GB2351503A - Zinc/manganese alloy plating bath; passivating - Google Patents

Abstract

There is disclosed an electroplating bath for depositing zinc/manganese alloys on a substrate characterized in that it comprises an aqueous bath free or substantially free of ammonium halide and of fluoroborate which is made up from 10-150 g/l, alkali metal salt, 30-90 g/l boric acid, 10-200 g/l water soluble zinc salt, 10-50g/l water soluble manganese salt, 60-140 g/l alkali metal gluconate or tartrate and a base e.g. an alkali metal hydroxide to bring the pH to the range 6.1-7.1. Aqueous compositions for forming black passivates on the surface of zinc alloy electrodeposits comprising hexavalent chromium, one or more carboxylic acids, eg acetic acid, a copper sulphate, and being free of silver ions are also disclosed.

Description

2351503 ALLOY PLATING The present invention relates to the deposition of

alloy deposits of zinc/ manganese alloys from electroplating baths which are at acid pH values close to neutral.

The problem with which the present invention is concerned is to obtain electrodeposits which have high contents of manganese, namely above 9% by weight, but which can be produced without the use of acid ammonium chloride or fluoroborate in the plating bath; these two ingredients being undesirable on 10 environmental grounds.

In addition the process must be able to plate components satisfactorily.

German OLS 2012774 describes a zinc plating process in which the plating bath contains 16.5g zinc sulphate heptahydrate, 1 10g sodium gluconate. 70g boric acid. 100g anhydrous sodium sulphate, 13g sodium hydroxide, 0. 2g benzaidehyde 1 and water to make up to one litre, the pH being 6.8. There is no reference to any allovin2 inaredients being present.

The prior art processes for plating zinc/manganese alloys contain ammonium chloride at acid pH's. We have attempted to replace the ammonium chloride by alkali metal chloride but found that this did not produce adequate amounts of manganese in the deposits.

Surprisingly we have found that if one uses alkali metal salts with gluconate or tartrate high contents of manganese can be obtained in the electrodeposit.

Thus according to the present invention an electroplating bath for depositing zinclmanganese alloys on a substrate comprises an aqueous bath free or substantially free of ammonium halide and of fluoroborate which is made up from 10-150 9,11 alkali metal salt. preferably 25-100 gil, preferably a sulphate 40-90 úrll boric acid, preferably 50-80 g/1, 10-200 uA water soluble zinc salt.

1 preferably 10-100 g.11 more preferably 20-40 g11, when the alkali metal salt 30 is a halide and 20-200gyll, preferably 45-100 g/1 when the alkali metal salt is a sulphate. 10-50 g/l water soluble manganese salt, preferably 20-40 RA.

7 60-140 9/1 alkali metal gluconate or tartrate, preferably 110-1330 g11.

Z and alkali metal hydroxide to bring the pH to the range 6. 1 to 7.2. preferably 6. 1 to 7.0. more preferably 6.3-6.9.

The alkali metal salt can be any such material but the sodium and potassium. chlorides or sulphates are the most economical and effective and the sulphates are preferred.

The water soluble zinc salt may be any of those used to electrodeposIt zinc but zinc sulphate is preferred.

The water soluble manganese salt may be any of those used to electrodeposit 1 () maneanese but manganese sulphate, which may be hydrated. is preferred. The zinc and the maneanese can be added to the plating bath in the form of salts other than the sulphates for example as sulpharnates, methane sulphonates. gluconates. tartrates. acetates. formates. or carbonates. When carbonates are added to acid systerns carbon dioxide will be released. This can be a way of avoiding the concentration of the sulphate conductivity salt rising to too high a level. Fairly high concentrations can have benefits in producing more even thickness distribution of the deposit as between high and low current density areas.

Gluconic and tartaric acids are hydroxy carbonic acids. and have been found effective as complexing agents for these systems. however citric acid does not seem =. -c good results. Other polyhydroxy compounds such as sorbitol might be expected to give stable complexes with zinc. as would amines such as tetra methylene pentarnine or EDTA. Triethanolarnine does not seem to be able to form a stable complex with zinc in this system.

Additional ingredients which may be added include grain refiners if desired.

Water soluble surfactants and polymers are well known in this art for this function and appropriate such materials may be added.

In a preferred form of the invention an electroplating bath is characterised in that it contains berizaldehyde as bisulphite in amount of 50 to 500M9.11. preferably i00 to ',00mg,,il. more preferably 175 to 225rng/1 e.g. about '00mglt'1. In another preferred form of the invention an electroplating bath is characterised in that it contains trimethviolpropane in an amount of 1 to 50 g/1. preferably 5 to 25g/1. more 1 - - - preferably 7.5 to 15g111 e.g. about 102/1.

The bath composition preferably comprises - 170 g/1 of salt anions preferably halide or sulphate anions, preferably 75 t - 140 g11, more preferably 80-120 g/1, 4 - 50 g/1 zinc ions. preferably 10 - 18 g11, g/1.

16 g11 of manganese ions, preferably 6 - 13 )5 - 90 e/l of borate ions. preferably 60 - 80 g/1, - 150 g/l of gluconate or tartrate ions, preferably 80 - 130 preferab. ly 175 to 225 mg/1 of benzaldehyde as bisulphite, or 7.5 to 15 all of trimethylolpropane.

io a pH in the range 6.1 to 7.2, preferably 6.1 - 7.0, more preferably 6. 3 - 6.9.

One specific embodiment of the invention is the following bath composition g/l zinc chloride, which provides 14.4 g/1 of zinc ions and 15.6 R/1 of chloride ions.

i - 31 g/1 manganese sulphate monohydrate. which provides 10. 1 2/1 of manganese ions and 17 gA of sulphate ions.

100g.11 potassium sulphate. which provides 55 gall of sulphate ions and 45 2,/1 of potassium ions.

2,1 boric acid. which provides 57 Q.11 of borate ions.

1-10 1 sodium gluconate. which provides 107 g/1 of gluconate ions and 13 C111 of k-)dium ions.

PH adjusted to 6.5 with sodium or potassium hydroxide.

A preferred specific embodiment of the invention is the following bath composition Z 6i -11 zinc sulphate heptahydrate. which provides 14.4 g/l of zinc ions and -21.71 -11 of sulphate ions. 30 9/1 manganese sulphate monohydrate, which provides 9. 8 g/1 of manganese ions and 6.5 g.A of sulphate ions. 100g111 potassium sulphate. which provides 55 g/l of sulphate ions and 45 g/1 of potassium ions. 75 gil boric acid. which provides 71.3) -11 of borate ions.

