GB2374088A

GB2374088A - Conversion treatment of zinc and zinc alloy surfaces

Info

Publication number: GB2374088A
Application number: GB0107881A
Authority: GB
Inventors: Anthony Rowan; Ernest Long; Trevor Pearson
Original assignee: MacDermid Ltd
Current assignee: MacDermid Performance Solutions UK Ltd
Priority date: 2001-03-29
Filing date: 2001-03-29
Publication date: 2002-10-09
Also published as: WO2002079539A2; WO2002079539A3; GB0107881D0

Abstract

A method of applying a phosphate conversion coating to a zinc or zinc alloy surface comprises contacting the surface with an acidic solution comprising phosphate ions, nitrate ions or nitrite ions and one or both of a molybdenum or vanadium compound. The resulting conversion coating is substantially black, coherent and adhesive to the substrate and contains zinc, phosphorus, one or both of molybdenum, and vanadium, and, when applied to a zinc alloy surface, the alloying element(s). The molybdenum or vanadium compound may be molybdenum oxide or vanadium oxide. An oxidising agent such as hydrogen peroxide.

Description

Treatment of Zinc and Zinc Alloy Surfaces Zinc or zinc alloy plating of metal substrates is well known as a means of reducing corrosion of the metal substrate. A particularly well known use is in the treatment of steel plate and other steel components in the automotive industry. In order to improve the corrosion protection provided by the zinc or zinc alloy plating, the zinc or zinc alloy surface is commonly subjected to a chromating step which can be successful in delaying the appearance of white corrosion products on the substrate.

The chromating process is advantageous in that it is relatively cost effective, reliable and efficient. However, conventional chromating treatments tend to use hexavalent chromium which is known to be both toxic and a carcinogen. The hexavalent chromium can leach over time from the chromated substrate, causing health problems to those who regularly handle chromated components (and providing an obstacle to the use of chromated components in the food and beverage industries) and further causing environmental problems. The treatment of effluent from the chromating process is also made more difficult.

In view of the above there is a need to provide an alternative to the chromating process. In order to be commercially successful, any such alternative process must be at least as cost effective and reliable as the chromating process and should avoid the health, handling and effluent treatment problems of the chromating process. A further requirement arises from the automotive industry where black deposits on the substrate are required, primarily for cosmetic reasons. Conventionally, such black deposits have been obtained by plating with a zinc/iron alloy followed by a black chromate coating (which is based on hexavalent chromium). After conventional chromating steps, an organic topcoat is often applied to provide further corrosion protection and thus alternatives to a chromating step must provide a coating to which an organic topcoat can be applied.

Accordingly, the present invention seeks to provide a process for coating zinc or zinc alloy plated articles which substantially overcomes the disadvantages of the

chromating process whilst also providing corrosion protection which is comparable to or better than the corrosion protection of a plated article with a chromate coating.

US 4,233, 088 describes a process for inhibiting corrosion on surfaces including steel, zinc, lead, copper and tin. The surface is treated with an acidic solution including phosphate ions and a dissolved metal oxide selected from the oxides of molybdenum, vanadium, tungsten, lead, titanium, manganese and copper. The resulting phosphate coating is further improved by the inclusion in the solution of a ligand forming organic polymer.

US 4,264, 378 discloses a phosphatizing process for metal surfaces, in particular for aluminium, in which the metal surface is treated with a solution at pH 1.5 to 3.0 containing at least one metal of valence 2 or higher and soluble molybdate, tungstate, vanadate, niobate and/or tantalate ions.

US 5,550, 006 describes compositions and methods for providing a phosphate coating on a copper surface for the preparation of printed circuit boards. The compositions contain at least one soluble compound containing vanadium, niobium, tungsten or tantalum, in addition to a source of phosphate ions.

In accordance with a first aspect of the present invention there is provided a method of coating an article having a zinc or zinc alloy surface, the method comprising contacting the zinc or zinc alloy surface with an acidic aqueous treatment solution comprising : (i) a source of phosphate ions (ii) a source of nitrate or nitrite ions, and (iii) a molybdenum and/or vanadium compound soluble in the treatment solution.

