CH641213A5 - Electroplating bath for electroplating chromium or chromium alloys - Google Patents

Description

The invention relates to a galvanic bath for electroplating chromium or chromium alloys, which contains chromium in trivalent form in aqueous solution and at least partially complex thiocyanate bound to the chromium.

The German patent application 2545654 describes a bath for electrolytic chrome plating, in which the chromium source consists of an aqueous solution of a thiocyanate-chromium (III) complex. A process for depositing chromium or a chromium alloy is also described there.

In the published patent application, aquothiocyanato-chromium (III) complexes are also mentioned as an exemplary embodiment, which complexes are prepared by bringing chromium perchlorate and sodium thiocyanate into chemical equilibrium in an aqueous solution. The complexes are represented by the general formula

((H20) 6_nCr (III) (NCS) np-n)

described, where n has a value between 1 and 6. Subscripts are always positive, subscripts can be positive, negative, or zero. In the solution the trivalent chromium is surrounded octahedrally with six ligands. The ligands occupy the inner coordination sphere. These ligands can only be exchanged very slowly with free ligands in the solution. For example, the reaction is running:

(Cr (H20> (NCS)) + 2 + * (NCS) ~

-► (Cr (mO) s * (NCS)) + 2 + (NCS) -

very slowly. This makes the chemical reactions of chromium (III) difficult and makes it necessary

Perform chemical equilibrium at high temperatures. Details can be found in the book by Basolo and Person with the title "Mechanism of Inorganic Reactions: Study of Metal Complexes in Solution", published by Wiley.

The linear thiocyanate anion (NCS-) has remarkable catalytic properties. It can form bonds through its nitrogen atom with metal ions and through its sulfur atom with metals (coordinate) and its electron density is extensively distributed across the three atoms (extensively-locally localized across the three atoms).

The thiocyanate anion presumably catalyzes the electron transfer reaction

Cr (III) + 3e ~ CR (O)

through the formation of multiple ligand bridges between a thiocyanato-chromium (III) complex and the cathode surface. The electrically active intermediate member can be referred to as

Cr (III) - NCS - M,

where M denotes the metal surface of the cathode, which consists of chromium (O) after a first monolayer of chromium is deposited. The “hard” nitrogen combines with the chromium (III) ion and the “soft” sulfur with the metal surface M of the cathode. The formation of multiple ligand bridges by means of thiocyanate in the electrochemical oxidation of chromium (II) on mercury electrodes is described in the journal Inorganic Chemistry, 9, 1024 (1970).

In practice, a chromium oxide (CrOä) and sulfuric acid bath is usually used for chrome plating. Baths in which chromium is hexavalent are very hazardous to health because they emit chromic acid vapors. They also have the disadvantage that an unacceptable milky precipitation of chromium occurs due to a power cut. The deposited chrome also tends to peel off. Accidental interruption of the current can result in significant losses. After all, it is difficult to deposit well-adhering layers that exceed a certain thickness.

Chromium plating baths that have been customary up to now have the disadvantage of limited effectiveness and low precipitation speed. It is also difficult to deposit uniform layers on larger areas. The layers are thicker in places of high current density, e.g. at corners or edges, and "fire" occurs. In addition, the acidic chromium baths have a high chromium concentration, i.e. e.g. Contain 100 to 200g chromium per liter. However, since chromium salts are quite expensive, the chromium concentration should be kept as low as possible in order to make the drip loss compensation bearable. The drip losses, i.e. Losses due to bath liquid, which adheres to them when the chromium-plated parts are removed, are approximately six times greater than the depletion of chromium caused by the plating of the bath.

Numerous efforts have been made to use trivalent chromium salts to deposit chromium or chromium alloys. British Patent 1144913 describes a solution for electroplating chromium, the chromium chloride in a dipolar, aprotic solvent such as e.g. Dimethylformamide, and contains water. British Patent 1333713 discloses a solution containing chromium ammonium sulfate in a dipolar, aprotic solvent and water. The solutions described could, however, be put into practice

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do not enforce. In particular, parts with complicated shapes could not be chromed satisfactorily, and because of the poor electrical conductivity due to the presence of the dipolar, aprotic solvent, voltage sources are required which are able to supply voltages up to 20 volts. Decreasing the amount of dipolar aprotic solvent present resulted in an unstable bath. In addition, the bathroom solution was relatively expensive. The chromium bath solution was between 0.5 and 1.5 molar, i.e. it had a relatively high chromium concentration. In addition, the use of dimethylformamide can lead to health problems.

