CORROSION
INTRODUCTION
Definition: Corrosion
is the degeneration of
materials by reaction
with environment.
Examples: Rusting of
automobiles,
buildings and
bridges, Fogging of
silverware, Patina
formation on copper.
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, cardio-vascular disease 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
COST OF CORROSION
Annual loss due to corrosion is estimated
to be 3 to 5 % of GNP, about Rs.700000
crores
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
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
DISSOLUTION OF ZN METAL IN HYDROCHLORIC ACID,
222 HZnClHClZn +=+ -------------------- -(1)
Written in ionic form as,
2
2 222 HClZnClHZn ++=++ −+−+ ----------------------(2)
The net reaction being,
2
22 HZnHZn +=+ ++ ------------------------- (3)
Equation (3) is the summation of two partial reactions,
eZnZn 2*2 +→ -----------------------------------------(4) and 222 HeH →++ ------------------------------------------(5)
Equation (4) is the oxidation / anodic reaction and
Equation (5) is the reduction / cathodic reaction
ELECTROCHEMICAL THEORY
The anodic & cathodic reactions occur simultaneously at different parts of the metal.
The electrode potentials of the two reactions converge to the corrosion potential by polarization
PASSIVATION
Many metals like Cr, Ti,Al, Ni and Fe exhibit areduction in theircorrosion rate abovecertain critical potential.Formation of aprotective, thin oxidefilm.
Passivation is thereason for the excellentcorrosion resistance of Aland S.S.
FORMS OF CORROSION
Corrosion may be classified in different ways
Wet / Aqueous corrosion & Dry Corrosion
Room Temperature/ High Temperature Corrosion
CORROSION
WET CORROSION DRY CORROSION
CORROSION
ROOM TEMPERATURE
CORROSION
HIGH TEMPERATURE
CORROSION
WET & DRY CORROSION
Wet / aqueous corrosion is the major form of
corrosion which occurs at or near room
temperature and in the presence of water
Dry / gaseous corrosion is significant mainly
at high temperatures
WET / AQUEOUS CORROSION
Based on the appearance of the corroded metal, wet corrosion may be classified as
Uniform or General
Galvanic or Two-metal
Crevice
Pitting
Dealloying
Intergranular
Velocity-assisted
Environment-assisted cracking
UNIFORM CORROSION
Corrosion over the entire exposed surface at a uniform rate. e.g.. Atmospheric corrosion.
Maximum metal loss by this form.
Not dangerous, rate can be measured in the laboratory.
GALVANIC CORROSION
When two dissimilar metals are joined together and exposed, the more active of the two metals corrode faster and the nobler metal is protected. This excess corrosion is due to the galvanic current generated at the junction
Fig. Al sheets covering underground Cu cables
CREVICE CORROSION
Intensive localized
corrosion within
crevices & shielded
areas on metal
surfaces
Small volumes of
stagnant corrosive
caused by holes,
gaskets, surface
deposits, lap joints
PITTING
A form of extremely
localized attack
causing holes in the
metal
Most destructive form
Autocatalytic nature
Difficult to detect and
measure
Mechanism
DEALLOYING
Alloys exposed to corrosives experience selective leaching out of the more active constituent. e.g. Dezincification of brass.
Loss of structural stability and mechanical strength
INTERGRANULAR CORROSION
The grain boundaries in
metals are more active
than the grains because
of segregation of
impurities and depletion
of protective elements.
So preferential attack
along grain boundaries
occurs. e.g. weld decay in
stainless steels
VELOCITY ASSISTED
CORROSION
Fast moving
corrosives cause
a) Erosion-Corrosion,
b) Impingement
attack , and
c) Cavitation damage
in metals
CAVITATION DAMAGE
Cavitation is a special
case of Erosion-corrosion.
In high velocity systems,
local pressure reductions
create water vapour
bubbles which get
attached to the metal
surface and burst at
increased pressure,
causing metal damage
ENVIRONMENT ASSISTED
CRACKING
When a metal is subjected to a tensile stress and
a corrosive medium, it may experience
Environment Assisted Cracking. Four types:
Stress Corrosion Cracking
Hydrogen Embrittlement
Liquid Metal Embrittlement
Corrosion Fatigue
STRESS CORROSION
CRACKING
Static tensile stress and specific environments produce cracking
Examples:
1) Stainless steels in hot chloride
2) Ti alloys in nitrogen tetroxide
3) Brass in ammonia
HYDROGEN EMBRITTLEMENT
High strength materials stressed in presence of hydrogen crack at reduced stress levels.
Hydrogen may be dissolved in the metal or present as a gas outside.
Only ppm levels of H needed
LIQUID METAL
EMBRITTLEMENT
Certain metals like Al and stainless steels undergo brittle failure when stressed in contact with liquid metals like Hg, Zn, Sn, Pb Cd etc.
Molten metal atoms penetrate the grain boundaries and fracture the metal
Fig. Shows brittle IG fracture in Al alloy by Pb
CORROSION FATIGUE
S-N DIAGRAM
Synergistic action of
corrosion & cyclic
stress. Both crack
nucleation and
propagation are
accelerated by
corrodent and the S-
N diagram is shifted
to the left
AirAir
CorrosionCorrosion
log (cycles to failure, Nf)
Str
ess
Am
pli
tud
e
CORROSION FATIGUE, CRACK
PROPAGATION
Crack propagation
rate is increased by
the corrosive action
Log (Stress Intensity Factor Range, −K
log
(C
rack
Gro
wth
Rat
e, d
a/d
N)
PREVENTION OF CORROSION
The huge annual loss due to corrosion is a
national waste and should be minimized
Materials already exist which, if properly used,
can eliminate 80 % of corrosion loss
Proper understanding of the basics of corrosion
and incorporation in the initial design of metallic
structures is essential
METHODS
Material selection
Improvements in material
Design of structures
Alteration of environment
Cathodic & Anodic protection
Coatings
MATERIAL SELECTION
Most important method – select the appropriate metal or alloy .
