Economic Efficiency of High-Entropy Alloy Corrosion-Resistant Coatings Designed for Geothermal Turbine Blades: A Case Study

Abstract

The aim of this paper is to establish the cost-effectiveness of high-entropy alloy coatings, using the electrospark deposition technique, designed for a geothermal turbine blade’s leading edge. The deposition of materials with high resistance to corrosion and erosion aims to increase the blade’s service life, reduce maintenance costs and improve production efficiency. According to our previous research on the CoCrFeNiMo_x high-entropy alloy system, the results showed a high corrosion resistance when in bulk or as a coating, and when tested in geothermal steam and in a saline solution. Based on the results, the high-entropy alloy was subjected to further analyses. The paper focused on two aspects of the research. The first direction was to explore the possibility of obtaining an effective, protective high-entropy alloy layer by the electrospark deposition method. To this end, various tests were performed to demonstrate that the new material possesses superior properties and is suitable for the geothermal environment’s demands. The second direction was to calculate the economic efficiency of coating the areas intensely subjected to wear, based on reports published by the geothermal power plants’ representatives. The final costs were compared with the commercially available equipment parts, and also with the general maintenance costs. From the calculation of the cost efficiency for the CoCrFeNiMo_0.85 high-entropy alloy, it resulted that the deposition method and the properties of the material are suitable for the operating conditions, representing an efficient and easy to apply solution to reduce maintenance costs in the geothermal industry.

1. Introduction

Corrosion represents a major concern in numerous industries (maritime, geothermal, petrochemical, etc.) due to the potential damage that can be caused by it [1,2,3]. The need for the constant replacing or repairing of various pieces of equipment or parts, leads to high maintenance costs. Research is focused on developing new materials, with improved properties, which can serve as options as commercially available alternatives.

One of the main concerns for the geothermal industry is represented by the erosion-corrosion effect that affects the in-work turbine blades, which are in direct contact with the geothermal steam. Due to their specific effects and superior properties, the use of high-entropy alloys is desired in multiple applications, where corrosion represents the main issue. The literature [4,5,6,7] presents these alloys as a viable solution for different applications or domains and they represent the subject of this paper.

According to our recent research on corrosive-resistant materials for the geothermal environment [8,9,10], promising results were obtained for high-entropy alloys from the CoCrFeNiMo system, produced from a liquid [11] and solid state [12]. The alloys were tested in corrosive media and the results published [12,13].

For this application, the CoCrFeNiMo_0.85 high-entropy alloy was analyzed in terms of economic efficiency, as a possible solution to improve the in-work geothermal power plant equipment. The initial powder’s composition was decided based on the theoretical calculation of the valence electron concentration (VEC). The results indicated that, for a value of 7.87, a mixture of both FCC and BCC phases were present and were later on confirmed [10]. According to G. Sheng et al. [14], the mixture of phases indicates there are properties such as high ductility and strength in the final alloy.

The coatings were subjected to corrosion testing and the results showed that after performing the testing in a saline solution (3.5 wt% NaCl), a corrosion rate of 0.00016 mm/year resulted. Good adhesion was observed from the pull-off test results where no cohesive or adhesive types of failure occurred. [8].

In an article published by Almar Gunnarsson et al. [15,16], the steam effect on the geothermal turbines was discussed, and it was mentioned that for a period of 15–20 years, the maintenance costs could reach USD 20 mil [17]. This case was studied for a geothermal power plant located in Hellisheiði, Iceland.

From an economical point of view, the main costs are represented by the turbine transportation from the power plant to the producer or service facility and by replacing the turbine blades when the damage cannot be undone by refurbishing.

The electrospark deposition method could represent a viable solution for the highly corrosive media due to the efficiency of the thin high-entropy layers. Advantages such as local repairs are also taken into consideration, where parts affected by the erosion effect could be adjusted.

By taking into consideration the presented factors, for this paper, economic calculations were performed in order to establish the cost efficiency of CoCrFeNiMo_x high-entropy alloy coatings by electrospark deposition.

2. Materials and Methods

In order to produce the CoCrFeNiMo_0.85 high-entropy coatings, the consolidated sample was machined into electrodes as presented in our previous research [12] and the coating process was conducted by using Spark Depo Model 300 (Fujieda-shi, Shizuoka-ken, Japan) electrospark deposition equipment with a miniature applicator. The shape and size of the electrodes was decided accordingly to the equipment applicator.

In Figure 1, the deposition method used is presented. This was manual process, where multiple passes were deposited in order to obtain a uniform coating over the entire surface of the subjected metallic material. The aim was to obtain a coating deposited by electrospark with a thickness between 25 μm and 100 μm due to the reported embrittlement when the thickness exceeds 100 μm and with no major improvements [18] when the thickness is under 25 μm. In order to minimize oxidation, the deposition process was carried out under a high purity argon atmosphere shield (more than 99.999%, according to the manufacturer).