Preferably, the soluble molybdenum or vanadium compound is present in an amount of from 0.05 to 5 g/l, more especially 0.5 to I g/l. It is, of course, particularly

preferred that hexavalent chromium compounds are excluded from the treatment solution of the invention. A particularly preferred source of phosphate ions is phosphoric acid. Other suitable sources of phosphate ions include trisodium phosphate, tripotassium phosphate, ammonium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, dipotassium phosphate and potassium dihydrogen phosphate. Preferably the phosphate concentration in the treatment solution according to the invention is from 2 to 20 g/l, more especially not less than 5 g/l. Preferably the pH of the treatment solution is from pH I to pH 3.5.

When a soluble vanadium compound is present in the treatment solution, it is preferred that the treatment solution further comprises an oxidising agent of sufficient oxidising strength substantially to maintain the vanadium present in its +5 oxidation state but of oxidising strength insufficient to significantly oxidise nitrate ions in the solution. Preferably the oxidising agent is hydrogen peroxide which is preferably present in an amount of from 0. 1 g/l to lOg/1 in the treatment solution.

In one preferred embodiment, the method of the invention preferably includes the step of maintaining the hydrogen peroxide concentration in the treatment solution by monitoring the intensity of colour of an orange coloured vanadium-peroxo complex formed in the treatment solution and, when the colour has faded to a predetermined extent, adding further hydrogen peroxide.

A preferred source of nitrate ions is sodium nitrate. Alternative sources of ions include nitric acid, potassium nitrate, nickel nitrate, cobalt nitrate, ferric nitrate and chromium nitrate. Suitable sources of nitrite ions include sodium nitrite and potassium nitrite. Preferably the nitrate or nitrite is present in the treatment solution in an amount of from 0.2 to 100 g/l. In many applications, the amount of nitrate or nitrite need not exceed lOg/1 and is preferably about 0.5 to 5g/l. However, for barrel plating processes a higher nitrate or nitrite content can be required in order to prevent powdery deposition. This may be due to the increased surface area/volume ratio. In

this case, the amount of nitrate or nitrite is preferably about 30g/l to about 70g/1, especially about 50g/l. The method of the invention is, in principle, applicable to any zinc or zinc alloy surface. However, particularly suitable zinc alloys include zinc/iron, zinc/cobalt, zinc/nickel, zinc/cobalt/iron, zinc/nickel/iron and zinc/nickel/cobalt/iron. Especially preferred alloys include zinc/iron alloys comprising from 0.5 to 5 wt % iron, zinc/nickel alloys comprising from 5 to 15 wt % nickel and zinc/cobalt alloys comprising from 0.2 to 2 wt % cobalt. Another preferred alloy is a zinc/cobalt/iron alloy comprising from 0.2 to 2 wt % iron and from 0.2 to 2 wt % cobalt.

In preferred forms of the method of the invention, the article to be coated is immersed in a bath of the treatment solution. It is particularly preferred that the article is immersed for a period of from 20 seconds to 5 minutes. Preferably the treatment composition is maintained at a temperature in the range of from about 15OC to about 70OC, more especially about 20 C to 30OC. Other means of contacting the article with the treatment solution include applying the treatment solution by means of a brush or roller or spraying the treatment solution onto the article.

The treatment solutions when containing vanadium according to the invention will usually have an initial yellow colouration. The inventors believe that this colour is attributable to the presence of the VO ion. However, the most stable oxidation state for vanadium is represented by the VO ion. As the VO2+ ion is thus an oxidising species, the treatment solution on standing may become green and then blue as the amount ofVO+ increases. Furthermore, in conditions of heavy use of the treatment solution, such as in the production of barrel plated components, a dark green precipitate may form in the process tank. Analysis of the precipitate by EDXA suggested that the precipitate has a composition of 61% vanadium, 38% phosphorus and 1% iron (oxygen content was not determined). This corresponds to a 1 : 1 mole ratio of V: P. Without wishing to be bound by theory, the inventors postulate that precipitate is vanadyl hydrogen phosphate, (VO) Hop04. Ultimately, a precipitate of

(VO) Hop04 will also form following the change of colour of the treatment solution from yellow to green to blue on standing. Precipitation of (VO) Hop04 can, in principle, be prevented by the addition of an oxidising agent to the treatment solution so that the vanadium is maintained in its +5 oxidation state, i. e. V02+. However, the choice of oxidising agent is restricted since oxidising agents which are too strong (such as"caroat"-i. e. potassium peroxo-mono sulphate) will oxidise nitrate ions present in the treatment solution thereby producing highly toxic NOx compounds.

