Rust only a matter of time
Page 52
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IT'S COMMON knowledge that if iron is in contact with a flow of water and a supply of air it will rust, and as soon as we get any amount of air in the cooling system, we have an ideal situation for rust, or the chemical process of corrosion, as it is called, to take place. For this reason, we explained in the previous article, the admission of air to the cooling system should be avoided. Unfortunately, the turbulent nature of the water flow in the water pump and in the top tank leads to some air being drawn into the system anyway, so some rust seems inevitable — unless a corrosion inhibitor is incorporated.
On top of this, there is electrolytic corrosion, a process which plays strange tricks as it attacks some areas and not others. It is likely to start wherever two different metals such as iron and copper are brought into contact with each other when immersed in water, or are in contact with water. The action is similar to that in a battery in that electric currents flow through the water from one metal to the other, and an effect called electrolysis takes place. The effect of this action is to wear away the metal in one area, and to build up a deposit in another — but the deposit is merely a pile of rust.
Although the flow of current is weak, over a period of time, severe local corrosion can appear, especially since there are a large number of different metals in a cooling system — cast iron, aluminium, brass or copper in the radiator, steel and copper tubes, copper gaskets and so on. Electrolytic corrosion takes place more readily at higher temperatures, unfortunately, with iron, copper and solder all corroding twice as quickly at 80 deg C as at 24 deg C.
In some cases, corrosion of this type is merely a nuisance, and in other cases, it can lead to premature failure. For example, it leads to the corrosion of aluminium castings, such as water outlets, to their retaining studs, which merely makes the job of removing the casting The metal is so thick in the area concerned that the corrosion is unlikely to extend right through the metal. On the other hand, any steel pipe with brazed fittings is likely to corrode through quite quickly, and so whenever the engine is being inspected, these should receive special attention.
Ideally, engines should be filled with distilled or rain water, but this is impractical for most workshops, so tap water is used normally. Water with acid or salt impurities should be avoided, as should water containing large amounts of lime, which will be deposited locally in the cooling system, However, the dilution of the water with anti-freeze, which includes anti-corrosion inhibitors, overcomes most of the minor impurities in tap water. Nevertheless, you need to keep an eye on those areas that are prone to electrolytic corrosion