Corrosion of a 3D-C/SiC composite in salt vapor environments

Corrosion behavior of the three-dimensional C/SiC composite was investigated in a Na₂SO₄ vapor environment at temperatures from 1000 to 1500°C. The degradation mechanisms for the C/SiC composite could be determined by dividing a complicated function describing the weight change of the composite with temperature into several monotone functions describing the mechanisms, and then identified by the strength change of the composite with temperature. There were three reactions between the C/SiC composite and the Na₂SO₄ vapor. The first one was the passive oxidation of the CND SiC, leading a small weight gain. The second one was the oxidation of the carbon phases, leading a small weight loss. The third one was the active oxidation of the CVD SiC, leading a large weight loss. The threshold temperature for these reactions was, respectively, 1080, 1100 and 1300°C. The transition temperature from passive to active was 1200°C. The activation energy for these reactions was, respectively, calculated by the weight change with temperature to be 114, 105 and 112 kcal/mol. The flexural strength loss of the composite reached its minimum value when the weight gain of the composite reached its maximum value at 1200°C. Below 1200°C, the C/SiC composite had a higher corrosion resistance to the Na₂SO₄ vapor. Above 1300°C, the poor corrosion resistance of the CVD SiC made the composite having a poor corrosion resistance to the Na₂SO₄ vapor.