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Electrochemistry IX
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Corrosion and factors affecting corrosion

Shaillee Kaushal is teaching live on Unacademy Plus

Shaillee Kaushal
Faculty in Chemistry with a teaching experience of 14 years. Specialised in teaching for boards and competitions.

Unacademy user
thank you mam for uploading the splendid lesson :-)
  1. XII -03 Electrochemistry IX

  2. Corrosion of metals: The process of weathering away of the metal due to attack of atomospheric gases on the surface of the meta The process of corrosion of iron is called rusting. Rest has the formula Fe203.xH20

  3. Mechanism of Rusting .Corrosion is a redox process by which metals are oxidized by oxygen in the presence of moisture. The theory of rusting is called electrochemical theory. "In this theory the process of rusting can be explained on the basis of formation of electrochemical cell on basis of formation of electrochemical cell on the surface of an iron object. Dropot moisture o,+ Fes. Rust (Cathode) Flow of Anode

  4. Step 1: The water vapours present in contact with iron surface has dissolved CO2 and 02 from air H20 CO2H2CO3 Thus the surface of iron is covered with an aqueous solution of carbonic acid, which undergoes dissociation to a small extent: H2CO3 2H + + C03-Ionisation of carbonic acid H2O H+ + OH-(ionisation of water) Step2: Second step involves oxidation of iron. Oxidation takes place at the point of strain as a steel nail first corrodes at the tip of the head. The tip of the nail acts as the anode Fe Fe2+ + 2e-

  5. Step3: The electrons flow along the nail to the areas containing impurities which act as cathode where oxygen is reduced to hydroxyl ions 02 + 2H20 + 4e- 4OH- The process of reduction involves the following two steps: First of all H eH]i) These hydrogen atoms combine with oxygen dissolved in water or from air 4[H 022H20 Combining both the equations we get 02 + 4H + + 4e--> 2H20(1) Adding equations (i) and (iv) we get the overall reaction of microcells established on the surface of iron 2Fe + 02 4H2Fe2+ 2H20 (iv)

  6. Step4: The ferrous ions (Fe2+) formed in the previous step reacts with dissolved oxygen or oxygen from air to form ferric oxide (Fe203) Hydration of ferric oxide gives rust Fe203 xH20Fe203. xH20

  7. Factors affecting corrosion 1. Standard reduction potentia: Lesser is the standard reduction potential, greater is the tendency of corrosion Strains and corrosion- corrosion of metals occur more readily at points of strain, bend, 3. Impurity of metal: Presence of impurity in metals corrosion 4. Salinity of water and corrosion:- If water is saline, it helps in the flow of current in 2. neck and scratches increases the probabilty of microelctro chemical cells on the surface of iron and hence, increase the process of corrosion. 5. Pollution and corrosion: The acidic oxides like etc. Present in air act as catalysts for corrosion

  8. PreventionofRusting: 1. Using Antirust solution:- Alkaline phosphate and alkaline chromate solution are the commonly used as antirust solutions. Alkaline phosphates and to form an insoluble, film of iron phosphate one the surface of iron, there by protecting it from corrosion, in addition the alkaline nature of an antirust solution decreases the availability of H ions which faciliate the oxidation of Fe to Fe2+ 2. Barrier protection In this method barrier or coating is applied to preventthe surface ofthe metallic object from coming in contact with the atmosphere (i) Oil paints on surface of metal prevents its contact with mosit air (ii) By oiling the surface (ii) Nichrome (Ni + Cr) plating on the surface of iron also acts as a barrier (iv) Bisphenol when applied on the surface also helps to avoid corrosion.

  9. 3. Sacrificial protenction: The surface of iron is coated with a more active metal than iron. The active metal looses electrons in preference to iron. Zinc metal is the most stable metal to cover iron surfaces. This process of coating the iron surface with zinc is called galvanisation. Zinc metal present on the surface of iron forms a thin protective layer of basic zinc carbonate i.e Zn03. Zn (OH)2 due to reaction between zinc and oxygen, CO2 and moisture in air. Sometimes the iron surface is coated with tin metal and this process is called tinning or tin plating. This method is not as effective as galvanisation. 4. Electrical or cathodic protection: If a buried steel pipe is connected to an active metal i.e highly electropositive metal, say magnesium a voltaic cell is formed; the active metal is anode and iron becomes the cathode. As the cathode, the iron containing steel pipe is protected from oxidation. The magnesium rod is eventually consumed and must be replaced this method is used to prevent the rusting of submarines and basline of oil refineries