Thermally - sprayed coatings for corrosion protection of steel operating in seawater environments

The corrosion behaviour of thermally-sprayed sacrificial coatings was investigated under different operating conditions which simulated those of an offshore structure in active service. Electrochemical measurements, combined with several characterisation techniques, such as SEM, EDX and Raman spectroscopy were used to assess the performance of the coatings during exposures to various conditions of simulated full and alternating seawater immersion, splash zone conditions and atmospheric corrosion conditions. The effect of temperature and graphene-containing sealants were also studied under artificial seawater immersion on aluminium-coated steel samples containing defects. The results of immersion tests of thermally sprayed aluminium (TSA) coatings with defects revealed significantly worse performance of the coatings exposed to cold temperatures (4C) compared with other temperatures tested. A significant amount of rust was observed in the defect region and required polarisation (-0.8 V vs Ag/AgCl) was not provided by the TSA. This was related to low solubility of aluminium and decreased precipitation of calcareous deposits.The application of graphene - containing sealants on TSA coatings turned out to be effective in decreasing the contact between the electrolyte and the steel substrate at warm temperatures (25C) under full immersion conditions. However, the addition of the sealants was found to be detrimental to the performance of TSA coatings operating at cold temperatures (4C) under full immersion conditions as well as warm temperatures (25C) under alternating immersion conditions. The results of atmospheric corrosion tests showed that cathodic and anodic regions were not easily distinguishable on TSA coatings. Droplets did not spread, and sulphur-containing corrosion products were detected after the test. This highlights the importance of using seawater, and not NaCl solutions, as a corrosive medium simulating marine environment