Passivation of Metal Surfaces Against Corrosion by Silica Scaling

Abstract

In high-enthalpy geothermal systems the precipitation of amorphous silica (SiO2) is the most common scaling problem. Together with corrosion, silica scaling affects the efficiency of geothermal power plants through increased maintenance costs and reduced equipment lifetimes. Despite their crucial importance for the development of better mitigation strategies, the relationship and inter-dependencies between scaling and corrosion are poorly understood. Here we present data from a time resolved study where the development of both silica precipitates and corrosion features on S316 stainless plates and S275 carbon steel coupons immersed in geothermal waters at the Hellisheiði power plant (SW-Iceland) were followed for between 1 day and 10 weeks. We chose four locations that differ with respect to fluid temperatures (55 to 120 C) and silica concentration (550 to 800 mg/L) and characterized the morphologies and textures of the precipitating silica and the corrosion products by high resolution imaging, while the solid precipitates and corrosion products were identified by X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). Silica precipitates were observed on all plates as uneven layers and rounded ‘bumps’ covering the plates completely, even after 1 day. With time and regardless of prevailing conditions, the size of the individual ‘bumps’ increased. Where silica precipitation was high, the carbon steel revealed only minor signs of corrosions, while where silica precipitation was minor ubiquitous corrosion was observed. The corrosion products were composed of iron sulfides (mackinawite and greigite), as well as their oxidized products (elemental sulfur, goethite and hematite). We show that in environments where silica precipitation is slow, corrosion dominates, while once an initial amorphous silica layer formed fast this will protect the underlying carbon steel from further reaction with the geothermal fluids and therefore from continuing corrosion. Thus, silica scaling can indeed act as a passivating agent for steel surfaces prone to corrosion.