Process for making bath composition for converting surface of ferrous metal to mixed oxides and organometallic compounds of aluminum and iron
The ferrous metal chemical conversion coating is comprised of mixed oxides and organometallic compounds of aluminum and iron. A ferrous metal substrate is immersed for a period of time in a bath composition, at a temperature, at a pH, and at a concentration for each constituent of the bath composition, that will form a coating with the desired characteristics. The bath composition comprises water, aluminum salt, oxalic acid, and an oxidizer. The conversion coating is amorphous in nature and can be formed as thin as 1 micron with a coating weight in the range of about 40-250 milligrams per square foot. When sealed with an appropriate rust preventive material, the coating enhances corrosion resistance of the ferrous metal substrate. It is an effective absorbent base for paint finishes. When top coated with a lubricant, the conversion coating aids (a) assembly of parts, (b) break-in of sliding surfaces, and (c) anti-galling.
1. A process for making a bath composition, comprising the steps of:
(a) dissolving the following in water (i) aluminum sulfate at a concentration of about 10 to about 30 grams per liter of solution, (ii) oxalic acid at a molar ratio of oxalic acid to aluminum sulfate in the range of about 1.5 to about 4, (iii) nitrobenzenesulfonic acid at a concentration of about 2 to about 4 grams per liter of solution;
(b) adjusting the pH into the range of about 3 to about 5 by adding sodium hydroxide after the aluminum sulfate, oxalic acid, and nitrobenzenesulfonic acid are fully dissociated in the water; and
(c) adding ferrous sulfate at a concentration of about 0.1 to about 0.5 grams per liter of solution.