Bimetal Corrosion
To whom it may concern.
Bimetal or Galvanic Corrosion occurs when two metals or alloys of different potential are in contact with each other, in the presence of an electrolyte. An electro-chemical reaction occurs, resulting of accelerated corrosion of the metal which has the higher galvanic potential. The greater the distance between the two metals, the greater the degree of galvanic corrosion.
In the case of Aluminium vs. AZ (Zinc/Aluminium coatings) we would make the following comments:-
- There is a very small potential difference between Zinc and Aluminium, with Zinc being more reactive. The instance of Bimetal Corrosion is small.
- An electrolyte, (Water or Soil) needs to be present for any reaction to occur.
- Aluminium forms a tight oxide layer in the presence of oxygen (air) which tends to insulate the two metals.
- When white rust, zinc oxide, does occur it also forms an insulating layer which inhibits further corrosion.
- In view of the above, risks of accelerated galvanic and bimetal Corrosion between Aluminium and AZ, is considered to be minimal.
Bruce Saxby
Metallurgical Consultant.
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