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What is galvanic corrosion and how can it be avoided?

   

Galvanic corrosion, also known as bimetallic corrosion, is an electrochemical process whereby one metal corrodes in preference to another metal that it is in contact with through an electrolyte.

Galvanic corrosion occurs when two dissimilar metals are immersed in a conductive solution and are electrically connected. One metal (the cathode) is protected, whilst the other (the anode) is corroded. The rate of attack on the anode is accelerated, compared to the rate when the metal is uncoupled.

For example, if aluminium and carbon steel are connected and immersed in seawater, the aluminium will corrode more quickly, whilst the steel will receive protection. Galvanic corrosion can be prevented by:

  1. Selecting materials with similar corrosion potentials.
  2. Breaking the electrical connection by insulating the two metals from each other.
  3. Applying coatings to both materials. The coating on the cathode is the most important and must be in good condition, otherwise the galvanic corrosion could be worsened.
  4. Separating the two materials by inserting a suitably sized spacer.
  5. Installing a sacrificial anode that is anodic to both metals.
  6. Adding corrosion inhibitor to the environment.

If these measures are not practical, the rate of attack can be minimised by keeping the anode to cathode area large, i.e. over 10. Alternatively, the anode could be designed with an appropriate corrosion allowance.

Galvanic action can cause preferential corrosion of welds in certain environments. For example, in seawater, carbon steel weld metal can be susceptible to severe corrosion, whilst the adjacent parent material is unattacked.

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