A Zn--Ni alloy having a high Ni content is used for supplying Ni.sup.2+ and Zn.sup.2+ ions into an acidic plating bath and for supplying Ni and Zn into a hot dip galvanizing bath. This alloy is characterized by being produced by using a flux consisting of a fused-salt former, which forms a salt having a melting temperature of 700.degree. C. or less, and Na.sub.2 B.sub.4 O.sub.7 and occasionaly additionally Na.sub.2 CO.sub.3. By using the inventive alloy, the bath can be quickly prepared, and Zn and Ni can be supplied to the bath without leaving the undissolved residue.