ABSTRACT

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/AgCl) even when a damage exposing 90% of steel surface is present. However, the potential became less negative with time reaching corrosion potential values (Ecorr) of steel in 35 days. Even after potential reached the Ecorr 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.

INTRODUCTION

Corrosion is a major concern for organizations operating in the offshore sector. To mitigate corrosion of offshore assets various mitigating methods such as use of coatings, cathodic protection etc are employed. One such coating system is thermally sprayed aluminum (TSA) and its alloys. It is well established that TSA coatings provide long-term corrosion protection of offshore steel structures.1,2 The low self-corrosion rate of aluminum in seawater coupled with its ability to act as an anode with respect to steel makes it an ideal candidate for such applications.3-5 The polarization of steel in seawater results in increased interfacial pH which leads to the deposition of calcareous matter on any exposed steel.6 Such deposits have the favorable effect of reducing the cathode area (exposed steel) and hence decreasing the corrosion rate.7

Performance of TSA-coated steel in marine environments has been widely investigated in long-term laboratory as well as on-site studies.2, 8-10 It is known that TSA has the ability to polarize steel in seawater and provide a substantial level of cathodic protection at ambient and elevated temperatures, even when damaged.5-7 The behavior of TSA-coated steel in natural seawater, with and without external cathodic protection, has also been explored.5 Additionally, work has been carried out to study the mechanism of calcareous deposit formation in seawater.4, 9-11 The influence of parameters, such as temperature, pH, pressure12 and various ions present in seawater has been studied. Although several papers have been published in the area, the research so far on damage tolerance has focused on small defects in the coating, typically 1-5%.7

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