Can any body tell me whats the roll of partial pressure in corrosion and specially in oil and gas industry? how we can calculate its percentage? and how much it is allowable?
The activity of gasses in solutions is usually referred to in partial pressure (or fugasity) instead of just a %, this is because the chemical impact is a function of how much of the gas is present, this is governed by both concentration and pressure.
You need to start with a good general corrosion chemistry texts (Uhlig or Fontana and Green) and learn some basics of solution chemistry and how they apply to corrosion.
For O&G, a lot of concerns center around partial pressure of H2S when selecting material, specifically for sour gas compliance. I think the relevant specification is NACE MR0175.