The Effects Of Water Depth On Cathodic Protection Of Steel In Seawater

Abstract The results of a laboratory study of the effect of hydrostatic pressure on the minerology of deposits formed under cathodic protection conditions on carbon steel are presented. Preliminary results indicate that primary deposits are of magnesium minerals and calcareous deposits, if any, are secondary. Introduction In 1824 Sir Humphrey Davy introduced cathodic protection as.a means of preventing the corrosion of iron nails used to fasten copper antifouling sheathing to the bottoms of wooden British naval vessels. He noted tha t barnacles adhered to "the calcareous deposits occasioned by the excess of negative electricity" (1). Today cathodic protection is still the principal means of corrosion control for submerged steel structures. The cathodic 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 providesprimary corrosion control, and the cathodic protection current demand drops to a level sufficient to repair this coating where itis damaged by mechanipal or other means. In warm shallow waters of the Gulf of Mexico this deposit is largely calcium carbonate and is fairly protective (2). Most ocean waters are supersaturated in calcium carbonate, but, due to reasons not completely understood, the precipitation does not normally occur spontaneously. Cathodic protection is necessary to cause a pH shift which initiates calcium carbonate deposition andat higher pH's, normally caused by higher cathodic protection current densities, magnesium hydroxide is coprecipitated (3). In deeper waters the effects of greater pressure and lower temperature retard the precipitation of calcium carbonate, because the seawater is no longer supersaturated (4-5). What this does for the structure and composition of calcareous material in a cathodic protection situation is not completely clear. If little or no calcareous material is deposited, considerably higher current densities may be needed to produce the required protection potential at the cathode. A greater rate of sacrificial anode depletion will occur or a larger impressed current supply will be required. The meager available qualitative evidence appears to point to the lack of buildup of a calcareous coating on exposed surfaces at deep depths. In one case, a cathodically protected deep sea mooring in the Gulf of Mexico, calcareous deposits were observed only on those portions of the mooring at depths less than 30 meters (6). In a second case, cathodically protected steel test panels exposed at 485 and 940 meters near the Bahamas did not develop calcareous coatings. Further, the current densities needed to protect the test panels were reported to increase markedly with depth (7).