Abstract
Steel and its alloys have been studied extensively where the inclusion of Ni and Cr have reportedly shown good corrosion resistance in high temperatures. Furthermore, Mo is known to provide pitting resistance. Geothermal environment has several corrosion initiators such as S, Cl, high temperature and pressure. In an S rich environment, the protective mechanism of the alloying elements differs. Mo is known to adsorb to S, reducing the diffusivity of the S into the bulk material, while Cr and Ni have antagonistic effects in presence of S. This paper seeks to confirm the importance of alloying elements in geothermal environment.
In the present work, EN 10028-2, S17400, S30403 and S31254 steel coupons were exposed in a geothermal wellhead at Hellisheiồi powerplant, Iceland, for 3 months. Temperature and pressure in the wellhead were 198 °C and 13,9 Bar gauge, respectively. The corrosion rate during exposure was evaluated using weight loss measurements and microstructural analysis. The results demonstrated that non-alloyed steel had by far the highest material loss rate. These findings indicate a linear correlation between Ni content in alloy and a low corrosion rate in an S rich environment.
