FI62347B - GALVANISERINGSBAD - Google Patents

Description

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England-England (GB) 35158/76 (71) Albright & Wilson Limited ,, P.0. 3ox 3> Oldbury, Warley, West Midlands, England-England (GB) (72) John Cooper Crowther, Stourbridge, West Midlands, Brian Allen, Bilston,

West Midlands, England-England (GB) (7I +) Oy Kolster Ab (5I +) Electroplating Baths - Galvaniseringsbad This invention relates to chromium plating of trivalent chromium solutions and is a modification of the invention described in GB Patent 1,455,580.

Our above application relates to chromium plating in a bath consisting of an aqueous solution containing chromium, formate or acetate, bromide and ammonia, preferably together with borates and electrically conductive salts such as alkali metal chlorides or sulphates. One problem that is sometimes encountered with these types of baths is the tendency of chromium to precipitate.

We have found that a very stable bath is obtained if formate and acetate are used as an addition in certain amounts.

Our invention comprises an aqueous electroplating bath for chromium plating comprising 0.1-1.2 moles per liter of colic chromium 0.5-3 moles of formate per mole of trivalent chromium, 0.1-0.5 moles of acetate per mole of formate, at least 0.01 moles per liter of bromide and at least 0.05 moles per liter of ammonia.

Preferably the bath contains at least 0.1 moles per liter of borate and also preferably at least 0.5 moles per liter of an alkali metal such as sodium or potassium. Preferably, the bath further contains 0.5 to 5 moles of 2,62347 sulfate and / or 0.5 to 5 moles of chloride. The bath may also contain fluoride and preferably a wetting agent.

The ingredients of the bath can be summarized as follows: (A) Trivalent chromium This is an essential ingredient in the solution of the invention. Concentrations of less than 0.1 mole or more than 1.2 moles of trivalent chromium cause a significant reduction in coating performance and the concentration should therefore be kept within these limits and preferably between 0.2 and 0.6 moles. Preferably, the solution is substantially free of hexavalent chromium, and preferably substantially all of the chromium in the solution is trivalent chromium prior to coating.

(B) Bromide This is an essential ingredient. The bromide content should be kept above 0.01 mole to avoid the formation of hexavalent chromium and to reduce the coating rate. The maximum concentration is not critical, but is typically less than 4 moles and preferably less than 1 mole. Iodide works in the same way as bromide, but has the disadvantage that the free iodide that would be formed during the coating is only 0.03% by weight soluble in water, while the bromide is 4% soluble. Thus, an attempt to use iodide instead of bromide will result in unacceptable precipitation of iodine. In addition, iodide is too expensive to be used economically in place of bromide. However, it is in principle possible to replace small portions of bromide with iodide, and references herein to bromide do not exclude bromide containing small amounts of iodide.

(C) Formate This is an essential ingredient. Typically, the molar ratio of formate to chromium should not exceed 3: 1 and preferably does not exceed 2.5: 1, in order to avoid unacceptably strong precipitation of the corresponding chromium salt. If the ratio is less than 0.5: 1, the coating power is undesirably reduced. Preferably, the total molar ratio of formate to acetate to chromium is between 3: 1 and 1: 1, and most preferably between 2.5: 1 and 1: 1, e.g. about 2: 1.

(D) Acetate This is an essential ingredient of the bath. The presence of acetate in addition to formate increases the stability of the bath. Less than about 0.1 moles of acetate per mole of formate poses a detrimental high risk of chromium precipitation from the bath. Too much acetate tends to lower the coating rate. The amount of acetate should not exceed 0.5 moles per mole of formate. However, for commercially satisfactory rates, the amount of acetate is preferably 0.2 to 0.4 moles per mole of formate, most preferably 0.2 to 0.3, e.g. 0.25 moles per mole of formate.

