Unit 3 corrosion & batteries


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Unit 3 corrosion & batteries

  1. 1. Unit 3 Corrosion And Batteries Sept. /06 MA/chem/RCET
  2. 2. <ul><li>Lecture 23 </li></ul>Sept. /06 MA/chem/RCET
  3. 3. CORROSION <ul><li>Any process of deterioration </li></ul><ul><ul><li>consequent loss of a solid metallic material </li></ul></ul><ul><ul><li>through an unwanted chemical or electrochemical attack by its environment , </li></ul></ul><ul><ul><li>started at its surface </li></ul></ul><ul><ul><li>is called corrosion. </li></ul></ul>Sept. /06 MA/chem/RCET
  4. 4. Sept. /06 MA/chem/RCET metal Metal oxide Corrosion ,oxidation Metallurgy ,reduction Higher energy Lower energy
  5. 5. Example <ul><li>Rusting of IRON </li></ul><ul><li>Fe 3 O 4 , reddish scale </li></ul><ul><li>Green film on surface of Cu </li></ul><ul><li>CuCO 3 + Cu(OH) 2 , </li></ul><ul><li>Basic copper carbonate </li></ul>Sept. /06 MA/chem/RCET
  6. 6. Classification Sept. /06 MA/chem/RCET dry wet other oxidation By gases liquid Evolution of H 2 Absorption of O 2
  7. 7. Dry / chemical corrosion <ul><li>Direct chemical action of environmental gases such </li></ul><ul><li>as oxygen ,halogen , hydrogen sulphide , sulphur di oxide , nitrogen or anhydrous liquid </li></ul><ul><li>with metal surface in immediate proximity </li></ul>Sept. /06 MA/chem/RCET
  8. 8. Oxidation corrosion <ul><li>Anode </li></ul><ul><li>2M 2ne- + 2M n+ </li></ul><ul><li>(Loss of electron ,oxidation process) </li></ul>Sept. /06 MA/chem/RCET
  9. 9. <ul><li>Cathode </li></ul><ul><li>n/2 O 2 + 2ne- nO 2- </li></ul><ul><li>(Gain of electron ,reduction process) </li></ul>Sept. /06 MA/chem/RCET
  10. 10. Over all reaction <ul><li>2M + n/2 O 2 2M n+ + nO 2- </li></ul>Sept. /06 MA/chem/RCET Metal ion Oxide ion Metal oxide
  11. 11. Oxidation corrosion Sept. /06 MA/chem/RCET M M n+ + ne - Reaction at metal – metal oxide interface Atmospheric oxygen metal
  12. 12. Chemical reaction <ul><li>2M 2M n+ + 2ne - </li></ul><ul><li>(loss of electron) </li></ul><ul><li>nO 2 + 2ne - 2nO 2- </li></ul><ul><li>(gain of electron) </li></ul><ul><li>2M + nO 2 2M n+ + 2nO 2- </li></ul>Sept. /06 MA/chem/RCET
  13. 13. TYPES <ul><li>Stable </li></ul><ul><li>Unstable </li></ul><ul><li>Volatile </li></ul><ul><li>porous </li></ul>Sept. /06 MA/chem/RCET
  14. 14. 1 .stable <ul><li>The oxide film on the surface of </li></ul><ul><li>Al , Pb , Sn , Pt, etc </li></ul><ul><li>stable, </li></ul><ul><li>Tightly adhering </li></ul><ul><li>and </li></ul><ul><li>impervious in nature. </li></ul>Sept. /06 MA/chem/RCET
  15. 15. 2. Unstable Sept. /06 MA/chem/RCET metal metal metal Exposed surface O 2 ,air Unstable metal oxide Film formed decomposed + O 2
  16. 16. 3. Volatile Sept. /06 MA/chem/RCET metal metal metal Exposed surface O 2 ,air volatile metal oxide Film formed decomposed + O 2
  17. 17. 4. Porous Sept. /06 MA/chem/RCET metal metal Exposed surface O 2 ,air porous metal oxide Film formed decomposed + O 2 Further attack through cracks ,pores
  18. 18. Corrosion by other gases <ul><li>SO 2 </li></ul><ul><li>CO 2 </li></ul><ul><li>Cl 2 </li></ul><ul><li>H 2 S </li></ul><ul><li>F 2 </li></ul>Sept. /06 MA/chem/RCET
