A number of papers have been written and presented dealing with the detailed methods of calculating and applying cathodic protection to offshore structures. In this paper it is our intention to consider the technical qualities of the materials and systems available and compare these commercially.
The term offshore structures as used in this paper specifically refers to fixed drilling and production platforms and mobile drilling rigs.
Cathodic Protection can be defined as the reduction or prevention of corrosion of a metal (e.g. steel) in an electrolyte prevention of corrosion of a metal (e.g. steel) in an electrolyte (e.g. sea water) by introducing an additional anode system from which electrical current will flow into the electrolyte and onto the corroding metal surface reducing or preventing the natural corrosion current flow from anodic areas to cathodic areas on that metallic surface. The protective current may be obtained from the use of highly electronegative materials such as magnesium, zinc or aluminium or by the use of relatively inert anodes with an external direct current power source. The former is termed sacrificial anode cathodic protection, the latter impressed current cathodic protection. protection. Steelwork Requiring Cathodic Protection Cathodic Protection should be considered for steel when it is:
Totally immersed in water.
Buried in the seabed or shore (i) in the bare condition or (ii) in the coated condition.
Associated with or immersed in concrete when it is either (i) in the bare condition or (ii) in the coated condition.
Combinations of the above may also require cathodic protesion. protesion.
Sacrificial Anode Materials The following alloys are used as sacrificial anodes:
These can be considered to be split into certain generic types. heat treatment aluminium zinc tin alloys, aluminium zinc mercury alloys and aluminium zinc indium alloys. Table 1 indicates some typical specifications.