Corrosion

• It is the degradation of a material due to a reaction with its

environment.

OR

• Process of Distruction of the material through chemical or

electrochemical attack by its environment.

• Slow process

• Measured in weight loss per unit time.• Measured in weight loss per unit time.

Classification:

1. Dry or Chemical Corrosion

2. Wet or Electrochemical corrosion

1.Dry or Chemical Corrosion

- Occurs due to chemical attack of by the environment such as dry

gas.

- Occurs due to high temperature and without liquid phase.

• It is of two types:

a) Oxidation corrosion b) Corrosion by gases

(a)Oxidation Corrosion:

- It is due to direct attack of oxygen on metals.

- Oxygen molecules are attracted to the surface by Vander Wall Force

Mechanism:-

1. When temp increases the metal undergoes oxidation and losses e-

2M → 2M+n + 2ne-

Metal IonMetal Ion

2. Electron are gained by the oxygen molecules forms oxide ions

nO2 + 4ne- → 2n O2-

Oxide Ion

3. Scale of metal oxide formed

2M + nO2 → 2M + 2n O2-

Metal Oxide

• Stable Corrosion: -Aluminium, Tin, Lead, Copper

• Non-stable corrosion:- Silver, Gold, Platinum

• Pilling – Bed Worth Ratio

Ratio of volume of oxide formed to the

volume of metal consumed.

(b)Corrosion by Gases

Carbon di-oxide, Chlorine, Hydrogen Sulphide, Sulphur di-oxide, Flourine

- Depends on chemical affinity b/w metal and the gas.

2. Wet or Electrochemical Corrosion

• Occurs when aqueous solution or liquid electrolytes are present

• Wet corrosion takes place in environments where the relative

humidity exceeds 60 %.

• Wet corrosion is most efficient in waters containing salts, such as

NaCl (e.g. marine conditions), due to the high conductivity of the

solution.

Mechanism Of Electrochemical Corrosion

Anodic Reaction:

Dissolution of metal takes place.

• As result metal ions are formed with the liberation of free electrons.

M ↔ M+n + e-

Metal Ion

Cathodic Reaction

(i) Hydrogen Evolution :- Occurs usually in acidic medium

2H+ + 2e- ↔ H2 (g)

(ii) Oxygen Absorption :- occurs when solution is aerated sufficiently.

O2+ 4H+ + 4e- ↔ 2H2O (In acidic medium)

O2+ 4H+ + 4e- ↔ 4OH- (In basic medium)

Forms of Corrosion:

(a) Galvanic Corrosion:- When two different metals are present in (a) Galvanic Corrosion:- When two different metals are present in

contact with each other in conducting medium e.g. Electrolyte

(b) Concentration Cell Corrosion:-

• Same as Galvanic corrosion

• Occurs when two different metals are exposed to different air conc.

(c) Pitting Corrosion:-(c) Pitting Corrosion:-

• Formed as a result of pit and cavities

• Localized attack and formed by cracking protective coating

Factors Affecting Corrosion

1. Nature of the Metal 2. Nature of the environment.

1. Nature of Metal

(i) Position in Galvanic Series:

If two metals are present in in electrolyte,

the metal with less reduction potential undergoes corrosion.

- Greater the difference faster the corrosion.

(ii) Over Voltage:

Due to high evolution of hydrogen, the rate is slow.

(iii) Area and Distance:

When anodic metal area is smaller than cathodic

area, rate of corrosion at anode is higher because of demand of

electron by cathodic area.

(iv) Physical and Mechanical properties of Metal:

(a) Pure metals are more corrosion resistant.

(b) Smaller grain size metal have high solubility and corrosion.

(c) Uniform distribution of stress on metal reduces rate of corrosion.

(d) Passive metals shows higher corrosion resistance because of

formation of protective oxide film on their surface.

(e) Polycrystalline forms are more sensitive.

