Because zinc is an amphoteric metal, soluble in acid but soluble in both alkali. The zinc alloy die casting in zinc accounted for 90%. Investment castings (Aluminum content of 4% or so) so by zinc etching is reasonable to consider.
As the zinc-Aluminum die castings dense surface layer is very thin (only 0.O5 ~ O.1mm) so the etching time to note. General erosion choose l%. 2% aqueous solution of hydrofluoric acid, or select 1% to 2% sulfuric acid aqueous solution at room temperature etching 5-30s can be transferred to the next step activation process.