US3466234A - Electrolytic formation of films of fe2o3 - Google Patents

Description

United States Patent 3,466,234 ELECTROLYTIC FORMATION OF FILMS OF Fe O Morris Cohen and Peter B. Sewell, Ottawa, Ontario,

Canada, assignors to Canadian Patents and Development Limited, Ottawa, Ontario, Canada, a corporation of Canada No Drawing. Filed June 20, 1966, Ser. No. 558,580 Int. Cl. C23b 9/00, 11/00 US. Cl. 204-56 6 Claims ABSTRACT OF THE DISCLOSURE There is provided a process for depositing ferric oxide films from an electrolytic bath containing ferrous iron in solution onto an anodic conductor. The conductors in the electrolytic bath are controlled as to the potentiostat voltage and current density whereby the ferrous iron is electrolytically oxidized to ferric oxide and the ferric oxide is deposited as a thin film on the anode. The films range in thickness from A. to about 10,000 A.

The present invention relates to the preparation of ferric-oxide-containing materials and particularly to the formation and deposition of relatively thin films thereof on anodes of various types. The invention also contemplates the transforming of the Fe O -type material into 'y-Fe O as well as Fe O Ferric oxide is used widely in the fields of metallurgy, gas purification, pigments, dyeing, and electronics, being employed in the latter instance in the manufacture of recording tapes and in television, radio and radar. It is also known to form good protective coatings but judging from the prior art the applying and aflixing of ferric oxide coatings has been difiicult to say nothing of the task of preparing the coating mixtures which must of course have good adhering qualities and must be compatible with the base material to which they are applied. Where coatings comprising a range of thicknesses are required it has been diflicult if not impossible in the past to obtain them.

Applicants have now discovered a method whereby ferric oxide coatings covering a wide range of thicknesses may be formed by electrochemical deposition from solution.

Applicants have found that fine grained compact Fe 0 -type films can be formed by making a conductor anodic under potentiostatic control in a solution containing dissolved ferrous ion. Films of this type may be used to enhance the protectiveness of the ordinarily formed oxide film on iron, to form colored films on iron or other metals by controlling the thickness to give various interference colors and to form thin magnetic or semiconductor films.

The films can be formed on iron, other metallic conductors such as platinum, zinc and silver. For nonconductors such as plastic tape it would be necessary to apply a thin film of a conducting material such as iron or other metal by evaporation, sputtering, etc. The properties of the oxide can be modified for example by further anodic oxidation in the absence of Fe++ to produce a cation-deficient outer layer, or if desired by incorporation of other ions during the anodic electrodeposition process, such as other oxidizable cations or complexforming anions.

In carrying out the process of the invention anode and cathode are immersed in an electrolytic bath containing ferrous (Fe++) iron in solution. A potentiostat is placed in the circuit and a reference electrode calomel half cell connected to the anode. The anode as indicated above Patented Sept. 9, 1969 ICC may be any conducting material, for example any metal, or any nonconducting material such as wood, paper, cloth, polymeric material, etc., coated with a conductor. The kinetics of deposition do not vary regardless of the nature of the anodic material, but can be varied by changing the nature of the solution.

Concerning the film of Fe O it is possible to control the thickness quite closely by adjusting the concentration of ferrous ion and the time of deposition, and it is most satisfactory to work under controlled potential conditions. The deposition takes place at essentially current efficiency and applicants have deposited films ranging from about 10 A. in thickness to several thousands of A. thickness. The thicker films for example, up to 10,000 A., contain some water which can be driven off by heating under a vacuum. The: conditions of heating to form Fe O or 'y-Fc O are well known. Other ions may also be incorporated into the film by controlling the composition of the electrolyte.

Very thin films in the range of about 10 A. to about 100 A. are of particular interest in the field of solid state devices. It has been found most satisfactory to use dilute solutions of ferrous ion, for example, 0.5 1O- molar, when depositing films in the very low thickness region.

Films are distributed evenly over the entire anode material and are the result of the ferrous ion (Fe++) in solution being oxidized at the anode to Fe O or other insoluble ferric compounds. The electrolyte solution concentration may range from very dilute to solutions saturated in ferrous ion for example 5.0 l0- molar in a borate buffer, pH 8. Where the pH is in the neutral range it is desirable to deaerate the solution.

The potential anode to calomel reference may vary widely, applicants having deposited films satisfactorily from 200.0 to +8000 millivolts.

The time required for deposition will be determined by the film thickness desired and also by the current density and the solution concentration. The current density may range from 10 to 1000 microamperes. per cmfl. The higher the Fe++ concentration the higher is the possible current.

EXAMPLE Colored Fe O films have been produced by anodic deposition of oxide film from ferrous ion in a deaerated borate buflfer solution of pH 8.0. The color of the film is dependent on the thickness in A. of the Fe O layer as shown in the following table, the results having been obtained using an iron anode at +300 millivolts with the solution being 1.0 10* molar in ferrous ion.

Thickness, A. Color Goldish-gray. Light-gray. Gray. Dark-gray. Blueish-gray. Grayish-blue. Grayish-green. Green. Yellowish-green. Pinkish-violet. 1 668. 0 Mauve. 1 1, 900-2, 000 Orange-yellow.

1 On pt.

Results identical with the above may be obtained using platinum as the anode.

If desired transparent coatings may be applied over the colored film layer, though the colors are quite stable and durable.

All of the films :set out in the table contain some boron derived from the buffer solution.

Buffers such as borate are not essential but may be helpful in controlling pH.

Generally the films are highly resistant to abrasion and the bonding to the underlying metal is excellent.

It will be obvious from the foregoing that applicants have devised an extremely versatile, easily manipulated process for applying films of Fe O of uniform thickness and distribution to any anodic material.

If desired the Fe O films may be heated under slightly reducing conditions in a controlled atmosphere to form 'Y'F6203.

We claim:

1. The method of depositing fine-grained, compact ferric oxide films comprising preparing an electrolytic bath containing ferrous iron in solution, conducting electrodes being immersed in the bath, making one of the conductors anodic under potentiostatic control, simultaneously electrolytically oxidizing the ferrous iron to ferric oxide and depositing the ferric oxide in the form of a film on the anode, the voltage of the potentiostat, anode to calomel reference, being in the range of about 200 to +800 millivolts and the current density being within the range of about 10 to about 1000 microamperes per cm.

2. The method claimed in claim 1 Where the anode is a metallic conductor or a nonconductor having a conducting film surface.

3. The method claimed in claim 1, including the step of heat-treating the Fe O films to form 'y-Fe O or Fe O 4. The method claimed in claim 2 wherein the nonconducting materials are selected from Wood, cloth, paper and olymeric materials.

5. The process of claim 1 Where the anode is selected from iron and platinum.

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

References Cited UNITED STATES PATENTS 3,124,520 3/1964 Juda 20486 3,294,665 12/1966 Roller 204227 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl. X.R. 20496