As many metallurgists will know, the bimetal effect, or bimetallic corrosion as it is sometimes referred to, will only happen when two metals of a dissimilar nature come into what is termed electrical contact with one another and they are bridged by liquid that is electrically conductive. Just like a simple battery would operate, this means a cell is produced that can result in corrosion. Frequently, this corrosive effect is noticed in one of the paired metals only. When it does, this can be enough to cause a drop in the structural stability of any aluminium door system. However, due to its exposure to rainwater and other fluids which might help to conduct an electrical charge, an aluminium entrance door system is even more likely to suffer from this technical issue. Although the bimetal effect can be an problem for any aluminium window system or aluminium facade, since this metal tends to be on the electronegative end of the electrode potential array, it is often most acute with doorways. Thankfully, the fenestration industry has multiple solutions to this engineering problem.
The Importance of the Knurling Process
The heightened bimetal effect aluminium doors can suffer from is sometimes caused by an increased surface area within the door frame's profile. Therefore, keeping the internal surface area to a minimum is advisable. Nonetheless, adding grooves and a pattern of angled lines within the extruded aluminium of a typical door frame profile is often done so that polyamide isolation strips can be rolled into them and held securely in place. This knurling process necessarily augments the amount of surface area, thereby adding to the overall cell-like effect. However, leading door manufacturers are able to mitigate for this issue by reducing the cutting depth of their knurling tool. This means that any increase in surface area created by knurling is kept as low as possible. In the car industry, low profile knurling and heated thermoplastic inserts have been developed for much the same purpose as is desired in aluminium door frames.
The Development of the Anti-Bimetal Insulation Profile
A polyamide thermal break should provide all of the insulating properties that modern aluminium door systems require, but the latest materials can provide a good deal more. Although some thermoplastics used in aluminium frames are susceptible to water and will swell up when they are in contact with moisture, others are much more resistant to this problem. Not only does this help with the shear strength of aluminium door frames being retained, but it means that the metal is not so exposed to trapped water. As a result, the inside section of an aluminium door frame should not suffer from the bimetal effect or any other form of corrosion due to water being retained as an undesired consequence of fitting inferior insulation strips.
"Although aluminium is a very reactive metal with a high affinity for oxygen, the metal is highly resistant to most environments... due to the inert character of the aluminium oxide." - ALFED.
Other Techniques Used to Handle Bimetallic Corrosion in an Aluminium Thermal Break Door
According to the Aluminium Federation in the UK, ALFED, so-called uniform attack of exposed metalwork can lead to oxidation caused by aggressive ions, for example chloride ions which might be in the atmosphere. To deal with this problem they suggest the application of cathodic protection, such as sacrificial zinc anodes, or covering the metal with a protective layer, for example powder coating. However, when it comes to bimetallic corrosion, ALFED recommend isolating the two metals surfaces from one another with a material such as neoprene. Some aluminium facade manufacturers favour this method, but others tend to design thicker sectional profiles for their framework instead. In extremely poor weather larger aluminium profiles are less susceptible to the bimetallic effect and deformation of them is consequently less likely. In some ocean-facing environments - a particularly difficult place to handle the bimetallic effect due to the greater conductivity of seawater - plastic insulating washers have been successfully placed between aluminium window frames and other metals, such as marine grade stainless steel, too.