US Pat. No. 3,917,517 describes a bath for electroplating chromium or chromium alloys which contains chromium chloride or sulfate and to which hypophosphite ions as an additive to or as a substitute for the dipolar, aprotic solvent disclosed in the aforementioned patents, are added. The addition of the hypophosphite ions to the trivalent chromium salt solution is said to eliminate the disadvantages of the solution containing the dipolar, aprotic solvent. However, the rate of precipitation is slower than if the solution contains a high proportion of the dipolar, aprotic solvent. Precipitation rates of only 0.05 to 0.15 µm / minute are achieved, which corresponds approximately to the rate that can be achieved with the best chromium (VI) acid baths. The bath temperature is below 55 ° C and a minimum chromium concentration of 0.5 mol per liter is required.

German Offenlegungsschriften 2612443 and 2612444 describe aqueous solutions for the electrolytic deposition of chromium which contain chromium sulfate and hypophosphite or glycine ions as weak complexing agents. The solution also requires chloride or fluoride ions. The best rate of precipitation is also around 0.15 (im / minute and the most favorable temperature is between 25 and 35 ° C. The most favorable chromium concentration for the production of satisfactory layers is given as 1 mol per liter.

German Offenlegungsschrift 2550615 describes a bath solution which contains chromium sulfate or chloride, ammonium sulfate or chloride, boric acid and a number of additional weak complexing agents, e.g. Contains glycine ions or hypophosphite ions. The concentration of the chromium ions and the concentration of the additional buffer is relatively high.

British patents 1455580 and 1455841 describe another way of precipitating chromium from aqueous solutions of its trivalent salts. The chromium ions are supplied by chromium chloride, chromium sulfate or chromium fluoride. As mentioned, bromide ions, ammonium ions and formate or acetate ions are additionally required. The rate of precipitation at a bath temperature between 15 and 30 ° C is 0.15 µm / minute. The chromium concentration is in the range between 0.1 and 1.2 mol / 1 and preferably 0.4 mol / 1.

It is the object of the invention to provide a galvanic bath for electroplating chromium or chromium alloys, which has a low chromium concentration and with which, at relatively high precipitation speeds, glossy and flawless layers can be produced in such a way that the apparatus used does not cause excessive corrosion and the operator are not exposed to harmful effects.

This object is achieved with a galvanic bath of the type mentioned with the features of the characterizing part of claim 1.

With the present bath, shiny deposits of chrome or chrome alloys are obtained in a current density range of 10 to 1000 + mA / cm2. The rate of precipitation can be up to 0.9 µm / minute. In addition, the range of bath temperature in which work can be carried out is very large and ranges from 20 to 70 ° C. Finally, the concentration of chromium ions in the bath is very low.

Previous attempts to plate chromium from solutions of its trivalent salts with reasonable precipitation rates are believed to have failed because a chromium (III) hydroxide compound was deposited on the cathode at high current densities. The deposition of chromium when using the bath according to the invention is possible even at high current densities with very favorable results, due to the catalytic effect of the thiocyanate and the action of the buffer on the cathode, which is released when the chromium atoms are formed on the cathode , is made possible.

An amino acid, such as e.g. Glycine (NH2CH2COOH) or a formate or an acetate. Amino acids are strong buffers and, when brought into chemical equilibrium with corresponding substances, can also complex with their nitrogen or oxygen atom with metal ions, e.g. trivalent chrome. For example, if an amino acid is brought into chemical equilibrium with a thiocyanato-chromium (III) complex, mixed amino acid-thiocyanato-chromium (III) complexes are formed.

In summary, it can be said that the essential progress of the bath according to the invention results from the combination of the catalytic action of the thiocyanate and the buffer action at the cathode of the buffer released at the cathode when the complex is broken down. Alloys of chromium with the added metal can advantageously be deposited by adding nickel, cobalt or other metal salts to the bath solution.

It is also advantageously possible to selectively deposit chromium and its alloys using photomasks.

The invention is described in detail below using exemplary embodiments.

Example I

To produce an electroplating bath according to the invention, a 0.05 molar aqueous solution of chromium chloride (CrCh • 6H2O) was produced. The solution was saturated with boric acid (H3BO3) (50 g / liter) and then at 80 ° C for one hour with 0.075 mol sodium thiocyanate (NaNCS) per liter solution, 0.16 mol glycine (NH2CH2COOH) per liter, 0.5 mol Potassium chloride (KCl) per liter and 2 moles of potassium bromide (KBr) per liter are brought into chemical equilibrium. The potassium chloride and potassium bromide were added to improve the conductivity of the solution. The chemical equilibrium solution was cooled, its pH was adjusted to 3.0 by the addition of a dilute sodium hydroxide solution, and finally 1 g sodium lauryl sulfate (wetting agent) was added per liter. The plating solution was placed in a Hull cell that contained a flat platinum-plated titanium anode and a flat Hull cell test cathode made of brass. No ion exchange membrane was used to separate the anode and cathode, and the temperature of the solution was 22 ° C. A 5 ampere plating current was passed through the solution for 2 minutes. The current density range was from 10 mA / cm2 to 580 + mA / cm2 (peak of the

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Plate [top of the plate]) received a shiny chrome deposit.

