Corrosion behavior of functionally graded cemented carbides with CoNiCrFe binder

The corrosion behavior of functionally graded cemented carbide (FGCC), with CoNiCrFe binder (WC-CoNiCrFe), has been investigated by performing electrochemical measurements and chemical characterization. The results showed that the CoNiCrFe binder was selectively dissolved in alkaline and acidic solutions. In the NaOH solution, the high contents of Ni and Co generated a passive film consisting of Co(OH)2, Ni(OH)2, Co3O4, and NiO that effectively isolated the corrosive medium from the materials. However, the contents of Fe and Cr were too low to provide satisfactory protection. In the HCl solution, the Cr element generated Cr0, Cr2O3, and Cr(OH)3. However, the Cr content was too low to form a passive film. In addition, Co, Ni and Fe in the binder were selectively dissolved in the acidic solution. Moreover, compared with the FGCC, the non-graded cemented carbide (CC) had a higher binder content and a more dispersed WC distribution. Thus, it formed a denser and thicker passive film, which was beneficial for the corrosion resistance in the NaOH solution. However, the CoNiCrFe binder was severely corroded in the HCl solution, which was due to the absence of a passive film. In comparison, the FGCC with a lower binder content and a denser WC distribution, showed a higher corrosion resistance. On the other hand, the WC grain size showed the opposite effect in alkaline and acidic solutions. In the NaOH solution, a small WC grain size increased the number of WC/CoNiCrFe interfaces, which favored the formation of a dense passive film. It, thereby reduced the probability for pitting corrosion. In the HCl solution, the corrosion generally started from grain boundaries with high interface energy as no passive film existed. Thus, a large WC grain size facilitated the improvement of the corrosion resistance. The results from the present study do not only provide a theoretical guidance for an understanding of the anticorrosion mechanism of WC-CoNiCrFe, but also a new approach for the preparation of cemented carbides with high corrosion resistance.