Therefore, any suitable oxidising agent will not be sufficiently oxidising to cause oxidation of nitrate ions to NOx compounds. A particularly suitable oxidising agent is hydrogen peroxide, which in addition to not being too strongly oxidising, is inexpensive and readily available. The use of hydrogen peroxide is further advantageous in that it forms a strongly coloured peroxo complex with vanadium ions. Thus, the maintenance of the desired concentration of hydrogen peroxide is simple in that as the orange colour begins to fade (as the hydrogen peroxide is consumed, so that there is less orange peroxo complex), the need for a further addition of hydrogen peroxide to the treatment solution is clearly indicated. The hydrogen peroxide (when used at 130 vol i. e. 35%) is preferably present in an amount of from 0. 3ml/l to about 30ml/l of treatment solution, and most preferably in an amount of about 5mol/1 of treatment solution.

In a particularly preferred embodiment of this aspect of the invention the coated article is subsequently treated with an organic topcoat or lacquer. Suitable organic topcoats include water-based acrylic lacquers and especially cathodic electrophoretic lacquers. Solvent based lacquers may also be used, but water based lacquers are preferred.

In a second aspect of the invention there is provided an article having a zinc or zinc alloy surface coated with a phosphate conversion coating according to the method of the first aspect of the invention.

A third aspect of the present invention provides an article having a zinc or zinc alloy plated layer and a phosphate conversion coating, said corr. ersion coating being dark grey or substantially black, coherent, adherent to the zinc or zinc alloy layer, and comprising the elements zinc, phosphorus, one or both of molybdenum and vanadium and (where the plated layer is a zinc alloy) the alloying metal or metals of the zinc alloy layer. Particularly preferably, hexavalent chromium is absent from the conversion coating.

A fourth aspect of the present invention provides an acidic aqueous treatment solution for contacting with a zinc or zinc alloy surface to form a phosphate conversion coating on said surface, said treatment solution comprising (i) a source of phosphate ions (ii) a source of nitrate or nitrite ions, and (iii) a molybdenum or vanadium compound which is soluble in the treatment solution.

Preferably the molybdenum or vanadium compound is a soluble molybdenum oxide or vanadium oxide. Preferably also the source of phosphate ions is phosphoric acid.

The pH of the treatment solution is preferably in the range of from pH I to pH 3.5. It is particularly preferred that hexavalent chromium ions are absent from the treatment solution of this aspect of the invention.

It is further preferred that when a soluble vanadium compound is present, the treatment solution further comprises an oxidising agent of sufficient oxidising strength substantially to maintain to vanadium present in the +5 oxidation state but of oxidising strength insufficient to significantly oxidise nitrate ions in the solution.

Thus in accordance with the present invention phosphate conversion coatings can be provided on zinc and zinc alloy substrates which meet industry requirements while avoiding the problems associated with hexavalent chromium based coatings. On zinc

substrates, the phosphate conversion coatings according to the invention tend to produce dark grey coatings. This colour can be modified by application of an organic topcoat which is not clear/colourless or the colour can be maintained by application of a substantially colourless clear organic topcoat. On zinc alloy substrates, in addition to other dark colours, black or substantially black coatings can be achieved.

The production of black coatings is especially advantageous in that, in order to obtain a black final finish, a substantially colourless clear organic topcoat can be used. In contrast, for other colours of the phosphate conversion coating if a black finish is required, a black lacquer (topcoat) must be used. As with zinc substrates, other coloured topcoats may also be used. The colour of the phosphate conversion coating is usually assessed visually and some variation in the absolute colour is to be expected. Thus, in this context and in this specification, although absolutely black coatings are preferred, the term"black"can include very dark coatings of other colours, such as very dark blue, grey or green which may also be iridescent in appearance.

