RU2734986C1 - Method for electrochemical deposition of chrome coatings from self-regulating electrolyte based on trivalent chromium compounds - Google Patents

Abstract

FIELD: machine building.

SUBSTANCE: invention relates to electroplating and can be used in machine building, instrument making and other industries for application of protective-decorative coatings on metals and alloys. Method of electrochemical deposition of chrome coatings on metals and alloys involves electrolysis in an aqueous solution containing, mol/l: chromium (III) chloride 0.25–0.3, formic acid 0.6–0.8, boric acid 0.7–0.9, ammonium or alkali metal chloride 3–4, ammonium or alkali metal bromide 0.4–0.6, dihydroxohexaformate tetriochrome (III) chloride 0.4–0.6. Electrolysis is carried out at electrolyte temperature of 20–22 °C, pH 1-2.5 and operating current density of 5–50 A/dm².

EFFECT: invention increases stability of the electrolyte composition during long-term operation and widens the range of operating current densities.

3 cl, 2 tbl, 3 ex, 1 dwg

Description

The invention relates to the field of electroplating and can be used in mechanical engineering, instrument making and other industries for applying protective and decorative coatings on metals and alloys.

A known method of electrochemical deposition of chromium coatings at a pH of 1.1-2.1, a temperature of 35-40 ° C and a current density of 20-50 A / dm ² from an electrolyte that contains, g / l: chromium (III) sulfate - 100- 150, sodium oxalate - 20-30, aluminum sulfate - 100, sodium sulfate - 80.

(YD Hamburg. Electroplated coatings. Handbook for application / M., Technosphere. - 2006. - S. 148).

The advantages of electrolytes based on chromium (III) salts include lower toxicity, a high electrochemical equivalent, which makes it possible to reduce energy consumption for coating deposition, and a higher current efficiency compared to electrolytes based on chromium (VI), as well as a significant improvement in working conditions. and simplification of the wastewater treatment process. At the same time, these solutions are inferior to the standard chromium plating electrolyte based on CrO ₃ in terms of stability in operation and the quality of the coatings obtained.

The closest in technical essence is a method of electrochemical deposition of chromium coatings from an electrolyte containing, mol / l: chromium (III) chloride - 0.3-0.6, formic acid - 0.56-0.78, boric acid - 0, 9-1.0, potassium chloride - 2.7-4.1, potassium bromide - 0.04-0.25, at pH 1.5-4, temperature 25-40 ° C and current density 8-12 A / dm ² .

(US Pat. 5294326 MKI C25D 3/06, NCI 205/287 / Shahin G.E .; Elf Atochem North America, Inc. - No. 960564; Appl. 13.10.92; Publ. 03.15.94).

The known method makes it possible to obtain high quality chrome coatings with a cathodic current efficiency of 22-28%. However, the proposed electrolyte is not sufficiently stable in terms of the content of chromium (III) ions and formate ions, which leads to a deterioration in the quality of coatings during its long-term use.

The objective of the invention and its technical result is to increase the stability of the electrolyte composition during long-term operation and to expand the range of operating current densities.

The technical result is achieved in that the method of electrochemical deposition of chromium coatings includes electrolysis at pH, temperature and cathode current density in an electrolyte containing chromium (III) chloride, formic and boric acids, chloride and ammonium or alkali metal bromide and chromium (III) hydroxoformate at the following ratio of components, mol / l:

chromium (III) chloride - 0.25-0.3,

formic acid - 0.6-0.8,

boric acid - 0.7-0.9,

ammonium or alkali metal chloride - 3-4,

ammonium or alkali metal bromide - 0.4-0.6,

chromium (III) hydroxoformate - 0.4-0.6.

The technical result is also achieved in that the complex compound is introduced into the electrolyte in the form of a powder, which is in non-woven polypropylene bags or in a special container through which the solution is circulated. This technique allows you to avoid stirring up the poorly soluble powder when mixing the solution and carrying it away with the parts.

The technical result is also achieved by the fact that the content of potassium (sodium) ions in the electrolyte in relation to the total concentration of alkali metal and ammonium ions is 0.08-0.13, which is ensured by the concentration of potassium (sodium) ions 0.32-0.5 mol / l with a total concentration of alkali metal and ammonium ions of 3.75-4 mol / l.