(E) Ammonia

The presence of ammonia is an essential feature of our invention. In general, if the ammonia content is less than 0.1 mole, a loss of coating power occurs at a high current density. The upper limit is not critical and ammonia may be present up to the saturation amounts, i. about 4 moles. Preferably ammonia is present in concentrations of 1-3 moles. Ammonia is present as the ion NH 4. Preferably, substituted such as hydroxylammonium, hydrazonium, alkylammonium, arylammonium, or heterocyclic ions such as pyridinium are not present as they cause harmful chromium precipitation.

(F) Borates

Although it is possible to coat chromium on the borate-free solutions of our invention, we have not been able to obtain results which we consider to be completely satisfactory commercially in the absence of borate. Concentrations below 0.1 mole provide an undesirably low coating power. The function of borate is unknown. Its beneficial effects may be due in part to its buffering action. However, other buffer salts such as phosphates and citrates prove to be relatively ineffective.

(G) Hydrogen ion

The best results are obtained if the bath is slightly acidic. At low pH values (less than 2), there is some loss of coating power, which becomes unacceptable at pH values below 1. If the pH is above 4, the coating rate is unfavorably slow. The optimum pH is between 2-3.5.

(H) Chloride and / or fluoride This is optional but preferred in the case of chloride.

The amount can range from zero to the maximum amount allowed by solubility. The chloride is generally added to the bath as an anion of a conductivity salt (e.g., sodium chloride), such as ammonium chloride, which is a suitable way to meet the ammonia needs of the bath, chromium chloride, which may optionally be used to meet at least part of the chromium requirement, and / or hydrochloric acid. The pH-value. Preferably the chlorine content is at least 0.5 moles, most preferably at least 1 mole, e.g. 1.5-5 moles. A particularly suitable range is 2-3.5 moles.

(I) Sulfate

It is an optional but preferred ingredient. The amount of sulfate can vary, such as the amount of chloride, between zero and the maximum amount soluble in the solution. In one type of bath, which is particularly preferred, the amount of sulfate is less than the total amount of halide, and preferably less than the total amount of chloride. However, in the second type of bath, the proportion of sulfate is higher than the proportion of halide, and may be the predominant anion in the bath. Like chloride, sulfate can be added to the bath as an anion or sulfuric acid of a conductivity salt or ammonium or chromium salts. Typical sulfate concentrations may be 0-5 moles, preferably 0.5-4, e.g. 0.6-3 moles, most preferably 0.6-1.2 moles. Preferably the combined chloride and sulphate concentrations are at least 1 mole, e.g. at least 2 moles, most preferably 2.5-3.5 moles.

(J) Trace metals

Iron and / or nickel are preferably present in the solutions of this invention in amounts ranging from 30 to 150 ppm, in total, for optimum coverage. The proportion of iron may exceed this value if it is desired to galvanize mixtures of chromium and iron. However, the proportion of nickel should not exceed 150 ppm alone or 100 ppm in the presence of substantial amounts of iron. Higher amounts of nickel cause the color of the deposit to deteriorate. Most of the other trace metals incl. cobalt, copper, lead and zinc are very harmful when any of them are present in excess of about 30 ppm. Preferably, the total content of such trace metals should be less than 30 ppm, most preferably less than 20 ppm.

(K) Alkali metals and / or alkaline earth metals These are optionally, but in the case of alkali metals, preferably present. They may comprise sodium, potassium or lithium, or less preferably alkaline earth metals such as calcium or magnesium or other metal ions which do not galvanize from solution with the chromium. The amount of such metals is not critical provided that they do not precipitate in the presence of other components. Their presence is useful to increase the electrical conductivity of the bath. Preferably they are present in an amount of at least 0.1 mol per liter until the solution is saturated.