“Natural” metal-corrosive combinations like
S. S.- Nitric acid, Ni & Ni alloys- Caustic
Monel- HF, Hastelloys- Hot HCl
Pb- Dil. Sulphuric acid, Sn- Distilled water
Al- Atmosphere, Ti- hot oxidizers
Ta- Ultimate resistance
IMPROVEMENTS OF
MATERIALS
Purification of metals- Al , Zr
Alloying with metals for:
Making more noble, e.g. Pt in Ti
Passivating, e.g. Cr in steel
Inhibiting, e.g. As & Sb in brass
Scavenging, e.g. Ti & Nb in S.S
Improving other properties
DESIGN OF STRUCTURES
Avoid sharp corners
Complete draining of vessels
No water retention
Avoid sudden changes in section
Avoid contact between dissimilar metals
Weld rather than rivet
Easy replacement of vulnerable parts
Avoid excessive mechanical stress
ALTERATION OF
ENVIRONMENT
Lower temperature and velocity
Remove oxygen/oxidizers
Change concentration
Add Inhibitors
Adsorption type, e.g. Organic amines, azoles
H evolution poisons, e.g. As & Sb
Scavengers, e.g. Sodium sulfite & hydrazine
Oxidizers, e.g. Chromates, nitrates, ferric salts
CATHODIC & ANODIC
PROTECTION
Cathodic protection: Make the structure more cathodic by Use of sacrificial anodes
Impressed currents
Used extensively to protect marine structures, underground pipelines, water heaters and reinforcement bars in concrete
Anodic protection: Make passivating metal structures more anodic by impressed potential. e.g. 316 s.s. pipe in sulfuric acid plants
COATINGS
Most popular method of corrosion protection
Coatings are of various types: Metallic
Inorganic like glass, porcelain and concrete
Organic, paints, varnishes and lacquers
Many methods of coating: Electrodeposition
Flame spraying
Cladding
Hot dipping
Diffusion
Vapour deposition
Ion implantation
Laser glazing
CONCLUSION
Corrosion is a natural degenerative process affecting metals, nonmetals and even biological systems like the human body
Corrosion of engineering materials lead to significant losses
An understanding of the basic principles of corrosion and their application in the design and maintenance of engineering systems result in reducing losses considerably
Gross and net calorific Value
Gross Calorific Value: It is the total amount of heat
generated when a unit quantity of fuel is completely
burnt in oxygen and the products of combustion are
cooled down to the room temperature.
As the products of combustion are cooled down to
room temperature, the steam gets condensed into
water and latent heat is evolved. Thus in the
determination of gross calorific value, the latent heat
also gets included in the measured heat. Therefore,
gross calorific value is also called the higher calorific
value.
The calorific value which is determined by Bomb
calorimeter gives the higher calorific value (HCV)
Net Calorific Value: It is defined as the net heat
produced when a unit quantity of fuel is completely
burnt and the products of combustion are allowed to
escape.
The water vapour do not condense and escape with
hot combustion gases. Hence, lesser amount than
gross calorific value is available. It is also known as
lower calorific value (LCV).
LCV=HCV-Latent heat of water vapours formed
Since 1 part by weight of hydrogen gives nine parts
by weight of water i.e.
OHOH222
12
→+
Therefore,
LCV=HCV-weight of hydrogen x 9 x latent heat of
steam
= HCV-weight of hydrogen x 9 x 587
Determination of Calorific value
1. Determination of calorific value of solid and non
volatile liquid fuels: It is determined by bomb
calorimeter.
Principle: A known amount of the fuel is burnt in
excess of oxygen and heat liberated is transferred to
a known amount of water. The calorific value of the
fuel is then determined by applying the principle of
calorimetery i.e. Heat gained = Heat lost
Bomb Calorimeter
Calculations
Let weight of the fuel sample taken = x g
Weight of water in the calorimeter = W g
Water equivalent of the Calorimeter, stirrer, bomb,
thermometer = w g
Initial temperature of water = t1oC
Final temperature of water = t2oC
Higher or gross calorific value = C cal/g
Heat gained by water = W x Dt x specific heat of water
= W (t2-t1) x 1 cal
Heat gained by Calorimeter = w (t2-t1) cal
Heat liberated by the fuel = x C cal
Heat liberated by the fuel = Heat gained by water and
calorimeter
x C = (W+w) (t2-t1) cal
C=(W+W)(t2-t1) cal/gx
Net Calorific value:
Let percentage of hydrogen in the fuel = H
Weight of water produced from 1 gm of the fuel =
9H/100 gm
Heat liberated during condensation of steam
= 0.09H 587 cal
Net (Lower calorific value) = GCV-Latent heat of
water formed
= C-0.09H 587 cal/gm
Corrections: For accurate results the following
corrections are also incorporated:
(a)Fuse wire correction: As Mg wire is used for
ignition, the heat generated by burning of Mg wire
is also included in the gross calorific value. Hence
this amount of heat has to be subtracted from the
total value.
(b)Acid Correction: During combustion, sulphur and
nitrogen present in the fuel are oxidized to their
corresponding acids under high pressure and
temperature.
DH = -144,000 Cal
DH = -57,160 Cal3
42
22
52
242
22
222
222
HNOOHON
SOHOHOSO
SOOS
→++
→++
→+
The corrections must be made for the heat liberated
in the bomb by the formation of H2SO4 and HNO3.
The amount of H2SO4 and HNO3 is analyzed by
washings of the calorimeter.
For each ml of 0.1 N H2SO4 formed, 3.6 calories
should be subtracted.
For each ml of 0.01 HNO3 formed, 1.43 calories must
be subtracted.
(C) Cooling correction: As the temperature rises
above the room temperature, the loss of heat does
occur due to radiation, and the highest temperature
recorded will be slightly less than that obtained.
A temperature correction is therefore necessary to get
the correct rise in temperature.
If the time taken for the water in the calorimeter to
cool down from the maximum temperature attained, to
the room temperature is x minutes and the rate of
cooling is dt/min, then the cooling correction = x dt.
This should be added to the observed rise in
temperature.
Therefore,
Gross calorific value (GCV)
GCV = (W+w)(t2-t1+Cooling correction)-[Acid+ fuse
corrections] / Mass of the fuel.
Theoretical calculation of Calorific value of a
Fuel: The calorific value of a fuel can be calculated if
the percentages of the constituent elements are
known.
Substrate Calorific value
Carbon 8080
Hydrogen 34500
Sulphur 2240
If oxygen is also present, it combines with hydrogen to
form H2O. Thus the hydrogen in the combined form is
not available for combustion and is called fixed
hydrogen.
Amount of hydrogen available for combustion = Total
mass of hydrogen-hydrogen combined with oxygen.
1g 8g 9g
Fixed Hydrogen = Mass of oxygen in the fuel
Therefore, mass of hydrogen available for combustion
= Total mass of hydrogen-1/8 mass of oxygen in fuel
=H-O/8
OHOH 2222
1→+
Dulong’s formula for calculating the calorific value
is given as:
Gross calorific Value (HCV)
Net Calorific value (LCV)
kgkcalSO
HC /]240,2)8
(500,348080[100
1+−+=
kgkcalHHCV
kgkcalH
HCV
/]58709.0[
/]587100
9[
−=
−=
FACTORS AFFECTING CORROSION
FACTORS AFFECTING CORROSION
The rate and extent of corrosion
depends mainly upon two factors–
1. Nature of the metal
2. Nature of the environment
NATURE OF THE METAL OR METALLIC
CONDITIONS
A.Position in Galvanic Series
Metal higher in the galvanic seriesare more likely to undergocorrosion
B. Relative anodic and cathodic areas
Corrosion is more rapid if theanodic area is small because thereis more demand for electrons bythe larger cathodic area.
a.
C.Purity of the metal
Impurities in a metal form minute
electrochemical cells and the anodic part
gets corroded.
D. Physical state of metal
The rate of corrosion is influenced
by the physical state of the metal
such as size, orientation of crystals,
stress etc. The smaller the size of
metal greater will be the corrosion
and the stressed port of the metal
also undergo more corrosion.
