Corrosion And Energy Storage System PDF

DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI UNIT-II: CORROSION AND ENERGY 1) Dry corrosion or direct chemical STORAGE SYSTEM: corrosion: This type of corrosion involves a Corrosion: Corrosion is a chemical process...

DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI UNIT-II: CORROSION AND ENERGY 1) Dry corrosion or direct chemical STORAGE SYSTEM: corrosion: This type of corrosion involves a Corrosion: Corrosion is a chemical process direct chemical attack, either of dry gases or in which deterioration or destruction of of anhydrous liquids on the surface of metals metallic material occurs due to unwanted or alloys. The chemical reaction between chemical or electrochemical attack of the corroding environment and metallic surface environment. For examples, rusting of iron, results in dry corrosion. Dry corrosion formation of green film on copper, etc. depends upon the following conditions. 1 Corrosion is a slow but continuous  Chemical affinity: More the affinity process. It depends upon the nature of existing between metals & environment, metallic material and the environmental more the corrosion & vice versa. condition. Normal metals like sodium,  Temperature: More the temperature calcium, etc. are corroded to very high extent. more the extent of corrosion. Heavy metals like tungsten, chromium, etc.  Corrosion Product: More volatile and are corroded to a considerable extent. Nobel porous corrosion product more the extent of like silver, gold, etc. are corroded to a corrosion. Stable corrosion product decreases negligible extent. the rate of corrosion. Rate of Corrosion Cause of corrosion: In nature, generally depends upon the nature of corrosion metals are always found in combined state, in products formed. the form of their ores such as oxides, Types of dry corrosion: Depending upon the hydroxide, sulphide, sulphates, carbonates, corroding environment, dry corrosion can be silicates, etc. In these combine states, metal is categorized into three types. present in positive oxidation state. Thus this a) Corrosion by oxygen (Oxidation is state of stability. During the process of Corrosion): This corrosion occurs when dry extraction of metals from their ores, metals oxygen or air, attacks directly the surface of ions are converted into neutral state by metal. Highly electropositive metals like nonspontaneous process. In neutral state sodium, calcium, etc. are corroded to very metal is unstable. Hence the metals exhibit a high extent. During this metal get oxidized natural tendency to pass into combined state, and form corresponding metal oxides of by their reaction with the environment different nature. Different nature of these factors. Thus the corrosion starts. Thus oxides or oxide layer are given below. corrosion is exactly opposite to that of i) Stable oxide: Heavy metal (aluminium, extraction. tin, lead, copper, etc.) forms the stable oxide. Types of Corrosion: Depending upon the The formed stable oxide layer avoids the types of the reactions occurring between the contact of metal surface with atmospheric metals and the environment, corrosion can oxygen, thus this layer behaves as protective be of two types. coating. 1) Dry Corrosion ii) Porous oxide: When metal oxide layer 2) Wet Corrosion have pores or cracks, the atmospheric oxygen DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI  DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI attacks nonproductive part and corrosion molecular hydrogen (H2). Due to high continues till the entire metal is converted pressure of this gas strength and ductility of into its oxides. For example, oxides of Li, Na, the metal is decreased. This phenomenon is K, Mg, are porous in nature. called hydrogen embrittlement. At high iii) Unstable oxide: The oxide layer formed temperature, the atomic hydrogen diffuses is unstable and it decomposes back into the inside the metal lattice becomes very much metal and oxygen. Thus temporary oxidation active and hence it combines with the of metal takes place. Hence oxidation element like C, N, O, S, etc. those are 2 corrosion is not possible in such a case. For present in small quantities, inside the metal examples silver, gold, platinum, etc. form lattice. For example in steel the atomic such type of oxide layer. hydrogen reacts with carbon to from iv) Volatile oxide: The oxide layer formed methane (CH4). Due to very high pressure of volatilized as soon as it is formed. After the gas product thus formed, intergranular volatilization of metal oxide layer, fresh, creaking and blistering of the metal occurs. metal surface is exposed air and gain metal This phenomenon is called decarbanisation oxide formed volatizes. This causes rapid and of steel. continues corrosion till the metal completely c) Corrosion by anhydrous liquids: This vanishes. Molybdenum oxide (MoO3) is corrosion occurs when the anhydrous liquids volatile. attack directly the surface of metallic b) Corrosion by other gasses: This compounds. Anhydrous liquids are of 3 corrosion occurs when dry gases like H2, Cl2 different type F2, H2S, SO2 etc. directly attack the surface of Organic liquids: