GB2167449A

GB2167449A - Passivation

Info

Publication number: GB2167449A
Application number: GB08429646A
Authority: GB
Inventors: Adriana G M Eggels
Original assignee: OMI International Corp
Current assignee: OMI International Corp
Priority date: 1984-11-23
Filing date: 1984-11-23
Publication date: 1986-05-29
Also published as: GB2167449B; GB8429646D0

Abstract

Black-passivated zinc-containing metal deposits can be obtained by contacting a zinc-cobalt alloy surface with (a) containing phosphate ions and being substantially free of chromate ions; or (b) containing chromate ions, phosphate ions and sulphate ions and being substantially free of formate ions; or (c) containing chromate ions, formate ions and sulphate ions and being substantially free of phosphate ions. o

Description

SPFCIFICATION Passivation This invention relates to the passivation of metal surfaces.

Zinc has long been used to plate various base metals, including steel and copper, for the purpose of enhancing both the functional and decorative characteristics of the metal. Further to improve the characteristics of the surface coating, in particular the corrosion resistance of the zinc itself, the deposit has frequently been passivated, that is to say treated with a composition to induce the deposition of various protective metal salts on the surface of the zinc.

Known zinc passivating compositions are based on hexavalent chromium. For example, a particular known passivafing composition contains hexavalent chromium ions, sulphate ions, acetate ions and nitrate ions. The treatment of a zinc surface with such a composition leads to a yellow passivate.

But if formate ions are added to the composition, an olive drab passivate results.

A further known olive drab passivating composition contains hexavalent chromium ions, sulphate ions, phosphate ions, nitrate ions and chloride ions. This composition yields an olive drab passivate which is somewhat darker than that obtained from the formate-containing composition described above.

For certain applications such as plating automotive components there has been a need for compositions for producing a corrosion resistant black passivate.

Certain black passivating compositions for zinc are available but they have disadvantages. For example, a known zinc black passivating composition contains hexavalent chromium ions, sulphate ions, acetate ions and silver ions. But because of the necessary presence of a silver salt, such a composition is expensive. Afurther composition for imparting black passivates to zinc surfaces contains molybdenum, but again the presence of this relatively rare element makes the cost of passivation high.

There is therefore a need for a cheaper and improved method of obtaining a black passivate coating on metallic deposits such as zinc. Whereas prior proposals have concentrated on the characteristics of the passivating composition itself, the present invention approaches the problem in a different way. It has been discovered that altering the constitution of the deposited metal can make a significant difference to the way in which passivating compositions affect the deposit. In particular, it has been found that if zinc-cobalt deposits are substituted for pure zinc deposits then the use of a certain class of passivating compositions results in a black passivate finish.

According to a first aspect of the present Invention, there is provided a method of producing a black passivate on a zinc-cobalt alloy surface, the method comprising contacting the surface with a composition either (a) containing phosphate ions and being substantially free of chromate ions; or (b) containing chromate ions, phosphate ions and sulphate ions and being substantially free of formate ions; or (c) containing chromate ions, formate ions and sulphate ions and being substantially free of phosphate ions.

The alloy may contain from 0.3 to 1.5% (by weight) cobalt, the balance being zinc, but it is especially preferred that 0.5 to 1.0% or even 0.6 to 0.7% cobalt be present.

When the composition contains phosphate ions and is substantially free of formate ions, the phosphate ion concentration is preferably such as results from the addition of from 3 to 15 ml/l, typically 7 to 12 ml/l, for example 10 ml/l, of concentrated phosphoric acid. The pH of the composition will preferably range from 1 to 2, typically from 1.2 to 1.8, and is desirably 1.5.

When the composition contains chromate ions, phosphate ions and sulphate ions and is substantially free of formate ions it may also contain acetate ions. The phosphate ion concentration is preferably such as results from the addition of 3 to 15 ml/l, typically 7 to 12 ml/l, for example 10 ml/l of concentrated phosphoric acid. The phosphate ions may be supplied by disodium orthophosphate.The chromate ion concentration (which may be provided by chromic acid or sodium dichromate), is preferably such as to provide a chromium (VI) concentration of from 8 g/l (which is approximately equivalent to 20 g/l chromic acid) to 50 g/l, typically from 10 g/l to 35 g/l and optimally 15.6 g/l in the absence of acetate ions or 30.6 g/l in the presence of acetate ions. The sulphate ion concentration may range from 5 g/l to 30 g/l, typically from 7 to 20 g/l and optimally 10 g/l in the absence of acetate ions or 17.5 g/l in the presence of acetate ions.The pH of the composition can range from about 0.8, below which the passivate may become unacceptably powdery, to about 2, above which discolouration may be unacceptable in some circumstances. The optimum pH range is from 1.3 to 1.5, particularly 1.5 when acetate ions are present. When present the acetate ions may be provided by from 50 to 100 ml/l glacial acetic acid, typically 60 to 80 ml/l, for example 70 ml/l. The presence of concentrated nitric acid has not been found to be detrimental up to about 5 ml/l, and the presence of concentrated hydrochloric acid present in an amount up to 5 ml/l was also found to be acceptable.