E. Nature of corrosion product
Metals like Fe, Mg etc form a nonprotective porous oxide film whichcauses maximum corrosion whilemetals like Al, Cr, Ni etc form aprotective coating which minimizescorrosion.
F. Solubility of corrosion product
In electrochemical corrosion, if thecorrosion product is soluble in themedium, then the corrosion proceedsat a faster rate.
G. Volatility of corrosion product
If the corrosion product is volatile
rapid and continuous corrosion
occurs.
NATURE OF THE ENVIRONMENT
a. Temperature
Rise of temperature increases the rate ofcorrosion.
b. Humidity
Humidity air is directly related to therate of corrosion. In humid conditionatmospheric gases easily formelectrochemical cell by which corrosionoccurs to a great extent.
C. Effect of PH
Generally acidic media is PH<7 ismore corrosive than alkaline andneutral media.
D.Formation of O2 concentration cell
If there is a difference in O2 concnaround the metal, then the lessoxygenated metal part becomesanode and the more oxygenatedpart becomes cathode and an O2concentration cell is set upresulting corrosion.
E.Nature of ions
Presence of anions like silicate ions in
the medium leads to the formation of
insoluble reaction products which
inhibit further corrosion. On the
other hand Cl- ions ions etc destroy
the protective surface film thereby
exposing fresh metal surface for
corrosion. Rapid corrosion of Al in
sea water is an example.
F.Presence of suspended particulars orcompounds
Particulars like NaCl, (NH4)2 SO4 etc alongwith moisture act as powerful electrolyteand promote corrosion.
G.Conductance of the corroding medium
In the case of underground andsubmerged structures, the conductance ofthe medium influences the rate ofcorrosion. Conductance of dry sandy soilis lower than that of clayey andmineralized soil. Hence the corrosion rateof metallic structures in lower in drysandy soil than in clayey and mineralizedsoil.
Battery :-
➢ A battery is a storage device used for the storage of chemical
energy and for the transformation of chemical energy into electrical
energy
➢ Battery consists of group of two or more electric cells connected
together electrically in series.
Battery acts as a portable source of electrical energy.
Energy produced by an electrochemical cell is not suitable for
commercial purposes since they use salt bridge which produce internal
resistance which results in drop in the voltage. The drop in voltage is
negligible only for a small interval of time during which it is being used.
Batteries are of 3 types. Namely
• Primary Batteries (or) Primary Cells
• Secondary Batteries (or) Secondary Cells
• Reserve Batteries
• Fuel Cells (or) Flow Batteries
Primary (Disposable) Batteries
➢ Leclanché Cells (zinc carbon or dry cell)
➢ Alkaline Cells
➢ Mercury Oxide Cells
➢ Zinc/MnO2 Cells
➢ Aluminum / Air Cells
➢ Lithium Cells
➢ Liquid cathode lithium cells
➢ Solid cathode lithium cells
➢ Solid electrolyte lithium cells
Secondary (Rechargeable) Batteries
➢ Lead–acid Cells
➢ Nickel/Cadmium Cells
➢ Nickel/Metal Hydride (NiMH) Cells
➢ Lithium Ion Cells
Lead-acid battery
Electrolyte – 20 % H2SO4
H2SO4 Concentration decreases with discharging and regained on charging
This can tested by specific gravity measurement of H2SO4
Cell voltage 1.88 – 2.15 V
PbO2 + Pb + H2SO4 2PbSO4 + 2H2Odischarging
charging
Basics-Cell Chemistry
• At the positive plate:
PbO2 + 4H+ + SO42- + 2e- PbSO4 + 2H2O
• At the negative plate: Pb + SO42- PbSO4 + 2e-
• Total Cell Reaction: PbO2 + Pb +2H2SO4 2PbSO4 +2H2O
Note: Active materials include lead dioxide, lead and sulfuric
acid.
D
C
D
C
D
C
There are four stages in the
discharging−charging cycle:
• Fully Charged
• Discharging
• Fully Discharged
• Charging
❑ Positive plate covered with lead oxide(PbO2)
❑ Negative plate covered with asponge lead (Pb)
❑ Electrolyte contains water (H2O)and a sulfuric acid (H2SO4)
FULLY CHARGED
❑ Current flows in the cell from the negativeto the positive plates.
❑ Electrolyte separates into hydrogen (H2) and sulfate (SO4).
❑ The free sulfate combines with the lead (both lead oxide and sponge lead)
and becomes lead sulfate (PbSO4).
❑ The free hydrogen and oxygen combine to form more water,
diluting the electrolyte.
DISCHARGING
❑Both plates are fully sulfated.
❑Electrolyte is dilutedto mostly water.
DISCHARGED
❑ Reverses the chemical reactionthat took place during discharging.
❑ Sulfate (SO4) leaves the positiveand negative plates and combineswith hydrogen (H2) to becomesulfuric acid (H2SO4).
❑ Hydrogen bubbles form at thenegative plates; oxygen appears at the positive plates.
❑ Free oxygen (O2) combines with lead (Pb) at the positive plate to become lead oxide (PbO2).
CHARGING
Liquid crystals are substances that exhibit a phase
of matter that has properties between those of a
conventional liquid, and those of a solid crystal.
Hence LC show anisotropy.
Note:Mesogen:
It is the fundamental unit of a liquid crystal that induces structural order in the crystals.
LIQUID CRYSTALS
i. NEMATIC LC
II. LYOTROPIC LC
ii. CHOLESTRIC LC iii. SMECTIC LC
I. THERMOTROPIC LC
I. THERMOTROPIC LIQUID CRYSTALS
Liquid crystals are said to be thermotropic if there liquid crystalline properties depend on the temperature.
i. NEMATIC LIQUID CRYSTALS
One of the most common LC phases is the nematic, where the molecules (mesogens) have no positional order, but they have long-range orientational order. (Most nematics are uniaxial: they have
one axis that is longer and preferred, with the other two being equivalent (can be approximated as cylinders)
Nematics have fluidity similar to that of
ordinary (isotropic) liquids but they can be easily
aligned by an external magnetic or electric field. An
aligned nematic has the optical properties of a
uniaxial crystal and this makes them extremely useful
in liquid crystal displays (LCD).
In Greek ‘nematic’ means thread. And hence the thread like structure of the nematic crystals.
ii. SMECTIC LIQUID CRYSTALS
In the case of Smectic type LC, the mesogens have both positional order and orientational order. The smectic
phases, which are found at lower temperatures than the nematic, form well-defined layers that can slide over one another like soap.
Smectic A Smectic C
iii. CHOLESTRIC LIQUID CRYSTALS
The cholestric phase can be defined as a special type of nematic LC in which the thin layers of the parallel mesogens have their longitudinal axes rotated in adjacent layers at certain angle.
II. Lyotropic LIQUID CRYSTALS
Liquid crystals which are prepared by mixing two or more substances, of which one is a polar molecule, are known as lyotropic liquid crystals.
Eg. Soap in water.
