Electrolytic processes for coating metal surfaces to provide high resistance to corrosion and abrasion

Abstract

A process for coating a metal article may include: preparing an electrolytic bath including a suspension of boron carbide particles in an aqueous solution including: at least one nickel (II) salt; and at least one phosphorous compound selected from: phosphoric acid, phosphorous acid, hypophosphorous acid, or their salts; and immersing, in the electrolytic bath, a cathode and an anode, and carrying out electrodeposition by passing direct current in the electrolytic bath. The cathode may include the metal article to be coated. The boron carbide particles may have an average size greater than or equal to 0.01 micron (μm) and less than or equal to 2 μm. The boron carbide particles may have been pretreated with at least one carboxylic acid or a derivative thereof. The at least one carboxylic acid or the derivative thereof may have a solubility in water at 20° C. greater than 0.10 grams/liter.

Claims

1. A process for coating a metal article, the process comprising: preparing an electrolytic bath comprising a suspension of boron carbide particles in an aqueous solution comprising: at least one nickel (II) salt; and at least one phosphorous compound selected from: phosphoric acid, phosphorous acid, hypophosphorous acid, or their salts; and immersing, in the electrolytic bath, a cathode and an anode, and carrying out electrodeposition by passing direct current in the electrolytic bath; wherein the cathode comprises the metal article to be coated, wherein the boron carbide particles have an average size greater than or equal to 0.01 micron (μm) and less than or equal to 2 μm, wherein the boron carbide particles have been pretreated with at least one carboxylic acid or a derivative thereof, and wherein the at least one carboxylic acid or the derivative thereof has a solubility in water at 20° C. greater than 0.10 grams/liter.

2. The process of claim 1, wherein the at least one carboxylic acid or the derivative thereof has a solubility in water at 20° C. greater than 10 grams/liter.

3. The process of claim 1, wherein the at least one carboxylic acid or the derivative thereof is selected from: (i) aliphatic and/or aromatic C.sub.1-C.sub.8 mono-carboxylic acids; (ii) aliphatic and/or aromatic C.sub.1-C.sub.12 di-carboxylic acids; or (iii) derivatives thereof.

4. The process of claim 3, wherein the aliphatic and/or aromatic C.sub.1-C.sub.8 mono-carboxylic acids are mono-hydroxylated, poly-hydroxylated, or mono-hydroxylated and poly-hydroxylated.

5. The process of claim 3, wherein the aliphatic and/or aromatic C.sub.1-C.sub.8 mono-carboxylic acids comprise one or more of formic acid, acetic acid, propionic acid, butyric acid, valerianic acid, capronic acid, enantic acid, caprylic acid, nonanoic acid, capric acid, acrylic acid, or methacrylic acid.

6. The process of claim 3, wherein the aliphatic and/or aromatic C.sub.1-C.sub.12 di-carboxylic acids are mono-hydroxylated, poly-hydroxylated, or mono-hydroxylated and poly-hydroxylated.

7. The process of claim 3, wherein the aliphatic and/or aromatic C.sub.1-C.sub.12 di-carboxylic acids comprise one or more of oxalic acid, adipic acid, phthalic acid, azelaic acid, sebacic acid, tartaric acid, or aldaric acid.

8. The process of claim 1, wherein the boron carbide particles have been pretreated with a mixture of tartaric acid and acrylic acid.

9. The process of claim 8, wherein a weight ratio between the tartaric acid and the acrylic acid is greater than or equal to 0.2:1 and less than or equal to 5:1.

10. The process of claim 1, wherein the boron carbide particles have been pretreated by suspending the boron carbide particles in water and adding the at least one carboxylic acid or the derivative thereof to the suspension thus obtained.

11. The process of claim 10, wherein the at least one carboxylic acid or the derivative thereof is added to the suspension of boron carbide particles in an amount greater than or equal to 1% by weight and less than or equal to 40% by weight with respect to a weight of the boron carbide particles.

12. The process of claim 1, wherein the boron carbide particles have been pretreated at temperature greater than or equal to 30° C. and less than or equal to 90° C., for time greater than or equal to 20 minutes and less than or equal to 120 minutes.

13. The process of claim 1, wherein the boron carbide particles have an average size greater than or equal to 0.05 μm and less than or equal to 1 μm.

14. The process of claim 1, wherein the boron carbide particles are purified before the boron carbide particles have been pretreated with the at least one carboxylic acid or the derivative thereof.

15. The process of claim 1, wherein the aqueous solution comprises Ni(II) ions in total concentration greater than or equal to 0.3 moles/liter and less than or equal to 3.0 moles/liter.

