Most of corrosion mecanism are oxydoreductions. Such phenomena are linked with thermodynamic science. In this way there is thermal activation. On your working temperature, all should be OK. It is what is said about atmospheric ammonia storage tank. Ammonia is know for SCC under severe conditions. Moreover at -20°C I hope molecules are not expected to broke, so you should not have any problem. But I coud be wrong, cause I do not know much about your fluid. pascal GRANIER