The development of novel and cost-effective corrosion-resistant coatings for high-temperature geothermal applications is being funded as a project with Horizon 2020—a European Union Research and Innovation program with nearly €80 billion of funding available over a span of seven years (2014 to 2020). TWI (Cambridge, United Kingdom), an independent research and technology organization with expertise in materials joining and engineering processes as applied in industry, is coordinating the recently awarded project, which is dubbed Geo-Coat.
According to TWI, the Geo-Coat target is to design new high-performance coatings to resist specified threats, or combinations of threats, as experimentally derived at key failure points within geothermal power plants. The coatings would be applied only to affected components. The specialized corrosion and erosion-resistant coatings will be based on selected high-entropy alloys (alloys comprising equal or almost equal amounts of five or more metals) and ceramic/metal mixtures. These coatings will be applied through thermal powder coating techniques (primarily high-velocity forms of high-velocity oxygen fuel/laser cladding) specifically developed to provide the bond strength, hardness, and density required for these challenging applications. Because of its low cost, good availability and weldability, carbon steel (CS) is the desired structural material for geothermal applications.
Geothermal energy, derived from the Earth’s heat, is one cornerstone for developing next-generation low-carbon energy technologies for renewable electricity, and drilling deeper wells to obtain higher enthalpy fluid is a way to increase the output of geothermal systems. TWI states, however, that the integrity of geothermal power plant components (e.g., liners and well casings, well heads, turbines, pumps, valves, heat exchangers, pipes, separators, condensers, hydrogen sulfide [H2S] abatement systems, etc.) can be at risk from geothermal sources because of the very aggressive nature of these natural environments, which experience high temperature and pressure conditions as well as corrosive salts. Additionally, geofluids become even more aggressive as wells move to deeper geothermal resources, and the corrosion, erosion, and scaling effects are increased.
CS is known to have poor resistance to corrosion and erosion experienced in various parts of the geothermal well system due to hot brine, low pH treatments to reduce silica scale, and pipe bends where erosive forces are highest. The main elements that cause corrosion include dissolved carbon dioxide (CO2), H2S, and ammonia (NH3) gases, as well as sulfate and chloride ions, which are heavily dependent on operational factors such as pressure, temperature, flow rate, oxygen concentration, suspended solids, flow regime (single or multi-phase, stratified, annular, bubbly, slug, etc.) and the pH level of the geothermal fluid.
Additionally, the high flow rates used to drive turbines, coupled with the presence of water, suspended solids, or precipitates caused by pH and pressure changes, can lead to erosive conditions that aggravate the corrosion effects. The material surfaces can be degraded even further by harsh physical or chemical treatments designed to address scaling formed from precipitates such as silica, sulfides and calcite. Increased CS wall thickness allowance, corrosion-resistant alloys, and titanium are also considered as solutions to long-term integrity issues caused by the corrosive nature of the geothermal fluids.
The project will last for three years, with the Geo-Coat partners coming from the United Kingdom, Iceland, Romania, and Norway.
Source: TWI, www.twi-global.com.