INTERNAL CHLORINATION OF NI BASED ALLOYS AND ITS RELATION TO VOLATILIZATION CORROSION

The kinetics of internal chlorination of Ni-based alloys between 400 d egrees C and 700 degrees C were investigated by optical and scanning e lectron microscopy. Internal chlorination of Ni-4Cr and Ni-4Cr-0.5Ce a lloy occurred by formation of solid precipitates such as CrCl2 and CeC l3 which are thermodynamically stable and have low vapor pressures at the temperatures within the alloy matrix. In the lower temperature ran ge, the kinetics of internal chlorination nearly followed a parabolic rate law. The degree of internal chlorination increased with increasin g temperature. In the higher temperature range, the internal attack wa s accompanied by the consumption of the alloy by volatilization. When the rate of internal chlorination equaled the rate of overall metal co nsumption, the depth of the internal chlorination zone would approach a steady state value. At high temperatures, where vaporization was pre dominant, the steady state depth of internal attack was even smaller t han that at lower temperature. When an alloy specimen was subject to l ittle volatilization, it showed more internal attack than when it was not, and vice versa.