Behavior of damaged thermally sprayed aluminum (TSA) in aerated and deaerated seawater

摘要

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 (R_p). 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 R_p 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.