CA1223547A - Electrodeposition from trivalent chromium bath - Google Patents

Abstract

IMPROVED ELECTRODEPOSITION FROM TRIVALENT CHROMIUM BATH

ABSTRACT OF THE DISCLOSURE
A trivalent chromium electroplating bath is improved by the addition thereto of a small amount of aliphatic sulfonic acid or the salt thereof. The presence of this compound in the bath increases the covering power of the bath and aids in the formation of a uniformly bright deposit. Sodium allyl sulfonate in a concentration between 0.5 and 2 grams/liter has been found to be a satisfactory additive for this purpose. Typically, this compound contains carbon to carbon unsaturation represented by, for ex-ample, allyl sulfonic acid, propargyl sulfonic acid, and vinyl sulfonic acid, as well as the alkali metal, ammonium and magnesium salts thereof.

Description

I

IMPROVED ELECTRODE POSITION FROM TRIVALENT CHROMIUM BATH

BACKGROUND OF THE INVENTION
, This invention relates to the electrode position of chromium onto a conductive substrate from a trivalent chromium electron plating bath.
for many years, jewelry, automotive components, furniture and numerous other articles have been electroplated with a thin layer of decorative chromium from a bath containing hexavalent chromium ions. The chromium typically was present in the plating bath as chronic acid. Traditionally, Or 6 plating solutions have suffered various shortcomings including limited coverage and throwing power, sensitivity to fluctuations and interruptions ox electrical current, the generation of harmful mists, and waste disposal problems associated with the relatively toxic hexavalent chromium ions.
Many of these problems have been overcome with the advent of trivalent chromium electroplating baths. These baths typically contain a source of trivalent chromium ions, a completing agent such as format or acetate ions to keep the Or 3 ions in solution, a suitable reducing agent to prevent or control the formation of Or 6, conductivity salts, and a buffer such as boric acid.
Despite the many advantages of using trivalent chromium as compared to hexavalent chromium, there continue to ye certain problems associated with trivalent chromium electroplating. One problem relates to the incomplete coverage of chromium in areas of the substrate where the current density is low, for example below 20 AS. One other problem concerns the formation of relatively dark areas on a surface that has been plated with trivalent chromium. These dark areas detract from the overall attractive appearance of the decorative finish. Although the reason that the .. I' ...

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dark areas are formed during plating is not understood, it is theorized that the presence of contaminants in the plating bath are a contributing factor. A large number of wetting agents have been tried in trivalent chromium baths in an effort to overcome the problems, however none has been found to be completely effect-ivy for its intended purpose, without creating other detrimental side effects. The presence of metallic impurities in the bath also appears to contribute to the problem of darkness.
In certain fields, such as the automotive industry, corrosion resistance is important and any improvements that can be made in the corrosion protection afforded by the layer of chromium is welcome.
SUMMARY OF THE INVENTION
It has now been discovered that the addition of bath soluble unsaturated aliphatic sulfonic acids or their salts to a trivalent chromium electroplating bath has very favorable effects on the chromium electrode posit. For years, this group of compounds has been used as a carrier or auxiliary brightener in nickel electron plating baths. However, these compounds have not been used in other types of plating baths nor for the purposes of the present invention. When a small but effective amount of one of these come pounds is used in a trivalent chromium bath, one or more of the following improvements are noted: 1) There is more efficient and more complete coverage, particularly at low current densities; 2) The formation of dark streaks and dark areas on the plated surface is reduced or eliminated, and 3) The resistance to corrosion is enhanced, particularly where the thickness of the deposit exceeds about 10 or 12 millionths of an inch.
DETAILED DESCRIPTION OF THE INVENTION
The present invention comprises a bath and a method of its use, involving immersing an object into an aqueous trivalent 3S~7 chromium electroplating bath containing (a) a source of Or 3 ions, (b) a complexes to keep the Cry ions in solution, (c) a reducing agent, (d) a buffer, and (e) ammonium ions. The bath is improved by the addition thereto of a small but effective amount of a bath soluble unsaturated aliphatic sulfonic compound.
Particularly satisfactory compounds for this purpose are bath soluble unsaturated aliphatic sulfonic acids and the alkali metal, ammonium and magnesium salts thereof, including compounds such as sodium ally sulfonate, vinyl sulfonic acid, and propargyl sulk ionic acid as well as mixtures of two or more of these compounds.
A typical trivalent chromium electroplating bath has the following composition:
Trivalent chromium ions 18-24 grams/liter Ammonium chloride 88-103 grams/liter Potassium chloride 98-114 grams/liter Ammonium bromide 9.8-11.3 grams/liter Boric acid 49-57 grams/liter Format ions 28~34 grams/liter Wetting agents 0.5-2.0 ml/liter According to the teachings of the present invention, a bath soluble unsaturated aliphatic sulfonic acid or the salt thereof is added to an aqueous acid trivalent chromium electroplating bath in an amount effective to improve the quality of the chromium electron deposit.
The chromium may be added as chronic sulfate, chronic chloride or chronic fluoride. Alternatively, it may be added to the bath as chrome tan, a by-product of leather tanning consisting of a mixture of sodium sulfate and chromium sulfate equivalent to 25-26% Cry.
The solution should be substantially free of hexavalent chromium.
I With the trivalent chromium maintained in the range of 18-24 grams/
liter, good plating results are achieved without the occurrence of undesirable precipitation of chromium from the bath.

