CA1111369A - Electroplating chromium and its alloys - Google Patents

Abstract

Chromium or chromium alloys can be electroplated from an equilibrated aqueous solution of a chromium (III) thiocyanate complex and a buffer rater1al, the buffer material in the equilibrated solution providing one of the ligands for the complex.

Description

1~113~i9 1 The present invention relates to the electroplating of chromium or a chromium containing alloy and is an improvement in or modifica-tion of the invention claimed in the specification of United King-dom Patent No. 1,431,639.
In United Kingdom Patent No. 1,431,639 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 cur-rent between an anode and a cathode in such a solution.
The preferred complexes described in the Patent No. 1,431,639are chromium (III) aquothiocyanate complexes prepared by equilibrat-ing chromium perchlorate and sodium thiocyanate in an aqueous solu-tion. The complexes so formed are described by the general formula:
((H20)6 n Cr(III) (NCS)n) (3 n) where n = 1 to 6 Subscripts are always positive but superscripts may be positive, negative or zero.
In the specification of our copending Canadian application for Letters Patent Serial No. 292,187, filed December 1, 1977, by Barclay et al, there is described and claimed a chromium or a chromium alloy electroplating solution, in which the source of chromium comprises ~ an aqueous solution of a chromium (III) thiocyanate complex having ; at least one ligand other than thiocyanate or water, in the innerco-ordination sphere of the complex. It should be noted that chromium (III) species in solution are octahedral with six ligands co-ordinated to the chromium atom. These ligands occupy and define at~
; the inner co-ordination sphere of the chromium atom and~inert inas-much as they exchange very slowly with free ligands in the solution, e.g., the reaction:
(Cr (H20)5 (NCS)) 2 + (NCS) -~ (Cr (H20)5 (NCS)) 2 + (NCS) is very slow. It is the slowness of reactions of this type which complicate the chemistry of chromium (ITI) and necessitate equilibra-tion at 1$1~3fi9 l high temperdtures. See the book by Basolo and Pearson "Mechanism of Inorganic Reactions: Study of Metal Complexes in Solution" published by Wiley.
The linear thiocyandte anion, NCS-, has unique catdlytic properties.
It is able to co-ordinate to metal ions through its nitrogen atom and to metal through its sulphur atoms, and its electron density is extensively localized across the three atoms.
The thiocyanate anion is be1ieved to catdlyze the electron transfer reaction:
Cr (III) + 3e -~ Cr(O) through the formation of multiple ligand bridges between a thiocyanate Cr(III) complex and the surface of the cathode. The electro-active inter-mediate can be identified as:

Cr(III) - NCS - M ~

where M is the metal surface of the cathode, which is Cr(O) after an ini-tial monolayer of chromium is plated. The 'hard' nitrogen co-ordinates to the Cr(III) ion and the 'soft' sulphur to the metal surface M of the cath-ode. Multiple-ligand bridging by thiocyanate in the electrochemical oxida-tion of chromium (II) at mercury electrodes is described in Inorganic Chemistry 9, 1024 (1970).
One embodiment of the invention described in our above mentioned application serial no. 292,187 comprises a particuldrly advdntdgeous 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) sulphatothiocyandte complex comprises mixed chromium (III) thiocyanate complexes having the formula:
(( 2)6-2m-n Cr(III) (S4)m (NCS)n)3 2m-n where M is 0, 1 or 2 and n is at least 1, but where 2m+n is not greater than 6.