Hydrophobic end of the mesogen
Hydrophilic end of the mesogen
Discontinuous cubic phase (micellar cubic phase) Hexagonal phase (hexagonal columnar phase) (middle phase) Bicontinuous cubic phaseLamellar phaseBicontinuous cubic phaseReverse hexagonal columnar phase Inverse cubic phase (Inverse micellar phase)
1. Liquid Crystal Displays: Used in display devices (LCDs) such as Laptops, watches, calculators, clocks, etc.
2. Liquid Crystal Thermometers: Chiral nematic (cholesteric) liquid crystals reflect light and the color reflected also is dependent upon temperature.
3. Optical Imaging: An application of liquid crystals that is only now being explored is optical imaging and recording.
3. Some of the liquid crystals are used in hydraulic break/clutch
system due to their high viscosity values.
Applications of Liquid CrystalsLiquid crystal technology has had a major effect many areas of science and engineering, as well as device technology. Applications for this special kind of material are still being discovered and continue to provide effective solutions to many different problems.Liquid Crystal DisplaysThe most common application of liquid crystal technology is liquid crystal displays (LCDs.) This field has grown into a multi-billion dollar industry, and many significant scientific and engineering discoveries have been made. Please refer to the LCD chapter for more detail.Liquid Crystal ThermometersAs demonstrated earlier, chiral nematic (cholesteric) liquid crystals reflect light with a wavelength equal to the pitch. Because the pitch is dependent upon temperature, the color reflected also is dependent upon temperature. Liquid crystals make it possible toaccurately gauge temperature just by looking at the color of the thermometer. By mixing different compounds, a device for practically any temperature range can be built.The "mood ring", a popular novelty a few years ago, took advantage of the unique ability of the chiral nematic liquid crystal. More important and practical applications have been developed in such diverse areas as medicine and electronics. Special liquid crystal devices can be attached to the skin to show a "map" of temperatures. This is useful because often physical problems, such as tumors, have a different temperature than the surrounding tissue. Liquid crystal temperature sensors can also be used to find bad connections on a circuit board by detecting the characteristic higher temperature. [Collings, 140-142]Optical ImagingAn application of liquid crystals that is only now being explored is optical imaging and recording. In this technology, a liquid crystal cell is placed between two layers of photoconductor. Light is applied to the photoconductor, which increases the material's conductivity. This causes an electric field to develop in the liquid crystal corresponding to the intensity of the light. The electric pattern can be transmitted by an electrode, which enables the image to be recorded. This technology is still being developed and is one of the most promising areas of liquid crystal research.Other Liquid Crystal ApplicationsLiquid crystals have a multitude of other uses. They are used for nondestructive mechanical testing of materials under stress. This technique is also used for the visualization of RF (radio frequency) waves in waveguides. They are used in medical applications where, for example, transient pressure transmitted by a walking foot on the ground is measured. Low molar mass (LMM) liquid crystals have applications including erasable optical disks, full color "electronic slides" for computer-aided drawing (CAD), and light modulators for color electronic imaging.As new properties and types of liquid crystals are investigated and researched, these materials are sure to gain increasing importance in industrial and scientific applications.
POLYMER COMPOSITE
What is Composites?
Combination of 2 or more materials
Each of the materials must exist more than 5%
Presence of interphase
The properties shown by the composite materials are differed from the initial materials
Can be produced by various processing techniques
A broad definition of composite is: Two or more chemically distinct
materials which when combined have improved properties over the
individual materials. Composites could be natural or synthetic.
Composites are combinations of two materials in which one of the
material is called the reinforcing phase, is in the form of fibers,
sheets, or particles, and is embedded in the other material called the
matrix phase.
Constituents of composite
materials1. Matrix phaseContinuous phase, the primary phase. It holds the dispersed phase and shares a load with it.
2. Dispersed (reinforcing) phaseThe second phase (or phases) is imbedded in the matrix in a continuous/discontinuous form. Dispersed phase is usually stronger than the matrix, therefore it is sometimes called reinforcing phase.
3. InterfaceZone across which matrix and reinforcing phases interact (chemical, physical,mechanical)
Matrix: Function
however the distribution of loads depends on the interfacial bondings
Reinforcement: Function
Reinforcement can be in the form
of: Continuous fiber
Organic fiber- i.e. Kevlar, polyethylene
Inorganic fiber- i.e. glass, alumina, carbon
Natural fiber- i.e. asbestos, jute, silk
Short fiber
whiskers
Particle
Wire
Interface: Function
To transfer the stress from matrix to
reinforcement
Sometimes surface treatment is carried out
to achieve the required bonding to the
matrix
a) Concentration (b) size (c) shape (d) distribution (e)
orientation
Characteristics of dispersed phase that might influence the properties of composites
Classification of composites
Examples of composites
a) Particulate & randomb) Discontinuous fibers & unidirectionalc) Discontinuous fibers & randomd) Continuous fibers & unidirectional
Classification based on Matrices
Composite materials
Matrices
Polymer Matrix Composites (PMC)
Metal Matrix Composites MMC)
Ceramic Matrix Composites (CMC)
Thermoset Thermoplastic Rubber
Widely used- ease of processing & lightweight
Composites – Ceramic Matrix
Ceramic matrix composites (CMC) are used in applications where resistance to high temperature and corrosive environment is desired. CMCs are strong and stiff but they lack toughness (ductility)
Matrix materials are usually silicon carbide, silicon nitride and aluminum oxide, and mullite (compound of aluminum, silicon and oxygen). They retain their strength up to 3000 oF.
Fiber materials used commonly are carbon and aluminum oxide.
Applications are in jet and automobile engines, deep-see mining, cutting tools, dies and pressure vessels.
Ken Youssefi
Mechanical
Engineering Dept.15
16
Composites – Metal MatrixThe metal matrix composites offer higher modulus of elasticity, ductility, and resistance to elevated temperature than polymer matrix composites. But, they are heavier and more difficult to process.
Properties of composites depend
on
Amount of phase
- Amount/proportion (can be expressed in weight fraction (Wf) or volume fraction (Vf))of phases strongly influence the properties of composite materials.
Xc = Xf Vf + Xm (1 - Vf ) - Rule of Mixture
Xc = Properties of composites
Xf = Properties of fiber
Xm= Properties of matrix
Voids
Free volume
Gas emission leads to voids in the final product
In composites- Voids exist in the matrix, interface and in between fiber & fiber
Voids create stress concentration points- influence the properties of the composites
Geometry of dispersed phase
(particle size, distribution,
orientation) Shape of dispersed phase (particle- spherical or
irregular, flaky, whiskers, etc)
Particle/fiber size ( fiber- short, long, continuous); particle (nano or micron size)
Orientation of fiber/particle (unidirection, bi-directions, many directions)- influence isotropic dan an-isotropic properties
Dictribution of dispersed phase (homogenus/uniform, inhomogenus)
Glass Fiber The types of glass used are as follows:
E-Glass – the most popular and inexpensive glass fibers. The designation letter “E” means “electrical” (E-Glass is excellent insulator). The composition of E-glass ranges from 52-56% SiO2, 12-16% A1203, 16-25% CaO, and 8-13% B203
S-Glass – stronger than E-Glass fibers (the letter “S” means strength). High-strength glass is generally known as S-type glass in the United States, R-glass in Europe and T-glass in Japan. S-Glass is used in military applications and in aerospace. S-Glass consists of silica (SiO2), magnesia (MgO), alumina (Al2O3).