  19. 19. Liquid metal corrosion <ul><li>Dissolution of solid metal </li></ul><ul><li>Internal penetration </li></ul>Sept. /06 MA/chem/RCET
  20. 20. <ul><li>Lect. 24 </li></ul>Sept. /06 MA/chem/RCET
  21. 21. Wet corrosion <ul><li>Conducting liquid </li></ul><ul><li>In contact with metal </li></ul><ul><li>Dissimilar metal , Alloys </li></ul><ul><li>Immersed , partially immersed </li></ul><ul><li>In solution </li></ul>Sept. /06 MA/chem/RCET
  22. 22. Chemical reaction <ul><li>Anodic reaction </li></ul><ul><li>M M n+ + ne - (oxidation loss </li></ul><ul><li>of e - s) </li></ul><ul><li>dissolves in solution </li></ul><ul><li>Forms compound such as oxide </li></ul>Sept. /06 MA/chem/RCET
  23. 23. Cathodic reaction <ul><li>1) Evolution of hydrogen </li></ul><ul><li>2) Absorption of oxygen </li></ul>Sept. /06 MA/chem/RCET
  24. 24. Evolution of hydrogen <ul><li>In acidic environment </li></ul><ul><li>Example </li></ul><ul><li>Fe Fe ++ ( ferrous) </li></ul>Sept. /06 MA/chem/RCET
  25. 25. Chemical reaction <ul><li>Anode </li></ul><ul><li>Fe Fe 2+ + 2e - (oxidation ,loss of electrons) </li></ul><ul><li>Cathode </li></ul><ul><li>2H+ + 2e- H 2 </li></ul><ul><li>(reduction, gain of electron) </li></ul><ul><li>Over all reaction </li></ul><ul><li>Fe + 2H + Fe 2+ + H 2 </li></ul>Sept. /06 MA/chem/RCET
  26. 26. Evolution of hydrogen Sept. /06 MA/chem/RCET M M n+ + 2e - Acidic solution ,electrolyte Small ,cathodic area Large anodic area
  27. 27. Absorption of oxygen <ul><li>In neutral environment </li></ul><ul><li>Rusting of iron , in presence of NaCl solution </li></ul><ul><li>Fe in presence of 0xygen forms iron oxide </li></ul><ul><li>Anodic area created on surface (smaller) </li></ul><ul><li>Metal part act as cathode (large) </li></ul>Sept. /06 MA/chem/RCET
  28. 28. Absorption of oxygen Sept. /06 MA/chem/RCET Aqueous neutral solution rust Large cathodic area Small anodic area
  29. 29. Anodic reaction <ul><li>Fe Fe 2+ + 2e - </li></ul><ul><li>(oxidation, loss of electron) </li></ul>Sept. /06 MA/chem/RCET
  30. 30. Cathodic reaction <ul><li>Fe 2+ ions (at anode) and OH - ions (at cathode) diffuse </li></ul><ul><li>Ferrous hydroxide is precipitated . </li></ul><ul><li>Fe 2+ + 2OH - ----------> Fe 2+ + 2e - </li></ul>Sept. /06 MA/chem/RCET
  31. 31. ( i ) <ul><li>If enough oxygen is present , ferrous hydroxide is easily oxidized to ferric hydroxide . </li></ul><ul><li>4 Fe(OH) 2 + O 2 + 2H 2 O---------> 4Fe(OH ) 3 </li></ul><ul><li>yellow rust , ( Fe 2 O 3 . H 2 O) </li></ul>Sept. /06 MA/chem/RCET
  32. 32. ( ii ) <ul><li>If the supply of oxygen is limited, </li></ul><ul><li>the corrosion product may be even </li></ul><ul><li>black anhydrous magnetite, Fe 3 O 4 . </li></ul>Sept. /06 MA/chem/RCET
  33. 33. <ul><li>Lect. - 25 </li></ul>Sept. /06 MA/chem/RCET
  34. 34. GALVANIC (OR BIMETALLIC) CORROSION <ul><li>When two dissimilar metals </li></ul><ul><li>(e.g., zinc and copper) </li></ul><ul><li>electrically connected and exposed to an electrolyte, </li></ul><ul><li>the metal higher in electrochemical series undergoes corrosion. </li></ul>Sept. /06 MA/chem/RCET
  35. 35. Galvanic / Bimetallic Sept. /06 MA/chem/RCET Zn Cu Electronic current Cathode ,protected Anode , attacked electrolyte
