CS259377B1 - Water-organic Cr (III) -based galvanic bath for chromium plating - Google Patents

Abstract

A chromium plating bath (based on chromium perchlorate), which eliminates some of the shortcomings of similar baths known to date (chlorine evolution at the anode, low bath conductivity due to the formation of ion pairs) while maintaining the quality of the obtained galvanic coatings. From the point of view of galvanic chromium deposition, a very advantageous electroactive complex in the proposed bath is the [Cr(DMF)6]3+ complex. The aforementioned complex is formed by ligand exchange from the [Cr(H2O)6]3+ complex in the absence of strongly complexing anions in a mixed solvent dimethylformamide — water. The aqueous-organic galvanic bath according to the invention can be used for such purposes where microdiscontinuous chromium coatings are required.

Description

259377

BACKGROUND OF THE INVENTION The present invention relates to the composition of an unconventional galvanic bath for chromium plating which overcomes some of the drawbacks of prior art similar baths (chlorine anode development, low conductivity of the bath due to ion pair formation) with unaltered galvanic coatings. In industrial practice, water and non-aqueous trivalent chromium baths (abroad) are also introduced and used (abroad) in addition to the classical hexavalent chromium base chromium baths [review by W. Immel, Galvanotechnik, 719 (1979) 495]. Cr (III) based baths have a number of advantages over classical Cr (VI) based baths since [-L. Gianeal, Plating and Surface Finishing, 69 (1982) 30; J. J. B. Ward. IRA Christie, Institute of Metal Finishing, 49, (1971) 148], that Cr (III) baths: 1. Working at current densities of about half lower and substantial (up to five times) faster as a Cr (VI) base bath. 2. They do not need barbecue adjustments to improve slip efficiency and the appearance of "burns" has not been observed. 3. Have better dispersion ability. 4. Bahko is handled, not a fire hazard. 5. They do not contain toxic hexavalent chromium and produce no high-chromium vapor and mist. 6. They require a lower chromium concentration and provide simplifications in the purification of waste water because they only require neutralization to precipitate Cr (OH) 3. 7. Provide shiny coatings, irrespective of whether the current is interrupted during plating and does not require unidirectional current sources. 8. In view of the structure of excreted vehicles, it should be noted that, unlike conventional baths, which use microdiscontinuous coatings in particular to use procedures, the Cr (III) baths are themselves microdiscontinuous. In favor of Cr (III) baths, the fact that, according to the multiannual medical research carried out on an employee of the company, which has been chromed with trivalent chromium baths, no damage to workers' health has been observed [U. Korallus, E. Ehrlicher, E. Wustefeld, Arbeitsmedizin, Sozialmedizin, Praventivme-dizin, 9 (1974) 51, 76 248],

Good results have been achieved in the world with Cr (III) mixed water-organic baths. Most commonly, the organic components of the bath are dimethylated amides, particularly Ν, β-diethylformamide (hereinafter DMF), and chromium is added to the baths in the form of chromium (III) chloride (hereinafter CrCl3) (Brit. Patent 1,144,913; U.S. Patent 3,772,167; U.S. Patent 3,772,170; U.S. Patent 3,788,957), exceptionally in the form of chromium sulfate (hereinafter Cr2 (SO4) 3), but with addition of chloride ammonium (hereinafter NH4 Cl) (U.S. Pat. No. 3,833,485).

A disadvantage of all previously used CrCl3-based baths (or with the addition of chloride in another form) is the development of chlorine on the anode during electrolysis, on the one hand the nature and properties of electroactive chromium complexes in baths on the other. The first of these drawbacks is eliminated by introducing a porous partition separating the cathodic space of the anodic (e.g., rit. Patent 1 144 913).

However, the introduction of the diaphragm results in an increase in the overall resistance of the system, so that electrolysis takes place at a higher level, which is economically disadvantageous. What remains problematic is the co-acting effect of chlorine, the developing naanode, and its adverse effect on workers' health and ecological safety.

