Water technology lecture notes

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Published Date:22-07-2017
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Unit I Water Technology 1.1 INTRODUCTION Water is essential for the existence of human beings, animals and plants. Though 80% of the earth‟s surface is occupied by water, less than 1% of the water is available for ready use. The main sources of water are  Rain  rivers and lakes (surface water)  wells and springs (ground water)  sea water Among the above sources of water, rain water is the purest form of water but it is very difficult to collect whereas sea water is the most impure form. Thus, surface and ground water are normally used for industrial and domestic purposes. Such water must be free from undesirable impurities. “The process of removing all types of impurities from water and making it fit for domestic or industrial purposes are called water treatment.” Before treating water one must know the nature as well as the amount of impurities. 1.2 HARD WATER AND SOFT WATER  Hard Water “Water which does not produce lather with soap solution, but produces white precipitate is called hard water”. This is due to the presence of dissolved Ca and Mg salts. ++ + 2C H COONa + Ca → (C H COO) Ca +2Na Water soluble Water 17 35 17 35 2 insoluble  Soft Water “Water which produces lather readily with soap solution is called soft water.” This is due to the absence of Ca and Mg salts. BOILER FEED WATER In Industry, one of the chief uses of water is generation of steam by boilers. The water fed into the boiler for the production of steam is called boiler feed water. Requirements of boiler feed water  It should have zero hardness.  It must be free from dissolved gases like O , CO , etc. 2 2  It should be free from suspended impurities.  It should be free from dissolved salts and alkalinity.  It should be free from turbidity and oil.  It should be free from hardness causing and scale forming constituents like Ca and Mg salts. FORMATION OF DEPOSITS IN STEAM BOILERS AND HEAT EXCHANGERS 1.5.1 Sludge and Scale formation in boilers In a boiler, water is continuously converted into steam. Due to this continuous evaporation of water, the concentration of soluble matters increases progressively. Then the salts separating out from the solution in the order of their solubility, the lease soluble ones separating out first. (i) Sludge  If the precipitate is loose and slimy it is called sludges.  Sludges are formed by substances like MgCl , MgCO , MgSO and CaCl . 2 3 4 2  They have greater solubilities in hot water than cold water.  Scale 1) If the precipitate forms hard and adherent coating on the inner walls of the boiler, it is called scale. 2) Scales are formed by substances like Ca (HCO ) , CaSO and Mg (OH) . 3 2 4 2 1.5.2 Disadvantages (i) Wastage of fuels Scales have a low thermal conductivity, so the rate of heat transfer from boiler to inside water is greatly decreased. In order to provide a supply of heat to water, excessive or over-heating is done. This causes increase in fuel consumption. The wastage of fuel depends upon the thickness and the nature of scale. (ii) Decrease in efficiency Scales sometimes deposit in the valves and condensers of the boiler and choke them partially. This results in decrease in efficiency of the boiler. Boiler explosion When thick scales crack due to uneven expansion, the water comes suddenly in contact with over-heated iron plates. This causes in formation of a large amount of steam suddenly. So sudden high-pressure is developed, which may even cause explosion of the boiler. 1.6 PREVENTION OF SCALE FORMATION (OR) SOFTENING OF HARD WATER The process of removing hardness – producing salts from water is known as softening or conditioning of water. Since water is a source for industrial purpose. It is mandatory to soften water to make it free from hardness producing substances, suspended impurities and dissolved gases, etc. Softening of water can be done by two methods.  