C-Glass – corrosion and chemical resistant glass fibers. To protect against water erosion, a moisture-resistant coating such as a silane compound is coated onto the fibers during manufacturing. Adding resin during composite formation provides additional protection. C-Glass fibers are used for manufacturing storage tanks, pipes and other chemical resistant equipment.
Fiberglasses (Glass fibers reinforced polymer matrix composites) are characterized by the following properties:
High strength-to-weight ratio;
High modulus of elasticity-to-weight ratio;
Good corrosion resistance;
Good insulating properties;
Low thermal resistance (as compared to metals and ceramics).
Fiberglass materials are used for manufacturing: boat hulls and marine structures, automobile and truck body panels, pressure vessels, aircraft wings and fuselage sections, housings for radar systems, swimming pools, welding helmets, roofs, pipes.
Glass Fiber
Carbon Fiber The types of carbon fibers are as
follows:
UHM (ultra high modulus). Modulus of elasticity > 65400 ksi (450GPa).
HM (high modulus). Modulus of elasticity is in the range 51000-65400 ksi (350-450GPa).
IM (intermediate modulus). Modulus of elasticity is in the range 29000-51000 ksi (200-350GPa).
HT (high tensile, low modulus). Tensile strength > 436 ksi (3 GPa), modulus of elasticity < 14500 ksi (100 GPa).
SHT (super high tensile). Tensile strength > 650 ksi (4.5GPa).
Carbon Fiber Reinforced Polymers (CFRP) are characterized by the following properties:
Light weight;
High strength-to-weight ratio;
Very High modulus elasticity-to-weight ratio;
High Fatigue strength;
Good corrosion resistance;
Very low coefficient of thermal expansion;
Low impact resistance;
High electric conductivity;
High cost.
Carbon Fiber Reinforced Polymers (CFRP) are used for manufacturing: automotive marine and aerospace parts, sport goods (golf clubs, skis, tennis racquets, fishing rods), bicycle frames.
Carbon Fiber
Kevlar Fiber Kevlar is the trade name (registered by DuPont Co.)
of aramid (poly-para-phenylene terephthalamide) fibers.
Kevlar fibers were originally developed as a replacement of steel in automotive tires.
Kevlar filaments are produced by extrusion of the precursor through a spinnert. Extrusion imparts anisotropy (increased strength in the lengthwise direction) to the filaments.
Kevlar may protect carbon fibers and improve their properties: hybrid fabric (Kevlar + Carbon fibers) combines very high tensile strength with high impact and abrasion resistance.
Kevlar fibers possess the following properties:
High tensile strength (five times stronger per weight unite than steel);
High modulus of elasticity;
Very low elongation up to breaking point;
Low weight;
High chemical inertness;
Very low coefficient of thermal expansion;
High Fracture Toughness (impact resistance);
High cut resistance;
Textile processibility;
Flame resistance.
The disadvantages of Kevlar are: ability to absorb moisture, difficulties in cutting, low compressive strength.
Kevlar Fiber
There are several modifications of Kevlar, developed for various applications:
Kevlar 29 – high strength (520000 psi/3600 MPa), low density (90 lb/ft³/1440 kg/m³) fibers used for manufacturing bullet-proof vests, composite armor reinforcement, helmets, ropes, cables, asbestos replacing parts.
Kevlar 49 – high modulus (19000 ksi/131 GPa), high strength (550000 psi/3800 MPa), low density (90 lb/ft³/1440 kg/m³) fibers used in aerospace, automotive and marine applications.
Kevlar 149 – ultra high modulus (27000 ksi/186 GPa), high strength (490000 psi/3400 MPa), low density (92 lb/ft³/1470 kg/m³) highly crystallinefibers used as reinforcing dispersed phase for composite aircraft components.
Kevlar Fiber
Zeolite process Zeolites are hydrated sodium alumino silicates capable of exchanging its sodium ions with hardness producing cations in water.
Na2O Al2O3.xSiO2.yH2O ,
where x=2 to 10 and y= 2 to 6
There are two types of Zeolites:-
(i) Natural Zeolites:- Are amorphous and non porous in nature.They are derived from green sand,by
washing,heating and treating with NaOH.
e.g. Natrolite- Na2O Al2O3.4SiO2.2H2O
(ii) Synthetic zeolites are porous and are prepared by heating together solutions of sodium silicate,sodiumaluminate and aluminium sulphate.
Principle:
Zeolites can be represented as Na2Z,from which Na can easily be replaced by Ca and Mg ions present in hard water.
Ca(HCO3) 2+ Na 2 Z CaZ + 2NaHCO3
Mg(HCO3) 2 + Na 2 Z MgZ +2NaHCO3
CaCl2+ Na2Z CaZ + 2NaCl
MgCl2+Na2Z MgZ + 2NaCl
CaSO4 +Na2Z CaZ + Na2SO4
MgSO4 +Na2Z MgZ + Na2SO4
Regeneration:-
After sometime, sodium Zeolites are completely converted into Calcium and Magnesium Zeolites i.e. get exhausted.The process by which exhausted Zeolite is converted into sodium Zeolite again by treating with 10% brine solution is known as Regeneration
CaZ + 2 NaCl Na2Z + CaCl2
MgZ + 2 NaCl Na2Z + MgCl2
Process:-
Hard water is percolated through Zeolite bed in a cylindrical tank.Sodium ions are replaced by Ca2+ and Mg2+ ions to form CaZ and MgZ.After sometimes the bed gets exhausted.At this stage supply of water is stopped and regeneration is carried out,by passing 10% brine solution.
Advantages of Zeolite process:-
1. Water of about 15 ppm hardness is obtained
2. The equipment is compact and occupies less space.
3. It requires less time for softening.
4. There is no danger of sludge formation because
impurities are not precipitated.
Disadvantages of zeolite process:-
1. Only cations are removed and not anions.
2. If water is turbid it clogs the pores of zeolite bed and makes it inactive.So the suspended impurities must be removed from hard water by coagulation and filtration first, before the water is fed to the zeolite bed
3. Mineral acids destroy the zeolite bed, so they must be
neutralised befor hand.
4. Acid radicals which are not removed during softening
cause caustic embrittlement and boiler corrosion. NaHCO3 NaOH + CO2
CO2 + H2O H2CO3
5. If large quantities of Fe2+ and Mn2+ are present in water ,the zeolite is converted into iron and manganese zeolitewhich can not be regenerated.
Limitations:
•If the supply of water is turbid in will
clog the pores of zeolite led
•Water contains large quantities of
colored ions such as Mn+2 and Fe+2
they may be removed first because
these ions produce Mn and Fe Zeolites
,which can’t be easily regenerated
•Mineral acids destroy the zeolite bed
Biodegradable Polymers: Introduction &
Applications
Definition
A “biodegradable” product has the ability to break down,
safely, reliably, and relatively quickly, by biological
means, into raw materials of nature and disappear into
nature.