When the composition contains chromate ions, formate ions and sulphate ions and is substantially free of phosphate ions, the chromate ion concentration is preferably such as to provide a chromium (VI) concentration of from 8 g/l to 50 g/l, typically from 10 g/l to 35 g/l and optimally about 19 g/l, and may be provided by chromic acid or sodium dichromate. The sulphate ion concentration is preferably from 0.5 to 5 g/l, typically from 1 to 3 g/l and optimally about 2 g/l and may be provided by magnesium sulphate.The formate ion concentration is preferably from 10 to 100 g/l, typically from 40 to 80 g/l and optimally about 60 g/l and may be provided by sodium formate. Acetate ions may be present, for example provided by sodium acetate or (glacial) acetic acid, at a concentration of 5 to 30 g/l, typically 10 to 20 g/l and optimally about 15 g/l. The presence of concentrated nitric acid up to about 15 mill has no adverse effect on the composition, but the pH of the composition is preferably within the range of from 2.0 to 3.5, optimally 2.5 to 3.2.

Preferably the contacting of the surface with the composition is achieved by submersing the surface in the composition. The submersion time may be from 30 to 120 seconds, preferably 50 to 90 seconds.

Mild mechanical or air agitation is desirable. The temperature of the composition may be 15O to 350C preferably 200-250C. Subsequently, the passivated surface may be rinsed in tap or demineralised water.

Many if not all of the benefits of zinc plating together with the additional benefits of improved corrosion resistance, can be obtained by plating a zinc-cobalt alloy instead of pure zinc. A process for plating such an alloy is disclosed in GB-A- 2116588. A method in accordance with the present invention is particuiarly effective when used on a deposit produced in accordance with the teaching of GB-A-21 16588.

As an indication of the radical departure of this invention from the state of the art, and to show that pure zinc and zinc-cobalt deposits are far from equivalent, it is noteworthy that the use of a conventional silver-containing black passivating composition for pure zinc or zinc-cobalt deposits does not give a satisfactorily black passivate when the cobalt content exceeds 0.3%. At cobalt concentrations of greater than 0.3% the appearance is poor and if there is more than about 0.6 to 0.7% cobalt in the alloy, the resulting passivate cannot be described as being black at all.

It has been found that the use of a method in accordance with the invention results in a highly satisfactory black passivate on zinc-cobalt alloys.

The corrosion resistance and the intensity of the black colour are not decreased with an increase in cobalt content in the alloy up to a maximum of about 1.5% cobalt. On the contrary, as the cobalt concentration in the alloy is increased from zero and especially from 0.3% up to an optimum of 0.6% to 0.8% cobalt, the betterthe black colour is.

It can therefore be seen that the present invention is the only way in the present state of technology to achieve a corrosion resistant black passivate finish on zinc-cobalt alloys, especially those containing more than 0.3% or 0.6% or 0.7% cobalt. According to a second aspect of the invention, therefore, there is provided a zinc-cobalt alloy preferably containing more than 0.3% or 0.6% cobalt (and preferably more than 0.7%, 0.8%, 0.9% or 1.0% but less than 1.5% cobalt) bearing a corrosion resistant black passivate.

Corrosion resistance can be measured by the standard salt spray test of DIN 50021 SS or ASTM B117, in which a 5% neutral salt spray is directed under controlled conditions at a test piece of material. The longer a test piece survives in the spray, the greater its resistance to corrosion. A test piece of passivated coated metal may be termed "corrosion resistant" if it survives more than 25 (or 35, 45, 55 or 65) hours in the spray test before the appearance of zinc corrosion products.

The following examples illustrate the invention and can be contrasted with the comparative examples.

EXAMPLE 1 Preparation of a Zinc-Cobalt Deposit.