External treatment  Internal treatment. EXTERNAL TREATMENT It involves the removal of hardness producing salts from the water before feeding into the boiler. The external treatment can be done by the following methods. Zeolite (or) Permutit process Zeolite (or) Permutit process Zeolites are naturally occuring hydrated sodium aluminosilicate minerals. The chemical formula is Na O.Al O .XSiO .YH O. The synthetic form of zeolite is called permutit and is 2 2 3 2 2 represented by Na Ze. 2 In this process the hard water is allowed to perlocate through sodium zeolite. The 2+ 2+ sodium ions which are loosely held in this compound are replaced by Ca and Mg ions. When zeolite comes in contact with hard water, it exchanges its sodium ions with calcium and magnesium ions of hard water to form calcium and magnesium zeolites. As sodium ions do not give any hardness to water, the effluent will be soft. The exhausted zeolite is again regenerated by treated with 5 to 10 percent of sodium chloride solution. CaZe + 2 NaCl → Na Ze + CaCl 2 2 MgZe + 2 NaCl → Na Ze + MgCl 2 2 Advantages  No sludge is formed during this process.  Water of nearly zero hardness is obtained.  This method is very cheap because the generated permutit can be used again.  The equipment used is compact and occupies a small space.  Its operation is also easy.  The process can be made automatic and continuous. Disadvantages  This process cannot be used for turbid and acidic water as they will destroy the zeolite bed. –  This treatment replaces only the cations, leaving all the anions like (HCO ) and 3 2– (CO ) in the soft water. 3  When such water is boiled in boilers, CO is liberated. Free CO is weakly acidic in 2 2 nature and extremely corrosive to boiler metal. Na CO + H O→2NaOH + CO 2 3 2 2  Due to the formation of sodium hydroxide, the water becomes alkaline and can cause cause caustic embrittlement.  Water containing Fe, Mn cannot be treated, because regeneration is very difficult.  This process cannot be used for softening brackish water. Because brackish water + also contains Na ions. So, the ions exchange reaction will not take place. DEMINERALIZATION 1.8.1 Ion exchange or Demineralisation process Ion exchange or demineralisation process removes almost all the ions (both anions and cations) present in the hard water. The soft water, produced by lime-soda and zeolite processes, does not contain hardness 2+ 2+ + + 2– – producing Ca and Mg ions, but it will contain other ions like Na , K , SO , Cl etc., 4 On the other hand demineralised (DM) water does not contain both anions and cations. Thus a soft water is not demineralised water whereas demineralised water is soft water. This process is carried out by using ion exchange resins, which are long chain, cross linked, insoluble organic polymers with a micro process structure. The functional groups attached to the chains are responsible for the ion exchanging properties. (i) Cation exchanger Resins containing acidic functional groups (–COOH, – SO H) are capable of 3 + exchanging their H ions with other cations of hard water. Cation exchange resin is represented as RH . 2 Examples:  Sulphonated coals  Sulphonated polystyrene R–SO H; R–COOH ≡ RH 3 2 (ii) Anion Exchanger Resins containing basic functional groups (–NH , –OH) are capable of exchanging 2 their anions with other anions of hard water. Anion exchange resin is represented as R (OH) . 2 Examples:  Cross-linked quaternary ammonium salts.  Urea-formaldehyde resin. R–NR OH; R–OH; R–NH ≡ R (OH) 3 2 2 Process The hard water first passed through a cation exchange which absorbs all the cations 2+ 2+ + + like Ca , Mg Na , K , etc. present in the hard water. RH + CaCl → RCa + 2HCl 2 2 RH + MgSO → RMg + H SO 2 4 2 4 RH + NaCl → RNa + HCl The cation free water is then passed through a anion exchange column, which absorbs – 2 – all the anions like Cl , SO , HCO , etc., present in the water. 4 3 R' (OH) + 2HCl → R'Cl + 2H O 2 2 2 R'(OH) + H SO → R'SO + 2H O 2 2 4 4 2 The water coming out of the anion exchanger completely free from cations and anions. This water is known as demineralised water or deionised water. Regeneration When the cation exchange resin in exhausted, it can be regenerated by passing a solution of dil.HCl or dil.H SO . 