Nature’s way: every resource made by nature returns to
nature. Nature has perfected the system we just need to
figure out how
What is Polymer Degradation?
polymers were synthesized from glycolic acid in 1920s
At that time, polymer degradation was viewed negatively as a process where properties and performance deteriorated with time.
Biodegradable Polymers
▪ Natural polymers
▪ Fibrin
▪ Collagen
▪ Chitosan
▪ Gelatin
▪ Hyaluronan ...
▪ Synthetic polymers
▪ PLA, PGA, PLGA, PCL, Polyorthoesters …
▪ Poly(dioxanone)
▪ Poly(anhydrides)
▪ Poly(trimethylene carbonate)
▪ Polyphosphazenes ...
Degradation Mechanisms
▪ Enzymatic degradation
▪ Hydrolysis
(depend on main chain structure: anhydride > ester >
carbonate)
▪ Homogenous degradation
▪ Heterogenous degradation
Polyesters
PGA: It is used in fishing industry, controlled release of pestisides, egg cartons,Razor handles, toys and in the medical field.
Poly(lactide-co-glycolide)
PCL (Poly caprolactone)It is a thermoplastic biodegradable polyester synthesized by chemical Conversion of crude oil, followed by ring opening polymerisation.PCL has good water, oil, solvent and chlorine resistance. It is manufactured under trade name “Tone Polymer”.
Polylactic acid
The skeletal formula of poly(lactic acid)
Poly(lactic acid) or polylactide (PLA) is a
thermoplastic aliphatic polyester derived from
renewable resources, such as corn starch (in the
United States), tapioca products (roots, chips or starch
mostly in Asia) or sugarcanes (in the rest of world). It c
Polyhydroxyalkanoates
HO OH
O
OHHO
O
O
O
O
O
n m
BacteriaCatalyzed
Polymerization
MicrobiallyCatalyzedDepolymerization
+HO OH
O
OHHO
O
O
O
O
O
n m
BacteriaCatalyzed
Polymerization
MicrobiallyCatalyzedDepolymerization
+
Polyhydroxy buterate valerate (PHBV)
BIOPOL
Medical Applications of Biodegradable Polymers
▪ Wound management
▪ Sutures
▪ Staples
▪ Clips
▪ Adhesives
▪ Surgical meshes
▪ Orthopedic devices
▪ Pins
▪ Rods
▪ Screws
▪ Tacks
▪ Ligaments
▪ Dental applications
▪ Guided tissue regeneration Membrane
▪ Void filler following tooth extraction
▪ Cardiovascular applications
▪ Stents
▪ Intestinal applications
▪ Anastomosis rings
▪ Drug delivery system
▪ Tissue engineering
CONDUCTING POLYMERS
Discovery of conducting polymers Discovered in the late seventies (1977) by Alan
Heegar , Dr. Hideki Shirakawa and Alan Macdiarmid
Before that polymers were used as insulators in the electronic industry
Advantages over conductors
Chemical - ion transport possible , redox behavior , catalytic properties, electrochemical effects, Photoactivity, Junction effects
Mechanical - light weight , flexible , non metallic surface properties
Polyacetylene1977
n
Polyaniline
1985
NHn
Polyphenylene1979
n
Polythiophene1982
S n
NH
n
Polypyrrole
1979
n
Poly(phenylene vinylene)
PPV 1979
Conductivity Polymers become conducting upon doping
Polymer becomes electronically charged
Polymer chains generate charge carriers
Concentration of dopant causes certain electrons to become unpaired
Formation of polarons and bipolarons
They have extended p-orbital system
Band structures for semiconductors and insulators
Semiconductors and Insulators have totally full valence bands and empty conduction bands with a bandgap between them. Ef exists in the bandgap.
The distinction between semiconducting and insulating materials is arbitrarily set to a bandgap of < or > 4 eV, respectively.
Energy
Ef,
Fer
mi
level
Metal
(Cu)
partially
filled 4s
(conduction)
filled
3p, 2p, 2s, 1p,
1s (valence)
Empty 4p
(conduction)
Band gap
Band gap
Filled
(deep valence)
Ef
Semiconductor
(Si)
Filled
(valence)
Empty
(conduction)
Band gap
Band gap
Filled
(deep valence)
Ef
Insulator
(Al2O3)
Filled
(valence)
Empty
(conduction)
Band gap
Band gap
105
S (-1 cm-1
100
10-5
10-10
10-15
Polyethylene
Polystyrene
PTFE
Nylon 6
Silica
silicon
Doped germanium
CopperIron
Graphite Doped PolyanilineDoped Polyacetylene
Doped Polypyrrole Doped Polythiophene
Doped Polyphenylene
Non-Doped Polyacetylene
Non-Doped Polythiophene
Non-Doped Polyphenylene
n NHn
n
S n
Electron-conducting polymersPolyacetylene
First conducting polymer to be synthesized
Best defined system
Reaction conditions allow to control the morphology of the polymer to be obtained as gel, powder, spongy mass or a film
Doped with iodine
Inherent insolubility and infusibility impose barriers to the processing of the polymer
Synthesized by
Dehydrohalogenations of vinyl chlorides:
Polymers prepared by this route have short conjugation length, structural defects and crosslinks
Precursor routes: Durham route
Polymers prepared by this route are continuous solid films, have controlled morphology range and can be stretched prior to conversion
Conduction mechanism R and L forms are interconverted through a charge
carrier soliton
Soliton is a mobile, charged or a neutral defect or a kink in the polymer chain
It propagates down the polymer chain
For short chains Kivelson mechanism is involved
Travel of a soliton by bipolaronmechanism
Contrast between isomers of
polyacetylene
170`C10^-7trans
-77`C10^-13cis
structureObtainable
temperature
Conductivity
(siemens/cm)
isomer
Reasons of trans’ stability
Two fold degeneracy
SOLITON formation due to symmetry
An unpaired electron at each end of an inverted sequence
of double bonds
Stability(contd.)
SOLITONS - Responsible for higher conductivity
Double bond next to a SOLITON may switch over to give
rise a moving SOLITON which leads to conduction
In presence of many SOLITONS , their sphere of
influence overlaps leading to conduction like metals
Doping in polyacetylene
• Amount of dopant used is significantly higher
• Doped polyacetylene is always in tans form
• Neutral polyacetylene can be doped in two ways
p type doping : oxidation with anions eg : ClO4(-)
n type doping : reduction with cations eg : Na(+)
- e
+ ClO4(-) + ClO4(-)
+ e
+ Na(+)(-)
Na(+)
Method of doping
•Chemical oxidants : iodine , nitronium species ,
transition metal salts
•Chemical reducing agents : sodium naphthamide
•Electrochemical methods : used dopants ClO4(-)
, BF4(-) and other complex species
Doping with Iodine
Effect of dopant
•Conductivity - increases upto a certain doping
level
•Stability - decreases
•Morphology : due to presence of charges shape
will not be retained - reason why doped
polyacetylene is always trans
Various
Applications
Coatings
• Prevents buildup of static charge in insulators
• Absorbs the harmful radiation from electrical
appliances which are harmful to the nearby
appliances
• Polymerization of conducting plastics used in
circuit boards
Sensors(to gases and solns.)
