The different varieties of iron and steel will not oxidise in dry air, or when wholly immersed in fresh water free from air, but they all rust when exposed to the action of water or moisture and air alternately.
"Very thin iron oxidises more rapidly than thick iron, owing to the scales of rust on the former being thrown off as soon as formed in consequence of the expansion and contraction from alterations of temperature.
"The comparative liability to oxidation of iron and steel in moist air, according to Mr. Mallet, is - 2
Cast iron .....
Wrought iron .....
Cast Iron does not rust rapidly in air. When immersed in salt water, however, it is gradually softened, made porous, and converted into a sort of plumbago.3
Mr. Mallet found that the rate of corrosion decreased with the thickness of the casting, being from 1/10 to 4/10 inch during a century in depth for castings 1 inch thick. Mr. D. Stevenson found the decay to be more rapid than this.
Wrought Iron oxidises in moist air more rapidly than cast iron.
The evidence as to its rate of corrosion in salt water is rather contradictory.
Mr. Rennie found that it corroded less quickly than cast iron, but Mr. Mallet's experiments showed that it corroded more quickly.
Recent experiments show that steel immersed in salt water is at first corroded more quickly than wrought iron, but that its subsequent corrosion is slower, and the total corrosion after a long period of immersion is less than that of wrought iron.
Galvanising consists in covering the iron with a thin coating of zinc.
The iron is cleaned by being steeped for some eight hours in water containing about 1 per cent of sulphuric acid, then scoured with sand, washed, and placed in clean water.
After this the iron is heated, immersed in chloride of zinc to act as a flux, and then plunged into molten zinc, the surface of which is protected by a layer of sal ammoniac.
The process differs slightly according to the size and shape of the article. It is a simple one, and may be applied to small articles in any workshop.
Mr. Kirkaldy found that galvanising does not injure iron in any way.
The zinc protects the iron from oxidation so long as the coating is entire; but if the sheet iron be bad, or cracked, or if the zinc coating be so damaged that the iron is exposed, a certain action is set up in moist air which ends in the destruction of the sheet.
"The sheets are generally galvanised before they are corrugated; but as in process of corrugation the sheets, especially the thicker ones, sometimes crack slightly on the surface (unless the iron is of the very highest quality), it is an advantage with all sheets thicker than 20 gauge (see p. 355) to galvanise after corrugation, so as to fill up with zinc any cracks that may have occurred. As, moreover, a larger quantity of zinc adheres to the corrugated than to the flat sheets, they have, when so coated, a distinctly higher value."1
1 Proceedings Inst. Civ. Eng. vol. xxvii. 2 Hurst.
3 A form of carbon known as graphite or blacklead.
4 Mr. Mallet in Proceedings Inst. Civ. Eng. vol. ii.
In order that the protection by painting may continue, the surface should be carefully examined from time to time, so that all rust may be removed. The paint may be renewed directly it is necessary (see Part IL, p. 284).
The following hints on the subject are condensed chiefly from the eminently practical book entitled Works in Iron, by Mr. Matheson.
Cast Iron should be painted soon after it leaves the mould, before it has time to rust. The object of this is to preserve intact the hard skin which is formed upon the surface of the metal by the fusing of the sand in which it is cast.
After this a second coat should be applied, and this should be renewed from time to time as required.
In any case, all rust upon the surface of castings should be carefully removed before the paint is applied.
Small castings are often japanned (see p. 435).
Before painting wrought iron care must be taken to remove the hard skin of oxide formed upon the surface of the iron during the process of rolling, and which, by the formation of an almost imperceptible rust, becomes partly loose and detached from the iron itself.
The scale is sometimes got rid of by "pickling," the iron being first dipped in dilute iron to remove the scale, and then washed in pure water.
"But somewhat the same result may be obtained by allowing the iron work to rust, and then scraping off the scale preparatory to painting. If some rust remains upon the iron the paint should not be applied lightly to it, but by means of a hard brush should be mixed with the rust."
The precautions to be taken in using these paints, and the objections to ordinary lead paints, are given in chapter VI (Fireproof Floors).
The ironwork for roofs, bridges, and similar structures, generally receives one coat of paint before it leaves the shops, and two or three more after it is fixed.
Dr. Angus Smith's process is an admirable means for preventing corrosion in cast-iron pipes.
The pipes having been thoroughly cleaned from mould, sand, and rust, are heated to about 700° Fahr. They are then dipped vertically into a mixture consisting of coal-tar, pitch, about 5 or 6 per cent of linseed oil; and sometimes a little resin, heated to about 300° Fahr.
After remaining in the mixture several minutes, long enough to acquire the temperature of 300°, the pipes are gradually withdrawn and allowed to cool in a vertical position.
Perfect cohesion should take place between the coating and the pipe, and the former should be free from blisters of any kind.
In practice the heating of the pipes before immersion is found to be very expensive, and is frequently omitted. However, many engineers consider it essential for really good work.
The Bower-Barff processes 2 protect the surfaces of iron and steel by covering them with a coating of black magnetic oxide.
In the original process, invented by Professor Barff, this was effected by subjecting the articles to be coated in a heated muffle to the action of superheated steam. The heated metal decomposes the steam and combines with some of its oxygen to form the coating of magnetic oxide.
A similar effect is produced by Mr. Bower's patent, under which the gas from a producer is burnt with a slight excess of air, and taken into a brick chamber, in which the articles to be coated are placed, a red coating of sesquioxide is produced soon after the articles are red, but after about 40 minutes the air is shut off, and the producer gases only admitted, when, in 20 minutes more, the sesquioxide is converted into magnetic oxide.
1 Matheson. 2 Proceedings Soc. Engineers, 1884, p. 59, Mr. Bower's paper.
"This alternate treatment goes on for different periods, depending upon the nature of the articles and the purpose for which they are required.
"For indoor work 4 hours are sufficient, but the time varies from 4 to 8 hours, or about half that necessary for coating by the aid of steam."1
Both processes are now worked by Mr. Bower, Prof. Barff's process being better for wrought iron, and that of Mr. Bower, which is much cheaper, for cast iron.