A 30 litre bath was made up as follows: Zinc chloride 78 g/l Cobalt chloride 20 gil(l) Sodium chloride 200 gIl Boric acid 20 g/l Sodium benzoate 4g/l(2) Polyethylene glycol 5 g/l(2) (mw 1500) Benzyl acetone 25 mg/1 Sodium naphthalene 200 mug/1 sulphonate Trimethylol propane 15git(3) pH 5 5(4 Notes ') The concentration of cobalt salt can be varied to alter the proportion of cobalt in the deposit.

(2) Sodium benzoate, polyethylene glycol, benzyl acetone and sodium naphthalene sulphonate together form a brightening system for the deposit.

(3) Trimethylol propane functions as a stabiliserfor anode control.

4) The pH can be adjusted with hydrochloric acid or sodium hydroxide as appropriate.

A 10 dm2 steel test panel was immersed in the above bath and cathodically electrified. Over a period of 10 to 20 minutes a zinc-cobalt deposit was formed on the panel. The current density during this time was 2 ASD and the bath temperature was kept between 28 and 32"C.

EXAMPLE 2 A zinc-cobalt coated steel panel prepared as in Example 1 was immersed for 75 seconds in an aqueous composition containing 10 ml/l concentrated phosphoric acid. The pH of the composition was 1.5. A black passivate film resulted.

EXAMPLE 3 A 1 litre composition was made up as follows: Hexavalent chromium 19 gil Sulphate ions 2.1 g/l Acetate ions 14.6 g/l Formate ions (as HCOONa) 62 g/l Concentrated nitric acid 14.6 ml/l A zinc-cobalt coated steel test piece was immersed in the composition for 1 minute. The zinc-cobalt alloy contained 0.7% cobalt. A satisfactorily black passivate resulted whose corrosion resistance was good: it survived 72 hours in the salt spray test before the appearance of zinc corrosion products.

EXAMPLE 4 A 1 litre composition was made up as follows: Hexavalent chromium ions 15.6 g/l Sulphate ions 8.5 g/l Phosphate ions (as Na2HPO4) 14.1 gil Concentrated nitric acid 5 ml/l Concentrated hydrochloric 5 mIll acid A zinc-cobalt plated steel test piece was immersed in the composition for 1 minute. The zinc-cobalt alloy contained 0.7% cobalt. A satisfactorily black passivate resulted whose corrosion resistance was good: it survived 80 hours in the salt spray test before the appearance of zinc corrosion products.

Comparative Example 1 A conventional black passivating composition for pure zinc coatings was made up as follows: Hexavalent chromium ions 30 g/l Sulphate ions 17 g/l Glacial acetic acid 70 ml/l Silver ions (as Ag NO3) 0.4 gil A zinc-cobalt coated steel test piece was immersed in the composition for 1 minute. The zinc-cobalt alloy contained 0.6% cobalt. A golden passivate resulted. The corrosion resistance of the surface coating was poor when measured by the conventional salt spray test.

Comparative Example 2 The bath of Comparative Example 1 was made up and into it was immersed a steel test piece coated with a zinc-cobalt alloy having a cobalt concentration of 0.2%. A black deposit formed, but the surface had poor corrosion resistance: cracks appeared after 24 hours in the salt spray test and the deposit turned green.

Claims

1. A method of producing a black passivate on a zinc-cobalt alloy surface, the method comprising contacting the surface with a composition either (a) containing phosphate ions and being substantially free of chromate ions; or (b) containing chromate ions, phosphate ions and sulphate ions and being substantially free of formate ions; or (c) containing chromate ions, formate ions and sulphate ions and being substantially free of phosphate ions.

2. A method as claimed in Claim 1, wherein the alloy contains from 0.3 to 1.5% (by weight) cobalt, the balance being zinc.

3. A method as claimed in Claim 1, wherein the alloy contains from 0.5 to 1.0 percent (by weight) cobalt, the balance being zinc.

4. A method as claimed in Claim 1, wherein the alloy contains from 0.6 to 0.7% (by weight) cobalt, the balance being zinc.

5. A method as claimed in any one of Claims 1 to 4, wherein when the composition contains phosphate ions and is substantially free of formate ions, the phosphate ion concentration is such as results from the addition of from 3 to 15 ml/l.

6. A method as claimed in any one of Claims 1 to 4, wherein, when the composition contains phosphate ions and is substantially free of formate ions, the phosphate ion concentration is such as results from the addition of from 7 to 12 ml/l.