2 4 RCa + 2HCl → RH + CaCl 2 2 RNa + HCl → RH + NaCl Similarly, when the anion exchange resin is exhausted, it can be regenerated by passing a solution of dil.NaOH. R'Cl + 2 NaOH → R'(OH) + 2 NaCl 2 2 Advantages  The water is obtained by this process will have very low hardness.  Highly acidic or alkaline water can be treated by this process. Disadvantages  The equipment is costly.  More explosive chemicals are needed for this process.  Water containing turbidity, Fe and Mn cannot be treated, because turbidity reduces the output and Fe, Mn form stable compound with the resins. INTERNAL TREATMENT Internal treatment involves adding chemicals directly to the water in the boilers for removing dangerous scale – forming salts which were not completely removed by the external Treatment for water softening. This method is used to convert scale to sludge which can be removed by blow-down operation.  Calgon conditioning  Carbonate conditioning  Phosphate conditioning  Colloidal conditioning Calgon conditioning Calgon is sodium hexa meta phosphate with a Composition Na (Na (PO ) ). A highly soluble complex containing Ca is formed by 2 4 3 6 replacing the sodium ions and thus prevents their formation of scale forming salts like CaSO . The reaction is as follows: 4 2CaSO + Na Na (PO ) → Na Ca (PO ) + 2Na SO 4 2 4 3 6 2 2 3 6 2 4 Since the complex is highly soluble there is no problem of sludge disposal. Carbonate conditioning Scale formation due to CaSO in low pressure boilers can be avoided by adding 4 Na CO to the boilers. 2 3 CaSO + Na CO → CaCO + Na SO 4 2 3 3 2 4 2- The forward reaction is favored by increasing the concentration of CO .CaCO 3 3 formed can be removed easily. Phosphate conditioning In high pressure boilers, CaSO scale whose solubility decrease with increase of 4 temperature. Such scale can be converted into soft sludge by adding excess of soluble phosphates. 3CaSO + 2Na PO → Ca (PO ) +2Na SO 4 3 4 3 4 2 2 4 There are three types of phosphates employed for this purpose. Tri-sodium phosphate – Na PO (too alkaline): used for too acidic water. 3 4 Di-sodium hydrogen phosphate – Na HPO (weakly alkaline): Used for weakly acidic 2 4 water. Mono sodium di hydrogen phosphate NaH PO (acidic) used for alkaline acidic water. 2 4 Colloidal conditioning The colloidal conditioning agents are kerosene, agar-agar, gelatin, glue, etc. They are Used in low pressure boilers. The colloidal substances convert scale forming substance like CaCO , CaSO into a 3 4 Non-adherent, loose precipitate called sludge, which can be removed by blow-down Operation. CAUSTIC EMBRITTLEMENT Caustic embrittlement is a form of corrosion caused by a high concentration of sodium Hydroxide in the boiler feed water. It is characterized by the formation of irregular intergranular cracks on the boiler metal, Particularly at places of high local stress such as bends and joints. Causes of caustic embrittlement Boiler water usually contains a small amount of Na CO . In high pressure boilers, 2 3 Na CO undergoes hydrolysis to produce NaOH. 2 3 Na CO + H O → 2NaOH +CO 2 3 2 2 This NaOH flows into the minute hairline cracks present on the boiler material by capillary action and dissolves the surrounding area of iron as sodium ferroate, Na FeO . 2 2 Fe + 2NaOH → Na FeO +H 2 2 2 This type of electrochemical corrosion occurs when the concentration of NaOH is above 100 ppm. This causes embrittlement of boiler parts, particularly the stressed parts like bends, joints, rivets, etc. Caustic embrittlement can be prevented by  Using sodium phosphate as the softening agent instead of sodium carbonate.  Adding chemicals such as tannin, lignin to the boiler water. They block the hairline cracks.  Adjusting the pH of the feed water carefully between 8 and 9. BOILER CORROSION Corrosion in boilers is due to the presence of  Dissolved oxygen  Dissolved carbon dioxide  Dissolved salts like magnesium chloride. Dissolved oxygen The presence of dissolved oxygen is responsible for corrosion in boilers. Water containing dissolved oxygen when heated in a boiler, free oxygen is evolved, which corrodes the boiler material. 