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.