The rusting of metals

It has long been known that metals can pass from a metallic to an earthy form. One must be careful not to expose unprotected iron to the open air, as under the influence of atmospheric conditions the outer layer of the iron quickly changes into a reddish crust called the rust. One can easily find that this new material differs from iron. The rust is very brittle, it is much lighter than iron and it is not attracted by a magnet. It was early observed that, although the specific gravity of the earthy product was less than that of the metal, yet there was a much greater bulk of it, and that, in fact, the earthy material weighed more than the original specimen of the metal.

It was Lomonosov who first proved that the extra material came from the air. He placed some tin in a flask and sealed up the mouth of the vessel. The tin was heated and converted into the white powder. Upon weighing the apparatus it was found that no change in weight occurred. It turned out, however, that when the mouth of the flask was opened, a good deal of air rushed in and the total weight was then greater. A portion of the original air had joined itself with the tin to form the powder.

Eighteen years later the same experiment was made and the same conclusion was drawn from it by Lavoisier. He named the gas taken from the air, oxygen. It is possible to confirm these conclusions in various ways. For example, when the air is pumped out of the flask before it is sealed, the metal can be heated in the vacuum indefinitely without rusting. Thus it can be seen that fundamentally rusting is the process in which metals oxidize and form compounds. It has been estimated that more than 25 % of the annual production of iron in the USA is consumed in replacing other iron that has been rendered unserviceable by rusting.

It is necessary, therefore, to wage a constant battle against this chemical process and our immediate concern is how to take care of metallic devices and structures once they enter our possession. We try to prevent or reduce rusting by various means, such as electroplating, painting or otherwise treating the metals that are susceptible to attack of this harmful action.

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