Q. What are the intended purposes of specifications MIL-C-10578 and QQ-P-35? Can the passivation be substituted for the phosphoric acid etch? I.D.
A. The military standard you referenced, MIL-C-10578, is primarily intended for phosphoric acid cleaning and rust removal of ferrous alloys, and can also be used to provide a slight etch on both ferrous and nonferrous alloys. It specifically references five different formulations, some of which contain detergents for degreasing and derusting, some of which are inhibited to minimize base metal attack and one considered to be non-foaming.
The other specification you referenced, QQ-P-35 (Passivation Treatments for Corrosion Resistant Steel), is a federal standard that was cancelled more than 10 years ago and now directs the reader to ASTM A967. Both the previous federal standard and the current ASTM specification are very similar in that they are intended for the passivation of stainless steel. In the case of the federal standard, all passivating solutions were nitric-acid based. In the case of the more-up-to-date ASTM A967, the reader has both nitric- and citric-acid options for passivating the stainless steel.
The passivating of stainless steel with nitric or citric acids is significantly different than the military standard intended to use phosphoric acid to etch or remove corrosion from mild steel. The chemistry of the materials, chemical treatments and final purpose are significantly different, so the two specifications should not be considered interchangeable.