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1 Preparation of this aqueous so1ution of chromium (III) sulphatothio-cyanate complex was by equilibrating an aqueous solution of chromic sul-phate (Cr2(S04)3.15H20) and sodium or potassium thiocyanate.
Another embodiment of the invention described in our above mentioned application comprises a chromium or a chromium alloy electroplating solu-tion 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)6 m n Cr(III) Clm (NCS)n)3-m-n where m is zero or 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 chromic chloride (Cr C13.6H20) 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 dif-ficult to apply more than very thin deposits of chromium and to ensure that the deposit covers and adheres to the articles to be plated.
Chromic acid plating baths have the further disadvantages that the plating efficiency is low and therefore the rate of deposition is low, the throwing power is limited and it is difficult to deposit layers of uniform thickness over substantial areas. More metal is deposited on high current density areas such as edges and in certain circumstances "burning" appears. It should also be noted that chromic acid plating baths contain a very high concentration of chromium, 100-200 g/litre.
However, since chromium salts are relatively expensive, the chromium con-1 ~entration should be kept as low as possible to minimize the cost of making up the bath and to reduce 'drag-out' on work pieces. ~he reduc-tion in drag-out loss in making decorative chromium deposits is important since drag-out can amount to six or more times the weight of metal plated. .
Numerous attempts have been made to use trivalent chromium salts to deposit chromium or a chromium containing alloy.
The specification of United Kingdom Patent No. 1,144,913 describes a solution for electroplating chromium which includes chromium chloride con-tained in a dipolar aprotic solvent (such as dimethylformamide) and water.
United Kingdom Patent No. 1,333,714 describes another solution which includes chromium ammonium sulphate in a dipolar aprotic solvent and water.
However, such solutions possess limitations which have hindered their industrial acceptance. In particular, parts of complex shapes could not be plated satisfactorily and the poor electrical conductivity, due to the presence of the dipolar aprotic solvent, required a power supply capable of supplying up to 20 volts. Reduction in the quantity of the dipolar aprotic solvent resulted in an unstable bath. In addition, the solution was relatively expensive. The plating solution also contained between 0.5 to 1.5M chromium ions a relatively high concentration. Also there are health hazards associated with the use of dimethylformamide.
United States Patent Specification No. 3,917,517 claiming priority from United Kingdom Patent No. 1,482,747, describes a chromium or chromium alloy electroplating solution comprising chromic chloride or sulphate having hypophosphite ions as a supplement to or replacement of the dipolar aprotic solvent disclosed in the last two mentioned United é
Kingdom patent specifications. The addition of hypophosphite ions to a trivalent chromium electroplating solution is said to "mitigate or over-come" many of the disadvantages of the solutions containing the dipolar aprotic solvent. However, the plating efficiency is stated to be lower than w;th high levels of the dipolar aprotic solvent and plating rates of 0.05 to 0.15 microns per minute, similar to the best rates available with the hexavalent chromic acid plating solutions, were obtained. Preferred ~,,,L,,~ '~
,~rr . .

