Methods to prevent corrosion
- 1. Electrochemical Protection or Cathodic Protection • Principle- To force the metal to behave like a cathode by connecting it to a powerful anodic metal plate • Two Kinds Sacrificial anodic protection- Metallic equipment to be protected is connected to a more anodic metal (Zn, Mg..) (underground pipelines an be protected) Impressed current cathodic protection • An electrical method, achieved by forcing a defined quantity of direct current • Positive terminal is connected to an insoluble anode like graphite 1(underground pipelines can be protected)
- 2. Cathodic Protection Sacrificial anodic protection Impressed current cathodic protection
- 3. Methods for application of Metallic Coating • Hot Dipping • Electroplating • Metal spraying • Metal cladding • Cementation 3 Prevention …
- 4. Hot Dipping • Base Metals(Fe, Cu) having higher melting point are immersed in the tank of molten metal like Zn, Sn, Pb, Al etc having low melting point – Coating of Zn on iron by hot dipping is called Galvanization. – Coating of Sn on iron is called Tinning. 4
- 5. Electroplating • Deposition of corrosion resistant metal (Au, Ag, Ni, Cr, Zn,Pb) over the base metal by passing electric current through an electrolytic solution containing soluble salt of coating metals. • Anode: rod of metal to be deposited • Cathode: base metal (thoroughly cleaned) • Electrolyte: metallic ions solution • e.g. Electroplating of Silver on another metal – At Anode- Ag (s) → Ag+ (aq) + e- – At Cathode- Ag+ (aq) + e- → Ag (s) 5 When current is passed metal ions from electrolyte deposit on cathode and equal number of metal ions are added to the electrolyte from anode
- 6. Electroplating Before - dull After - shiny 6
- 7. Metal Spraying • Spraying guns - for coating of metals. • Zn, Al, Sn, Pb (low melting) are sprayed on the base metal in molten state Wire gun method- wire of metal to be coated is melted by oxy-acetylene flame and blast of compressed air release molten metal as spray. Advantage: Can be done on rough surfaces and required thickness is easily obtained Technique can be used where metal dipping is not possible like bridges. 7
- 8. Wire gun 8
- 9. Metal-Cladding Process in which thin layer of coating metal get bonded to base metal on one or both sides permanently when they are passed through heavy rollers under the action of heat and pressure Application : In aircraft industry, petroleum, equipments used for handling of chemicals 9
- 10. Some non-ferrous metals are particularly resistant to corrosion, e.g. Cu and Zn They form strong oxides on their surfaces (as do aluminium and lead) and these protect the metal from further oxidation. Cladding on the buildings above Copper Cladding Zinc Cladding Steel Cladding
- 11. Cementation • Base metal is heated after packing it in powder of coating metal in a box. • Heating diffuses the coating metal into base metal forming an alloy • Thickness of coating is controlled by varying the temp and time of treatment – Coating of Cr over base metal: Chromising – Coating of Si over base metal : Siliconising 11
- 12. Organic Coatings• Paints • Varnishes • Enamels • Lacquers Mechanical dispersion mixtures of finely divided solids called pigments in a fluid medium. Pigments e.g. Cr2O3 for Green Pigment, ZnO for white pigment •Fluid medium consist of non-volatile film forming material called drying oil (glyceryl esters of higher fatty acids) and a highly volatile solvent called thinner (kerosene, benzene to reduce the viscosity) Paints
- 13. Drying oil is used to hold the pigment on metal surface by forming a protective film Plasticizers can be added in paints to provide elasticity to film and to minimize the cracking of film
- 14. • Colloidal solution of Natural & Synthetic resins – RESIN- Cross linked, high mol weight organic polymers – Natural: Rosin C15H20O6 - a solid form of resin obtained from pines and some other plants – Synthetic: Phenol formaldehyde Varnishes Enamels Dispersion of pigments in a varnish- provides lustrous finish Lacquers Colloidal dispersion of a solution of cellulose derivatives like cellulose nitrate, resins & plasticizers in a solvent like acetone Provides a water proof film