Abstract
A new electrochemical method was proposed for an evaluation of the protective ability of the inner paint film of doubly coated metal specimen with two different types of paint. The method consisted of the measurement of A. C. impedance of the doubly coated specimen and that of singly coated specimens with the constituent paint film. The impedance of the imagined doubly coated specimen was calculated from the measurements of the singly coated specimens. A comparison was made between the calculated and the observed impedances of doubly coated specimen in order to elucidate the influence of direct contact with the salt solution upon the impedance of inner paint film.
The method was applied to three typical paint systems immersed in 0.5M NaCl solution, that is, the primer-top coat paint system, the wash primer-primer system and the zinc rich paint-primer system. No actual difference was observed in the first system. Moreover, in the second system, the inner layer of wash primer was found to exist in a highly conductive state even when it was covered with a primer paint film.
The most remarkable change was observed in the last system, since the zinc rich paint was highly conductive when it was directly in contact with salt solution and, in contrast, highly resistive when indirectly in contact. From this observation, it is expected that, from the initial period of immersion, zinc rich paint acts in accordance with the so-called resistant control mechanism for a reasonably long while until the beginning of the breakdown of primer or top coat paint film.
