GB1582711A - Electroplating chromium and its alloys - Google Patents

Description

(54) ELECTROPLATING CHROMIUM AND ITS ALLOYS (71) We, IBM UNITED KINGDOM LIMITED, a British Company of P O Box 41, North Harbour, Portsmouth, Hampshire, England, P06 3AU, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to the electroplating of chromium and its alloys and is an improvement in or modification of the invention claimed in the specification of our United Kingdom Patent 1431639.

In our United Kingdom Patent Specification No. 1431639 there is described and claimed a chromium or chromium alloy electroplating solution in which the source of chromium comprises an aqueous solution of a chromium (III) thiocyanate complex and a process of plating chromium or a chromium containing alloy comprising passing an electric plating current between an anode and a cathode in such à solution.

The preferred complexes described in the Patent Specification 1431639 are chromium (III) aqflothiocyanate complexes prepared by equilibrating chromium perchlorate and sodium thiocyanate in an aqueous solution. The complexes so formed are described by the general formula: ((H20)6-nCr(III) (NCS)n) (3-n) where n = 1 to 6 NB Subscripts are always positive but superscripts may be positive, negative or zero.

Chromium (III) species in solution are generally octahedral with six ligands coordinated to the chromium atom. These ligands occupy and define the inner coordination sphere of the chromium atom and are inert inasmuch as they exchange very slowly with free ligands in the solution e.g. the reaction: (CR(H20)s (NCS))+2 + *(NCS)-(Cr(H2O)5 *(NCS))+2 + (NCS)- is very slow. It is the slowness of the reactions of this type which.complicate the chemistry of chromium (III) and necessitate equilibration at high temperatures. See the book by Basolo and Pearson "Mechanism of Inorganic Reactions: Study of Metal Complexes in Solution" published by Wiley.

One embodiment of the invention described in the specification of our application 3179/77 (Serial No. 1576479) comprises a particularly advantageous chromium or a chromium alloy electroplating solution, in which the source of chromium comprises an aqueous solution of a chromium (III) sulphatothiocyanate complex. More particularly the chromium (III) sulphatothiocyanate complex comprises mixed chromium (III) thiocyanate complexes having the formula: ((H20)6-2m nCr(III) (S04)m (NCS)n)3.2m-n where m is 1 or 2 and n is at least 1, but where 2m +n is not greater than 6.

Preparation of this aqueous solution of chromium (III) sulphatothiocyanate complex was by equilibrating an aqueous solution of chromium (III) sulphate (Cr2(SO4)3.15H20) and sodium or potassium thiocyanate.

Another embodiment of the invention described in the specification of our application 3179/77 (Serial No. 1576479) comprises a chromium or a chromium alloy electroplating solution in which the source of chromium comprises an aqueous solution of a chromium (III) chlorothiocyanate complex. More particularly the chromium (III) chlorothiocyanate complex comprises mixed chromium (III) thiocyanate complexes having the formula: ((H20)G m n Cr(III) Clm (NCS)n)3.rn.fl where m is positive and n is at least 1, but where m + n is not greater than 6. Preparation of this aqueous solution of chromium (III) chlorothiocyanate complex was by equilibrating an aqueous solution of chromium (III) chloride (Cr Cl3.6H20) and sodium or potassium thiocyanate.

Commercially, chromium has been plated from aqueous chromic acid baths prepared from chromic oxide (Cr 03) and sulphuric acid. Such baths, in which the chromium is in hexavalent form, present a considerable health hazard as a result of the emission of chromic acid fumes.

Further if the plating current is interrupted for any reason a deposit of unsatisfactory milky appearance is produced. In addition delamination of the deposited chromium occurs. Thus accidental interruption of the plating current can cause significant losses and barrel chromium plating is rendered extremely difficult since it is difficult to apply more than very thin deposits of chromium and to ensure that the deposit covers and adheres to the articles to be plated.

The present invention provides a process of plating chromium or a chromium containing alloy comprising providing an electroplating solution in which the source of chromium comprises an aqueous solution of a chromium (III) thiocyanate complex, passing an electric current between an anode and a cathode in the aqueous solution and interrupting the current one or more times during the deposition of a chromium layer.

The interruption of the plating current permits the process to be used for barrel chromium plating and for the inspection of chromium deposit during plating since provided the deposit is kept wet and grease-free it may be removed from the solution.

Interrupting the plating current has another important but quite unexpected result. It has been found that the range of current density over which bright plating occurs is extended if the plating is interrupted one or more times for a period of 5 to 10 seconds, although the period itself is not critical. It is thought that prior to the current interruption bright plating occurs over a particular range, but that after an interruption bright plating occurs over a wider range of current density since plating appears to occur preferentially on the previously un-plated parts. It is thought that changes in the electrochemical nature of the surface of the initially plated portion causes this preferential plating. However no difference can be detected in the quality of final deposit.

