Poly(phenylenesulfide)-based coatings for carbon steel heat exchanger tubes in geothermal environments

To inhibit corrosion and fouling by calcium silicate and silica scales of the carbon steel heat exchanger tubes in geothermal power plants operating at brine temperature of 160°C, their internal surfaces were lined with high-temperature performance poly(phenylenesulfide) (PPS)-based coating systems. The systems included the PPS containing polytetrafluoroethylene (PTFE) as an anti-oxidant additive, silicon carbide (SiC) as a thermally conductive filler, and aluminum oxide-rich calcium aluminate (ACA) as an abrasive wear resistant filler. Then, the lined tubes underwent an eleven-month-long field exposure at the site of power plant. The results from the post-test analyses of the exposed liners revealed that these PPS coating systems had an excellent thermal stability, and satisfyingly withstood this brine temperature, and also greatly resisted the permeation of brine, demonstrating that they adequately protect the tubes against corrosion in a wet, harsh geothermal environment. Furthermore, modifying the surfaces of the PPS top layer with PTFE, retarded the hydrothermal oxidation of the liner, thereby reducing significantly the rate of scale deposition and creating surfaces unsusceptible to reactions with scales. Thus, all the scales deposited on the liner's surfaces were readily scoured off by hydroblasting at a very low pressure. By contrast, although the stainless steel tubes had a great protection against corrosion, the formation of passive oxide layers at their outermost surface sites was detrimental in that they became more susceptible to scale deposition and developed strong adherence to the scales. As a result, the high-pressure hydroblasting was required to remove this scale adhering to the tubes' surfaces.