KEY POINTS

  • Avoid contact between dissimilar metals
  • Avoid a difference of more than a 0.015 anodic index value
  • Use isolators to avoid electrical contact between metals

What is galvanic corrosion?

Galvanic (or bi-Metallic) corrosion is the preferential corrosion that occurs when dissimilar metals are in contact in the presence of an electrolyte. The effects are like conventional corrosions of a single metal but generally proceeds at a higher rate depending on the difference in electrochemical reactivity of the anode and cathode metal.

The intensity of galvanic corrosion is affected by the electrolyte pH and conductivity and occurs when there is:

  • An electrolyte bridging two metals
  • Electrical contact between two metals
  • A difference in potential between the metals to enable significant galvanic current
  • Sustained cathodic reaction on the more noble of the two metals

How can galvanic corrosion be prevented?

One way is to electrically insulate the two metals from each other (e.g. using plastic insulators). Unless they are in electrical contact, there can be no galvanic corrosion.

Another way is to keep both metals dry (e.g. powder coating), or coating the more noble (the material with higher potential).

When contacting metal’s potentials are closely matched, the galvanic current is less, reducing the corrosion. Using the same metal throughout is best, but externally avoid more than .015 V difference in the “Anodic Index”.

Sapphire use isolations between galvanised arms and the aluminium frame of the Glide-On Cassette balcony structure.

Reference should be made to the Zinc Millennium Map which categorises corrosion into 5 groups according to their geographic location within the United Kingdom.