4 g/1 sodium gluconate or sodium tartrate, which provide 107 2/1 of gluconate ions, and 96 g/1 of tartrate ions respectively, pH adjusted to 6.5 with sodium or potassium hydroxide.

Effective plating conditions are room temperature, without agitation, using a zinc anode with a plating current of 2A. However higher or lower temperatures may be used e.g. up to WC or down to WC. Agitation may be used if desired. Plating currents in the range 0.5 to 4A may be used.

The invention also extends to passivating compositions for zinc/manganese alloys which unexpectedly give a black passivate and improved corrosion resistance.

Thus according to this aspect of the present invention an aqueous composition for forming a black Dassivate on the surface of a zinc/manganese electrodeposit is characterised in that it comprises hexavalent chromium. one or more carboxylic acids and a copper sulphate and is free of silver ions. The hexavalent chromium may be provided by a mixture of Cr03 and concentrated sulphuric acid. e.g. it may contain 30 to 70 a/1. preferably 40 to 60 g111 e.g. about 50 iTI of Cr03 and 2 to 15 m11/1. preferably 5 to 10 m/1 of 96% FIS01.

The composition preferably contains 40 to 100 ml/l. preferably 50 to 70 ml/l. more preferably 60 to 80 ml/l of acetic acid as the carboxylic acid.

The composition preferably contains 10 to 225 g/1 of copper sulphate. c. g.

4-3 - - CuSQ.5M0 preferably 14 to 20 g/1 more preferably 15 to 18 g, 1.

The invention also extends to a method of providing a zincl;man,,o,,anese allov electrodeposit with a black passivate which comprises treating the electrodeposit with a passivate composition as claimed herein. Preferabiv the zinCImanganese clectrodeposit contains 14 to 20% by weight of manganese.

The invention also extends to a zinc.,manú!anese electrodeposit especially one made in accordance with the present invention whenever provided with a black passivate finish by a method as claimed herein.

The invention mav be put into practice in various ways and a number of specific embodiments will be described with reference to the accompanying 0 examples to illustrate the invention. All references to room temperature mean 25'C.

Examples 1-8

Electroplating bath compositions were made up from the ingredients set out in Tables 1A and 1B Table IA

2 4 Example 1

Inaredient "0 Zinc chloride g11 30 30 -30 31 31 31 -Manganese sulphate. M0 g/1 1 Potassium chloride all 100 100. 100 1 loo Boric acid (H3B03) =0/1 60 i 60 60 i 60 Sodium eluconate a/l J.)o 120 120 Cationic polymer rnl/l (1) i -; 1. 5 Carboxylated ethoxylated alcohol ml/l (2) 24 Carbowax 4000g/l (3) 4 Sodium benzoate 9.4 BenzAidene acetone mLyl'l anilin ma/l (4) Sodium hydroxide to adjust pH to pH 6.5 6.5 6 5 6.5 Plating temperature "C.: 25 1 25 6 Table IB

Example 5 6 7 8 Ingredient Zinc chloride A 30 30 30 30 Manganese sulphate. 1H20 g/1 31 31 31 31 Potassium chlo ride gA 100 100 100 100 Boric acid (H3B03) 9/1 60 60 60 Sodium gluconate g11 120 120 120 120 Cationic polymer n-ii/1 (1) Carboxylated ethoxylated alcohol ml/1 (2) Carbowax 4000 g11 (3) 14 4 14 4 Sodium benzoate 0 14 Benzylidene acetone rng/1 Vanilin mc/l (4) Sodium hvdroxide to adjust pH to pH 6.5 6.5 6.5 6.5 5- Plating temperature 'C if 22 5 3. 5 33 53 i i Notes on Table 1 1) urelviene quaternary ammonium polymer. sold as MIRAPOL WT, which contains 64 %w/w of the said polymer dissolved in water. Mirapol WT has a CAS number 68555-3 36-.2 and is sold by Rhone-Poulenc and is stated to have the formu!a CH, 0 C H, I ! I I lo --N--CH,CH,CH,NHCNHCH,CH,CH.N.-CH,CH,OCH2CH2--- 2n Cl CH, CH3 n Where n = 6 (average) (2) This is supplied as AKYPO LF4 by Kao Corporation as an 86 % minimum active inaredient solution in water. The active inUedient is indicated by the 7 supplier to be a mixture of Capryleth-9 carboxylic acid and Hexeth-4 carboxylic acid. (see International Cosmetic Ingredient Dictionary. 6"' Ed. p 137 and p 445) (-3) Carbowax 4000 is 100% w/w polyethylene glycol of NIW 3500 supplied as a solid powder by Union Carbide.

(4) The vanilin is added in the form of a bisulphite adduct to bring it into solution.

Each of the baths given in Table I were used to plate Hull cell panels in Hull cells. which panels afforded a mild steel substrate and are of flat rectangular shape 10 being 10 cms lono, by x 6.7 cms wide.

A zinc anode was used with a plating current of 2A and a plating time of 10 minutes without aeitation. In all the tests gassing occurred at the mild steel cathode indicatina that the efficiency was less than 100%.

The mild steel Hull cell panels have high, medium and low current density re,-,ions and can be considered as havin2 ten re2ions located from the hi2hest to the 10west current density region along the panel. In the following Table -1 the highest density realon will be called re2ion 10 and the lowest densitv reinon will be called realon 1.

The appearance of the deposits is indicated by the following letter codes which have the meanin2s tliven below Table Ailoy compositions are also given for two of the examples namely Examples 4 and 6 at four different Positions namely positions 9.7.4 and Table 2

Example 1 2 3 4 5 6 7 8 Panel position bu bu bbs bl bl bl bu bl 9 bl iyv bbs SB ar SB bu SB composition %Mn 28.1 2 0. 6 8 SB gy bbs SB gr SB bu SB Z:?, - 7 SB gy bbs SB ar SB SB SB 25.6 composition % Mn - 18.9 6 SB 2Y bbs SB gr SB SB SB i SB BR bbs SB SB SB SB SB 4 SB BR SB SB SB SB SB SB composition %Mn 20.7 17.3 11 av Z= BR SB SB SB SB SB SB - 2Y BR SB SB SB SB SB bl composition % Mn 15. 3 9.6 1 gy BR BR BR BR bl BR bl Table 2

Appearance Codes fo bu = burnt. bl = black. SB = semi-bris!ht. zv = grey. BR = bricht.

nbs bright brown streakv. -)rr = grainy.

The ranking of these appeprances from best to worst is BR> SB > LY-Y > bbs > bil > Lyr > bu.