Phenols, picric acid etc. the metal. Rate of dry corrosion of a metal by attack direct the metal surface and cause this other gases depends on following facts type of corrosion even at ordinary  More the affinity between metal and temperature. reacting gasses, more the extent of corrosion. Inorganic liquids: Anhydrous liquids like  If the corrosion product is volatile then HF, Br, etc. directly attack the metal surface. the corrosion occurs with high speed till Even at ordinary temperature and causes this whole metal is destroyed. For example, when type of corrosion. Cl2 attacks Tin (Sn), volatile stannic chloride Liquid metals: Following liquids metals at is formed and hence corrosion continues high temperature attack directly the metal with a high speed till whole metal is lost. surface and causes this type of corrosion. Corrosion by Hydrogen: In some reactions, The corrosion reaction occurring here occurring over the surface of metals, atomic involves either dissolution of solid metal hydrogen (H) is formed. This atomic surface by the liquid metal or inter-granular hydrogen has a specific action on metals. The penetration of the liquid metal atom into action depends upon temp. At ordinary solid metal. The cause of this type of reaction temperature, the atomic hydrogen diffuses is due to the existence of the temperature inside the metal and recombine to from the gradient within the solid-liquids system. DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI  DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI More the temperature gradient, more the Hence corrosion takes place at anodic area. corrosion. At the cathodic area, reduction takes place. Mechanism of Oxidation Corrosion or Dry The corrosion product (such as metal oxide Corrosion by Oxygen: Oxidation corrosion etc.) is formed between anodic and cathodic involves reaction between metals and oxygen. areas. The electrochemical corrosion can During this process, initially oxygen molecule occur in the following two ways. rapidly adsorbed on the surface of metal  Galvanic corrosion (by the formation of which results into evolution of heat. Because galvanic cell) 3 of it, the temperature rises and the adsorbed  Concentration cell corrosion (by the oxygen molecules are dissociated to oxygen formation of concentration cell) atoms. Now metal atoms lose their electrons Mechanism of Wet or Electrochemical to oxygen atoms. The metal ions and the Corrosion: In wet or electrochemical oxygen ions thus formed combine together to corrosion, there is formation of two areas i.e. form metal oxide. The reaction involved cathodic and anodic areas. The general during oxidation corrosion can be reaction of anode is oxidation of metal. represented as M → M n+ + ne− But at cathode, the reaction depends on the nature of electrolyte medium. If the medium is acidic then corrosion occurs by the These metal oxide molecules form a layer evolution of hydrogen at cathode. If the over the surface of the metal. As corrosion medium is neutral then corrosion occurs by continues, several layers are formed which the absorption of oxygen at cathode. results into formation of thick coating of Hydrogen Evolution Mechanism: In this metal oxide on the surface of the metal. The type of electrochemical corrosion, always H2 rate of corrosion decreasing as thickness gas is evolved at cathode. This corrosion increases on the metal surface. However generally occurs when the corrosive corrosion still continues because metal ions environment is acidic in nature and free formation takes placed continuously due the from oxygen. Generally, during this type interaction between fresh metal atom and mechanism, anodic area is quite large while oxygen. Hence the process of corrosion is cathodic area is very much small. For the said to be a slow but continuous process. understanding of such mechanism consider 2) Wet Corrosion or Electrochemical iron metal on which such corrosion is Corrosion: This type of corrosion occurs due occurring. to the contact of conducting solution Reaction at anode: At large anode area the (electrolyte) with metal or alloys. It involves ‘Fe’ atoms undergo oxidation with the the formation separate anodic and cathodic liberation of electron, Fe → Fe2+ + 2e−. areas between which current flows through Thus the ferrous (Fe2+) ions are formed the conducting medium. At anodic areas, during the oxidation reaction and the formed metal is destroyed by oxidation reaction. electron flow toward the cathode area. DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI  DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI Reaction at cathode: At cathode, the If enough oxygen is present, ferrous electrons are accepted by the H+ ions (from hydroxide is easily oxidized to ferric electrolyte medium) and hydrogen gas is hydroxide, 4Fe(OH)2 + O2 + 2H2 O → 4Fe(OH)3. evolved at cathode, 2H+ + 2e− → H2 ↑ If supply of O2 is limited corrosion Overall reaction: product may be black anhydrous magnetite Fe3O4. Most commonly corrosion occurs at anode, and the rust is deposited at or near the cathode. 