THE CORROSION OF METALS 25 



products. For iron this is somewhere between 40 and 65 per cent 

 relative humidity, probably nearer the latter figure." The atmos- 

 pheric corrosion products of the non-ferrous metals are in general less 

 deliquescent, i.e., have higher critical humidities; for nickel it appears 

 to be above 70 per cent relative humidity and for zinc and copper 

 above 75 per cent. It has been suggested that the marked increase in 

 the resistance of copper containing about 0.5 per cent arsenic to atmos- 

 pheric corrosion is due to the fact that the presence of arsenic renders 

 the corrosion product less hygroscopic." The effect of copper in 

 copper bearing steel may be of the same nature; ^^ apparently the 

 inhibiting action in this case does not lie in the production of a film 

 which is any more resistant to attack initially than that on ordinary 

 steel. ^^ In dust-free air, even at high humidities, iron does not corrode 

 but forms an invisible protective film. Electron beam studies ^^ 

 have indicated the structure of this film to be a form of ferric oxide 

 which has been designated as a Fe203 in contrast to the composition 

 of a non-protective form which appears to be 7 FeOOH. 



The presence of dust particles in the atmosphere greatly increases 

 the rate of corrosion of iron. In this case, as well as in the accelerated 

 attack which occurs above the point of critical humidity, the process 

 involves the displacement of hydrogen from water. Other common 

 examples of this type are the solution of metals in acids or alkalies, 

 the reaction of sodium with water, the deposition of metallic copper 

 from copper sulfate solution by metallic zinc and in general the 

 corrosion of metals in moist air, in soils and in water. It is well estab- 

 lished that the process of corrosion in these cases is electrolytic in 

 character, i.e., that corrosion occurs by means of the operation of small 

 galvanic cells at the surface of the metal. The primary reactions of 

 these cells may be and generally are followed by important secondary 

 chemical reactions of the products of electrolysis with the constituents 

 of the environment. Between the anode and cathode areas there is a 

 flow of current through intervening electrolytic paths of greater or 

 lesser resistance. Naturally the amount of corrosion is proportional 

 to the amount of current flow.^^ It is largely the distribution and size 

 of anode areas which determines the character of the corrosive attack. 

 For a given amount of metal dissolution the existence of relatively few 

 anode areas small in size obviously leads to pitting, while if there are 

 numerous anodes uniformly distributed, corrosion will likewise be 

 uniform. The distribution of anodes is determined by the inhomo- 

 geneity of the base metal, the character of the films which are formed, 

 the accidental contact of inert bodies and the conductivity of the sur- 

 rounding electrolytic media. 



