Lesson 5: Corrosion and Electrolysis

• Corrosion

  The breakdown or deterioration of metals through redox reactions

  
• Rusting

  Oxidation of iron in steel

  
• Corrosion of steel

  1. Non-uniformities on the surface become the anodic region

  2. Rest of the metal becomes the cathodic region

  3. Water acts as a salt bridge

  
• Oxidation of iron

  Fe (s) → Fe2+ (aq) + 2e-

  
• Reduction at cathode

  O2(g) + 2H2O (l) + 4 e- → 4OH- (aq)
• Non-uniformities

  • Bumps, dents
• Anodic region

  • The hole, gets smaller and smaller
• Cathodic region

  • Orange dust, gets larger
• High humidity and water

  More rust
• Copper corrosion

  • Turns green
  • Copper oxide protective layer
• Water pipe corrosion

  • Lead/iron reacts with water, causes health issues
  • Hard water has high iron

  
• Galvanizing

  Coating iron with zinc to protect from oxidation

  
• Cathodic protection

  Electrons are continually supplied to the metal that is being protected, making it the cathode
• Uses of cathodic protection

  • Buried pipelines, fuel tanks, sewage tanks
• Electrolysis

  Applying an electric current to a system to produce a chemical change
• Electrolytic cells

  Galvanic cells that use electricity to produce a chemical change that would not occur spontaneously

  
• Uses of electrolytic processes

  • Galvanizing
  • Aluminum production
  • Electrorefining metals
  • Electroplating metals
• sometimes the cathode might not be a solid, such as hydrogen gas, so the cathode is made of platinum (very unreactive) (so the cathode would be a gas + Pt)
• Electroplating

  1. Submerge steel cathode in anode in zinc solution

  2. Induce current from cathode through zinc solution to anode

  3. Dissolved zinc precipitates and forms plate on cathode

  4. Oxygen bubbles rise from anode
• Cathode

  The material going to be coated, attached to positive terminal of electrical energy supply
• Anode

  Immobile, oxygen bubbles rise from
• Electrolyte

  Zinc solution
• Electroplating

  • Coating iron with zinc to protect from oxidation
• Electroplating is a type of surface treatment using electrolysis