Patent ID: 6899956

Claim:
A process for forming a hybrid conversion coating on a ferrous metal substrate, comprising the steps of: (a) applying to the substrate an intermediate coating rich in molecular iron and oxygen by contacting the substrate with a reagent of (1) an aqueous solution of oxalic acid at a concentration of about 0.5-35 grams per liter, a pH of about 0.5-6.5, a temperature of about 50-150° F., and a contact time of about 0.5-10 minutes; (2) an accelerator selected from organic and inorganic nitro compounds, a hydroxylamine accelerator, and mixtures thereof; and (3) a wetting agent; optionally by a slurry deposition; and (b) contacting the coated substrate of step (a) with an aqueous solution of oxidizing agents to form a surface that is predominantly magnetite, Fe 3 O 4 .