The manganese content was determined by cutting a lem by 4cm sample r'l', )m [he Hull panel. The rear face of the sample is masked and then the deposit is stripped off with 40 nil of hydrochloric acid (500m1111 HCl 3 5 % and 500m111 water). This solution is then diluted down to 100m.1 with dernineraliZed water. Induced plasma emission spectroscopy (ICP) is used to determine the zinc and manganese content. Standard apparatus tmodel OPTIMA 3000 manufactured by Perkin Elmer) is calibrated usinLY standard procedure against a hydrochloric acid blank (20% by 9 volume) and a standard containing 250 m---/1 zinc ions and 2.5 m-Lyll manganese ions in 20% by volume HCL The wavelenaths for the elements to be measured are selected to have a good sensitivity and to not be interfered with by other elements which may be present. The wavelen-th for zinc was 206 rim, that for maneanese was 279nin..

As can be seen from Table 2 zinc/manganese electrodeposits containing, between 15 and 28 % manganese can be obtained. The deposits are generally semibri2ht in appearance. which is useful as a technical finish e.g. for functional components such as fasteners, bolts, screws, nuts and brackets.

It will be noted that the amount of manganese in the deposit is lower at a plating bath temperature of 53'C than at -25'C.

The solution of Example 1 -5 of Table 1 was left to stand open to air in the laboratory for several weeks and it remained clear without any colour changes indicailn2 Rood stability. Examples 9-14 Resistance to corrosion by neutral salt spray testing was carried out on flat plate samples 10x7cms in area plated in a 2.5 litre beaker having the composition of c example 4 above. using a zinc anode of plate form. and mechanical agitation at 25'C. The anode was parallel to the workpiece and 13cms therefrom. The face of -iie worKpiece which faced the anode was the face exposed to the salt spray. The deposits contained 1 -1 to -1 1 % Manganese. balance zinc and were 10 micrometres thick. Table 3 below gives a comparison of a conventional alkaline zinc deposit with no passivate (ex 9) and with two proprietory passivares PERMAPASS 3080 - ( a trivalent chromium passivate) (hereafter PP3080) -5 i PERv1APASS is a Trade Mark of Enthone OMI Inc and is rey-istered in a number ofeountries) (exIO) and P2 (MOLYPHOS 66) - (a chrome-free passivate.) ( supplied by Centre for Advanced Electroplating. Copenhagen. Denmark.) (.hereafter P2) (ex h and the said sample of example 4 with the same three degrees of passivation (ex and 14).

P2 is a chrome free conversion coatina in which the ratio of molybdenum to phosphorus is 0.66. The pH is 4.6. and itis used at 60C for 3 minutes.

Table 3.

Neutral salt Spray test Example Passivate Time to Time to Time to Commence 5% WCP Commence ment of WCP merit of RR (2) hrs (3)hrs (4) hrs 9 none <24 48 48 (i,) 3 72 240 PP '080 24 P2 24 48 48 none <24 <24 168 13 PP3080 48 72 24 8 14 P- '14 24 168 \Ores on Table The neutral salt spray test consists of continuously exposing' the plated article to a salt fog formed by nebulizing neutral 5% w.1w sodium chloride solution at 35C using the Standard procedure of ASTM B 117. WCP means white corrosion products, and cornrriencement occurs at the eYes of the plate.

dg WCP means that 5,c of the area of the plate is covered with WCP RR means red rust. The difference in protection against red rust for the product in accordance,,-ith the present invention (Ex 14) of 168 hours is a significant improvement - 5,-x,er the prior product (Ex 11) of 48 hours.

Carbowax 4000 was present in each of examples 4-8 and. as can be seen 2 rom Table 2. these have the largest extent of semi-bright appearance. and are preferred. Whilst the present invention is not dependent on the accuracy or otherwise of any theory. Carbowax 4WO is believed to act as a grain refiner. which serves to promote the formation of uniform. adherent deposits. Examples 15 to 25 These were made up to develop a chloride free near neutral zinc-manganese plating, process. It is anticipated that metal concentration in chloride baths will be a problem. The plating efficiency is less than 100% and a considerable amount of the metal deposited is likely to be manganese rather than zinc. The use of zinc anodes would cause a build-up of zinc. Inert anodes could not be used because they would cause evolution of toxic chlorine gas.

Table 4A below sets out the ingredients and amounts for examples 15 to 18. and Table 4B for examples 19 to 22, and Table 4C for examples 23 to 25.

Table 4A

Example 15 16 17 18 Ingredient Zinc chloride 9/1 30 Zinc sulphate. M0 g11 I-! 6 65 1 i 1 1 Potassium chloride g/1:i 100 loo 1 - i Sodium sulphate anhydrous g/I i! 100: loo 1 - i 1 1 Manganese sulphate. 111,0 g1I 1 30: 30: 30. 11 -0 Boric acid (H3B03) 75 7 5 75 75 Sodium gluconate g/1 120 1-10 1210 120 Cationic polymer ml/l (1) i - - CarboxvIaied ethoxviated alcohol ml/l (2) Carbowax 4000 ft/l (3) 4 4 Sodium benzoare afl Benzylidene acetone mg/1 1 15 Vanilin mR, 1 1(4) PEG 400 (5) Pluriol E-1500 g/1 (6) Lutron LIF-1 gil (7) - - - - R.Avmin G-35 8) Potassium ihiocyanate 2A Sodium allyl sulphonate 9,1 (9) Sodium hydroxide to adjust pH to pH 6.5 6. 5 6. 5 6.

-)5 Plating temperature 'C 25 - -15 13 Table 4B

Example 19 20 21 22 Ingredient Zinc chloride g11 Zinc sulphate. 7H20 9/1 165: 65! 65 165 Potassium chloride g/1 Sodium sulphate anhydrous g/1 11 100 100 100; 100 1 i 1 - 1 Manganese sulphate. 1H20 9/1 130 i 30! 30!' 30 Bodc acid (H3B03)9/1 75 75 75 75 Sodium gluconate g/1 120 120 120 120 Cationic polymer m111 (1) Carboxylated ethoxylated alcohol rni/1 (2) Carbowax 4000 g11 (3) Sodium benzoate g11 Benzylidene acetone mgA Vanilin mg/1 (4 PEG 400 g/1 (5) Pluriol E-1500 gA (6) 4 - Lutron HF-1 gA (7) - 4 Polymin G-35 g "1 (8) - Potassium thiocyanate g.11 Sodium allyl sulphonate g/1 (9) - - Sodium hydroxide to adjust pH to pH 6.5 6.5 6.5 6.5 Plating temperature C 25 25 25 25 14 Table 4C

