The Use of Cathodic Protection In Conjunction With Paint Coatings—A Progress Report^★

Attempts to mitigate corrosion of steel tanks housing hundreds of transformers and network protectors have led to two main investigations: first, the selection of the best available paint coating system from a corrosion protection standpoint that can readily be applied, and second, the determination of the feasibility of applying cathodic protection in conjunction with this coating. The selection of the best paint coating system has involved laboratory testing of approximately one hundred different systems under conditions simulating those encountered in service. As a result, the present coating system consists of three coats of an asphalt base paint. Recently, two plastic coating systems, both vinyls, have shown up equally good on test but application difficulties have have prevented their adoption at present. The study of the feasibility of applying cathodic protection in conjunction with the protective coating is still in its initial phases although it has, been going on for over a year. To date, the investigation has considered the following phases: a) Choice of a suitable electrode, b) Effect of cathodic currents from magnesium electrodes on four different paint coatings, c) Effect of electrolytes containing various mixtures of salt and fresh waters, d) Effect of magnitude of cathodic current, e) Actual field installation of a magnesium electrode in a tidal transformer manhole. Tests on four different protective coatings, using a magnesium electrode and a straight sea (East River) water electrolyte with no control of the cathodic current, gave widely divergent results. Tests made on the standard asphalt base coating with electrolytes consisting of all East River water, equal parts of East River and tap waters, and one part of East River water to three parts of tap water showed no major differences although the resultant cathodic currents • were roughly proportional to the amount of East River water in the electrolyte. Tests made on the standard coating with cathodic currents maintained at 1.0 and 0.5 milliampere for comparison with the original test at 47-67 milliamperes indicate that deterioration of the coating around the window starts within 4 days and that the rate of deterioration is roughly proportional to the cathodic current in the ranges considered. However, at both lower currents, rusting was observed on the uncoated sections whereas none was observed at the highest current. The rapid and almost complete removal of the protective coating was confirmed by a trial installation of a magnesium electrode in a tidal transformer man-hole in the Coney Island section of Brooklyn. Although indications to date are that cathodic currents of sufficient magnitude to prevent corrosion at breaks in protective coating surfaces cause deterioration of the coating, it is planned to investigate other electrode materials, and the effect of holding the galvanic potential below the hydrogen over-voltage point.