Electrochemical Corrosion Resistance of Ni and Co Bonded Near-Nano and Nanostructured Cemented Carbides

Tamara Aleksandrov Fabijanić,Irbas Škrinjarić,Johannes Pötschke,Markus Mayer,Marin Kurtela

doi:10.3390/met10020224

Tamara Aleksandrov Fabijanić, Irbas Škrinjarić + Show 3 more

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https://doi.org/10.3390/met10020224

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Abstract

The advantages of nanostructured cemented carbides are a uniform, homogenous microstructure and superior, high uniform mechanical properties, which makes them the best choice for wear-resistant applications. Wear-resistant applications in the chemical and petroleum industry, besides mechanical properties, require corrosion resistance of the parts. Co as a binder is not an optimal solution due to selective dissolution in an acidic environment. Thus, the development of cemented carbides with alternative binders to increase the corrosion resistance but still retaining mechanical properties is of common interest. Starting mixtures with WC powder, grain growth inhibitors GGIs; VC and Cr3C2, and an identical binder amount of 11-wt.% were prepared. GGIs were added to retain the size of the starting WC powder in the sintered samples. The parameters of the powder metallurgy process were adapted, and samples have been successfully consolidated. A very fine homogeneous microstructure with relatively uniform grain-size distribution and without microstructural defects in the form of carbide agglomerates and abnormal grain growth was achieved for both Ni-bonded and Co-bonded samples. Achieved mechanical properties, Vickers hardness, and Palmqvist toughness, of Ni-bonded near-nanostructured cemented carbides are slightly lower but still comparable to Co-bonded nanostructured cemented carbides. Two samples of each grade were researched by different electrochemical direct current corrosion techniques. The open circuit potential Ecorr, the linear polarisation resistance (LPR), the Tafel extrapolation method, and the electrochemical impedance spectroscopy (EIS) at room temperature in the solution of 3.5% NaCl. From the carried research, it was found that chemical composition of the binder significantly influenced the electrochemical corrosion resistance. Better corrosion resistance was observed for Ni-bonded samples compared to Co-bonded samples. The corrosion rate of Ni-bonded cemented carbides is approximately four times lower compared to Co-bonded cemented carbides.

Highlights

Despite the constant search for adequate alternative binders, Co-based cemented carbides are still widely used because of their unique set of properties such as high hardness, toughness, and Metals 2020, 10, 224; doi:10.3390/met10020224 www.mdpi.com/journal/metalsMetals 2020, 10, 224 thermal shock resistance [1]
It is not a prime requirement of cemented carbides, corrosion resistance is important in various applications of cemented carbides especially in the chemical and petroleum industry where the parts are exposed to an aggressive environment [8,9]
Due to excellent oxidation and corrosion resistance, Ni-based cemented carbides can find an advantage in several applications such as wear-resistant parts for pumps, cutting tools in contact with emulsions, elements subjected to organic compounds, or choke valves [10,11,12,13,14,15]

Summary

Introduction

Despite the constant search for adequate alternative binders, Co-based cemented carbides are still widely used because of their unique set of properties such as high hardness, toughness, and Metals 2020, 10, 224; doi:10.3390/met10020224 www.mdpi.com/journal/metalsMetals 2020, 10, 224 thermal shock resistance [1]. Ni-bonded cemented carbides show superior resistance to oxidation and to a wide variety of other corrosive media compared to Co-bonded hard metals [5,6,7] It is not a prime requirement of cemented carbides, corrosion resistance is important in various applications of cemented carbides especially in the chemical and petroleum industry where the parts are exposed to an aggressive environment [8,9]. The effects during abrasion or erosion processes are enhanced by electrochemical corrosion and, corrosion resistance is meaningful criteria in the selection of cemented carbide products in oil and gas production or a similar environment [5,10,11] It is important for alternative binders to feature good mechanical properties like hardness and wear resistance even at an elevated temperature for various kinds of tools [10]

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