US3293158A - Anodic spark reaction processes and articles - Google Patents

Description

Dec. 20, 1966 w. M NEILL ETAL ANODIC SPARK REACTION PROCESSES AND ARTICLES 2 SheetsSheet 2 Filed Sept. 17, 1963 BSVL'IOA INVENTORS WILLIAM McNElLL BY LEONARD L. GRUSS ATTORN EYSI United States Patent 3,293,158 ANODIC SPARK REACTION PROCESSES AND ARTICLES William McNeill, Philadelphia, and Leonard L. Gruss, Willow Grove, Pa., assignors to the United States of America as represented by the Secretary of the Army Filed Sept. 17, 1963, Ser. No. 309,580 21 Claims. (Cl. 204-56) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of us of any royalty thereon.

This invention relates to anodic spark reaction and particularly, to processes for coating metals by anodic spark reaction and to coated articles produced by such processes. The invention also relates to a process for coating assembled combinations of different metals and to assemblies coated thereby.

The phenomenon of anodic sparking has been observed by many investigators and is described in detail in the prior art. The effect appears when the voltage across an anodic barrier film is raised to a point where film thickness can no longer increase uniformly, and dielectric breakdown occurs. The term anodic barrier film is used to indicate a continuous film which covers an anode surface and acts as a barrier to the passage of electric current in the anode direction. Most of the prior art on anodic barrier films, US. Patent 2,364,964 being representative thereof, is concerned with the phenomena which occur at voltages below the spark potential, and consequently little is known about the chemical and structural properties of the products of anodic reactions that occur above the spark potential.

Application of the principals of anodic spark reaction has been made in the protective finishing of light metals, particularly magnesium. In the HAE process (US. Patent 2,723,952) an electrolyte containing OH, PO F1 AlO and MnO ions is used and magnesium is anodized at voltages above the spark potential. A hard, adherent layer containing a mixture of refractory magnesium compounds is thereby deposited on the magnesium surface. The same type of reaction is employed in the Cr-22 process (US. Patent 2,778,789) but the electrolyte is an ammoniacal solution of CrO HPO and P ions and the resulting coating is non-alkaline. While the prior art applications of anodic spark reaction suggest the possible formation of anodic barrier films, there has been little evidence as to the structure or composition of the anodic reaction products and attempts to utilize anodic spark reaction to form protective or other useful coatings on other than the usual metals of Al, Mg and alloys thereof have met with little success. The problem becomes increasingly complex where, as in the production of many hardware items, it is necessary prior to finishing to complete an assembly of components made from different metals, e.g., Al and Mg assemblies containing threaded steel inserts.

Accordingly, a principal object of the present invention is to provide improved anodic spark reaction processes for providing useful coatings on a variety of metals.

Another object of the invention is to provide an improved anodic spark reaction process for providing useful coatings on assemblies of components made from different metals.

Other objects of the invention will in part be obvious and in part appear hereinafter in the following detailed description of the principles of the invention and several of the embodiments thereof.

In the drawings accompanying and forming a part of the specification FIG. 1 depicts voltage-time curves for various metals subjected to anodic spark reaction in aluminate solution,

3,293,158 Patented Dec. 20, 1966 FIG. 2 depicts voltage-time curves for various metals subjected to anodic spark reaction in tungstate solution, and

FIG. 3 depicts voltage-time curves for various metals subjected to anodic spark reaction in silicate solution.

In the course of the investigation leading to the present invention, a number of metals were subjected to anodic spark reaction in various electrolytes including aqueous solutions of NaAlO Na WO -ZH O, or Na SiO '9H O. The anodic reactions were carried out at a current density in the range of 0.1 to 1.0 amps/in. with the voltage across the electrolytic cell being increased until sparks appeared on the anode surface, and thereafter increased; manually to maintain the required current as the anode film resistance increased. Electrolyte temperature was maintained in the range 0 to 40 C., and treatment time required for coverage of the anode varied with current density, the process being more rapid as current density increased. FIGS. 1, 2 and 3 depict the anodic voltages plotted as a function of time for the various metal parameters in 0.1 N solutions of NaAlO Na Wo -2H O, and Na SiO 91-1 0, respectively.