~111369 1 range of temperature for plating is stated to be 25 - 5C with a practi-cal maximum being 35C for a chromic chloride solution and 55C for a chromic sulphate solution. The concentration of chromium was given as being 0.5M to 1.75M with a preferred range of 0.7M to 1.3M.
German Offenlegungsschift 2,612,443 and 2,612,444, claiming priority from United Kingdom patent specifications 1,498,533 and 1,498,53~ respec-tively, describe an aqueous electroplating solution comprising chromic sulphate having hypophosphite or glycine ions as "weak complexing agents".
In addition, this solution requires chloride or fluoride ions respectively.
The maximum plating rate was again approximately 0.15 microns per minute and the preferred temperature range 25 to 35C. The preferred concentra-tion of chromium for decorative plating was given as lM.
German Offenlegungsschift 2,550,615, which corresponds to United Kingdom specification 1,488,381, also discloses a trivalent electroplating solution containing chromic sulphate or chloride, ammonium sulphate or chloride, boric acid, and a variety of alternative additional 'weak com-plexing' materials including glycine ions and hypophosphite ions. However, in the examples, the concentration of chromium ions and the concentration of the additional buffer material was relatively high.
United Kingdom patent specifications 1,455,580 and 1,455,841 described another approach that has been used to deposit chromium from aqueous solu-tions of trivalent salts. In these patents the source of chromium ions was chromic chloride or chromic sulphate or chromic fluoride. In addition, bromide ions, ammonium ions and formate or acetate ions are stated to be essential. The plating rate was stated to be 0.15 microns per minute and a temperature in the range 15 to 30C. The concentration of chromium was given as between 0.1 and 1.2M, the preferred value being given as 0.4M
chromium ions.
The present invention provides a chromium or a chromium alloy electro-plating solution, in which the source of chromium comprises an aqueous equilibrated solution of chromium (III) thiocyanate complex and a buffer material, the buffer material providing one of the ligands for the complex.
UK9-77~009 -6-1 The buffer material is preferably an amino acid such as Glycine NH2CH2COOH. The amino acids are strong buffering agents, but also are able to form, during equilibration, complexes with metal ions such as chromium (III) by co-ordination through its nitrogen or oxygen atom.
Thus by equilibrating an amino acid with a chromium (III) thiocyanate complex, mixed amino acid chromium (III) th;ocyanate complexes are formed.
In use, the electroplating solution of the present invention has been found to have a number of highly desirable properties enhancing the cata-lytic characteristics of the chromium (III) thiocyanate plating solutions described above. Firstly, the plating range can be extended and bright deposits have been produced over the range 10 to 1000~ mA/cm2; secondly, plating rates of up to 0.9 microns per minute have been achieved; thirdly, the temperature range over which bright chromium can be deposited is very wide i.e. 20 to 70C and fourthly, the concentration of chromium ions in the solution is very low.
It is believed that in earlier attempts to deposit chromium from trivalent solutions, plating was inhibited at high current densities by the deposition of a hydroxy chromium (III) species on the cathode. The deposition of chromium from the solution of the present invention is facilitated at high current densities both by the catalytic effect of the thiocyanate and by the intimate buffering at the cathode by the amino acid released from the chromium atom as it discharges onto the cathode.
Other buffer matçrials could be used such as formates, acetates, etc.
However, the combination of the catalytic properties of thiocyanate and the intimate buffering of the complexed buffer material is what achieves the remarkable improvements provided by the present invention.
The chromium (III) thiocyanate complexes of the present invention may be chromium (III) sulphatothiocyanate complexes or chromium (III) chlorothiocyanate complexes.
It will be clear that by the addition of nickel, cobalt or other metal salts to the solution, alloys of chromium and these metals can be plated. In addition it will be understood that chromium and chromium 3qi9 1 alloys can be plated through photoresist masks.
The invention will now be described by way of example with reference to the following examples:
Example I
Preparation of a plating solution according to the invention comprised preparing an 0.05M aqueous solution of chrom;c chloride (CrC13.6H20). The solution was saturated with boric acid (H3B03) (50g/litre) and equilibrated at 80C for 1 hour with 0.075M sodium thiocyanate (NaNCS), 0.16M glycine (NH2CH2COOH), 0.5M potassium chloride (KCl) and 2M potassium bromide. The potassium chloride and bromide were added to improve the conductivity of the solution. The equilibrated solution was cooled, its pH adjusted to 3.0 by the addition of dilute sodium hydroxide and lg/litre sodium lauryl sulphate (wetting agent) was added.
