METHOD FOR ELECTRODEPOSITING A FUNCTIONAL OR DECORATIVE CHROMIUM LAYER FROM A TRIVALENT CHROMIUM ELECTROLYTE

Abstract

A method for the electrodeposition of a functional or decorative chromium layer onto a metallic substrate in an electrodeposition process from a halide-ion free and boric acid free aqueous electrolyte solution and to the coated product obtained thereby.

Claims

1. A method for the electrodeposition of a functional or decorative chromium layer onto a metallic substrate in a batch or a continuous electrodeposition process from a halide-ion free and boric acid free aqueous electrolyte solution, the electrolyte comprising: i) a trivalent chromium compound provided by a water-soluble chromium(III) salt wherein the electrolyte solution contains at least 50 mM and at most 1000 mM Cr3+-ions; ii) a total amount of from 25 to 2800 mM of sodium sulphate or potassium sulphate; iii) a formate salt as a complexing agent at a $\left(\frac{\mathrm{complexing}\mathrm{agent}}{{\mathrm{Cr}}^{3+}}\right)$ molar ratio of at least 1:1 and at most 4.0:1; iv) optionally sulphuric acid or sodium hydroxide or potassium hydroxide to adjust the pH to the desired value; v) optionally a surfactant to facilitate the release of hydrogen gas bubbles from the substrate, wherein the aqueous electrolyte solution has a pH of between 1.50 and 3.00 measured at 25° C. and wherein the temperature of the aqueous electrolyte solution during electrodeposition is between 30 and 60° C., wherein the substrate acts as a cathode and wherein one or more anodes comprising a catalytic coating of i). iridium oxide or ii). a mixed metal oxide comprising iridium oxide and tantalum oxide for reducing or eliminating the oxidation of Cr3+-ions to Cr6+-ions, and wherein the electrodeposition is performed by means of pulsed electrodeposition comprising two or more current pulses at a selected current density for a selected pulse duration, wherein each current pulse is followed by an interpulse period wherein the current density is set to 0, wherein the interpulse period is at least 0.1 seconds and wherein the pulse duration is at least 0.1 seconds.

2. The method according to claim 1, wherein the electrolyte solution consists of: i) the trivalent chromium compound provided by a water-soluble chromium(III) salt wherein the electrolyte solution contains at least 50 mM and at most 1000 mM Cr3+-ions; ii) a total amount of from 25 to 2800 mM of sodium sulphate or potassium sulphate; iii) a formate salt as a complexing agent at a $\left(\frac{\mathrm{complexing}\mathrm{agent}}{{\mathrm{Cr}}^{3+}}\right)$ molar ratio of at least 1:1 and at most 4.0:1; iv) optionally sulphuric acid or sodium hydroxide or potassium hydroxide to adjust the pH to the desired value; v) optionally a surfactant to facilitate the release of hydrogen gas bubbles from the substrate; vi) remainder inevitable impurities.

3. The method according to claim 1, wherein the pH is adjusted to a value of 2.00 or more.

4. The method according to claim 1, wherein in the batch electrodeposition process the pulse duration is between 0.5 and 2.5 seconds, and wherein the interpulse period is between 0.5 and 5 seconds.

5. The method according to claim 1, wherein in the continuous electrodeposition process the pulse duration is between 0.5 and 2.5 seconds, and wherein the interpulse time is between 0.5 and 5 seconds.

6. The method according to claim 5, wherein the pulse duration in the continuous electrodeposition process is between 0.5 and 2 seconds, and wherein the interpulse time is between 0.5 and 2 seconds.

7. The method according to claim 1, wherein the water-soluble chromium(III) salt is basic chromium(III)sulphate and/or wherein the complexing agent is sodium formate.

8. The method according to claim 1, wherein the amount of chromium deposited is at least 1 g/m2.

9. The method according to claim 1, wherein the temperature of the electrolyte during electrodeposition is at least 35° C., preferably wherein the temperature of the electrolyte during electrodeposition is at most 50° C.

10. The method according to claim 1, wherein the line speed of the electrodeposition line in the continuous electrodeposition process is at least 50 m/min, preferably at least 100 m/min.

11. The method according to claim 1, wherein the molar complexing agent/Cr ratio is 2.0:1.

12. The method according to claim 1, wherein the metallic substrate is an unalloyed or low-alloyed steel strip or sheet, preferably a nickel coated steel strip or sheet or a copper coated steel strip or sheet.

13. The method according to claim 1, to provide a metallic substrate with a functional or decorative chromium layer having a gloss value of at least 800 when measured under an angle of 20° in accordance with ISO 2813:2014.

14. The method according to claim 1, to provide a metallic substrate with a functional chromium layer for use in a photovoltaic application the chromium layer having a thickness of between 75 and 1000 nm.

15. A method of producing a photovoltaic device comprising including in the photovoltaic device the metallic substrate with the functional chromium layer produced according to claim 14.

16. The method according to claim 15, wherein the photovoltaic application is a solar cell.

17. The method according to claim 14, wherein the chromium layer has a gloss value of at least 800 when measured under an angle of 20° in accordance with ISO 2813:2014.

18. The method according to claim 1, wherein the pH is adjusted to a value of 2.00 or more, and to a value of 2.75 or less.

19. The method according to claim 12, wherein the metallic substrate is a nickel coated steel strip or sheet or a copper coated steel strip or sheet.

Description

FIGURES

[0077] The invention is further explained by means of the following, non-limitative figures.

[0078] FIG. 1: Single pulse electrodeposition process with pulse duration of 1 second as a function of current density. Left hand side: Chromium coating weight in mg/m.sup.2, right hand side: gloss expressed in GU (Gloss Units).

[0079] FIG. 2: Relation between the number of pulses and the chromium coating weight deposited for a current density of 26 A/dm.sup.2, pulse durations of 1 s and an interpulse time of 10 s. The top line presents the ICP-MS measurements, the middle line presents the benchtop XRF measurements and the lower line presents the handheld measurements.

[0080] FIG. 3: Loss in process efficiency with increasing interpulse time. S means that the underlying nickel layer of the Hilan was shiny (see table 3) and D that the underlying nickel layer was dull. 8 and 20 mean the number of is pulses of 26 A/dm.sup.2 used to deposit the chromium layer.

[0081] FIG. 4: SEM images of chromium surface obtained by single-pulse process vs multi-pulse process. Both images have been made at the same magnification. The measurement bar represents 1 μm. The Zeiss equipment was operated at an EHT of 5.00 kV, Signal A=SE2, Magnification of 11430 x, and the size of the observed specimen is 10.00×7.500 μm.sup.2. The I Probe was 150 pA, and the WD is 4.6 mm. Pixel size 9,766 nm).

[0082]