Highly corrosion-resistant zincate type zinc-iron-phosphorus alloy plating bath and plating method using the plating bath

A method for electro-depositing a zinc-iron-phosphorus alloy film, comprising:

• a) immersing an object to be plated in a plating bath having a pH of not less than 13, and comprising 2 to 40 g/l of a zinc source as expressed in terms of the amount of zinc, 0.02 to 10 g/l of an iron source as expressed in terms of the amount of iron, 1.0 to 60 g/l of a phosphorus source as expressed in terms of the amount of phosphorus, and 10 to 200 g/l of an auxiliary agent for depositing phosphorus, which is selected from the group consisting of alkylenepolyamines having 4 to 12 carbon atoms, reaction products of alkyleneamines having 4 to 12 carbon atoms with alkylene oxides having 2 to 3 carbon atoms, reaction products of alkyleneamines each having 4 to 12 carbon atoms with epihalohydrins, alkanolamines having 2 to 6 carbon atoms, aminopolycarboxylic acids having 6 to 14 carbon atoms and salts thereof, and oxycarboxylic acids having 2 to 8 carbon atoms and salts thereof;
• and
• b) passing an electric current through the object serving as a cathode to electro-deposit, on the object, a zinc-iron-phosphorus alloy film having an iron content ranging from 0.1 to 30% by weight, a phosphorus content ranging from 0.001 to 1.5% by weight and a balance of zinc.