Design - internal surfaces in contact with liquid

General principle of design

Cathodic protection inside surfaces is done with galvanic anodes or by impressed current. The nature, the location and the number of anodes depend on the chemical composition of the liquid, on its conductivity, on the temperature... In practice, the conductivity of the liquid must be higher than 0.001 S.m ^-1.

The electrical resistivity of the internal coatings of surfaces is relatively high if we use, for example, enamel or epoxydic resin. However if such coatings are degraded, their resistivity decreases. The electrical resistivity of coatings containing cement is relatively low.

When the resistivity of the coating is high, the current density taken into account during the design is about 5 mA.m ^-2 . It is about 1 A.m ^-2  for non-covered surfaces.

When the conductivity of the liquid is low (strong resistivity), and that the structure to be protected contains non-covered metal elements, those ones must be isolated from the whole structure. The location of the anodes needs to be studied carefully.

Secondary reactions

In the case of an internal protection, the electrochemical reactions on the anode and cathode (surface to be protected) can modify the composition of the liquid which is in a confined volume. This is to be taken into account for sanitary or industrial reasons. Gaseous emissions can also occur, it is in particular hydrogen, when the potential of the structure is too negative. This is why vent cocks and visit openings are sometimes required, as well as potential measurements. Chart 3 points out some requirements.