Corrosion 1

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Institute www.uniqueinstitutes.org CORROSION The Process of constant eating up of the metals from their surface by the action of surrounding is known as corrosion of metals. Only noble metals are not corroded from

the

surrounding. Example of corrosion: (i)

Rusting of Iron.

(ii)

Appearance of Green film of copper plate.

(iii) Corrosion of boiler plates Types of Corrosion: There are two types of corrosion: (i)

Atmospheric Corrosion

(ii)

Immersed Corrosion

(i)

Atmospheric Corrosion: This type of corrosion occurs when metal are come in contact with air and take place due to active gases and moisture. The active gases (O2, Co2, H2S, and SO2) react with metal

and form metal salts. Iron reacts with oxygen and water forming hydrated oxide. In the presence of Co2, A green layer of film will be formed on copper. Atmospheric corrosion also happens due

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Institute www.uniqueinstitutes.org to moisture. It reacts with metal in the presence of oxygen and Co2. The suspended particle is increased the rate of corrosion. Dust, carbon are the suspended particle. Immersed Corrosion: When metal are present in water or other liquid for sometime. Then it reacts and corroded. The rate of corrosion depends on: (i)

Nature and concentration of cation and anion.

(ii)

Concentration of dissolved oxygen.

(iii) Temperature (iv) Nature of corrosion product Theory of Corrosion: Corrosion depends on following two theories: (i)

Direct Chemical Action Theory: Corrosion is only happened due to presence of acid particle like carbonic acid (CO3). Rusting of Iron is due to moisture, oxygen and carbon-dioxide. The iron converts in ferrous bicarbonate which is further oxidized to basic ferric carbonate. Fe + O + CO2 + H2O 2Fe (HCO3)2 + H2O + O

2Fe(OH) CO3 +2Co2 +2H2O

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Fe +(HCO3)2

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Institute www.uniqueinstitutes.org 2Fe (OH) Co3 + H2

2Fe (OH)3 + 2CO2

Iron first reacted with oxygen, carbon dioxide, and water and forms ferrous bicarbonate. It converts in Ferric carbonate. After reaction with water, it becomes rust.

(ii)

Electro Chemical Theory: This is due to uneven surface of reactive metals like Iron behaves in small cell, In which O2, CO2, H2O are electrolytes. At anode :

Fe

At Cathode

:

Fe+2 + 2eHCO-3 + H+

H2CO3

H2O

OH- + H+

The overall reaction by electrolytic method: 4Fe+2 + O2 + 4H2O Fe2O3 + XH2O (Rust)

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2Fe2O3 + 8H+ Fe2O3

X

H2O

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Institute www.uniqueinstitutes.org PROTECTION OF METAL: The process of preventing the metallic surfaces from being corroded by their surrounding is known as protection of metals. 1.

Alloy: Alloys resist corrosion better than metals. The corrosion takes place only on the surface of metal. The metal is alloyed at the surface only to form protective film. Steel is an example of alloy.

2.

Metallic Coating: The life of metal can be improved by applying coating of some corrosion resistance metals on their surface. The metallic coating is generally done by Zn, Ni, Tin, Copper, Aluminium and Chromium. These coatings act as a barriers and stop the effect of gases. But before apply metallic coating, we should do some pretreatment.

Pretreatment: The surface of metal should be cleaned before applying coating. It should be free from dust .There is no film of compound should exist on surface. (i)

Hot alkali solution is used to remove greasy spots.

(ii)

Dilute acid (HCl, H2So4) are used to remove the films of metal salts. These are also removed by rubbing.

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Institute www.uniqueinstitutes.org HOT Dipping: The metal to be coated is passed through a bath of molten coating metal and thickness of the coating metal is adjusted by adding out the excess of coating metal with roller. These are non uniform. There are two methods: 1.

Galvanising: It is the process of coating of Iron with a thin coat of zinc.

2.

Tinning:

it is coating of tin over the iron.

(a)

Spraying: It is done on bridges, tanks, ships, and tower.

(b)

Clading: Duralium claded with aluminium produces a material which combines the strength of alloys copper cladded steel is used in electrical industry. Alclad steel is used in aircraft industry.

Electroplating: The rate of dissolution at the anode and rate of deposition at the cathode are equal. It is a process of depositing a noble metal on another metal by electrolytic method. CEMENTATION: The metal is surrounded from all sides by the powerdered metal to be coated and packed in a box. It is heated for a long time to a temp below the point of fusion. The

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Institute www.uniqueinstitutes.org superior metal diffuses in the article and form alloy and prevent corrosion. The cementation of Zn with Iron is called SHERADIZING and with aluminium is called colorizing. CATHODIC PROTECTION: This method is to force the metal to be protected to behave like cathode. i.