4Fe + 6H O + 3O → 4 Fe (OH) 2 2 3 Dissolved carbon dioxide When water containing bicarbonates is heated, carbon dioxide is evolved which makes the water acidic. Carbon dioxide dissolved in water forms carbonic acid. This leads to intense local corrosion called pitting corrosion. Ca(HCO ) → CaCO + H O + CO 3 2 3 2 2 CO + H O → H CO 2 2 2 3 Dissolved magnesium chloride When water containing dissolved magnesium chloride is used in a boiler, hydrochloric acid is produced. HCl attacks the boiler in a chain-like reaction producing hydrochloric acid again and again which corrodes boiler severely. MgCl + 2H O → 2HCl + Mg (OH) 2 2 2 Fe + 2 HCl → FeCl + H 2 2 FeCl + 2H O → Fe (OH) + 2 HCl 2 2 2 Corrosion by HCl can be avoided by the addition of alkali to the boiler water. Prevention of boiler corrosion Removal of dissolved oxygen and carbon dioxide can be done either chemically or mechanically. Chemical method For the removal of dissolved oxygen, sodium sulphite, hydrazines are used. 2Na SO + O → 2Na SO 2 3 2 2 4 N H + O → N + 2H O 2 4 2 2 2 Hydrazine is the ideal compound for the removal of dissolved O as it forms only 2 water and inert nitrogen gas during the reaction. Dissolved CO is removed by the addition of ammonium hydroxide. 2 2NH OH + CO → (NH ) CO + H O 4 2 4 2 3 2 Mechanical method Oxygen along with carbon dioxide can be removed mechanically by the de-aeration method In this method, water is allowed to fill in slowly on the perforated plates fitted inside the tower. To reduce the pressure inside the tower, the de-aerator is connected to a vacuum pump. The sides of the tower are heated by means of a steam jacket. This is based on the principle that the solubility of a gas in water is directly proportional to pressure and inversely proportional to temperature. High temperature, low pressure and a large exposed surface, reduces the dissolved gases (O and CO ) in water. 2 2 The water flows down through a number of perforated plates and this arrangement exposes a large surface of water for de-aeration. 1.12 PRIMING AND FOAMING (CARRY OVER) During the production of steam in the boiler, due to rapid boiling, some droplets of liquid water are carried along with steam. Steam containing droplets of liquid water is called wet steam. These droplets of liquid water carry with them some dissolved salts and suspended impurities. This phenomenon is called carry over. It occurs due to priming and foaming. Priming Priming is the process of production of wet steam. Priming is caused by  High steam velocity.  Very high water level in the boiler.  Sudden boiling of water.  Very poor boiler design. Prevention Priming can be controlled by  Controlling the velocity of steam.  Keeping the water level lower.  Good boiler design.  Using treated water. Foaming The formation of stable bubbles above the surface of water is called foaming. These bubbles are carried over by steam leading to excessive priming. Foaming is caused by the  Presence of oil and grease.  Presence of finely divided particles. Prevention Foaming can be prevented by  Adding coagulants like sodium aluminate, aluminium hydroxide.  Adding anti-foaming agents like synthetic polyamides. DESALINATION Depending upon the quantity of dissolved solids, water is graded as Fresh water has 1000 ppm of dissolved solids. Brackish water has 1000 but 35,000 ppm of Dissolved solids. Sea water has 35,000 ppm of dissolved solids. Water containing dissolved salts with a peculiar salty or brackish taste is called brackish water. It is totally unfit for drinking purpose. Sea water and brackish water can be made available as drinking water through desalination process. The removal of dissolved solids (NaCl) from water is known as desalination process. The need for such a method arises due to the non-availability of fresh water. Desalination is carried out either by electro dialysis or by reverse osmosis. 