Example 23 24 25 Ingredient Zinc chloride a/I Zinc sulphate. 7H.0 g/1 65 165 1 6 -5 : Potassium chloride g11 100 i 100 w i - - i Sodium sulphate anhydrous g/l i - 1 i; 100 :,0: "0 Manganese sulphate. IM0 g/1 1 3! 30 Boric acid (14,B03) 0:11'/1 75 75 75 Sodium gluconate L7/1 120 1 120 120 Cationic polymer ml/l (1) i Carboxylated ethoxylated alcohol ml/l (2) Carbowax 4000 9/1 (3) Sodium benzoate g.1 BerizAidene acetone m2/1 Vanilin mg/l (4) PEG 400 c., 1 (5) Pluriol E-1500 g/l (6) Lutron HF-1 iy,"1 (7) Polvmin G-35 g'/1 Potassium thiocyanate g.'I 6 - Sodium all-y-1 suiphonate g/l (9) '10 Sodium hvdroxide to adjust pH to pH 6. 5 6.5 6. -5 Plating temperature T 1 25 -75 25 Notes on Table 4 1). (2). (3:5). (4) same as table 1.

(5) PEG 400 is a polyethylene glycol which has a Molecular weight of 400. and is sold by BASF as Pluriol E-400 as a 100% active ingredient liquid.

(6) Pluriol E-1500 is a polyethylene glycol of MW 1500 sold by BASF as a 100% active ingredient liquid. -1 is a modified polyglycol ether sold by BASF as a 100% active (7) Lutron HF ingredient liquid.

(8) Polymin G-35 is a polyethylene imine of low MW sold by BASF as a 50% w/w active ingredient solution in water.

(9) Added as 300 g/1 solution in water. 10 In Examples 17 to 25 which use sulphate salts rather than chlorides a similar compound to Carbowax 4000 is used namely PEG 400. It has a better solubility in the sulphate bath than does Carbowax 4000.

Water soluble polymers and surfactants are preferred.

Each of the baths given in Tables 4A and 4B were used to plate Hull cell panels in Hull cells. as described for examples 1-8. using a zinc anode with a plating current of 2A and a plating time of 10 minutes without agitation. except for Example 16 which used air agitation. The appearance of the panels was generally semi-bright with some dull areas in the high current density region.

These Hull cell panels were then analysed by the ICP technique described for examples 1-8 and the example number. position of analysis and alloy content are alven in Table 5. For each example the table first gives the total content in pprn of zinc plus manganese and then below this the % of manganese. these figures are listed in columns below the position at which the analysis was done namely Hull 2 postrions.4.7 and 9.

16 Table 5.

Hull position 2 4 7! 9 Example no

97.7 234.9 369.3 431.1 % Mn 11.3 19.6 24.8 19.6 16 79.7 225.8 445. 581.9 16 % Mn 6. 1 18.1 22.8 3 27.-) 17 Not tested (1) 17 18 93. 3, 250.7 1416.3 523.3) 18 -,, M n 10.4 15.3 j 20.8 2) 4. 5 i 9 Not tested (2) 19 9 1. 3 11 232.5 3)71.4 3398.7 2 0 /-c M n 7.5 15. 25.1 67.1 108.2 1 166.9 Not tested (3) v-1 n: 0.4 1 0.5; 2. 3 i 1 51.6 196.8 Not tested (3) 47.4 0.4 1.7 In 19.6 1-7 133.9 M.6 33 3 1. 1 Not tested (3) -, -- C/-, \, In: 1.8 0.4 ,0.9 24 114.7 3 94. 7 18.0 18.4 24 1J.8 108.0 338.9 Not tested (3) Mn 5. 1 12.9 17.5 1 1 17 Example 26

A 25 litre bath was made up for barrel plating using the composition of example 18 with the p11 adjusted to 6.6 with sodium hydroxide.

Barrel plating was carried out on steel bolts as the workpiece using one steel anode of 20x 25 cins and one zinc anode of 4.5x 6 cins. at 1 Aildm2. for 70 minutes at 14.6 A, 11 volts, and 25'C. The plated bolts were semibright in appearance with dull heads. The plating solution discoloured from pink to yellow and inspection of the steel anode showed some pitting indicating attack on the steel anode. which was confirmed by analysis of the bath which was shown to contain 43ppm of iron.

Analysis of the deposit by 1CP as for examples 1-8 indicated 15.6% manganese in the deposit, which was 8.6 micrometres thick. The plating efficiency was 43.5 %.

ExaLnples 27. 28 and 29.

Three samples of the plated bolts of example 26 were subjected to 1 27) (see Example 10 pass vation for 30 seconds with PERMAPASS 33080 (Ex above). 3, minutes with P.' (Ex 228) (see Example 11 above) and 30 seconds with another proprietary passivate ENTHOX 7748 (Ex 29). The resulting passivated bolts were respectively "bright uniform, purply blue",---flecky iridiscent yellowand -lridiscent yellow" in appearance.

ExiMple 30.

Hull cell plating was carried out with the bath composition of Example 18 to which was added 50 mú,./1 of benzylidene acetone as the active ingredient (predissolved in isopropyl alcohol). This gave a slight improvement in brightness.

Exa=le 3 1.

Hull cell plating was carried out with the bath composition of Example 18 to which was added 20 ing/1 of vanilin added as the bisulphite adduct. This produced a clear improvement in brightness. especially in the high current density area.

Exg.mple 32.

The barrel plating of Example 26 was continued using the same bath but with the addition of 20 ing.11 of vanilin added as the bisulphite adduct. In addition the steel anode was replaced and instead as the anodes two platinized titanium mesh anodes were used. 15x 20 cms in size. 1CP analysis of the alloy deposit indicated 18 20% manganese. The thickness was 8.8- 10.3 micrometres. The plated bolts were brighter than in example 26 but the heads were still slightly dull. The passivation procedures of Examples 27-29 were repeated but the appearance of the passivated bolts did not change.

The amount of iron in the bath at the beginning of this plating run was 43ppm and at the end of the run had not changed, indicating that no iron was lost from the steel workpieces.

Example 32 used inert anodes and demonstrated that this sulphate process can be carri ed out without evolution of chlorine gas. Steel anodes should be 10 avoided. Mixed inert and zinc anodes could be used.

Exa=les 33-47 Sulphate plating baths similar to Example 18 were made up with the compositions shown g11 in tables 6A, 6B and 6C below, and Hull cell plating was carried out as for examples 1-8 namely 2A but 20 minutes plating time.