The colours of the coatings of the invention on zinc or zinc alloys may be exemplified using the well known system of RAL numbers. Typically the colours achieved on zinc alloy substrates will include: RAL 5004 Blue/Black RAL 5008 Blue/Grey RAL 5011 Steel Blue RAL 6007 Bottle Green RAL 6008 Brown/Green RAL 6009 Fir Green RAL 6012 Green/Black RAL 6015 Olive/Black RAL 6022 Olive Drab RAL 7021 Grey/Black

Examples of the most preferred colours on zinc alloys include : RAL 8022 Black/Brown RAL 9005 Jet Black RAL 9011 Graphite Black RAL 9017 Traffic Black RAL 9004 Signal Black Typically, the colours achieved on zinc substrates will include : RAL 7011 Iron Grey RAL 7012 Basalt Grey RAL 7015 Slate Grey RAL 7016 Anthracite Grey RAL 7031 Blue/Grey It will be understood that these colours from the RAL system are quoted by way of example only and although some of these colours represent particularly preferred colours according to the invention, the invention is not limited to any one or any group of the above colours.

The following examples are illustrative of the invention: Example 1 A steel panel coated with 8 microns of zinc/iron alloy containing approximately 0.8% iron was immersed in a solution containing 20 g/l of phosphoric acid (S. G. 1.65), 1 g/l sodium nitrate and 1 g/l sodium vanadate at a temperature of 25 C for I minute.

An excellent black coating resulted. Examination of the deposit by scanning electron

microscope revealed the structure illustrated in Figure 1 (magnification approximately 2500x). The slightly porous structure seen in Figure I was considered to be ideal for subsequent application of organic lacquers or other topcoats.

Analysis of the surface by Energy Dispersive X-ray Analysis at 15kV gave the following composition: Zinc 87 % Iron 5 % Phosphorus 6 % Vanadium 2 % This clearly illustrates that both vanadium and phosphorus are deposited on the surface of the metal. The iron content at the surface was much higher than the bulk iron concentration (0.8%) indicating that the formation of the phosphate coating causes the alloying elements in the zinc alloys to become enriched at the surface.

The coating was tested to ensure that it was coherent and adherent to the underlying zinc/iron alloy layer by applying the adhesive side of a piece of adhesive tape to the surface of the coating, and then removing the tape. The adhesive side of the tape was then examined visually and it was found that substantially none of the coating had adhered to the tape. The coating was thus clearly both coherent and adherent to the underlying alloy.

Example 2 A steel panel coated with 8 microns of zinc/iron alloy containing approximately 0.8% iron was immersed in a solution containing 20 g/l of phosphoric acid (S. G. 1.65), 1 g/l sodium nitrate and 4 g/l sodium vanadate at a temperature of 25OC for 1 minute. An excellent black coating resulted. Examination of the deposit by scanning electron microscope revealed the structure illustrated in Figure 2 (magnification

approximately 2500x). The coating was also tested using the adhesive tape test described in Example I and was found to be coherent and adherent to the underlying alloy.

The structure shown in Figure 2 has a similar structure to that of Example I and Figure 1, but slightly coarser in nature.

Analysis of the surface by Energy Dispersive X-ray Analysis at 15kV gave the following composition: Zinc 82 % Iron 7. 5 % Phosphorus 6. 5 % Vanadium 4 % Again, this shows codeposition of vanadium but in greater quantities than Example 1.

Example 3 The bath of Example 1 was used to process a steel panel with 8 microns of a zinc/cobalt/iron alloy deposit having the approximate composition of 0.3% cobalt and 0.5% iron. The results were the same as in Example 1.

Example 4 The bath of Example I was used to process a steel panel coated with 8 microns of a zinc/nickel alloy deposit having the approximate composition of 11% nickel. A black deposit was obtained when the sample was immersed in the bath at a

temperature of 65 C for 2 minutes. In the adhesive tape test, the deposit (coating) was found to be coherent and adherent to the underlying alloy.