(L) Surfactants These are optionally, but preferably, present in effective and suitable amounts. Wetting agents and defoamers are used throughout the electroplating technology, and many suitable examples are well known to those skilled in the art. Any wetting agents commonly used in the galvanizing of hexavalent chromium can be used in this invention. However, since the solutions of this invention are much less oxidizing than solutions of hexavalent chromium, it is also possible and advantageous to use less expensive wetting agents commonly used in electroplating solutions of the less potent type. The main limitation on the effectiveness of wetting agents consists of the presence of free bromine in solution. Surfactants that are prone to bromination are therefore not recommended, e.g., most of the nonionic surfactants. The surfactants used according to our invention are typically cationic or preferably anionic, e.g. succinic acid sulphonates, alkyl benzene sulphonates having 8 to 20 aliphatic carbon atoms, such as sodium dodecylbenzene sulphonate, alkyl sulphates having 8 to 20 carbon atoms and sodium sulphate, e.g. sodium , such as sodium lauryl polyethoxysulfates. If the solution has undesired foaming tendencies, it is also possible, optionally, to include suitable antifoaming agents, e.g. fatty alcohols such as ethyl alcohol. The selection of surfactants for use in our solution is a routine matter within the ordinary skill of those skilled in the art. According to normal practice, the amount of wetting agent used is, for example, 0.1 to 10 parts per thousand.

It is preferred that the solutions of our invention consist essentially of the ingredients listed above. However, we do not rule out the presence of small amounts of other materials which are suitable for solutions and which do not adversely affect the galvanizing properties to a significant extent. In general, it is preferred that the nitrate ion be substantially absent, as it tends to prevent chromium deposition from the team. Sulfite t-ion, which has been recommended above as an additive for chromium plating baths, also has a detrimental effect if present in amounts greater than about 0.01 mole, but other substances, organic or inorganic, which do not inhibit the deposition of chromium or significantly impair masking effect or creates unacceptable toxicity problems, may optionally occur. Whether a particular substance can be accepted in solution can be routinely determined by simple experimentation.

The bath may conveniently be prepared by dissolving the desired water-soluble salts in water in amounts sufficient to give the desired concentration. Typical salts that can be used include chromium chloride, chromium sulfate, potassium, sodium, ammonium bromide, potassium formate, sodium borate, ammonium chloride, ammonium sulfate and sodium chloride.

Cationic substances may, if desired, be added in whole or in part as bases, such as aqueous ammonia. A particularly suitable form of chromium is basic chromium sulfate, which is a commercially available chromium pickling solution. For example, 33% basic chromium sulfate obtained by reduction of sodium dichromate with sulfur dioxide is a common commercial article and the advantage of the invention is that it is possible to use such relatively inexpensive and readily available chromium sources. However, we do not rule out the use of salts such as chromium formate or acetate.

Anionic species can be added, at least in part as acids, for example hydrochloric, sulfuric, boric, formic or acetic acid. Preferably, once the essential ingredients have been dissolved, the pH can be adjusted by adding e.g. ammonium, sodium or potassium hydroxide. Preferably, the pH is initially adjusted to the top of the preferred range, e.g. 2.5-4. In use, the pH tends to lower and should be maintained, occasionally adjusted, in the range of 2.5-3.5.

The bath can be prepared at room temperature. Alternatively, the bath can be prepared at an elevated temperature (e.g., 70 ° C) and allowed to cool. Typical cooling may take 10-24 h. Unlike previous formate-based baths, it is not necessary to perform the electroplating step first as soon as the bath is made, or to add excess formate during manufacture.

The solution is preferably used at temperatures of 15-30 ° C, for example 20-25 ° C. Current densities in the range of 5H-10,764 A / s 2 (5-1,000 A / sq. Ft.) (E.g., about 1,076 A / s? (100 A / sq. Ft.) Can be used.

The system is useful for electroplating plastics and non-ferrous materials as well as ferrous and nickel-containing materials. Galvanizing for plastics (usually ABS) is common with hexavalent chromium solutions. The methods used are well known and are described, for example, in William Goldie's book "Electroplating of Plastics" and in Sauvestre's presentation to the Society of Automotive Engineers in January 1965 entitled "Electroplating of Plastics". Preferably, we use the same procedures hitherto used to deposit hexavalent chromium on plastic, but by replacing the solution of our invention with hexavalent chromium solutions used hitherto.

