Salt-bath nitriding (Tufftride / Melonite / QPQ) — fast, dirty, effective

QPQ stands for Quench, Polish, Quench. After the FNC immersion and the first quench, the part is mechanically polished to knock down the rougher outer porous zone of the compound layer, then re-immersed in an oxidizing salt bath at 350-450°C for 15-30 minutes. The oxidizing bath grows a thin (1-3 µm) magnetite-type Fe₃O₄ layer over the compound layer and into its residual porosity. The dark gray-to-black oxide is the visible signature of QPQ.

Corrosion data from QPQ-treated mild steel typically shows 200-1000+ hours of neutral salt-spray resistance (ASTM B117) on parts that would rust within 24 hours bare. That number is the polish step plus the oxide, not the nitrogen. The polish closes the porous zone; the magnetite layer is itself corrosion-resistant. Strip either step and the corrosion numbers do not apply.

QPQ is the right call when the part needs corrosion resistance on top of galling resistance: hydraulic shafts, valve internals, firearm components, fastener tooling, and stamping dies running corrosive coated stock. It is the wrong call when the compound layer needs to be ground or lapped off in finishing, since the polish and the oxide are the corrosion claim, and finish grinding removes both.