Both S31803 (F51) and S32205 (F60) describe 2205 duplex stainless steel. The difference is compositional range: S32205 was introduced to capture material made to the higher-performing top end of the original S31803 specification. S32205 always meets S31803, but the reverse is not true, so S32205 can be dual-certified as S31803.
When duplex stainless steels were first developed, their initial specification was captured as UNS S31803. Many leading manufacturers consistently produced the grade towards the upper end of the allowable specification to maximise corrosion performance, helped by the AOD steelmaking process, which allowed tighter control of composition.
AOD also allowed the level of nitrogen additions to be influenced, rather than nitrogen being present only as a background element. The highest-performing duplex therefore sought to maximise chromium (Cr), molybdenum (Mo) and nitrogen (N).
The difference between a duplex alloy whose composition meets the bottom of the specification and one that hits the top can be several points on the formula PREN = %Cr + 3.3 %Mo + 16 % N. To differentiate the top-of-range material, a further specification was introduced, namely UNS S32205. Duplex stainless steel made to S32205 (F60) will fully meet S31803 (F51), whereas the reverse is not true, so S32205 can be dual-certified as S31803.