It is preferred, when galvanizing the solutions of our invention, to use inert anodes, such as carbon anodes. Other inert anodes such as platinum titanium or platinum can be used, but are more expensive. However, for alloy galvanizing, it is possible to use, for example, iron or chromium / iron anodes.

7 6 2 3 4 7

The invention is illustrated by the following examples:

Example 1

The solution was prepared by dissolving the following substances in water at laboratory temperature (20-25 ° C), and diluting to 1 liter.

The chromium pickling solution used in this experiment was a 33% alkaline solution. This is a commodity used to pickle leather and is made by reducing sodium dichromate with sulfur dioxide. The reaction product has a "basicity" of 33%, with "basicity" being a measure of the replacement of sulfate with hydroxyl to give products of different configurations. The solution contained 130 g / l Cr.

. .

Average molarity

Chromium pickling solution 150 ml 0.4

Wetting agent 1 ml

Ammonium chloride 90 g 1.7

Potassium chloride 75 g .1.0

Ammonium bromide 10 g 0.1

Boric acid 50 g 0.8

Sodium acetate 15 g 0.2

Ammonium formate 55 g 0.87

Sulfuric acid (density 1.84) 2 ml i_1_

In preparation, the pH was 3.4 and electroplating with a carbon anode was started within 30 minutes at a volume flow rate of 0.5 A / liter. After 1 hour of electroplating (i.e., after 0.5 Ah per liter), a sample was taken for evaluation in a Hull cell, a cell lined with circulating cooling to maintain the temperature in the range of 20-25 ° C.

A 10 A current was controlled for 3 minutes using a carbon anode and the following distribution was observed in the panel by measuring the standard amount and thickness of electricity.

Current density (A / m2 4 300 2 150 1 080 540 220

Thickness (nm) 305 203 152 203 102

The electroplating range was evaluated in the range of 1,000-8 ASF with a pH working value of 3.0.

Example 2

The comparative experiment was performed as follows:

Solutions A and B were prepared by dissolving the ingredients in cold water and electroplating using a carbon anode for 10 Ah per liter at a volume flow rate of 0.5 A / liter.

62347 8

A B

Sodium chloride 150 150 g Ammonium chloride 4 0 40 g

Boric acid 50 50 g

Sodium bromide 10 10 g

Sodium acetate - 15 g

Ammonium formate 55 55 g

Chromium pickling solution (150 gCr / l) 130 130 ml jWetting agent 1 1 ml j pH 2.9 3.3

The pH of the two solutions was adjusted as shown, and 5 g / liter of ammonium formate was added to solution B. The combination of a high pH and a large amount of formate is known to promote the precipitation of chromium formate.

The solutions were stored in polyethylene for two weeks, after which time solution A o precipitated while solution B was clear.

After 8 weeks, there had been a strong precipitation in solution A, but no precipitation had occurred in solution B.

Claims (7)

9,62347 Aqueous chromium plating bath containing 0.1-1.2 moles per liter of trivalent chromium, at least 0.01 moles per liter of bromide, at least 0.05 moles per liter of ammonia and a carboxylic acid complexing agent for trivalent chromium, characterized in that the carboxylic acid complexing agent is formate and acetate a mixture of 0.5 to 3 moles of formate per mole of trivalent chromium and 0.1 to 0.5 moles of acetate per mole of formate. Bath according to Claim 1, characterized in that the proportion of acetate is 0.2 to 0.4 moles per mole of formate. Bath according to one of Claims 1 or 2, characterized in that the total proportion of formate and acetate is between 2.5 and 1 mole per mole of trivalent chromium. Bath according to one of the preceding claims, characterized in that it contains at least 0.1 mol of borate. Bath according to one of the preceding claims, characterized in that it contains at least 0.5 moles of chloride and / or sulphate. Bath according to one of the preceding claims, characterized in that it contains 30 to 150 ppm of total iron and / or nickel. Bath according to one of the preceding claims, characterized in that it contains at least 0.1 mol of sodium and / or potassium.