It is clear that the ability of the process of the present invention to interrupt plating, to remove the parts for inspection and then continue; or to perform barrel chromium plating has considerable commercial value. In addition, the process enables more complex shape parts to be plated with less complex anode arrangements. Further it has been found that regulation of the magnitude of the cell voltage is no longer critical.

A preferred electroplating solution for the process according to the present invention comprises an aqueous solution of a chromium (III) sulphatothiocyanate complex prepared as described in Example XI of the specification of our copending application 3179/77 (Serial No. 1576479) referred to above.

A matt or white streaky surface is often obtained at high current densities when plating chromium from a chromium (III) thiocyanate aqueous solution. It is thought that the deposition of chromium hydroxy species, which can occur in these circumstances, as a result of a high pH at the cathode surface, causes this effect. However, when the cathode has an initial thin deposit of chromium on the normally high current density portion the matt or steaky surface does not occur. In extreme cases the white streaky deposit delaminates from the cathode surface.

The present invention further provides a process of plating chromium or a chromium containing alloy comprising providing an electroplating solution in which the source of chromium complexes comprises an aqueous solution of a chromium thiocyanate complex and passing an electric current between an anode and a cathode in the aqueous solution in at least two steps, during the first step the current having a low magnitude relative to that normally used for plating and during the second step the current being increased to that normally used, whereby during the first step the current density on the cathode is substantially below that during the second step.

The process according to the present invention allows the normally high current density portions of the cathode to be covered with an initial thin bright chromium deposit plated at a low current density by substantially reducing the cell current. Thereafter the cell current is increased and the normally high current density portions of the cathode are plated at the normal high current density without producing a white streaky surface on an otherwise bright chromium deposit.

By using the process of the present invention parts of more complex shapes can be plated without producing a white streaky matt deposit of delamination at the high current density portion of the articles to be plated. A preferred electroplating solution for the process according to the invention comprises an aqueous solution of chromium (III) chlorothiocyanate complex prepared as described in Example I of the specification of our copending application 3179/77 (Serial No. 1576479) referred to above.

EXAMPLE I An aqueous solution of a chromium (III) sulphatothiocyanate complex was prepared from the following materials: 0.075 M Chromium (III) sulphate (Cr2(SO4)3.15H20) 0.15 M Sodium thiocyanate (NaNCS) 0.5 M Sodium perchlorate (NaClO4.H2O) 50 g/litre Boric acid H3BO3 The above solution was equilibrated at 850C, cooled, the pH adjusted to 2.5 with 10% NaOH and 0.5 g/litre sodium lauryl sulphate added.

(A) The above solution was placed in a Hull cell and chromium was deposited on a brass Hull cell plate using a cell current of 1 Amp for 5 minutes. A bright chromium deposite was obtained from 26mA/cm2 to the top of the plate.

(B) The Hull cell test described above at (A) was repeated with a new brass Hull cell plate, but with the current being interrupted every minute and the brass plate being removed from the cell each time the current was interrupted for a period of five seconds. A bright chromium deposit was observed from 16mA/em2 up to the top of the plate.

In other tests substantial extension of the plating range has been observed on both the upper and lower current density limits.

EXAMPLE II An aqueous solution of a chromium (III) chlorothiocyanate complex was prepared from the following constitutents: 0.1M Chromium (III) chloride (CrCl3.6H20) 0.4M Sodium thiocyanate (NaNCS) 1M Sodium chloride (NaCl) 50 g/litre Boric acid (H3BO3) The above solution was equilibrated at 850C, cooled, the pH adjusted to 3.5 with 10% NaOH and 0.5 g/litre of sodium lauryl sulphate added.

(A) The equilibrated solution was placed in a Hull cell and chromium was deposited on a brass Hull cell plate using a cell current of 2 Amps for 2 minutes. Bright chromium was deposited from 10 to 160 mA/cm2 however white streaks occurred above 60 mA/cm2 and some delamination at current densities over 100 mA/cm2.

(B) The Hull cell text described at (A) was repeated with a new brass Hull cell plate, but with the current initially at 0.7 Amps for 30 seconds and then at 2 Amps for 2 minutes. Bright chromium was deposited from 10 mA/cm2. No white streaks or delaminated deposits were observed.

The current may be interrupted and the cathode may be removed when the cell current is switched between the low and high currents.

WHAT WE CLAIM IS: 1. A process of plating chromium or a chromium containing alloy comprising providing an electroplating solution in which the source of chromium comprises an aqueous solution of a chromium (III) thiocyanate complex, passing a current between an anode and a cathode in the aqueous solution and interrupting the current one or more times during the deposition of a chromium layer.

2. A process as claimed in claim 1 including removing the cathode from the aqueous solution during the interruption or interruptions of said current.

