Superheater corrosion becomes a problem when tube surface temperatures are more than first melting temperature (FMT) of the ash deposits. Especially when environment is reducing.

If Cl in BL DS > 2.5 wt% (which is > 8 mol% Cl/(Na+K)) FMT is also sticky temperature

At sticky temperatures ⇒ massive depositions

Ash mostly: Na₂SO₄; Na₂CO₃ and some Na₂S, NaCl and K salts
Typical FMT: 520 to 580 ^C which limits final steam temperature, which is rarely above 480C (900F)