Download General - Department of Mechanical Engineering

Constraints on materials
 Temperature
 Galvanic compatibility
 Atomic oxygen
 Moisture absorption/desorption
 Fluid compatibility
 Thermal cycling
 Chemical (corrosion)
Constraints on materials, cont..
 Temperature
 The range of temperatures experienced will play a large part in the
materials selection. Extremes are illustrated by the examples of cryogenic
tanks and thermal protection systems for re-entry applications.
Temperatures below room temperature generally cause an increase in
strength properties, however the ductility decreases. Ductility and
strength may increase or decrease at temperatures above room
temperature. This change depends on many factors, such as temperature
and time of exposure.
 Materials shall be compatible with the thermal environment to which they
are exposed.
 Passage through transition temperatures (e.g., brittle-ductile transitions
or glass transition temperatures including the effects of moisture or other
phase transitions) shall be taken into account.
Constraints on materials cont..
Galvanic compatibility
 If two or more dissimilar materials are in direct electrical contact in a
corrosive solution or atmosphere galvanic corrosion might occur. The less
resistant material becomes the anode and the more resistant the cathode.
The cathodic material corrodes very little or not at all, while the corrosion
of the anodic material is greatly enhanced.
Atomic oxygen
 Materials are exposed to a flux of atomic oxygen. The flux level varies
with altitude, velocity vector and solar activity. The fluence levels vary
with the duration of exposure.
Moisture absorption/desorption
 The properties of materials are susceptible to changes induced by the
take-up of moisture. Moisture absorption occurs during production and
application of components.
Constraints on materials cont..
 Fluid compatibility
 In some occasions materials are in contact with liquid oxygen, gaseous oxygen or other
reactive fluids or could come into contact with such a fluid during an emergency
situation.
 Thermal cycling
 Thermal cycling can induce thermal stresses and due to the difference in coefficient of
thermal expansion for example between fibres and matrix for composites and between
base metal and coating micro-cracks can form which could jeopardise long-term
properties.
 Materials subject to thermal cycling shall be assessed to ensure their capability to
withstand the induced thermal stresses and shall be tested.
 Chemical (corrosion)
 The chemical environment to which a material is subjected in its life span may cause
changes in the material properties. Corrosion is the reaction of the engineering
material with its environment with a consequent deterioration in properties of the
material. Corrosion will include the reaction of metals, glasses, ionic solids, polymeric
solids and composites with environments that embrace liquid metal, gases, nonaqueous electrolytes and other non-aqueous solutions, coating systems and adhesion
systems.
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 stressleading 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
WHY DO METALS CORRODE?
 Any spontaneous reaction in the universe is
associated with a lowering in the free energy of the
system. i.e. a negative free energy change
 All metals except the noble metals have free energies
greater than their compounds. So they tend to
become their compounds through the process of
corrosion.
Fundamental Components
 Corrosion can be defined as the deterioration of material by reaction to
its environment.
 Corrosion occurs because of the natural tendency for most metals to
return to their natural state; e.g., iron in the presence of moist air will
revert to its natural state, iron oxide.
 4 required components in an electrochemical corrosion cell:
1) An anode; 2) A cathode; 3) A conducting environment for ionic
movement (electrolyte); 4) An electrical connection between the anode and
cathode for the flow of electron current.
 If any of the above components is missing or disabled, the
electrochemical corrosion process will be stopped.
Example – The Daniell Cell
 This example illustrates some of the basics of corrosion.

On the surface of the Zn
bar we have the following
2
Zn  Zn  2 e


On the surface of the Cu bar
we have the following
2

Cu  2 e  Cu
Note the current path. The salt bridge provides for ion exchange.
Driving force
 A driving force is necessary for electrons to flow between the anodes
and the cathodes.
 The driving force is the difference in potential between the anodic and
cathodic sites.
 This difference exists because each oxidation or reduction reaction
has associated with it a potential determined by the tendency for the
reaction to take place spontaneously. The potential is a measure of
this tendency.
Summary:
What’s needed for Corrosion:
1.
2.
3.
4.
An anode. This is where the damage occurs. Oxidation takes place.
A cathode. Here’s where the reduction reaction takes place.
An electrolyte. (Almost any moisture will do.)
A current path between the cathode and anode.
ELECTROCHEMICAL NATURE
 All metallic corrosion are electrochemical reactions i.e.
metal is converted to its compound with a transfer of
electrons
 The overall reaction may be split into oxidation (anodic)
and reduction (cathodic) partial reactions
 Next slide shows the electrochemical reactions in the
corrosion of Zn in hydrochloric acid
ELECTROCHEMICAL REACTIONS
IN CORROSION
REDOX reactions

Here is an oxidation reaction. Fe is the symbol for iron.
Note that metal looses electrons.
Fe  Fe 2  2 e 
 Here is a typical reduction reaction involving hydrogen ions
in solution. Note that the H gains electrons.


2 H  2e  H2
These Reactions want to occur in Pairs
We are assuming that the Fe is
surrounded by a weak acid in which
H+ ions are abundant.
This acid is called an electrolyte. It
provides a home for the dissolve Fe+2
ion.
Note that there has to be an internal
movement of electrons through the
Fe.
General Reactions
 Anode: (Metal basically dissolves in the electrolyte.)
M M
n
 ne

 Cathode: (This is a very common reaction!)
2 H 2O  O2  4 e  4 OH 


Surfaces near high O2 concentration are cathodic!
PASSIVATION
 Many metals like Cr, Ti, Al,
Ni and Fe exhibit a reduction
in their corrosion rate above
certain critical potential.
Formation of a protective,
thin oxide film.
 Passivation is the reason for
the
excellent
corrosion
resistance of Al and S.S.
Galvanic Series
more cathodic
(inert)
more anodic
(active)
• Ranking the reactivity of metals/alloys in seawater
Platinum
Gold
Graphite
Titanium
Silver
316 Stainless Steel (passive)
Nickel (passive)
Copper
Nickel (active)
Tin
Lead
316 Stainless Steel (active)
Iron/Steel
Aluminum Alloys
Cadmium
Zinc
Magnesium
Relationship between the rate of corrosion, corrosivity of an
environment and corrosion resistance of a material.
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.
Corrosion influenced by flow – 2
This is a bend in a copper pipe-work cooling
system. Water flowed around the bend and then
became turbulent at a roughly cut edge.
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.
Influence of corrosion on value
A very slight amount of corrosion may not interfere with the usefulness of
an article, but can affect its commercial value. At the points where these
scissors were held into their plastic case some surface corrosion has
occurred which would mean that the shop would have to sell them at a
reduced price.
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.
Significance of Corrosion
on Infrastructure
Engineer finds corrosion in collapsed bridge at
North Carolina speedway (2000)
Corrosion & Catastrophic Failure
A Concrete bridge failure