ABSTRACT
The offshore cathodic protection industry became aware of the susceptibility of zinc anodes in 1977 when it was discovered on anode attached to a hot pipeline riser in the Ekofisk field in the North Sea. It was studied by Ashworth et al. who posited that the inter-granular attack might be a result of grain boundary segregation of aluminium in alloy. Unlike the tin-containing formulations, mercury-containing anodes could be used in the as-cast condition. Through to end of the 1970s, aluminium alloys containing zinc and mercury dominated the offshore anode market. An approach to achieving rapid polarisation is to make use of very negative operating potential of magnesium anodes. Then, as the short-lived magnesium depletes, continuing through-life protection is sustained by longer-lived, more economic aluminium or zinc anodes. Sacrificial anodes operate in neither the constant current nor constant potential mode. They form part of a galvanic couple in which they are usually connected to a much larger area of bare steel.
