Corrosion behaviour of plasma sprayed Fe based metallic glass (Fe73Cr2Si11B11C3 (at%) coatings in 3.5% NaCl solution
Introduction
Amorphous metallic alloy systems, also known as metallic glasses (MGs), have created interest worldwide due to their unique physical, mechanical and chemical properties [1], [2], [3], [4], [5]. Among them, Fe based MGs are more popular due to their low material cost, ultra-high hardness, superior strength, good soft magnetic properties, excellent wear, and corrosion resistance [[6], [7], [8], [9], [10], [13], [14], [15], [11], [12]]. However, due to poor glass-forming ability and intrinsic brittleness limits its industrial applications as structural materials [16], [17], [18]. In this regard, to overcome the drawbacks of the existing bulk metallic glasses, it is promising to develop coatings from the MGs to impart a high hardness, superior strength and excellent corrosion and wear resistance to the structural materials [6,7,[19], [20], [21], [22]].
Fe based amorphous and nanocrystalline alloy coatings are deposited using different thermal spray technologies, such as high velocity oxy-fuel spraying (HVOF), atmospheric plasma spraying (APS), arc spray, and Kinetic spray deposition process for long term surface protection of industrial components in various sectors, including automobiles, marine environment, nuclear industry, oil and gas industries, etc. [23], [24], [25], [26]. Among all the available thermal spray technologies, the APS is considered to be a simple, versatile, and industrially viable process for the deposition of amorphous/nanocrystalline composite coatings, owing to the high cooling rate (107–108 K/s) [23,[27], [28], [29]].
In the past few decades, many Fe based MGs have been synthesised using Fe-(Cr, Mo)-(C, B) multicomponent alloys [30,31]. The choice of alloying elements has a significant impact on the physical and chemical properties of the synthesised coatings. It has been presented by many researchers that the addition of Cr [32], [33], [34], [35], Mo [36], Nb [37], Al [38], Ni [16], Co [16], Y [39] and Si [40], [41], [42], [43] in the Fe based MG systems enhances the corrosion resistance by increasing the amorphous phase content and forming the passivating films on the metal surface in the aggressive corrosion environment. The metalloids (such as B, C, P) are used to increase the glass-forming ability (GFA) [44], [45], [46], [47]. However, the high material cost is associated with these expensive elements (like Ni, Mo, Nb, Hf, Y, etc.). This has fostered to a research drive on synthesizing an economical Fe based MG system using relatively inexpensive metalloids, viz. B, C, Si, and P, with the minimal addition of expensive elements, such as Cr in a small amount to improve the GFA as well as corrosion resistance. Despite many researches on the corrosion behaviour of Fe based MG coating systems, however, the extensive study on long-term corrosion performance of Fe based MG coating systems are very limited [48], [49], [50], [51]. Moreover, the effect of variation in the coating thickness on the long-term corrosion resistance has also not been carried out extensively.
In our previous research [18,52] an economical low Cr containing Fe73Cr2Si11B11C3 coatings were successfully prepared by the APS process on a steel substrate at various plasma powers varying from 18– 35 kW. It was found that the coatings deposited at the lowest plasma power (18 kW) possessed the highest corrosion resistance among other coatings deposited at higher plasma power due to the presence of high amorphous phase content, enabling the formation of a uniform passive film. However, the performance of these coatings for immersion corrosion and the corresponding corrosion mechanism have not been reported yet. It has been known that the service life of the coatings decreases with time in the corrosive environment. Hence, the knowledge of long term corrosion behaviour and corrosion mechanism is of utmost importance. Therefore, in continuation, the current investigation aims at assessing the immersion corrosion and electrochemical corrosion performance of the FeCrSiBC coatings and mild steel substrate as reference material. The effect of variation in the coating thickness on the long-term corrosion resistance of the coatings has also been analysed. Moreover, the changes in the potentiodynamic polarization and EIS with the immersion time have been presented and a probable corrosion mechanism has been proposed.
Section snippets
2.1 Coating preparation
Commercially available gas atomized Fe-based amorphous powders having a nominal composition of 87.6 Fe-2.5Cr-6.7-Si-2.5B-0.7C (wt%), with the powder size of 5–70 μm obtained from Epson Atmix Corporation, Japan, was selected as a feedstock powder for the formation of the coating. Low carbon mild steel coupon samples with a dimension of 50 mm x 20 mm x 2 mm were served as a substrate. The chemical composition of the mild steel substrate is shown in Table 1. Prior to the spraying, the substrates
Morphology and phase composition of the feedstock powders
Fig. 1(a) shows the morphology of the gas atomised feedstock amorphous powder used for coating preparation. The powder particles are regular and nearly spherical in shape and illustrate a smooth surface. This spherical shape of powder particle is the characteristic of an inert gas atomized iron-based powder particle. The particle size is in the range of 5 to 70 μm. The typical spherical morphology of the feedstock powders with a smooth surface is ideal for good flowability during the APS
Conclusions
In this study, the plasma sprayed compact Fe-based composite coatings of different thicknesses consisting of mostly amorphous phase with a minor fraction of nanocrystalline phases have shown excellent corrosion resistance to that of the mild steel substrate in NaCl solution due to the formation of a highly protective passivating film. Further, it has been observed that the Coating II with 110 ± 12 μm thickness exhibits superior corrosion resistance to that of the Coating I (70 ± 15 μm) and
CRediT authorship contribution statement
Pavan Bijalwan: Conceptualization, Methodology, Investigation, Formal analysis, Data curtion, Visualization, Writing – original draft, Project administration. Charu Singh: Investigation, Formal analysis, Data curtion. Anil Kumar: Investigation, Formal analysis, Data curtion. Kuntal Sarkar: Formal analysis. Nitu Rani: Formal analysis. Tapas Laha: Conceptualization, Methodology, Resources, Validation, Supervision. Atanu Banerjee: Conceptualization, Methodology, Resources, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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