1.13.1 Reverse Osmosis When two solutions of different concentrations are separated by a semi-permeable membrane, flow of solvent takes place from a region of low concentration to high concentration until the concentration is equal on both the sides. This process is called osmosis. The driving forces in this phenomenon are called osmotic pressure. If a hydrostatic pressure in excess of osmotic pressure is applied on the higher concentration side, the solvent flow reverses, i.e., solvent is forced to move from higher concentration to lower concentration .This is the principle of reverse osmosis. Thus, in reverse osmosis method pure water is separated from its dissolved solids. Using this method pure water is separated from sea water. This process is also known as super-titration. The membranes used are cellulose acetate, cellulose butyrate, etc. Advantages  The life time of the membrance is high.  It can be replaced within few minutes.  It removes ionic as well as non-ionic, colloidal impurities.  Due to simplicity low capital cost, low operating, this process is used for converting sea water into drinking water. UNIT – 2 ELECTROCHEMISTRY&CORROSION CONTROL Emf Series / Electrochemical Series: Definition The arrangement of various metals in the order of increasing values of standard reduction potential is called emf series. Applications of Emf Series: o Calculation of Standard emf of the cell o Relative ease of Oxidation or Reduction o Displacement of one element by the other o Determination of equilibrium constant for the Reaction o Hydrogen Displacement Behaviour o Predicting Feasibility/ Spontaneity of the cell 1. Calculation of Standard emf of the cell ° Using E , the standard emf can be calculated ° ° ° ECell E = E - E RHE LHE 2. Relative ease of Oxidation or Reduction Standard Reduction Reaction Potential Positive Reduction Negative Oxidation Eg. F ( + 2.87V) Reduction Li ( - 3.01V) Oxidation 3. Displacement of one element by the other Metal with more negative reduction potential can displace those metals with less negative or positive potentials from the solution ° 2+ Eg. E / = + 0.34 V Cu Cu 4. Determination of equilibrium constant for the Reaction Equilibrium constant can be calculated using the formula ° WKT, G = RT ln K = 2.303 RT log ° K Log K = - G 2.303 RT = nFE 2.303 RT 5. Hydrogen Displacement Behaviour Metal with negative reduction potential will displace hydrogen from the solution ° Eg. Zn + H SO ZnSO + H ( E zn 2 4 4 2 ° Ag + H SO No Reaction (E Ag 2 4 6. Predicting Feasibility/ Spontaneity of the cell ° Spontaneity depends on E value ° E - Positive ( Reaction Spontaneous) ° E - Negative ( Reaction non Spontaneous) ° E - Zero (Reaction Equilibrium) Measurement Of Emf Of A Cell: Principle: Poggendorff‟s Compensation Principle: Emf of the cell is just opposed or balanced by an emf of standard cell (external emf) so that no current flows in the circuit. Diagram: (Refer Book) Construction:  Potentiometer consists of a uniform wire AB  A Storage battery(K) is connected to the ends A and B of the wire through a rheostat(R)  U n k n o w n emf (x) – connecting its  Positive to Pole A  Negative to Sliding contact (D) through galvanometer G.  S l i d i n g contact is free to move along the wire AB till no current flows through the galvanometer.  Emf of unknown cell is directly proportional to the distance AD E  x AD  Unknown cell (x) is replaced by a standard cell (s) in the circuit  S l i d i n g contact is again moved, till there is null reflection in the galvanometer  Emf of the standard cell E is directly proportional to the distance AD‟ s E  s AD‟ Emf of the unknown cell x = Length AD Enf of the unknown cell s Length AD‟ E = AD x E AD‟ s Using this formula EMF of the unknown cell is calculated. = -0.76 V) = +0.80 V) Eg: Oxides of heavy metals II Corrosion by H 2 Corrosion by H 2 H embrittlement ( ordinary temperature) Decarburization (high temp) 2 Fe + H S FeS + 2H H 2H 2 2 H + H H C + 4H CH 2 4 Formation of cracks and blisters The process of