• Polypyrroles can detect NO2 and NH3 gases by
changing its conductivity
• Biosensor : polymerization of polyacetylene in
presence of enzyme glucose oxidase and
suitable redox mediator like triiodide will give
rise to a polymer which acts as glucose sensor
Polymeric Ferroelectric
RAM(PFRAM)
• Uses polymer ferroelectric material
• Dipole is used to store data
• Provides low cost per bit with high chip
capacity
• Low power consumption
• No power required in stand by mode
• Isn’t a fast access memory
Biocompatible Polymers
• Artificial nerves
• Brain cells
Batteries
• Light weight
• Rechargeable
• Example - Polypyrrole - Li & Polyaniline - Li
Displays
• Flat panels
• Related problems : low life time & long switching
time
Conductive Adhesive
• Monomers are placed between two conducting plates
and it allows it to polymerize
• Conducting objects can be stuck together yet allowing
electric current to pass through the bonds
1. Proper selection and designing
2.Cathodic and anodic protection
3.Protective coating
(a ) Metal Coating
(b) Inorganic coating
(c) Organic coating
4. Corrosion Inhibitors
Design an equipment by avoiding contact between two dissimilar metals.
Maintaining a larger anodic area of the metal.
Metals should be close in the galvanic series.
2. Cathodic ProtectionIn this method, the corroding metal is
forced to behave like a cathode. There aretwo types of cathodic protection
In this method, the metallic structure
which is to be protected from corrosion is
connected to a more anodic metal by a
wire so that the entire corrosion is
concentrated on this more active metal.
The more active metal loses and get
corroded and this metal is called
sacrificial anode. Metals commonly
employed as sacrificial anode are Mg, Zn,Al and their alloys.
Applications
Important applications of sacrificial anodic
method include protection of buried pipe
lines, underground cables, marine structuresetc.
In this method, an impressed current isapplied in the opposite direction to nullifycorrosion current so as to convert thecorroding metal from anode to cathode.Impressed current can be derived from adirect current source like battery. An inertor insoluble electrode like graphite or silicaact as anode to complete the circuit. Thesurroundings of anode should be filled withsalts and carbon to increased theconductivity.
This type of cathodic protection has been
applied to water coolers, water tanks,
buried oil and water pipes, transmission
towers etc. This type of protection isemployed when
1. Long term protection is needed
2. Large structures are to be protected
3. There is a cheap source of electrical
power.
3. Protective coatingsAn important method for protecting a metal from
corrosion is to apply a protective coating. The
protective coatings may be of metal, inorganic or
organic. The coated surface isolates the metal from the
corroding medium. The coating applied must be
chemically inert towards the environment.
Metallic coatings are mostly
applied on Iron and steel because
these are cheap and commonly
used construction materials.
There are two types of metalliccoatings.
The base metal which is to be protected is
coated with a more anodic metal for eg.
Coatings of Zn on steel is anodic because
their electrode potentials are lower than that
of the base metal ie. Fe.
It is obtained by coating a more inert metal
having higher electrode potential. Than the
base metal. Eg. Coating of Sn, Cr, Ni on Fe
surface. The coating should be continuous
and free from pores and cracks. These
coating metals usually have higher
corrosion resistance than the base metal.
It is used for producing a coating of lowmelting metal such as Zn, Sn, Ph, Al etcon relatively higher melting metals suchas iron, steel, copper etc. This is done byimmersing the base metal covered by alayer of molten flux. The flux is used tokeep the base metal surface clean andalso to prevent oxidation of the moltenmetal. Most widely used hot dippingmethods are : (i) galvanization and (ii)tinning
It is the process of coating Zn over ironor steel sheet by immersing it in moltenZn. The procedure involves the followingstages.
The iron or steel article is Ist cleaned bypickling with dil H2So4 for 15 – 20 min.at 60 – 900C in an acid bath. Thistreatment also removes any oxide layerpresent on the surface of the metal. Thearticle is then washed with water in awashing bath & dried in a dryingchamber.
It is then dipped in a bath of molten Znkept at 425 – 4350C. The Surface of thebath is covered with NH4Cl flux to preventoxide formation.
The article gets coated with a thin layer ofZn. It is then passed through a pair of hotrollers to remove excess of Zn and to getuniform thickness for coating. Then it isannealed at about 6500C & cooled slowly.In the case of Zn coating even if theprotecting layer has cracks on it, ironbeing cathodic does not get corroded.
Applications
This method is widely used for protection of Fe
from atmospheric corrosion in the form of
articles like roofing sheets, wires, pipes, nails,
screws, tubes etc. It is to be noted that
galvanized utensils should not come incontact with acids.
It is an eg. For cathodic coatings. It is the process
of coating Sn over Fe or steel articles by immersing
it in molten Sn. The process consists in Ist treating
the iron sheet with dil H2So4 to remove any oxide
film. After this it is passed through a bath of ZnCl2flux which helps the molten Sn to adhere to the
metal sheet. Next the sheet passes through palm
oil which prevents through a pair of hot rollers to
remove excess of Sn & produce uniform thicknessfor Sn coating.
Tinning is widely used for coating steel, Cu
and brass sheets which are used for
making containers for storing food studs,
oils, kerosene & packing food materials.
Tinned Cu sheets are used for making
cooking utensils & refrigerationequipments.
In this process, a thick homogeneous layer ofcoating metal is bonded firmly & permanently tothe base metal on one or both the sides. Thismethod cnhanceds corrosion resistance. Thechoice of cladding material depends on thecorrosion resistance required for any particularenvironment. Nearly all existing corrosionresisting metals like Ni, Cu, Al, Ag, Pt and alloyslike stainless steel, Ni alloys, Cu alloys can beused as cladding materials. Cladding can bedone by different means.a. Fusing cladding material over the base
metalb. Weldingc. Rolling sheets of cladding material over
base metal
In this process, the coating metal in themolten state is sprayed on the previouslycleaned base metal with the help of asprayer. The sprayer coatings arecontinuous but somewhat porous asealer – oil is applied on such a coatingto provide a smooth surface. However,adhesion strength of metallic spraying isusually lesser that obtained by hotdipping or electroplating. It is thereforeessential to have a cleaned metal surface.Spraying can be applied by the followingtwo techniques.
In this method, the coating metal in the
form of thin wire is melted by an oxy –
acetylene flame and vaporized by a blast of
compressed air. The coating metal adheres
to the base metal. Al is coated on aircraftsteel parts using this techniques.
In this method, the coating metal is supplied in the
form of fine powder which is converted in to a
cloud of molten globules by a blower and areadsorbed on the base metal surface.
it is probably the most important andmost frequently applied industrialmethod of producing metallic coatings.Electroplating is carried out by a processcalled electrolysis. Thus in this process,the coating metal is deposited on the basemetal by passing direct current throughan electrolyte containing the soluble saltof the coating metal. The base metal to beelectroplated is made the cathode of theelectrolytic cell whereas the anode iseither made of the coating metal itself oran inert material of good electricalconductivity like graphic.
