Cathodic Protection of Offshore Structures by Extreme Damage Tolerant Sacrificial Coatings

摘要

Sacrificial coatings such as thermally sprayed aluminium (TSA) have been used in offshore oil and gas platforms for decades. They provide a barrier layer when intact and cathodic protection when damaged or in the presence of surface-connected porosity. The damage tolerance of these coatings have been known from experience, but the level of damage these coatings can tolerate whilst still providing sacrificial protection to steel is unexplored. To explore the damage tolerance of TSA, coated carbon steel bar with a high degree of damage was exposed to synthetic seawater and the potential was monitored. It was observed that TSA is capable of polarising the steel bar to below -800mV (Ag/AgCI) even when a damage exposing 90% of steel surface is present. However, the potential became less negative with time reaching corrosion potential values (E_(corr)) of steel in 35 days. Even after potential reached the E_(corr) of steel no rust was seen on the exposed steel surface due to the deposition of a fine layer of calcareous matter. However, after 100 days rust spots began to emerge. The work carried out indicates the extreme damage tolerance of TSA.