THE CORROSION OF METALS 37 



of lead in tenth normal sulfuric acid breaks rather abruptly after 

 about 500 minutes to the potential of a gas electrode. Apparently 

 the anodic areas become progressively covered with a film or sulfate 

 until substantially the entire surface is passive. Upon the introduc- 

 tion of a drop of thousandth normal copper sulfate, the sulfation of a 

 similar lead specimen is consumated in about 200 minutes at the end 

 of which time the potential breaks to that of the cathodic areas of 

 copper which have been formed by replacement deposition. A still 

 higher concentration of copper brings about sulfation still more rapidly 

 and when the solution is contaminated with platinum a break to the 

 potential of platinum occurs after a still shorter period. In the same 

 manner, the relative corrodibilities of leads of various purities and 

 certain lead alloys ^^ has been compared. 



The foregoing discussion of the application of electrochemical 

 methods to corrosion investigations outlines techniques by means of 

 which it is possible to get information of the following kind. By the 

 position of the potential of a metal against its environment and the 

 trend of this potential with time it is possible to determine whether the 

 corrosion process is controlled by reactions occurring at the anodic 

 areas, the cathodic areas or both, that is, whether there is a tendency 

 toward passivity, inhibition or progressive attack. Measurements of 

 film stability whether in terms of the leakage current which may be 

 passed through the film, or in terms of the amount of film-forming 

 material (such as chromates) required to produce passivity or the 

 amount of film destroying material (chlorides) required to render the 

 metal active, furnish information as to the quality of corrosion resistant 

 films. Finally measurements of the rate at which a film forms on a 

 metal when placed in a film-forming environment also throws light 

 upon the relative surface reactivity of the metal. Such information 

 is of assistance in determining the rates of corrosion in homogeneous 

 corrosive environments or the rate of passivation in film-forming 

 environments. It is evident in all of these cases that the interpretation 

 of the experimental data which are obtained and the application of 

 the findings to practical corrosion problems is considerably facilitated 

 by a chemical knowledge of the environments in which metals are 

 used as well as the composition and physical state or structure of the 

 metallic material. With such measurements and such knowledge it 

 is possible to predict corrosion behavior and to obtain an understanding 

 of corrosion problems usually not possible by ordinary empirical 

 corrosion tests. 



To summarize, the process of corrosion may be one of direct com- 

 bination of a metal and a non-metal or it may be one in which hydrogen 



