Experimental CaCO3 scaling of different substrate materials: Unravelling the effects of temperature, corrosion, crystal nucleation and growth

Thermal water is increasingly used for heat and electric
power production and provides base-load capable renewable
and virtually unlimited geothermal energy. The generally
highly mineralized deep thermal water can, however, induce
the formation of mineral precipitates along the water circuit
of geothermal power plants, which can considerably impact
on the plant’s efficiency. In order to explore the processes and
effects of temperature, corrosion, crystal nucleation and
growth on carbonate (CaCO3) scaling, we performed basic
laboratory experiments, in which we exposed different
substrate materials (e.g. corroded and uncorroded carbon steel
coupons) to synthetic geothermal solutions at temperatures
ranging from 30 to 90°C. The incorporation of divalent Mg
and Sr cations into the precipitated carbonate scales are
thereby investigated as environmental proxies. Fluid
chemistry and solid phase analysis (e.g. XRD, FT-IR, SEM)
complement our approach.
The experiments, based on solid and liquid phase
analysis, as well as variable Mg/Ca and Sr/Ca ratios, revealed
a strongly temperature-dependent precipitation behaviour of
different CaCO3 polymorphs (calcite, aragonite, vaterite) also
depending on the particular substrate being present. At lower
and intermediate temperatures, stainless steel and polyamide
substrates seem to favour calcite formation, whereas the
carbon steel substrate supports aragonite formation. Vaterite
formation is clearly promoted by polyamide substrates.
Contrary to observed wall crystallization on substrates,
homogenous (particulate) nucleation of aragonite further
occurs from aqueous solution. Importantly, the different
polymorphs and crystallization modes result in a strong effect
on the actual scale material characteristics, e.g. density,
mechanical and chemical resistance. Our experimental results
clearly indicate that CaCO3 crystal nucleation and growth, as
well as element fractionation strongly depend on the materials
used in geothermal applications.