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That can be a rather complex issue, and may be best summed up in Non-mandatory Appendix E of ASME VIII-1:

E-1

From the standpoint of corrosion, pressure vessels may be classified under one of the following groups:

(1) vessels in which corrosion rates may be definitely established from information available to the designer regarding the chemical characteristics of the substances they are to contain. Such information may, in the case of standard commercial products, be obtained from published sources, or, where special processes are involved, from reliable records compiled from results of previous observations by the user or others under similar conditions of operation.
(2) vessels in which corrosion rates, while known to be relatively high, are either variable or indeterminate in magnitude;
(3) vessels in which corrosion rates, while indeterminate, are known to be relatively low;
(4) vessels in which corrosion effects are known to be negligible or entirely absent.

E-2
When the rate of corrosion is closely predictable, additional metal thickness over and above that required for the initial operating conditions should be provided, which should be at least equal to the expected corrosion loss during the desired life of the vessel.

E-3
When corrosion effects are indeterminate prior to design of the vessel, although known to be inherent to some degree in the service for which the vessel is to be used, or when corrosion is incidental, localized, and/or variable in rate and extent, the designer must exercise his best judgment in establishing a reasonable maximum excess shell thickness. This minimum allowance may, of course, be increased according to the designer's judgment.

E-4
When corrosion effects can be shown to be negligible or entirely absent, no excess thickness need be provided.

E-5
When a vessel goes into corrosive service without previous service experience, it is recommended that service inspections be made at frequent intervals until the nature and rate of corrosion in service can be definitely established. The data thus secured should determine the subsequent intervals between service inspections and the probable safe operating life of the vessel.

E-6
For parts which are essential to vessel strength such as stiffener rings, the attachment of the part to the shell must provide adequate corrosion allowance or protection to assure the required strength throughout the service life. Some attachments, such as intermittent welds, require protection on both face and root sides; alternatively, continuous welds or a suitably sized seal weld between the strength welds will provide protection for the root side.

in simple terms you define the corrosion allowance based on expected corrosion rate and design life. You've to find corrosion data for your combination of metal/fluid/operating conditions/environments and then calculated the Corrosion Allowance

corrosion rate (mm/y) x Design life (years) = Corrosion allowance (mm)

usually for Carbon steel the max corrosion allowance is 3 -6 mm for item and piping in a plant and 8-10 mm for pipeline.

one important think is that you've to identify all the possible deterioration mechanisms for your material.

http://www.hse.gov.uk/comah/sragtech/techmeasmaterial.htm

corrosion prevention & control