Phase Evolution and Microstructure Analysis of CoCrFeNiMo High-Entropy Alloy for Electro-Spark-Deposited Coatings for Geothermal Environment

Ioana Csaki,Fridrik Magnus,Laura E. Geambazu,Radu Stefanoiu,Sigrun N. Karlsdottir,Andri I. Thorhallsson,Cosmin Cotrut

doi:10.3390/coatings9060406

Ioana Csaki, Fridrik Magnus + Show 5 more

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https://doi.org/10.3390/coatings9060406

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Abstract

In this work, a CoCrFeNiMo high-entropy alloy (HEA) material was prepared by the vacuum arc melting (VAM) method and used for electro-spark deposition (ESD). The purpose of this study was to investigate the phase evolution and microstructure of the CoCrFeNiMo HEA as as-cast and electro-spark-deposited (ESD) coating to assess its suitability for corrosvie environments encountered in geothermal energy production. The composition, morphology, and structure of the bulk material and the coating were analyzed using scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD). The hardness of the bulk material was measured to access the mechanical properties when preselecting the composition to be pursued for the ESD coating technique. For the same purpose, electrochemical corrosion tests were performed in a 3.5 wt.% NaCl solution on the bulk material. The results showed the VAM CoCrFeNiMo HEA material had high hardness (593 HV) and low corrosion rates (0.0072 mm/year), which is promising for the high wear and corrosion resistance needed in the harsh geothermal environment. The results from the phase evolution, chemical composition, and microstructural analysis showed an adherent and dense coating with the ESD technique, but with some variance in the distribution of elements in the coating. The crystal structure of the as-cast electrode CoCrFeNiMo material was identified as face centered cubic with XRD, but additional BCC and potentially σ phase was formed for the CoCrFeNiMo coating.

Highlights

IntroductionThe concept of complex compositionally alloys, referred to as high-entropy alloys, has been recently proposed waiving the idea of solute and solvent and adopting the concept of a mixture of multi-principal elements in an equimolar or nearly equimolar ratio
The concept of complex compositionally alloys, referred to as high-entropy alloys, has been recently proposed waiving the idea of solute and solvent and adopting the concept of a mixture of multi-principal elements in an equimolar or nearly equimolar ratio. This is a radical departure from the conventional notions that opens up a vast alloy design space yet to be fully explored [1]
We present the results obtained for corrosion in saline water and hardness testing of the bulk alloy, and the phase evolution and microstructure analysis of the bulk material and the coating fabricated by electro-spark deposition with an electrode manufactured with the CoCrFeNiMo alloy

Summary

Introduction

The concept of complex compositionally alloys, referred to as high-entropy alloys, has been recently proposed waiving the idea of solute and solvent and adopting the concept of a mixture of multi-principal elements in an equimolar or nearly equimolar ratio. This is a radical departure from the conventional notions that opens up a vast alloy design space yet to be fully explored [1]. The main maintenance problems are due to the wear, erosion, and corrosion of plant components due to the corrosive nature and high temperatures of the geothermal steam [12]. For example, chloride (Cl− ) and sulfur ions

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