GB2148951A

GB2148951A - Phosphating processes and compositions

Info

Publication number: GB2148951A
Application number: GB08426012A
Authority: GB
Inventors: Kevin Brown
Original assignee: Pyrene Chemical Services Ltd
Current assignee: Pyrene Chemical Services Ltd
Priority date: 1983-11-02
Filing date: 1984-10-15
Publication date: 1985-06-05
Also published as: ZA848506B; CN85101750A; KR850004132A; GB8329250D0; KR890003586B1; IN159935B; GB2148951B; AU3489284A; GB8426012D0

Abstract

A protective coating can be formed on steel or other metal surfaces at low temperatures, for instance below 40 DEG C, by contact with the solution that is generally free of chlorate and chloride and that contains 0.5 to 1.5 parts silicofluoride or, less preferably, borofluoride, 0.8 to 2.5 parts zinc, 10 to 25 parts phosphate, 1.5 to 10 parts nitrate, 0.1 to 1.2 parts nickel and 0.25 to 2 parts sodium nitrobenzene sulphonate. The composition may also contain 0 to 0.7 parts manganese.

Description

SPECIFICATION Phosphating processes and compositions It is customary to provide a phosphate coating on a metal surface prior to application of a paint or other lacquer coating so as to provide corrosion resistance and increased adhesion of the lacquer to the surface. Zinc phosphate coatings are generally used, especially prior to electro-immersion lacquering. They are generally provided using aqueous zinc phosphate solutions containing oxidising agents.

Traditional zinc phosphate solutions are operated with zinc:phosphate ratios of the order of 1:1 to 4 in the presence of oxidising agents such as nitrate and chlorate and at elevated temperatures, for example 70 to 90"C. These processes therefore require a large amount of thermal energy and the properties of the coating may not be entirely satisfactory, especially when the surfaces are part of car or other auto bodies. Accordingly, the auto industry is now using specially developed zinc phosphate processes which mainly fall into one of two main classes.

One class is a process that operates with conventional zinc: phosphate ratios but with the addition of a substantial amount of manganese and at very low temperatures. These processes have the advantage of very low energy consumption and give results that, by most standards, are very satisfactory. Such a process was first described in British Patent Specification 983,924. In British Patent specification 2,072,225 it is described how the process is improved by using, as oxidising agent, a combination of chlorate and nitro benzene sulphonate. For instance the process may use a solution containing 3.5 g/l zinc, 0.5 g/l manganese, 0.2 g/l nickel, 11.6 g/l phosphate, 4 g/l nitrate, 1 g/l sodium nitro benzene sulphonate, 3.5 g/l sodium chlorate, 1 g/l tartrate and 0.7 g/l borofluoride.

The other main class of processes used for phosphating auto bodies are the low zinc processes, that is to say, processes in which the amount of zinc is much lower than conventional with the amount always being below 1.6 and often below 1.2 g/l. Such processes are described in for instance, German patent application 2232067 and British Patent specifications 2,046,312, 2,074,611 and 2,093,075. In these processes the ratio zinc: phosphate is generally in the range 1: 8 to 100. It is generally accepted that low zinc processes require powerful oxidising accelerators and so normally chlorate, generally also with nitrite and/or nitrate, are used in commercial practice. Manganese is an optional ingredient in some of the processes but is generally absent. Nickel is generally present, usually in amounts greater than in conventional process, for instance 1 g/l.

A low zinc process having particularly high amounts of oxidising accelerators is described in British Patent Specification 2,074,611 and in this the maximum amount of zinc is specified to be lower than the maximum normally specified in low zinc processes. Thus, it must not be above 1.3 g/l.

These low zinc processes normally operate at temperatures of 40 to 70"C and so consume more thermal energy than the low temperature processes described above. However, the processes are sometimes considered advantageous since, under some circumstances, the properties of the coatings may be considered superior. For instance, they may give slightiy better corrosion protection under prolonged corrosion conditions than is obtained with the described low temperature processes.

There have been numerous descriptions in Patent Specifications of accelerating systems for low zinc processes and of accelerating systems for conventional zinc phosphate processes. See, for instance, European Patent Specification No. 6071 6. In these various patent specifications there are occasionally general references to the use of one or more oxidising agents selected from a list of several, and this list often includes nitrobenzene sulphonate, chlorate and nitrate. In practice it is accepted that whenever nitrobenzene sulphonate is used it should be used in combination with chlorate (which is known to be a very powerful accelerator). See for instance British Patent Specifications Nos. 1 542222 and 2072225.Although this accelerator system is extremely satisfactory for many purposes, in some instances it is considered undesirable to include chlorate, because of the risk of contamination by chloride ion.

It has been our object to devise a composition that can be used at lower temperatures than those normally suitable for low zinc processes whilst retaining the very desirable corrosion resistance and other properties of the coatings obtained by low zinc processes.

It might be thought that the obvious way of achieving this would be merely to increase the amount or power of the accelerator system, for instance by increasing the amount of chlorate, and/or by relying upon the presence of substantial amounts of manganese.