decrease in on metal surface due to high carbon content in steel is pressure of H gas termed as decarburization of 2 steel III Liquid metal corrosion Occurs due to a) Either dissolution of solid metal by a liquid metal (or) b) Liquid metal may penetrate into the solid metal Electrochemical or Wet Corrosion: Wet corrosion occurs in two conditions i) When two dissimilar metals are contact with each other ii) When a metal is exposed to varying concentration of oxygen Mechanism: At Anode Oxidation occurs 2+ - M M + 2e At Cathode Reduction occurs. Depends on nature of the environment a) Acidic environment: Hydrogen evolution occurs + - 2H + 2e H 2 b) Neutral environment: Hydroxide ion formation takes place - - ½ O + 2e + H O 2OH 2 2 Hydrogen Evolution Type: All metals above H in electrochemical series undergo this type of corrosion 2 Example: Iron metal in contact with HCl At Anode: 2+ - Fe Fe + 2e (Oxidation) At Cathode: + - 2H + 2e H (Reduction) 2 Absorption of oxygen or Formation of hydroxide ion type corrosion: Example: Iron metal in contact with a neutral solution At Anode: 2+ - Fe Fe + 2e (Oxidation) At Cathode: - - ½ O + H O + 2e 2OH 2 2 Net Corrosion reaction is 2+ - Fe + 2OH Fe(OH) (Reduction) 2 Electroplating: Principle: It is the process in which the coating metal is deposited on the base metal by passing a direct current through an electrolytic solution containing the soluble salt of coating metal. Objectives: On Metals: i) To increase the resistance to corrosion. ii) To improve the hardness On Non-Metals: i) To increase strength ii) To preserve and decorate the surfaces of wood, glass, plastics etc., Process: Object to be coated first treated with dil.HCl or dil.H SO 2 4 Cathode Object to be coated Anode Gold foil Electrolyte AuCl 3 Additives for strong and Glue, Gelatin adherent coating To improve brightness Brightening Agents o Optimum temperature 60 C 2 Optimum current 1 – 10 mA/cm density When current is passed gold dissolves in solution and deposits on the object Various chemical reactions: AuCl ionises as 3 3+ - AuCl Au + 3Cl 3 At Cathode: 3+ Au ions moves to cathode and deposits as Au metal 3+ - Au + 3e Au At Anode: Free chloride ions moves to the - - anode. Au + 3Cl AuCl + 3e 3 Corrosion Control By Cathodic Protection a) Sacrificial Anodic protection method: The metal to be protected is made cathode. It is connected with more active metal. Corrosion will concentrate only on the active metal. Thus original metal is protected. Hence this process is known as sacrificial anodic protection method. Applications: i) Used for the protection of ships and boats. ii) Protection of underground pipelines, cables from soil corrosion iii) Calcium metal is employed to minimize soil corrosion. b) Impressed current cathodic protection method: The corroding metal is converted from anode to cathode. This is done by applying a current in opposite direction of corrosion current. Method: Negative terminal of the battery is connected to the metal. Positive terminal of the battery is connected to an inert anode. Inert anode used is graphite, platinised Titanium. The anode is buried in a “back fill” to provide electrical contact. Applications: Structures like tanks, pipelines, ships etc can be protected. Corrosion Inhibitors: Anodic inhibitors: It prevents corrosion occurring at the anode. It forms an insoluble Compound with metal ions. These precipitate forms a protective film and prevents corrosion. Example: Chromates, nitrates, phosphates of transition elements. Cathodic inhibitors: It prevents corrosion occurring at cathode. In acidic solution: Hydrogen evolution occurs. + - 2H + 2e H 2 Corrosion can be reduced by + i) Slowing the diffusion of H ions. This is done by adding amines, pyridines. ii) Increasing over voltage. This is done by adding antimony and arsenic oxides. In a neutral solution: Formation of hydroxide ion occurs. - - ½ O + 2e + H O 2OH 2 2 Corrosion can be reduced by i) Eliminating O . Done by adding Na SO , N H 2 2 3 2 4 - etc., ii) Eliminating OH . Done by adding Mg, Zn or Ni