Downhole pipes utilised for geothermal systems can encounter corrosion when operating in high-temperature/low-pH environments. Potential geothermal sources existing in Singapore and Japan contain acidic geothermal fluids mixed with sulphuric acid (H2SO4) stemming from the mixing of underground waters and magmatic gases. Present knowledge gap showcases minimal understanding towards the suitability of materials in H2SO4 geothermal surroundings particularly in the pH 2 to pH 4 range.
This thesis improves the knowledge gap by contributing in research towards identifying the corrosion resistance of various materials exposed to H2SO4 geothermal environments. Low-pH/high-temperature experiments were conducted with a tantalum-lined autoclave in pH 2, 3 and 4 H2SO4 geothermal solutions and at 80 °C, 120 °C and 150 °C temperatures. Corrosion behaviours of N80 carbon steel, 13 Cr stainless steel, 22 Cr duplex steel, Inconel 625 and Grade 2 Titanium were evaluated with in-situ electrochemical measurements and surface analytical techniques.
The N80 carbon steel encountered very high general and localised corrosion rates in the pH 2, 3 and 4 H2SO4 solutions encountering almost 100 mm/year corrosion rates in the most extreme environment of pH 2 at 150 °C. Alternatively, corrosion rates reduced significantly with the 13 Cr and 22 Cr in the 150 °C conditions. Physical material degradation observed with white light interferometry and focused ion beam characterisation techniques highlighted large localised features on the N80 carbon steel and less corrosion effects on the 13 Cr and 22 Cr.
Amongst the most aggressive test geothermal environment of pH 2 and 150 °C Inconel 625 and Grade 2 Titanium also indicated excellent corrosion resistance, with Titanium experiencing the lowest corrosion rates of 0.01 mm/year and showing superior passive film characteristics compared to the different materials. A collection of results was incorporated to create material performance maps relating corrosion behaviour with respect to pH and temperature in order to support material selection within the energy sector.