2. Nature of Environment

(i) Temperature: directly proportional

(ii) Humidity: faster in humid conditions

(iii) pH : If less than 7 rate is high. Al, Zn, Sn, Pb, and Fe are affected by

both acid and bases.

(iv) Impurities and Suspended Particles: When these will get dissolved

in moisture, provides electrolyte for conductivity and hence

corrosion increases.

Corrosion Control:

1. Selection of metal and alloy:

- Using pure and noble metals

- Practically not possible because of low strength of pure metal

- Use of metal alloys which are homogeneous

2. Proper design of metal:

(i) Minimal contact with medium

(ii) Prevention from moisture(ii) Prevention from moisture

(iii) Adequate ventilation and drainage

(iv) Welding

(v) Avoid cervices b/w adjacent parts

(vi) Bend should be smooth

(vii) Bimetallic contacts should be avoided

(viii)Paint cathodic portion

(ix) Prevent uneven stress

3. Cathodic Protection:

Force the metal to be protected to behave like cathode.

(i) Sacrificial anodic protection:

- Metal to be protected from corrosion connected to more anodic

metal

- Commonly used metals Mg, Zn, Al and their alloys

(ii) Impressed current method:

- Direct current is applied in opposite direction to nullify the

corrosion currentcorrosion current

- Converts the corroding metal from anode to cathode.The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again.

The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again.

4. Modifying Environment

(i) Eliminating dissolved oxygen:

- De-aeration

- By using chemical substances like sodium sulphite and hydrazine.

Also called Deactivation.

(ii) Reducing Moisture:

- Dehumidification by using silica gels

(iii) Reducing Acidity:

- Neutralizing the acidic environment by adding lime, NaOH,

Ammonia

- Commonly used in refineries

5. Protective coating:

- Application of coating

- Coating material should be chemically inert under particular temp

and pressure.

6. Use of corrosion Inhibitor

(i) Anodic Inhibitor:

- These are oxygen and oxidizing agent.

- They combine the anodic metal forming an oxide film which reduce

corrosion

(ii) Cathodic Protection:

- Organic inhibitors like amines, mercaptans, urea and thiourea

reduces the H ion diffusion by adsorption

- Mercury, arsenic and antimony deposits films at cathodic area

which raise the hydrogen over volume.

- Eliminating Oxygen from the medium by adding sodium sulphate

and hydrazine.

The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again.

Protective Coating

Surface preparation for Coating:

1. Cleaning:

- To prepare for suitable condition

- Removing contaminants to prevent detrimental reaction product

- E.g. de-greasing, sand blasting, vapour degreasing, pickling and

alkaline cleaning.

2. Solvent Cleaning:

- Must be non-inflammable and nontoxic

- Trichloro trifluoroethane which has low toxicity are costlier

- Vapour de-greasing is economical and advantageous because of

continuous cleaning with small quantities of solvent.

3. Electrolyte Pickling:

- Provides better and rapid cleaning by increasing hydrogen evolution

resulting in agitation and blasting action

- Sand blasting is mechanical cleaning.

4. Alkaline Cleaning:

- Cheaper and less hazardous

- Used in conjunction with surface active (wetting) agent

- Ability depends on pH, rapidly decreases below 8.5

- Other abilities are rinsability, detergent properties, sequestering,

wetting etc.

5. Acid Cleaning

- Acid such as HCl, H2SO4, H3PO4 is very effective.

- 5-10% H2SO4 and HCl used to remove inorganic contaminants.

- Pickling are performed at high temp. (60 ̊C)

- Is effective for removal of grease, oil , dirt and rust.

Methods of Application of Metallic Coating

1. Hot Dipping:

- Metal is kept in molten state and base metal is dipped into it.

- Used for producing a coating of low M.P

- E.G. Tinning (Tin coating on Iron)

- Process is followed by cooling the coating through a palm oil to

prevent oxidation of tin plate to its oxide.

- Palm oil layer is removed by alkaline cleansing agent.

