CORROSION BEHAVIOR OF PAINTED IRON 345 



It will be noted that the potentials of the test electrodes are initialK' 

 quite similar, but diverge with time and form two distinct groups of 

 curves, which ultimately become separated by about 0.7 volt. It was 

 observed that invariably electrodes did not corrode if their potentials 

 became more electropositive (more noble) over a long period of time, 

 while, on the other hand, marked corrosion accompanied a negative 

 trend of potential. A state of equilibrium was reached ultimately by 

 the passive electrodes between 0.25 and 0.30 volt, and by the active 

 electrodes between —0.40 and —0.45 volt. 



Red oxide and red lead paints were selected for study because 

 practical experience indicates that they are representative of the two 

 types of protective paint, viz., (1) those which protect merely because 

 they serve as physical barriers, and (2) those which exert a chemical 

 inhibiting action as well. 



The test electrodes were commercial iron, of high purity, in the form 

 of 1/8 inch rods. The pigments were technical grades of red oxide 

 (FeaOs) and red lead (Pba O4) of high quality. Raw linseed oil and a 

 lead-cobalt dryer were used in the preparation of all of the paints, 

 which were formulated and compounded in the customary manner. 

 Approximately 20 per cent of a flexible type varnish and 10 per cent of 

 blown linseed oil w^ere incorporated with raw linseed oil to form the 

 vehicle in one of the red oxide paints. 



As a rule, the primary purpose of a protective paint is to shield iron 

 from the corrosive action of water and water vapor. Total immersion 

 is an extreme condition, but a condition to which all such paints are 

 frequently subjected. For this reason, and also in order to speed up 

 possible reactions and save time, all of the potential measurements on 

 painted iron recorded in this paper were made on submerged specimens. 

 Similar measurements on painted iron exposed to the atmosphere are 

 equally possible and can be made without disturbing service conditions. 

 It is planned to extend this study to include such measurements. 



The time-potential curves obtained in the study of primers are pre- 

 sented in Fig. 2. There are included for reference typical curves (6 and 

 7) for iron in the active and passive states, and a curve (curve 5) for 

 iron coated with a dried film of linseed oil. It will be noted that the 

 linseed oil coated electrode behaved in much the same manner as bare, 

 active iron, except that a much longer time w^as required for the po- 

 tential changes to take place. Several days elapsed before the po- 

 tential reached the equilibrium value attained by bare iron in a few 

 hours, and at this point rust was clearly visible. 



The potential of iron painted with red oxide primer (curve 1), 

 immediately after immersion in water, was approximately 0.33 volt. 



