An aqueous anti-rust surface treatment composition used for forming a film on a surface of a metal member of the present invention contains a silane coupling agent; a water-soluble transition metal compound containing a water-soluble titanium compound or a water-soluble zirconium compound; a condensed phosphate; and a solvent containing water, in which the condensed phosphate contains a highly condensed phosphate which is a salt of high condensate of equal to or more than four phosphoric acids.

An aqueous anti-rust surface treatment composition used for forming a film on a surface of a metal member of the present invention contains a silane coupling agent; a water-soluble transition metal compound containing a water-soluble titanium compound or a water-soluble zirconium compound; a condensed phosphate; and a solvent containing water, in which the condensed phosphate contains a highly condensed phosphate which is a salt of high condensate of equal to or more than four phosphoric acids.

A powder for a powder magnetic core, being composed of iron-base soft magnetic powder particles which each have a phosphoric acid-based chemical conversion coating on the surface. In this powder, the maximum thickness of the phosphoric acid-based chemical conversion coating is 20 to 200 nm, and recessed portions are formed on the surface of the chemical conversion coating with the total area of openings of the recessed portions being 0.5 to 50% by area relative to the total area of the observation visual fields, as determined by observing ten or more parts of the surface of the phosphoric acid-based chemical conversion coating through a scanning electron microscope with a magnification of 10000 or more.

A powder for a powder magnetic core, being composed of iron-base soft magnetic powder particles which each have a phosphoric acid-based chemical conversion coating on the surface. In this powder, the maximum thickness of the phosphoric acid-based chemical conversion coating is 20 to 200 nm, and recessed portions are formed on the surface of the chemical conversion coating with the total area of openings of the recessed portions being 0.5 to 50% by area relative to the total area of the observation visual fields, as determined by observing ten or more parts of the surface of the phosphoric acid-based chemical conversion coating through a scanning electron microscope with a magnification of 10000 or more.