Under these conditions, shiny decorative chrome coatings are obtained with a current efficiency of 10-15% while maintaining the stability of the solution with respect to chromium ions and formate ions and the stability of the quality of the coatings during long-term operation.

To prepare solutions of a given composition, a weighed portion of KBr (NaBr, NH ₄ Br) is dissolved in a working bath filled with 1/3 of distilled water heated to 70 ° C. Then HCOOH is added. After that, with constant stirring, the calculated volume of the previously prepared chromium chloride concentrate (1.0 M solution of CrCl ₃ ⋅6H ₂ O, aged for 2 weeks after preparation) and the weighed portion of H ₃ BO ₃ , NH ₄ Cl (NaCl, KCl ). The temperature is kept at 50 ° C to increase the rate of complex formation. Then the solution is brought to the specified volume with distilled water and stirred until the salts are completely dissolved. The solution is allowed to stand until the color changes from green to blue. After preparing a solution of a given composition, the pH of the solution is adjusted using a concentrated KOH solution (NaOH, NH ₄ OH) to pH = 2.5. The electrolyte is processed at i _k = 15 A / dm ² (the cathode is a copper plate, the anode is graphite) without separating the anode and cathode spaces. The amount of passed electricity is about 1A⋅h / l. In this case, the pH can drop from 2.5 to 2.0.

A chromium complex compound is prepared separately. In 500 ml of warm (30-40 C) distilled water with stirring, dissolve 240 g (0.9 mol) CrCl ₃ × 6H ₂ O (solution A). In 400 ml of warm (30-40 C) distilled water with stirring, dissolve 82 g (1.2 mol) of sodium formate (HCOONa) (solution B). With continuous stirring, Solution B is added to solution A and the pH is adjusted to 2.6-2.8 using 10% NaOH solution. The resulting solution is heated to 50-60 ° C and kept at this temperature for 1 hour. After this treatment, the solution is cooled, the pH is adjusted again to 2.6-2.8 and left for 1-2 days to complete the complexation reaction and the formation of a precipitate of dihydroxohexaformate trichrome chloride. The formed precipitate is filtered off, washed with cold 10-15 ° C distilled water, removed from the filter and dried in air. The resulting precipitate, in accordance with the results of elemental analysis, corresponds to the crystal hydrate of the complex compound [Cr ₃ (OH) ₂ (HCOO) ₆ ] Cl, which is further used for the preparation of the electrolyte and the deposition of chromium coatings.

The poorly soluble complex compound [Cr ₃ (OH) ₂ (HCOO) ₆ ] Cl is involved in maintaining equilibrium between the solid phase and the solution according to the reaction

which ensures the maintenance of a constant concentration of the main components of the solution and pH during prolonged electrolysis.

The range of permissible current densities, ensuring the production of high-quality chromium coatings, significantly depends on the ratio between potassium (sodium) and ammonium ions. The largest range of current densities, leading to an increase in the opacity of the electrolyte and the formation of shiny decorative coatings on products of complex configuration, corresponds to the content of potassium (sodium) ions 0.32-0.5 mol / l with a total content of alkali metal and ammonium ions of 3.75- 4 mol / l, which corresponds to the ratio of the concentrations of potassium (sodium) ions to the total concentration of alkali metal and ammonium ions equal to 0.08-0.13.

The invention can be illustrated by the following examples.

Example 1. According to the above method, chromium plating solutions were prepared without a complex (prototype) and with the addition of a complex (the composition is shown in Table 1). Chromium plating was carried out at room temperature in 100 ml cells with graphite anodes with an area of 3 cm ² , copper cathodes. The current density at the anode was 8 A / dm ² , pH = 1–2.5, the amount of electricity passed through a unit volume was 10 A h / L. Then the content of chromium ions Cr ³⁺ and formic acid НСООН was analyzed in solutions. The data in Table 1 show that the administration of a poorly soluble complex helps to maintain the initial concentration of chromium ions and formic acid.