Considering the advantages and shortcomings of the existing Cr (III) baths (patented abroad), the possibilities of obtaining galvanic coatings have been found to possess properties that would not only satisfy the requirements of practice, but also from the viewpoint of environmental protection. environment.

SUMMARY OF THE INVENTION The present invention is based on the use of chromium (III) chloride (hereinafter Cr (C104) 3) for the preparation of a water-organic chrome pellet. This chromic salt is sufficiently soluble in the DMF-water mixtures for galvanic chromium-plating purposes. The following are added to the chrome bath: ammonium perchlorate (hereinafter NH4C104) to increase the stability of the bath, sodium chloride (hereinafter NaC104) and boric acid (hereinafter H3BO3) to increase the flow efficiency. The solvent is a mixture of DMF and water.

A series of chrome pellets was prepared with the above components, varying the concentrations of the individual components. Testing tests have shown that high-quality, whitish-coated coatings with good adhesion and toughness are excluded from the baths where the concentration ranges of the individual inorganic constituents of the jev limits

Cr (C 104) 3i 0.3-0.8 mol. dm "3, preferably 0.6-0.6 mole dm" 3,

NaClO 4 0.5-0.7 mol. dm "3, preferably 0.6-0.7 mole dm" 3, NH 4 ClO 4 0.4-0.6 mole. dm "3, preferably 0.5-0.6 mol. dm" 3, H 3 BO 3 0.08 - 0.12 mol. dm-3, preferably 0.1 mol. dm "3.

LiC104 (lithium perchlorate) can also be used instead of NaC104. The preferred non-mixed solvent DMF-water is a 30: 70 to 90: 10 volume ratio, preferably 50: 50 to 75: 25.

Note that all perchlorate, ammonium and sodium perchlorates are added to the bath in the form of hydrates, and as far as these are significant, in the above-mentioned resultant ratio of DMF mixed solvent. Water is included both the hydrate (crystal and coordination) water added with said inorganic salts.

The freshly prepared bath, consisting of the above-mentioned components, cannot be used immediately for galvanic chromium plating, since in the fresh bath all trivalent chromium is in the form of aquocomplexes [Cr (H2O) g] 3+. Coatings with satisfactory properties are eliminated after a bath of at least 10 days at a temperature greater than 273 K (preferably between 288 to 298 X). It is preferred to leave the bath for 15 days. The same is needed for a sufficient amount of ligand exchange to take place in the bath [Cr (H2O) g] 3+ + 6 DMF -> [Cr (DMF) e] + 6 H 2 O (1)

Reaction (1) proceeds slowly but spontaneously because DMF molecules have more substantial donor properties than H2O molecules (V. Gutmann, Coordination Chemistryin Non-aqueous Solutions, Springer Verlag, Wien - New York, 1968, p. 19).

The excretion of chromium from a bath containing a complex of [Cr (DMF) e] 3+ is preferable to excretion from a bath containing ibacomplex [Cr (H2O) 6] 3+ for the following reasons: a) Complex [Cr (DMF) 6] 3 + reduces the more positive potentials (about 200mV) and more reversible than the complex [Cr (H2O) s] 3 + - b) After reduction of the [Cr (DMF) 6] 3+ complex and the exclusion of chromium metal at the cathode, the incorporated DMF ligands contribute to buffer the solution around the electrode (while reducing and excreting chromium from the complex [Cr (H2O) 6] 3+, water molecules are released that have an adverse effect on pH formation around the electrode, while the current excretion of hydrogen with chromium, leading to deterioration of the coating It should be pointed out that the complex [Cr (DMF) 6] 3 + is formed only when the solution does not contain molecules or ions with such strong donor properties (complex which may compete with the molecular ligand DMF, such as chlorine Otherwise - and this is the case for all of the above (cited) CrCl3-based patented baths in the mixed solvent DMF - water - complexes with mixed chloride and dimethylformamide ligands are formed, le-bo the donor strength of the DMF and Cl ligands is about the same (V. Gutmann, Coordination Chemistry in Non-aqueous Solutions, Sprinter Verlag, Wien-New York, 1968, p. 168). In the case of a CrCl3 based bath, it should be ground. U.S. Patent 1114,913 discloses the formation of complexes [Cr (DMF) 4Cl 2] + and [Cr (DMF) 3 Cl 3].

