Electrolyte for electroplating

Abstract

There is provided an electrolyte for the electrodeposition of chromium comprising: (A) water; (B) at least one chromium salt; and (C) at least one complexing agent,
wherein the molar ratio of components B:C is in the range of 1:1 to 1:50. There is also provided a method for electrodepositing chromium metal onto a conductive substrate.

Claims

1. An electrolyte for the electrodeposition of chromium comprising: (A) less than 50 wt % water; (B) at least one chromium salt; and (C) at least one complexing agent selected from the group consisting of acetamide, urea, ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol and glycerol, wherein the molar ratio of components B:C is in the range of 1:1 to 1:50.

2. The electrolyte according to claim 1, wherein the chromium salt comprises at least one salt selected from the group consisting of CrCl.sub.3.6H.sub.2O, KCr(SO.sub.4).sub.2.12H.sub.2O and Cr.sub.2(SO.sub.4).sub.3.10H.sub.2O.

3. The electrolyte according to claim 1 further comprising an additive comprising at least one selected from the group consisting of boric acid, lactic acid, citric acid, ethylene diamine, sodium borate, sodium citrate, sodium phosphate, nicotinic acid, dimethyl hydantoin and methyl nicotinate.

4. The electrolyte according to claim 3 wherein the concentration of the additive is in the range of from 0.05 to 0.5 mol dm.sup.3.

5. The electrolyte according to claim 1 further comprising at least one bromide or iodide salt.

6. The electrolyte according to claim 5 wherein the salt is present is in a concentration of from 0.05 to 0.2 mol dm.sup.3.

7. The electrolyte according to claim 5, wherein the salt is sodium iodide or lithium iodide.

8. A method comprising the steps of: (i) contacting a conductive substrate and a counter electrode with the electrolyte as defined in claim 1; and (ii) passing a current through the electrolyte to electrodeposit chromium metal onto the conductive substrate.

9. The method according to claim 8 wherein the conductive substrate comprises a material selected from the group consisting of mild steel, copper, aluminium, stainless steel, brass, cobalt and alloys thereof.

10. The method according to claim 8 wherein the current has a density in the range 50 to 300 mAcm.sup.2.

11. The method according to claim 8 wherein the electrodeposition is carried out at a temperature of between 30 and 60 C.

12. The method according to claim 8 further comprising the step of: moving the conductive substrate through the electrolyte either by (i) rotation, wherein the rotation frequencies are in the range 0.1 to 10 Hz; or (ii) oscillating horizontal motion, wherein the oscillation frequencies are in the range 0.1 to 10 Hz.

13. The method according to claim 8 wherein the chromium metal electrodeposited during the passing step has a thickness of 5 to 500 m.

14. The method according to claim 8 wherein the chromium metal electrodeposited during the passing step has a hardness greater than 600 HV.

15. The electrolyte according to claim 1, wherein the electrolyte comprises from 10 to 25 wt % water.

Description

DETAILED DESCRIPTION AND BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows an optical photograph, SEM image, thickness cross section and plating conditions of chromium deposit obtained from the electroreduction of 2 urea: CrCl.sub.3.6H.sub.2O with and without additives, for 1 hour at 40 C. and 4-5 V.

(2) FIG. 2 shows an optical photograph, SEM image, thickness cross section and plating conditions of chromium deposit obtained from the electroreduction of 2 urea: KCr(SO.sub.4).sub.2.12H.sub.2O with and without additives, for 1 hour at 40 C. and 4-5 V.

(3) FIG. 3 shows the effect of current density and potential pulse sequences on deposit morphology.

(4) FIG. 4 shows the effect of current density on deposit morphology obtained in a flow cell with a flow rate of 72.2 cm.sup.3/s.

(5) FIG. 5 shows the effect of current density on the deposit morphology obtained using the flow cell with a flow rate of 72.2 cm.sup.3/s using chrome alum:urea:water based eutectic.

(6) The optimum current density is in the range 50 to 300 mAcm.sup.2.

(7) The temperature can affect speciation and mass transport. The temperature at which the above-described electrodeposition methods are conducted may be, for example, any temperature between 20 and 60 C. The optimum temperature is between 30 and 60 C.

(8) Mass transport is vital in controlling morphology and optimum hardness and appearance are obtained when the cathode is moved through the electrolyte during the electrodeposition process. Movement is controlled by rotation (where rotation frequencies are in the range 0.1 to 10 Hz) or horizontal motion (where oscillation frequencies are in the range 0.1 to 10 Hz). This replenishes the electrolyte close to the electrode surface.

(9) In relation to the above-described electrodeposition method, the conductive substrate may be any suitable solid, conductive material such as mild steel, copper, aluminium, stainless steel, brass, cobalt or alloys thereof.

(10) Further, the reducing potential applied to the conductive substrate may be, for example, a constant potential. Alternatively, the deposition can be achieved by utilising a constant current. The current density is calculated based on the size of the substrate which is being plated.

(11) In particular embodiments of the invention, the electrodeposition in the above-described methods is conducted under an inert atmosphere (e.g. under an atmosphere of argon or, particularly nitrogen).

(12) In a preferred embodiment, the electrolyte comprises 20 wt % water 1CrCl.sub.3.6H.sub.2O and 2ChCl.

(13) As discussed above, deposit morphology can be significantly affected by mass transport. By mechanically moving the sample in the solution this provides better deposit morphology and improved hardness.

(14) In an experiment, the plating was conducted from 40 litres volume of Chromline 50 (20% H.sub.2O w/w) with 0.1 M NaBr and 0.1 M H.sub.3BO.sub.3. The conditions were as follows: One cathodemild steel plate (1 mm thickness for all samples) Two anodesIrO.sub.2 coated Ti mesh (Electrode area=1056 cm.sup.2), anode/cathode distance was 13 cm Bath temperature was at 40 (3) C. Plated sample was moved laterally at ca. 0.5 Hz frequency

(15) Examples of deposits obtained by this process are shown in FIG. 3. Pulsing the applied potential also affected the deposit morphology as shown in FIG. 3.

(16) A flow cell can also improve deposit morphology and thickness at lower current densities, as shown in FIG. 4.

(17) In a further experiment, the plating was conducted from 11.8 litres volume of Chromline 50 (20% H.sub.2O w/w) in a flow cell. The conditions were as follows: One cathodemild steel plate (1 mm thickness for all samples) One anodeIrO.sub.2 coated Ti mesh (EA 35=cm.sup.2), anode/cathode distance set at 3.6 cm Reaction temperature was controlled at 38 (4) C. Voltage was at 15 (4) V but lower current densities were required Flow rate was at 72.2 cm.sup.3/s

(18) The adhesion of the chromium layer onto a mild steel substrate can also be dependent upon the pre-treatment protocol. A suitable protocol to achieve effective degreasing involves the following process. Degrease for 1 minute in hexane at room temperature with stirring Degrease for 10 minutes in Anapol C with stirring at 60 C. Rinse with water Rinse with acetone Dry with compressed air

(19) The use of chrome alum based liquids with water produces coatings with less cracks and a harder surface (see FIG. 5). In a further experiment, the plating was conducted from 0.3 litres volume of chrome alum/urea DES with 30% weight water. The conditions were as follows: One cathodemild steel plate (1 mm thickness for all samples) One anodeIrO.sub.2 coated Ti mesh (area=4 cm.sup.2), anode/cathode distance was 2.5 (0.2) cm Reaction temperature was controlled at 17 (2) C. Carried out in the same cell flow cell as discussed above.