Corrosion

• Corrosion


  A complex electrochemical (galvanic) action that causes metals to be transformed into their salts and oxides

  
• Requirements for electrochemical corrosion

  • Metal anode
  • Cathode
  • Electrolyte
  • Direct electrical contact between dissimilar metals

  
• Corrosion is a natural process, and its prevention is almost impossible, but it can be controlled
• Dry corrosion


  A slow chemical reaction, not electrochemical in nature

  
• Wet corrosion


  An electrochemical process that takes place in environments where the relative humidity is high
• Direct chemical attack


  A form of corrosion where an alkaline or acidic liquid comes into contact with metal

  
• Mercury corrosion


  Mercury attacks aluminium by a chemical reaction known as amalgamation

  
• Electrochemical series


  If the anode is smaller than the cathode, the anode will give up electrons more easily and the corrosion will be more rapid
• Galvanic corrosion


  Corrosion that can take place where dissimilar metal skins are riveted together or where aluminium inspection plates are attached to a structure with steel screws

  
• Stress corrosion


  Corrosion that can occur due to stresses from a fitting that has been pressed into a structural part with an interference fit

  
• Surface corrosion


  Uniform attack over an entire surface area, causing a dulling of the surface

  
• Pitting corrosion


  Localized anodic areas that can eat completely through the metal
• Intergranular corrosion


  Corrosion that occurs along the grain boundaries of the metal, often showing up as a blister on the surface

  
• Filiform corrosion


  Corrosion that can propagate beneath an organic coating, with the front of propagation acting as the anode and the defect neighbourhood acting as the cathode

  
• Fretting corrosion


  Erosion that occurs when two surfaces fit tightly together but can move relative to one another

  
• Exfoliation corrosion


  An extreme case of intergranular corrosion that occurs chiefly in extruded materials

  
• Microbiological corrosion


  Corrosion caused by the metabolic activity of microorganisms
• Metal corrosion susceptibility


  • Aluminium
  • Anodised aluminium
  • Magnesium
  • Titanium
  • Cadmium
  • Chromium
  • Nickel
  • Copper and copper alloys
  • Silver
  • Gold
  • Tin

  
• Corrosion can severely affect the strength and integrity of metal structures
• Fretting corrosion


  Wear caused by the continued rubbing action between two surfaces, where the debris between them does not have an opportunity to escape and acts as an abrasive to further erode the surfaces
• By the time fretting corrosion appears on the surface, the damage is usually done and the parts must be replaced
• Fretting corrosion may occur around rivets in a skin, indicated by dark deposits around the rivet heads, streaming out behind, giving the appearance of the rivet smoking
• Rivets showing signs of fretting corrosion must be replaced as soon as possible
• Exfoliation corrosion


  An extreme case of intergranular corrosion that occurs chiefly in extruded materials, where the grain structure is more laminar, causing the material to separate or delaminate

  
• By the time exfoliation corrosion is evident on the surface, the strength of the metal has been greatly decreased, and the metal no longer has any strength

  
• The only remedy for exfoliation corrosion is complete removal of the affected sections
• Microbiological corrosion


  Corrosion caused by microbes (microscopic life forms, including fungi) that live in the water condensed in aircraft fuel tanks and feed on the hydrocarbon fuel
• The scum formed by the microbiological growth holds water in contact with the tank structure, where corrosion of the concentration cell type will inevitably form
• If the scum forms along the edge of a seal in an integral fuel tank, the sealant may pull away from the structure, causing a leak and an expensive resealing operation
• Aluminium


  • Highly anodic, subject to pitting, intergranular corrosion and stress corrosion cracking, but the formation of a tightly adhering oxide film offers increased resistance under mild corrosive conditions

  
• Anodised aluminium


  An electrochemically applied oxide coating that increases the thickness of the naturally occurring protective aluminium oxide film

  
• Magnesium


  • Highly susceptible to corrosion when the metal surface is exposed to the environment without a protective finish, with corrosion deposits tending to raise slightly and spread rapidly
• Titanium


  • Highly corrosion resistant due to the formation of an extremely adherent oxide film, but can accelerate corrosion of dissimilar metals it is coupled with

  
• Cadmium


  • Used as a protective coating on steel hardware, anodic to steel and protects it by galvanic action, with corrosion evident by white-to-brown-to-black mottling of the surface

  
• Chromium


  • Used as a protective plating, forms a continuous oxide coating that can protect itself and any underlying metal, but may contain cracks where corrosion can originate

  
• Nickel


  • Used as a plating metal, subject to pitting corrosion, and when added to stainless steel increases its stress corrosion resistance, but nickel-based alloys are subject to hot corrosion attack and embrittlement

  
• Copper and copper alloys

  • Considered corrosion resistant, with corrosion usually limited to staining and tarnishing, forming a protective oxide crust

  
• Silver


  • Does not corrode but tarnishes in the presence of sulfur, forming a brown-to-black film, and can accelerate corrosion of underlying copper through galvanic action

  
• Gold


  • Highly corrosion resistant, but can accelerate corrosion of underlying less noble metals like silver or copper at pores or pinholes in the gold plating
• Tin


  • Offers good solderability and corrosion resistance, but tends to grow 'whiskers' on plated applications