The proposed Cr (C104) 3-based bath also exhibits a multiply higher conductivity compared to a CrCl3-based bath (British Patent No. 1,144,913) at the same molar ratio of the respective electrolytes [Cr (C104) 3, NH4Cl, NaC104 and H3BO3i, respectively] . CrCl 3, NH 4 Cl, NaCl and H 3 BO 3] in the same mixed solvent of DMF - water. This can be explained by the increased formation of ion pairs in chloride-containing solutions. It is known, for example, that the value of the association constant of the tonic pair {[Cr (DMF) g] 3+, CI- {is more than 30 times higher than the value of the association pairing tone tone {[Cr (DMF) g] 3+, C1O4 -}. T. D. Lo T. W. Swaddle, Inorg. Chem., 15, (1976) 1881).

Based on the experience with the proposed Cr (C104) 3 based binder, we can conclude that the advantages of the proposed bath based on Cr (III) compared to conventional Cr (VI) baths are that Cr (C104) 3 in contrast to the above-mentioned CrCl3-based patent bath, which consists of the following: a) Chlorine is not produced when working with a Cr (C104) 3-based bath at the anode. b) Excretory potential of the [Cr (DMF) 6] 3+ complex from the Cr (C104) 3-based bath is significantly more positive than the excretion potential of chlorocomplexes [Cr (DMF) 4Cl3j + and [Cr (DMF) 3Cl3] from CrCl3-based powders. c) Experimentally, it was confirmed that Cr (C104) 3-based ceffer is more substantial than a CrCl3-based bath, in which formationion pairs are more widely used. 239377

Bearing in mind points b, c, using a proposed Cr (C104) 3 based bath assumes substantial lower electricity consumption (compared to a bath based on

CrCl3).

Experimentally, the results in the laboratory instrumentation showed that the quality of the galvanic plants obtained from the proposed Cr (C104) 3 base bath is matched by the quality of the coatings obtained from the foreign patented CrCl3-based bath with NH4Cl, NaCl and H: BO3. (British Patent No. 1,144,913),

The plating tests were carried out in a two-electrode connection, the cathode being a glass graphite disc electrode embedded in teflon (supplied by a rotating electrode RDE, Laboratory Equipment Prague).

The cathode was polished and copper-plated before it was chromed, or just polished. In addition to said precisely defined geometrical cathode, copper baffle cathodes were also used. The quality of the electrodeposited coating did not depend on the cathode material. The large-scale carbon anode was separated from the cathodic by a glass frit (S1j. Air-carbon black and ambient temperature were used. High-gloss to quartz trains with excellent adhesion and toughness were eliminated at current densities in the interval from The microscopic disposition of the coatings obtained was confirmed by the primary microscope (using an electron microscope at 2,000-3,000 plating).

At current densities outside this range, the visual appearance of the coatings is aggravated: they lose gloss and darkness.

It should be further noted that the quality of the galvanic coatings obtained from the Cr (C104) 3 based bath is not deteriorated with its age. The functionality of the bath whose flooding composition is given in Example 1 was carefully controlled after one and two years at ambient temperature (fluctuating between 288 and 298 K). The quality of the coatings remained unchanged, although a significant amount of the complex salt [Cr (DMF) 6] (C1O4) 3 was crystallized from the bath over time. (The solubility of [Cr (DMF) e] (ClO 4) 3 in pure DMF is about 130 gm, dm-3, i.e. about 0.15 mole, dm-3, in pure water, this salt is not soluble at all.

