Corrosion of Aluminium
The widespread use of aluminium for use in the atmosphere and in natural waters is to a large extent due to its generally excellent corrosion resistance. This is because of the formation of a protective alumina (Al2O3) surface film, which has very high electrical resistivity. However, because aluminium is an amphoteric metal, this oxide film becomes increasingly unstable outside the neutral pH range, between about 4.5 and 8.5. If the film is removed, then high dissolution rates on aluminium can occur. Therefore, in natural waters, aluminium should not normally be used outside of this pH range.
Even when used in neutral waters, however, aluminium can undergo pitting attack, which can lead to wall perforation and failure of the component or system. This usually occurs in waters containing high chlorides (>200mg/L) and is thought due to the chloride ion entering the oxide film at naturally occurring flaws in the film. Pitting attack can also occur if there are any copper ions in the water. Because of their relative positions in the electrochemical series, copper ions will be reduced on aluminium and deposit as metallic copper on the aluminium surface. This takes place at discrete sites, which are the flaws in the alumina film. Since the copper acts as efficient cathodes for oxygen reduction, it causes intense anodic attack on the surrounding aluminium leading to pit initiation.
Protection of Aluminium in Fernox Inhibitors
The use of correct water treatment can protect aluminium from the risk of pitting attack and also direct galvanic corrosion if aluminium is coupled directly to a more noble metal. Inhibitors used in central heating systems should inhibit corrosion of all metals used in the system, including ferrous metals, copper and copper alloys and aluminium.
Fernox inhibitors, such as F-1 Protector use a special amine to protect aluminium. This forms a microscopically thin coating on the aluminium plugging the flaws, which are the sites for pit initiation. The amine itself is alkaline and will protect aluminium itself in alkaline conditions. (Fernox in the past have produced an alkaline cleaner, which because it contained silicates, gave excellent corrosion inhibition of aluminium even at pH 11). However, blending the amine with the acidic form of ferrous metal inhibitors (such as carboxylic acids) brings the pH of the F-1 and Protector formulations back to more neutral pH’s. Furthermore, the pH of the inhibited solution is buffered using borate, so that the circulating water remains at around neural during use. Additions of copper corrosion inhibitors ensure that there is no risk of copper ions entering the circulating water and causing pitting of aluminium.
The present formulations of F-1 and Protector have been used for several years in the field giving excellent all-round protection of central heating systems against corrosion and scale. During this time, there has not been one reported incidence of failure of aluminium heat exchangers or boilers in systems treated with Fernox products.
Summary
- Aluminium gives normally excellent corrosion resistance due to a surface oxide film
- This film can be destroyed in acidic or alkaline conditions
- High chlorides or copper ions in the water cause pitting of aluminium
- Good quality water treatment can prevent this pitting attack and protect aluminium at non-neutral pH’s.
- Fernox inhibitors contain inhibitors to prevent attack on Al and to protect other metals in the system. They are buffered to a neutral or near neutral pH.
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www.fernox.com