Anode Sacrificial Protection: In this method, the metallic plate is connected by wire to more anodic metal. Therefore all corrosion is concentrated on more active metal. Due to this, more active metal gets corroded slowly, while original metal remains safe. Due to this, it is called sacrificial protection. Metals employed as sacrificial anode are magnesium, zinc,

Aluminium and their alloy. In cathodic protection, an anode of more strongly reducing metals is sacrificed to maintain the object.

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Institute www.uniqueinstitutes.org ii.

Impressed Cathodic Protection: In this method, an impressed current is applied in opposite direction to nutrilize the corrosion current and convert the corroding metal from anode to cathode. A sufficient D.C. current is applied to an insoluble anode and dip in the soil. Then connected to the metallic structure

be protected. In this electrons are supplied from an external cell so that object itself becomes cathodic and is not oxidized. It is useful for large structures for long terms operation.

FUEL

AND COMBUSTION

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Institute www.uniqueinstitutes.org Fuel is a combustible substance containing carbon which on burning in air produces large amount of heat that can be used economically for domestic and industrial purpose.. Classification of Fuels: There are three types of Fuels: i.

Solid Fuels

ii.

Liquid Fuels

iii.

Gaseous Fuels

Primary Coal: Wood, Peat, Lignite, Bituminous, Anthracite etc. Natural Gas, crude Petroleum, Gasoline. Artificial Coal: Water gas, Producer gas, coal gas, oil gas, biogas. Analysis of coal: In coal, we determine ash, moisture, volatile matter and fixed carbon present in coal. (a)

Determination of Moisture: Weight accurately some quantity of coal in Crucible lid. . Place crucible inside an electric hot air oven at temp. 1050C. It remains for one hour. Then taken out and cooled;

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Institute www.uniqueinstitutes.org Weight of the empty crucible = m1g. Weight of crucible + Coal sample = m2g Weight of crucible + anhydrous coal left as residue = m3g Weight of coal sample = (m2-m1) g Loss in weight = (m2-m3) g % of moisture = m2-m3 / m2-m1 X 100 (b)Similarly determine volatile matter % volatile =

Weight of volatile matter removed Weight of moisture free coal

ASH:

Weight of ash formed X 100 Weight of dry coal

(c)

Percentage of Fixed Carbon: The percentage of carbon alone in the sample of coal is called percentage of carbon. To get carbon, the sum of percentage of moisture, volatile matter, ash is subtracted from 100. This gives percentage of carbon.

IMPORTANCE:

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Institute www.uniqueinstitutes.org i.

Moisture tells us about amount of water, lesser the moisture, better the quality of coal. It increases the transport charge.

(ii) Ash: Due to ash i.

Calorific value is decreased.

ii.

The removal and disposal of ash is problem.

Volatile matter: A large percentage of volatile matter indicates that a large proportional of fuel is changes into gas. It produces smoke. ULTIMATE Analysis: It is perfect method to determine carbon, hydrogen, nitrogen, sulphur, oxygen. (a)

Determination of carbon and hydrogen: Weight of coal sample taken = m gm. It consist a known quantity of coal is burnt and passes through two bulbs. In which one contain CaCl2 and other containing KOH. Due to increase in the wt of CaCl2 represent water, while KOH represent CO2. Wt. of coal sample = m gm Wt. of KOH sample = a gm Wt of CaCl2 bulb = b gm

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Institute www.uniqueinstitutes.org Amount of carbon = 12/44 X a gm, Percentage of carbon = 12a/44m X 100 Amount of hydrogen = 2/18b gm % of Hydrogen = 2/18 X b/m X 100

(b)

Determination of % of Nitrogen: A powdered coal is treated with conc. H2So4 in the presence of K2SO4. It is heated in KJeldahl15 flask till the solution becomes clear. The nitrogen converted into (NH4)2 So4. The ammonia produced is absorbed in a known quantity of standard H2So4. The amount of acid neutralized by ammonia. Let weight of coal taken = m gm. Weight of NH3 calculated = a gm Weight of nitrogen = 14a/17 gm %of nitrogen = 14a/17 X 100

(c)

% of Sulphur: A known quantity of powdered coal is heated nitric acid. The sulphur becomes sulphuric acid. It reacts with BaSo4. Wt of coal taken = m gm Wt of BaSo4 = a gm

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Institute www.uniqueinstitutes.org Amount of sulphur in BaSo4 = 32a/233 % of sulphur = 32a/233m X 100

(d)

Determination % of oxygen: We add percentage of carbon, hydrogen, nitrogen, sulphur and sum is subtracted from 100.

