## 1 Introduction  
Clean energy sources, such as offshore wind, are key to decarbonise energy production [1]. The material of choice for construction of offshore structures is carbon steel due to its low cost, high strength, machinability and weldability. However, this material is prone to corrosion and associated degradation in offshore environments, hence the need to employ corrosion mitigation methods [2; 3]. These corrosion mitigation methods include coatings and/or cathodic protection. Thermally sprayed aluminium (TSA) is a widely used metallic coating system that acts as a barrier when sufficiently thick and offers cathodic protection to immersed offshore steel structures [4]. Several long-term coastal studies beginning in the 1940s, and offshore oil and gas exploration sector experience since the mid-1980s, indicate that TSA provides excellent long-term mitigation of splash and tidal zone corrosion [5; 6; 7]. As a result, at least one operator has reduced the splash zone corrosion allowance offshore by 3 mm on a number of its projects since 1999 [8]. Outside the oil and gas sector, several offshore wind turbine operators currently specify a coating that includes a first layer of thermally sprayed Zn-15 Al (85%Zn-15%Al) for the topside, which is well above the splash zone in the marine atmosphere. Although commercially pure aluminium is a coating considered by the oil and gas sector, one long-term study incorporating tidal, splash and submerged test panels reports that Zn-15 Al behaves equally well [7; 9]. To date, long-term assessment of TSA coating performance has been in the form of visual examination, reporting evidence of corrosion products (such as rust formation in steel, aluminium oxide/hydroxide on TSA), blistering and dis-bondment, but there are limited data indicating that submerged TSA coatings freely corrode at 2-3 \(\upmu\)m per annum in the North Sea [10]. There is also evidence that, when applied correctly, sealants can significantly extend the life of TSA coatings, but the occasional reported failure of sealed coatings implies that further optimisation of this aspect is required [11; 12]. The vast majority of TSA coatings are deposited by twin wire arc spraying or wire flame spraying, and their relative merits in terms of coating quality (primarily defined as porosity, oxide content and adhesive strength), process deposition rate and deposit efficiency are well-documented and can be related to coating behaviour [13; 14]. It has been predicted that an optimised 200 \(\upmu\)m thick TSA coating could provide a service life exceeding 30 years in a splash zone [6]. The work suggested optimizing some key factors such as coating composition, surface preparation, coating application, service application, choice and application of sealer, galvanic exposure condition and environmental impacts. This was based on service experience of TSA applied to Hutton risers and other performance experiences discussed previously. The choice of coating alloy for splash zone or continuously submerged service was noted to be either Al (99.5%) or Al-5 Mg. The use of Al-Zn alloys, such as Zn-15 Al and Zn-30 Al, is likely to increase the cathodic polarizability while decreasing coating lifetime due to the higher dissolution rate of Zn-alloys in seawater [15]. To achieve a good performance coating system, stringent control of surface preparation and application are required [16]. SINTEF reported corrosion rates of 2-3 \(\upmu\)m/y after 11 months of exposure in natural seawater in Norway for both 99.5% Al and Al-5 Mg. One might argue that this equates to a service life >60 years for a 200 \(\upmu\)m TSA coating [6]. In humid industrial or marine atmospheres, corrosion rates of up to 15 \(\upmu\)m/y have been reported for thermally sprayed Zn coatings. Hence, in very corrosive environments an unsealed 150 \(\upmu\)m thick Zn coating may corrode away in 10 years [17; 18]. There are other reports that offer performance data of TSA in different applications.