The Corrosion of Metals — I. Mechanism of Corrosion 



Processes 



By R. M. BURNS 



This paper outlines the application of electrochemical methods to corro- 

 sion investigations. It discusses the position of the potential of a metal 

 against its environment and the trend of this potential with time, pointing 

 out that it is thereby 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 required to produce passivity or the 

 amount of film destroying material required to render a metal active, fur- 

 nish information as to the quality of corrosion resistant films. Measure- 

 ments of the rate at which a film forms on a metal when placed in a film- 

 forming environment throws light on its relative surface reactivity, and 

 such information is of assistance in determining the rate of corrosion in 

 homogeneous corrosive environments or the rate of passivation in the film- 

 forming environments. On the basis of such measurements and with a 

 chemical knowledge of the environments in which metals are used as well 

 as the composition and physical state or structure of the metals, it is pos- 

 sible to predict corrosion behavior and to obtain an understanding of cor- 

 rosion problems usually not possible by ordinary empirical corrosion tests. 



ALL metals are corrodible under the appropriate circumstances. 

 The most important metal industrially, iron, is probably the most 

 corrodible under ordinary conditions. Many estimates have been 

 made of the value of iron and steel products destroyed by corrosion.^ 

 While much depends upon the basis of calculation it seems reasonable 

 to conclude that the annual cost of corrosion in this country is of the 

 same order as the interest on the public debt or nearly one third of the 

 cost of the federal government in normal times. The common non- 

 ferrous metals — zinc, lead, copper, aluminum, nickel and tin — are more 

 resistant to corrosion largely because of their tendencies to form pro- 

 tective surface films. In the atmosphere under favorable circum- 

 stances tests have indicated, for example, that in the form of sheet 

 0.03 inch in thickness and exposed on one side as in the case of roofings, 

 zinc, copper and lead if mechanically undisturbed would resist cor- 

 rosion for more than one, two and three centuries respectively.^ Once 

 a protective film is formed it may preserve the metal indefinitely. 

 Under other circumstances these metals may readily corrode. Contact 

 with large inert soil particles may result in the perforation of cable 

 sheathing 0.10 inch in thickness in about 8 years.' Tin, although 

 resistant to corrosion in air and pure water, is severely corroded by 

 alkalies, and aluminum is attacked by both alkalies and acids. The 



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