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.
A compositionally complex alloy (CCA) was developed in powder form and applied as a coating onto a carbon steels substrate by using thermal spray. The purpose of this study was to investigate the effect of microstructural modification induced by using two different powder production methods, mechanical alloying and gas atomisation, onto the corrosion resistance of the coatings for a CoCrFeMo 0.85 Ni composition. The evolution of microstructure from powders to coatings was analysed using scanning electron microscopy coupled with energy-dispersive spectroscopy and X-ray diffraction. In order to evaluate the corrosion performance of the coatings, electrochemical corrosion tests were performed in a 3.5 wt % NaCl solution at pH = 4. The study demonstrates that the powder production method has a significant influence on the phase composition and, in turn, corrosion behaviour of the resulting coating, with the gas atomising route imparting better corrosion resistance properties. Nevertheless, the appearance of the face-centered cubic (FCC) phase characteristic of the CoCrFeMo 0.85 Ni alloy within the coating produced from the mechanically alloyed powder, opens the possibility for this powder manufacturing technique to effectively produce compositionally complex alloys.Coatings 2019, 9, 695 2 of 16 and consisted of Laves phase within an FCC solid solution. Elemental segregation was observed, with higher melting point elements (Cu, Nb) enriching the interdendritic regions. Better corrosion performance than previously studied coatings was measured in 3.5 wt % NaCl solution, with the Cu and Nb-rich regions representing the areas of preferential corrosion. In another work by Gao et al. [6], CCA of the CoCrFeNiAl 0.3 composition was produced by using radio frequency magnetron sputtering and tested in 3.5 wt % NaCl solution. The coatings consisted of a polycrystalline FCC structure with homogeneous element distribution and showed increased hardness and improved corrosion performance as compared to wrought 304 stainless steel. Some CCAs have demonstrated excellent performance in both H 2 SO 4 and NaCl solutions. Similar to conventional alloys, it is interesting to note that Cr, Ni, Co, Ti in CCAs enhance corrosion resistance in acid solutions, Mo tends to inhibit pitting corrosion, whereas Al and Mn display a negative effect [7]. Wang et al. [8] applied thermal spray technology to fabricate coatings of the Ni x Co 0.6 Fe 0.2 Cr y Si z AlTi 0.2 composition. Results indicated that the hardness of the CCAs prepared by using the thermal spraying in combination with annealing at 1100 • C for 10 h was significantly increased compared to that of the cast alloy. Moreover, the alloy exhibited excellent corrosion resistance, resulting from the presence of the Cr 3 Si phase and several other (unidentified) phases. More recently, the effect of grain refinement and elemental partitioning onto the strength and corrosion resistance of a friction stir processed Cu-containing CCA has been evaluated by Nene et al. [9]. Their work shows that grai...
High-entropy alloys could provide a solution for corrosion resistance due to their impressive properties. Solid-state processing of high purity Co, Cr, Fe, Ni and Mo metallic powders and consolidation resulted in a bulk material that was further machined into electro spark deposition electrodes. After the stainless steel substrate surface preparation, thin successive layers of the high-entropy alloy were deposited and Pull-Off testing was performed on the newly obtained coating, for a better understanding of the adhesion efficiency of this technique. Good adhesion of the coating to the substrate was proved by the test and no cracks or exfoliations were present. Corrosion resistance testing was performed in a liquid solution of 3.5 wt.% NaCl for 6 hours at room temperature and the results obtained validated our hypothesis that CoCrFeNiMo0.85 high-entropy alloys could provide corrosion resistance when coating a stainless steel substrate.
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