Electropolishing of stainless steel

Claim 1

A method for the formation of a mirror-like finish on a stainless steel item, which comprises (1) immersing said item in an electropolishing bath to a depth such that the hydrostatic pressure on said item is less than about 1.2 psi determined at 85° C., said electropolishing bath comprising orthophosphoric acid and sulphuric acid in a relative weight proportion of about 1:1 to about 2:1 and a total amount of about 55% to about 75% by weight of the bath and at least one corrosion inhibitor, said electropolishing bath having a specific gravity determined at a temperature of about 85° C., in the range of about 1.63 to about 1.78 and a dissolved iron concentration of at least about 0.5 by weight up to saturation of said bath by iron salts; (2) subjecting said immersed item to anodic electrochemical treatment in said electropolishing bath at an applied voltage of about 6 to about 15 volts and a current density of about 2 to about 9 amp/sq. in at a bath temperature of about 70° to about 100° C. using a specific gravity and dissolved iron concentration within the above-specified ranges and for a time sufficient that an external surface of the treated item has a surface roughness of less than about 2 microinches and a mirror-like appearance; and (3) forming a corrosion-resistant external transparent oxide coating on the item by immersing said electropolished item in an aqueous nitric acid bath having a concentration of about 20 to about 40 vol.% HNO 3 at a temperature of about 45° to about 70° C. for at least about 20 minutes.

Claim 2

A method for the formation of a mirror-like finish on a stainless steel item, which comprises (1) immersing said item in an electropolishing bath to a depth such that the hydrostatic pressure on said item is less than about 1.2 psi determined at 85° C., said electropolishing bath comprising orthophosphoric acid and sulphuric acid in a relative weight proportion of about 1:1 to about 2:1 and a total amount of about 55% to about 75% by weight of the bath and at least one corrosion inhibitor, said electropolishing bath having a specific gravity determined at a temperature of about 85° C., in the range of about 1.63 to about 1.78 and a dissolved iron concentration of at least about 0.5 by weight up to saturation of said bath by iron salts; (2) subjecting said immersed item to anodic electrochemical treatment in said electropolishing bath at an applied voltage of about 6 to about 15 volts and a current density of about 2 to about 9 amp/sq. in at a bath temperature of about 70° to about 100° C. using a specific gravity and dissolved iron concentration within the above-specified ranges and for a time sufficient that an external surface of the treated item has a surface roughness of less than about 2 microinches and a mirror-like appearance; and (3) forming a corrosion-resistant external transparent oxide coating on the item by immersing said electropolished item in an aqueous nitric acid bath having a concentration of about 20 to about 40 vol.% HNO 3 at a temperature of about 45° to about 70° C. for at least about 20 minutes.