Corrosion resistance
Corrosion of cemented carbide leads generally to a surface depletion of the binder phase and, thus, the surface region will remain only as a carbide skeleton. The bonds between adjacent carbide grains are rather weak, and the deterioration rate will increase accordingly. At low binder phase contents, the carbide skeleton is more developed and, accordingly, such grades exhibit a combined wear and corrosion resistance somewhat higher than those of corresponding grades with higher binder phase contents.
However, this effect is insufficient to offer a significant life improvement. The limited corrosion resistance of straight WC-Co grades often makes them unsuitable in applications where the corrosive conditions are severe. For these applications, like carbide cutting tools, Hyperion Materials & Technologies has developed a series of highly corrosion resistant grades.
Corrosion rate as a function of the pH value for different types of cemented carbides tested in buffered solutions. These tests include a final surface wear treatment by tumbling in order to obtain a true value of the depth of the corroded surface zone
As shown in the diagram, straight WC-Co grades are resistant down to pH 7. This is also valid for WC-Co grades containing cubic carbides like TiC, TaC, and NbC. The highest corrosion resistance is obtained for certain alloyed TiC-Ni based grades, which are resistant down to about pH 1, but compared to the straight WC-Co grades they are brittle and have inferior thermal conductivity. They also have the disadvantages of being difficult to grind and braze; therefore, they are only used in specific applications with high demands on corrosion and wear resistance but in which mechanical strength and thermal shock resistance are less important.
In most corrosion-wear situations, (i.e., carbide cutting tools) the better choice is specially alloyed WC-Ni grades, which are resistant down to pH 2 to 3. Even in certain solutions with pH values < 2, they have proved to be resistant to corrosion. As they have WC as the hard principle and Ni and Co are similar metals in most respects, their mechanical and thermal properties are comparable to those of the straight WC-Co grades.
The pH value is one of the most important parameters when determining the corrosivity of a medium, but other factors also significantly influence corrosivity, such as temperature and electric conductivity of the medium. The latter is dependent on the ion concentration (i.e., the amount of dissolved salts in the solution).