The plating solution was introduced into a Hull cell having a flat platinized titanium anode and a ~lat brass Hull cell test cathode. No ion exchange membrane was used to separate the anode and cathode and the temperature of the solution was 22C. A plating current of 5A was passed through the solution for 2 minutes. Bright chromium was deposited from lOmA/cm2 to the top of the plate (580 + mA/cm2).
Example II
To illustrate the effect of equilibrating glycine with chromium (III) thiocyanate producing an aqueous equilibrated mixed glycine chromium (III) sulphato thiocyanate complex solution, the preparation of the solution was carried out in two stages A and B below:
(A) A plating solution was prepared by providing an 0.075M chromic sul-phate solution (Cr(S04)3.15H20). The solution was saturated with boric acid (H3B03) (80g/litre) and equilibrated at 80C for 1 hour with 0.15M
sodium thiocyanate and 0.8M sodium sulphate. The sodium sulphate was added to improve the conductivity of the solution. The equilibrated solu-tion was cooled, its pH adjusted to 2.5 by the addition of dilute sodium hydroxide and 0.6g/litre sodium lauryl sulphate (wetting agent) was added.
This plating solution was introduced into a standard Hull cell, as 1 in Example I, and the temperature of the solution Maintained at 25C. A
plating current of 5A was passed for 2 minutes. Bright chromium was deposited on the Hull cell plate from 5mA/cm2 to 125 mA/cm2.
(B) The solution prepared in (A) above was re-equilibrated at 80C for 1 hour with the addition of 5g/litre of glycine (.065M). The pH was adjusted to 2.5 by the addition of dilute sodium hydroxide.
A current of 5A was passed through solution B at a temperature of 25C in a standard Hull cell for 2 minutes. Bright chromium was now deposited over the range 5 to 275 mA/cm .
(C) The solution preapred in (A) above was re-equilibrated at 80C for 1 hour with the addition of lOg/litre of glycine (0.13M). The pH was adjusted to 2.6 by the addition of dilute sodium hydroxide. A current of 1.6A was passed through solution (C) at a temperature of 47C using a 12 cm2 cathode (130 mA/cm2) for 30 minutes. A chromium deposit 10 microns thick was deposited (i.e. 0.33 microns per minute).
(D) The effect of temperature is illustrated by the following:
The solution B was heated to 45C and a current of 5A was again passed through the solution in a standard Hull cell for 2 minutes. Bright chromium was now found to be deposited from 12 mA/cm2 to 400 mA/cm2.
Example III
A plating solution was prepared substantially as described in Example I of our UK patent specification 1,431,639 i.e. 150 grms. of sodium dich-romate (Na2Cr207) was added to 485 mls of perchloric acid (HC104) and 525 mls water. About 400 mls hydrogen peroxide was added in dropwise fashion until the solution became deep blue. When this state was reached, the solution was boiled down to half its volume driving off hydrogen peroxide and leaving the required solution of chromium perchlorate Cr(C104)3. This solution provides a source solution of chromium (III) for plating.
10 grms glycine were dissolved in water and the pH adjusted to 2.0 with perchloric acid. 100 mls of the chromium source solution was added to the glycine solution, the pH of which was again adjusted to 2.0 with sodium hydroxide solution and the volume adjusted to 1 litre by the addi-1~L1369 1 tion of water. This solution was equilibrated with sodium thiocyanate (0.3M) and sodium perchlorate (lM) for 1 hour at 80C. The solution was cooled to 40C, saturated with boric acid (H3B03) 70g/litre and lg/litre of sodium lauryl sulphate was added.
The following plating results were obtained with the solution pre-pared in Example III.
(A) Bright chromium could be deposited at temperatures in the range 25C
to 70C, best results being attained at temperatures above 35C.
(B) The solution was introduced into a Hull cell and heated to 70C. A
brass Hull cell cathode was plated at a total current of lOA for 2 minutes using a flat platinized titanium anode. Bright chromium was deposited on the brass plate from the 20 mA/cm2 position to the top of the plate (1000 + mA/cm ). There was no sign of burning or poor deposit.
(C) The solution was heated to 70C and a 6.3 mm diameter brass rod was plated at 300 mA/cm2 for 10 minutes, the rod being agitated during plating.
The thickness of the chromium deposit, measured by weighing, was 9 microns.
(D) The solution was heated to 70C and a 6.3 mm diameter brass rod was plated at 135 mA/cm for 10 minutes, the rod being agitated during plating.
The thickness of the chromium deposit, measured by weighing, was 6 microns.
Example IV
A plating solution was prepared as in Example IIC except that lM
sodium perchlorate was used to improve the conductivity of the solution in place of the 0.8M sodium sulphate. A bright chromium deposit 0.85 microns thick was plated on both sides of a brass strip 2 x 5 cm under the following conditions:
pH = 2.55, temperature 46C, current 2A and time 2.5 minutes.
The current density was 100 mA/cm2 and the chromium was deposited at 0.3 micron/minute.

