Corrosion resistance of W and Ta based amorphous coatings in a geothermal steam environment – A combinatorial study
Introduction
Geothermal fluids are increasingly being used for electricity generation and heating (Bertani, 2016) as they are generally considered to be a sustainable and renewable source of energy. However, geothermal environments are corrosive to metals due to the high temperatures and pressures involved and the aggressive chemical composition of the steam, which can result in a high maintenance cost of geothermal installations. Corrosive substances commonly found in geothermal steam include H2S, NH3, CO2 and Cl−, but the chemical composition is highly variable between geothermal sites (Karlsdóttir, 2012, Zarrouk and Moon, 2014) which can result in unexpected failure of tried and tested components when installed at a new site. It is therefore important to develop new materials which are corrosion resistant and efficient methods to tailor them to specific environments.
Amorphous metals are highly promising in terms of corrosion resistance due to their atomic structure (Bakare et al., 2012, Scully et al., 2011, Wang et al., 2012). Most metals have a polycrystalline atomic arrangement which means that they are composed of crystalline grains separated by grain boundaries. The grain boundaries are under-dense regions where even the chemical composition can differ from the bulk. Grain boundaries are often the preferred locations for the onset of corrosion and can be pathways for the diffusion of impurities (Ohring, 2001, Roberge, 2008). Amorphous materials on the other hand have no long-range periodic atomic arrangement and no grain boundaries (Choi et al., 2005). As a result these preferred corrosion sites are removed resulting in enhanced corrosion resistance. Furthermore, amorphous materials are highly uniform structurally and chemically and therefore have fewer defects than crystalline materials. This homogeneity means that the passivating layer which forms on the surface of corrosion resistant metals (usually an oxide) is also free from defects. Since amorphous materials are not formed in a thermodynamic equilibrium they can be oversaturated by the element which gives good corrosion resistance and the passivation layer will as a result also be oversaturated by this element (Hashimoto, 2011).
The most well known amorphous materials are oxides such as SiO2 (silica glass) and Al2O3 (alumina) which have good corrosion resistance and high hardness. However, oxides such as these tend to be brittle and can therefore crack during thermal cycling. Amorphous metals on the other hand are more ductile which makes them more suitable for high or variable temperature environments (Schuh et al., 2007, Hufnagel et al., 2016) but high cooling rates and very specific material combinations are typically required to stabilize amorphous metallic phases (Inoue and Takeuchi, 2011). Here, we explore the corrosion resistance of W- and Ta-based amorphous metal films in a geothermal power facility, using combinatorial material synthesis (Ding et al., 2014, Green et al., 2013, Frisk et al., 2016). W and Ta are refractory metals which are resistant to heat and wear, as well as being corrosion resistant (Lichti and Wong, 2014, Cardonne et al., 1995, Erik Lassner, 1999), which are the key factors for materials in geothermal installations. We show that alloying with Si is effective in producing an amorphous structure and that the combinatorial approach allows a large range of compositions to be screened in terms of amorphicity, temperature stability and corrosion resistance with high efficiency.
Section snippets
Experimental methods
WSi and TaSi films were deposited by combinatorial DC magnetron sputtering. An ultrahigh vacuum chamber was used, with a base pressure of 1 × 10−8 mbar, and the sputtering gas was 99.999% pure Ar at 5 × 10−3 mbar. The films were deposited on thin strips of either naturally oxidized single crystal Si(100) or 304 stainless steel. In both cases the film thickness was approximately 1 μm. The deposition was performed at room temperature (no substrate heating) and without applying a substrate bias,
Results and discussion
A selection of XRD measurements on the as-grown samples is shown in Fig. 2 for a range of compositions. The broad peak centred at 2θ values of 37–40° is indicative of an x-ray amorphous structure with no long-range atomic order (Magnus et al., 2013). The position of this peak can be interpreted as the average atomic spacing and together with its full width at half maximum we can determine the coherence length σ (in crystalline materials often referred to as the grain size), using the Scherrer
Conclusions
WSi and TaSi binary alloys with a large range of compositions have been screened for temperature stability and corrosion resistance in a geothermal environment using combinatorial material synthesis. Both materials systems form amorphous alloys over a large range of compositions, when prepared by DC magnetron sputtering at room temperature. The temperature stability is composition dependent but the most stable phases are amorphous when annealed in air up to approximately 500 °C in the case of
Acknowledgements
This work was funded by the Icelandic Centre for Research, grant no. 163799-0611. A.S.I, U.B.A and F.M. are shareholders in Grein Research. The authors wish to thank HS Orka for access to the Reykjanes Geothermal Power Plant corrosion testing facility, and G. H. Gudfinnsson for assistance with the EPMA.
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