If the latter has begun, it promotes its own further formation, as rust, like other porous bodies, absorbs gases and therefore takes up moisture and acids from the air. Besides, where rusting has already begun, the change from the first-formed ferrous compound to ferric hydrate is attended by a setting free of the active acid, which is then in a condition to act powerfully in the formation of fresh rust. Rust already formed must therefore be quickly removed, in order that a new layer shall not be produced. Rusting, being promoted by acids existing in the air, is also accelerated by those present in water, and for this reason iron is destroyed more quickly in marshes and bogs than in lakes or considerable currents of water, which are generally comparatively free from acids. The tendency of iron to rust is also increased by some salts dissolved in water. Thus it is explained why pieces of cast iron can, by long immersion in sea water, be changed to loose masses, retaining the same outward form, but consisting essentially of carbon. Iron which has been metamorphosed in this manner contains more carbon in proportion to the completeness with which the iron itself has been dissolved.
The mass, when taken out of the water, possesses a low specific gravity, and such great porosity that a condensation of air and simultaneous rise of temperature take place, sometimes sufficient to cause the spontaneous ignition of the whole. If a substance negative to iron, such as scale, tin, etc, partially covers its surface, the portions coated are of course protected, but the uncovered portions are only so much the more liable to rust: therefore, before coating pieces of iron with oil-paint one should free them from all scale by the action of dilute acid. If, as appears to be the case, contact with impurities renders iron positively electrical, their existence in its interior must also promote rusting. Thus, forged iron appears to rust first along the bands of impurity occurring in it. A partial coating of a metal positive to iron, such as zinc, not only protects the covered portions of the iron, but also hinders the rusting of the unprotected parts, the more completely, indeed, the smaller they are. A coating of fat also protects iron, for some time; but when the fat, by absorption of oxygen from the air, has become rancid and in part changed into fatty acids, the tendency of the iron to rust is increased.
From the part which galvanic influences take in the rusting of iron, it follows that those substances positive to iron, which, when in mere contact with it, prevent it from rusting, promote the same if they are alloyed with the iron, because such alloys are in general more positive than iron itself. Thus, manganese alloyed with iron promotes the tendency of the latter to rust. So long, however, as the quantity of manganese is uniform and not too large, its effect in this direction is inconsiderable. If, on the other hand, the quantity is uniformly distributed, the rusting of the portions of iron richer in manganese, and therefore more positively electrical, must be greatly promoted by contact with those parts poorer in manganese; hence the presence of unequally distributed quantities of manganese appears to largely augment the tendency of iron to rust. Association with electro-negative substances, such as carbon and phosphorus, diminishes the tendency of iron to rust, if the quantities of the electro-negative substances be evenly distributed through its whole mass. But iron poorer in metalloids in contact with iron richer in these .bodies, becomes more positively electrical, and thus the rusting of the poorer portions proceeds more rapidly.
Sulphur is an exception among the metalloids, inasmuch as it promotes rusting. Forged iron rusts more easily. With an increase in the amounts of carbon, silicon, and phosphorus present in iron, the tendency to rust diminishes, so that cast iron is the more capable of resistance to rusting, accordingly as it contains mote combined carbon, silicon and phosphorus, and becomes denser. Grey cast iron is, as is well known, poorer in combined carbon and less dense than white. Both characters induce a greater tendency to rust. Perhaps the mechanically-contained graphite also contributes'to this, as galvanic action may arise from its contact with the iron. In spite of its lower density, and the contained graphite, grey cast iron withstands rusting better than steel, although the amount of combined carbon in the latter is probably at least as large as in the former. This is probably accounted for by the greater freedom of steel from silicon and phosphorus. The circumstance that grey iron smelted with coke is less soluble than that obtained from charcoal may be similarly explained, viz., by its greater proportion of silicon, and probably also, phosphorus. Spiegeleisen withstands rusting better than granular white cast iron, by reason of its greater density and larger amount of carbon.
Elaborate experiments in connection with this subject were made by Parker, whose results are for the most part in accordance with the foregoing statements. (Akerman.)
(1) Galvanising consists in covering the iron with a thin coating of zinc. The iron is cleaned by being steeped for some 8 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. 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.