In one embodiment, a retarded acid system comprises an aqueous acid and a retarding surfactant. The aqueous acid may comprise from 5 wt. % to 25 wt. % of a strong acid, that is, an acid having a K.sub.a greater than or equal to 0.01. The aqueous acid may further comprise from 75 wt. % to 95 wt. % water. The retarding surfactant may have the general chemical formula R(OC.sub.2H.sub.4).sub.xOH where R is a hydrocarbon having from 11 to 15 carbon atoms and x is an integer from 6 to 10. The retarding surfactant may have a hydrophilic-lipophilic balance from 8 to 16.

A mixed powder for powder metallurgy comprises: an iron-based powder; and a lubricant, wherein the lubricant consists of a low-melting-point lubricant having a melting point of 86 C. or less and a high-melting-point lubricant having a melting point of more than 86 C., the low-melting-point lubricant has at least one of an amide group, an ester group, an amino group, and a carboxyl group, a ratio R1 of the low-melting-point lubricant to whole of the lubricant is 5 mass % or more and less than 90 mass %, a ratio R2 of a mass of a free lubricant to a mass of a binding lubricant is 0 or more and 15 or less, and an amount R3 of the low-melting-point lubricant contained as the free lubricant is less than 0.10 parts by mass with respect to 100 parts by mass of the iron-based powder.