©2010 John Wiley & Sons, Inc. M P
Groover, Fundamentals of Modern
Manufacturing 4/e
Electroplating
For electroplating of Ni, NiSO4 and NiCl2 areused as the electrolyte. For electroplating ofCr, chromic acid is used as the electrolyte.For Au plating, AuCl3 solution is taken as theelectrolyte. For Cu plating CuSO4 solution isused as the electrolyte. In silver plating,AgNO3 solution is used as the electrolyte.
Metallic plating process driven entirely by chemical reactions - no electric current is supplied
Deposition onto a part surface occurs in an aqueous solution containing ions of the desired plating metal ◦ Workpart surface acts as a catalyst for the reaction in the
presence of reducing agent
Metals that can be plated: nickel, copper, and gold
Notable application: copper for plating through-holes of printed circuit boards
Inorganic coatings
The coated surface isolates the metal from the
corroding medium. The coating applied must be
chemically inert towards the environment. Inorganic
coatings are further classified in to chemical
conversion coatings and vitreous coatings.
These coatings are produced on the surface of ametal or alloy by chemical or electrochemicalreaction. The metal is immersed in a solution ofsuitable chemical which reacts with the metalsurface producing and adherent coating. Thesecoatings protect the base metal from corrosion.Moreover many of these coatings areparticularly useful to serve as excellent basesfor the application of paints, enamels and otherprotective coatings. The most commonly usedsurface conversion coatings are chromatecoatings, phosphate coatings and chemicaloxide coatings.
There are produced by the immersion of thearticle in a bath of acidic potassiumchromate followed by immersion in a bathof neutral chromate solution. The surfacefilm consisting of a mixture of trivals andhexavalent Cr is formed. Chromate coatingspossess more corrosion resistance and canalso be used as a base for paints. Theseare applied on Zu, Cd, Mg and Al
These are produced by the chemical reaction of
base metal with aqueous solution of phosphoric
acid and a phosphate of Fe, Mn or Zn. The reaction
results in the formation of a surface film consisting
of phosphate of a surface film consisting of
phosphates of the metal. These coatings are
usually applied by immersing or spraying or
brushing. These coating do not give complete
corrosion resistance but can serve as base for
painting. These are applied on metals like Fe, Zn,
Cd, Al and Sn.
These types of coatings are formed on the
surface of metals like Fe, Al, Mg etc by treating
the base metal with alkaline oxidizing agents
like potassium permanganate. This treatment
increases the thickness of the original oxide film
on the metal, there by increasing the corrosion
resistance. Oxide coatings form a good base for
paints. These oxide coatings have got only poor
corrosion resistance. However, for better
protection the thickness of the oxide film can be
increased 100 to 1000 times by electrolyticoxidation or anodisation.
Ceramic protective coatings can be broadlydivided into vitreous enamel coatings andpure ceramic coatings. These coatings havethe following advantages.
1.They posses high refractoriness andinertness
2.They are wear resistant & easily be cleaned
3. They are glossy in appearance
4.They are good thermal & electricalinsulators
Vitreous enamels are defined as glossy
inorganic composition that can adhere to
metals by fusion and protect them from
corrosion, abrasion, oxidation and hightemperature.
Vitreous enamel coatings consists of a
ceramic mixture of refractories and large
proportion of fluxes. These coatings are
usually applied on steel and cast iron
equipments. The raw materials used for thevitreous coatings are the following.
Vitreous coatings
1. Refractories like quartz (SiO2), clay etc.
2. Fluxes like borax (Sodium tetra borate
Na2B4O7), cryolite (Na3AlF6) (Sodium
alumino fluoride), Soda ash (anhydroussodium carbonate Na2CO3) etc.
3. Opacifiers like TiO2, SnO2, Al2O3 etc
4. Pigments like metallic oxides organic
dyes etc
5. Floating agents like plastic, clay, gum etc
6. Electrolytes like MgSO4, MgCO3, Na2Co3
etc.
Polymers and resins (natural or synthetic) usually formulated to be applied as liquids that dry or harden as thin surface films on substrate materials
Advantages:
◦ Wide variety of colors and textures available
◦ Capacity to protect the substrate surface
◦ Low cost
◦ Ease with which they can be applied
©2010 John Wiley & Sons, Inc. M P
Groover, Fundamentals of Modern
Manufacturing 4/e
1. Binders - give the coating its properties
2. Dyes or pigments - provide color to the coating
3. Solvents - dissolve the polymers and resins and add proper fluidity to the liquid
4. Additives
©2010 John Wiley & Sons, Inc. M P
Groover, Fundamentals of Modern
Manufacturing 4/e
Chemicals which are added in small
quantities to the corroding medium in
order to reduce the corrosion rate are
called corrosion inhibitors. They reduce
corrosion by forming a protective film
either at the cathode or anode. Thus there
are two types of corrosion inhibitors –anodic inhibitors and cathodic inhibitors
Anodic inhibitors
Chromates (CrO42-), phosphate (PO4
3-) and
Tungstates (WO42-) of transition metals are
used as anodic inhibitors. They react with
the newly produced metal ions at the anode
forming a protective film or barrier there by
preventing further corrosion.
Cathodic inhibitors
Cathodic reaction takes place with either
evolution of H2 or absorption of O2
depending on the nature of the corrodingmedium.
1. Evolution of H2 in acid medium
2H+ + 2e- → H2 (g)
Evolution of H2 can be prevented by slowing
down the diffusion of H+ ions to the cathode
or by increasing H2 over voltage. Diffusion of
H+ ions can be prevented by adding organic
inhibitors such as amines, urea, thiourea etc.
These are adsorbed at the surface as a film.
Arsenic oxide or antimony oxide is added to
increase the H2 over voltage. These oxides
form adherent film of metallic arsenic orantimony at the cathodic areas.
II. Absorption of O2 in metal or alkalinemedium
H2O + ½ O2 + 2e- → 2 OH-
The formation of OH- ions can be prevented eitherby removing O2 from the medium or by decreasingthe diffusion of O2 in to the cathode. O2 is removedeither by adding reducing agents like Na2SO3, N2H4etc or by mechanical dearation.
2 Na2SO3 + O2 → 2 Na2SO4
N2H4 + O2 → N2 + 2H2O
Salts of Zn, Mg or Ni are added to the corrodingmedium to reduce the diffusion of O2 towardscathode. These salts react with OH- ions at thecathode forming insoluble hydroxides which areadsorbed at the cathode.
II. Absorption of O2 in metal or alkalinemedium
H2O + ½ O2 + 2e- → 2 OH-
The formation of OH- ions can be prevented eitherby removing O2 from the medium or by decreasingthe diffusion of O2 in to the cathode. O2 is removedeither by adding reducing agents like Na2SO3, N2H4etc or by mechanical dearation.
2 Na2SO3 + O2 → 2 Na2SO4
N2H4 + O2 → N2 + 2H2O
Salts of Zn, Mg or Ni are added to the corrodingmedium to reduce the diffusion of O2 towardscathode. These salts react with OH- ions at thecathode forming insoluble hydroxides which areadsorbed at the cathode.