  36. 36. Explanation <ul><li>zinc (higher in electrochemical series) forms the anode and is attacked and gets dissolved; </li></ul><ul><li>whereas copper (lower in electrochemical series or more noble) acts as cathode. </li></ul>Sept. /06 MA/chem/RCET
  37. 37. Mechanism <ul><li>In acidic solution, the corrosion occurs by the hydrogen evolution process; </li></ul><ul><li>while in neutral or slightly alkaline solution, oxygen absorption occurs. </li></ul><ul><li>The electron-current flow from the anodic metal zinc, </li></ul><ul><li>Zn Zn 2+ + 2e- (oxidation) </li></ul>Sept. /06 MA/chem/RCET
  38. 38. Examples <ul><li>(i) Steel screws in a brass marine hardware; </li></ul><ul><li>(ii) Lead - antimony solder around copper wire; </li></ul><ul><li>(iii) A steel propeller shaft in bronze bearing; </li></ul><ul><li>(iv) Steel pipe connected to copper plumbing. </li></ul>Sept. /06 MA/chem/RCET
  39. 39. CONCENTRATION CELL CORROSION <ul><li> corrosion is due to electrochemical attack on the metal surface, </li></ul><ul><li>exposed to an electrolyte of varying concentrations or of varying aeration. </li></ul>Sept. /06 MA/chem/RCET
  40. 40. Concentration cell corrosion Sept. /06 MA/chem/RCET Water line corrosion Zn rod Corroding anode NaCl solution More oxygenated part Less oxygenated part
  41. 41. <ul><li>This may be the result of local difference in metal - ion concentrations, </li></ul><ul><li>caused by local temperature differences or inadequate agitation or slow diffusion of metal-ions, produced by corrosion. </li></ul>Sept. /06 MA/chem/RCET
  42. 42. <ul><li>It has been found experimentally that “poor-oxygenated parts are anodic” </li></ul><ul><li>Consequently, a differential aeration of metal causes a flow of current, </li></ul><ul><li>called the differential current. </li></ul>Sept. /06 MA/chem/RCET
  43. 43. <ul><li>Zinc will dissolve at the anodic areas, </li></ul><ul><li>and oxygen will take up electrons </li></ul><ul><li>at the cathodic areas to form </li></ul><ul><li>hydroxyl ions. </li></ul>Sept. /06 MA/chem/RCET
  44. 44. Chemical reaction <ul><li>anodic </li></ul><ul><li>Zn Zn 2+ + 2e - (Oxidation) </li></ul><ul><li>cathodic </li></ul><ul><li>1/2 O 2 + H 2 O + 2e- 2 OH- (Reduction) </li></ul>Sept. /06 MA/chem/RCET
  45. 45. <ul><li>The circuit is completed by migration of ions, </li></ul><ul><li>through the electrolyte, </li></ul><ul><li>and flow of electrons, </li></ul><ul><li>through the metal, from anode to cathode. </li></ul>Sept. /06 MA/chem/RCET
  46. 46. <ul><li>Corrosion is accelerated under accumulation of dirt sand, </li></ul><ul><li>scale or other, contamination </li></ul><ul><li>The result is localized corrosion, </li></ul><ul><li>due to non - uniform corrosion. </li></ul>Sept. /06 MA/chem/RCET
  47. 47. <ul><li>Metals exposed to aqueous media corrode under blocks of wood or pieces of glass, </li></ul><ul><li>which screen that portion of metal from oxygen access. </li></ul>Sept. /06 MA/chem/RCET
  48. 48. <ul><li>Lect . -25 </li></ul>Sept. /06 MA/chem/RCET
  49. 49. PASSIVITY <ul><li>. Passivity is the </li></ul><ul><li>“ phenomenon in which a metal or an alloy exhibits a much higher corrosion - resistance than expected from its position in the electrochemical series.” </li></ul>Sept. /06 MA/chem/RCET
