Abstract In this study, the parameter space where a magnetite (Fe3O4) film forms on the surfaces of steels exposed to non-isothermal oxygen-controlled Pb and LBE coolant loops while avoiding PbO contamination has been quantified by a consistent set of 6 equations. These equations provide both the allowable temperature range of operation as well as the range of oxygen concentrations that must be maintained within the loop. The equations allow for a set of margins on the oxygen and iron concentrations in the system. Given a value for the maximum temperature within the loop, the minimum allowable temperature can be calculated and vice versa. The “protective triangle” concept was introduced to visualize the principles of the conditions promoting corrosion protection, and how these are affected by changing temperatures, concentrations and applied margins.