In obtaining the data plotted in FIGS. 1, 2 and 3, a current density of 0.16 amp/cm. was employed in all reactions, except with Ni in Na SiO for which the current density was 0.32 amp/cmn": The reaction cell was a glass vessel which was thermostatted at 25il C., and was equipped with a stirring device and a cathode comprising .an inert conductor, e.g., platinum or graphite. The cathode surface area was at least equal to the anode surface area in all experiments. The power supply for this series of experiments was a rectifier having a variable DC output of 0 to 5000 volts. The voltage output was unregulated, but fluctuations were less than 6 percent.

Anodes were prepared in the form of cylindrical rods and mounted in tight-fitting Teflon sleeves which served to mask the anode surface at the air-electrolyte interface. Anodes varied slightly in size, but typical dimensions were 0.8 cm. dia. with a total surface area of about 6.5 cm. exposed to the electrolyte. The only exceptions were the gold electrodes which were 2 cm. in length and 0.1 cm. dia., and the Mn electrode which was a fiat plate of approximately 4 cm. surface area. Surface preparation also varied, depending on the anode metal, but the general procedure was to degrease in an organic solvent, etch in an appropriate acid or alkaline solution and rinse in distilled water. Cleaning procedures were carried out immediately prior to anodizing.

The analyses of anodic spark reaction products were performed on insoluble material which adhered to the anode. Powder X-ray diffraction patterns were obtained (Copper Ka radiation and at 57.3 mm. camera) on products of all the anodic spark reactions shown in Table I. Chemical and spectrographic analyses were made of anode products from aluminate solutions. spectrographic analyses were performed on the samples to provide a qualitative check on their elemental composition. These analyses showed that the only metal ions in the anode products were those of aluminum and the anode metal.

The spark reaction products adhering on anodes treated in NaAlO solutions were generally light in color, very hard, and had the appearance of porous sintered powders.

Chemical analyses were made of the spark reaction products obtained in NaAlO solutions and the results are summarized in Table II. It is evident from the results that major components of the spark reaction products are derived from the anion constituent ofthe electrolyte.

X-ray analyses of the spark reaction products from NaAlO solutions are given in Table III. The X-ray data do not account for all the anode products which were shown in the chemical analyses, and the probable reason is that there were significant quantities of noncrystalline material present. It was in fact, surprising that the X-ray patterns in the present study were sufficiently Well-defined to permit unambiguous identification of the major anode products ll-A1 MgA1 O and ZnAl O and MgO.

Spark reactions were obtained with Al, Cd, Zn, Bi, and Cu in Na WO solutions, but not with Mg, Ni, Co, Fe, and Ag. The main spark reaction product was in all cases a chalky yellow or yellow-green powder that formed a thick coating on the anode. The coating on the cadmium anode contained a blue powder which was shown to be of the same composition as the yellow-green material. X-ray diffraction patterns obtained of the TABLE TABLE IL-CHEMICAL ANALYSES OF ANODIO SPARK REACTION PRODUCTS IN 0.1 N NaAlOz 1 The oxidation state of the anode metal ions is assumed to be plus 2 'lhe MgO in the anode product is assumed to result from the Mg constituent in the aluminum alloy anode.

IIL-ANODIC SPARK REACTION PRODUCTS Anode Metal Electrolyte 0.1 N NaAlO 0.1 N NagWO; 0.1 N Na SiO MgAl O4+MgO N.I. ZnSiO Bi 1 1 Metallic phase.

N.I. X-ray difiraction pattern could not be identified. Not investigated.

N.R. no spark reaction.

products from the Na WO solutions could be superimposed on a pattern for 99% pure monoclinic W0 which showed that the crystalline anode product was essentially this material.

Tungsten trioxide, W0 in its polycrystalline form is useful as a catalyst in a number of chemical syntheses,

TABLE L-ANODIC GLOW SPARK INITIATION VOLT- Electrolyte Anode Metal 0.1 N NaAlOz 0.1 N NflzWO-1 0.1 N N a SiO Glow Spark Glow Spark Glow Spark 285 N.R NR.

