I left my saw hanging up in my damp shed. Much later I found it was covered in these seemingly random lines of rust (see photo, right). Why did they form like that?
鈥� The picture of the saw shows relatively large areas of bright metal. This suggests that the tool was originally coated with a protective lacquer, probably cellulose acetate butyrate. As lacquers age they loose plasticity and become brittle. The natural expansion and contraction of the blade with changes in temperature would split the coating, forming random fine cracks and exposing the metal surface to corrosion.
Allan Whatling, St Mawgan, Cornwall, UK
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鈥� The tool pictured has undergone what is known as , a process which can affect metals that have been given a thin protective coating. The phenomenon has been the subject of extensive study since as long ago as the 1940s. Thread-like filaments of corrosion up to half a millimetre wide grow in the coating at a constant rate of about 0.4 mm per day in random directions. They do not cross each other, and if one filament approaches another it will either stop dead or veer off at an angle.
The process occurs thanks to a differential aeration cell 鈥� which illustrates a neat little example of electrochemistry in action. In the head of each filament is a tiny pocket containing a concentrated solution of ferrous salts which can draw water from the atmosphere. Oxygen also diffuses through the coating and an electrochemical cell is set up. Iron oxidises under the head of the filament and a trail of rust forms behind the head.
Because of the nature of the reactions, the filament propagates with the head acting as a moving anode and the tail as a cathode. The potential at the head of a filament would be neutralised were it to encounter the tail of a different filament, so the only way it can propagate is to change direction.
鈥淭he only way it can propagate is to change direction鈥�
Filiform corrosion can be minimised by applying a phosphate or chromate primer to the surface of the saw, although these are now being replaced by safer organic alternatives at an industrial level. But even the lowly shed owner can take proactive steps: wipe your tools down with an oily rag every now and again and you will stop the corrosion in its tracks.
I infer from the photo that the shed must have had a relative humidity of between 65 and 95 per cent. Any lower and the corrosion would not have happened; any higher and you would probably just see rust blisters instead.
Peter Barnes, Penarth, South Glamorgan, UK