Prediction of Thermal Spray Coatings Performance in Marine Environments by Combination of Laboratory and Field Tests

Abstract

Cost-effective corrosion mitigation of offshore steel structures can be achieved by thermal spray coatings. These coatings, when comprised of Al, Zn and their alloys, provide a physical barrier against the environment when intact, and cathodic protection to underlying steel when damaged. Due to the complexity of marine environments, laboratory tests should be combined with field work in order to understand the corrosion protection offered by these coatings. The work presented here was carried out with thermal spray coatings of aluminum alloys (AA1050, AA1100, Al-5Mg) and Zn-15Al prepared by Twin Wire Arc Spray onto low carbon steel substrates. The resulting coatings were ~300 μm in thickness, and 5% of surface area defects were artificially machined in order to expose the steel substrate, simulating mechanical damage or erosion of the coating. Electrochemical data collected over a 90 days period showed a good correlation between laboratory and real marine environment results. Aluminum alloys showed better corrosion protection in fully immersed conditions, while zinc alloys performed better in atmospheric and splash zones. Overall, these results aim to improve design of thermal spray coatings to protect carbon steel in marine environments.

Effect of Corrosion Products and Deposits on the Damage Tolerance of TSA-Coated Steel in Artificial Seawater

“…The simulated potential and corrosion rate were compared with experimental data available in the literature of the electrochemical performance of TSA coating with an intentional damage area of 5% [6,10,14]. All comparison tests evaluated the 300-µm thick coating of pure aluminium (1050 alloy, 99.5 wt% Al) obtained by spraying arc wire deposited on low carbon steel samples at room temperature under conditions of total immersion using seawater artificial according to ASTM D1141-98 [23].…”

“…All comparison tests evaluated the 300-µm thick coating of pure aluminium (1050 alloy, 99.5 wt% Al) obtained by spraying arc wire deposited on low carbon steel samples at room temperature under conditions of total immersion using seawater artificial according to ASTM D1141-98 [23]. Details of the coating obtaining process and experimental methods can be found elsewhere [6,10,14]. Figure 1a shows schematically the shape of the physical specimen.…”

“…This technological innovation led to the installation of 60 offshore wind structures in the German Baltic Sea using TSA coating with sealant as the primary corrosion control system instead of conventional organic coating and galvanic anodes [4]. This advance is also due to the ability of TSA coating to polarise the steel in seawater, which has been demonstrated from experience [5], laboratory tests in simulated environments, and trials on site [6,7]. It is now well established from several studies that TSA coating provides corrosion protection, in the atmospheric, splash, and submerged zones [8,9].…”

“…TSA coating acts as a barrier and offers cathodic protection even if some form of damage occurs during the operational life, such as by floating debris or accidental impact. The protection mechanism offered by TSA coating with an exposed steel surface in simulated marine conditions has been explained from several investigations [6,10,11]. The process involves multiple and simultaneous reactions however, the protection can be attributed mainly to two reasons: (i) When aluminium and steel are in electrical contact, a galvanic couple is established between these metals (TSA as anode and exposed steel as cathode) hence, TSA provides cathodic protection by acting as a sacrificial anode, and (ii) the formation of deposits on steel reduces the cathodic area, and the diffusion of dissolved O 2 is hindered [12].…”

“…However, the quantification of the damage level that TSA can tolerate is not fully explored. While long term tests conclude that TSA provides protection when the defect surface is between 1% and 5% [6,10,14], there are limited data on damage tolerance in the presence of a large cathode.…”