Example 5 A steel panel coated with 8 microns of zinc/iron alloy containing approximately 0.8% iron was immersed in a solution containing 20 g/l of phosphoric acid (S. G. 1. 65), 1 9/1 sodium nitrate and 4 g/l sodium molybdate at a temperature of 25 C for I minute. An excellent black coating resulted, which was both coherent and adherent to the underlying alloy.

Example 6 Steel panels coated with 8 microns of zinc/iron alloy deposit (0.8% iron) were processed with various baths in order to illustrate the scope of the invention. The results are shown in the table below:

H3PO4 (gel) NaN03 (g/l) NaVO3 (g/I) Results 20 0 1 Black. Powdery and non adherent 20 1 1 Black. Compact coherent deposit 20 5 1 As above 20 1 0 Black. Slightly powdery 20 1 4 Slightly iridescent black deposit 1 1 1 No coating after 1 minute 2 1 1 Grey/green coating 5 1 1 Slightly iridescent purple/black 10 1 1 Slightly iridescent black deposit Example 7

Steel panels coated with 8 microns of zinc/iron alloy deposit with an iron content of approximately 1. 2% were treated with the bath of Example 3 in order to produce black deposits. The panels were then rinsed thoroughly and dipped in a water-based lacquer consisting of a self-emulsifying ethylene/acrylic acid copolymer dispersed in water (Lugalvan DC from BASF). The panels were allowed to drain and oven dried

for 15 minutes at 100Oc. Half of the panels were scribed with an X that penetrated the lacquer in order to simulate damage. The panels were then subjected to neutral salt spray testing in accordance with ASTM B-117 or equivalent protocols. The results were surprisingly good and the panels had less than 5% white corrosion after 460 hours salt spray testing as shown by Figure 3. The phosphate coating performed particularly well on the scribed panels and resisted corrosive "undercutting"of the lacquer.

Example 8 The procedure carried out in Example 7 was repeated but using a cathodic electrophoretic laquer instead of the water based acrylate lacquer. The lacquer used

in this example was a commercially available product (Electrolac-supplied by MacDermid PLC). This consists of an emulsion of an acrylic co-polymer neutralised with lactic acid in water together with a blocked polyisocyanate. This lacquer was applied in accordance with the manufacturers directions and stoved at 150OC to cross link it. The cosmetic appearance of the panels was excellent and salt spray resistance was in excess of 300 hours to 5% white"rust".

Example 9 2kg of M8 bolts were barrel plated in a small plating barrel with 8 microns of a zinciron alloy deposit (0. 8-1% Fe). The bolts were rinsed and then immersed in a solution containing 20g/1 of phosphoric acid (S. G. 1.65), 50g/1 sodium nitrate, 0. 5g/1 sodium metavanadate and 5mol/1 of 35% hydrogen peroxide solution at a temperature

of 25 C for I minute. The bolts were then rinsed and dried. An excellent uniform black coating was formed on the bolts which could not be removed by the adhesive tape test described above.

Comparative Example 1 A steel panel coated with 8 microns of zinc/iron alloy containing approximately 0.8% iron was immersed in a solution containing 20 g/l of phosphoric acid (S. G. 1.65) at a temperature of 25 C for 1 minute. A vigorous reaction took place and a black deposit was formed on the surface of the panel. After drying the panel, the deposit was found to be powdery and unsuitable for commercial use.

Comparative Example 2 A steel panel coated with 8 microns of zinc/iron alloy containing approximately 0.8% iron was immersed in a solution containing 20 g/l of phosphoric acid (S. G. 1.65) and 1 g/l sodium nitrate at a temperature of 25 C for 1 minute. A compact black coating was formed on the panel. However, this coating could be removed by rubbing.

