RIASSUNTO
ABSTRACT
Thermally sprayed aluminum (TSA) has been used in offshore oil and gas infrastructure for decades. Their effectiveness in mitigating corrosion of steel structures in presence of seawater has been proven over the years. However, very little work is reported on the performance of TSA in other sectors where conditions are different and damage of such coatings is more likely, such as monopoles of offshore wind turbines. Furthermore, data on the performance of damaged TSA in conditions with limited dissolved oxygen, such as in internal walls of monopiles, is scarce. This paper addresses these knowledge gaps and reports the corrosion performance of damaged TSA in a simulated marine immersion environment, both in the presence and absence of dissolved oxygen. In this work, holidays amounting to 3% of the sample area were drilled to uncover the underlying carbon steel and the samples were exposed to synthetic seawater at 25°C for 18 days. Open circuit potential (OCP) was monitored and linear polarization resistance (LPR) method was used to measure the polarization resistance (Rp). The OCP values for the sample in deaerated condition was more active (or anodic) than the ne in aerated condition through the entire duration of the test. However, the Rp values indicated that the corrosion rate of the sample in aerated condition is likely to be significantly higher. After testing, examination of the sample revealed the formation of calcareous deposit, Mg- and Ca- containing layers, in aerated condition on the exposed steel surface. In the deaerated condition, Mg-containing layers were predominantly formed on the exposed steel surface with no visible steel corrosion product. Detailed microstructural characterisation of the deposit confirmed the formation of protective Ca- and Mg-based layers in different conditions. Thus, one can conclude that TSA can protect carbon steel in both aerated and deaerated conditions.
INTRODUCTION
Several coating systems have been used in marine environments mitigate corrosion of offshore assets. One such coating system is thermally sprayed aluminum (TSA) and its alloys. It is fairly well established that TSA coatings provide long-term corrosion protection of offshore steel structures.1-3 The uptake of TSA has grown in the UK since the development of the Hutton Field in 1981, wherein a tension leg platform had flame sprayed aluminum as the corrosion mitigation system.1 It is established that in Cl- containing solutions, aluminum protects steel cathodically in the pH range 0-14.4 As the pH of seawater is ~8, this works well for offshore environments. This, combined with the low self-corrosion rate of aluminum makes it an ideal candidate for such applications.5-7 The cathodic polarization of steel in seawater results in increased interfacial pH by the cathodic release of OH- which leads to the deposition of calcareous matter on any exposed steel thereby reducing the cathode area (exposed steel) and hence further decreasing the corrosion rate.8,9