Chemical Corrosion of Liquid-Phase Sintered SiC in Acidic/Alkaline Solutions Part 1. Corrosion in HNO₃ Solution

The corrosion behavior of the liquid-phase sintered SiC (LPS-SiC) was studied by dipping in 3.53 mol/L HNO₃ aqueous solution at room temperature and 70 °C, respectively. The weight loss, strength reduction and morphology evolution of the SiC specimens during corroding were revealed and also the chemical corrosion process and mechanism of the SiC specimens in the acidic solution were clarified. The results show that the corrosion of the LPS-SiC specimens in the HNO₃ solution is selective. The SiC particles are almost free from corrosion, but the secondary phases of BaAl₂Si₂O₈ (BAS) and Y₂Si₂O₇ are corroded via an acid-alkali neutralization reaction. BAS has a higher corrosion rate than Y₂Si₂O₇, resulting in the formation of the bamboo-leaf-like corrosion pits. As the SiC specimens etched in the HNO₃ solution at room temperature for 75 days, about 80 μm thickness corrosion layer forms. The weight loss and bending strength reduction of the etched SiC specimens are 2.6 mg/cm² and 52%, respectively. The corrosion of the SiC specimens is accelerated in the 70 °C HNO₃ solution with a rate about five times bigger than that in the same corrosion medium at room temperature.