4 Thus the corrosion takes place with the evolution of hydrogen at cathode when the medium is acidic. Figure: Mechanism of wet corrosion by oxygen absorption. Types of Corrosion: Figure: Mechanism of wet corrosion by 1) Pitting corrosion hydrogen evolution 2) Waterline corrosion Oxygen Absorption Mechanism: In this 3) Inter – granular corrosion corrosion always O2 gas is absorbed at 4) Galvanic corrosion cathode. This corrosion generally occurs, if 5) Stress corrosion O2 gas is present in dissolved state in the 1) Pitting corrosion: It is the corrosion natural aqueous corrosive environment, and occurring inside the pit of the metal. This is a generally anodic area is very much small and sort of oxygen concentration cell i.e. cathodic area is quite large. corrosion due to aeration difference. Reaction at anode: At smaller anodic area, It is a localized accelerated attack Fe atoms undergo oxidation with liberation resulting in the formation of pits (pin holes of electrons, Fe → Fe2+ + 2e−. These liberated or cavities) the pits forms may penetrate deep electrons flow towards cathodic areas. into metal. Pit being less aerated acts as small Reaction at cathode: At cathode, the anodic area while rest of the portion of the electrons are accepted by dissolved oxygen. metal being more aerated comparatively acts 1/2O2 + e− + H2 O → 2OH − as cathodic area. At small anodic area, metal Fe2+ ions from anode and OH from cathode undergoes oxidation, M → M n+ + ne− , while at diffuse and meet to form ferrous hydroxide cathode reduction takes place. The actual precipitate, Fe2+ + 2OH− → Fe(OH)2 ↓. reduction reaction depends upon the nature of the aqueous corrosive environment. If the DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI  DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI medium is neutral water then reduction of tank above the water meniscus is highly water with absorption of oxygen will occurs. oxygenated as compared to below the 1/2O2 + e− + H2 O → 2OH − meniscus. More aerated (upper portion) acts In this type of corrosion, the part of the as cathode while the less aerated portion acts metal which is oxidized gets deposited as as an anode, thus the formation of oxygen metal hydroxide inside the pit. Thus intense concentration cell takes place. At anode, corrosion occurs inside the pits as it is a small oxidation of metal occurs, M → M n+ + ne−, area while rest of the area is protected from while at cathode reduction reaction occurs. 5 the corrosion. The actual reduction reaction depends upon the nature of the aqueous corrosive environment. If the medium is neutral water then reduction of water with absorption of oxygen will occurs. 1/2O2 + e− + H2 O → 2OH − In this type of corrosion, the part of the Formation of the pits on the surface of metal which is just below the water level gets the metal is due to the following reasons. dissolved and deposited as metal hydroxide  Presence of sand, dust etc. embedded just below the water line. Hence it is called as over the surface of the metal. water line corrosion. This type of corrosion  Formation of the cavities inside the metal mainly found in ships, marine plants, water due to the evolution of the trapped air storage tank, etc. bubbles during solidification of metals.  Due to stressing and straining of the metals while bending.  Due to scratching and cut edges.  Due to sliding under the load. Waterline corrosion can be  Due to non-uniform finishing and the controlled by the use of antifouling paints surface roughness. and by using sacrificial anodes.  Due to intense flow of liquids on the 3) Inter–granular corrosion: It is the surface of the metal corrosion which occurs along the grain Control of pitting corrosion: The pitting boundaries of metal and alloys. corrosion can be avoided by using During the solidification of a metal  Pure and homogenous metal with highly or an alloy, initially, aggregation of atoms of polished surface the metal takes place to form number of  Surface cleanliness granules. These granules further aggregate to  Selection proper materials form a solid. However, certain impurities 2) Waterline corrosion: Generally it occurs present in the metal (or one of the when water is stagnant in a metal or steel components of the alloy) get precipitated tank for long time. The portion of the metal between the inter-granular spaces. Due to this DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI  DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI the region of the grain in contact with the water then reduction of water with 1 inter-granular space (grain boundary), absorption of oxygen will occurs. 2O + e− + 2 becomes the region of less concentration as H2 O → 2OH. In − acidic solution, the corrosion compared to the rest of the portion (grain occurs by the hydrogen evolution (2H+ + center) of the solid metal. Hence the grain 2e− → H2 ). boundaries act as anodic area with while the grain centre acts as cathodic area. Therefore, corrosion takes place at grain boundaries. 6 During galvanic corrosion, anode gets dissolved into electrolyte that the degradation (corrosion) of anode takes place while cathode gets protected. 