When the ligand exchange reaction (1) produces a higher amount of [Cr (DMF) 6] 3 + complex than is capable of dissolving in the respective DMF-water mixture, the excess complex is dissolved out of the supersaturated solution (bath). -circuits in the form of crystals [Cr (DMF) e] (C1O4) 3. Removal of electroactive complex

AND

[Cr (DMF) 6] 3+, caused by the galvanic extraction of chromium from the bath, is then cooked by chemical reaction (1), one by dissolution of crystalline [Cr (DMF) phi (C1O4); the life of the bath-1'a is increased. Example 1 0.6 mol. Dm "3 Cr (ClO 4) 3, 0.5 mole. dm-3NH 4 ClO 4, 0.6 mole. dm-3 NaC10 4 and 0.1 mole. dm" 3 H3BO3 Dissolve 50 ml of DMF-water with stirring. :: 50 and we'll stand for 15 days. (After this time, galvanic coatings are eliminated from the bath.) Electrolysis takes place under air at ambient temperature (298 K). Copper or glass-carbon cathodes, graphite graphite, cathodic and anodic spaces separated by frit (S1). High-quality galvanic coatings are eliminated in the interval of current densities of 800 - 2000 A. m ~ 2. Example 2 0.5 mol. dm "3 Cr (C 104) 3, 0.6 mol. dm" 3 NH 4 ClO 4, 0.7 mol. dm "3 NaClO 4 and 0.1 mole dm-3 H3BO3 are dissolved under stirring to mix DMF-water with a volume ratio of 75:25 and grown for 15 days. The electrodes used in Example 1 are used. at a flow density range of 800-1,800 A m @ 2. Example 3 0.3 mol. dm "3 Cr (C 104) 3, 0.4 mole, dm 3 NH 4 ClO 4, 0.5 mole dm" 3 NaClO 4 and 0.08 molar. Dm ~ 3 H3; Dissolve the mixture of DMF-water (90: 10 by volume) with stirring for 15 days. The electrodes used, as in Example 1. Highly galvanic coatings are deposited at a current density of 600-1,500 A. m "2. During e-electrolysis, the bath is heated slightly. Example 4 0.8 mole dm 3 Cr (C1O 4) 3, 0.6 mole dm 3 NH 4 ClO 4, 0.7 mole dm" 3 NaC10 4 and 0, 12 mole. Dm 3 Dissolve H3BO3 with a mixture of DMF-water 30: 70 by volume and leave to stand for 20 days. The electrodes used in Example 1 are used. Highly galvanic coatings are deposited at a current density of 550-900 A m. "2.

The water-organic galvanic bath for chromium plating according to the invention can be used for purposes where microdiscontinuous coatings are required, characterized by better corrosion resistance.

Claims (2)

Cr-III-based galvanic bath for chromium plating, characterized in that it consists of chromium perchlorate in a concentration of 0.3 to 0.8 mol. dm ^-3 , ammonium perchlorate at a concentration of 0.4 to 0.6 mol. dm ^-3 , sodium or lithium perchlorate at a concentration of 0.5 to 0.7 mol. . dm ³ and boric acid at a concentration of 0.08 to 0.12 mol. dm ^-3 , dissolved in the dimethylformamide-water mixed solvent, wherein the composition of the dimethylformamide-water mixed solvent is in the range of volume ratios of 30: 70 to 90: 10. 2. A water-organic galvanic bath on A Cr (III) base for chromium plating according to claim 1, characterized in that it preferably consists of chromium perchlorate in a concentration of 0.5 to 0.6 mol. dm · ³ , ammonium perchlorate at a concentration of 0,5 to 0,6 mol. . dm ^-3 , sodium or lithium perchlorate at a concentration of 0.6 to 0.7 mol. dm ^-3 and 0.1 molar boric acid. dm ³ dissolved in the dimethylformamide-water mixed solvent, wherein the composition of the dimethylformamide-water mixed solvent is in the range 50:50 by volume ratio