WORLD DECAY

Wanganui Chronicle, Volume 79, Issue 306, 31 December 1935, Page 8

WORLD DECAY

INFLUENCE OF AIR AND FUMES BATTLE AGAINST THEM The slow but persistent .destructive effect which air. and moisture, smoke, fumes, and chemical solutions exercise on all materials ifc well known (says a writer in the Melbourne Age), inis corrosion has its useful aspects; it Helps to convert rock into soil and to get rid of some of our rubbish; but generally we combat it, as when it tends to destroy our siiuctiiral materials, or interferes with our aesthetic ideas. Efforts to prevent such corrosion, or at least to slow down its effects, are age-old, and naturally have iu the past Deen directed mainly to the preservation of iron, but latterly the scope of such efforts has been extended, partly because so many new materials, such as, for exampie, aluminium, are being used, and also because ot the increasing costs and extensive importance of many structures.

One naturally associates the use ol protective measures with some display of aesthetic consideration, and it is quite likely Xhat the use of paint for ornamentation preceded its use as a protective agent. Anto-coerosion problems fall roughly into two broad classes. There are those rather specialised and technical problems which the chemical engineer has to meet, and then those one to everyday attack by the atmosphere, the type which annoy the householder. This latter class only will be considered here. Oldest Method. The oldest and most universal ►method of protection is, of course, the use of paints, which aim at covering the sunace of the material in question with a layer which shall be as impervious as possible to air and moisture. All paints contain, besides possibly colouring matter, a liquid which is organic in nature —that is, contains compounds of carbon, for example, linseed oil, varnish, rubber solution, duco bases, coal tar or its constituents. Paints have the advantage of being simple to use, and are easily renewable. On the other hand, they are, too, easily removable by mechanical means, lose their nature at higher temperatures, and like all complex organic matter, undergo slow deterioration by continued exposure to light and air. In other words, they are lacking in one important desideratum, permanency. To remedy this defect one must avoid the use of organic materials, and must substitute something wholly inorganic—that is, mineral in nature. Ju practice the choice is limited to substances of the nature of glass, glass itself or enamel, or a metal itself which will not corrode. Enamel is au old friend and may be dismissed with a mere mention; the use of metal protectors for iron, as being one of the most easily corrodible and widely used metals, may be discussed here. It was obviously a matter of early observation that certain metals do not, under ordinary conditions, rust as appreciably as does iron, and so naturally these metals were used when possible instead of iron—lead being used for roofs and the lining of water cisterns, copper kettles used instead of iron. Mechanical and economic considerations, however, limited the extent to which such metals could replace iron; lead would be useless, and nickel too expensive for bridge building, and so one pasesd to the obvious expedient of coating iron with a rustresisting metal. “Galvanising” Iron. It is not always easy to get any metal to adhere firmly to iron, but a favourable case is that of tin. The modern process probably arose from observations made in the tin smelting industry; the crude tin, for purification purposes, was melted iu iron kettles. The industry can be traced back, to England, to Norman times? and in Henry \ Ill’s time —.1509-1547 —was sufficiently important to warrant the prohibition of the importation of tinware from abroad, Saxony being a serious rival. Incidentally, the industry flourished iu the west of England partly because of the propinquity of the iron works of the Forest of Dean to the tin mines of Cornwall.

Zinc, commercially speaking, is a much younger metal than tin—the art of extracting it from its ores was evolved much later; hence we find that it was as recently as 1837 that a patent was granted for “galvanising” iron by dipping it into molten zinc. The advent of cheap current electricity, about the middle of last century, made possible the coating of metals with gold, silver, or copper, but this electro-deposition had a restricted, although useful field; hence, until recently, we have had to rely hiainly on the use of tin and zinc for ordinary big-scale work. Latterly, however, the list of available metals has been extended considerably to include nickel, cobalt, lead, cadmium, chromium, tantalum, ahiminiums, and alloys such as brass. The advance which has been made is simply a chapter in the general advance of applied chemistry. The chemical engineer has had to solve some very specialised problems in corrosion, and the success of his inquiries has been passed on to the general public. The problems met with concerned not only the question of au economic supply of the particular metal, but also a thoroughly reliable i ec.unque. Four Methods. In general, four methods may be used—hot dipping, electro-deposition, metal spraying, and burying the iron in, for example, aluminium powder, and heating. Metal spraying is especially useful, since a completed structure can be covered in situ, and all joints covered over. Each method has its own particular advantages and limitations, depending on the metal to be used and the type of deposit required. Electro-deposition can be made to give a brilliiint mirror-like deposit, while a sprayed surface is generally matt. The method by which thin layers of metal protect iron might at first sight be ascribed to the possibility of these metals being less liable to attack by atmospheric oxygen than is iron. This is not necessarily so; aluminium unites more vigorously with, oxygen than does iron. The apparent anomaly is explained by noting that iron rust does not in all cases adhere firmly to the iron; air gains access to the underlying layers, and rusting continues. But with aluminium the reverse holds, the film adheres tenaciously and further attack is prevented. It is hardly an

exaggeration to say that every metal corrodes to some extent, but the nature of the initial oxide film and its degree of adhesion determine the actual extent of the corrosion. As far as visual estimation is concerned, the transparency of the film is important, thus the brilliancy of chromium plating is dependent on the transparency of a very thin film; the reverse holds in the case of lead. All of the above-cited protective metals are more expensive than iron; tin, for example being about thirty times the price of steel, so, for economic reasons, a very thin layer has to suffice. In the. case of tin plates the thickness of the layer is of the order of one ten-thousandth of an inch, or in other words, a ton of tin can be made to cover about fifteen acres of sheet steel..