Example II

To illustrate the effect of balancing glycine with chromium (III) thiocyanate, resulting in an aqueous, chemical equilibrium solution containing mixed glycine-sulfato-thiocyanato-chromium (III) complexes , the solution was prepared in the two further process steps A and B described in detail below:

A. A plating solution was prepared by first providing a 0.075 molar chromium sulfate solution (Cr (SC> 4) 3-15H2O). The solution was saturated with boric acid (H3BO3) (80 g / liter) and then brought into chemical equilibrium for one hour at 80 ° C. with 0.15 mol sodium thiocyanate per liter and 0.8 mol sodium sulfate per liter. The sodium sulfate was added to improve the conductivity of the solution. The equilibrated solution was cooled, its pH was adjusted to 2.5 by the addition of dilute sodium hydroxide solution, and finally 0.6 g sodium lauryl sulfate (wetting agent) per liter was added.

This plating solution was filled in a standard hollow cell as in Example I, and then the temperature of the solution was adjusted to 25 ° C and kept at that temperature. A 5 ampere plating current was then passed through the solution for 2 minutes. Here, shiny chrome deposits were obtained on the Hull cell plate in the current density range between 5 and 125 mA / cm2.

B. The solution, which is prepared as described in A, was again chemically balanced at 80 ° C for one hour with the addition of 5 g glycine per liter (0.065 mol per liter). The pH of the solution was then adjusted to 2.5 by adding a dilute sodium hydroxide solution. A current of 5 A was passed through solution B, which was in a standard hollow cell, at a temperature of 250 C for two minutes. A shiny chrome deposit was obtained in the current density range between 5 and 275 mA / cm2.

C. The solution prepared as described in A was chemically equilibrated at 80 ° C for one hour with the addition of 10 g glycine per liter (0.13 mol per liter). The pH was adjusted to 2.6 by the addition of dilute sodium hydroxide solution. A current of 1.6 A was passed through solution C at a temperature of 47 ° C. for 30 minutes, using a cathode with an area of 12 cm 2 (corresponding current density = 130 mA / cm 2). A 10 mm thick chrome deposit (corresponding to 0.33 jim / minute) was obtained.

D. The temperature effect is illustrated below: A solution of 5 A was passed through solution B, which was in a standard hollow cell and heated to 450 ° C., for two minutes. Here, shiny chrome deposits were obtained in the current density range between 12 and 400 mA / cm2.

Example III

A plating solution was prepared essentially as described in Example I of German Offenlegungsschrift 2545654, i.e. 150 g of sodium dichromate (Na2Cr20?) were added to 485 ml of perchloric acid (HC104) and 525 ml of water. 400 ml of hydrogen peroxide were added dropwise until the solution turned deep blue. After this state was reached, the solution was boiled down to half its volume, the hydrogen peroxide being expelled and the chromium perchlorate required

(Cr (C104) 3) solution remained, this solution serves as a chromium (III) source solution for plating.

10 g of glycine was dissolved in water and the pH was adjusted to 2.0 with perchloric acid. 100 ml of the chromium (III) source solution was added to the glycine solution, then the pH was adjusted again to 2.0 by the addition of a sodium hydroxide solution, and finally the volume of the solution was brought to 1 liter by the addition of water. This solution was brought into chemical equilibrium for one hour at 80 ° C. with 0.3 mol of sodium thiocyanate per liter and 1 mol of sodium perchlorate per liter. The solution was cooled to 400 C, saturated with boric acid (H3BO3) (70 g per liter) and 1 g sodium lauryl sulfate per liter was added.

The following plating results were obtained with the solution prepared in Example III.

A. Shiny chrome deposits were obtained at temperatures between 25 and 70 ° C, with the best results at temperatures above 35 ° C.

B. The solution was placed in a Hull cell and heated to 70 ° C. A brass Hull cell cathode was plated at a total current of 10 A for two minutes using a flat platinized titanium anode. A shiny chrome deposit was obtained on the brass plate from the 20 mA / cm2 position to the top of the plate (1000 + mA / cm2). No signs of fire or unsatisfactory precipitation were found.

