Method and composition for phosphatizing steel under pressure

Claim 1

A method for treating ferrous surfaces to form a manganese phosphate coating thereon characterized by the ability to withstand, subsequent to being heated to a temperature as high as 450*F, the corrosive effect of a 5% salt spray test for more than 750 hours, said method comprising the steps of, formulating a manganese phosphatizing bath wherein the ratio of total acid to free acid is between 5 to 1 and 15 to 1 and the pH ratio is in excess of 2.0, adding from 0.1 to 2.0 percent by weight of the salt obtained by reacting manganese with an organic acid selected from the carboxylic group containing 4 to 6 carbon atoms with the particular carbon atom adjacent to at least one of the carboxyl groups being replaced by a hydroxyl group, the amount of said added manganese salt being selected to maintain said acid ratio and said pH without causing the total manganese content of said bath to exceed 5.0 percent, confining said bath and the ferrous surfaces to be coated within a sealed container, and heating the interior of said container to provide a steam pressure of between 1 and 22 psig for at least 15 minutes.

Claim 2

A method for treating ferrous surfaces to form a manganese phosphate coating thereon adapted to resist the corrosive effect of a standard 5% salt spray test comprising the steps of, formulating an aqueous manganese phosphatizing bath to include between 0.01 and 1.0 percent of ferrous iron, enriching the bath with from 0.1 to 2.0 percent by weight of the manganese salt of a carboxylic acid selected from the group consisting of citric, tartaric, gluconic, and saccharic acids, confining the bath and the ferrous surfaces to be coated thereby within an autoclave, and subjecting the contents to steam pressure in excess of one psig for at least 15 minutes during the formation of the phosphate coating on the ferrous surfaces.