110 N.R N.R 265 180 i Not investigated. N.R., no spark reaction.

and as a high temperature corrosion protective coating for titanium. It is thus seen that the present invention provides a method of directly forming polycrystalline coatings of anhydrous W0 on several metals. While it has been shown in the prior art that hydrated W0 can be precipitated from tungstate solutions by anodic reaction, it is necessary in such cases to provide an additional dehydration step in order to obtain anhydrous material.

The anode products from reactions in Na SiO solutions bore some resemblance to the slags which form on furnace refractories. The results of X-ray diffraction analyses of these materials are given in Table III. Crystalline phases were found on all the anodes except Cd. A very heavy background in the films for Bi, Ni, Cu, Zn, and Fe anode products pointed to the presence of amorphous material, e.g., fused silicates. The crystalline phases reported in Table III were well characterized despite the high background level and represent major anode product components. The presence of metallic Cu, Fe, and Bi in the coatings on these anodes did not impart electrical conductivity to them, and it is probable that the metallic phases were embedded in other anode products, e.g., amorphous silicates.

While silicate coatings for the protection and decoration of metal surfaces are well known, typical processing steps in the application of such coatings are slip-casting or spraying of powdered silicate slurries to obtain soft coatings which must then be fired in order to be hardened and bonded to the underlying metal surface.

The present invention provides a method for producing hard, adherent silicate coatings on various metals by a one-step electrochemical process, which consists of anodization of metal articles in a relatively dilute aqueous silicate electrolyte. While it is recognized that, in general, silicate solutions are not new in electrochemical processes, heretofore there has been no recognition of the critical importance of the nature and concentration of the anion constituent in determining the composition and structure of anodic reaction products.

The aforementioned results showed clearly the operability of anodic spark reaction to form useful coatings on a variety of metals when the critical nature and concentration of the anion constituent in the electrolyte is understood. These results cannot be attributed to the relatively high current density which was employed (0.1-6 amp/cm?) because in a number of cases, e.g., Mn in NaAlO and Mg, Ni, Co, Fe, and Ag, in Na WO solution, no such phenomena could be observed. This was true even when the current density was increased to several times the above value. Furthermore, the growth of a barrier film was found to occur in a number of cases, e.-g., Ni and Co in NaAlO solution, at current densities less than one tenth the above value.

Confirmation of the almost universality of. the inventive process was obtained when anodic spark reaction was employed to :form useful coatings on refractory metals, e.=g., Ti, Zr, V, Mo, W, Nb and Ta. Visible sparking on anodes of the refractory metals was observed in all cases at a DC. initiation voltage in excess of 190 volts. Anodic reaction products forming coatings on the refractory metal anodes comprised corundum (oi-A1 0 or structurally related analogues (MlgAl Q ZnA1 O etc), anhydrous W0 and fused silicates, respectively, where the anion constituent of the electrolyte employed consisted of aluminate ion, tungstate ion, and silicate ion, respectively. Concentrations of anion constituents were varied from an effective amount up to about 0.3 N 11501 aluminate ion and from an effective amount up to about 0.2 N for tungstate ion and silicate ion.

In accordance with another embodiment of the invention, the aforementioned coatings of corundum and structurally related analogues thereof may be formed on assembled combinations of the aforedescribed metals in an electrolyte wherein the anion constituent consists of aluminate in an effective amount up to about 0.3 N. In one demonstration a bar of aluminum exposing inserted pins of Cu, Fe, Ni, Mg and Zn was anodized at a current density of approximately 0.5 amp/in. in a 0.1 N sodium aluminate solution, the temperature of which ranged from 25 to 40 C. during treatment. The anode voltage rose from O to approximately 420 volts, DC, in a period of to minutes at the end of which time all the metal surfaces of the assembly were covered with hard coatings of corundurn or spinel, said coatings being sufficiently hard to scratch glass.