IMPORTANCE: It helps in classify of coal. The combination of ‘CHONS’.

1.

Carbon: Greater the %of carbon, greater calorific value.

2.

Hydrogen: Lesser the % of H, Better the quality of coal.

3.

Oxygen: High oxygen coals are characterized by high moisture, low calorific value and absence of cooking powder. Greater percentage of oxygen in not suitable.

4.

Sulphur: It is also undesirable. It pollutes the atmosphere.

UNIT S of Heat: C.G.S. Calorie is amount of heat required to raise the temp. of one gram of water through one degree. S.I. Unit: S.I. unit of heat is joule.

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Institute www.uniqueinstitutes.org Calorific value of fuel: It is totally quantity of heat produced by complete combustion of a unit mass of fuel in air when products of combustion are cooled down to a definite temp. Higher calorific value: It is defined as total amount of heat produced when one unit mass of the fuel has been burnt completely and products of combustion are called down to room temp. Net or Lower calorific value: It is difference between gross calorific value and latent heat of water vapour. Theoretical value: Hydrogen – 34500 Carbon – 8000 Sulphur – 2240 H.C.V. = 1/100 [8080C + 34500 (H- 0/8) + 22405] Kcal/k L.C.V = H.C.V – 9/100 H x 587 Kcal/Ks Solid Fuels: 1.

Wood: It is obtained form forest. It consist mainly [C1H1O]. It also contains some resins, waxes, inorganic matters.

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Institute www.uniqueinstitutes.org Composition: Carton = 50%; Oxygen = 73%, Hydrogen = 6%, Ashes = 0.5 to 15%, Nitrogen = 1.2% It burns with a long luminous and smoky flame and very little around of ash. It calorific values 3500-4400 Kcal/Kg.

2.

Peat: When wood is changing into coal, Peat is first stage its colour brown to black.

Composition: Carbon – 56%, Nitrogen = 15%, Oxygen = 35%, Hydro = 5%, Sulphur = 0.5%. Calorific value of Peat is 4500-5400 Kcal/Kg. It is unsuitable for industry.

3.

Lignite: It contains two parts (I) Brown Coal (ii) Black coal. In black sample, carbon content is greater than brown.

Composition: Carbon = 60-70%; Sulphur – 1.5%, Oxygen = 15.1g Nitrogen = 1.5%, Hydrogen = 5%, Ash = 4% Calorific value of Lignite is 5500-7000K. It is household fuel and for steam raisins in boiler.

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Institute www.uniqueinstitutes.org 4.

Bitumines coal (ii) Anthracite coal. There are also artificial solid fuels. (i)

Charcoal (ii) Coke

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Institute www.uniqueinstitutes.org GASEOUS FUEL: Natural gas: It is important gaseous fuel which is

(i)

obtained from underground. Crude oil. A mixture of gaseous and volatile constituent of petroleum is called natural gas. It is mainly contained methane, ethane other hydrocarbon. It is also called marsh gas. LPG is a mixture of propane and butane. Properties: 1.

It is a mixture of methane (70-90%), ethane, propane, butane and other like Co2, H2S1N2 and gases.

2. It is excellent gaseous fuel and burn with blue flame. 3. Its calorific value is 1200 Kcal. Per cubic meter. Use: 1. It can be sued for generation of electricity by using fuel. 2. It is used for manufacturing black carbon. WATER GAS: It is a mixture of carbon monoxide and hydrogen. It gives blue flame and mixture of C and H2. C+2H2O

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CO+ H2O-29000cal

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Institute www.uniqueinstitutes.org This reaction is exothermic: At temp. 10000C, steam reacts. C to form CO2. 2H2O+C

2H2O+ CO2-19000Cal.

When temp. Below 10000C, the steam blast is stopped. An air blast is passed through the red hot bed of coke for some time to raise the temp. Upto 14000C.

Properties: 1.

The composition of water gas-CO= 44%, H2= 48%, CO2 = 3%. N = 45%, CH4 = 0.5%.

2.

It burns with non luminous flame.

3.

It has high calorific value of 3200 Kcal/Cubic meter.