Example 2. According to the above method, chromium plating solutions were prepared without a complex (prototype) and with the addition of a complex (the composition is shown in Table 1). The complex was introduced into the solution in a plastic container with a fabric filter. Chromium plating was carried out at room temperature in 1000 ml cells with graphite anodes with an area of 3 cm ² , copper cathodes. The current density at the anode is 8 A / dm ² , pH = 1-2.5. The electrolysis time for coating one sample was 15 min, the operating current density was from 5 to 50 A / dm ² . The solution temperature was 20-22 ° C. The content of chromium ions Cr ³⁺ and formic acid НСООН in solutions was estimated under conditions of long-term electrolysis - when passing from 10 to 35 A. h / L. The results of studies of the influence of the amount of passed electricity on the ratio of the concentrations of formic acid and chromium and the concentration of these substances in the absence (solution 1) and in the presence of poorly soluble chromium hydroxoformate (solution 2) are shown in Table 2.

The experiments carried out show that the concentration of chromium and formic acid in a solution without the addition of a poorly soluble salt decreases in proportion to the amount of electricity passed). When chromium plating electrolyte is saturated with chromium hydroxoformate, the concentrations of formic acid and chromium decrease slightly due to the establishment of ionic equilibrium between the solution and the poorly soluble salt, but with a further increase in the amount of passed electricity, the change in the concentrations of HCOOH and Cr ^{3+ is} insignificant.

Example 3. According to the above technique, chromium plating solutions were prepared with the addition of a complex (the composition is shown in Table 1) and with a different ratio (X _{K +} ) of the concentrations of potassium (sodium) ions in the electrolyte to the total concentration of alkali metal and ammonium ions in the range from 0 to 0, 2, which was ensured by a change in the concentration of potassium (sodium) ions from 0 to 0.77 mol / l with a change in the total concentration of alkali metal and ammonium ions from 3.15 to 4.25 mol / l. The complex was introduced into the solution in a plastic container with a fabric filter. Chromium plating was carried out at room temperature in 1000 ml cells with graphite anodes with an area of 3 cm ² , copper cathodes. The current density at the anode is 8 A / dm ² , pH = 1-2.5. The electrolysis time for coating one sample was 15 min, the operating current density was from 5 to 50 A / dm ² . The solution temperature was 20-22 ° C.

The figure shows the effect of the ratio (X _{K +} ) of the concentrations of potassium (sodium) ions in the electrolyte to the total concentration of alkali metal and ammonium ions on the limiting current density at pH = 2.5, t = 20-22 ° C, the compositions of the solution are shown in Table 1 ...

The figure illustrates the dependence of the maximum permissible current density on X _{K +} in solutions. As follows from the figure, with an increase in X _{K +, the} value of the limiting current density increases to 50 A / dm ² and passes through a maximum at X _{K +} equal to 0.08-0.13.

Based on the research results, it can be concluded that to obtain high-quality coatings with the highest chromium current efficiency in a wide range of operating current densities in the electrolyte composition, it is necessary that the ratio (X _{K +} ) of the concentrations of potassium (sodium) ions in the electrolyte to the total concentration of alkaline ions metals and ammonium was in the range of 0.08-0.13, while the permissible concentration of K ⁺ in solution is 0.32-0.5 mol / l, and the total concentration of alkali metal and ammonium salts is 3.75-4 mol / l.

Claims (5)

1. A method of electrochemical deposition of chromium coatings on metals and alloys, including electrolysis in an aqueous electrolyte containing chromium (III) chloride, formic and boric acids, chloride and bromide of ammonium or alkali metal, characterized in that the electrolyte additionally contains chloride of dihydroxohexaformiate trichrome (III) with the following ratio of components, mol / l: chromium (III) chloride 0.25-0.3 formic acid 0.6-0.8 boric acid 0.7-0.9 ammonium or alkali metal chloride 3-4 ammonium or alkali metal bromide 0.4-0.6 dihydroxohexoformate trichrome (III) chloride 0.4-0.6, and electrolysis is carried out at an electrolyte temperature of 20-22 ° C, pH 1-2.5 and at a working current density of 5 to 50 A / dm ² . 2. The method according to claim 1, characterized in that the chloride of dihydroxohexa formate trichrome (III) is introduced into the electrolyte in the form of a powder, which is in non-woven polypropylene bags or in a container through which the electrolyte is circulated. 3. The method according to claim 1, characterized in that the content of potassium or sodium ions in the electrolyte in relation to the total concentration of alkali metal and ammonium ions is 0.08-0.13, which provides a concentration of potassium or sodium ions 0.32-0 , 5 mol / l with a total concentration of alkali metal and ammonium ions of 3.75-4 mol / l.

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