Thermal Sprayed Aluminium for Subsea Heat Exchanger Surfaces: Effect of Temperature on Protection Current Requirement and Calcareous Development
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More and more equipment in the Oil and Gas Industry are being placed Subsea. Thisincludes subsea coolers with high internal temperatures. With high temperatures comewith challenges within material selection and corrosion. Traditional material choices like carbon steel and organic coatings with cathodic protection (CP) is not an option for subsea coolers. This is due to insulating properties to the organic coatings and the dense calcareous deposits which form on the surface of the steel.Thermally sprayed aluminium (TSA) is known to have good corrosion resistance inseawater, and to have a small current demand under CP, which makes the probability ofprecipitating calcareous layer on the surface low. The surface topography for thermallysprayed aluminium is rough, which increases the total surface area for heat transfer. The rough surface also produces turbulent flow close to the surface, which may increase the heat transfer. The combination of these properties makes thermally sprayed aluminium a attractive candidate as a coating for subsea coolers.The drawback of thermally sprayed aluminium is that there is little empirical data onits behaviour at high temperatures, especially when in contact with CP. Therefore, the purpose of this Master Thesis is to provided more documentation on the subject of the properties of thermally sprayed aluminium at high surface temperatures in seawater, with and without CP.For providing more documentation a literature study and experiments on TSA willbe carried out. The principle behind the experimental part of this project is very simple. Pipes of UNS S31245 stainless steel with a TSA coating were internally heated to different temperatures and immersed in slow flowing seawater, to simulate the conditions a subsea cooler would be subjected to. The purpose of the experiments was to find the corrosion potential, the current density requirement for TSA connected to an CP system and the corrosion rate of the freely corroding TSA. After exposure the samples was analysed to quantify the amount, if any, calcareous deposits form on TSA, and to see what effect the exposure had on the thermal conductivity of the samples.This thesis discovered that the corrosion rate of TSA increases with temperature. Initially the corrosion rate of TSA is quite high, but it quickly decreases for all temperatures. The corrosion rate for the 90 C internal temperature was initially 50 micrometers per year but decreased to 8 micrometers per year after 65 days.The current density requirement for TSA is very low compared to steel, with a currentdeensity of 3-5 mA/m2 obtained in this thesis. Temperature increases the current demandslightly. Based on both the experiments and the literature it is safe to say that using theDNV recommended practice for current density requirement for TSA is acceptable, eventhough the recommended practice may be conservative.Calcareous deposits form on TSA at all temperatures. For both the samples connectedto an anode and the freely corroding samples, however, not as a continuous protectivelayer that precipitates on steel, but mostly as thin layers in small areas around intermetallic particles. The temperature affects the amount and type of calcareous deposits which form on TSA. At high temperatures the calcareous deposits mainly consist of Mg(OH)2 and at low temperature the calcareous deposits consist of both CaCO3 and Mg(OH)2.Thermal conductivity measurements shows that the TSA coating and calcareous depositsare negligible when it comes to the overall thermal conductivity of the pipe. The combination of negligible effect on the thermal conductivity, low corrosion rate, and small current density requirement makes TSA a solid choice for corrosion protection of subsea heat exchangers.