4.

It is mixed with hydrocarbon is called carbureted water gas which is good used for lighting.

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Institute www.uniqueinstitutes.org 5. It is good heating fuel. Use: (i)

It is used in manufacture of CH3OH. Coke.

(ii)

It is used for welding

(iii)

It is used for obtaining H2 on commercial scale. It is used for lighting and heating fuel.

BIO GAS: It is a mixture of methane, carbon dioxide, hydrogen, hydrogen sulphide. Biogas contains 65% of methane gas. It is obtained by anaerobic fermentation of animal waste. Like animal dung in the presence of water and bacteria. Animal dung and plant wastes contain a lot of carbon compounds like carbohydrates, proteins and fats. Manufacture: The raw material is a mixture of cattle dung and water. Fixed dome type consists of a well shaped, under ground. Its domes act as gas holder for bio gas. There is gas outlet “S” at the top on value.

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Institute www.uniqueinstitutes.org There is an inlet chamber in left side and outlet chamber in right side. The inlet chamber is connected to a mixing tank. M while outlet chamber is connected to overflow chamber.

Properties: 1.

CH4 = 65%, H2 = 7.4%, CO2 = 35%, N2 = 2.6%.

2.

It is obtained by anaerobic fermentation.

3.

The formation value is more than dry dung.

4.

The cattle dung and water are mixed in equal proportional in mixing tank to prepare slurry. The dome being left free from the collection of bio gas. It takes 50 to 60 day for new gas plant to become.

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Institute www.uniqueinstitutes.org Operative: Due to formation, bacteria in the presence of water, bio gas become. Due to large quantity of bio gas, it exerts pressure in diagster tank and slurry comes out from overflow tank. The bio gas which is collected in the dome of the diagster is taken out through outlet. Use: i.

Biogas is used for cooking food.

ii.

It burns with smoke and hence does not produce air pollution.

iii.

It has high calorific value.

iv.

It is very clean and convenient to use.

v.

It is cheaper in use.

vi.

It is used for running engine.

vii. It is used for street lighting. PRODUCER’S GAS: It is a mixture of carbon mono-oxide nitrogen. It calorific value is low.

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Institute www.uniqueinstitutes.org Manufactures: It is prepared due to red hot coke. The furnace in which gas is produced called produced. There is a arrangement for blowing air inside and extra arrangement for ash. C+ O2 CO2 + C

CO2 + 97 Kcal 2CO-39K Cal

COMPOSITION: CO = 30%, N2 = 50% H2 = 8-12% Properties: It is poisonous gas, insoluble in water, its calorific value of gas is 1200 Kcal/Cubic meter

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. Use: i.

It is used as heating fuel for furnace.

ii.

It is used to provide reducing atmosphere.

iii.

It is used for heating iron retorts.

CHARACTERISTICS OF GOOD FUEL: 1. It should high calorific isalue. 2. It should low moisture content. 3. It should be readily available in bulk at lowest. 4. It should posses moderate ignitation temp. 5. Its cost of storage in bulk should be low. 6.

It should not undergo spontaneous combustion.

7. Its combustion easily controllable. 8. Its product of combustion should not harmful. 9. It should be easily transport. Advantages of Gaseous fuel: (i)

The gaseous fuels do not produce any ash.

(ii)

The nature of flame can be regulated to oxidizing.

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Institute www.uniqueinstitutes.org (iii) They can be lighted easily. (iv) They are conveyed to the place of need. (v)

They are economical than solid and liquid.

(vi) They burn with more efficiency and high flame is obtained.

LUBRICANTS A lubricant is a substance which reduces for of friction between two relatively moving surface sin contacts with each other. Effects: It reduces the force of friction. The lubricants form a thin film between the suding surfaces and keep them apart. It reduces the force of friction between the sliding surfaces. 2. It reduces the expansion of metal by local frictional heal. 3. It minimizes the power loss due to friction. 4. It absorbs shock between bearing and other engine parts.