1 -. 19 Table 6A

Example 33 34 35 36 37 Ingredient i Z11S04.71120 60 60 60 60 160 Na2 Sol 100 100 100 100! loo 1 Anhydr.

MnSO, 1H.0 30 30 30 JO 60 11,B03 0 37.5 37.5 75 75 Na gluconate 120 120 60 120 120 Na tartrate 1 - - 1 Na citrate Sorbitol TEA (1) TEPA (2) EDTA-2Na PEG 400 4 14 4 4 4 pH 6.3 6.6 6.7 6.6 6.7 Notes on Table 6 1) Triethanolamine Tetra ethylene pentarnine Ethylene diamine tetra acetic acid disodium salt -)o Table 6B

Example 38 39 40 j 41 42 -7 In-redient ZnS0,7H.0 60 60 60 Na, SO, 100 100 100 100 1 loo Anhydr.

MnSO,.M.0 30 1 30 30 1 -30 30 H,B0, 75 75 75 75 Na cluconate Na tartrate i Na citrate Sorbitol TEA (1) 60 - TEPA (2) 60 EDTA -2Na 120 PEG 400 4 14 4 4 4 pH 6.5 6.8 6.5 6.6 Notes on table 6B 4) a precipitate was formed which did not redissolve so the plating was not carried out 21 Table 6C

Example 43:E4 45 46 47 Ingredient ZnSO,. 7H.0 1: 60 60 60 60 1 90 Na, SO,! 100 100 100! 100 100 1 Anhydr.

MnS04. 1H20 1 30 30 30 1: 60 60 H B03 1 75 75 75 75 75 3 Na gluconate 120 J I i Na tartrate - i - Na citrate Sorbitol TEA (1) 1 - i TEPA (2) EDTA-2Na 120 PEG 400 4 4 4 4 4 pH 6.9 6.6 6.6.6.6 6.6 ExaLnples 48-54 A bath of the composition of example 36 was modified hy adjusting it's pH.

Examples 48 and 49 had pH 3.4. Ex 50 pH 5.3:%: Ex 51 pH 5 - 9: Ex -52 plI 6.4: Ex 53 3 pH 7. 1. Ex 54 was example 36 to which was added 10 nil of N-ainino ethyl ethanol arnine and the pH was then adjusted to 6.5 with sodium hydroxide.

When the pH was above 7.5 a precipitate was formed.

The appearance of the Hull panels of examples 333-54 was that generally the panels show burning or non-adherent black deposits in the high current density g/l areas. Acceptable results were only obtained with gluconate and tartrate. 1. uluconate gave better uniformity than 60 g11. 75 g/1 boric acid gave better results than lower values. Higher pH values gave better results with regard to appearance especially in the low current density areas.

1CP analysis as for examples 1-8 was carried out on the Hull cell panels which had adherent deposits. The locations of theanalysts on the lx 4 cm area were as follows in Table 7.

Table 7

Position on Hull cell panel Cm distance from low Comment current density edge 2 1-2 cm This is the low current density area 4 3-4 cm -7 6-7 cm.

9 8-9 cm This is the hieh current density area q The results of the analyses are given in Table 8 below as % manganese content of the deposit.

2 3 Table 8

Hull 4 7 9 Platina rate position Example

0.8 8.0 34 11.3 18.8 10.5 16.4 36 14.5 18.7 25.6 27.6 37 14.6 19.6 i! -3,8 15.8 18.2 21.9 23.7 41 Not suitable

Not plated 1.7 3.6 9.8 44 12.

17.9 25. 3 4 5 i 2 4. -3) 60.7 Verv low efficiency 46 14.4 19.1 47 3.9 19.2 0.3 14.

48 l!.0 49 7 14.3) 17.3 -)0.' 0 9.5 16.1 1 20.6 18.9 23.8 2 7. 1 14.6 2 25. 3 1 _3. 7 18.5 252 15.6 20.9 24.3 54 0.1 0.1 0.1 q 24 The above results and ICP analyses indicate that boric acid makes the alloy distribution more uniform because it increases the % mancanese content In low current density areas and the medium current density areas.

Higher gluconate amounts give slightly higher % manganese and better high C1 current density appearance.

Tartrate gives slightly more uniform manganese distribution than gluconate.

Citrate gives high % manganese but very low efficiency.

TEPA and N-amino ethyl ethanolarnine suppress the % manganese in the deposit.

Doubling the manganese concentration in the bath only produces a slight increase in % manganese in the deposit, and thus has no economic benefit.

Hicagher zinc plus manganese concentration in the bath produces a less uniform appearance.

Higher pH within the range up to 7.1 results in more uniform distribution of maniranese in the deposit.

Sorbitol can be used as a complexor but results in less good distribution of manganese in the deposit and a less CFood appearance than is obtained with 2luconate.

Example 55

A 20 litre bath was made up for barrel plating using the composition set out in Table 9 below with the pH adjusted to 6.8 with sodium hydroxide.

Table 9

Example 55 i Ingredient Zinc sulphate. M0 g/1 60 1: Sodium sulphate anhydrous g/1 100 i 1 ! Manganese sulphate. 1H,0 g/1 30 Boric acid (H3B03) g/1 75 Sodium gluconate a/l 120 Benzaidehyde (as bisulphite) mg/1 200 Barrel platimg was carried out on steel nuts with an attached washer as the workplece using two platinised titanium anodes of 20 x 25cms and onezinc anode of x 6ems. at 1.73 A/drn2. for 60 minutes at lOA, 9.5 volts. and 291C. The solution did not change colour during electrolysis. No attack was observed on the platinised titanium anodes. The zinc anodes were attacked sufficiently to maintain the zinc concentration in the bath at a stable level. The plated nuts were grey to semi-bright in appearance.

Analysis of the deposit by ICP as for examples 1 -8 indicated 17.8-18.8 % manganese in the deposit, which was 10 micrometres thick. The plating efficiency was 34 %. ExaMles 56-65 The nuts produced by Example 55 were passivated by immersion in the passivating agents listed in Table 10A which identifies the passivates by a number P3 or P4 or by their trademark identifications and gives the pH used, the time of immersion and the resultant colour.

1 26 Table lOB gives the composition of passivates P3 and P4.

Table lOA

Ex Passivate pH Time Appearance identification (mins) ENTHOX 775 1.6 45 Black with some dull patches 56 57 P3 1.98 120 Bright black 58 P3 3.0 120 'grey black 59 P4 Bright black slightly lighter than Ex 57 Frappaz Passival 1.3 30 red-yellow patchy iridescent LP108 61 Triazur 200 2.0 50 blue with yellow patches (Chemmetal) 62 ENTHOX VOZ 1.0 60 brown. partially stripped, too mgressive. dissolves too much of the deposit.

