Degradation Behaviour of Metallic Materials in Geothermal Environments

Abstract

The corrosion behaviour of carbon steels exposed to the Sulphur containing environment is complicated, especially in geothermal environments. The preliminary tests show that the corrosion rates of carbon steels in geothermal areas such as Taupo Volcanic Zone, New Zealand are much higher than the corrosion in a clean air condition. We studied the morphology and microstructures of the corroded surfaces and corrosion products by using scanning electron microscopy (SEM/EDS) and X-ray diffraction (XRD). Climatic factors such as rainfalls, exposure season, sulphur (S) concentration and wind direction are the most important factors that influence the corrosion behavior of steel sheets and zinc coating. This research is to understand the corrosion mechanisms of common metals (mainly mild steel sheets and zinc coating layer in atmosphere polluted by sulphur containing species, including sulfur catalytic mechanism, hydrolysis mechanism and direct corrosion reaction mechanism. Other factors such as exposure duration, rainfall and the influence of chlorine ions under geothermal conditions are also discussed. The main results of this study include: (1) Sulfur plays a leading role in corrosion in the first three months, but the sulfur content is nonlinearly related to the corrosion rate. (2) The longer the corrosion time, the higher the corrosion penetration depth, but the sulfur content in the corrosion products does not increase. (3) Corrosion intensity is high in areas with high rainfall. (4) Pure zinc plate is more resistant to corrosion than carbon steel under the same sulfurcontaining wet conditions. This research aims to (1) improve our understanding of the degradation behaviours, kinetics and processes of steels when exposed to the areas with various geothermal influences; (2) help to develop methodologies to evaluate the short-term and long-term performance of typical metal parts in buildings, constructions and infrastructures in well-characterized geothermal environments in New Zealand and worldwide; and (3) seek affective methods to control these types corrosion.