To provide parts comprising a silicon-containing substrate having a barrier layer to prevent formation of gaseous species of Si when exposed to a high temperature aqueous environment.

The barrier layer for the silicon-containing substrate to prevent formation of gaseous species of silicon when exposed to a high temperature aqueous environment comprises aluminosilicate of group IIA and/or group IIIB such as barium - strontium aluminosilicate and an oxide of group VB.