Abstract: Process for the treatment by chemical conversion, within very short spaces of time of substrates of zinc or of one of its alloys especially of continuously zinc coated steel coils, wherein the bath employed in the phosphatization step proper comprises, besides the conventional constituents, a proportion of 0.3 to 2 g/l of Fe.sup.2+ ions, this bath comprising also from 0.2 to 1.5 g/l, preferably from 0.2 to 1.2 gl/ of Zn.sup.2+ ions and from 0.3 to 2 g/l, preferably from 0.3 to 1.2 g/l of Ni.sup.2+ ions, the weight ratiosZn/Ni and Zn Febeing comprised between 0.1 and 5, these ratios being preferably comprised respectively between 0.4 and 2 and between 0.25 and 4.