What is it?
Bimetallic corrosion occurs when two metals, with different potentials, are in electrical contact while immersed in an electrically conducting corrosive liquid, Because the metals
have different natural potentials in the liquid, a current will flow from the anode (more electronegative) metal to the cathode (more electropositive), which will incease the corrosion on the anode.
This additional corrosion is bimetallic corrosion. It is also referred to as a galvanic corrosion, dissimilar metal corrosion or contact corrosion.
The effect of coupling the two metals together increases the corrosion rate of the anode and reduces or even suppresses corrosion of the cathode. Hence, coupling a component to a sacrificial anode can prevent corrosion, and this is the principle of cathodic protection.
Checklist:
In situations where contact between dissimilar metals cannot
be avoided the following steps should be considered:
- Select metals that are close together in the galvanic series for the particular environment.
- Avoid relatively small areas of the less noble metal and large areas of the more noble metal.
- Insulate the metals from each other or separate them with internally coated spool pieces.
- Exclude electrolyte from around the bimetallic junction, for example, by painting.
- Paint both metals where possible; where impractical, paint the more electropositive metal (cathode).
- Make extra allowance for corrosion by increasing the thickness of the electronegative metal.
- Apply compatible metal or sacrificial metal coatings.
- In equipment containing dissimilar metals that are not in contact, ensure that localised corrosion does not result from deposited noble metals such as copper, by inhibiting the electrolyte or by coating susceptible surfaces.
- If electrical insulation is adopted, check that it is effective after installation of equipment is complete.
- With critical items of plant and equipment, include electrical insulation checking in routine inspection schedules.