7. A method as claimed in any one of Claims 1 to 4, wherein, when the composition contains phosphate ions and is substantially free of formate ions, the phosphate ion concentration is such as results from the addition of 10 ml/l.

8. A method as claimed in any one of Claims 1 to 7, wherein, when the composition contains phosphate ions and is substantially free of formate ions, the pH of the composition is from 1 to 2.

9. A method as claimed in any one of Claims 1 to 7, wherein, when the composition contains phosphate ions and is substantially free of formate ions, the pH of the composition is from 1.2 to 1.8.

10. A method as claimed in any one of Claims 1 to 7, wherein, when the composition contains phosphate ions and is substantially free of formate ions, the pH of the composition is 1.5.

11. A method as claimed in any one of Claims 1 to 4, herein, when the composition contains chromate ions, phosphate ions and sulphate ions and is substantially free of formate ions, it also contains acetate ions.

12. A method as claimed in any one of Claims 1 to 4 or 11, wherein, when the composition contains chromate ions, phosphate ions and sulphate ions and is substantially free of formate ions, the phosphate ion concentration is such as results from the addition of from 3 to 15 ml/l.

13. A method as claimed in any one of Claims 1 to 4 or 11, wherein, when the composition contains chromate ions, phosphate ions and sulphate ions and is substantially free of formate ions, the phosphate ion concentration is such as results from the addition of from 7 to 12 ml/l.

14. A method as claimed in any one of Claims 1 to 4 or 11, wherein, when the composition contains chromate ions, phosphate ions and sulphate ions and is substantially free of formate ions the phosphate ion concentration is such as results from the addition of 10 ml/l.

15. A method as claimed in any one of Claims 1 to 4 or 11, wherein, when the composition contains chromate ions, phosphate ions and sulphate ions and is substantially free of formate ions the phosphate ions are supplied by disodium orthophosphate.

16. A method as claimed in any one of Claims 1 to 4 or 11 to 15, wherein, when the composition contains chromate ions, phosphate ions and sulphate ions and is substantially free of formate ions, the chromate ion concentration is preferably such as to provide a chromium (vi) concentration of from 8 to 50 gil.

17. A method as claimed in any one of Claims 1 to 4 or 11 to 16, wherein, when the composition contains chromate ions, phosphate ions and sulphate ions and is substantially free of formate ions, the sulphate ion concentration ranges from 5 g/l to 30 g/l.

18. A method as claimed in any one of Claims 1 to 4 or 11 to 17, wherein, when the composition contains chromate ions, phosphate ions and sulphate ions and is substantially free of formate ions, the pH of the composition ranges from 0.8 to 2.

19. A method as claimed in Claim 11, wherein acetate ions are provided by from 50 to 100 ml/l of glacial acetic acid.

20. A method as claimed in any one of Claims 1 to 4, wherein, when the composition contains chromate ions, formate ions and sulphate ions and is substantially free of phosphate ions, the chromate ion concentration is preferably such as to provide a chromium (vi) concentration of from 8 to 50 g/l.

21. A method as claimed In any one of Claims 1 to 4 and 20, wherein, when the composition contains chromate ions, formate ions and sulphate ions and is substantially free of formate ions, the sulphate ion concentration is from 0.5 to 5 g/l.

22. A method as claimed in any one of Claims 1 to 4, 20 and 21, wherein, when the composition contains chromate ions, formate ions and sulphate ions and is substantially free of phosphate ions, the formate ion concentration is from 10 to 100 g/l.

23. A method as claimed in any one of Claims 1 to 4 and 20 to 22, wherein, when the composition contains chromate ions, formate ions and sulphate ions and is substantially free of phosphate ions, acetic acid is present.

24. A method as claimed in Claim 23, wherein the acetate ions are provided by glacial acetic acid at a concentration of from 5 to 30 g/l.

25. A method as claimed in any one of Claims 1 to 4 and 20 to 24, wherein, when the composition contains chromate ions, formate ions and sulphate ions and is substantially free of phosphate ions, the pH of the composition is within the range of from 2.0 to3.5.

26. A method as claimed in any one of Claims 1 to 25, wherein the contacting of the surface with the composition is achieved by submersing the surface in the composition.

27. Azinc-cobalt alloy bearing a corrosionresistant black passivate.

28. A passivated alloy as claimed in Claim 27, containing from 0.3% to 1.5% cobalt.

29. A method of producing a black passivate on a zinc-cobalt alloy surface substantially as herein described with reference to any one of the examples.

30. A passivated zinc-cobalt alloy substantially as herein described.