An impregnation liquid comprises
diacetone alcohol, in a proportion of between 20 wt. % and 50 wt. %,
glycerol trioleate, in a proportion of between 20 wt. % and 50 wt. %,
castor oil, in a proportion of between 10 wt. % and 40 wt. %,
acetone, in a proportion of between 3 wt. % and 5 wt. %, and at least one
sulfonate, in a proportion of between 0 wt. % and 1.5 wt. %. A method for surface treatment of a
ferrous part includes an impregnation step by such an impregnation liquid, and the treated part obtained.