22 BELL SYSTEM TECHNICAL JOURNAL 



is not constant.^ For example, iron, magnesium and zinc corrode in 

 conductivity water exposed to the air in the order given; in sodium 

 chloride solution the order is magnesium, zinc and iron, while finally 

 in strong alkali solutions these metals corrode in the order: zinc, iron 

 and magnesium. 



It is apparent that the occurrence of corrosion depends upon both 

 the character of the environment and of the metal. While the environ- 

 ment in which a metal is used is usually complex, it is generally possible 

 to recognize those constituents which exert a controlling influence on 

 the course of corrosion. In the ordinary atmosphere water vapor and 

 oxygen are major factors in the process. Other substances such as 

 sulfur dioxide, chloride ions and dust also influence the character of the 

 corrosive attack. The green patina, a basic sulfate, which forms in 

 the course of time on copper exposed to the air and which may preserve 

 the metal for centuries, owes its origin to traces of sulfur dioxide in the 

 atmosphere.^ On the other hand, the higher concentrations of sulfur 

 dioxide prevailing in the neighborhood of smelters which treat sulfur 

 bearing ores may rapidly corrode copper telephone wires to destruction. 

 Steel containing up to 0.25 per cent copper is about two-fold more 

 resistant to corrosion than non-copper bearing steel in most industrial 

 atmospheres; in New York City, however, the very small chloride ion 

 content of the atmosphere, an otherwise typical industrial atmosphere, 

 largely destroys the corrosion resistance conferred by the copper. 

 Rainfall is an influential factor in the corrosion of metals exposed to 

 the atmosphere. It may increase corrosion by removing soluble 

 corrosion products from the surface of the metal, or it may retard 

 corrosion by washing away dust particles and electrolytes, both of 

 which promote corrosive attack. For example, in New York City the 

 daily application of a water spray increased the rate of corrosion of zinc 

 by 30 per cent but decreased the rate of corrosion of difi"erent ferrous 

 materials from 30 per cent to 46 per cent, the amount of corrosion 

 being determined by loss of weight measurements. The corrosion 

 products of zinc are appreciably more soluble than those of iron and 

 presumably were largely removed by the frequent washing. On the 

 other hand, the deliquescent nature of the corrosion products of iron ^ 

 at humidities prevailing a large part of the year provide, in contrast 

 to the relatively less deliquescent corrosion products of zinc, a film of 

 moisture more or less saturated with corrosive electrolytes and dust 

 particles. This film is diluted or otherwise removed by the water 

 spray. 



Indoor environments differ from the outside atmosphere mainly in 

 being drier, cleaner and subject to less pronounced temperature varia- 