4) Stress corrosion: It is combined effect of This is a sort of localized corrosion static tensile stress and the selective corrosion process. It is slow and depends upon the environment on a metal. This is sort of following factors. localized corrosion process because only the  Nature of the component material portion of the metal under the tensile stress precipitated inside the inter-granular spaces. undergoes this type of corrosion. Such  Nature of the corrosive environment. portion is generally small, narrow and in the  Metallic Structure state of high energy as compared to rest of 3) Galvanic corrosion: When two dissimilar the portion of the metal. metals, say zinc and copper, are electrically connected and exposed to an electrolyte, the metal with low values of electrode potential undergoes corrosion. This type of corrosion is called galvanic corrosion. In the above example, zinc (low value of electrode potential) acts as anode while copper (high value of electrode potential) acts as cathode. At anode, oxidation of zinc takes place, Zn → Zn2+ + 2e− , and released electrons move Therefore, the electrode potential of these toward copper metal (cathode) where portion is generally higher than that of the reduction reaction occurs. The actual remaining large portions. This small portion reduction reaction depends upon the nature act as anodic areas and remaining large of electrolyte. If the electrolyte is neutral portion acts as cathodic areas. Intense and DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI  DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI localized corrosion occurs at anode while free access of oxygen to the underlying metal cathode is protected from the corrosion. surface (through cracks and pores) for fresh Stress corrosion occurs due to presence of action and thus, corrosion continues non- more tensile strength and specific corrosive stop. Metals like Aluminium forms oxide, agents. The corrosion agents are highly whose volume is greater than the volume of specific and selective such as a) caustic alkalis metal. Consequently, an extremely tightly- and strong nitrate solution for mild steel b) adhering non-porous layer is formed. Due to traces ammonia for brass c) acid chloride the absence of any pores or cracks in the 7 solution for stainless steel. oxide film, the rate of oxidation rapidly Some examples of stress corrosion: decreases to zero. a) Season cracking: It is stress corrosion of Corrosion Control: Corrosion is a slow the copper alloys. Alloying components process. However it is very harmful because present in copper like P, As, Sb, Zn, Al, Si there are tremendous losses due to corrosion etc. create the anodic areas of high potential of metals and alloys. It is impossible to stop under the strain in presence of ammonia or the corrosion process, hence it becomes amines and undergoes stress corrosion. The essential to minimize or control the metals or season cracking in brasses increases with Zn alloys from corrosion. Corrosion control can contents. be achieved by following methods: b) Caustic embrittlement: It is the stress 1) Design and Material selection corrosion of the mild steel in presence of 2) Cathodic Protection alkaline solution at high temperature. It is 3) Protective surface coatings always found in steam boilers and heat 1) Material selection and Design: transfer equipment in which alkaline water is a) Material Selection: Selection of the metal under treatment. or metallic material for the construction of Pilling Bed worth rule: According to it “an the machines, equipment etc. is the main oxide is protective or non-porous, if the factor of corrosion control. It should possess volume of the oxide is at least as great as the high corrosion resistance. The corrosion volume of the metal from which it is resistance can be increased by formed”. On the other hand, “if the volume  Improving purity of metals: The of the oxide is less than the volume of metal, presence of a very small amount of an the oxide layer is porous (or non-continuous) impurity inside the metal decreases it is and hence, non-protective, because it cannot corrosion resistance and causes intense and prevent the access of oxygen to the fresh localized pitting corrosion. Metals like Al, metal surface below”. Alkali and alkaline Mg, Zn, Pb etc undergo rapid corrosion in earth metals (like Li, K, Na, Mg) form oxides impure state due to formation of galvanic of volume less than the volume of metals. cell. However their corrosion resistance Consequently, the oxide layer faces stress and increases with increasing its purity. In extra strains, thereby developing cracks and pores pure state, metal shows very high corrosion in its structure. Porous oxide scale permits resistance. DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI  DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI  Alloying of metals: Corrosion resistance should be free from obstructions and as well as strength of many metals (likes Fe, crevices. Cu, Co, Cr, Ni etc.) can be increased on alloying them with suitable alloying components.  