Thermally sprayed aluminum (TSA) has been used to protect offshore structures for many years at ambient temperatures. However, the melting point of aluminum, 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 is known regarding the protection mechanism offered by TSA at elevated temperatures. This paper reports micro-scale surface characterization data on damaged TSA-coated carbon steel exposed to boiling synthetic seawater. The corrosion rate, calculated using linear polarization 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 characterization of the calcareous was also carried out and the kinetic information regarding the rate of formation of calcareous deposits is also presented.

Corrosion rate determination is often based on linear polarization (LPR) technique, carried out on small specimens in a controlled environment. The current is measured when the specimen is polarized away from its corrosion potential and the corrosion current gives the corrosion rate using Faraday's law. An important parameter in this calculation is the specimen area exposed. When testing rough, porous specimens such as thermally sprayed aluminum (TSA), the geometrical surface area does not represent the real surface area and hence the corrosion rate is not the actual corrosion rate. To measure the actual corrosion rate aluminum was thermally sprayed onto glass and the surface was characterized using a confocal microscope. These specimens were exposed to seawater at 25, 50 and 100°C, and the linear polarization resistance was determined using a potentiostat at room temperature. This was converted to a corrosion rate based on geometric and the actual surface area.