Electrocoating process for depositing a corrosion retardant layer on a metal substrate and sequentially electrocoating a cationic film-forming polymer coating

In the process for electrocoating a cationic film-forming polymer composition on a metal electrode of an electrocoating cell, said electrode consisting of a metal article and being electrically connected to a direct current power source, by immersing said electrode in a bath of an aqueous coating composition comprising a synthetic cationic film-forming polymer composition which is at least partially neutralized with an acidic material which forms anions in said bath, electrocoating a coating on the electrode, removing the electrode from the bath and drying the electrode to form a continuous film thereon, the improvement used therewith comprises:

• 1. immersing the metal article in the bath which is connected to the positive side of the direct current source thereby providing the article with a positive polarity and electrocoating a coating of anions on the surface of the metal article;
• the anions combine with the surface of the metal article to form a corrosion retardant layer on the metal;
• and
• 2. changing the polarity of the metal article by switching to the negative side of the direct current source thereby providing the article with a negative polarity and electrocoating a coating of the cationic film-forming polymer onto the metal article to form a uniform film thereon;
• wherein the aqueous coating composition has a pH of about 3 to 8 and comprises 5-50% of film-forming constituents uniformly dispersed in an aqueous medium;
• wherein the film-forming constituents consist essentially of
• a cationic film-forming polymer of
• 1. an alkyl methacrylate or styrene or a mixture thereof;
• 2. an alkyl acrylate;
• 3. an amino alkyl methacrylate or an amino alkyl acrylate;
• and
• 4. a hydroxy alkyl acrylate or a hydroxy alkyl methacrylate;
• and a water-dispersible cross-linking agent;
• and contains sufficient phosphoric acid or chromic acid to provide said pH.