Proper heat treatment: Internal stress of a metal decreases its corrosion resistance. Hence by proper heat treatment like 8 annealing or shot preening, metal can be made free from stress and hence its corrosion resistance can be increased. b) Material Design: While preparing design, 2) Cathodic Protection: The metal to be following things should be considered for protected is forced to behave like a cathode corrosion control. by passing opposite external current. Since,  Nature of design material: The metallic anodic area on the metal is converted to used for the construction of a design should cathodic area, corrosion does not occur. be in the state of uniform stability. It can be Cathodic protection can be achieved by two achieved by proper selection of material. methods:  Composition of design material: Only a) Sacrificial Anode method single metallic material should be used. If not b) Impressed current method. possible, then contact of dissimilar metal a) Sacrificial Anode method: In this should be avoided. When two dissimilar method, the metallic structure to be metals are to be in contact, the anodic protected is connected by a wire to a more material should have as large area as possible; active metal, so that all the corrosion is whereas the cathodic metal should have as concentrated at this more active metal. much smaller area as possible. It decreases the rate of corrosion. The joint of dissimilar metal should be coated with insulating material in order to avoid contact between dissimilar metals. Anodic metal should be not be completely coated or painted by protective Coating because if any break or The more active metal itself gets corroded pinhole is formed, then rapid pitting slowly, while cathodic structure is protected, corrosion of anodic metals starts. hence the more active metal is called as  Shape of design: Equipment design sacrificial anode. The sacrificial anode when should avoid accumulation of corrosive consumed completely, replaced by another product or dirt on it. Equipment design similar metal block and thus metal structure should avoid localized stresses, sharp bends, is always protected from the corrosion. battles and lap joints. Tanks and pipelines DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI  DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI Metals commonly used for sacrificial anode  Can be automatically controlled which are Zn, Mg, Al and their alloys. reduces maintenance &operating cost. Application of cathodic protection: 3) Protective surface coatings: It is done by  Protection of ship hulls, cables, piers galvanizing and tinning from marine corrosion. a) Galvanizing: It is process of coating iron  Protection of underground pipe from soil or steel sheets with a thin coat of zinc to corrosion. prevent them from rusting. b) Impressed current method: In this Process: The iron or steel article is first 9 method, an impressed current is applied in cleaned by pickling with dilute H2SO4 opposite direction to nullify the corrosion solution for 15-20 minutes at 60-900C. This current, and convert the corroding metal treatment also removes any scale, rust (Oxide from anode to cathode. Usually the layer) and impurities. The article is then impressed current is derived from a direct washed and dried. It is then dipped in bath current source like battery or rectifier. When of molten zinc, maintained at 425-4300C. battery is used as D.C. source, the platinum The surface of the bath is kept covered with a anode is used while in case use of rectifier flux ammonium chloride to prevent oxide insoluble graphite anode is used. The anode formation. When the article is taken out is buried in backfill (composed of coke breeze from the bath, it is coated with thin layer of or gypsum) to increases its electrical contact zinc. It is then passed through a pair of hot with surrounding soil. rollers. This process removes any excess of zinc and produces a thin film of uniform thickness. It is the annealed at a temperature of 6500C and finally cooled slowly. This type protection is used for protection of condensers, open water box coolers, water-tanks, harbor ship hulls, Uses: It is most widely used for protection of industrial pipelines etc. iron from atmospheric corrosion in the form Advantages of Impressed current method: of roofing sheets, wires, pipes , nails ,bolts ,  Useful when current requirements and screws , buckets tubes, etc. electrolyte resistivity are high. d) Tinning: It is coating tin over iron or  Require a cheap source of electrical steel articles. During this process initially power. steel sheet treated with dilute sulphuric acid  Can be used for large structures & long to remove any oxide film. Then, it is passed term operation. through a bath of zinc chloride flux. The flux DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI  DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI helps the molten metal to adhere to the The palm oil protect the hot tin coated metal sheet. This sheet then passes through a surface against oxidation. The rollers remove tank of molten tin and finally through a any excess of tin and produce a thin film of series of rollers underneath of palm oil. uniform thickness on the steel sheet. Uses: It is use for coating steel, copper and brass sheets which used for manufacturing containers for food stuffs, ghee, oils, kerosene, etc. 