PRESERVATION OF IRON.
BY PROF. HENRY E. COLTON.
"What is wanted is something equally applicable to large or small pieces of iron, and which will answer to ward off the attacks not only of the common atmospheric oxygen, but also remain unaffected by acids or salt waters."
The above from a late number of the SCIENTIFIC AMERICAN states not only the writer's ideas but also one of the greatest wants of the age. Iron is daily being put to more and more varied uses. On land the great question is what will prevent rust; on water, what will prevent rust and fouling of bottoms of iron vessels. We will briefly summarize the many patents granted for this purpose.
Eight are for sheathing of various kinds put on in varied modes. The most practicable of these is a system prepared by Daft. Most iron vessels are now constructed by every other plate lapping the edges of the one between. He proposes, instead of having the plates all the same width, to have one wide and one very narrow plate. This would leave a trough between the two wide plates of the depth of the thickness of the plates. He proposes to force into this trough very tightly pieces of teak, and to the teak, thus embedded, he nails a sheathing of zinc. The zinc is kept clean by slowly wearing away of its surface from action by contact with the iron and salt water.
There are four patents, in which various, so-called, non-conducting coats are put on the iron, and copper pigment in some form put on over them. These have been specially condemned in England, as no matter how good the non-conducting substance--and many are so only in name--it will become rubbed off at some points, and there the bottom will be eaten both by salt water and action of copper.
Coal tar and asphaltum are the subjects of patents in various forms.
One patent claims rubber or gutta-percha dissolved in linseed oil as a vehicle in which to grind the pigment; another the same dissolved in naphtha or bisulphide of carbon as a pigment; another hard rubber, ground.
Enameling with different materials is proposed by some, while one proposes to glaze the bottoms so that barnacles and grass would find a slippery foothold.
Combinations of tallow, resin, and tar--mineral and pine--are patented mostly to use over other paints.
Coal tar, sulphur, lime, and tallow, are the subject of one patent; guano, red lead, and oil of another; while sulphur and silica are claimed by a third.
Paints containing mercury, arsenic, and even strychnine, are the subjects of several patents. A mixture of coal tar and mercurial ointment of one.
Galvanism is proposed to be used in various ways--strips of copper and zinc, or by galvanizing the plates before use. Black lead finds a place in many compositions.
One patent, by a complicated process, effects a union of metallic zinc and iron; this, granulated and ground fine, then mixed with red lead and oil, makes the paint. It is said to be the best of all the patented stuffs.
It is astonishing how many use oils derived from coal, peat, or resin, and tars of the same.
There are about fifty patents for this object and with all of them before their eyes, the British Society for the Advancement of Art still hold the $5,000 reward for a pigment or covering which will perfectly protect from rust and fouling. However they may puff their products for selling, no one has the temerity to claim that they deserve the reward.
We think it would be difficult to find so many expedients ever before adopted for the accomplishment of any one object. These are all English patents, England having necessarily been obliged to use iron for vessels from its cheapness as well as its consequent first introduction there. In the United States no patents worth mentioning have been granted.
The first requisites for a pigment or coating for iron are, that it should not contain any copper--the corrosive action of that metal on iron being intense. Then if for work exposed to air it should form such a coating as to be impervious to that gaseous fluid, and be so constituted chemically as not to be oxidizable by it; if under water--especially sea water--to be impermeable to moisture, so elastic as not to crack, so insoluble as not to chloridize; to form a perfect, apparently hard, coating: and yet wear just enough to keep off incrustation, barnacles, or growth of grass. In fact, this slow wearing away is the only preventive of fouling in iron vessels. Wooden bottoms may be poisoned by solutions of copper--and that metal has no superior for such uses, especially when it is combined in mixture with mineral or resinous tars and spirits--these compounds, however, are not only useless on iron bottoms, but also injurious. What then is the substance: 1st. One of the oxides of lead (red lead). 2d. The purest oxide of iron to be found. If properly made these articles can be carried to no higher state of oxidation, and respectively, as to order named, they have no superiors for body and durability. By preference, 1st, red lead, either out of or under water; 2d, Prince's oxide of iron only, out of water. The color of these paints--the first red, the latter brown, may be hidden by a coat of white or tinted color. If there were to be had in combination as a white paint, an oxide of lead and an oxide of zinc, it would be immensely superior to either, but that such has not been produced is rather the fault of carelessness than of possibility. Zinc protects iron with great effect, but it is too rapidly worn in the effort to be of lasting value. Hence the great desideratum, the yet to be, the coming pigment is a white oxide of lead or a combined white oxide of lead and white oxide of zinc, without sulphates or chlorides.
Those materials answer very well for work exposed to atmospheric air, and perhaps nothing will ever be found better; but a different need is that for salt water. No mere protector of the iron from rust can be found superior to pure red lead and linseed oil. We have seen a natural combination of zinc, lead, and iron, which, in our experience, ranks next; but the zinc is acted on by the chloride of sodium, and wears away too much of the material. Red lead, however, while covering the iron perfectly and effectually preventing rust, and also having but little disposition to chloridize, when it does, will foul both with grass and barnacles. Hence, the first desideratum being obtained, how shall we accomplish the other. The prevention of fouling may be accomplished in two ways: First, cover the vessel's bottom with two or even three coats of red lead, and give each time to dry hard. Then melt in an iron pot a mixture of two parts beeswax, two parts tallow, and one part pine resin; mix thoroughly, and apply hot one or two coats. This mixture may be tinted with vermilion or chrome green. It is not necessary to use any poisonous substance, as it is only by its softness and gradual wear that it is kept clean. Second, mix red lead and granular metallic zinc, ground fine, or such a mineral as we have mentioned--crystalline and granular in its character. Put on two or three coats, and allow each to set--they will never dry hard. The zinc will slowly wear off, keeping the whole surface clean, while there will be left enough coating of the lead to preserve the iron from rust. The oil I would urge for these pigments is linseed--as little boiled as possible, to be thinned with spirits of turpentine. There seems to have been a mania for mixtures of tar and resins, their spirits and oils; my experience fails to show me any advantage for them on an iron bottom. They have neither elasticity nor durability, while linseed oil has both in a pre-eminent degree, and is no more likely to foul than they, when in a combination that does not dry hard. Besides they are difficult to grind, inconvenient to transport, and offensive to use.
Perhaps we have not, in the opinion of some, answered the want expressed in the first paragraph. No pigment with the requisites of durability and cheapness will resist the attacks of strong acids on iron. The first we have mentioned will--all such as may float in our air from factories or chemical works. Chemically it is converted by nitric acid and chlorine into an insoluble substance--plumbic acid or the cyanide of lead. An experience of more than three years, with almost unlimited means at our command for experiment, demonstrates to us that we have indicated the means of filling the other requisites asked for. It may be that something new will be discovered, but we doubt it. Let any one tread the road we have trod, investigate and experiment where and as much as we have, and, if that place is, where we have not, and their experience will be the same as ours.