Bimetallic (galvanic) corrosion is an electrochemical process that occurs when dissimilar metals are in contact in the presence of an electrolyte. This process is similar to the conventional corrosion of a single uncoupled metal but it generally proceeds at a higher rate depending on the differences in electrochemical reactivity of the different types of metal (anodes and cathodes).
What Impact Does Galvanic Corrosion Have on Construction Projects?
Galvanic corrosion can have significant negative impacts on construction projects, including:
- causing structural weakness and safety hazards
- increasing maintenance costs
- shortening the lifespans of materials
- damaging the aesthetic appeal (through staining, discolouration, material degradation).
What Methods Can Reduce This Type of Corrosion?
Where contact between dissimilar metals cannot be avoided, consider the following steps:
- Select metals that are close together in the galvanic series for the relevant environment.
- Avoid relatively small areas of the less noble metal and large areas of the more noble metal in electrical contact with each other.
- Insulate the metals from each other.
- Exclude electrolytes from the area around the bimetallic junction, for example by painting to exclude water ingress.
- Paint both metals where possible; if that’s not practical, paint the most noble metal.
- Provide additional corrosion allowance on the less noble metal.
- Apply compatible metal or sacrificial metal coatings.
What Steps Can Be Taken to Prevent Galvanic Corrosion?
- Electrically insulate the two metals from each other – if they’re not in electrical contact, there can be no galvanic couple set-up. Electrical insulation can be done using plastic or another insulator to separate steel water pipes from copper-based fittings or by using a coat of grease to separate aluminium and steel parts.
At MyDek, we recommend specifying an isolation barrier between the cleat and the steel. We offer Neoprene 110X110 and non-combustible pads for this purpose.
- Keep the metals dry, for example by painting or encasing the protected metals in plastic or epoxy and allowing them to dry.
- Coat the two materials. If it’s not possible to coat them both, apply the coating to the material with the higher potential, that is, the most noble metal.
- Remember that the more closely matched are the individual potentials, the lesser will be the potential difference and hence the galvanic current. Use the same metal for all construction, if possible; it’s the most precise way of matching potentials.
- Finally, connect an electoral power supply to oppose the corrosive galvanic current. For example, in a system composed of 316 SS (a 300 series stainless steel, i.e., a very noble alloy that is quite resistant to corrosion and has a high potential) and a mild steel (a very active metal with lower potential), the mild steel will corrode in the presence of any electrolyte such as salt water. If a sacrificial anode is used (such as a zinc alloy, an aluminium alloy, or magnesium), these anodes will corrode instead, protecting the other metals.
As ever, the MyDek team is on hand to help you with technical solutions for your project. You can check out our product range here or contact our team; we will be delighted to support you.
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