Criticality of the concentration of the aluminate anion constituent in the electrolyte employed is shown by results of anodic spark reaction attempts set forth in the following table:

TABLE IV.RESULTS OF ANODIC SPARK REACTION Having thus described the invention so that others skilled in the art may be able to understand and practice the same, it is expressly understood that the invention is not limited to the preferred embodiments but may be otherwise embodied or practiced within the scope of the following claims.

We claim:

1. An electrochemical process for producing a hard, adherent coating on the surface of an article comprising at least one metal selected from the group consisting of Al, CO: a V, M0 W; and Ta, said process comprising subjecting .said metal to anodic spark reaction in an aqueous electrolyte wherein the anion constituent consists of alnminate ion in a concentration ranging from an effective normality up to 0.3 N.

2. An electrochemical process according to claim 1 wherein an anodic current density between about 0.1 and 1.0 amp/in. is maintained during said anodic spark reaction.

3. An electrochemical process according to claim 1 wherein said electrolyte is maintained at a temperature between 0 and 40 C. during said anodic spark reaction.

4. An electrochemical process for producing a hard, adherent coating comprising a substance selected from the group consisting of corundum and structurally related analogues thereof on the surface of a metal selected from the group consisting of Al, Zn, -Bi, Ni, Co, Fe, Cn, Ag, Au, Ti, Zr, V, Mo, W, Nb, and Ta, said process comprising subjecting said metal to anodic spark reaction while maintaining anodic current density between about 0.1 and 1.0 amp/in. in an electrolyte consisting of an aqueous solution of sodium aluminate ranging in concentration from an effective normality up to 0.3 N and maintained at a tempertaure between 0 and 40 C.

5. An electrochemical process for producing a hard, adherent coating on the surface of. an assembly of different metals, said process comprising subjecting said assembly to anodic spark reaction in an electrolyte wherein the anion constituent consists of aluminate ion in a concentration ranging from an effective normality up to 0.3 N.

6. An electrochemical process according to claim 5 wherein said coating comprises a substance selected from the group consisting of corundum and structurally related anolo-gues there-of.

7. An electrochemical process according to claim 5 wherein an anodic current density between about 0.1 and 1.0 amp/in. is maintained during said anodic spark reaction.

8. An electrochemical process according to claim 5 wherein said electrolyte is maintained at a temperature between 0 and 40 C. during said anodic spark reaction.

9. An electrochemical process for producing a hard, adherent coating comprising a substance selected from the group consisting of corundum 'and structurally related analogues thereof on the surface of an assembly of different metals selected from the group consisting of Mg, Al, Zn, Bi, Ni, Co, Fe, Cu, Ag, Au, Ti, Zr, V, M-o, W, Nb, and Ta, said process comprising subjecting said assembly to anodic spark reaction while maintaining anodic current density between about 0.1 and 1.0 amp/in. in an electrolyte consisting .of an aqueous solution of sodium aluminate ranging in concentration from an effective normality up to 0.3 N and maintained at a temperature between 0 and 40 C.

10. The method of coating the surface of a metal selected from the group consisting of Al, Cd, Zn, Bi, Cu, Ti, Zr, W, Nb, and Ta, which comprise electrolytically treating which comprises electrolytic treatment of an anode of said metal in an electrolyte consisting of an aqueous tungstate solution at a voltage sufliciently high to cause sparking on the anode surface, the concentration of tu-ngstate anion in said solution ranging from an effective normality up to 0.2 N.

11. The method of claim 10 wherein the electrolyte temperature during treatment is maintained between 0 and 40 C.

12. The method of. claim 10 wherein the anodic current density is maintained between 0.1 and 1.0 amp/in 13. The method of producing an anhydrous tungsten brioxide coating on the surface of a metal selected from the group consisting of A1, Cd, Zn, Bi, Cu, Ti, Zr, V, Mo, W, Nb, and Ta, which comprise selectrolytically treating said metal at a voltage sufliciently high to cause sparking on said surface and at a current density between 0.1 and 1.0 amp/in. in an aqueous tungstate solution maintained at a temperature between and 40 C., the concentration of tungstate anion in said solution ranging from an effective normality up to 0.2 N.