  50. 50. Passivity falling order <ul><li>Tl --------> Al ------> Cr -------> Be ------- </li></ul><ul><li>>Mo -------> Mg ------> Ni ------> </li></ul><ul><li>Co -------> Fe ------> Mn ------->Zn ------- </li></ul><ul><li>> Cd -------> Sn ------> Pb ------> Cu </li></ul>Sept. /06 MA/chem/RCET
  51. 51. <ul><li>The action of more concentrated </li></ul><ul><li>solution of HNO 3 on active metals (Fe and Al) produces a thin protective oxide film, </li></ul><ul><li>thereby stifling the anodic reaction and making them passive </li></ul>Sept. /06 MA/chem/RCET
  52. 52. 8. UNDERGROUND OR SOIL <ul><li>The corrosiveness of a soil depends upon : </li></ul><ul><li>(i) its acidity, </li></ul><ul><li>(ii) degree of aeration, </li></ul><ul><li>(iii) electrical conductivity, </li></ul>Sept. /06 MA/chem/RCET
  53. 53. <ul><li>(iv) its moisture and salts content, </li></ul><ul><li>(v) presence of bacteria and micro- </li></ul><ul><li>organisms, and </li></ul><ul><li>(vi) soil texture. </li></ul>Sept. /06 MA/chem/RCET
  54. 54. (a) <ul><li>Gravelly or sandy solis are very porous and strongly- aerated. </li></ul><ul><li>If a metal pipe is buried in such a soil, corrosive conditions are similar to those under wet condition, </li></ul><ul><li>and the corrosion rate will be governed by amount of moisture content in the soil. </li></ul>Sept. /06 MA/chem/RCET
  55. 55. (b) <ul><li>In water-logged soils, </li></ul><ul><li>the amount of free oxygen available is very small, </li></ul><ul><li>but various bacteria and micro- organisms can grow, </li></ul><ul><li>which may lead to micro-biological corrosion. </li></ul>Sept. /06 MA/chem/RCET
  56. 56. corrosion depends <ul><li>(i) the pH (or acidity) of the soil, </li></ul><ul><li>(ii) the presence of salt, and </li></ul><ul><li>(iii) the presence of oxygen, etc. </li></ul><ul><li>oxygen facilitates the evolution of hydrogen and, hence, accelerates the rate of attack. </li></ul>Sept. /06 MA/chem/RCET
  57. 57. 9. PITTING CORROSION <ul><li>Pitting corrosion is a localized accelerated attack, resulting in the formation of cavities around which the metal is relatively unattached. </li></ul><ul><li>pinholes, pits and cavities in the metal. </li></ul>Sept. /06 MA/chem/RCET
  58. 58. <ul><li>pitting is, </li></ul><ul><li>usually, the result of the breakdown or cracking of the protective film on a metal at specific points. </li></ul>Sept. /06 MA/chem/RCET
  59. 59. Breakdown of the protective film <ul><li>caused by : </li></ul><ul><li>(i) surface roughness or non-uniform finish, </li></ul><ul><li>(ii) scratches or cut edge, </li></ul><ul><li>(iii) local straining of metal, due to non-uniform stresses, </li></ul>Sept. /06 MA/chem/RCET
  60. 60. <ul><li>(iv) alternating stresses, </li></ul><ul><li>(v) sliding under load, </li></ul><ul><li>(vi) impingement attack (caused by the turbulent flow of a solution over a metal surface), </li></ul><ul><li>(vii) chemical attack. </li></ul>Sept. /06 MA/chem/RCET
  61. 61. <ul><li>1. Differential amount of oxygen in contact with the metal the small part </li></ul><ul><li>(underneath the impurity) become the anodic areas </li></ul><ul><li>2 . Surrounding large parts become the cathodic areas. </li></ul>Sept. /06 MA/chem/RCET
