As the name calls for, protective gas is for protecting products which are heat-treated. Normally this protection refers to undesired oxidation. Protective gases may be nitrogen, argon or mixtures of nitrogen and hydrogen. The desired oxidation potentials are in the range of ppm to 10E-30 bar oxygen partial pressure.
With classical neutral annealing, the required oxygen partial pressure of the protective gas has to be lower than the equilibrium partial pressure of the treated metal. With it, that alloy element is considered which is most sensitive to oxygen.