Choke valves used in oil and gas production can be exposed to serious corrosive conditions during oil well stimulation when concentrated acids are used. These valves are also exposed to extreme erosive conditions. Hardmetals with varying binder content (5–7 wt%), binder composition (cobalt and a mixture of cobalt, nickel, and chromium), tungsten carbide (WC) particle size (0.6–3.5 µm) and particle size distribution were tested. Six hardmetals were first tested for resistance towards pure erosion. The hardmetals were later tested by first corroding the test specimens for 4, 8 and 16 h in 3.7 wt% hydrochloric acid, that is, a pH of approximately 0, and then subsequently eroding them. A liquid jet erosion rig was used for erosion using silica particles at 74 m/s jet velocity and 90° impact angle. The effect of corrosion time, t, on the relative mass loss, MR, after erosion was found to follow the equation , where k is a constant. The relative synergetic effects registered when measuring mass loss due to corrosion with subsequent erosion are higher for the more erosion resistant materials. Reduced binder percentage reduces the relative synergy. A more varied tungsten carbide grain size gives a less synergetic effect. Although the synergetic effect is stronger the more resistant the hardmetals are towards pure erosion, the same ranking, as to the resistance to corrosion with subsequent erosion, is obtained for the three types of materials.