Relationship of Chemical Components and Impurities of Aluminum Galvanic Anodes Upon the Cathodic Protection of Marine Structures

Aluminum galvanic anodes form the principal basis for cathodic protection of offshore structures and marine vehicles. A satisfactory cathodic protection (CP) design using the galvanic mode must employ a valid anode. It follows, therefore, that highly responsible compositional consideration be associated with such anodes. Since applications extend from very low temperature to above ambient seawaters, this factor is addressed in the paper. Applications in sea muds and its effect upon anode behavior is also considered. Primary emphasis will be given the indium-activated alloys with occasional comparisons to the mercury-activated aluminum anodes. The grounds for this emphasis lies in the fact that the indium-activated anodes constitute the great majority of use. The data and observations discussed are based primarily on low temperature, ambient and higher temperature laboratory and pilot tests supplemented by user field applications over the last 25–30 years.

Dependable aluminum anode performance in the subject waters and muds point to careful control of iron and copper impurities, coupled with the correct balance of indium, zinc and silicon. Generalized recommendations are presented with regard to composition. The future challenges of cathodic protection at greater seawater depths are also addressed. Possible effects upon anode current capacity and voltage are so related.