As to the pressure of steam in the boiler, 40 lb. is the extreme; but they are obliged to use considerable force to effect the double object of keeping out atmospheric air and to efficiently oxidise the surfaces treated. The oxidation only proceeds till the pores of the iron are all filled up with black oxide; but with very thin objects, great caution has to be taken, lest they should be oxidised through.
(15) The method of preserving iron by forming an inoxidisable film or coat upon the surface has been tried in France, the process adopted being modifications of those patented by Barff and Bower. According to Krafft, Bourdon encloses the articles to be preserved in a cylinder closed at both ends by riveted plates, into one of which the steam supply pipe enters, while the other is supplied with 3 openings. Into one of these a thermometer is fitted; the second Is supplied with a stopcock through which to allow the water condensed to run off. This must be done frequently, as the steam must be as dry as possible. To the third is fitted an escape-valve for the steam. The most favourable conditions for success are the following: - The pressure must amount to 2 or 2 1/2 atmospheres, the temperature must be from 626o to 644° F. (330° to 340° C), and 5 hours must be allowed for the completion of the operation. A covering of a greenish-black colour is obtained, which adheres firmly and is perfectly stable.
The cylinder is placed in a sort of oven, maintaining its shell at about 930° F. (500° C). The thermometer, plunged in the steam of the interior with its registered part protruding so as to allow observations, showed, however, only 644° F. (340° C). If the current of steam is stopped, the thermometer will almost instantly rise to 930° F. (500° C). The coating is a perfect success; care must, however, be taken that no parts of the articles are soldered together by tin solder, as the latter melts at 442° F. (228° C). Even if the connection remains intact, there will always be a few minute globules of solder detached and stains caused. Copper must be used instead. In further following, up his experiments, Bourdon conceived the idea of replacing the steam by hot air. He proceeded as follows: - A coil of pipe communicating at one end with the open air ascends gradually through a reservoir heated to 248° F. (120° C), whence it enters the cylinder in which the articles to be operated upon are enclosed. This cylinder is identical with that used for steam. The escape-valve leads into a tank containing water, permitting a better regulation of the air current. This must pass very slowly.
The interior pressure is but a little above one atmosphere, as the apparatus communicates with the open air. The temperature of the air in the cylinder is 536° F. (280° C); the time consumed, 5 hours. A layer 1/5in. in thickness is obtained, capable of resisting the action of emery-paper, and unaffected by dilute sulphuric acid. The layer possesses a fine greenish-black colour. To ensure perfect success, the articles must be suspended completely free. After removing them' from the apparatus, they are rubbed with a greasy cloth; stains, if any, are removed with emery-paper or iron-dust. It has been found that with an elevation of temperature under pressure of one atmosphere a very thick layer is obtained, which, however, scales off easily. The adherence is, therefore, a question of temperature and not of pressure, as was formerly supposed. Those pieces coated by hot air were for one month exposed to the weather without being attacked in the least. On removal of the exterior black rind, a grey layer is discovered below the same, which to some extent becomes rusty on exposure. The rust, however, does not adhere as on metallic iron, but is easily removed by scraping with a piece of wood. This fact also applies to articles coated by steam.
Last June, Bourdon tried the process on 400 rifle barrels at once. Similar trials have since been made, showing the practicability of using it on a large scale. The principal point is to obtain a current of air sufficiently abundant to secure a proper thickness of the layer, but of a circulation slow enough to allow the air to act on the iron. The French Government has already adopted the process at some of its arsenal manufactories.
(16) A process of painting, as a substitute for galvanising, has been invented by Neujean and Delaite, of Liege. It is specially intended for objects of large dimensions, which cannot easily be moved, and therefore cannot well be dipped into a bath of melted zinc The zinc, when finely pounded, is simply mixed with oil and siccative. In this way a Tarnish is obtained, which is applied with a brush in the usual manner. A single layer is sufficient, but two are preferable. The coated objects can be left as they are, or bronzed and painted as required.
(17) An anti-corrosion paint for iron. If 10 per cent, of burnt magnesia, or even baryta or strontia, is mixed cold with ordinary linseed oil paint, and then enough mineral oil to envelope the alkaline earth, the free acid of the paint will be neutralised, while the iron will be protected by the permanent alkaline action of the paint. Iron to be buried in damp earth may be painted with a mixture of 100 parts of rosin (colophony), 25 of gutta-percha, and 50 of paraffin, to which 20 of magnesia and some mineral oil have been added. (Neueste Erfind.)
(18) A simple and economical way of tarring sheet iron pipes to keep them from rusting is the following: - The sections as made should be coated with coal tar and then filled with light wood shavings, and the latter set on fire. The effect of this treatment will be to render the iron practically proof against rust for an indefinite period, rendering future painting unnecessary. In proof of this assertion, the writer cites the example of a chimney of sheet iron erected in 1866, and which, through being treated as he describes, is as bright and sound to-day as when erected, though it has never had a brushful of paint applied to it since. It is suggested that by strongly heating the iron after the tar is laid on the outside, the latter is literally burnt into the metal, closing the pores and rendering it rust proof in a far more complete manner than if the tar itself was first made hot and applied to cold iron, according to the usual practice. It is important, of course, that the iron should not be made too hot, or kept hot for too long a time, lest the tar should be burnt off.