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Recipe for corrosion Active metal Water Oxygen (atmospheric corrosion) Acid (chemical corrosion) Salt High temperature

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TYPES OF CORROSION Dry / Chemical / Wet / Atmospheric Electrochemical Corrosion Dry / Chemical / Wet / Atmospheric Electrochemical Corrosion

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DRY / CHEMICAL / ATMOSPHERIC CORROSION Occurs mainly by direct contact of atmospheric gases such as O 2, SO 2,CO 2,halogens,etc. Subtypes A) Corrosion due to oxygen - Occurs due to attack of atmospheric oxygen on metal surface on metal surface, forming metal oxide - Metal + O 2 Metal oxide

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The oxide films formed are further classified in 3 categories: a) Stable oxide film 1) Porous oxide film - metals react with O 2 forming oxides - here the volume of oxide formed is less than volume of reacted metal - hence,pores are developed and corrosion continues - Eg: Ca, Mg, Na

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2) Non-porous oxide film The oxide film formed covers the underlying metal completely Thus there is no access for further attack of oxygen, thereby stopping further corrosion. Hence, metals develop a protective oxide layer and become passive till the layer is interrupted. E.g.: Al, Cr

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b) Unstable oxide film These films gets decomposed on the metal surfaces forming back the metal and oxygen. Metal + Oxygen Metaloxide Metal + Oxygen Eg: Au,Pt,Ag Such metals do not get corroded, but loss of metals continues slowly.

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Volatile oxide film These gets vaporized from metal surfaces as soon as they are formed. Thus the metal is available for further attack. Hence, corrosion continues till the metal is available. E.g.: Mo

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(iii) Liquid metal corrosion Due to chemical action of flowing liquid metal at high temperatures on solid metal or alloy. The corrosion reaction involves either: (i) dissolution of a solid metal by a liquid metal (ii) internal penetration of liquid metal inti the solid metal

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Wet or Electrochemical Corrosion.- Mechanism: Electrochemical corrosion involves flow of electrons between anode and cathode. The anodic reaction involves dissolution of metal liberating free electrons. M M n + + n e- The cathodic reaction consumes electrons with either evolution of hydrogen or absorption of oxygen which depends on the nature of corrosive environment.

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(A)Evolution of hydrogen: This type of corrosion occurs in acidic medium e.g., considering the metal Fe, anodic reaction is dissolution of iron as ferrous ions with liberation of electrons. Fe Fe 2+ + 2e- ( Oxidation at anode) 2H + + 2e - H 2 (Reduction at cathode) Anode electrons flow Cathode The overall reaction is Fe + 2H ++ Fe 2+ + H 2 This type of corrosion causes displacement of hydrogen ions from the solution by metal ions. All metals above hydrogen in electrochemical series have a tendency to get dissolved in acidic solution The anodes are large areas, whereas cathodes are small areas.

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(B) ABSORPTION OF OXYGEN Rusting of iron in neutral aqueous solution of electrolytes in presence of atmospheric oxygen. Usually the surface of iron is coated with a thin film of iron oxide. If the film develops cracks, anodic areas are created on the surface. While the metal parts act as cathodes. Anodes are small areas, while the rest metallic part forms large cathodes.

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At anode: Fe Fe 2+ + 2e - (Oxidation) At cathode: The released electrons flow from anode to cathode through iron metal. O 2 + H 2 O + 2e - 2OH - (Reduction) Fe 2+ + 2OH - Fe(OH) 2 If oxygen is in excess, ferrous hydroxide is easily oxidized to ferric hydroxide. Fe(OH) 2 + O 2 + 2H 2 O 4Fe (OH) 3 The product called yellow rust corresponds to Fe 2 O 3. xH2O

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TYPES OF ELECTROCHEMICAL CORROSION 1. WATERLINE CORROSION This is also known as differential oxygen concentration corrosion. When water is stored in a steel tank, it is observed that the maximum amount of corrosion takes place along a line just beneath the level of the water meniscus. The area above the waterline (highly oxygenated) acts as cathodic and is not affected by corrosion. If the water is relatively free from acidity, little corrosion occurs. This type of corrosion is prevented to a great extent by painting the sides of the ships by antifouling paints.