3. A process as claimed in claim 1 or 2 in which the electroplating solution is contained in the anode is provided by an electroplating barrel apparatus.

4. A process as claimed in any one of claims 1 to 3 in which the chromium (III) thiocyanate complex is a chromium (III) sulphatothiocyanate complex.

5. A process of plating chromium or a chromium containing alloy comprising providing an electroplating solution in which the source of chromium comprises an aqueous solution of a chromium thiocyanate complex and passing an electric current between an anode and a cathode in the aqueous solution in at least two steps, during the first step the current having a low magnitude relative to that normally used for plating and during the second step the current being increased to that normally used, whereby during the first step the current density on the cathode is substantially below that during the second step.

6. A process as claimed in claim 5 including interrupting the current between the first and second steps.

7. A process as claimed in claim 5 or 6, in which the chromium (III) thiocyanate complex is a chromium (III) chlorothiocyanate complex.

8. A process substantially as described with reference to either of Examples I (B) and II

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. copending application 3179/77 (Serial No. 1576479) referred to above. EXAMPLE I An aqueous solution of a chromium (III) sulphatothiocyanate complex was prepared from the following materials: 0.075 M Chromium (III) sulphate (Cr2(SO4)3.15H20) 0.15 M Sodium thiocyanate (NaNCS) 0.5 M Sodium perchlorate (NaClO4.H2O) 50 g/litre Boric acid H3BO3 The above solution was equilibrated at 850C, cooled, the pH adjusted to 2.5 with 10% NaOH and 0.5 g/litre sodium lauryl sulphate added. (A) The above solution was placed in a Hull cell and chromium was deposited on a brass Hull cell plate using a cell current of 1 Amp for 5 minutes. A bright chromium deposite was obtained from 26mA/cm2 to the top of the plate. (B) The Hull cell test described above at (A) was repeated with a new brass Hull cell plate, but with the current being interrupted every minute and the brass plate being removed from the cell each time the current was interrupted for a period of five seconds. A bright chromium deposit was observed from 16mA/em2 up to the top of the plate. In other tests substantial extension of the plating range has been observed on both the upper and lower current density limits. EXAMPLE II An aqueous solution of a chromium (III) chlorothiocyanate complex was prepared from the following constitutents: 0.1M Chromium (III) chloride (CrCl3.6H20) 0.4M Sodium thiocyanate (NaNCS) 1M Sodium chloride (NaCl) 50 g/litre Boric acid (H3BO3) The above solution was equilibrated at 850C, cooled, the pH adjusted to 3.5 with 10% NaOH and 0.5 g/litre of sodium lauryl sulphate added. (A) The equilibrated solution was placed in a Hull cell and chromium was deposited on a brass Hull cell plate using a cell current of 2 Amps for 2 minutes. Bright chromium was deposited from 10 to 160 mA/cm2 however white streaks occurred above 60 mA/cm2 and some delamination at current densities over 100 mA/cm2. (B) The Hull cell text described at (A) was repeated with a new brass Hull cell plate, but with the current initially at 0.7 Amps for 30 seconds and then at 2 Amps for 2 minutes. Bright chromium was deposited from 10 mA/cm2. No white streaks or delaminated deposits were observed. The current may be interrupted and the cathode may be removed when the cell current is switched between the low and high currents. WHAT WE CLAIM IS:

1. A process of plating chromium or a chromium containing alloy comprising providing an electroplating solution in which the source of chromium comprises an aqueous solution of a chromium (III) thiocyanate complex, passing a current between an anode and a cathode in the aqueous solution and interrupting the current one or more times during the deposition of a chromium layer.

2. A process as claimed in claim 1 including removing the cathode from the aqueous solution during the interruption or interruptions of said current.

3. A process as claimed in claim 1 or 2 in which the electroplating solution is contained in the anode is provided by an electroplating barrel apparatus.

4. A process as claimed in any one of claims 1 to 3 in which the chromium (III) thiocyanate complex is a chromium (III) sulphatothiocyanate complex.

5. A process of plating chromium or a chromium containing alloy comprising providing an electroplating solution in which the source of chromium comprises an aqueous solution of a chromium thiocyanate complex and passing an electric current between an anode and a cathode in the aqueous solution in at least two steps, during the first step the current having a low magnitude relative to that normally used for plating and during the second step the current being increased to that normally used, whereby during the first step the current density on the cathode is substantially below that during the second step.

6. A process as claimed in claim 5 including interrupting the current between the first and second steps.

7. A process as claimed in claim 5 or 6, in which the chromium (III) thiocyanate complex is a chromium (III) chlorothiocyanate complex.

8. A process substantially as described with reference to either of Examples I (B) and II

9. An article or articles plated with chromium or a chromium alloy by the process claimed in any one of the preceeding claims.