CORROSION OF IRON AND STEEL. 513 



chirk analyses made in accordance with Section 8 (b) shall conform to the requirements as to 

 phosphorus and sulphur specified in Section 6. 



CORROSION OF IRON AND STEEL. If iron or steel is left exposed to the atmosphere 

 it unites \sitli oxygen and water to form rust. Where the metal is further exposed to the action 

 of corrosive gases the rate of rusting is accelerated but the action is similar to that of ordinary 

 rust inij. Nrit her dry air nor water free from oxygen has any corrosive effect. While not essential 

 to corrosion acids greatly hasten its action. It seems evident that some weak electrolysis is 

 c.-ntial for corrosive action. Where iron or steel are in contact with water electrolytic action 

 will always take place, although the amount is very small under ordinary conditions. Where a 

 considerable electrolytic force exists the corrosion is greatly hastened. The increase in the use 

 of electricity has doubtless had a tendency to increase the corrosion of iron and steel and to make 

 the problem of the preservation of iron and steel from corrosion of great importance. 



In an article on "The Corrosion of Iron " in Proceedings of American Society for Testing 

 Materials, vol. VII, 1907, pages 211 to 228, Mr. Allerson S. Cushman shows that the two factors 

 wfthout which the corrosion of iron is impossible are electrolysis and the presence of hydrogen 

 in the electrolyzed or " ionic " condition. The electrolytic action can only take place in the 

 presence of oxygen or some other oxidizing agent. Rust is a hydroxide of iron ferric hydroxide, 

 FeOsHi. The corrosion of iron or steel may be prevented or retarded by covering it with a coating 

 that will protect it from the water or the air. 



It is commonly believed, with good reason, that cast iron corrodes less rapidly than either 

 wrought iron or steel. The graphite in the cast iron and the silicious coating that the cast iron 

 receives in molding doubtless assist in protecting the cast iron from corrosion. 



It is also commonly believed that steel corrodes more rapidly than wrought iron. The tests 

 that have been made to determine the relative corrosion of wrought iron and steel are very con- 

 flicting, but it appears certain that the difference in the corrosion of well made steel and well made 

 wrought iron is very slight. The acid test as a measure of natural corrosion has been used, es- 

 pecially by firms manufacturing and selling " ingot iron " (very low carbon Bessemer or open- 

 hearth steel). Committee A- 5 on the Corrosion of Iron and Steel of the American Society for 

 Testing Materials in the Proceedings of the Society, vol. XI, 1911, page 100, states tliat it considers 

 the acid test as unreliable as a measure of natural corrosion and does not recommend its use. 



In the paper on " The Corrosion of Iron " above referred to, Mr. Cushman states: " A 

 very widespread impression prevails that charcoal iron or a puddled wrought iron are more re- 

 sistant to corrosion than steel manufactured by the Bessemer and open-hearth processes. It is 

 by no means certain that this is the case, but it would follow from the electrolytic theory that in 

 order to have the highest resistance to corrosion a metal should either be as free as possible from 

 certain impurities, such as manganese, or should be so homogeneous as not to retain localized 

 positive and negative nodes for a long time without change. Under the first condition iron would 

 appear to have the advantage, but under the second much would depend upon the care exercised 

 in manufacture, whatever process was used." 



From the preceding discussion it would appear that neither " ingot iron " nor wrought iron 

 any advantage in resisting corrosion over a well made structural steel. 

 PAINT.* The paints in use for protecting structural steel may be divided into oil paints, 



tar paints, asphalt paints, varnishes, lacquers, and enamel paints. The last two mentioned are 



too expensive for use on a large scale and will not be considered. 



OIL PAINTS. An oil paint consists of a drying oil or varnish and a pigment, thoroughly 

 mixed together to form a workable mixture. " A good paint is one that is readily applied, has 

 good covering powers, adheres well to the metal, and is durable." The pigment should be inert 

 to the metal to which it is applied and also to the oil with which it is mixed. Linseed oil is com- 

 monly used as the varnish or vehicle in oil paints, and is unsurpassed in durability by any other 

 drying oil. Pure linseed oil will, when applied to a metal surface, form a transparent coating that 

 offers considerable protection for a time, but is soon destroyed by abrasion and the action of the 

 elements. To make the coating thicker, harder and more dense, a pigment is added to the oil. 

 An oil paint is analogous to concrete, the linseed oil and pigment in the paint corresponding to the 



* This discussion on paints is taken from the author's " The Design of Steel Mill Buildings." 

 34 



