| abstract |
The present invention addresses the problem of providing a surface-treated steel material which is capable of exhibiting excellent rust prevention performance for a long period of time, while additionally exhibiting excellent moisture resistance. The present invention provides a surface-treated steel material which is obtained by forming a coating film on the surface of a steel material with a base layer being interposed therebetween, said base layer containing at least an aluminum/zinc alloy plating layer, wherein: the aluminum/zinc alloy plating layer contains, as constituent elements, Al, Zn, Si and Mg; the Mg content is from 0.1% by mass to 10% by mass; the aluminum/zinc alloy plating layer contains from 0.2% by volume to 15% by volume of an Si-Mg phase; the mass ratio of Mg in the Si-Mg phase to the total Mg amount in the plating layer is 3% or more; the coating film is formed from a rust preventive coating material composition that contains (A) a coating film forming resin, (B) a crosslinking agent and (C) magnesium oxide; the magnesium oxide (C) has a lattice constant of 0.4214 nm or less; and the coating film has a BET specific surface area of 2.0 m2/g or less. |