[K.O.B.タイタン]

Does anyone know if Co₃(PO₄)₂ is soluble in iron?

[Enthalpy]

Welcome!

To my surprise, cobalt phosphate can be melted around 1100°C without decomposing.

Intuition, and nothing more, suggests that Co would dissolve separately in liquid Fe, O would react with Fe, and P dissolve in Fe before slowly evaporating.

An other worry is that a solid can lose or change its colour by dissolving.

But you might try to sinter a thin powder mix of cobalt phosphate and steel. This can happen well below 1100°C.

If you get nice colours, please show us!