Abstract
Over a dozen commercial and new polymer, polymer-like carbon (PLC), and diamond-like carbon (DLC) coatings originally developed for the Oil & Gas industry show promising characteristics against geothermal carbonate scales. This paper discusses the major findings of both (a) calcium carbonate scale tests and (b) corrosion tests at two temperatures and with multiple brines, including a carbonate-forming brine and a high-chloride acidic brine. Coating performance Vs. characteristics such as thickness, surface roughness, and water contact angles, is also investigated. In general, thin coatings (less than 5 μm), whether polymer, PLC, or DLC, tend to be unfit for service due to an inadequate gas permeation resistance, even when the coatings are applied onto corrosion-resistant substrates such as Alloy 718. In contrast, the thicker polymer and DLC coatings (e.g., ~15 μm) are found to be more fit-for-service, and more appealing when also strongly hydrophobic (i.e., with high water contact angles). This paper also shows that hydrophobicity, surface roughness, and a low carbonate scale deposition behavior are not noticeably correlated. Importantly, when scale formation is accompanied by strong carbon-dioxide degassing (e.g., 150°C), the more gas-impervious coatings have been found to resist better blistering and gas-induced removal or delamination.
