Development and Characterization of Natural Chromite Coating on Metal Substrate Using the Plasma Spray Process

Abstract

Natural materials are gaining interest as coating feedstock because their “quality to cost” ratio is better and they are more environmentally friendly than most of the synthetic ceramics. They give sufficient protection to metal surfaces against harsh conditions such as corrosion, wear, and high temperature. In the current study, chromite mineral was beneficiated and reduced to two different sizes to be used as feedstock material for thermal spray coating. Powders were upgraded by gravity and magnetic separation, respectively, and thermally sprayed onto mild steel samples by using atmospheric plasma spray (APS) equipment. Morphology, structure, phases, elemental distribution of chromite powder, and coatings were studied using field emission scanning electron microscopy, X-ray diffraction, X-ray fluorescence spectroscopy, and energy-dispersive X-ray spectroscopy. Tribological properties of APS chromite coatings were investigated by using a ball-on-disk tribometer, and corrosion resistance properties were evaluated by carrying out potentiodynamic polarization testing in 3.5% NaCl solution. It is observed that the coating has better wear and corrosion resistance and is worn by abrasive wear that includes scratching and particles pull out. Coating efficiency, surface morphology, and microhardness of the coating developed by fine powder were better than those of coarse powder coating.

Corrosion Behaviour of a Cr2O3 Coating on Mild Steel in Synthetic Mine Water

“…The Cr 2 O 3 coating exhibited cracks (Figure 3(b)) due to residual stress during solidification [31] and cooling of the coating [32]. Additionally, there were pores observed between the top and bonding layers (Figures 3(b) and 3(c)) resulting from unmelted or partially melted particles, attributed to the difference in melting points of the powders (Cr 2 O 3 : 2435 °C [33] and NiCrAlY: 1356 °C [31], as reported by Abbasi et al [11], Nu et al [4], and Gerald et al [34]. Figure 3(c) shows that the Cr 2 O 3 coating displayed alternate dark and lighter splats with some unmelted particles.…”

“…This is because the Cr 2 O 3 coating has a tendency to produce a protective oxide film that acts as a barrier to the corrosive media, resulting in a low corrosion rate [3,4,41]. When [4] and Abbasi et al [11]. At pH 1, the Cr 2 O 3 coating had the highest E corr , i corr , and corrosion rate, whereas its lowest corrosion rate was at pH 3, which was not expected.…”

“…The higher hardness of the coating can be attributed to its splat-like structure and a good interlayer bonding of single-phase composition (Figure 3(a)) [13]. The high standard deviation of Cr 2 O 3 coating hardness was due to pores and unmelted particles [11], giving different local hardness values. The mild steel substrate exhibited a low hardness level of 180 ± 14 HV 3 (Figure 4(b)), as expected, necessitating the application of a harder Cr 2 O 3 coating for its protection.…”

“…To mitigate these issues, various techniques such as thermal plasma spraying, high-velocity oxy-fuel spraying (HVOF), chemical vapour deposition, magnetron sputtering, and ion implantation have been used to provide corrosion and wear-resistant surfaces for mild steel under different operating conditions [7][8][9]. Plasma coating is considered the most reliable method as it produces coatings with high bond strength, thicknesses ranging from ~100 to 300 μm, and are free from oxides and porosity [3,10,11].…”

“…In many acidic and alkaline environments, the Cr 2 O 3 coating exhibits passivation corrosion behaviour, whereby it forms a stable passive film on its surface during its interaction with the environment [19]. This passive film acts as a protective barrier, hindering the occurrence of further corrosion processes [4,11,20,21].…”