In short – the thermal oxides formed after Minimox® treatment have:
1. Better surface chemistry
410 stainless steel. Recommended maximum service temperature for continuous exposure in air = 620°C (1150°F). Partially treated sample below was heated to 850°C for 30 hours.
Untreated and oxidized 410 stainless. Thermal oxide is thick, flaking Fe2O3.
Minimox-treated and oxidized 410 stainless. Thermal oxide is thin, adherent Cr2O3 + Cr-Mn-O spinel, even though there is no Cr or Mn in the solution.
2. Better surface structure – no flaking or protrusions
Ni 600 alloy; 1000°C, 500 hours in air
Minimox-treated and oxidized (100x)
Untreated and oxidized (100x)
Untreated (1000x). Exposed microvoids for corrosion nucleation
The composition of the protective layer is primarily a function of the alloy, not the coating material. For example, the protective layer that forms on aluminum alloys is different from the protective layer that forms on stainless steels.
The temperature of oxidation is set by the alloy, not by any Minimox solution requirements.