Refractories exposed to corrosive melts at high temperatures experience wear due to dissolution. The presented work deals with the dynamic corrosion of magnesia fine ceramics in a CaO–Al2O3–SiO2–MgO silicate slag with a CaO/SiO2 weight ratio of 0.65. Finger tests at 200 rpm at three different temperatures, 1450, 1500, and 1550 °C, were performed. A contemporary rotary finger test (RFT) device with in-situ wear profile measurement using a laser device was used. The precise dimension measurements with high-resolution facilitate the determination of total mass flux densities, which allow for determination of effective binary diffusivities. Thereby, two methods based on Sherwood relations were applied. The comparison shows the benefit to better consider deviations from pure cylinder shape. The use of contemporary RFT devices and magnesia fine ceramics allows reliable determination of the effective binary diffusivity.