2. Metal Cladding:

- The surface to be protected is sandwiched between two layers of

the coating metals and pressed between rollers.

- E.g. Alclad Sheeting– Plate of duralumin is sandwiched between

99.5%pure aluminum

3. Electro Plating:

- Pure metal is made as cathode and base metal as anode.

- Electrochemically coat metal is deposited on base metal.

- This metal gives smooth, fine and uniform coating

- It depends on

(i) Temperature (ii) Current density (iii) Electrolyte Concentration

(iv) Nature of base metal (v) Time

4. Electroless Plating:

- Nobel metal is deposited catalytically on less noble metal by using

reducing agent without using electrical energy.

- Advantage over Electro plating

(i) More economical since no electricity required

(ii) Irregular shape can be plated uniformly

(iii) Plating on plastics can also be done

5. Metal Spraying:

- Coating is applied by means of spraying device

- E.g. Aluminum is plated in this way on Aircrafts.

Chemical Conversion Coating

• These are formed on metal surface by chemical reaction b/w metal

surface and inorganic salt solution

• Coating base metal is converted into one of the resultant protective

film.film.

• These films are insoluble, adherent, crystalline or amorphous in

nature.

• Can be done in 3 ways

1. Phosphate coating

2. Chromate coating

3. Anodized coating

1. Phosphate Coating

- Produced by chemical reaction b/w base metal and aq. H3PO4, Zn or

Fe or Mn Phosphate

- Phosphate coating are applied Iron, Steel, and Zinc

- Film formed on base metal after coating consist of Zn-Fe, Mn-Fe

Phosphates.

2. Chromate Coating2. Chromate Coating

- Produced by dipping the base metal in Potassium chromate (acidic)

followed by immersion in neutral chromate bath.

- Resulting film consist of trivalent and hexavalent chromium.

- Used as base for paints, lacquers and enamels.

3. Anodized Coating

- Formed by anodic oxidation process

- This is produced on non-ferrous metals like Al, Zn, Mg

- In this method base metal is made as anode

- Process is carried out by passing moderate direct current through a

bath in which the metal is suspended as anode.

- Coating are formed as a result of Progressive oxidation starting at

surface of base metal.The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again.

Solutions

When substances dissolve to form

a solution, the properties of the

mixture change.

A solution is a type of mixture.

⚫Solution: a type of mixture that is the

same throughout.

A solution can be physically separated

All portions of the solution have the same

properties

⚫Homogeneous: the same

⚫Examples: salt water, blood, sugar

water, gasoline

Solutes and Solvents

⚫Solute: a substance that is dissolved to

make a solution.

when dissolved it separates into individual

particles

⚫Solvent: is a substance that dissolves a

solute.

when the solute is dissolved into the solvent it

is not possible to identify the solvent and solute

as individual parts

Examples of Solutes and Solvents

⚫ Saltwater:

salt=solute,

water=solvent

⚫ Blood: calcium ions,

sugars=solutes,

water =solvent

saltwaterfishing365.com

ucdavismagazine.ucdavis.edu

Types of Solutions⚫ Solutions can be made from

solids, liquids, and gases

⚫ Air: solute=oxygen, solvent=nitrogen (oxygen is dissolved in nitrogen)

⚫ Humidity: solute=liquid, solvent=gas (water is dissolved into air)

⚫ Stainless steel: solute=chromium metal, solvent=iron (chromium metal is dissolved in iron to form a shiny steel)

www.germes-online.com

askville.amazon.com

Suspensions

⚫ Suspension: a mixture in which

different parts are identifiable as

separate substances

(heterogeneous mixture)

⚫ Heterogeneous: different, not the

same

⚫ Flour and water. The suspension

looks cloudy and can be separated

by using a filter. www.chemistrydaily.com

Solvent and Solute Particles Interact

⚫The parts of a solution can be physically

separated because they are not changed

into new substances.

⚫The way a solid compound dissolves in a

liquid depends on the bond type.

