Galvanic corrosion is an electrochemical process that occurs when two dissimilar metals are in contact through an electrolyte. Because it needs two different metals, it is also known as bimetallic corrosion.
One of the metals corrodes in preference to the other, and how much depends on how dissimilar the two are in terms of their galvanic potential. It usually happens by accident, when the interaction of dissimilar metals is overlooked in the original design. It can also be used deliberately for protection: a dissimilar metal is connected to the metal you want to protect and corrodes sacrificially, so it can be inspected and replaced as required.
How to avoid galvanic corrosion
- Choose materials with similar corrosion potentials.
- Insulate the two metals from each other to break the electrical connection.
- Apply coatings to both materials. The coating on the cathode matters most and must stay in good condition, or it can make galvanic corrosion worse.
- Insert a suitably sized spacer to separate the two materials.
- Install a sacrificial anode that is anodic to each metal.
- Add a corrosion inhibitor to the environment.
Where these measures are not practical, the rate of corrosion can be lowered by keeping the anode area large relative to the cathode, or by designing the anode with an appropriate corrosion allowance.