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Institute www.uniqueinstitutes.org 5. It acts as seal. 6. It acts as coolant, which cool the engine. There are three types of lubrication. 7. Hydrodynamic lubricants: In these types of lubrication. The moving surfaces are separated by filter of thicker than 5000A0. The moving parts never touch other under fluid lubrication. If the rate of relative motion between two moving surfaces is moderate, there is sufficient amount of supply of lubricant. Minerals oil are used for this purpose. 2. Boundary lubrication: When an oil film is subjected to a high load, the speed of moving parts gets reduced. The film decreases in thickness to a point where a fluid lubricant breaks down. The fiction force in presence of a boundary film is usually about of forty times than under fluid lubrication. Films of ferrous oxide Feo, MgAl2O4 are used. Graphite is largely used a solid lubricant. It provides a surface layer on metals and the friction between metal surfaces is minimized. Extreme Pressure Lubricant: These are those lubricants is which both temp. and pressure are extremely high when surface

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Institute www.uniqueinstitutes.org are sliding under conditions of boundary lubrication, some metals function still pressure. Extreme Pressure lubrication is that substance which reacts within metal surfaces at high tem. These consist of minerals oil with the addition of compounds containing sulphur or chlorine or phosphorus atom under some condition, graphite, MOS2 (holybraclisulphide and born nitride are used. Types: i.

Solid lubricant

ii.

Semi solid

iii.

Liquid.

Solid lubricants: Graphite, zincoxide, molybdenum disulphide are used solid lubricant. a)

Graphite: It is soapy to touch and elastics. It is not affected by acid, alkalies or heat. It does not catch fire easily when it is mixed with watch it is called aqua dog and mixed with oil, called oil dag.

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Institute www.uniqueinstitutes.org Zinc oxide is also an effective lubricant. It is mixed with oil and lubrication for those machines which used for food processing. Semi solid lubricants: These are used where does not stick to sliding surface. They are used for bearing working under high temperature. The examples of senisolid are greerse, Vaseline, waxes. LIQUID: (i) Vegetable oil (ii) Animal Oil (iii) Mineral oil (iv) Synthetic oil (iv) Blended oil SELECTION of lubricants: The properties of a properly selected lubricant should not change under service condition: 1.

Lubricants for cutting tools: Cutting fluid are lubricants used in machining, cutting, turning and grinding of metals. The main functions of cutting fluids are (a) the cool the metal work piece (c) To reduce power consumption.

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Institute www.uniqueinstitutes.org 2.

Lubricants for Gears: They should posses’ good oiliness, high resistance to oxidation and high load carrying capacity.

3.

Lubricants for delicate instruments: Delicate instruments like watches, clocks and sewing machine are not exposed to high temp. There For vegetable oil, palm oil are used.

4.

Lubricants for steam engine: Lubrication of the steam engine cylinders is done by ejection of lubrication by oil drop wise. The oil posses high viscosity. It should not hydrolyze in presence of steam at high temp.

5.

Lubricant for steam turbines: These are compounded oil containing additives which provide high oxidation stability, corrosion resistance.

1.

Viscosity: It is defined as opposition offered by liquid. It is

also defined as the tangential force in newton required to move 1m2 of liquid over another surface with velocity 1cm./s. It measures internal friction between moving parts sliding each other. A good lubricant body at high temp. The apparatus used for measurement viscosity is called viscometer. 2.

Viscosity index: This properly changes with temp. The

rate of change of viscosity of oil with temp is known as

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Institute www.uniqueinstitutes.org viscosity index. A good lubricant should have www.visces index, therefore change of viscosity with temp. is very small. At low temp viscosity will increase in such that the oil well fails to move into the machine parts. 3.

FLASH POINT: It is defined as the lowest temp. at which

it gives sufficient amount of vapours as to form an inflammable mixture with air and vapour. It gives a flash upon application of flame. Fire point of a liquid is the lowest temp. to which the oil must be heated to burn continuously when a test flame is applied to it. The fire point of a lubricant is higher than flash point. 4.

Pour point and cloud point: It is the temp. below which

oil can not be used as a lubricant. A good lubricant should have low pour point. The can be used for a longer span of temp. the cloud point is that temp. at which the cloudiness appears due to separation of wax from the oil. 5.

Neutralization value: It is an indication of acidic an basic

impurities in the lubricant. It is also defined as milligrams of kilo of oil. Cause: (I) Product of oxidation of oil.

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Institute www.uniqueinstitutes.org (ii) Contamination of oil by SO2 from combustion of fuel. (iii) Additive used in improving the properties of oil. A good lubricant should have low and value because it the acid value a high, the acid will correct the machine parts. Emulsification: It is property of absorbing dust particle and other impurities of oil. Emulsion act as abrasive and responsible for wear and tear of and. It should not remain for a ling time and should break quickly. It is useful when lubricating oil is used in the presence of water in the machinery. Thick oil has high steam emulsion should be low viscous emulsion oil. Steam emulsion should be low.

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