14. The method of producing a silicate coating on the surface of a metal selected from the group consisting of Al, Zn, Bi, Ni, Fe, Cu, Ti, Zr, V, Nb, Ta, M0, and W, which consists of the electrolytic treatment of an anode of said metal in an electrolyte consisting of an aqueous silicate solution at a voltage sufiiciently high to cause sparking on the anode surface, the concentration of siltcate anion in said solution ranging from an etfective normality up to 0.2 N.

15. The method of claim 14 in which the electrolyte temperature is in the range 0 to 40 C.

16. The method of claim 14 in which the anodic current density is in the range 0.01 to 1.0 amp/i11 17. The method of producing a silicate coating on the surface of a metal selected from the group consisting of Al, Zn, Bi, Ni, Fe, Co, Ti, Zr, V, Nb, Ta, Mo, and W, which comprises electrolytically treating said metal at a voltage s-ufficiently high to cause sparking on said surface and at a current density between 0.1 and 1.0 amp/in. in an aqueous silicate solution maintained at a temperature between 0 and C., the concentration of silicate anion in said solution ranging from an efiective normality up to 0.2 N.

18. A coated article produced by the process of. claim 1.

19. A coated article comprising an ass. 'bly of different metals produced by the process of claim 5.

20. A coated article produced *by the process of claim 10.

21. A coated article produced by the process of claim 14.

References Cited by the Examiner UNITED STATES PATENTS 1,954,000 4/ 1937 Truesdale et al. 2045 6 2,196,161 4/1940 Frasch 204-5-6 2,215,167 9/1940 Sumner et a1. 20456 X 2,313,755 3/1943 Loose 20456 2,348,826 5/1944 Kra-use et a1. 20456 2,364,964 12/ 1944 Frasch 204-58 2,780,591 2/1957 Frey 2 0456 X FOREIGN PATENTS 543,726 3/ 1942 Great Britain.

JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner.

Claims (3)

1. AN ELECTROCHEMICAL PROCESS FOR PRODUCING A HARD, ADHERENT COATING ON THE SURFACE OF AN ARTICLE COMPRISING AT LEAST ONE METAL SELECTED FROM THE GROUP CONISTING OF AL, ZN, BI, NI, CO, FE, CU, AG, AU, TI, ZR, V, MO, W, NB, AND TA, SAID PROCESS COMPRISING SUBJECTING SAID METAL TO ANODIC SPARK REACTION IN AN AQUEOUS ELECTROLYTE WHEREIN THE ANION CONSTITUENT CONSISTS OF ALUMINATE ION IN A CONCENTRATION RAGING FROM AN EFFECTIVE NORMALITY UP TO 0.3 N.

10. THE METHOD OF COATING THE SURFACE OF A METAL SELECTED FROM THE GROUP CONSISTING OF AL, CD, ZN, BI, CU, TI, ZR, W, NB, AND TA, WHICH COMPRISES ELECTROLYTICALLY TREATING WHICH COMPRISES ELECTRLYTIC TREATMENT OF AN ANODE OF SAID METAL IN AN ELECTRLYTE CONSISTING OF AN AQUEOUS TUNGSTATE SOLUTION AT A VOLTAGE SUFFICIENTLY HIGH TO CAUSE SPARKING ON THE ANODE SURFACE, THE CONCENTRATION OF TUNGSTATE ANION IN SAID SOLUTION RANGING FROM AN EFFECTIVE NORMALITY UP TO 0.2 N.

14. THE METHOD OF PRODUCING A SILICATE COATING ON THE SURFACE OF A METAL SELECTED FROM THE GROUP CONSISTING OF AL, ZN, BI, NI, FE, CU, TI, ZR, V, NB, TA, MO, AND W, WHICH CONSISTS OF THE ELECTROLYTIC TREATMENT OF AN ANODE OF SAID METAL IN AN ELECTRLYTE CONSISTING OF AN AQUEOUS SILICATE SOLTION AT A VOLTAGE SUFFICIENTLY HIGH TO CAUSE SPARKING ON THE ANODE SURFACE, THE CONCENTRATION OF SILICATE ANION IN SAID SOLUTION RANGING FROM AN EFFECTIVE NORMALITY UP TO 0.2 N.

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