Electrochemical monitoring of damaged thermally sprayed aluminium coatings on steel in a simulated marine immersion environment at different temperatures

摘要

In this paper the corrosion behaviour of damaged thermally sprayed coatings is presented. Commercially pure Al was arc-sprayed onto carbon steel substrates and exposed to synthetic seawater at two different temperatures (5°C and boiling seawater (~101.5°C)) for 5000h. Prior to exposure circular defects were generated in the coating to expose 4-5% area of the substrate. The samples were left at their free corrosion potential (vs saturated calomel electrode (SCE)) and the corrosion rate was monitored using linear polarisation resistance (LPR) technique for 2000h. Once the tests were completed the samples were sectioned and microstructural characterisation was carried out. The potential of the TSA-coated steel with a 4-5% holiday was found to decrease initially and then reach a higher value with time at both temperatures. At 5°C, the potential decreased from -820 mV (SCE) at the start to -1080 mV (SCE) after ~300h, reaching -970 mV (SCE) after 1200h. The minimum (-1200mV SCE) was reached earlier (~16h) in boiling seawater, finally reaching values between -800mV (SCE) and -840mV (SCE) after 640h. Microstructural examination revealed a thin layer of calcareous deposit on the exposed steel surface. This calcareous deposit had two-layer structure with Mg(OH)_2 close to the substrate and CaCC>3 away from it at 5°C, and comprised of Mg(OH)_2 only in boiling seawater. The layer formed in situ seems to have provided a barrier and protected the steel from further corrosion giving low corrosion rates of 4-5um/y at 5°C and 6-15um/y in boiling seawater after 2000h. Thus, Al coating is able to protect steel substrates at extreme temperatures even when defects are present.