CORROSION BEHAVIOR OF PAINTED IRON 317 



primer containing a moderate amount of varnish was subjected to test, 

 riie results are represented by curve 2 in Fig. 2. 



This curve is of especial significance, not only because its character- 

 istics are so much more pronounced than those of curve 1, but ajso 

 because it furnishes an explanation for the divergence of results which 

 have been obtained with red oxide primers in practice. The marked 

 increase in the length of time required for this curve to pass through its 

 various phases as compared to curve 1, is evidence of the water- 

 excluding effect of the added varnish. The period of definite passivity 

 has been extended to at least three times its former length, and the 

 momentary halt in curve 1 at approximately -0.09 volt has been 

 prolonged to an hour and a half at a slightly more positive potential. 

 Curve 2 continues somewhat erratically in the active direction after 

 the half-way halt in its course. E.xamination of the iron after several 

 days' immersion revealed corrosion. The broken line represents, in 

 days rather than hours, the quite different behavior of a duplicate 

 specimen, which prior to this time had acted similarly except for the 

 fact that it had required a somewhat longer time to pass through its 

 various phases. Apparently a slightly less permeable paint film made 

 it possible for the corrosive action to be stifled, temporarily at least. 

 Alternations of corrosive attack and film formation were observed 

 generally when iron corroded in contact with red oxide pigments and 

 primers. In a relatively dry atmosphere there is no doubt that iron 

 painted with red oxide is maintained in a passive condition, but ex- 

 posure to excess moisture must result eventually in active corrosion. 

 Since, then, the protective value of red oxide primers is dependent 

 primarily upon their ability to exclude moisture, they must be classed 

 as physical inhibitors of corrosion. 



The corrosion behavior of iron painted with red lead is clearh- indi- 

 cated in Fig. 2 by curves 3 and 4. The presence of a few small pores or 

 imperfections in the paint film on one of the test electrodes did not 

 materially affect the results. The initial potentials were somewhat 

 lower than was the case for red oxide, and the initial trend of the curves 

 was in the active direction, but a reversal soon took place and the iron 

 became definitely and permanently passive. An equilibrium potential 

 of approximately 0.25 volt was attained. Inspection of the iron after 

 several weeks' immersion failed to reveal any sign of corrosion. 



Red lead primer continues to inhibit corrosion even after moisture 

 has fully penetrated the film. On the basis of its action, both as 

 pigment and primer, red lead must be classed as a chemical inhibitor of 

 corrosion. The reason for its passivating action is a disputed question. 

 Paint chemists have inclined to the view that a highly protective lead 