16. The process of claim 1, wherein the at least one phosphorous compound is a mixture of phosphorous acid with hypophosphorous acid, phosphorous acid with one or more salts or hypophosphorous acid, one or more salts of phosphorous acid with hypophosphorous acid, or one or more salts of phosphorous acid with one or more salts or hypophosphorous acid.

17. The process of claim 1, wherein pH of the aqueous solution is greater than or equal to 0.5 and less than or equal to 4.

18. The process of claim 1, wherein the electrodeposition is carried out for a time so as to obtain a coating thickness greater than or equal to 5 μm and less than or equal to 200 μm.

19. The process of claim 1, wherein the boron carbide particles have been pretreated at temperature greater than or equal to 40° C. and less than or equal to 80° C., for time greater than or equal to 40 minutes and less than or equal to 80 minutes.

20. The process of claim 1, wherein the electrodeposition is carried out for a time so as to obtain a coating thickness greater than or equal to 10 μm and less than or equal to 75 μm.

Description

(1) The present invention will now be further illustrated with reference to the figures accompanying the present description merely for exemplifying purposes, in which:

(2) FIG. 1 shows a scanning electron microscope (SEM) microphotograph of a section of a coated sample according to the invention (200× magnification), where the black portion outside the coating layer is the resin in which the sample was incorporated, lapped and polished to perform the SEM analysis, while the underlying white part is the base metal on which electrodeposition was carried out; the presence of black particles consisting of boron carbide can easily be appreciated;

(3) FIG. 2 shows a SEM microphotograph obtained on the same sample at front view, with an enlargement of a portion that highlights the typical mammillary structure of Ni/P alloys; the presence of black particles consisting of boron carbide can easily be appreciated.

(4) The following working examples are provided merely to illustrate the present invention and should not be construed in a sense that would limit the scope of protection defined by the accompanying claims.

EXAMPLE 1

(5) Boron carbide in the form of particles having average size of 0.4 μm was suspended, in a quantity of 10 g/l, in an aqueous phase having a pH value of 2.0 (obtained by adding a H.sub.2SO.sub.4 aqueous solution), containing 1 g/l of tartaric acid and 1 g/l of acrylic acid. The suspension thus obtained was maintained for about 1 hour in an ultrasonic sonicator at 60° C.

(6) To the suspension was then added an aqueous solution consisting of:

(7) TABLE-US-00001 nickel sulphate (NiSO4 * 6H.sub.2O) 75 g/l phosphorous acid 15 g/l sodium hypophosphite 40 g/l sodium acetate (CH.sub.3COONa) 120 g/l.

(8) The electrolytic bath thus obtained was brought to a pH value of 2.0 with a H.sub.2SO.sub.4 aqueous solution.

(9) The electrodeposition process was carried out on a steel plate, used as cathode, while the anode used was a titanium anode coated with mixed oxides. During the electrodeposition, the electrolytic bath was maintained for three hours at a temperature of about 80° C. and the current density at a value of about 10 A/dm.sup.2.

(10) At the end of the electrodeposition process, a coating layer of the plate was obtained with a thickness of about 25 μm, containing about 15% by weight in phosphorus, having Vickers hardness of 900 HV. It should be noted that such a high hardness was obtained without subjecting the specimen to a subsequent heat treatment (annealing) step, which is usually carried out for this type of coatings for the purpose of increasing its hardness.

(11) On the specimen thus treated, corrosion resistance was measured in neutral saline fog, according to the ISO 9227:2017 standard, obtaining a value of 1000 hour (rating: 10).

(12) The sample thus coated was subjected to scanning electron microscope (SEM) analysis, and the microphotographs thus obtained are shown in FIG. 1 (section) and FIG. 2 (surface). The coating layer showed a substantially homogeneous distribution of boron carbide particles in the Ni/P matrix, with a mammillary surface structure that is typical of the Ni/P alloy.

EXAMPLE 2 (COMPARATIVE)

(13) Example 1 was repeated in the same operating conditions, the only difference being that the boron carbide particles were not pre-treated with tartaric acid and acrylic acid, but rather used as such.

(14) At the end of the electrodeposition process, a coating layer of the plate was obtained with a thickness of about 25 μm, containing about 15% by weight of phosphorus, having Vickers hardness of 550 HV (without annealing).

(15) On the specimen thus treated, corrosion resistance was measured in neutral saline fog, according to the ISO 9227:2017 standard, obtaining a value of 120 hours (rating: 8).