Corrosion Behavior of Powder Metallurgical Cobalt-Based Alloys in Hydrochloric Acid

Ding, Jinhui (Changzhou University) | Zhang, Yuexiang (Huiyue New Materials Technology Company)

OnePetro 

ABSTRACT

Cobalt-based alloys, though possessing relevant resistance to oxidation and erosion, inevitably encounter corrosion in acidic environments. In this paper, the cobalt-based alloys containing Cr-Ni-Mo- W-Co ingredients were produced by hot isostatic pressing (HIP) method and their corrosion behavior in hydrochloric acid was investigated using potentiodynamic polarization (PDP) curves, electrochemical impedance spectroscopy (EIS) measurements and immersion corrosion tests. Post-test, the alloys surface was characterized by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD), the ions concentration in corrosion solution was characterized by inductively coupled plasma (ICP). The results indicated that corrosion resistance of three alloys follows the order 3.0Mo > 1.0Mo > 0.5Mo. During corrosion process, constituent Ni, Fe, Cr and Co in base material tend to dissolve preferentially, while carbides precipitates will be protected due to galvanic coupling in hydrochloric acid solution. Different alloy constituents and microstructures will result in different corrosion behaviors.

INTRODUCTION

Owing to the high hardness and good toughness over a wide temperature range, cobalt-based alloys present excellent wear resistance and valued industrial applications, such as cutting tools or structural materials at high temperature [1]. However, they do not show desirable corrosion resistance in harsh corrosive environment. Undergoing corrosion usually deteriorates the surface properties and wear resistance of a tool in service, accelerating its failure [2]. Therefore, it is greatly valued to improve its corrosion resistance and select appropriate materials for service in harsh environment.

The corrosion properties of WC-Co hard-metals received great concern from researchers [3]. A.M. Human etc. have investigated the electrochemical behavior of tungsten carbide based cemented carbides, found that the binder corrodes faster than the carbide and is leached out in spite of exhibiting a pseudopassive behavior [4]. Sutha Sutthiruangwong etc. have examined the corrosion properties of cemented carbides with cobalt binder phase in HCl and H2SO4 solution, found the corrosion resistance of cemented carbide increases with decreasing magnetic saturation, and lower cobalt binder content exhibits better corrosion resistance [5]. S. Hochstrasser etc. have carried a systematic investigation on the corrosion mechanisms of the WC-Co composite in solutions with different pH, they found that the solution pH plays more dominant role on corrosion susceptibility than the specific ions. The corrosion process of WC-Co consists mainly of Co dissolution in neutral and acidic solution, while WC dissolution becomes more significant at alkaline pH [6]. The grain size of WC-10Co alloy influences its electrochemical corrosion behaviors, the alloys with smaller WC grain sizes exhibit better corrosion resistances in solutions of NaOH and Na2SO4, while the alloys with larger WC grain sizes exhibit better corrosion resistance in H2SO4 [7].