Stories About Chemistry
52. …And How to Fight It
There is a remarkable column in Delhi which has stood there for many centuries. It is remarkable because it is made of pure iron. Time has no effect on it. Ages have passed, but the column still looks quite new. It does not rust. As if corrosion had in this case betrayed its habits. . .
How the ancient metallurgists succeeded. in producing pure iron is quite a puzzle. Some hotheads have contended that it was not even made by man. That visitors from other worlds had put up this obelisk to commemorate their arrival on Earth.
But if we deprive the column of the mysterious aureole of its origin there remains a fact which is very important to chemists, namely, that the purer a metal is, the slower it is attacked by corrosion. If you want to conquer corrosion, use the purest metals possible.
And not only purity is important; it is also necessary to give the surface of the metallic article as high a finish as possible. It appears that separate “hills” and “valleys” can play the part of foreign inclusions. Scientists and engineers have succeeded in obtaining almost ideally smooth surfaces. Articles with such surfaces have already found usage in the construction of rockets and spaceships.
And so is the problem of corrosion prevention solved? By no means. Very pure metals are expensive and difficult to obtain, especially in large quantities. Besides, engineering prefers alloys, because their range of properties is much wider. And an alloy is at least two metals.
Chemists have studied all kinds of corrosion mechanisms in sufficient detail. And when they intend to obtain a new alloy with predetermined properties, they thoroughly consider the “corrosion” aspect among others. There are at present I many alloys which resist corrosion very well.
In our everyday life we often have to do with galvanized and tin-plated articles. Iron is coated with a film of zinc or tin to protect it from rust, which helps for some length of time. Besides, we have all seen iron house roofs coated with a dense layer of oil paint.
To weaken or decrease corrosion also means to decrease in some way the velocity of the electrochemical reaction constituting the corrosion process. Special organic and inorganic substances called inhibitors are used for this purpose.
At first they were sought by trial-and-error methods and were found by accident.
Even before Peter the Great’s time Russian gunsmiths used a curious method. To remove scale from gun barrels they would wash (pickle) them with sulphuric acid mixed with wheat husks. In this primitive manner they kept the acid from dissolving the metal.
The search for new inhibitors is now no longer an inspired art, nor a matter of luck, but an exact science. Hundreds of chemical corrosion inhibitors of all kinds are known today.
We must look after the “health” of metals before they are “infected” by corrosion. This is the main task of the “metal-doctor” chemists.