Underprotection of Mild Steel in Seawater and the Role of the Calcareous Film

Under normal conditions cathodically protected mild steel in seawater is also protected by a precipitated film of calcium carbonate and magnesium hydroxide, the so-called calcareous film. Under certain circumstances, underprotection can occur, for example; where current demand values are under estimated, or where structures are approaching the end of their design lives and anodes are severely reduced in size, or at cold deep water situations where the calcareous film is more soluble and reluctant to form.  In these situations, a calcareous film might well occur but complete protection is probably not possible. These conditions are being studied in the laboratory using artificial seawater and protection current densities less than would be normally applied in practice. Experiments involving weight loss, ac impedance and surface analysis will be described to attempt to clarify the situation under these interesting conditions.