Concerns Over Utilizing Aluminium Alloy Anodes in Sealed Environments

摘要

In 2012, a cathodic protection (CP) trial was undertaken to establish the current requirements to protect the submerged and buried bare steel internal surfaces of monopile windturbine structures in the North Sea. Aluminium alloy anodes were utilized for the CP system and a remote monitoring system was also installed to measure the time dependent variations in anode current, structure to seawater potential, cathode current density and monopile hydrogen gas concentration produced from operation of the CP system. The possibility of hydrogen production within a confined space from the operation of a sacrificial anode CP system was one of the projects primary concerns, as it could create a hazardous area within the monopile. However, within a few weeks of the trial commencing, the seawater pH inside the monopile, had changed from near neutral pH 8 to less than pH 5. Furthermore, when access to the confined space within the monopile was attempted, toxic gas alarms were energised. Water pHs of the values observed had never been found or reported before when using aluminium alloy anodes and neither had the production of toxic and flammable gases from the operation of a sacrificial CP system, namely Hydrogen Sulphide, although trace hydrogen was expected and was a concern hence one of the factors that lead to the offshore trial being undertaken. This paper outlines the results from the trial and discusses theories why the pH had decreased and briefly describes the solution adopted to overcome the low water pH obtained with the use of aluminium alloy anodes.