Ionic( e- are lost or taken)

⚫Separate into ions (+ or – charged particles) in the

solution

Covalent ( e- are shared)

⚫Separate into individual molecules (2 or more

elements combined together)

Properties of Solvents change in

Solutions

⚫A solution’s physical properties are

different from the physical properties of the

pure solvent.

⚫The amount of solute in the solution

determines how much the physical

properties of the solvent are changed

Lowering the freezing point

⚫Freezing point: temperature at which a liquid becomes a solid

⚫The freezing point of a liquid solvent decreases when a solute is dissolved in it.

⚫ExampleWater, pure = 0 degrees C.

Water + salt = a freezing point lower than 0 degrees C.

coolcosmos.ipac.caltech.edu

Lowering the freezing point

⚫Making Ice Cream

Depends on lowering the freezing point of a

solvent

Canister hold liquid ice cream ingredients

Put in a larger container containing ice and salt

⚫Salt lowers the freezing point of the mixture

⚫Causes ice to melt (absorbing heat from

surroundings)

⚫Ice cream mix is chilled when its contents are

constantly stirred

⚫Tiny ice crystals form all at once instead of gradually

⚫Causing the ice cream to be smooth and creamy

Raising the boiling point

⚫ Boiling point: temperature

when a liquid turns into a

gas

⚫ A solution’s boiling point can

be raised by the amount of

solute in the solvent.

⚫ Example:

Antifreeze added to cars

prevent it from

overheating or having the

liquid’s reach their boiling

point

www.state.tn.us

www.classic-car-magazine.co.uk

The amount of solute that dissolves can

vary

⚫ Concentration: The amount of

solute dissolved in a solvent at

a given temperature.

⚫ Examples:

Hot chocolate…the more

powdered mix you add the higher

the concentration of chocolate

Lemonade…the more frozen

lemon concentrate or powdered

mix you add the more tart the

drink becomes

www.thesunblog.com

Degrees of Concentration

⚫Dilute: a solution has a low concentration of solute

⚫Saturated: a solution that contains the maximum amount of solute that can be dissolved into the solvent at a given temperature.

⚫Supersaturated: a solution can contain more solute than normal by raising the temperature of the solvent.

www.seroundtable.com

Solubility

⚫Solubility: the amount of the substance

that will dissolve in a certain amount of

solvent at a given temperature.

⚫The solubility of a solute can be changed

by raising the temperature

If solute is a gas…then you can change the

pressure…higher pressure of gas in a liquid

increases the amount of gas that can be

dissolved

Solubility and Temperature

Solute Increased

Temperature

Decreased

Temperature

Solid Increase in

solubility

Decrease in

solubility

Gas Decrease in

solubility

Increase in

solubility

Solubility and Pressure

Solute Increased

Pressure

Decreased

Pressure

Solid No effect on

solubility

No effect on

solubility

Gas Increase in

solubility

Decrease in

solubility

Solubility depends on Molecular Structure

⚫When a substance

dissolves, its molecules

(covalent bonds) or ions

(ionic bonds) separate

from one another and

become evenly mixed with

molecules of the solvent

⚫Water contains polar

covalent bonds.

Negative region (O)

Positive region (H)

www.emc.maricopa.edu

Polar and Nonpolar

⚫Water and oil do not mixWater is polar…negative and

positive regions

Oil is nonpolar...no charge

Molecules are not attracted to each other (opposite charges attract each other) so they do not mix

⚫Water and sugar or salt mixWater is polar

Salt and sugar are also polar

Opposite charged molecules are attracted to each other

www.historyforkids.org

Solutions can be acidic, basic, or neutral

Acid Base

Donate H+ ion

(proton…atomic number

is 1=1 proton)

Can accept a H+

ion...usually release an

OH- ion than can accept

a H+ ion.

