Corrosion resistance of W and Ta based amorphous coatings in a geothermal steam environment – A combinatorial study

Highlights

• WSi and TaSi are amorphous in a large composition range when prepared by sputtering.

• TaSi amorphous coatings are highly corrosion resistant in geothermal steam.

• WSi amorphous coatings undergo severe corrosion in geothermal steam.

• Combinatorial methods are extremely efficient in screening for corrosion resistance.

Abstract

Amorphous films of WSi and TaSi have been deposited by DC magnetron sputtering in order to determine their corrosion resistance in geothermal steam. A combinatorial approach is employed whereby samples with composition gradients are deposited and analysed for amorphicity, temperature stability and corrosion resistance. WSi and TaSi form amorphous metal alloys over the composition range 4–60 at.% W and 12–80 at.% Ta, respectively. The amorphous structure of both alloys is stable up to temperatures well exceeding those encountered in a geothermal power generation environment. W-based amorphous alloys undergo severe corrosion when subjected to 30 days in geothermal steam whereas Ta-based amorphous alloys are not significantly affected by the same conditions. This shows that combinatorial methods can be employed to screen efficiently corrosion resistant materials and that Ta-based amorphous alloys are promising for applications in geothermal power generation.

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.