63 7778 1.1 60 gareen/grey 64 IMMUNOX 3K 2.0 30 reddish blue ENTHOX 961 2.0 30 red green iridescent 66 ENTHOX 747 1.6 30 Yellow iridescent The passivates listed in Table lOA under trademark are commercialIv available under such marks. Enthox 775 is a hexavalent chromium passivate containing silver ions.

Enthox VOZ and If 778 are hexavalent chromium passivates containing inorganic acids. Enthox 961 is a hexavalent chromium passivate containing inomanic salts and an oreanic additive.

Enthox 747 contains chromium oxide, carboxylic acid, inorganic acid and metal salt.

Immunox 3K contain nitric acid and various metal salts, phosphate and fluoride.

17 2 Table lOB

Passivate P3 P4 Ingredient Cr03 9/1 50 i 50 1 HS04 (96%) ml/l 7 8 Acetic acid m-1/1 60 80 Formic acid m/1 7 CuS0,5 H209 /1 16 16 Examples 55. 56 to 61 and 63 to 66 (without any preliminary heat treatment) were then subjected to neutral salt spray testing when suspended on a grid spaced apart 1 - from each other by 2cms 30 such nuts being treated and arranged in such a wa so c y that drips from the top of the array did not fall on nuts at the bottom of the array.

The results are given in Table 11 below. the test being carried out as described for Table 3 and the abbreviations WCP and RR havinly the same mcanin!is as given for Table 3.

Z7 29 Table 11

It can be seen from Table 11 that the passivates in accordance with the present invention (P3 and P4) which give a black finish also give much improved orrosion resistance as compared to the other black passivare namely ENTHOX 775 which contains silver ions.

1 Time to 1 Time to 5 % Time to : Ex commencement 5% commencement of RR of WCP (hrs) WCP (hrs) i (hrs) L56 146 142 286 57 1 478 670 238 58 238 478 670 59 238 1478 622-670 238-334 286-478 574-670 61 5 22 142 63 238 238 574-670 64 T? 238 334 238 286 574-621 66 286 334 574 -670 142 )o Examples 56A-61A and 63A to 66A Examples 56 to 61 and 63 to 66 were subjected to heat treatment, namely 1 hr at 12101'C. This is to simulate conditions in the engine compartment where parts get exposed to elevated temperatures in automotive applications. They were then subjected to neutral salt spray treatment as in Table 11 and the results are given in Table 12.

Table 12 to 5% Time to Ex Time to Time commencement WCP (hrs) commencement of RR 1 of WCP (hrs) (hrs) 56 5 5 142 57 22 46 574 58 142 238 i 621 59 23 8 238 670 46 142 334-406 61 2 22 22 14 6 3 22 22 142 64 22 46 3 3 4 4 0 6 22 238-406 -2 66 142 142 478 1 1 ion for a blue colour was achieved by In unox 3K. for a vellow The best passivar un iridescent colour by Enthox 747 and for a black colour by P3 or P4.

Exa=le 67 Example 55 was repeated and barrel plating was carried out on Jern long steel screws using two platinized titanium anodes (20 x 25 em) and three zinc anodes i - (4.5 x 6cm) at 0.8A,,dm for 60 minutes at 7.4 volts, 8 Amps and 251C. The plated screws had bright heads and points. The threaded area was Uey. Analysis of the deposit by 1CP as for examples 1-8 indicated 16.7 % manganese in the deposit which was 6 micrometres thick. Scanning electron microscope (SEM) analysis indicated that the heads had 19.4 % manganese content and the centres of the threads 6.3 % o manganese content.

Examples 68 to 78 The screws produced by Example 67 were passivated with the same passivates under the same conditions as given in Table 10 with the same appearances C1 being produced.

The screws of examples 67, and 68 to 76 (without any preliminary heat treatment) were then subjected to neutral salt spray testing as for examples 55. 56 to 61 and 63 to 66 and the results are given in Table 13. Table 13 Ex Time to Time to 5 % Time to commencement WCP (hrs) commencement of RR of WCP (hrs) (hrs) 68 192 192 1 288-360 69 192 192 360-528 192 192 '60-528 71 192 192 5-8-648 72 192 192 4332-576 73 96 96 192 74 192 192 360-576 1 192 360-576 76 1 192 240 432-576 67 <96 <96 96 Examples 68A to 76A Examples 68 to 76 were subjected to heat treatment. namely 1 hr at 12011C to anneal the coatings. They were then subjected to neutral salt spray treatment as in Table 11 and the results are 2iven in Table 14.

Table 14

Ex Time to Time to 5 % Time to commencement WCP (hrs) commencement of RR of WCP (hrs) (hrs) 68A 96 96 19,? 69A 96 192 528 70A 96 192 720 > 720 IIA 96 192 72A 192 192 720 96 96 19-7 715A 74A 96 192 528 192 75A 196 720 76A 96 192 576 1 1) j 1 For Examples 68 to 76 the best black colour is obtained in Examp'e 68. Examples 69-71 are slightly more brown.

A comparison of Tables 13 and 14 with Tables 11 and 12 indicate that heating the screws improved resistance to red rust though white corrosion was s 1 ightIv worse overall. This effect was not observed with the nuts.

The overall conclusions on passivation is that Permapass Immunox ')K (Ex

64) gives better appearance and corrosion results than Chemmetal Triazur 1-00(ex 61). For iridescent passivation Entliox 747 (Ex 66) gives the best results.

The best black colour is with Enthox 775 (Ex 56) but P3 or P4 (Ex 57, 58 and 59) give much better corrosion results but with a sliLyhtly brownish black colour.

Exgk=les 77-96 The Hull plating procedures used for Examples 1-8 were carried out on the compositions given in Tables 15A. 15B. 15C and 15D set out belokk,.

Table 15A

77 78 79 80 81 Example

Ingredient ZnS0,7H.0 60 60 60 60 60 1 Na. SO,. 100 1 loo 1 loo: 100 100 Anhvdr.

MnSO- IH.0 3 0 i 30 30 -310 60 q WB0-, 75 75 75 75 75 x M 120 120 1 120 Na Lyluconate Heliotropine (as 200 hisulphite) ppm Benzaldehvde (as 200 hisulphite) ppm Salicylaldehyde (as 200 bisulphite) ppm PEG 400 ml/l 14 716- 6.8 6.8.6.8 Ph 6.8.8 Table 15B

Example 82 83 84 8 5 86 i Ingredient ZnS0,711,0 60 60 160 160 60 i Na, SO, 100 100 100 1 100 i 100 1 Anhydr.

