Corrosion Basics and Components

Corrosion

• The tendency of metals to return to their original state, usually as metal oxides.
• Occurs when metals react with oxygen in an oxidation-reduction reaction.
• Deterioration of a material due to reaction with its environment.

Components of Corrosion

• Anode: Where corrosion occurs.
  • Releases positively charged metal ions into the electrolyte.
  • Electrons are left behind in the metal and flow to the cathode.
  • Oxidation (loss of electrons) takes place at the anode.
• Cathode: The part of the metal where electrons flow.
  • Remains intact and protected.
  • Reduction takes place at the cathode.
• Electrolyte: A solution capable of conducting electrical current (e.g., moisture or salt solution).
• Electrical Path: A connection between the anode and the cathode, allowing electron flow.

Types of Corrosion

• Galvanic Corrosion: Occurs when two dissimilar metals are in contact in the presence of an electrolyte.
  • The more anodic metal corrodes, while the more cathodic metal remains intact.
  • The further apart the metals are in the galvanic series, the greater the corrosion difference.
• Stress Corrosion Cracking: Cracking of a material under static stress due to corrosion.
  • Occurs in many plastics, aluminum alloys, copper alloys, magnesium alloys, carbon steels, stainless steels, titanium alloys, and others.
  • High applied or residual stresses increase the likelihood of stress corrosion cracking.
• Atmospheric Corrosion:
  • Moisture: Condensation and dew can create an aggressive electrolyte environment that promotes corrosion.
  • Chemicals: Acids in the atmosphere accelerate corrosion.
  • Temperature: Every 10°C increase in temperature can double corrosion activity.
  • Mechanical Stress: Stress can exacerbate corrosion by opening up small cracks.

Prevention and Control

• Inhibitors: Substances that, when added in low concentrations, decrease the corrosiveness of the environment.
  • Specific inhibitors depend on the alloy and the corrosive environment.
  • Examples: Hydrazine in boiler feed water, nitrite in cooling systems.
• Protective Coatings:
  • Electroplating: Common coatings used for corrosion resistance, wear resistance, and decorative purposes.
    • Cadmium plating: Excellent corrosion resistance, especially in marine applications, but toxic.
    • Chromium plating: Widely used for corrosion, wear, and heat resistance, as well as decorative purposes.
    • Nickel plating: Used as a base for chromium plating, provides wear and corrosion resistance.
    • Tin plating: Used for products in contact with foodstuffs, protects against corrosion and aids soldering.
    • Zinc coating: Galvanizing is a hot-dip process used for industrial and construction applications.
  • Paints: Provide corrosion resistance and a decorative finish.
    • Chlorinated-rubber paints: Excellent chemical resistance and water penetration resistance.
    • Acrylic paints: Fast drying, tough, and durable, widely used in vehicle applications.
    • Epoxy resins: Used for high chemical resistance.
• Cathodic Protection:
  • Sacrificial Anodes: Highly active metals (e.g., magnesium, zinc, aluminum) used to prevent corrosion of a less active material.
    • The anode is consumed instead of the material being protected.
  • Galvanic Corrosion Control: Sacrificial pipe sections can be used to concentrate corrosion in specific areas.
    • Zinc and aluminum are preferred metals for marine industry pipelines.