The use of cathodic protection with metal protective paints is of interest to industry where steel strucrtures are immersed in water or buried in soil. While both systems of protection offer many advantages, both have some disadvantages. Hence the question, “Are they compatible and would a combination of the two systems overcome some or all of the disadvantages of each?”
It was felt that with conventional paint systems, blistering could be minimized by applying a constant potential between an anode and the painted surface rather than a constant current. Four groups of tests were set up in fresh and salt water. Details of tests are given.
In fresh water it was noted that blistering of paint increased rapidly with increasing potential measured against the surface of the paint film with a saturated KCl calomel half cell. Although considerable blistering was observed where the potential measured against a calomel half cell did not exceed —850 millivolts, the paint films materially assisted cathodic protection.
In sea water it is also noted that vinyl paint films blistered increasingly with increasing potential pressure across the film. At potentials less than 1000 millivolts measured against a KCl calomel half cell very little if any blistering was encountered after 1½ years over that observed on a panel hot under cathodic protection. On cathodically protected panels small pinpoint white deposits formed, indicating effective policing by cathodic protection.
To establish more definitely the best combination of cathodic protection and paint, the study should extend several years beyond the time of initial failure of the paint film.
