US3832292A - Catalytic cathodic hardening of oxide films - Google Patents

Abstract

OXIDIC FILMS ON STAINLESS STEEL AND SIMILAR ALLOYS FORMED BY TREATMENT OF THE ALLOYS IN AQUEOUS SOLUTIONS OF SULFURIC ACID AND PITTING INHIBITING OXIDIZING AGENTS ARE HARDENED BY BEING TREATED AS CATHODES IN AQUEOUS ELECTROLYTES CONTAINING HEXAVALENT CHROMIUM AND A SUBSTANCE CAPABLE OF PROMOTING THE DEPOSITION OF OXIDIC CHROMIUM DEPOSITS AT THE CATHODE IN PREFERENCE TO CHROMIUM METAL.

Description

United States Patent Office Patented Aug. 27, 1974 3,832,292 CATALYTIC CATHODIC HARDENING F OXIDE FILMS William Heald Sutton, Stourbridge, Thomas Ernest Evans, Solihull, and Anthony Christopher Hart, Sedgley, England, assignors to The International Nickel Company, Inc., New York, N.Y.

No Drawing. Filed Oct. 30, 1972, Ser. No. 301,810 Claims priority, application Great Britain, Oct. 12, 1972, 47,048/72 Int. Cl. C23b 11/00 US. Cl. 204--56 R 12 Claims ABSTRACT OF THE DISCLOSURE Oxidic films on stainless steel and similar alloys formed by treatment of the alloys in aqueous solutions of sulfuric acid and pitting inhibiting oxidizing agents are hardened by being treated as cathodes in aqueous electrolytes containing hexavalent chromium and a substance capable of promoting the deposition of oxidic chromium deposits at the cathode in preference to chromium metal.

The present invention is concerned with hardening oxidic films formed on iron-base, chromium-containing, corrosion-resistant alloys, e.g., stainless steels, by treatment of said alloys in aqueous sulfuric acid solutions containing a pitting inhibiting oxidizing agent (advantageously chromic acid).

A known method of treating stainless steel and other chromium-containing alloys comprises two main steps, in the first of which an oxidic film is formed on the surface of the alloy by immersion of the alloy in an aqueous solution of chromic and sulfuric acids, with or without other constituents, and in the second of which the alloy bearing the film is subjected to electrolysis as the cathode in an electrolyte from which chromium can be deposited. This method is the subject of US. application Ser. No. 114,357, now US. Pat. 3,755,117 and 252,459, now US. Pat. 3,766,023, filed in the name of Anthony C. Hart, in which it is explained that the electrolytic treatment is short, lasting for a period of time adequate to harden the film, but not so long that any chromium becomes visible on the surface as a white deposit.

In practice of the Hart process, there is an increasing tendency for chromium metal to be deposited from. aqueous electrolytes containing chromic and sulfuric acids as the current density increases, and, at the comparatively low current density required to ensure that no chromium metal is deposited, it is difficult to harden the film completely. Our object in the present invention is to eliminate or at least to reduce the deposition of metallic chromium on cathodes during electrolysis of solutions containing hexavalent chromium.

It is an object of the invention to provide a novel process for hardening oxidic films on stainless steel and similar alloys.

Generally speaking, the present invention contemplates a process for hardening an oxidic film produced on an iron-base, chromium-containing, corrosion-resistant alloy by treatment in an aqueous sulfuric acid bath containing a pitting inhibiting oxidizing agent comprising treating as a cathode the surface of such an alloy bearing such an oxidic film in an aqueous solution containing hexavalent chromium (introduced advantageously as CrO and an agent capable of promoting the cathodic deposition of chromium oxide in preference to metallic chromium. In highly acidic aqueous chromic acid electrolytes, e.g., baths containing about 25 to about 750 grams per liter (gpL) of CrO ions produced by addition of substances from theigroup of CrCl HF, HNO H PO and CH COOH and ions derived from reaction of these substances with water and/or hexavalent chromium are capable of promoting the cathodic deposition of oxidic deposits such as chromic oxide, hydrated chromic oxide or chromic hydrpxide in preference to chromium metal.

When employed in this specification and claims, the term iron-base, corrosion-resistant, chromium-containing alloy includes stainless steels and other iron-containing alloys which also contain greater than about 11% and up to about 30% by weight of chromium. Stainless steels can be ferritic or austenitic and usually contain about 13% to about 25% (by weight) chromium.

