Mechanism of Calcareous Deposit formation on TSA-coated Steel Structures at Elevated Temperatures

Thermally sprayed aluminium (TSA) has been used to protect offshore structures for many years at ambient temperatures. However, the melting point of aluminium, and the limited corrosion rate data indicate that they are likely to perform well in high temperature seawater as they tend to form calcareous deposits. Nonetheless, not much in known regarding the protection mechanism offered by TSA to at elevated temperatures. The paper reports micro-scale surface characterisation data on damaged TSA-coated carbon steel exposed to boiling synthetic seawater. The corrosion rate, calculated using linear polarisation resistance (LPR) technique of TSA was ~10µm/year.  This is very similar to the TSA corrosion rate observed at room temperature. The formation of a calcareous deposit which covers the exposed steel seems to reduce the corrosion rate. Detailed microstructural characterisation of the calcareous was also carried out and the kinetic information regarding the rate of formation of calcareous deposits is also presented.