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.
Key words: Cathodic protection, underprotection, weight loss, calcareous films, impedance,
SEM