It is thought that when the metal carrying the film to be hardened is made the cathode in a hardening solution as contemplated in the present invention and cathodic electrolysis carried out, the hexavalent chromium solution is cathodically reduced and an oxidic deposit such as oxide, hydrated oxide or hydroxide is formed in the pores of the film. Local pH changes brought about electrolytically in proximity to the film are considered to assist in the oxide deposition. The color of the oxide deposited has little or no effect on the color of the film, although the color of the film is usually slightly advanced in shade.

Examples of mirror finish Type 304 stainless steel plate given a blue film coating by immersion in a chromic and sulfuric acid solution, and hardened by cathodic electrolysis in an electrolyte according to the invention will now be described. in these examples, the experimental variables, apart from the solution, include current density, time of process and temperature, and the effects of the hardening treatment are to harden the film and slightly to advance the color of the film in shade. Undesirable hardening effects are chromium metal deposition, oxide deposition on top of the film and too great or uneven color advancement. Hardness of the film is measured in the examples by a standard rub-test which consists of rubbing the film surface with a pencil-type eraser, preferably Remington, loaded with a 400 gram weight. As a basis for comparison, an unhardened film would fail in one ,or two rubs, whereas a film hardened according to the, invention can withstand up to as many as six hundred rubs before failure.

- EXAMPLE 1 Chro'mic acid plus chromic chloride in aqueous solution was employed as the hardening electrolyte. The solutiomwas made up of 250 gpl. of CrO and 2.1 gpl. of CrCl '6I-I O to provide a 2.5M CrO and 0.025N chloride ion, i.e., a molar ratio of about 100 to 1 hexavalent chromium to chloride ion.

It is thought that all chromic acid based hardening solutions work on the same principle as the Hart Gro /H solutions. The important cathode reactions are: 1) reduction of Cr Cr (2) PH discharge leading to pH rise and the precipitation of Cr as oxide or hydroxide; and (3) a proportion of the Cr is reduced to Cr and deposited as chromium metal. Whereas the presence of SO anions seems to catalyze both the Cr Cr and Cr Cr, it can be that other ions can preferentially or solely catalyze Cr Cr Test conditions are as set forth in Table I.

1 Amperes per square decimeter.

The data in Table I show it is possible to obtain remarkably increased hardness using CrO /Cl of 100:1.-.

The increased hardness does not result in unacceptable color advancement. Provided that the process is not operated for too long a time, no deposit is formed on top of the color 'fi1m. Raising the operating temperature to 40 C. or 60 C. is beneficial, enabling longer process times to be used without surface deposition.

Changing the CrO /Cl ratio to 20:1 ofiers no advantage.

EXAMPLE 2 Chromic acid plus hydrofluoric acid in aqueous solution was employed as the hardening electrolyte. The electrolyte was made up with 250 gpl. of CrO and 2.5 gpl. of 40% hydrogen fluoride in water to provide a 50 to 1 molar ratio of CrO to fluoride ion. Test results are set forth in Table II.

TABLE II Hardness Current (No. of nsity Time Temp. rubs to (an/din!) 1 (min.) 0.) failure) Remarks 2 2 20 200. Trace of Cr deposit around edges; slight color advancement. 2 20 150 Slight color advancement. .1 so 20 300 Do. 0.05 30 20 360 Do.

At 20 C. it is advantageous to work at lower current densities to avoid Cr metal deposition. Surprisingly the lower current densities produce better hardening than higher current densities. As in the CrOg/Cl solution, better hardening is obtained than in CrO /H SO solution.

Some experiments were carried out using a 20: 1 ratio of CrO :F, with similar results.

EXAMPLE 3 Chromic acid plus nitric acid in aqueous solution was employed as the hardening electrolyte. The test solution contained 250 gpl. CrO and 2.2 gpl. of 70% by weight HNO solution giving a 100:1 molar ratio of CrO :NO Table III shows the results obtained.

The data in Table III shows that the hardness obtained in chromic acid-nitric acid aqueous electrolytes is comparable with that given by the Gro /H 80 solution.

EXAMPLE 4 Chromic acid plus orthophosphoric acid in aqueous solution was employed as the hardening electrolyte.

