Hydrothermal oxidation of fluoroelastomer bearings after a year-long exposure to geothermal environments

Abstract

In previous work, we investigated the mechanisms of hydrothermal oxidation of fluorinated ethylenepropylene (fluoroelastomer, Viton R ©) that was used for four months as the bearing elastomer in down-hole pumps extracting the energy resource from hot geothermal brine reservoirs (∼150 ◦C) [1]. As a result, the polymeric fluorocarbon structure in the superficial layers of the Viton elastomers was oxidized in the following sequence: fluorocabon → fluorocarbonyl → fluorocarboxylic acid → fluorocarboxylate complexes. Oxidation not only converted the topographically smooth surface of the elastomers into a roughly textured one, but also resulted in considerable loss of their elastic and resilient properties which are necessary in bearing elastomers. One important question arising from that information was how fast and deeply the oxidation-caused degradation of bearings occurs during exposure in a wet, harsh geothermal environment. If the kinetics of the oxidation degradation is defined, we then may be able to predict the useful lifetime of the Viton elastomers. Thus, our emphasis in this letter report was directed toward estimating the degree of hydrothermal oxidation of Viton bearing that was used for a year in the Mammoth Pacific geothermal power station. In this experimental work, the surfaces and cross-sectional areas of the 1year-old elastomers were explored by scanning electron microscopy (SEM) coupled with the energy-dispersive X-ray (EDX) spectrum. Quantitative data on the respective chemical elements were obtained by comparing the EDX k-ratios for the elements in the samples. The kratio of an element is obtained by dividing the net X-ray peak counts of that element by its standard X-ray counts [2, 3]. Computer programs were prepared to calculate theoretical k-ratios and to give us the atomic fraction of each element present in the bearings. Fig. 1 shows the SEM microphotograph together with the EDX spectrum for the surfaces of the bearing. Its surface texture closely resembles that observed in the 4mo.-old one (not shown); namely, it had a rough surface, covered by numerous fragments sized between 5 and 100 μm. Fig. 1 also shows the EDX atomic fractions of these fragments. As is seen, the principal elements