MnS0,111,0 30 30 30 J0 H3BO, 75 75 75 75 75 Na gluconate 120 120 12, 0 120 Heliotropine (as 200 bisulphite) ppm Benzaidehyde (as 1200 bisulphite) ppm SalicylaIdehyde (as 200 bisulphite) ppm SCO, ppm 400 KSCN iy/l 4 4 ESAIEK 4 20289 sA PEG 400 14 4 pH 6.8 16.8 6.8 6.8 6.8 4 Table 15C seo, ppm KSCN a/l ESA/EK::4 0M9 LY1 (1) TMP g."l 10 i 10 PT-5 mi."l (3) PEG 400 (ml/D pH 6.8 :6.8 i 6.8:6.8 Example 87 88 89 90 Ingredient ZnSO,.71-1.0 60 60 60 60 il Na. SO, 100 1100 100 100 Anhydr.

MnSO,. 11-1,0 30 30 30 1-1,1303 75 75 75 175 i Na gluconate 120 120 120 120 Heliotropine (as 200 bisulphite) pprn Benzaldehyde (as 200 bisulphite) pprn Salicylaldehyde (as j hisulphite) ppm 1 )5 Table 15D

1 Example 191 92 93 94 95 96 Ineredient ZnS0,7H,0 160 60 60 60 60 60 Na,S04 1 100 100 100 100 100 100 Anhydr.

MnS0,1H.0 30 30 30 no 60 160 1 H,,B03 75 75 75 75 175 i 75 Na gluconate 120 120 120 120 120 I'm Heliotropine (8) 200 (as bisulphite) ppm Benzaldehyde (as tiisuiptlite) PPM Sallevialdehyde (as bisulphite) ppm SeO. ppu KSCN efl ESA/EK W'Y89 z/1 (1) TMP (2) PT-5 rfil,,"1 (3) LuLTalvan HS1000.:4 Rewoquat CPEM L.." 1 (5) Anisaldehyde as 200 bisulphite, ppm (6) C-36 (7) ml/I PEG 400 (rnl/l) 4 14 4 4 4 4 pH 6.8 6.8 6.8 16.8 16.8 6.8 36 )ies on Table 15 ESA/EK 20289 is supplied by Bayer and is described as a quatemary amine product (.) TMP is trimethylolpropane (3)) PT-5 is a quaternary poly alkylene imine 4) Lugalvan HS 1000 is a thio diglycolethoxylate (5) Rewoquat CPEM is (N-methyl-N-pentaethoxy)-N-coco ammonium methosulphate (6) Anisaldehyde is 4-methoxy benzaldehyde (7) C-36 is 36% benzyl nicotinate solution (8) Heliotropine is piperonal also called 1,3-benzodioxole-5 carboxaldehyde Each of compositions 79 to 96 were used to plate Hull cell panels in Hull cells as described for Examples 1-8 using a zinc anode with a plating current f of 2A and a platincy time of 10 minutes without aLyitation. The example number. the appearance of the 10cm long panels and the length of the panel exhibiting Lhat appearance are given in Table 16.

1 The Hull cell panels were then analysed by the ICP technique described for examples 1-8 and the example number. position of analysis. the deposit wt in rng/4 cm- area at that position (labelled "N-C) and the '5c' wt manganese content of that deposit (labelled " % "'i at,- given in Table 17.

37 Table 16

Example Appearance length (cms) 1 77 semi bright 8.5 78 bright + semi 8.5 bright 1 79 streaky bright 8.5 bright 7.5 1 81 bright 8 82 streaky bright 9 83 fully black - i 84 1 semi bright 6 semi bright J) 86 bright full lerinh 87 bright full length 88 semi bright 8 89 swili bright 8 semi bright 7 91 irregular 7 92 streaky bright 8 91 semi bright j 8 JY 94 bright area 7 bright 6 96 brown bright 38 Table 17A

Hull position 6 8 2 Example

77 wt 169 174 190 228 79 16.0 26.7 32.0 36.3 wt 152 158 217 2 13 % 10.9 19.2 22.1 22.4 6 j k U. _ i 1,0 i 84 wt 182! _150 162 l 228 84 % 0.4 3.4 5 9.3 wl 1.1) 1 151 1 174 265 % 4.6 12.1 15.0 20.5 86 wt 76 5 8 86 118 86 % 0.4 0.3 10.4 0. 3 51 31 87 wt 52 47 0.

87 % 0.5 0.6 88 A,r 170 186 232 283 88 % 3.4 14.8 18.1 19.4 89 wt 146 110 140 198 89 % 0.1 0.1 0.4 10.2 wl 107 86 ill i 187 % 0.2 0.8 15.2 0- 39 Table 17B

Hull position 2 4 6 8 Example

91 wt 75 209 257 1 3352 91 % 10.7 18.3 22.7 1 25.4 92 wt 36 114 155 T192 92 % 15.6 26.4 8.9 93 wt 120 199 1 257 348 93 % 11.6 17.7 21.6 j_24.8 j 94 wt 69 139 182 '776 94 % 17.7 25.0 28.4 31.7 wt 60 123 155 22 3 % 18.9 25.4 128.1 31.3 96 wt 10 5 8 3 3 96 % 6.4 33). 6 1 57.1 146.0 The preferred range of alloly composition is in the range 14-20% Mn. This should be as uniform as possible over the whole panel. The deposit weight i.e. the thickness should be as uniform as possible and as high as possible.

The thicker the deposit the more efficient is the process and the quicker can a desired thickness be deposited.

The passivates P3 and P4 are also effective in producing black deposits on zlnc,,;iron alloy electrodeposits e.g. containing 04-0.8% by weight iron. They are also elfective on zinclcobalt alloy electrodeposits e.g. containing 0.6 to 1.-"c cobalt.

They are also effective on zinc/nickel electrodeposits.

Claims (1)

1. I. An electroplating bath for depositing zinc/manganese alloys on a substrate characterized in that it comprises an aqueous bath free or substantially free of ammonium halide and of fluoroborate which is made up from 10-150 a/I alkali metal salt. 30-90 2,11 boric acid, 10-200 a/l water soluble zinc salt 10-50g/l water soluble manganese salt, I () 60-140 g/l alkali metal gluconate or tartrate, and a base e. 2. an alkali metal hydroxide to bring the pH to the range 6.1-7. 1.

