The corrosion rate (Cr) of commercially available ball bearings made of silicon nitride (Si3N4) of high degree (HD) and standard quality (S) was measured at hydrothermal conditions in different aqueous concentrations of sulphuric acid (H2SO4) and sodium hydroxide (NaOH). The Cr magnitude for the materials diminished when increasing the acid concentration. At higher concentrations of H2SO4 (98%) the formation of SiO2 on the surface of the balls inhibited the progress of corrosion. The corrosion rate of the materials was increased when the sodium hydroxide solution concentration was increased. The surface of the materials was analyzed by means of scanning electron microscopy (SEM), while the corrosion rate was determined based on weight losses.