  62. 62. <ul><li>Intense corrosion, therefore, </li></ul><ul><li>start just underneath the impurity. </li></ul><ul><li>Once a small pit is formed, </li></ul><ul><li>the rate of corrosion will be increased. </li></ul>Sept. /06 MA/chem/RCET
  63. 63. INTERGRANULAR CORROSION <ul><li> along grain boundaries and only where the material, especially sensitive to corrosive attack exists, </li></ul><ul><li>and corrosive liquid possesses a selective character of attacking only at the grain boundaries, </li></ul>Sept. /06 MA/chem/RCET
  64. 64. <ul><li>but leaving the grain interiors </li></ul><ul><li>untouched or </li></ul><ul><li>only slightly attacked. </li></ul>Sept. /06 MA/chem/RCET
  65. 65. <ul><li>WATERLINE CORROSION </li></ul><ul><li>When water is stored in a steel tank, it is generally found 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 the cathodic and is completely unaffected by corrosion. However, if the water is relatively free from acidity, little corrosion takes place. </li></ul>Sept. /06 MA/chem/RCET
  66. 66. <ul><li>The problem of waterline corrosion is also that concerns marine engineers. In the case of ships, this kind of corrosion is often accelerated by marine plants attaching themselves to the sides of ships. The use of special antifouling paints restrict this to some extent. </li></ul>Sept. /06 MA/chem/RCET
  67. 67. Sept. /06 MA/chem/RCET
  68. 68. <ul><li>It is characteerized by a highly localized attack occurring, when overall corrosion is negligible. For stress corrosion to occur : (i) presence of tensile stress, and (ii) a specific corrosive environment are necessary. The corrosive agents are highly specific and selective such are: (a) caustic alkalis and strong nitrate solution for mild steel; (b) traces of ammonia for brass; (c) acid chloride solution for stainless steel. </li></ul>Sept. /06 MA/chem/RCET
  69. 69. <ul><li>This type of corrosion is seen in fabricated articated articles of certain alloys (like high-zinc brasses and nickel brasses) due to the presence of stresses caused by heavy working like rolling, drawing or insufficent annealing. However, pure metals are relatively immune to stress corrosion. </li></ul>Sept. /06 MA/chem/RCET
  70. 70. Sept. /06 MA/chem/RCET
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  72. 72. Sept. /06 MA/chem/RCET
  73. 73. <ul><li>These become so chemically- active that they are attacked, even by a mild corrosive environment, resulting in the formation of a crack, which grows and propagates in a plant (perpendicular to the operating tensile stress), until failure occurs or it may stop, after progressing a finite distance. </li></ul><ul><li>Some typical examples </li></ul>Sept. /06 MA/chem/RCET
  74. 74. <ul><li>V(1) Seeason cracking in a term applied to stress corrosion of copper allays, mainly brasses. Pure copper is immune to stress corrosion, but presence of small amounts of alloying element (like P,As,Sb, Zn, Al, Si) result in marked sensitivity. For examples, alpha brass (which when highly stressed) undergo intergranular cracking in an atmosphere, containing traces of ammonia or amines. The attack occurs along the grain boundaries, which become more anodic with respect to the grain themselves (probably </li></ul>Sept. /06 MA/chem/RCET
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  77. 77. Sept. /06 MA/chem/RCET