Corrosion rates are dependant on the materials of construction and the environment.  One thing I can suggest is that all materials should be the same if possible, if not then as close together on the galvanic series as possible to prevent galvanic corrosion (which is worse in a saltwater atmosphere).  One good source for data is the Kennedy Space Center Corrosion Testbed (http://corrosion.ksc.nasa.gov/).  They have done extensive testing with samples exposed to the ocean atmosphere.  In my dealings with NASA they are extremely helpful.

BLN is correct in stating that the corrosion rates can be extremely variable.  In the chemical industry, the common corrosion allowance on carbon steel tanks is 0.125".  Most tank corrosion failures tend to be in the floor plates.  It is not uncommon to double the corrosion allowance on the floor to 0.25".  

tahnks for the tip- the tank is proposed to be welded steel

Adamo,
If this tank is for storing sea water, the average corrsion rates would be in the order of 10 - 15 mpy (mils per year). Atmospheric corrosion (bare carboon steel) in a marine environment is about 25 - 20 mpy (Measure at Point Reyes CA @ 19.7 mpy over two years.)

It is always good to consider the use of internal epoxy linings and external paint systems for corrosion protection. Cathodic protection should also be considered.