P71624GB 5

Claims

Claims I. A method of coating an article having a zinc or zinc alloy surface with a phosphate conversion coating, the method comprising contacting the surface with an acidic aqueous treatment solution comprising: (i) a source of phosphate ions (ii) a source of nitrate or nitrite ions, and (iii) a molybdenum and/or vanadium compound soluble in the treatment solution.
2. A method as claimed in claim 1 wherein the molybdenum or vanadium compound is a soluble molybdenum oxide or a soluble vanadium oxide.
3. A method as claimed in claim 1 or claim 2 wherein, when a soluble vanadium compound is present in the treatment solution, the treatment solution further comprises an oxidising agent of sufficient oxidising strength substantially to maintain the vanadium present in its +5 oxidation state but of oxidising strength insufficient to significantly oxidise nitrate ions in the solution.
4. A method as claimed in claim 3 wherein the oxidising agent is hydrogen peroxide.
5. A method as claimed in claim 4 wherein the hydrogen peroxide is present in an amount of from 0.1 g/l to log/1 in the treatment solution.
6. A method as claimed in claim 4 or claim 5 including the step of monitoring the hydrogen peroxide concentration by monitoring the intensity of colour of an orange coloured vanadium-peroxo complex and, when the colour has faded to a predetermined extent, adding further hydrogen peroxide.
7. A method as claimed in any of claims 1 to 6 wherein the molybdenum or vanadium compound is present in an amount of from 0. 05 g/l to 5 g/l.
8. A method as claimed in any of claims 1 to 7 wherein the source of phosphate ions is phosphoric acid.
9. A method as claimed in any of claims 1 to 8 wherein the phosphate concentration in the treatment solution is from 2 g/l to 20 g/l.
10. A method as claimed in any of claims I to 9 wherein the pH of the treatment solution is from about pH 1.5 to about pH 3.
11. A method as claimed in any preceding claim wherein the source of nitrate ions is sodium nitrate.
12. A method as claimed in any preceding claim wherein the nitrate or nitrite ions are present in an amount of from 0.2 g/l to 100 g/l.
13. A method as claimed in any preceding claim wherein the zinc alloy is selected from zinc/iron, zinc/cobalt, zinc/nickel, zinc/cobalt/iron, zinc/nickel/iron and zinc/nickel/cobalt/iron.
14. A method as claimed in any preceding claim further comprising the step of applying an organic top coat to the conversion layer.
15. An article having a zinc or zinc alloy surface coated with a phosphate conversion coating produced by the method of any preceding claim.
16. An article having a zinc or zinc alloy plated layer and a phosphate conversion coating, said conversion coating being dark grey, dark brown, dark blue, dark green or substantially black, coherent, adherent to the zinc or zinc alloy layer and

comprising the elements zinc, phosphorus, one or both of molybdenum and vanadium and, where the plated layer is a zinc alloy, the alloying metal or metals.
17. An article as claimed in claim 16 having a zinc alloy plated layer.
18. An articles as claimed in claim 17 wherein the phosphate conversion coating is substantially black.
19. An acidic aqueous treatment solution for contacting with a zinc or zinc alloy surface to form a conversion coating on said surface, said treatment solution comprising : (i) a source of phosphate ions (ii) a source of nitrate or nitrite ions, and (iii) a molybdenum or vanadium compound which is soluble in the treatment solution.
20. A treatment solution as claimed in claim 19 wherein the molybdenum or vanadium compound is a soluble molybdenum oxide or a soluble vanadium oxide.
21. A treatment solution as claimed in claim 19 or 20 wherein, when a soluble vanadium compound is present, the treatment solution further comprises an oxidising agent of sufficient oxidising strength substantially to maintain the vanadium present in the +5 oxidation state but of oxidising strength insufficient to oxidise nitrate ions in the solution.
22. A treatment solution as claimed in claim 21 wherein the oxidising agent is hydrogen peroxide.
23. A treatment solution as claimed in claim 22 wherein the hydrogen peroxide is present in an amount of from 0.1 g/l to lOg/1.
24. A treatment solution as claimed in any of claims 19 to 23 wherein the source of phosphate ions is phosphoric acid.
25. A treatment solution as claimed in any of claims 19 to 24 wherein the pH of the solution is from about pHI to about pH3.5.
26. A treatment solution substantially as described with reference to any of Examples 1 to 5.
27. A coated article prepared in accordance with the method of any of Examples 1 to 5,7 and 8.