4 (1) Solutions of CrO /H PO of the following compositions were investigated:

CrO 250 gpl.: H PO 0.5 gpl., i.e., 500:1 ratio CrO 250 gpl.: H PO 2.5 gpl., i.e., :1 ratio CrO 250 gpl.: H PO 5.0 gpl., i.e., 50:1 ratio CrO 250 gpl.: H PO 12.5 gpl., i.e., 20:1 ratio (2) Each solution composition was tested at temperatures of 20 C., 40 C., and 60 C.

(3) Each solution and temperature was tested at current densities of 0.1, 0.2, 0.5 and 1.0 a./dm. for 20 minutes. Typical results are set forth in Table IV.

TABLE IV Hardness Current (No. of ClO3/H3PO4 solution Temp. density Time rubs to ratio C.) (aJdJI'L (in min.) failure) The data in Table IV show that the hardening was greater than that given by a CrO /H SO solution. The color advancement of the hardened films was acceptable and no oxide or chromium metal was deposited on top of the color film.

The following trends within the ranges of variables tested were observed:

EXAMPLE 5 Chromic acid plus acetic acid in aqueous solution was employed as the hardening electrolyte. A solution of 250 gpl. CrO plus 5.25 gpl. acetic acid, i.e. a molar ratio of 28.5: 1, was used at 40 C. for treatment times of from 2 minutes at a current density of 15 a./dm. to 15 minutes at a current density of 4 a./dm.

H able V shows some of the results obtained.

TABLE V Hardness (No. of Current Time rubs to density (a./dm. (in min.) failure) 1 Solution allowed to stand for 2 days.

The resulting hardnesses were comparable to or in excess of those achieved with chromic acid plus sulfuric acid hardening solutions, for example, equal to or better than 100-150 rubs to failure of the film. There was some evidence that a freshly prepared solution was more effective than a solution that had been allowed to stand, possibly, because on standing acetic acid may have been oxidized by the chromic acid.

Although the present invention has been described in conjunction with preferred embodiments, it is to be under stood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be with the purview and scope of the invention and appended claims.

We claim:

1. A process for hardening an oxidic film on iron-base, chromium-co taining, corrosion-resistant alloy, aid xidic film having been formed by immersion of said alloy in an aqueous solution of sulfuric acid and a pitting-inhibiting oxidizing agent comprising making said alloy bearing said film a cathode in an aqueous solution containing hexavalent chromium and an agent capable of promoting the cathodic deposition of chromic oxide, hydrated chromic oxide or chromic hydroxide in preference to metallic chromium and passing sufiicient electric current across the solution cathode interface so as to deposit said chromic oxide, hydrated chromic oxide or chromic hydroxide in said oxidic film to harden said oxidic film.

2. A process as in claim 1 wherein the agent capable of promoting the cathodic deposition of chromic oxide, chromic hydroxide or hydrated chromic oxide in preference to metallic chromium is selected from the group of chloride ion, fluoride ion, nitrate ion, phosphate ion and acetate ion and ionic species derived from reaction of said ions with water and hexavalent chromium.

3. A process as in claim 2 wherein the mole ratio of hexavalent chromium to ion of said agent is about 20 to 1 to about 500 to 1.

\4. A process as in claim 1 wherein the agent added to the aqueous solution is a chloride ion donor.

.5. A process as in claim 4 wherein the cathode current density is about 2 to about 4 amperes per square decimeter and the molar ratio of CrO to chloride ion is in the range of about 100 to 1 to about 20 to 1.

6. A process as in claim 1 wherein the agent added to the aqueous solution is a phosphate ion donor.

7. A process as in claim 6 wherein the cathode current density is about 0.1 to about 1.0 ampere per square decimeter.

8. A process as in claim 6 wherein the aqueous solution is maintained at a temperature of about 20 C. to about 60 C.

9. A process as in claim 1 wherein the aqueous solution containing hexavalent chromium is a solution of chromic acid containing about 25 to about 750 grams per liter of CI'O3.

10. A process as in claim 1 wherein the agent added to the aqueous solution is a fluoride ion donor.

11. A process as in claim 1 wherein the agent added to the aqueous solution is a nitrate ion donor.

12. A process as in claim 1 wherein the agent added to the aqueous solution is an acetate ion donor.

References Cited UNITED STATES PATENTS 3,535,213 10/1970 Okada 2044l JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl. X.R.