Galvanic Corrosion

Galvanic Corrosion is a process of when two different metals of equal surface area are combined in the presence of an electrolyte (a fluid that conducts electricity). Hydrogen is absorbed at the cathode and a current flows, thus the less noble metal migrates into the solution (the ‘anode’ is attacked).

This exemplifies the basic mechanism for simple batteries.

The size of the cathode relative to the anode controls the power generated and the corrosion level at the anode.

The three main causes for galvanic corrosion are:

  1. Different corrosion potentials between the two metals
  2. Direct electrical contact between the two metals
  3. An electrolyte solution which connects and is able to break down the two metals such as condensation, rain or fog exposure.

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Galvanic Attack on Roofing & Cladding Materials

Bad design for fastening assemblies

Large cathode absorbs lots of hydrogen and drives corrosion against small anode; cathode is ‘protected’.

‘Sacrifical anode’ effect = fastener at risk.

*Area rule applies

Zinc plated fastner [anode]
Aluminium sheet [cathode]
Galvanic attack

Good design for fastening assemblies

Small cathode attracts small level of hydogen and so has weak effect against large anode.

Cathode (fastner) is ‘protected’ and little corrosion effect on large anode.

*Area rule still applies

Stainless fastner [cathode]
Aluminium sheet [anode]
Good fastner protection

Galvanic Series

The Galvanic Series helps to determine the presence of corrosion by identifying semi-metal and metal nobility. Thus, when two different metals are combined in the presence of an electrolyte, it is the metal with the lesser nobility which will experience corrosion. It is important to note that unless situated in low humidity, adverse effects can occur when incorrect metals are combined and exposed to an electrolyte.

The Galvanic Series provides a beneficial guide for when choosing which metals to combine. Subsequently, appropriate material selection can assure the metals chosen minimalize risk of undergoing a harmful galvanic reaction, or prompt users to reduce possible potential reactions by having relevant protection in place.