Corrosion Resistance of Tungsten Carbide Grades

Cobalt is by far the most widely used binder metal or “cement” in cemented tungsten carbides because it most effectively wets tungsten carbide grains during liquid phase sintering. For this reason cobalt is believed to be superior to other binder metals in terms of eliminating residual porosity and achieving high strength and toughness values in sintered products. The failure of cemented carbides in corrosive environments, however, is generally due to the chemical reaction of cobalt with corrosive agents. The corrosion process involves the dissolution of the cobalt binder at exposed surfaces leaving a loosely knit skeleton of tungsten carbide grains having little structural integrity. This mechanism is often referred to as cobalt “leaching” and is typically accompanied by a flaking off of unsupported carbide grains in the affected surface areas.
Although WC + Co grades have fairly good resistance to attack by acetone, ethanol, gasoline and other organic solvents as well as by ammonia, most bases, weak acids, and tap water. Exposure to formic, hydrochloric, hydrofluoric, nitric, phosphoric, sulfuric, and other strong acids, however, can result in a relatively rapid deterioration of the binder phase. Corrosion rates are affected also by temperature, the concentration and electrical conductivity of the corrosive agent, and by other environmental factors. Alloying cobalt with chromium, molybdenum, and/or nickel can significantly improve the resistance of the binder to corrosion, but the complete substitution of nickel for cobalt has proven to be the most effective means of extending the life of cemented carbides in highly corrosive environments. WC + Ni grades and other special formulations designed to resist corrosion without compromising strength or other attributes are therefore ideal for applications such as seal rings, fluid control components, choke valves, nozzles, and bearings.
The charts below are intended to serve only as general indicators of the corrosion resistance of cemented carbide grades. Table 1 shows general corrosion ratings for Co and Ni binder grades as a function of pH value. Table 2 shows the relative corrosion resistance of selected Federal Carbide grades based on the weight loss of test coupons immersed in various solutions for a specific period of time.