Corrosion behavior of Si3N4-TiN composite in sulphuric acid

The corrosion behavior of a hot pressed Si3N4-TiN electroconductive composite, with Al2O3 and Y2O3 as sintering aids, was studied in 1.8 M H2SO4 aqueous solution at 70 and 90 °C up to 200 h in an open system. At 70 °C an outer corrosion zone with strong removal of the glassy grain boundary phase is developing on top of a zone of leaching. The outer zone exposes the formerly protected TiN-grains and results in an increasing level of Ti removal with time and overall nearly linear kinetics. At 90 °C the leaching of the network modifiers from the grain boundary phase dominates. A growing silica-rich residue inside the corrosion zone develops, while TiN exposure is developing very slowly: the overall kinetics is nearly parabolic. Thus simple extrapolation from lower to higher temperature behavior is not correct and at long times damage can even be lower at higher temperature.