-dependent" deformation.
 Temperature Dependence

The temperature range in which creep deformation may occur differs in

various materials.

Lead can creep at room temperature

Tungsten requires a temperature in the thousands of degrees before creep

deformation can occur, while ice will creep at temperatures near 0 °C
(32 °F).
As a general guideline, the effects of creep deformation generally become
noticeable at approximately 35% of the melting point and at 45% of melting
point for ceramics.

Virtually any material will creep upon approaching its melting temperature.

Since the creep minimum temperature is related to the melting point, creep
can be seen at relatively low temperatures for some materials.

Plastics and low-melting-temperature metals, including many solders, can

begin to creep at room temperature, as can be seen markedly in old lead hot-
water pipes.

Glacier flow is an example of creep processes in ice.

Stages of creep
 Service failures during high temperature

The creep rate of hot pressure-loaded components in a nuclear reactor at

power can be a significant design constraint, since the creep rate is enhanced
by the flux of energetic particles.

Creep in epoxy anchor adhesive was blamed for the Big Dig tunnel ceiling
collapse in Boston, Massachusetts that occurred in July 2006.

In steam turbine power plants, pipes carry steam at high temperatures (566 °C
(1,051 °F)) and pressures (above 24.1 MPa or 3500 psi).
In jet engines, temperatures can reach up to 1,400 °C (2,550 °F) and initiate creep
deformation in even advanced-design coated turbine blades. Hence, it is crucial for
correct functionality to understand the creep deformation behavior of materials.

In the design of many everyday objects - for example, metal paper clips are stronger
than plastic ones because plastics creep at room temperatures.

The design of tungsten light bulb filaments attempts to reduce creep deformation.
Sagging of the filament coil between its supports increases with time due to the
weight of the filament itself. If too much deformation occurs, the adjacent turns of the
coil touch one another, causing an electrical short and local overheating, which
quickly leads to failure of the filament. The coil geometry and supports are therefore
designed to limit the stresses caused by the weight of the filament.
 Wear Failures

Bicycle sprockets – (left is new one and right is used one)

Wear is the damaging, gradual removal or deformation of material at solid
surfaces.

Causes of wear can be mechanical (e.g., erosion)

or chemical (e.g., corrosion).

The study of wear and related processes is referred to as tribology.

Wear in machine elements, together with other processes such

as fatigue and creep, causes functional surfaces to degrade, eventually
leading to material failure or loss of functionality. Thus, wear has large
economic relevance.
Wear of metals occurs by plastic displacement of surface and near-surface
material and by detachment of particles that form wear debris.

The particle size may vary from millimeters to nanometers. This process may
occur by contact with other metals, nonmetallic solids, flowing liquids, solid
particles or liquid droplets entrained in flowing gasses.

The wear rate is affected by factors such as type of loading (e.g., impact,
static, dynamic), type of motion (e.g., sliding, rolling), and temperature.
Depending on the tribosystem, different wear types and wear mechanisms can
be observed.