Exposures were conducted of iron, nickel, ferritic 1-18%Cr steels, austenitic 18%Cr-9%Ni-and 20%Cr-31%Ni-steels and a 16%Cr-Ni-base alloy at 500 8C in He-30%H 2 O and 70%H 2 O-30%NH 3 , to compare the corrosion behaviour of these materials in water vapor as in conventional power plants with their behaviour in a NH 3 -H 2 O mixture, i.e. under conditions of the "Kalina-cycle". After 50 h in He-H 2 O generally a dense oxide scale had grown on iron and on the steels, whereas the scale grown in NH 3 -H 2 O was porous, due to initial formation of the c 0 -and e-nitrides, which are converted to Fe 3 O 4 later. The porous scale allows internal nitridation of the Cr-steels, nitrogen is transferred into the metal phase and reacts to finely dispersed CrN-precipitates. This process causes stresses in the material and formation of cracks. The higher the Crcontent of the material, the worse is the damage of the materials surface.
Least corrosion damage occurs for iron and the 1%CrMo-steel, however, the inward penetration of nitridation is greatest, and after 5 years on the low Cr-steel a layer of about 15 mm would be embrittled by internal nitridation, formation of c 0 and e-nitride layers and external oxidation.
Nickel is strongly damaged by intermediate formation of instable Ni 3 N, which causes internal stresses and cracking, but also pore formation by its decomposition. The surface region of the 15%Cr-Ni-base alloy is also destroyed by internal nitridation and extrusion of Ni-particles, while for this material the inward penetration of nitridation is relatively slow due to the low solubility and diffusivity of N in Ni and Ni-alloys.