SUPPLEMENTARY DISCLOSURE
At line l of page 7, it is stated that the buffer material is preferably an amino acid such as Glycine. It has been confirmed that further buffers such as aspartic acid: HOOCCH2CH(NH2)COOH, Arginine: H2NC(=NH).NH.CH2.CH2.CH2.CH(NH2)COOH, Histidine: N:CH.NH.CH.C.CH2.CH(NH2).COOH, and Hydroxyproline: NH.CH2.CH(OH).CH2.CH.COOH.
Additionally, the dipeptides Diglycine NH2CH2CO.NH.CH2.COOH and Triglycine NH2.CH2.CO.NH.CH2.CO.NH.CH2.COOH can be considered.
All of these six buffers are operable in the present invention and improvements in efficiency and variations in colour of the deposit could be possible using one of the six on its own or as an addition to glycine.
Example V
Preparation of another plating solution, according to the present inven-tion, comprises the following steps, the amounts of the constituents are for 1 litre of plating solution:
1. Add 60 grams boric acid (H3B03) to 600 ml deionized or distilled water.
Heat and stir until dissolved. Adjust pH to between 2 and 2.4 with 10% NaOH
or 10% H2S04.

2. Add 33.12 grams chromium (III) sulphate (Cr2(S04)3.15H20) to the boric acid solution prepared in step l.

3. Add 32.43 grams sodium thiocyanate (NaSCN) to solution of step 2. Heat and maintain at 85C + 5C, stirring for 90 minutes.

4. Cool to room temperature. Add 10 grams glycine (NH2.CH2COOH) to solution.
Adjust pH as in step 1. Heat and maintain at 85C + 5C, stirring for 90 minutes.

5. Cool to room temperature and add 140 grams sodium perchlorate (NaCl04.H20).Heat and stir until dissolved.

6. Adjust to pH 2.5 as in step l.

7. Make up to one litre with distilled or deionized water.

8. Add l gram sodium lauryl sulphate or 0.1 gram FC.98 and stir to dissolve.

UK9-77-009 -ll-The solution is now ready for plating.
N.B. FC98 is a wetting agent and is a product of the 3M Corporation.
Suitable plating conditions are as follows:
The bath can be operated over a range of current density, pH and tempera-ture. Suitable conditions are 50-150 mA/cm2, pH 2.0-4.0 and temperature 25-60C.
At 105 mA/cm , pH 3.5 and temperature 50C, 0.6 microns chromium is deposited in 2 minutes (20-23% efficiency).
The anode current density should be maintained at about 40 mA/cm2. The anodes can be of platinized titanium or carbon.
Fume extraction should be used as small electrochemical breakdown of the thiocyanate anion occurs at the cathode.
Example VI
A plating solution was prepared as in Example V except that the ligand buffer glycine was replaced by glycilglycine or glycilglycilglycine (diglycine or triglycine respectively).
The concentration of each ligand buffer was varied between 1 gram/litre to 20 grams/litre. Bright plating was obtained over this range. The efficiencyincreased from 11 percent at 1 gram/litre to 16 percent at 10 gram/litre and decreased to 5 percent at 20 grams/litre.
Example VII
A plating solution was prepared as in Example V except that the quantity of NaSCN was reduced to 16.Z grams and glycilglycine was substituted for glycine. The concentration of glycilglycine was varied from 1 gram/litre to 5 grams/litre. Bright plating was obtained with efficiencies similar to those obtained in Example VI.
Example VIII
A plating solution was prepared as in Example V except that the ligand buffer glycilglycine was added to the solution already containing 10 grams of the ligand buffer glycine. The concentration of glycilglycine was varied from 1 gram to 5 grams per litre. Bright plating was obtained with the efficiency decreasing from 19 percent to 13.5 percent with increasing concentration of glycilglycine in the range 1 to 5 grams per litre.

