With an increase in sulfuric acid concentration passivation of maraging steels Fe-Cr-Ni-Mo and Fe-Cr-Ni-Mo-Cu is facilitated in the general case as a result of the strengthening of oxidizing properties. However, aging is an important regime. For a weakly oxidizing medium aging close to 450-degrees-C is most critical, accompanied by layering of the solid solution with respect to chromium, and after soaking for 6 h the steel acquires susceptibility towards pitting. The opposite is true for oxidizing media after aging in the intercritical temperature range, probably connected with formation of two solid solutions-alpha and gamma and different solubility in them of such alloying elements as molybdenum. Alloying the steel with copper gives different results depending on the oxidizing properties of the medium and it may be desirable if a given steel without copper is active.