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Corrosion

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  • 1. GROUP NO 6 Group Members Name Waqas Ahmad Umair Aslam Tayyab Naveed Muhammad Umair Muhammad Mudeser khalid Mujahid Ali
  • 2. Definition of Corrosion Corrosion is the slow process of decay of the metal, due to the attack of the atmosphere gases on the surface of the metal, resulting in the formation of metallic compounds such oxides, hydroxides, carbonates, sulphides etc.
  • 3. FORMS OF CORROSION CORROSION WET CORROSION DRY CORROSION CORROSION ROOM TEMPERATURE CORROSION HIGH TEMPERATURE CORROSION
  • 4. 1. Dry or Chemical Corrosion: It is due to the direct chemical action of environmental atmosphere gases such as O2. H2S, SO2, N2, halogens or anhydrous inorganic liquids with metal surfaces. 2. Wet or Electrochemical Corrosion: It is due to the existence of separate “anodic” and “cathodic” areas in the system between which current flows through the conducting liquid and the anode gets oxidized and wasted.
  • 5. WET / AQUEOUS CORROSION Based on the appearance of the corroded metal, wet corrosion may be classified as  Uniform or General  Galvanic or Two-metal  Crevice  Pitting
  • 6. UNIFORM CORROSION  Corrosion over the entire exposed surface at a uniform rate. e.g.. Atmospheric corrosion.  Maximum metal loss by this form.  Not dangerous. Rate can be measured in the laboratory
  • 7. GALVANIC CORROSION  When two dissimilar metals are joined together and exposed, the more active of the two metals corrode faster and the nobler metal is protected. This excess corrosion is due to the galvanic current generated at the junction  a) Steel plates with copper rivets in seawater 15 months.  b) ) Copper plates with steel rivets, same environmental conditions
  • 8. CREVICE CORROSION  Intensive localized corrosion within crevices & shielded areas on metal surfaces  Small volumes of stagnant corrosive caused by holes, gaskets, surface deposits, lap joints
  • 9. PITTING A form of extremely localized attack causing holes in the metal Most destructive form Autocatalytic nature Difficult to detect and measure
  • 10. Electrochemical Theory of Corrosion (Mechanism Of Corrosion) Corrosion is an electrochemical process. Galvanic cells are setup between dissimilar metals in contact with each other or between dissimilar parts of he same metal when surrounded by moist air or liquid. Anodic area oxidation takes place Cathodic area reduction takes place. Metallic ions formed at the anodic part and ions formed at the cathodic part diffuses towards each other through conducting medium and the corrosion product is formed somewhere between the anodic and cathodic areas.
  • 11. Anodic iron gets oxidised to Fe2+ M Mn+ + ne- The oxygen at the cathode changes to OH- by reduction. ½ O2 + H2O +2e2 OH-
  • 12. Factors Affecting Corrosion Rate & Extent of corrosion
  • 13. 1. The position of metals in the electrochemical Series More oxidation Decreasing tendency to loose electrons Metal SRP, Eo Lithium Potassium Calcium Sodium Magnesium Aluminum Zinc Nickel Tin Hydrogen--------------- 0.00 Copper Silver Platinum Gold---------------------- +1.15 V Less Reduction Increasing order of std reduction potential
  • 14. 2. Purity of the metal A pure metal is more corrosion resistant than an impure metal. The rate and extent of corrosion increases with the amount of impurities present.
  • 15. 3.Physical state of the metal Smaller the size of the metal, more the area under stress and greater is the corrosion.
  • 16. 4. Solubility and volatility of corrosion product If the product of corrosion is soluble in the corroding medium and also is volatile, corrosion occurs faster.
  • 17. 6. Nature of the corroding environment A.Temperature: Corrosion generally increases with rise in temperature of environment. B.Humidity of the air: Rate of corrosion increases with presence of moisture in the atmosphere. C.Impurities: Presence of impurities like CO2, H2S, SO2, acid fumes etc increases corrosion rate. D.Influence of pH: In acid medium corrosion is more and in alkaline medium it is less.
  • 18. Prevention of Corrosion
  • 19. 1. Maintaining the purity of the metal Pure metals are generally corrosion resistant.
  • 20. 2. Alloying with other elements It to found that alloying with suitable elements increases corrosion resistance of most metals.
  • 21. 3. Modification of environment Deaeration of aqueous environment and dehumidification decreases the rate of corrosion.
  • 22. 4 . Protective coatings 1.Metallic Coating 2.Non-metallic coatings 3.Organic coatings
  • 23. i. Metallic Coating a) Using less active metal b) Using a more active metal
  • 24. ii. Non-Metallic Coating a) Phosphate coating by Alkaline solution of phosphate. b) Chromate coating using chromate solutions.
  • 25. iii. Organic Coating Plastics, polythene, rubber etc, are used for coating to prevent corrosion. Mainly on articles like, ship, submarines. Etc.
  • 26. ELECTRICAL PROTECTION
  • 27. Zn2+ + 2e- Zn ½ O2 + H2O + 2e- Anode 2OH- Cathode Cathod e Zn Anode
  • 28. THANK YOU FOR ATTENTI ON