I suppose you are speaking about the flange thickness t, as of course you should deduct the corrosion allowance on neck thickness.
Personally I consider that corrosion in the flange may take place only up to the inner diameter of the gasket, so that in my opinion the effect of that corrosion may be neglected.
Consider also that if you have a raised face (and of course you shall not include the raised face thickness into the flange thickness for calculation), then you may assume that corrosion will only consume the raised face.
Moreover, as always, the answer could depend also on how much corrosion allowance is to be provided. In highly corrosive conditions(CA>3 mm) it may be wise to consider a corroded flange thickness (but still using the raised face, if any, as a kind of 'consumable'). prex
