Process for the treatment by chemical conversion of substrates of zinc or of one of its alloys, concentrate and bath used for performing this process

In a process for the treatment by chemical conversion of substrates of zinc or of one of its alloys including zinc coated steel coils intended to be painted, which process comprises successively at least:

• a preactivation step by means of titanium salts,
• the phosphatization step proper,
• one or several rinses with water,
• a drying step,
• each of the various steps of the treatment being performable by stream of by dipping, the improvement enabling the phosphatization step proper to be achieved in less than about 10 seconds and according to which the phosphatization step proper is carried out by way of a phosphatization bath comprising, besides the conventional constituents, a proportion from 0.3 to 2 g/l of Fe.sup.2+ ions, a proportion from 0.2 to 1.5 g/l of Zn.sup.2+ ions and a proportion from 0.3 to 2 g/l of Ni.sup.2+ ions, said proportions being selected in such a way that the weight ratios
• Zn/Ni and Zn/Fe
• are comprised respectively between 0.4 and 2 and between 0.25 and 4, the said bath comprising also a sufficient proportion of a reducing agent for the reduction of the Fe.sup.3+ ions, which result from the reaction of the Fe.sup.2+ ions with air.