Corrosion resistant alloys for geothermal applications
Geothermal energy is heat energy generated and stored in the Earth. Temperatures at the core–mantle boundary may reach over 4000 °C (7,200 °F). Geothermal fluid is a naturally occurring mineralised mixture of pressurised water and steam heated underground to between 200–325degC. The steam and hot water are drawn up from a geothermal field by production wells from depths of up to three kilometres.
Worldwide, 11,700MW of geothermal power is online, plus more than 28GW of geothermal heating capacity is installed for district heating, space heating, spas, industrial processes, desalination and agricultural applications. Geothermal power is cost-effective, reliable, sustainable, and environmentally friendly, but has historically been limited to areas near tectonic plate boundaries. The Earth’s geothermal resources are theoretically more than adequate to supply humanity’s energy needs, but only a very small fraction may be profitably exploited due to the high costs of drilling and exploration for deep resources.
Due to the variability of fluid properties across different locations, an understanding of local conditions is needed in order to safely specify materials. Temperatures, fluid compositions and flow conditions of the geothermal resource can all vary significantly and create differing levels of corrosivity. Precautions have to be taken in order to utilise metals, as dissolved CO2, H2S, NH3 and chloride ions may lead to significant corrosion.
Geothermal plant technologies:
a) Dry steam plants
Such plants are relatively rare as they are used where reservoirs liberate predominantly vapour rather than water, from drilled wells up to 10,000ft / 3km deep. Natural steam comes to the surface to power turbine generators directly, allowing a relatively simple plant design. The depleted steam turns to condensed water and can be used in the plant’s cooling system, before being re-injected back into the well.
b) Flash steam plants
Here, water is sent through an injection well to the geothermal source, where it is heated to above 360 degrees F (182 C°). A production well carries the hot water to the surface, where it is sprayed into a tank, causing some of the water to rapidly vaporise, or “flash.” The vapour drives a turbine, which powers a generator that creates electricity. Excess liquid in the first tank can be flashed in a second tank to recover further energy in the same cycle. The resulting water then is sent back down into the injection well to repeat the process.
c) Binary-cycle plants
These plants are called “binary” because they use both hot water from their geothermal sources and a fluid that has a much lower boiling point than water. This allows operators to bring the water -which is not hot enough to flash on its own – up from the geothermal source from 600 to 2,000 feet (183m – 610m) under the surface, then use it to heat the tank containing the secondary fluid, which flashes and drives the plant’s turbine. By containing the hot water in a closed system that doesn’t flash, binary plants avoid the problems that flashing can bring— including calcium carbonate scaling and metal corrosion that result from impurities carried in the geothermal brine.
i) Carbon steels
Because of their low cost and wide availability, carbon steels are an obvious starting point for materials selection, particularly for thick-walled systems where a degree of material loss can be accommodated. However, in thinner-walled systems, any risk of local corrosion or cracking would pose an unacceptable risk.
For relatively benign projects, where the pH value is greater than 6 and the Cl– concentration is lower than 2%, a uniform corrosion rate between 1-10 mpy / 25-250µm per year may be observed. However, Cl– will increase the likelihood of pitting corrosion, along with and hydrogen sulphide. The presence of either compound will be exacerbated by low amounts of oxygen in solution, accelerating uniform corrosion and triggering accelerated pitting. High flow rates and solid particles in the fluids result in erosion corrosion, therefore the optimum flow rate for carbon steel materials is typically limited to 5-7 fps / 1-2m/s.
The use of protective linings is widespread to facilitate the use of carbon steels, particularly when needed in thicker sections. A wide variety of lining choices exist, from concrete, rubber and different polymers such as PTFE, PVC and FRP or weld overlay with a more corrosion-resistant alloy such as Alloy 625.
ii) Stainless steels
The use of stainless steels over carbon steels obviously decreases the probability of uniform corrosion formation in geothermal fluid environment. However, more serious corrosion problems may still occur, such as pitting, stress corrosion cracking and erosion corrosion dependent upon the conditions. An increase in the Cl– concentration results in an increase in the effect of local corrosion, whilst rising temperatures increases the pitting potential. The resistance of stainless steel against pitting and cracking corrosion depends on its Cr and Mo content. Therefore, more highly-alloyed grades such as Alloy 2205, Alloy 32750, Alloy 32760 and Ferralium® 255 have found favour over lower-alloyed grades such as Alloy 316, where increasing alloy content infers increasing pitting resistance. It is common for several grades to be used in combination across a project, as the temperature reduces through the process.
iii) Nickel Alloys
More corrosive or higher temperature geothermal fluids often call for the frequent usage of nickel alloys over and above stainless steels, despite the increased alloy costs. For high temperature geothermal fluids, super duplex stainless steels would not be suitable for use above 250degC, therefore Ni-Cr-Mo alloys such as Alloy 625 and Hastelloy C-256 are used. They provide very strong corrosion resistance with a Pitting Resistance Equivalent number (PREN) of 50 and 70 respectively, compared with >40 for super duplex stainless steels.
Despite the expense, titanium and titanium alloys have been successfully deployed in the most aggressive areas within geothermal plants. Corrosion rates typically lower than 0.3 mpy / 7µm per year have been observed experimentally, with limited increase in corrosion rate with increasing temperature, Cl– concentration or flow rate. Titanium is also generally resistant to cavitation and impact damage.
Pitting may be observed where Cl- concentration >10%, and here various titanium alloys may offer stronger performance, such as Gr29 (Ti-6 Al-4 V0.1 Ru). The service life of this material is above 15 years and it does not have renewable costs compared to low alloy steel and in addition they do not form corrosion and accumulation products containing radioactive and heavy metals.
Additionally, titanium is the preferred selection where there is oxygen entrainment, as fluids containing oxygen and hot Cl– ions can result in component failures in both stainless steels and nickel alloys. In such conditions, titanium alloys are used in wellhead valves, pressure gauges, pipes and blow-out preventers.