   I - 2. An electroplating bath for depositing zinc/manganese alloys on a substrate characterized in that It comprises an aqueous bath free or substantially free of halide and of fluoroborate which is made up from 1 1550 g.,, I alkall metal salt. other than a halide. -1,0-90 il-/l boric acid. '0-200 E!./I water soluble zinc salt 10-50L-,il water soluble manganese salt. 6o-140 L,,;l alkali metal aluconate or tartraLe. and a base e. 2. an alkali metal hydroxide to bring the pH to the range 6. -5-6. 9.

   ) characterized in that it contains An electroplating bath as claimed in claim g.'] of alkali metal salt.

   An electroplating bath as claimed in claim 1. 2 or 3 characterized in that it contains 50-70 -/1 boric acid.

   An electroplating bath as claimed in claim 1. 2. 3) or 4 characterized in that it contains 50-90 a/l water soluble zinc salt.

   - "I 41 6. An electroplating bath as claimed in anyone of claims 1 to 5 characterized in that it contains 20-40g/1 water soluble manganese salt.

   7. An electroplating bath as claimed in any one of claims 1 to 6 characterized in that it contains 110-130 g11 alkali metal gluconate or tartrate.

   8. An electroplating bath as claimed in any one of claims 1 to 7 characterised in that it contains benzaldehyde as bisulphite in an amount of 50 to 500 m0A.

   9. An electroplating bath as claimed in any one of claims 1 to 7 characterised in that it contains trimethylolpropane in an amount of 1 to 50 g/1.

   10. An electroplating bath as claimed in anyone of claims 1 to 9 characterized in that it contains alkali metal hydroxide to bring the pH to the range 6.3-6.9.

   An electroplating bath composition characterized in that it comprises an aqueous bath comprising 15 - 170 gIl of sulphate ions. 4 - 50 a/l of zinc ions.

   16 ty-11 of manaanese ions. - 90 a.;'1 of horate ions.

   1 -,0 ti."! of!zluconaie or tartrate ions. and pH in the range 6.1 - 7.2.

   An electroplating bath composition characterized in that it comprises an aqueous bath comprising 5,5 to 75 e.g.65 2/1 zinc sulphate heptahydrate.

   1 - - to 40 e. a. 30 g/1 manganese sulphate monohydrate.

   = j -- - to 110 e.g. 100gr/l potassium sulphate, to 85 c.a. 75 e/1 boric acid.

   f) 110 to 130 e.g. 120 e/l sodium gluconate or sodium tartrate, and in that the 1 - 1 42 pH is adjusted to 6.4 to 6.9 with a base e.g. sodium or potassium hydroxide and In that the composition is free or substantially free of alkali metal halide and of ammonium halide and of fluoroborate.

   13. An electroplating bath composition as claimed in claim 11 or 12 characterised in that it contains 175 to 225 mg/1 of benzaldehyde as bisulphite.

   14. An electroplating bath composition as claimed in claim 11 or claim 12 characterised in that it contains 7.5 to 15 g11 of trimethylolpropane.

   15. A method of making a zinc/manganese alloy electroplate on workpicces which comprises contacting workpieces with an electroplating bath and providing an electrode and passing an electroplating current between the electrode and the workpieces characterised in that the electroplating bath is a bath as claimed in any one of claims 1 to 14.

   16. A method as claimed in claim 15 characterised in that the bath is a bath as claimed in claim 2 or any of claims 3 to 10 when dependant on claim 2 or claim 11 or claim 121 or 13 or 14 and the electrode is an inert electrode or a zinc electrode or 10 a mixture thereof.

   1 -.

   1 A work-piece whenever provided with an electroplate of zlncilmanganese allov by a method as claimed in claim 15 or claim 16.

   118. An aqueous composition for forming a black passivate on the surface of a zinc..manganese electrodeposit characterised in that it comprises hexavalent chromium. one or more carboxylic acids and a copper sulphate and is free of silver ions.

   J0 19. A composition as claimed in claim 18 characterised in that the hexavalent and concentrated sulphuric acid.

   chromium is provided by a mixture of Cr03 1 1 111 43) -'YO. A composition as claimed in claim 19 characterised in that it contains 30 to 70 g/1, Cr-03 and 2 to 15 ml/l, of 96% HS04 -) 1.

   A composition as claimed in claim 18, 19 or 20 characterised in that it contains 40 to 100 mi/1 of acetic acid.

   212. A composition as claimed in any one of claims 18 to 21 characterised in thaE it contains 10 to 25 g/1 of copper sulphate, e.g. CuSO, 5HO.

   3. A method of providing a zinc/manganese alloy electrodeposit with a black passivate which comprises treating the electrodeposit with a composition as claimed in any one of claims 18 to 22.

   24. A method as claimed in claim 23 characterised in that the zinc/maneanese electrodeposit contains 14 to 20% by weight of manganese.

   25. A zinc/manganese electrodeposit whenever provided with a black passivate hv a method as claimed in claim 23 or claim 24.

   6. An aqueous composition for forming a black passivate on the surface of a zinc/lron electrodeposit or a zinc/cobalt clectrodeposit or a zlnc,,; nlckcel electrodeposit wherein the composition comprises hexavalent chromium. one or more carboxylic acids and a copper sulphate and is free of silver ions.

   76 which contains 70 s A composition as claimed in claim 30 to 3-11 CrOand - to 15 ml/l of 96 % MSO, 28. A composition as. claimed in claim 26 which contains 40 to 100 mJ/1 of acetic acid.

   -'9.

   w. A composition as claimed in claim 26 which contains 1 to -15 2.11 of copper sulphate.

   44 30. An aqueous composition for forming a black passi-,,,ate on the surface of a zinc,liron. zinc/cobalt or zinc/nickel electrodeposit which comprises 30 to 70 g/1 CrO, and 2 to 15 m111 of 96% MS0,: 40 to 100 ml/l of acetic acid: and 10 to 25 g/1 of copper sulphate.5H,0 and which is free of silver ions.

   31. A method of providing a zine.,iron. zinc/cobalt or zinc/nickel electrodeposir with a black passivate which comprises treating the electrodeposit with a composition as claimed in claim 26 or claim 30.

   3 )2. A method as claimed in claim 31 in which the electrodeposit is a zine. ,"iron allov containinz 0.4 to 0. 8 % by weight iron.

   A method as claimed in claim 31 in which the elecirodeposit is a zinc,, cobalt alloy containing 0.6 to 1.2 bv weiLht cobalt.

   1 - 'C 34. A zinc/iron. zinc/cobalt or zinc/nickel electrodeposit whenever provided ".lih a black passivate by a method as claimed in claim 3 1 -