10 Difference between galvanizing and Tinning: Galvanizing Tinning A process of covering iron and steel with a thin A process of covering steel with a thin coat coat of zinc to prevent it from rusting. of tin to prevent it from corrosion. Zinc protects the iron from corrosion by Tin protects the iron from corrosion due to scarification (anode action) because it is more its noble nature because it is less electropositive than iron. electropositive than iron and have higher corrosion resistance. If the coating of zinc is broken at some place, it If the coating of tin is broken at some place, continue to protect underlying iron by galvanic it cannot protect underlying iron since the cell action. iron causes rapid corrosion. Galvanized containers cannot be used for Tin coated containers can be used for storing storing acidic food stuffs because zinc react with acidic food stuffs because tin is non-toxic food acid, forming highly toxic compounds. and avoid any food poisoning. DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI  DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI ENERGY STORAGE SYSTEM: applying the external voltage electrodes can be Basic principles of Batteries: A battery is a reconstructed to their original state, hence, the device that converts the chemical energy into batteries can be rechargeable. Thus, the electrical energy by means of an rechargeable batteries act as energy source as electrochemical reaction called redox reaction. well as energy storage systems. In a redox reaction, oxidation and reduction Nickel–cadmium battery: It is the first small reaction take place simultaneously. Each redox sealed rechargeable battery or secondary reaction is associated with the standard cell battery. Ni-Cd batteries exhibit long cycle life, potential (EO). high rates of charge and discharge with constant 11 Looking at the very basics of battery discharge voltage and also better low technology, a battery is a combination of two or temperature performance. However, the cost more electrochemical cells. These of the nickelcadmium battery construction, electrochemical cells store energy in the form healthily risks associated with the manipulation of chemical energy, and this is converted into of cadmium and also the memory effect lead to electrical energy when connected to an the search of alternate secondary battery electrical circuit in which an electrical current system. can flow. Construction: Ni-Cd batteries usually have a A cell consists of two electrodes with an metal case with a sealing plate equipped with a electrolyte placed between them. The electrode self-sealing safety valve. The cathodic (negative) on which reduction takes place is known as and anodic (positive) plates are isolated from cathode, while the electrode at which oxidation each other by the separator and are rolled in a takes place is known as anode. The electrolyte spiral shape inside the case. This is known as between them can either be a liquid or a solid. the jelly-roll design and allows a Ni–Cd cell to Many cells are enclosed in a special container, deliver a much higher maximum current than and there is an element known as a separator an equivalent size alkaline cell. placed between the anode and cathode. This is porous to the electrolyte and prevents the tow electrodes from coming into contact with each other. The potential difference across the terminals of the battery is known as the terminal voltage. If the battery is not passing any current (that is when it is not connected to any circuit) then the terminal voltage seen is the open circuit voltage and this equals the EMF or electromotive force of the battery. Working: In alkaline (OH) electrolyte, the Cd Types of Batteries: There two types of batteries acts as anode, where oxidation of cadmium 1) Primary Batteries: In primary batteries, the takes place. Upon oxidation, cadmium electrode reactions are not reversible. That is precipitated as Cd(OH)2. The Ni acts as applying the external energy will not cathode, where reduction nickel oxide reconstruct the electrodes, hence, the cells are hydroxide, NiO(OH), takes place. The not rechargeable. After the discharge, they are electrochemical reactions of Ni-Cd battery discarded. during it discharge are shown below. While 2) Secondary Batteries -In secondary batteries, during recharging of the battery, the reactions the electrode reactions are reversible. That is DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI  DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI are reverse to the reactions taking place during discharging. At Anode: Cd + 2OH− → Cd(OH)2 ↓ +2e− At Cathode: 2NiOOH + 2H2 O + 2e− → 2Ni(OH)2 + 2OH− Overall reaction: 2NiOOH + 2H2 O + Cd → 2Ni(OH)2 + Cd(OH)2 Applications: Ni–Cd is used in portable electronic devices, photographic equipment, hand-held lamps, computer- 12 memory standby, toys, and novelties. It also used in cordless and wireless telephones, emergency lighting, remote-controlled electric model airplanes, boats, and cars, as well as cordless power tools and camera flash units. Lithium ion battery: Construction: DEPARTMENT OF CHEMISTRY, PRMIT&R BADNERA-AMRAVATI