Solid stainless steel for MSF once-through plants

A field test in a multi-stage flash (MSF) once-through plant has been evaluated. Six metallic construction materials, five stainless steels and one nickel base alloy, were exposed for a period of 28 months in the hot chlorinated and air saturated seawater stream of the first flash chamber at a temperature of 100°C. The lowest alloyed grades, i.e., type 316, S31726 (“317LMN”), and S31803 suffered pitting and crevice corrosion while the highly alloyed grade S31254 suffered superficial crevice corrosion. The most highly alloyed stainless steel S32654 and the nickel base alloy N06625 were fully resistant. No material suffered stress corrosion cracking. A 6Mo grade such as S31254 is a good choice for the first stage of evaporator vessels. The first six stages of the plant where the test was performed are made of solid S31254.

1. Stainless steels of type S31254 and S32654 have adequate corrosion resistance to be used for the first-stages evaporator shells in MSF once- through plants. 2. It is advisable to use S32654 for flange facings at the first stage to minimize the potential for crevice corrosion at gasket joints. 3. Stainless steels of type 316, S31726 and S31803 should not be used for the first stages of MSF once-through plants. 4. The results indicate that there are still some residual chlorine and oxygen in the water leaving the first stage of a once-through MSF plant.