Composition and process for inhibiting corrosion of ferrous or non-ferrous metal surfaced articles and providing receptive surface for synthetic resin coating compositions

DETAILED DESCRIPTION OF THE INVENTION In the practice of the invention ferrous or non-ferrous metals such as cold rolled steel, galvanized steel and aluminum which contain surface coatings of zinc phosphate, iron phosphate, or manganese-iron phosphate are rinsed with an aqueous solution of urea or a urea derivative containing a sufficient amount of the urea or urea derivative, usually about 0.
1%-0.
2% by weight of the solution, to provide a corrosion resistant barrier on the resultant surface which enhances the corrosion resistance and at the same time provides a surface which is receptive to synthetic resinous coatings applied in any number of ways but especially synthetic resinous coatings applied by electrodeposition where the metal article is the cathode in an electrodeposition bath.
While the invention is not limited to any theory, it is thought that the urea (or its derivatives) complexes with the metal and its phosphate coating and polymerizes providing a corrosion resistant barrier on the surface.
The residual active sites of zinc and iron are chemically bound to the urea.
Any unreacted phosphates can react directly with the urea.
Thus, the urea acts as a chemical bonding and polymerizing agent to passivate the final surface and provide better corrosion resistance.
The temperature of the treatment can vary from room temperature to the boiling point of the solution being applied.
The pH of the coating solution can vary depending on the concentration and in the case of the urea solution alone, is slightly alkaline, However, it is preferable to use a solution in which the pH range is from about 0.
8 to 6.
5.
The treating solution can also contain one or more oxidizing agents to wash and/or purify the surface and for this purpose nitric acid and alkali metal and alkaline earth metal salts thereof are effective.
Any number of acids may be used to control the pH including, for example, sulfuric acid, boric acid, and fluosilicic acid (50% by weight H.
sub.
2 SiF.
sub.
6).
While the invention is preferably practiced by the use of the simple compound urea, either alone or in combination with acidulating ingredients, compounds represented by the general formula ##STR1## wherein X represents 0 or S, R