EN 12516-2 Corrosion allowance

[Sjimmie]

In section 4 the standarized corrosion and erosion allowance (c2) is taken up.
I am wondering why the following values are used:
- 1 mm for ferritic and ferritic-martensitic steels
- 0 mm for all other steels
- 0 mm if e_co ≥ 30 mm, or if e_c1 ≥ 30 mm

Why has ferritic and ferritic-martensitic steels only the corrosion allowance?
Coould anyone give a plausible reason for this?

 

[XL83NL]

Assuming the other steels meant are austentic (300 series) and austenitic-ferritic (duplex), they have far better corrison resistance over carbon steels, and usually are set to 0 mm.