Taste sour Taste bitter

Produce burning or

prickling sensation on

skin

Feel slippery

React with most metals

www.mhhe.com

pH scale

⚫pH scale: potential of Hydrogen

⚫Higher H+ lower number (Acid)

⚫Lower H+ higher number (Base)

⚫Range is 0-14, 7 is neutral

⚫Common Acids and Bases

Base: Soap pH 10

Acid: lemon juice pH 2

⚫Acids and Bases neutralize each other

Metal alloys are solid mixtures

⚫ Alloy: a mixture of one or

more metals and one or more

other elements…made from

melting and mixing

⚫ Examples:

Brass: zinc and copper

Bronze: tin and copper

Stainless steel: chromium

and iron

www.kengem.com

art-foundry.com

Fresh Water resources

Global Overview

⚫ While 67% of Earth’s surface is covered by

water, only less than 2.7% of global water is

freshwater. Most of the freshwater (2.05%)

are locked in ice caps and glaciers. Only

less than 0.7% is available for human use.

Over two thirds of the

earth's surface is covered

with water, 97.2% of which

is contained in the five

oceans. The Antarctic ice

sheet, containing 90% of all

fresh water on the planet, is

visible at the bottom.

Atmospheric water vapour

can be seen as clouds,

contributing to the earth's

albedo.

Iceberg and Polar cap store most of the fresh water

on Earth

Volume of water stored in

the water cycle's reservoirs

Reservoir Volume of water

(106 km³)

Percent

of total

Ocean 1370 97.25

Ice caps & glaciers 29 2.05

Groundwater 9.5 0.68

Lakes 0.125 0.01

Soil Moisture 0.065 0.005

Atmosphere 0.013 0.001

Streams & rivers 0.0017 0.0001

Biosphere 0.0006 0.00004

Scarcity of fresh water

⚫ On a global basis, fresh water is a

increasingly scarce resource. It is partially

caused by increasing population coupled by

change of consumption pattern and climate

changes.

Water consumption for food

production (I)

⚫ Meat production use a lot of water when

compared to growing food crops.

⚫ A shift in food consumption pattern toward

more meat consumption will cause a

substantial increase in water consumption.

Water consumption for food

production (II)

Competing water uses (I)

Competing water uses (II)

⚫ Industrialized / developed countries tend to

use more water in their industrial

production.

⚫ Other countries tend to use more water for

agricultural uses.

Fresh Water supply

Problems related to Water crisis

⚫ Inadequate access to safe drinking water by over 1.1 billion people

⚫ Groundwater overdrafting leading to diminished agricultural yields

⚫ Overuse and pollution of water resources harming biodiversity

⚫ Regional conflicts over scarce water resources sometimes resulting in warfare.

Threats to fresh water resources

⚫ Climate change causes change in

frequencies of droughts and floods.

⚫ Depletion of aquifers caused by over-

consumption as a result of population

growth.

⚫ Pollution and contamination by sewage,

agricultural and industrial runoff.

Distribution of population and

water resources

Pressure of freshwater ecosystem

Fresh Water Outlook

⚫ Estimated from existing data, some

countries are going to experience serious

shortage of fresh water supply in the

coming 20 years time.

⚫ China, India and South Africa and Middle

East countries may among the most

adversely affected countries.

Water in China

⚫ According to the World Bank forecast, Mainland China has only a per-capita share of 2700 cubic meters per annum, one fourth of the world's average at present.

⚫ Half of China's 617 largest cities face water deficits. Beijing is among the most water-short.

⚫ The areas south of the Yangtze River, China's longest, which account for only 36.5 per cent of the country's total territory, have 80.9 per cent of its total water resources. However the areas north of the Yangtze, which make up 63.5 per cent of China, possess only 19.1 per cent of total water resources.

Desalination of sea water as

fresh water supply

⚫ Desalination of sea water can be done either

via distillation or membrane process.

⚫ Both process requires large amount of

energy and thus costly, which means

desalination remains an expensive option

for providing reliable fresh water supply,

restricted to only economically well-off

countries.