THE CORROSION OF METALS 29 



their salts in which the metal ion concentrations are molal and normal 

 respectively. A table of the values of such potentials constitutes the 

 so-called E.M.F. series. This comparison of metals is of little value in 

 predicting either the driving force or the rate of operation of practical 

 corrosion cells, the electrodes and ionic concentrations of which bear 

 little correspondence to those defined in the E.M.F. series. Moreover, 

 the practical case is complicated in many instances by the gas electrode 

 behavior of the metal and by the flow of current during the corrosion 

 process. For example, the potentials of many metals when measured 

 in the atmosphere are much more noble or cathodic than might be 

 expected from a knowledge of the E.M.F. series. This is due to the 

 fact that in the presence of moisture and oxygen, metals may function 

 wholly or in part as oxygen electrodes. The exact values of these 

 electrodes depend upon the concentrations of oxygen present, and 

 upon the acidity of the solution. This explains the origin of the 

 potential difference of the differential aeration cells to which reference 

 has been made previously. 



If the corrodibility of copper in the presence of moisture were judged 

 solely from the position of the metal in the E.M.F. series no attack 

 would be expected, since in this series copper is more noble than 

 hydrogen, the element which must be displaced in the corrosion process. 

 As a matter of fact copper does not corrode even in hydrochloric or 

 sulfuric acids in the absence of available oxygen. It is readily corrodible, 

 however, in nitric acid because in effect under these circumstances the 

 position of the hydrogen electrode is rendered cathodic to copper 

 (i.e., more noble than copper) owing to the depolarizing influence of 

 oxygen. It is probably for this same reason that oxygen markedly 

 accelerates the corrosion of monel metal in 3 per cent sulfuric acid.^^ 



The change in electrode potential with current flow, polarization, 

 may be illustrated in a simple experiment as follows: If the zinc coating 

 is removed from a portion of the surface of a strip of galvanized iron, 

 exposing thereby the underlying iron surface, one has what amounts 

 to a simple galvanic zinc-iron couple. If this couple is completely 

 immersed in a dilute salt solution and potential measurements are 

 made at different points on the iron and zinc surfaces by means of a 

 calomel half-cell, it will be observed that the potentials of iron and zinc 

 at some distance from the iron-zinc interface are approximately those 

 values obtained for these metals separately in the same electrolyte; 

 while, on the other hand, the value of the iron potential near the inter- 

 face has become more anodic and the potential of zinc near the interface 

 has moved in the cathodic direction, i.e., the difference in potential 

 between iron and zinc near the interface of the two metals is appreci- 



