The aluminium oxide layer produced by anodising can mitigate the effects of bimetallic corrosion.
In a nutshell:
• Bimetallic corrosion occurs when two dissimilar metals are in contact with an electrolyte, such as water.
• The more reactive metal transfers ions to the less reactive metal and corrodes as a result.
• Anodisation can mitigate corrosion by providing an insulating oxide layer that is also abrasion-resistant.
• Bimetallic corrosion risks arise under prolonged submersion or exposure to electrolytes.
• Environmental conditions like seawater increase corrosion risk, whereas acidic rain is less of a concern.
What is bimetallic corrosion?
Bimetallic corrosion occurs when two dissimilar metals are in contact in the presence of an electrolyte like water. The electrolyte encourages the transfer of ions from one metal to another, creating a galvanic reaction and causing corrosion. The more reactive metal forfeits the ions and corrodes whereas the less reactive metal that receives the ions is protected.
This process can occur within certain architectural systems such as façades where aluminium panels are in contact with stainless steel fixings in the presence of an eletrolyte. Aluminium is higher in the galvanic series than stainless steel so the aluminium would act as the sacrificial anode, corroding preferentially to protect the stainless steel fixings. This would result in early depreciation of a façade leading to costly repairs.
Effect of anodising on bimetallic corrosion
One way this can be adequately mitigated is through anodisation, which is the electrolytic process that thickens the naturally occurring oxide layer on aluminium. The oxide layer acts as a thin insulator which prevents contact with the electrolyte, so in some situations, it can help mitigate bimetallic corrosion. The anodic surface is also abrasion-resistant although it is quite thin and therefore isn’t wholly immune to breakage.
Likelihood of bimetallic corrosion
However, for bimetallic corrosion to pose a serious risk to architectural systems they would have to be submerged in or exposed to the electrolyte for long periods of time. Acidic rain for example wouldn’t facilitate bimetallic corrosion whereas the electrical connection of stainless steel to aluminium in seawater is much greater which is something to bear in mind system in sub-marine or coastal conditions.