## 4 Discussion
### Design Life of Steel Structures Coated with TSA  
The corrosion rate of unprotected C-Mn steel in seawater is reported to be \(\sim\)0.1 to 0.5 mm/y [40]. Although this value depends on various factors such as temperature, salinity, agitation, wet and dry conditions etc., it nonetheless gives a good guide. The presence of a thermally sprayed anodic material, such as a TSA coating, will protect the steel while the coating is being preferentially consumed. Little or no measurable corrosion of the C-Mn steel plate is likely to occur in the absence of holiday or damage. Even when a holiday is present, the TSA coating is likely to protect the steel provided the holiday is in electrical and electrolytic contact with the TSA. Even when no apparent holiday is present, the thermal spray process produces a coating with porosity. This porosity, if surface connected, causes precipitation of calcareous material on the surface of exposed steel. The calcareous deposits plug the pores close to the steel surface causing a sealing effect. Similar sealing effect is also expected due to the generation of Al corrosion products on the pore walls. The test data presented in this paper showed that the TSA coatings provided sufficient protection to mitigate corrosion of the C-Mn steel substrate during immersion in artificial seawater, with the thermal spray layer acting both as a barrier and sacrificial coating. Electrochemical measurements during immersion showed that the calculated corrosion rate of TSA was initially high at -0.1 mm/y, in all the coated samples, but decreased significantly after 300 days to -0.005 mm/y, as shown in Figure 4. The initial high corrosion rate was likely to correspond to exposure of the active as-sprayed coating and the steady reduction of the corrosion rate due to the formation of corrosion products and associated calcareous deposits. This change in corrosion rate corresponded to a steady increase in potential of the coated samples from approximately \(-\)1100 mV to \(-\)900 mV, which corresponds to the coatings becoming less active. Nevertheless, potentials were maintained below 800 mV (Ag/AgCl) for all cases for the entire duration of the tests, which is generally considered to be the threshold for the cathodic protection of C-Mn steel in seawater immersion. Estimates of the loss of coating thickness are difficult to obtain from physical thickness measurements due to the build-up of corrosion product and calcareous deposit which fill the pores present in the thermal spray coatings and the relatively small losses of material during the test periods. However, from electrochemical measurements during alternate immersion, corrosion rates <0.005 mm/y were calculated after 300 days of exposure. For the sake of simplicity, if we assume that the TSA consumption occurs at a rate of 0.005 mm/y over the nominal coating thickness of 0.3 mm, one would estimate coating service life to be -60 years. However, one must note that although the corrosion rates are calculated and presented in units such as mm/y, is not constant over the material's lifetime. In the case of TSA in service, the corrosion rate is likely to change further as the calcareous deposits form or if mechanical damage of the coating occurs. Realistically, if a safety factor of two is assumed, a coating life exceeding 30 years can be expected using unsealed Al with holidays. The values of estimated coating life in the previous paragraphs are based on constant immersion in quiescent laboratory conditions and thus are not entirely representative of real life immersed zones. The seawater velocity has a bearing on the corrosion rate. However, they give a good indication of corrosion performance in seawater immersion conditions. In other areas of an offshore structure, such as the splash zone or the tidal zone, the environmental conditions are different. In such environments, the seawater splash or alternate immersion combined with temperature fluctuations, UV radiation, biofouling, etc. provide different service conditions to the ones presented in this paper.