ZZ35~'7 The ammonium ions are present in the bath to assist in the oxidation reduction reaction that takes place at the anode.
Preferably, the ions are introduced as N~13 although certain substituted ammonium compounds may be used instead.
The bromide is present as a reducing agent and as such, helps to avoid the formation of hexavalent chromium. Preferably, it is added as ammonium bromide. Iodide or fluoride can be used in place of the bromide.
The boric acid serves as a buffering agent in the plating lo bath and is preferred over other buffering salts such as pros-plates and citrates which are relatively ineffective. Generally, if the borate is absent, or is present in concentrations below about Owl molar, the bath possesses undesirably low covering power.
The potassium chloride is present in the bath as a conduct tivity salt. Other alkali metal or alkaline earth metal salts of strong, readily soluble, acids such as chlorides or sulfates may be used. By increasing the electrical conductivity of the bath, the electric power consumption requirements of the bath are substantially reduced.
The format ions are essential to the successful operation of the bath. They form a complex with the chromium ions to keep the chromium in solution.
There are many examples of wetting agents that are suitable for use in the bath, including those surfactants that typically are used in hexavalent baths. Cat ionic or anionic agents are preferred over non ionic agents and include such compounds as sulfosuccinates, alkyd Bunsen sulfonates and alkyd sulfates the latter two having between 8 and 20 carbon atoms. If the bath has a tendency to foam, anti-foaming agents such as fatty alcohols may be added.

,. , 3S9~7 Plating improvements have been achieved by the use of an effective amount of a bath soluble unsaturated aliphatic sulfonic acid compound. A particularly suitable compound is the sodium salt of ally sulfonic acid, CH2=CH-CH2S03Na added to the bath in an amount of between about 0.5 and 2 grams per liter. Another very effective compound, in a concentration ranging from a trace to about 1 gram per liter is the sodium salt of propargyl sulfonic acid, }~C-C-CH2S03H. These compounds; in the ranges indicated serve to increase the covering power of the bath, thus minimizing or eliminating misplace and skipping particularly at low current densities. Furthermore, they aid in the elimination from the deposit of dark areas and dark streams, smudges and smears. At the same time, they appear to contribute to enhanced corrosion protection of the metallic substrate, particularly noticeable at lo chromium thicknesses above 10 or 12 millionths of an inch. Also, there are some indicators that the compounds may contribute to an increase in the rate of chromium deposition.
Vinyl sulfonic acid, CH2=CH-S03H, when used in a concentra-lion of 2 to 10 grams per liter also contributes to the improved operation of a trivalent chromium bath, although the improvements are not as substantial as those obtained by the use of ally and propargyl sulfonates.
Typically, the compounds that work most effectively have 2 or 3 atoms in the carbon chain with the carbon to carbon unsaturation occurring at the end of the rain that is remote from the cellophane-ate radical.
The invention is not limited, however, to the use of the aforementioned compounds but is broad enough to cover other bath soluble straight and branched chain unsaturated aliphatic sulfonic acids and the salts thereof. or example, other alkali metal, ammonium, and magnesium salts may be used in place of the Sydney.

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Furthermore, one or more of the hydrogen ions in the compound may be replaced with a halogen ion.
For best results, plating of trivalent chromium is carried out at a temperature between about 15 and about 35C preferably in a range of between 20 and 25C. Because of the heat generated during plating, the bath is maintained at a proper operating temperature by heat exchange or refrigeration as needed.
The pi of the bath is maintained between 2.0 and 3.5, pro-fireball between 3.1 and 3.3. At the lower phi the plating speed lo increases but coverage at low current density is lost while the converse is true at higher pi levels. The pi may be adjusted downward or upward as needed, by the addition of, e.g. concern-treated hydrochloric acid or ammonium hydroxide.
For the successful plating from a trivalent chromium plating bath, a current density in the range of between about 50 and about 250 amps per square foot is recommended. The use of an average current density between 80 and 150 amps per square foot provides a balance between deposition rate, consumption of additives and the temperature control of the bath, and results in the deposition of chromium at the rate of between 3 and 5 millionths of an inch per minute.
During plating, the bath is agitated using mild air agitation to help uniformity of metal distribution. Perforated plastic air lines, installed in the plating tank beneath the cathode area, are suitable for this purpose.
The following examples are presented to more fully illustrate the present invention.
EXAMPLE I
A trivalent chromium bath having the following composition is .~2~35~

