jayantasm2003

Your question is so general. You can't find something straight forward that tells you when you should select what. As far as I know, there are numerous things to consider while selecting a material for a purpose. NACE and EFC has got valuable sets of publications (standards and guidelines) for materials and materials selection. You might find the book "materials selection for refineries and associated facilities" quite helpful. You should be able to find it in this forum somewhere!
Again, ASM handbooks are an ocean of information about materials and their properties. These handbooks also contains a lot of information about corrosion and corrosion mechanisms.
Corrosion allowance should be specified based on the design life and corrosion rate of vessel material in its intended service (process fluide, temperature, etc.). NACE have got some practical and useful data bases for corrosion rates. Some of large organizations and companies use their corrosion modelling softwares (normally in-house developed programs) to predict corrosion rates.
I don't want to make you disappointed. However, I were you, I would consult an experienced materials engineer (or even consultant company) to select materials for process equipment.

Regards,