Deep Water Effects on Cathodic Protection

ABSTRACT The results of a laboratory study of the effect of hydrostatic pressure on the mineralogy of deposit formed under cathodes protection conditions are presented. This report is an update of a previous report (1) which described mineral deposits formed at room temperature at pressures equivalent to depths of up to 645 meters. The present report contains information on deposits formed at low temperatures (6°C). It confirms the earlier report which showed that mineral deposits are likely to be higher in magnesium and lower in calcium than had been previously reported. INTRODUCTION Cathodes protection is the principal means of corrosion control for submerged steel structures. The cathodes protection current causes a pH shift in the seawater near the steel structure and a hard, dense mineral deposit is formed on the steel surface, This mineral deposit then provides primary corrosion control, and the cathodes protection current demand drops to a level sufficient to repair this coating where it is damaged by mechanical or other means (2). In warm shallow waters of the Gulf of Mexico this deposit is largely calcium carbonate and is fairly protective (3). Host ocean waters are supersaturated in calcium carbonate, but, due to reasons not completely understood, the precipitation does not normally occur spontaneously. Cathodes protection is necessary to cause a pH shift which initiates calcium carbonate deposition and at higher pH's, normally caused by higher cathodes protection current densities, magnesium hydroxide is co precipitated (4). In deeper waters the effects of greater pressure and lower temperature retard the precipitation of calcium carbonate, because the seawater is no longer supersaturated (5-6). A more detailed discussion of the possible consequences of changes in mineralogy of cathodes-protection deposits is contained in our earlier report (l). EXPERIMENTAL All exposures in this article were conducted in a laboratory pressure vessel described in detail in an earlier report (1). Sargasso Sea Surface Water was pressurized to the desired level. This did not simulate actual deep-water conditions, because no attempt was made in these experiments to eliminate any dissolved oxygen present in the unpressurized water. Cathodes current was supplied by means of an electrochemical cell with a zinc anode and an iron cathode (l). Current densities were maintained within the ranges specified for cathodes protection of stationary offshore structures (l-2). RESULTS AND DISCUSSION Table 1 summarizes the exposure conditions for the experiments which have been completed. Our previous report contained scanning electron micrographs showing deposits formed at room temperature (22°C). These results were interpreted as indicating that magnesium was more likely to be found in deep-water deposits than in deposits formed in warmer surface waters (l). Figure 1 is a high-magnification scanning electron micrograph of a surface water (unpressurized) cathode surface. Two types of deposits are apparent in this photograph, which is typical of most of the cathode surfaces. Energy-dispersive X-ray spectra of these deposits show them to be primarily magnesium minerals with small amounts of iron, zinc, silicon, calcium and chlorine. While calcium is present, it is the fifth most abundant metal on these surfaces.