prepared.
Or 3 22 grams/liter NH4Cl 98 grams/liter Clue 109 grams/liter NH4Br 10.8 grams/liter H3B03 54 grams/liter HC02 32 grams/liter Wetting Agent 1 milliliter/liter C3H5S03Na As noted in Table I
lo pi 2.8 A series of I inch x 6 inch brass panels, electroplated with a layer of bright nickel, each panel containing a V-shaped recess, I inches wide x 1 inch deep, are immersed in a I liter beaker containing 1 liter of the plating solution and a graphite anode.
Each panel is plated at 5 amps for 3 minutes using mild air agitation, with a bath maintained at a temperature of between 20 and 25C. With the V-shaped recess, the plating current density across the brass panels varies from a low of 8 or 9 amps per square foot at the bottom of the recess and 18-25 amps per square foot on the sides of the recess to a high of about 117 amps per square foot at -the top edge of the recess and 100 amps per square foot on the planer surface of the panel. Table 1 will show -that, with each successive panel, incremental amounts of sodium ally sulfonate are added to the beaker.
TABLE I
CONCENTRATION OF SODIUM
ALLY SULFONATE
PANEL NO. (GRAMS/LITER) APPEARANCE
1 0 Dark color in recess, black smears or streaks at all current densities.

SLY

2 0.2 Slightly smaller dark area in recess; black smears less noticeable.

3 0.5 Recess area improved over Panel No. 2; black smears nearly eliminated.

4 1.0 Dark color in recess nearly gone; no black streaks.

1.5 Recess the same as in panel #4 6 2.0 Small dark area in recess;
some smear noticeable.

7 2.5 Dark recess ring; smear more prominent.

8 3.0 About the same as panel #7.

Panels 2, 3 and 4 show successively better improvement as the concentration of sodium ally sulfonate is increased from 0.2 to 1.0 g/l while panels 6, 7 and 8 show that the improvement becomes less noticeable at concentrations of 2.0 g/l or higher.
Although sodium ally sulfonate is effective in a concentra-lion of between about 0.5 and about 2 grams per liter in plating bath, each of the other compounds covered by this invention will be effective in a concentration range that can readily be deter--.~22~

mined by following the procedures of Example I substituting the other compound for the sodium ally sulfonate.
EXAMPLE II
On a commercial basis, plating is carried out it a plastic or rubber lined tank, using graphite anodes fitted with titanium anode hangers. In one plating sequence, steel automotive bumpers are electroplated with successive layers of copper, semi-bright nickel and bright nickel after which they are immersed into the trivalent chromium plating bath of the instant invention contain in approximately 1 gram/liter of sodium ally sulfonate. With the pi of the bath maintained between 2.7 and 3.5, plating is conducted at an average current density of approximately 150 amperes per square foot with mild air agitation of the bath.
Plating is continued until a deposit of chromium having a typical average thickness of between 10 and 30 millionths of an inch is formed. The bumper is then removed from the plating bath and is rinsed. A visual inspection of the bumper shows a uniformly shiny electrode posit of chromium with an absence of black streaks and black areas, and improved coverage of chromium in recesses.
EXAMPLE III
A series of 4" x 6" cold rolled steel panels, plated with a duplex layer of semi-bright nickel and bright nickel were electron plated in a plating solution of Example 1 at an average current density of 100 amps per foot said solution containing 1 gram per liter of sodium ally sulfonate. The panels were then subjected to copper accelerated salt spray (CUSS) test, an accelerated corrosion test used to correlate with longer service exposure of articles of commercial manufacture. These panels were compared with similar panels plated from a bath identical to that in Example 1, containing no sodium ally sulfonate.

