THE CORROSION OF METALS 27 



potentials of lead in identical solutions exposed respectively to oxygen 

 and to an inert gas, nitrogen. An even more striking example of the 

 operation of oxygen concentration cells is observed when glass beads 

 are in contact with a lead surface wet by a film of sodium chloride 

 solution exposed to the air.^" After a period of a few months a ring 

 of bright red corrosion product forms around an anodic pit located 

 directly beneath each bead. This red corrosion product, a red 

 tetragonal form of litharge, is characteristic of an alkaline attack. 

 Apparently sufficient caustic soda to cause corrosion (a solution of 

 pH 12 approximately) was produced by the differential aeration cell 

 resulting from the contact of the bead with lead. 



The foregoing paragraphs have described some of the complexities 

 encountered in corrosion processes. In view of these complexities it 

 has been one object of these Laboratories for some years to advance 

 the development of a generalized theory of corrosion applicable to all 

 cases of corrosion of the replacement type, since it is this type of 

 process which prevails in atmospheric, soil and w^ater exposures. 

 Direct combination of metals with non-metallic elements is limited 

 largely to somewhat extreme conditions such as those of industrial 

 processing. Attention will now be directed to the general theory of 

 corrosion in its present state of development. 



The fundamental reactions of corrosion processes of the replacement 

 type as represented by the operation of corrosion cells are as follows: 



At the anode: M ^ M+ ; 2(0H)'^' O + H2O. 



That is, the metal sends ions into the solution or there are plated out 

 non-metallic elements such as oxygen. Either process is accompanied 

 by a loss of electrons. If corrosion is induced by an externally applied 

 potential, as for example when stray electrical currents flow from 

 underground metallic structures to earth, oxygen atoms may leave 

 the surface in the form of molecular oxygen. 



At the cathode: M+ -^M\ H+ A H ; 2H -> H2 gas. 



This process consists in plating out either metal or hydrogen atoms. 

 In the latter case atomic hydrogen either leaves the metal surface as 

 hydrogen molecules or acts as a reducing agent, being in turn oxidized. 

 The electrode reactions may be combined in equations as follow^s: 



1. Solution of the metal: M + H+ -^ M+ + H. 



2. Removal of hydrogen: (a) 2H — > Ho, 



{b) 2H + O -> H2O. 



