The corrosion properties of cemented carbides with cobalt binder phase have been examined in HCl and H 2 SO 4 solution at room temperature. Potentiodynamic polarization technique with saturated calomel reference electrode was employed in this study. Air and inert argon were used as a circulating media. The effect of magnetic saturation property of cemented carbide on corrosion behavior is described. Specimens were prepared in industrial sinter furnaces under various conditions to obtain different magnetic saturation at various binder contents. According to aerated experiment, there was a difference of anodic behavior of cemented carbides between HCl and H 2 SO 4 solution. The specimen in H 2 SO 4 solution shows lower current density than in HCl by up to two orders of magnitude. This can be explained by the effect of anion on corrosion behavior of cemented carbides. A large difference between aerated and deaerated acidic solution was not observed. There was a small change of polarization curve in cathodic regime due to different extent of cathodic reaction. In addition, free corrosion potential was slightly shifted to more noble values in aerated solution. In anodic polarization, both curves were almost identical. This shows that dissolved oxygen has small influence on anodic behavior of cemented carbides. Chronoamperometric measurement as well as electrochemical investigations showed that pseudopassivity is caused by a diffusion controlled process, which is in contradiction to literature where coverage of surface is claimed. Unstable precipitates are formed in cemented carbides with high tungsten containing binder during anodic dissolution.

Corrosion properties of Co-based cemented carbides in acidic solutions

Effect of tannic acid on corrosion behavior of carbon steel in NaCl solution

Cemented tungsten carbide (WC/Co) holds a successful past as abrasion and wear-resistant components in mining industries for their wonderful combination of very high hardness and good fracture toughness as well as comparatively extraordinary wear resistance. Generally, the tungsten carbide drill bits/blades are used in rock drilling, mineral cutting, gas oil drilling and even tunneling industries. The service environments of the WC/Co tool bits are terribly complicated because of totally different hardness of the drilling objectives at different working conditions, consequence various movement patterns of the WC/Co drill bits. As a result, the failure mechanism of the tool bits is quite different. The mechanism of hole drilling and different mining operation and processes, a tool-bit gradually degrades till it breaks at the end of its life. Replacing a drill-bit once its breakage is often expensive in certain special applications. At the same time an early tool replacement decision could lead to cause of lower tools life. This type of claims is marking the ways that modify the accurate prediction of tool failures. In circumstances, where degradation signals using the appropriate features are utilized to make the tool replacement decision. Intensive investigation of the performance of tungsten carbide tools in hard metal industries and tool industries is being conducted worldwide. Tungsten carbide alloyed with cobalt (WC/Co) shows unique characteristics like high strength at elevated temperature and high mechanical and chemical resistance that makes carbide tools appropriate for cutting, drilling, mining and machining operation. A whole failure study is revealed within the paper. This study also discusses the failure mode of a tungsten carbide tools, its prediction and remedies.

Modes of failure of cemented tungsten carbide tool bits (WC/Co): A study of wear parts

During corrosion of cemented carbides the binder phase is selectively dissolved from the material. After the binder has been dissolved into the solution the remaining tungsten carbide skeleton can not withstand a mechanical force any longer and these loose grains can be easily removed even by light abrasion. In principle Co-based cemented carbide does not passivate. However the potentiodynamic curve of the material shows a conventional anodic behavior. It exhibits critical potential and breakthrough potential. Since the current density of WC–Co cemented carbides after reaching critical potential is several orders of magnitude higher when compared to true passivity this phenomenon is called pseudopassive behavior. Some precipitations could be formed in pseudopassive region at the interface of intact cemented carbide and the skeleton but they are not stable and re-dissolve into the corrosive solution. Modification of the binder with chromium carbide addition improves corrosion resistance of cemented carbides significantly. Chromium acts as an alloying element and yields to a formation of a mixed Co–Cr oxide layer as shown by EF-TEM analysis. This oxide layer is considered as a true passive layer decreasing the rate of dissolution of the binder. Vanadium has been found to dissolve into the binder besides of forming thin layer between binder and WC grains. Vanadium in the binder also improves corrosion resistance of cemented carbide.

Passivity and pseudopassivity of cemented carbides

The evaluation of tribo-corrosion synergy for WC-Co hardmetals in low stress abrasion

The influence of a corrosive wood-cutting environment on the strength of hardmetals and on their behaviour under static loading conditions has been investigated. Two commercial hardmetals were tested, which differed in hard phase composition. The results show exposure to this environment to have a highly detrimental influence on the strength of both hardmetals investigated, due to localised corrosive attack which results in the formation of stress raisers. The relative loss in strength as a function of corrosion time is the same for both hardmetals. However, the performance of the two grades differ significantly when a static load is applied in this corrosive environment: the strength of the WC-Co grade remains unaffected by the applied load, while the grade containing a mixed carbide phase exhibits stress corrosion cracking which results in a further reduction in strength.

G. Korn

Papers

The influence of a corrosive wood-cutting environment on the mechanical properties of hardmetal tools