After the coating was obtained, it was microstructurally analyzed with SEM Quanta Inspect S, Fei, Holland, type: FP201711 and EDAX Apex equipment. Mapping and topography analyses were performed with the same equipment and the results are presented. The samples were cleaned and decontaminated with high purity alcohol (99.99%) and dried with hot airflow in order to remove the impurities.

The roughness of the coating was measured with a Pocket Surf Namicon, nondestructive testing, Varese, Italy. Measurements were performed both longitudinally and transversally for a minimum of 10 measurements for each direction and 5 minimum for each area.

The economic efficiency calculation of the CoCrFeNiMo_0.85 high-entropy alloy coating deposited by electrospark method was performed based on the literature reports regarding the corrosive–erosive effect of the geothermal steam and on the geothermal turbine blades’ market value.

3. Results

After the deposition process led to the desired high-entropy alloy coatings, the samples were analyzed, and the results are presented here.

The roughness test results are presented in

Table 1. Roughness measurement results for the CoCrFeNiMo_0.85 coating.

Sample Name	Longitudinal		Transversal
	Center (Ra (µm))	Edge (Ra (µm))	Center (Ra (µm))	Edge (Ra (µm))
HEA_M	5.96	4.09	5.28	4.96
	6.4	4.52	5.1	5.72
	5.84	5.15	4.68	4.95
	6.03	6.3	3.43	5.29
	6.03	5.17	4.61	6.1
Average (Ra (µm))	6.052	5.046	4.62	5.404

. The obtained values indicate a relatively uniform roughness in both the longitudinally- and transversally-measured directions.

Considering that the electrospark deposition is a manual process, the roughness of the coating was rather uniform, having a min. of 4.62 µm and a max. of 6.05 µm. The roughness did not influence the quality of the coating for this specific case.

The electrospark-deposited coating was analyzed, and the results are presented in Figure 2. From the microstructural analyses results obtained on the sample’s surface, a continuous deposition could be observed (Figure 2a), with homogenous plateaus (Figure 2b) where the surface topography presents the roughness degree. Due to the specific cooling conditions of the electrospark deposition process, some micro cracks and pores were observed on the surface of the deposited layers (Figure 2c), this phenomenon being often found in other fast solidification processes, such as laser deposition.

The dendritic structures from Figure 2d are specific for the high-entropy alloys, which confirm the presence of the HEA coating. Due to the dendritic structures’ reduced dimensions, the high-entropy alloy presents a high alloying degree and an improved homogeneity degree, where supplementary thermal treatments are not necessary.

In the analyses results from Figure 3, a homogeneous deposition can be observed, where the particles are distributed uniformly on the entire surface with no separation on components or compounds being noticed. Even though the process took place in an argon atmosphere, the oxygen was present due to the deposition method. The oxygen quantity, for this case, did not affect the coating functionality.

From the previous analyses performed on cross-sections taken from the coated areas, it was observed that the micro cracks and pores were present on the surface of the sample, with the structure not being affected [12].

4. Economic Efficiency of High-Entropy Coatings

According to Almar Gunnarsson et al. [15], the corrosive, erosive and abrasive effects of the geothermal steam were researched at Hellisheiði geothermal power plant in Iceland, where currently there are seven in-work geothermal turbines. Due to the very aggressive environment created by the geothermal steam chemical composition, temperature, pressure and abrasive particles [16], the maintenance costs for the geothermal turbine can go as high as USD 20 mil for an operating period of 15–20 years [17]. The main costs are represented by the turbine blades’ replacement and turbine transportation for refurbishing, but also the downtime necessary for the mentioned reasons [15], which can lead to a cost of approx. USD 190,000/turbine/year.

The main concerns regarding the high-entropy alloys are represented by the costs when comparing them with the commercial alloys and materials used currently for components or equipment parts. In order to reduce costs while benefiting from the superior properties of these materials, coating the in-work equipment was proposed as an alternative, viable solution. Due to the direct contact of the turbine blades with the geothermal steam, by depositing HEA on the leading edge, the corrosion–erosion effect could be reduced.

According to the data published by Yoshihiro Sakai et al. [19], the standard dimensions for the next-generation low-pressure geothermal steam turbine blades range from 348 mm to 789 mm for 50 Hz and from 290 mm to 665 mm for 60 Hz. There are usually 90 blades on the last row of the rotor, in the area of low-pressure steam, resulting in high replacement costs [20].

For replacing a carbon steel single turbine blade, the price is approx. USD 1.344 according to the India Mart website [21]. By considering the mentioned data, the cost calculation for coating a single blade was performed, where material and labor costs were included.

The raw, pure (approx. 99.5%) elemental metallic powder materials were purchased from Laboratorium^® Company, and the prices for the necessary quantity to produce 100 g of the high-entropy alloy material are presented in

Table 2. Costs for elemental metallic powder materials purchased from Laboratorium^®.

