Pipe Wall Thickness for Steam Jacketed Piping

I have a query. I am currently working on a project where we are dealing with steam jacketed sulfur piping.
The question is while arriving at corrosion allownce, shall I consider internal as well as external corrosion allownce for performing pipe wall thickness calculation? The sulfur core corrosion allowance is 3mm and for the steam jacket it is 1.5mm.

I am of the opinion that while performing wall thickness calculation (Internal as well as External Pressure) I shall consider 4.5 mm Corrosion Allowance.