STEEL, THE PRESERVATION OF STRUCTURAL. Introduction.— The treat ment necessary to secure the preservation of the steel used in buildings, bridges and other engineering structures is of quite as much im portance as the original construction if the structure is to stand the test of time. The durability of a steel structure will depend upon the quality of the metal, upon its mass and upon the measures taken to prevent its de terioration ; and its strength, measured either by the stress it will bear or by its resistance to corrosion, is only the strength of its weakest member. Consequently the surface inspection of steel is one pf the most important functions of the inspector, and will remain so until the steel manufacturer succeeds in producing a non-corrodible surface upon his product.
Rust.—More through ignorance of the chem ical action involved than for any other reason, the appearance of rust upon iron is regarded generally as merely the effect of corrosion, rather than as a contributing and continuing cause of further deterioration. Rust on iron or steel is formed by electrolytic action in which the iron is attacked by hydrogen ions obtained from water, the oxygen set free unit ing with the continually forming iron hydrate. It is a loose, porous mass, containing about 20 per cent of combined water, with varying amounts of hygroscopic moisture, and it per mits the passage of water and gases through itself to the metal underneath, while facilitating the passage of the destructive electric current. By keeping the agencies of corrosion al ways in contact with the surface of the metal not yet affected, this characteristic of rust to absorb and to persistently retain moisture makes possible the progressive corrosion which goes on beneath it, and furnishes a reason for the more or less complete corrosion of rivers and screws frequently noted in steel structures. In the light of the now widely accepted hydrolytic theory of corrosion this moisture-holding prop erty of rust becomes of the greatest importance, for except in the presence of moisture electro lytic action is impossible.
Rust has a tendency to grow and spread out from a centre. Each such centre is a spot at
which water has been deposited in the form of drops, or, if the metal has been completely covered with water, it may be the spot from which the water has last evaporated, and, there fore, where the liquid drop remained the longest time. uSweat," the condensation of the mois ture permeating the air, is always present on a surface of steel or iron, when its temperature is lower than that of the surrounding atmos phere. A wash of rain is less apt to be In jurious to steel than the deposition of water in drops or in a thin film over the surface (dew).
Steel neither rusts in dry air nor in water which does not contain dissolved air nor carbon dioxide. The best protection will, therefore, be obtained from the most impervious coating. It may be noted that the general cause for the failure of preservative coatings to protect steel from rust is that they are permeable to moist air and carbon dioxide, which are the most common agencies for the corrosion of the metal itself.
The contributing causes of corrosion may be classified thus: (1) lack of purity or homo geneity in the metal ; (2) two or more diverse varieties of iron or steel in electrical contact: (3) exposure to hydrogen ions from moisture or any other source in the immediate presence of active oxygen ; (4) a chemical or electro lytic condition in the preservative coatings em ployed; (5) mechanical injury to the treated metallic surface; (6) stray electric currents using the metal as a conductor.
The preservative processes in general use may be grouped as follows : (1) development of a resistant surface of magnetic oxide on the metal; (2) plating with other metals, as in gal vanizing and tinning; (3) applications of inde pendent coatings—as of asphalt, oil paints, varnishes or waterproof cements. The pre liminary requisite in all cases is the absolute cleanness of the steel from all deteriorated metal at the time the protection is applied: otherwise the destructive electrolytic action may continue beneath the preservative film, with no outward sign of what is going on.