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Galvanic Cell Anode Zn ( 0.76) Cathode Cu (+0.34) e flow Zn Zn 2+ + 2e oxidation Cu 2+ + 2e Cu Reduction or 2H + + 2e H 2 or O 2 + 2H 2 O + 4e 4OH Zn will corrode at the expense of Cu

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This type of corrosion is due to electrochemical attack on the metal surface exposed to an electrolyte of varying concentrations or of varying aeration. This causes a difference in potential between the differently aerated areas. Experimentally, it has been observed that poor oxygenated parts are anodic. Differential aeration of metal causes a flow of current called the differential current. e.g. Zn is partially immersed in NaCl solution, then the parts above and adjacent to the waterline are strongly aerated and hence become cathodic Whereas parts immersed show a smaller oxygen concentration and become anodic. Zinc will dissolve at anodic areas and oxygen will take up electrons at the cathodic areas forming hydroxyl ions. Zn Zn 2+ + 2e - (Oxidation) O2 + H 2O + 2e - 2OH - (Reduction)

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3. CONCENTRATION CELL CORROSION

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Following are the facts about differential aeration corrosion: (a) Less oxygenated part is the anode. (b) Corrosion is accelerated under accumulation of dirt, scale or other contaminations. (c) Metals exposed to aqueous media corrode under blocks of wood or glass which restricts the access.

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6. INTERGRANULAR CORROSION Occurs along the grain boundaries when: -material is sensitive to corrosive attack - corrosive liquid possess a selective character of attacking along the grain boundaries. A solid solution gets adhered at centre, which is anodic to grain centres. Sudden failure of metal occurs. Observed in alloys- steel (Fe,Cr,C), chromium carbide gets precipitated. REMEDY: -Proper heat treatment

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FACTORS INFLUENCING RATE OF CORROSION 1 ) Nature of metal a) Position of metal in galvanic series The higher the electrode potential and position in the galvanic series of metal,it act as an anode and thus undergo corrosion. b) Potential difference Greater the difference in the electrode potential, higher is the rate of corrosion c) Overvoltage The difference between the potential of the electrode when the gas evolution was actually observed and the theoretical value of the same solution is called overvoltage. d) Areas occupied by the anode and cathode Corrosion at anode = Area of cathodic part Area of anodic part

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e ) Purity of the metal If metals are impure,then the impurities present in them cause heterogeneity, which gives rise to small electrochemical cells hence corrosion starts. Eg: impurities of Fe or Pb on Zn

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NATURE OF CORROSION PRODUCT a) Temperature Rate of corrosion is greater at higher temperature b) Moisture Corrosion enhances in moist conditions c)Influence of pH In acidic pH, rate of corrosion is higher d)Concentration of Electrolyte Presence of other ions also influences the rate of corrosion eg: Cl ions in the vicinity of metal leads to corrosion

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METHODS TO DECREASE RATE OF CORROSION 1) Proper Designing Avoid sharp bends, stresses,use of screws, nuts,bolts Prefer welding Surfaces of joining parts should be smooth

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2) Using pure metal Impurities cause heterogeneity Thus use of pure metals prevents corrosion 3) Using metal alloys Alloying gives a better product eg: Iron is alloyed with Cr and C to give steel eg: Brass is an alloy of Cu and Zn 4) Cathodic protection method To reverse the flow of current between two dissimilar metals, thereby reversing the action of the metal sin contact. Methods include: (a)Sacrificial anode method/ Auxillary anode method (b) Impressed current method

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(I) SACRIFICIAL ANODIC METHOD In this method, the metal structure can be protected from corrosion by connecting it with wire to a more anodic metal. As this more active metal is sacrificed in the process of saving metal from corrosion, it is known as sacrificial anode. The metals, commonly used as sacrificial anodes are Mg, Zn, Al and their alloys. Applications of this method: 1. Protection of underground cables and pipelines from soil corrosion. 2. Protection of ships and boat hulls from marine corrosion. 3. Prevention of rusty water by inserting Mg sheets or rods into domestic water boilers or tanks

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SACRIFICIAL ANODIC METHOD

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(B) IMPRESSED CURRENT METHOD An impressed current is applied to convert the corroding metal from anode to cathode. The applied current is in opposite direction to nullify the corrosion current. This can be accomplished by applying current source like battery or rectifier. And connected to the corroding metal structure which is to be protected. Thus,the object itself becomes cathodic and not oxidized. ADVANTAGES: Operating and maintenance costs are less. Suited for large structures and long term operations. APPLICATIONS: Tanks and pipelines, transmission line-towers, marine piers, ships etc.

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IMPRESSED CURRENT METHOD IMPRESSED CURRENT METHOD

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(II) ANODIC PROTECTION METHOD PRINCIPLE: Growth of protective oxide surface film by the application of anodic current on the metal/alloy in a suitable oxidising agent. To protect the structure anodically, potentiostat is used. Potentiostat has 3 terminals: 1) the object to be protected 2) An auxillary electrode 3) A reference electrode

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Important parameters in anodic protection are: The metal to be protected should possess high passivity over a wide range of potential. The current density needed to start protection should be as low as possible. The cathodic metal should possess adequate resistant to corrosion under working conditions.

