THE CORROSION OF METALS 21 



noble metals such as gold, silver and platinum, being less reactive 

 chemically than the more basic metals, are as a group the least cor- 

 rodible, yet silver tarnishes markedly in moist atmospheres containing 

 volatile sulfur compounds; gold is attacked by halogens in solution 

 and platinum by fused alkalies. 



The protection of metals from corrosion may be accomplished in 

 general either by maintaining a non-corrosive surrounding environ- 

 ment or by coating the metallic parts with paints, lacquers or more 

 corrosion-resistant metals. Such measures as the control of humidity 

 and dust in interior atmospheres, deoxygenation of boiler waters, the 

 use of passivators such as chromates, carbonates, phosphates, silicates 

 and alkalies in the water-cooling systems or water scrubbers of air 

 conditioning equipment and the use of cathodic protection which con- 

 sists in setting up an electrolytic cell in which the metallic part subject 

 to corrosion is made the cathode, are typical examples of environmental 

 control designed to inhibit corrosion. Another well-known example of 

 avoiding corrosion by control of environment is the protection of 

 underground cables which is afiforded by the proper drainage of elec- 

 trical stray currents which have been picked up by the cable network. 

 Where it is infeasible to maintain an inert environment the use of non- 

 ferrous metallic coatings is of great value in the preservation of steel 

 products. Much metallurgical work has been devoted in recent 

 years to the development of corrosion-resistant alloys. In both of 

 these cases the protective feature consists in a naturally developed 

 surface film. Where natural films afiford insufficient protection it 

 becomes necessary to resort to coatings of organic materials such as 

 paints, lacquers, enamels, complex structures of such materials as 

 pitches or asphalts with jute felt or paper, etc. It has been estimated 

 that one hundred and twenty million gallons of paint are used annually 

 for corrosion prevention.'* 



Corrosion may be defined in most general terms as the chemical 

 reaction of a metal with the non-metallic constituents of its environ- 

 ment. In this sense any reaction in which a metal is degraded to 

 one of its compounds, such as an oxide, hydroxide, acid or salt is a 

 corrosion reaction. The nature of the reaction which occurs in any 

 given case depends both upon the reactivity of the metal, its purity, 

 physical state and surface condition and upon the character of the 

 environment, that is, upon the chemical components present, their 

 physical phases and concentrations. It also depends upon the tem- 

 perature. Corrodibility is not wholly an inherent property of a metal 

 which can be determined by a single arbitrarily chosen corrosion test 

 of any sort; even the relative order of corrodibility of a series of metals 