C. In the solution heated to 70 ° C, plating was carried out on a 6.3 mm diameter brass rod at a current density of 300 mA / cm 2 for ten minutes, while moving the rod in the solution during plating. The chrome deposit thickness determined by weighing was 9 µm.

D. In the solution heated to 70 ° C, plating was carried out on a 6.3 mm diameter brass rod at a current density of 135 mA / cm 2 for ten minutes, moving the rod in the solution during plating. The thickness of the chrome deposit, which was determined by weighing, was 6 (in.

Example IV

A plating solution, except that instead of 0.8 moles of sodium sulfate per liter, 1 mole of sodium perchlorate per liter was used as in Example HC to improve the conductivity of the solution. Using this solution, a shiny 0.85 µm thick chrome deposit was obtained on both sides of a 2x5 cm brass strip under the following conditions: pH: 2.55, temperature: 46 ° C, current: 2A, and time: 2, 5 minutes. The current density was 100 mA / cm2 and the chrome precipitation rate was 0.3 \ im / minute.

Example V

The procedure for producing another plating solution according to the invention was as follows, the stated amounts of the ingredients relating to 1 liter of plating solution:

1. 60 g of boric acid (H3BO3) are added to 600 ml of deionized or distilled water. The solution is heated and stirred until the boric acid dissolves. The pH is adjusted to a value between 2 and 2.4 by adding a 10% NaOH or 10% H2S04 solution.

2. 33.12 g of chromium (III) sulfate (Crî (S04) 3-15H2O) are added to the boric acid solution prepared in step 1.

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3. 32.43 g of sodium thiocyanate (NaSCN) are added to the solution generated in step 2. The mixture is heated to 85 ± 5 ° C and then stirred at this temperature for 90 minutes.

4. Then cool to room temperature and add 10 g glycine (NH2CH2COOH) to the solution. The pH is adjusted as in step 1. The mixture is then heated to 85 ± 50 C and stirred at this temperature for 90 minutes.

5. The mixture is then cooled to room temperature and 140 g of sodium perchlorate (NaClO-t-H2O) are added. The mixture is then heated and stirred until completely dissolved.

6. The pH is adjusted to 2.5 as in step 1.

7. The volume of the solution is brought to 1 liter with deionized or distilled water.

8. Add 1 g of sodium lauryl sulfate or 0.1 g of FC98 (a wetting agent sold by 3M Corporation under this trade name) to the solution. The mixture is then stirred until completely dissolved.

The plating solution is now ready. Suitable plating conditions are listed below:

Favorable results are achieved with current densities in the range between 50 and 150 mA / cm2 at pH values in the

Range between 2.0 and 4.0 and at temperatures in the range between 25 and 60 ° C. With a current density of 105 mA / cm2, a pH value of 3.5 and a temperature of 500 C, 0.6 µm chromium is deposited within 2 minutes (efficiency 20 to 23%).

The anode current density should be kept at around 40 mA / cm2. The anodes can be made of placed titanium or carbon.

It should be done under a fume hood, since there is only a small amount of electrochemical cleavage of the thio-cyanate anion at the cathode.

Advantageously, aspartic acid, arginine, histidine and hydroxyproline can also be used as the buffer in addition to the glycine shown

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(NH.CH2.CH (OH) .CH2.CH.COOH) and also by di- or tripeptides formed from two or three amino acids by condensation, such as e.g. Diglycin or triglycin can be used. These last-mentioned six buffers can be used alone or as an additive to glycine in order to adapt the galvanic bath according to the invention to special applications and to influence the color of the precipitate.

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Claims (7)

641 213 1. Galvanic bath for electroplating chromium or chromium alloys, which in aqueous solution contains chromium in trivalent form and at least partially complex thiocyanate bound to the chromium, characterized in that the bath contains a buffer which is in chemical equilibrium with the complex solution and one which provides ligands complexly bound to the chromium. 2. Bath according to claim 1, characterized in that the buffer consists of an amino acid, a formate or an acetate. 2nd PATENT CLAIMS 3. Bath according to claim 2, characterized in that the buffer consists of glycine. 4. Bath according to one of claims 1 to 3, characterized in that sulfate or chlorine ions are present in it, which are at least partially in the form of complex ligands bound to the chromium. 5. Bath according to one of claims 1 to 4, characterized in that it contains portions of chromium sulfate, sodium thiocyanate and glycine which are in chemical equilibrium in an aqueous solution saturated with boric acid. 6. Bath according to claim 5, characterized in that sodium perchlorate is additionally contained in the bath solution. 7. Bath according to one of claims 1 to 4, characterized in that it contains portions of chromium perchlorate, glycine and sodium thiocyanate, which are in chemical equilibrium in an aqueous solution saturated with boric acid.