.~

1~11369 Example IX
A plating solution was prepared as in example VIII except that the quantityof NaSCN was reduced to 16.2 grams. The efficiency decreased from 15 percent to 13.5 percent with increasing concentration of glycilglycine in the range 1 to 5 grams per litre. Bright plating was obtained over this range.
Example X
A plating solution was prepared as in Example V except that the quantity of NaSCN was reduced to 16.2 grams per litre and aspartic acid at a concentration of 0.05 M was substituted for the ligand buffer glycine. This produced good bright plating over a wide range 10 mA/cm2 to 500 plus mA/cm2 and an efficiency of 23 percent was obtained.
Example XI
The concentration of aspartic acid in Example X was increased to O.lM.
Bright plating produced complete cover over a lOA Hull cell plate. Plating could be carried out up to a temperature of 80 at an efficiency of 23 percent.
Example XII
A plating solution was prepared as in Example V except that the ligand buffer glycine was replaced by either Arginine or Histidine and that the quantity of NaSCN was reduced to 16.2 grams/litre. With the Arginine, and with the Histidine, bright plating was achieved at concentrations of 0.05M and an efficiency of 13.8 percent was obtained.
The following examples show the use of acetates, formates and hypophos-phites either alone or with glycine as the ligand buffer:
Example XIII
A plating solution was prepared as in Example V, except that the quantity of NaSCN was reduced to 16.2 grams and sodium acetate (CH3.C00 NA) was sub-stituted for the ligand buffer glycine. The concentration of sodium acetate was 8.2 grams per litre, i.e., O.lM. The solution produced bright clean deposits over the range 5 mA/cm2 up to approximately 600 mA/cm2 with an efficiency of 14 percent. In this example, the solution temperature was 40C and had a pH
of 2.5.

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Example XIV
A plating solution was prepared as in Example V, except that the quantity of NaSCN was reduced to 16.2 grams and sodium formate (H.C00 NA~ was substitutedfor the ligand buffer glycine. The concentration of sodium formate was 6.8 grams per litre, i.e., O.lM. The solution produced bright clean deposits over the range 5 mA/cm2 up to approximately 600 mA/cm2 with an efficiency of 14 percent. In this example, the solution temperatue was 40C and had a pH of 2.5.
Example XV
A plating solution was prepared as in Example V, except that the quantity of NaSCN was reduced to 16.2 grams and sodium hypophosphite (NAH2P02) was substituted for the ligand buffer glycine. The concentration of sodium hypo-phosphite was 8.8 grams per litre, i.e., O.lM. The solution produced bright clean deposits over the range 15 mA/cm2 up to approximately 300 mA/cm2 with an efficiency of 14 percent. In this example, the solution temperature was 40C
and had a pH of 2.5.

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Claims (19)

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

1. A chromium or a chromium alloy electroplating solution, in which the source of chromium ions comprises an aqueous equilibrated solution of a chromium (III) thiocyanate complex and a buffer material, the buffer material providing one of the ligands for the complex.

2. A solution as claimed in claim 1, in which the buffer material is an amino acid, a formate or an acetate.

3. A solution as claimed in claim 2, in which the amino acid isglycine.

4. A solution as claimed in claim 1, 2 or 3, in which the chromium (III) thiocyanate complex is a sulphatothiocyanate complex or a chlorothiocyanate complex.

5. In a method of plating chromium wherein an aqueous equilabrated solution of chromium (III) thiocyanate complex provided the source of chromium and wherein a current is passed through the solution to cause electrodeposition of chromium, the improvement comprising, providing a buffer material in the solution, said buffer material providing one of the ligands for said complex.

6. In a method as claimed in claim 5, said buffer material being an amino acid, a formate or an acetate.

7. The invention as claimed in claim 6 wherein the amino acid is glycine.

CLAIMS SUPPORTED BY SUPPLEMENTARY DISCLOSURE

8. A solution as claimed in claim 1 in which the buffer material is a peptide or a hypophosphite.

9. A solution as claimed in claim 2 in which the amino acid is aspartic acid, arginine, histidine or hydroxyproline.

10. A solution as claimed in claim 8 in which the peptide is diglycine or triglycine.

11. The invention claimed in claim 5 wherein the buffer material is a peptide or a hypophosphite.

12. The invention claimed in claim 6 wherein said amino acid is aspartic acid, orginine or histidine.

13. The invention claimed in claim 5 wherein the buffer includes an amino acid and a peptide.

14. The invention claimed in claim 13 wherein said amino acid is glycine and said peptide is diglycine.

15. The invention claimed in claim 5 wherein the buffer is peptide.

16. The invention claimed in claim 5 wherein the peptide is di-glycine or triglycine.

17. The invention claimed in claim 2 or claim 6 wherein the acetate is sodium acetate.

18. The invention as claimed in claim 2 or claim 6 wherein the formate is sodium formate.

19. The invention as claimed in claim 8 or claim 11 wherein the hypophosphite is sodium hypophosphite.