From the standpoint of corrosion, pressure vessels may be classified under one of the following groups:
a) vessels in which corrosion rates may be definitely established.
b) vessels in which corrosion rates, while known to be relatively high, are either variable or indeterminate in magnitude;
c) vessels in which corrosion rates, while indeterminate, are known to be relatively low;
d) vessels in which corrosion effects are known to be negligible or entirely absent.
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.
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.
When corrosion effects can be shown to be negligible or entirely absent, no excess thickness need be provided.
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.
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.