Pure Elemental Metallic Powder	Price (USD/100 g)	Total Price for CoCrFeNiMo0.85 (USD/100 g)
Cobalt	143.9	123.11
Chromium	322.66
Iron	21.36
Nickel	59.02
Molybdenum	100.73

.

The mechanical alloying processing cost for producing 100 g of the high-entropy alloy, processed in Fritch Pulverisette 6 Mono mill is presented in

Table 3. Elaboration costs for 100 g of HEA processed by mechanical alloying.

Process	Functioning Time (h)	Energy Consumption (kWh)	Total Consumption (kW)	Energy Costs (USD/kW)	PCA (USD)	Total Price (USD)
Mechanical Alloying	30	1	30	0.2	0.14	5.4

. To the processing costs, the price of the process control agent (PCA) was added. For milling 100 g of high-entropy alloy, 2 wt% N-Heptane was used.

After obtaining the alloyed powder, samples were consolidated by pressing and sintering. For the pressing process, the costs were low and they were added to the other costs section in the final calculation. The calculation of economic efficiency also included the material loss resulted from the ball milling process, which was estimated at 3%. For the sintering process, the costs are presented in

Table 4. Furnace sintering process costs for the HEA samples.

Process	Functioning Time (h)	Energy Consumption (kWh)	Total Consumption (kW)	Energy Costs (USD/kW)	Total Price (USD)
Furnace Sintering	6	3	18	0.2	3.64

.

The furnace sintering process was performed under an argon atmosphere, with a gas flow of 2 L/min, purchased from Linde^®, with a purity of 99.99% and a price of USD/L 0.01. The samples were sintered for 360 min in a sintering furnace, which resulted in a total argon cost of USD 7.42. After adding the costs of the argon to the sintering process costs, this resulted a total of USD 11.06 per batch of HEA samples.

After the samples were consolidated and mechanically processed into electrodes, multiple successive layers of the high-entropy alloy were deposited on the stainless steel substrate. The calculations were performed for the standard coating applicator, which is often used to cover large areas in a shorter time. Working time and related costs for coating an area of 100 × 100 × 0.05 mm with CoCrFeNiMo_0.85 HEA are shown in

Table 5. Electrospark deposition costs for 100 × 100 × 0.05 mm surface deposition.

Process	Functioning Time (h)	Energy Consumption (kWh)	Total Consumption (kW)	Energy Costs (USD/kW)	Total Price (USD)
Electrospark Deposition	2	1.6	3.2	0.2	0.65

.

To the cost of the deposition, the argon consumption cost was added, namely USD 1.24, resulting in the final cost of USD 1.79. The final cost to deposit an area of 100 × 100 × 0.05 mm with CoCrFeNiMo_0.85 HEA by the specified methods, which included the labor cost of USD 22.49 and other expenses of USD 11.24, was calculated with Equation (1).

Solid State Processing + Material Loss + Consolidation + Deposition + Labor + Other Costs = USD 55.7

(1)

For this case study, covering the leading edge of the geothermal turbine blades with a width of 20 mm along its entire length, as shown schematically (Figure 4), was taken into consideration.

The established length was represented by the arithmetic mean of the existing dimensions for 50 HZ turbines [18], and the result is presented in Equation (2).

L_m = (L₁ + L₂ + L₃ + L₄ + L₅ + L₆)/6 = 581 mm,

(2)

where L_m is the average length of the low pressure geothermal steam turbine blades.

The deposition area was calculated in Equation (3) for a length of 581 mm and a width of 20 mm.

S = L ∗ l = 581 mm ∗ 20 mm = 11,620 mm² = 116.2 cm²

(3)

The total cost of coating the leading edge of a geothermal turbine blade was calculated, and for a length of 581 mm this resulted in a total cost of USD 64.72.

5. Conclusions

Multiple successive layers of the obtained high-entropy alloy were applied in order to obtain the desired coating. Even though minor cracks and pores were observed on the surface, the coating had a compact and homogeneous structure with good adhesion. The surface defects were subjected to further testing in order to be reduced.

The obtained results of the roughness testing present a rather uniform roughness of the surface on the tested directions. Considering the final destination of the produced coating, the roughness does not affect the functionality.

The economic efficiency of the proposed method in this paper was calculated taking into account the market value of a commercially available geothermal turbine blade. Theoretical calculations demonstrated that in order to cover the leading edge of a geothermal turbine blade with an average length of 581 mm resulted in a cost of USD 64.72, which is lower than the cost of replacing it. By applying the results on a larger scale, the costs could be reduced significantly when comparing them with the commercially available replacement blades, which are currently listed at approx. USD 1.344.

As well as the economical assessment, the deposition method could also be used for repairs, which is suitable for this industry.

In conclusion, the experimentation and the economical case study results lead to a possible viable solution for reducing the overall maintenance costs in geothermal power plants.