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ADVANTAGES Low operating cost Wide range of corroding environments can be tackled Needs few auxillary Complicated metal structures can be protected LIMITATIONS High installation cost Suitable only for metals showing active-passive behaviour If system goes out of control, high corrosion rates may occur

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PROTECTIVE COATINGS (A) METALLIC COATINGS Metallic coatings can be divided in two categories: 1 ANODIC COATINGS 11 CATHODIC COATINGS

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(1) ANODIC COATINGS Coating of metals which are anodic to the base metal. As long as coat layer is intact, corrosion of base metal is under control. If discontinuities occur, a galvanic cell is formed. eg: Coating of Zn on Fe # Zn dissolves anodically, whereas Fe (being cathodic) is protected.

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(2)CATHODIC COATINGS Coating of metals which are cathodic (noble) to the base metal. They protect the base metal as they have higher corrosion resistant than the base metal. If the coatings are punctured, than more corrosion damage occurs, since galvanic cell will be set up. Thus leads to formation of small anode and large cathode,resulting in severe corrosion. eg: Coating of Sn on Fe Sn is lower in electrochemical series than Fe

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H OT DIPPING (A) GALVANIZING Galvanizing is a process in which the iron article is protected from corrosion by coating it with a thin layer of zinc. Iron or steel is cleaned with dil H 2 SO 4 at 60-90C (15 --20 min), which removes scale, rust and other impurities present if any. Then washed with water and dried. Dipped in the molten zinc bath (425-450C). To prevent it from oxide formation, the surface is covered with a flux (NH4Cl). When the iron piece is taken out it is coated with a thin layer of zinc. To remove excess zinc, it is passed through a pair of hot rollers. Lastly, it is annealed at a temperature of 450C and then cooled slowly.

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(B) TINNING Process of coating tin on iron. Iron is cleaned with dil H 2 SO 4. It is cleaned with water & dried properly The article is passed through ZnCl 2 flux. Next it is dipped in the molten mass of tin(232C). To remove excess tin, it is passed through a pair of hot rollers. Lastly, it is annealed and then cooled slowly. APPLICATIONS Tinned containers are used to store food, pharmaceuticals. Tinned wire is used for soldering.

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TINNING OF STEEL SHEET

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ELECTROPLATING ELECTROPLATING Electroplating is the electro-deposition of metal, by means electrolysis over surface of metals, alloys or non-metals. The article to be plated is cleaned with organic solvents Treated with dil.HCl/H 2 SO 4. Then made cathode of electrolytic cell. The anode is either the coating metal or an electrode of inert material like graphite. Under the influence of electric current,coating metal ions migrate to the cathode and gets deposited. FAVOURABLE CONDITIONS Low temperature High current density Low metal ion concentration

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APPLICATIONS 1. Plating for protection from corrosion and chemical attack. 2. Plating for decoration. 3. Plating for special for special surface and engineering effects. 4. Electroforming 5. Plating on non-metallic materials.

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METAL CLADDING: The surface to be protected is sandwiched between two thin layers of coating metal and pressed between rollers. The finished product may be welded at the edges or riveted at somepoints. This method is used for coating Al, Cr, Ni, etc. ALCLAD: Coating of a thin homogeneous sheet of a duralumin on Al sheets and fused so that it strongly binds permanently either on one side or on both sides and passed through rollers under heat and pressure. Al used is 99.5% pure Cladded sheets (alclad) are widely used in aircrafts industry.

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METAL SPRAYING The coating metal is filled in spraying gun/piston and sprayed on metal. The gun is equipped with oxyacetylene flame & supply of compressed air. Melting of metal wire and spraying occurs simultaneously and a uniform coating is developed. Widely used for applying coating of Cu,Pb,Ni,Sn, Al,Zn,etc. ADVANTAGES: High speed Capable to cover larger area in short time Uniform coating can be obtained Can be easily applied to restricted areas

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ORGANIC COATINGS PAINTS: Paint may be defined as the mechanical dispersion mixture of pigments and fillers which are in a liquid medium and later becomes film forming oil. CONSTITUENTS OF PAINT: a. Pigment b. Vehicle or drying oil c. Thinners d. Driers d. Fillers or extenders e. Plasticizers f.. Anti-skinning agents

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Requirements of a Paint: 1. The adhesion capacity of the paint should be high to the material on which it is to be used. 2. The paint should spread easily over the surface to be protected. 3. On drying, the paint film should not be cracked. 4. The paint film should have high corrosion resistance property so as to protect the painted surface from the corrosion environment. 5. The paint film should be stable. 6. The paint film should be prepared such a way as to be applicable easily by spraying or brushing. 7. The paint film should yield a smooth and uniform surface. 8. The paint film obtained on the surface should be tough, uniform and adherent. 9. The colour of the film should be stable and should not get affected by the environment conditions. 10. The covering power of the paint should be high.

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