:~ZZ3~7 A first group of panels was plated with thin layers, i.e. 0.3 and 0.1 mix respectively, of semi-bright and bright nickel follow-Ed by 3 minutes and 6 minutes of plating from a trivalent chromium bath. The plated panels were CAYS tested for 189 hours.
A second group of panels was plated with a thick duplex nickel comprising 0.9 mix of semi-bright and 0.3 mix of bright nickel, followed by 3 and 6 minutes trivalent plating. A chromium layer of 7-10 millionth of an inch was deposited over the nickel at 3 minutes and 18-20 millionth was deposited at 6 minutes.
These panels were CUSS tested for 492 hours.
A third group of panels contained a heavy, e.g. 35 millionth inch layer of chromium over same plating system used for a thick duplex layer of nickel. These panels were CUSS tested for 192 hours.
In all instances, the semi-bright nickel was plated from a Powerful nickel plating bath and the bright nickel from a Nova bright nickel plating bath, both marketed by The Horatio Chemical Company.
All of the panels were then evaluated using the ASTM test 20 B537-70 entitled "Standard Recommended Practice for Rating of Electroplated Panels Subjected to Atmospheric Exposure". The rating is based on a scale of 1 to 10. As with other applications of this rating system, 10 represents a perfect specimen free of basis metal corrosion and the decreasing numbers represent in-creasing degrees of corrosion.
In the first group, those panels which were plated from the chromium bath containing sodium ally sulfonate were rated 1 to 2 units better -than those plated from the bath containing none of this compound.

In the second group, with the thicker nickel deposit, the panels plated from the improved bath were rated 2 to 3 units higher.
In the third group, plated with a thick layer of chromium, S the bath containing sodium ally sulfonate gave panels which were rated a 9 while the bath without the sulfonate produced panels which were rated a 4 or a 5.
Thus, this test gives evidence that as the thickness of the chromium layer and/or the thickness of the total deposit increase en, the resistance of the layer to corrosion appears to be en-hanged by the presence of an unsaturated aliphatic sulfonic acid compound in the chromium plating bath.

Claims (14)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An aqueous acid trivalent chromium electroplating bath con-taining a source of trivalent chromium ions, a complexing agent, a reducing agent, and a buffer, the bath including a bath soluble un-saturated aliphatic sulfo-organic compound having two or three car-bon atoms, said compound selected from the group consisting of sul-fonic acids, alkali metal, magnesium and ammonium, salts of sulfonic acids, and mixtures thereof, present in an amount effective to in-crease the covering power of the bath during electrodeposition.

2. The bath according to Claim 1 wherein the compound is sel-ected from the group consisting of sodium allyl sulfonate and allyl sulfonic acid.

3. The bath according to Claim 2 wherein the compound is sod-ium allyl sulfonate and is present in a concentration of between about 0.5 and about 2 grams/liter.

4. The bath according to Claim 1 wherein the compound is sel-ected from the group consisting of sodium vinyl sulfonate and vinyl sulfonic acid.

5. The bath according to Claim 1 wherein the compound is sel-ected from the group consisting of propargyl sulfonic acid and the sodium salt thereof.

6. The bath according to Claim 5 wherein the compound is pro-pargyl sulfonic acid and is present in a concentration ranging from a trace amount up to about 1 gram per liter.

7. The process of depositing a layer of decorative chromium on the surface of a conductive substrate from an aqueous acid elec-troplating bath containing:

a) A source of trivalent chromium ions, b) A complexing agent to maintain the chromium ions in solu-tion, c) A reducing agent, and d) A buffer, to which is added up to 10 g/l of a bath soluble unsaturated alipha-tic sulfonic acid compound, said process comprising:
1) Immersing the substrate in the electroplating bath while agitating the same, 2) Passing a direct electrical current through the bath, main-tained at a temperature of between about 15°C and about 35°C, said current being sufficient to produce a current density of between 50 and 250 amps per square foot on the surface of the substrate, and 3) Maintaining the current through the bath until the desired thickness of chromium is obtained.

8. The process according to Claim 7 wherein the sulfonic acid compound is selected from the group consisting of sulfonic acids, and the alkali metal, ammonium and magnesium salts thereof.

9. The process of Claim 8 wherein the aliphatic sulfonic com-pound is selected from the group consisting of allyl sulfonic acid and sodium allyl sulfonate.

10. The process of Claim 9 wherein the compound is sodium al-lyl sulfonate and is present in the aqueous bath in an amount of between about 0.5 and about 2 grams/liter.

11. The process of Claim 8 wherein the compound is selected from the group of sodium vinyl sulfonate and vinyl sulfonic acid.

12. The process of Claim 11 wherein the compound is sodium vinyl sulfonate and is present in a concentration of between about 2 and about 10 grams per liter.

13. The process of Claim 8 wherein the compound is selected from the group consisting of propargyl sulfonic acid and the sod-ium salt thereof.

14. The process of Claim 13 wherein the compound is propargyl sulfonic acid and is present in a concentration ranging from a trace amount up to about 1 gram per liter.