salts. Vapour Phase inhibitors: This readily vapourise and form a protective layer on the metal surface. Example: Dicyclohexyl ammonium nitrate, benzotriazole etc., Differential Aeration Corrosion: This type of corrosion occurs when a metal is exposed to varying concentration of O or any electrolyte on the surface of the base 2 metal. Example: Metals partially immersed in water (or) conducting solution. Mechanism: Metal part which is above the solution is more aerated and hence becomes cathodic. Metal part which is inside the solution is less aerated and hence becomes anodic. At Anode: Corrosion occurs 2+ M M + - 2e At Cathode: Hydroxide ions are produced. - - ½ O + 2e + H O 2OH 2 2 Example: Pipeline corrosion: Pipelines passing from one type of soil example from clay(less aerated) to sand (more aerated) undergo differential aeration corrosion. Factors Influencing the Rate of Corrosion: i) Nature of the metal a) Position in emf series: Metals above hydrogen in emf series undergo corrosion easily. Greater the difference in their position faster is the corrosion rate. b) Relative areas of the anode and cathode: The rate of corrosion will be more when the cathodic area is larger. c) Purity of the metal: 100% pure metal will not undergo corrosion. Higher the percentage of impurity, faster is the rate of corrosion. Example: %Purity of Zinc 99.999 99.99 99.95 Corrosion Rate 1 2650 5000 d) Nature of corrosion product: If the corrosion product is soluble in the medium, the corrosion rate will be faster. If it is volatile, then also corrosion rate will be faster. e) Nature of the surface film If the surface film is porous and non-protective, it bring about further corrosion. If the surface film is non-porous and protective, it prevents further corrosion. f) Over voltage or over potential: The over voltage of a metal in the corrosive environment is inversely proportional to corrosion rate. ii) Nature of the environment a) Temperature: The rate of corrosion is directly proportional to temperature. b) Humidity: The rate of corrosion will be more, when the humidity in the environment is high. c) Presence of corrosive gases: Acidic gases like CO , SO , H S and fumes of HCl, H SO 2 2 2 2 4 etc increases corrosion d) Presence of suspended particles: Particles like NaCl, (NH ) SO with moisture increases 4 2 4 corrosion. e) Effect of pH: The rate of corrosion will be maximum when the corrosive environment is acidic i.e. pH is less than 7. Electroless Plating Principle Electro less plating is a technique of depositing a noble metal on a catalytically active surface of the metal to be protected by using a suitable reducing agent without using electrical energy. Metal ions + Reducing Agent Metal + Oxidized product Process: Step 1: Pretreatment and activation of the surface: The surface to be plated is first degreased by using organic solvents or alkali, followed by acid treatment. Example: The surface of stainless steel is activated by dipping in hot solution of 50% dil.H SO 2 4 Step 2: Plating bath : Nature of the Name of the Quantity(g/l) Function compound compound Coating solution NiCl 20 Coating metal 2 Reducing agent Sodium 20 Metal ions reduced hypophosphite Complexing agent Sodium succinate 15 Improves the quality Buffer Sodium acetate 10 Control the pH Optimum pH 4.5 Optimum 93˚C temperature Step 3: Procedure The pretreated object is immersed in the plating bath for the required time. At Cathode: 2+ - Ni + 2e Ni At Anode: - - + - H PO + H O H PO + 2H + 2e 2 2 2 2 3 Net Reaction: 2+ - - + Ni + H PO + H O Ni + H PO + 2H 2 2 2 2 3 Applications: 1. Electro less Ni plating is extensively used in electronic appliances. 2. Electro less Ni plating is used in domestic as well as automotive fields. UNIT -3 ENERGY SOURCES Nuclear fission: It is the nuclear reaction in which heavy isotopes are split into lighter nuclei on bombardment by 235 neutrons. Fission reaction of U is given below 141 1 92 + 56Ba + 3 0n + energy 235 +0n1 → 36Kr (Structure of fission reaction)

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