We have now surprisingly found that if we either omit the manganese or keep its amount at very low levels it is possible to formulate a composition that will give coating properties comparable to those obtained with conventional low zinc processes but at a bath temperature lower than has been permissible with low zinc processes even though the composition can be, and preferably is, free of chlorate. In particular it is necessary to have zinc in an amount at or slightly above the top end of the ranges normally specified for low zinc processes and it is necessary to use nitrobenzene sulphonate and a complex fluoride and nickel, all in amounts generally as specified below.

We have also surprisingly found that whereas low zinc processes are normally conducted by spray, and the solution has to be specially formulated if it is to be applied by dip, the compositions used in the invention can easily be formulated so as to be equally effective for spray or dip applications.

A composition according to the invention is preferably substantially free of chlorate and chloride, contains 0.8 to 2.5 parts zinc, 10 to 25 parts phosphate, 1.5 to 10 parts nitrate, 0.1 to 1.2 parts nickel, 0.5 to 1.5 parts silicofluoride and/or borofluoride and 0.25 to 2 parts nitrobenzene sulphonate (measured as sodum salt). Generally this is introduced as sodium nitrobenzene sulphonate. The composition may be formulated initially as a concentrate but in use is formulated as a working solution by dilution with sufficient water to provide 1000 parts. All parts are by weight and so typical working solutions contain, for instance, 0.8 to 2.5 g/l zinc.

The amount of zinc is preferably above 1.4 parts and is generally below 2 parts. The amount of phosphate is preferably 1 2 to 1 8 parts. The amount of nitrate is preferably 3 to 6 parts. The amount of nickel is preferably 0.15 to 0.35 parts. The amount of nitrobenzene sulphonate is usually 0.5 to 1.5 parts.

The composition may be free of manganese but preferably contains manganese in an amount of at least 0.05 parts but generally not more than 0.7 parts (i.e. 0.7 g/l in the working solution). If manganese is present it is highly preferred that the amount should not exceed 0.25 parts as higher amounts tend to give a rather coarse coating and the amount of manganese is preferably from 0.1 to 0.25 parts.

The composition may contain hydroxy carboxylic acid, for instance tartrate, typically in an amount of up to 1.5 parts.

The ratio Zn: P04 is preferably from 1:4 to 1:30, most preferably 1:6 to 1:1 3. Often it is below 1:8 or, at most, below 1:10. The total acid value of the working solution is generally from 10 to 40 points, preferably 1 5 to 25 points and the free acid content of the solution is generally from 0.5 to 5 points, preferably 0.5 to 3 points. The ratio of free acid : total acid is generally 1:10 to 30, preferably around 1 :15 to 25.

Although fluoroborate is recommended in British Patent specification 2,046,313, we surprisingly find that it is preferable in the invention to use silicofluoride since this gives a faster coating rate and a more uniform coating A process according to the invention comprises forming a protective coating on a metal surface by contacting the surface with a working solution as described above. The solution can be used for forming coatings on ferrous, zinc or aluminium surfaces. It is of particular value on steel surfaces. Before contacting the surface with the solution the surface should be cleaned and rinsed, for instance in conventional manner.

Contact between the solution and the surfaces may be by spray or immersion or other conventional techniques, and is generally continued for periods of from 45 seconds to 3 minutes when spraying or 2 to 5 minutes when immersed. Combined processes, for instance spray, immerse, spray, may be used.

The solution temperature during the contact is generally from 25 to 55"C but is preferably below 40"C, normally around 25 to 35"C.

After the contact with the solution the surfaces may be rinsed with water and may be given a chemical after rinse in conventional manner, for instance with a dilute aqueous solution of chromium compounds. The after rinsed surface may then be water rinsed and dried in conventional manner.

The resultant coatings give good corrosion resistance, even during prolonged exposure to corrosive conditions, and serve as an excellent base for paint.

During use the solution is replenished in conventional manner, generally with an alkaline accelerator replenishment containing sodium hydroxide and nitrobenzene sulphonate and with a separate acidic replenishment containing the other components.

The following is an example of the invention.

A working solution is prepared containing 1.5 g/l zinc, 13.9 g/l phosphate, 5.1 g/l nitrate, 0.24 g/l nickel, 0.17 g/l manganese, 1 g/l silicofluoride and 1 g/l sodium nitrobenzene sulphonate and having a total acid content of 21 points and a free acid content of 1 point. A succession of steel surfaces were sprayed with this solution while maintaining it at a temperature of around 30"C. The surfaces had previously been cleaned and, after spraying, were rinsed with water, given an after rinse of chromium compounds, rinsed with water and dried. They were then painted by cathodic electrodeposition.

When the bath needed replenishment it was replenished with two replenishments. One replenishment was a solution of 39.95 g/l sodium hydroxide and 39 g/l sodium nitrobenzene sulphonate. The other replenishment was the solution of 108 g/l zinc oxide, 282 g/l phosphoric acid, 7 g/l manganese carbonate, 24.7 g/l 35% H2SiF6 and having a total acid content of 32.5 points and a free acid content of 7.9 points.

The coated surfaces had excellent corrosion resistance and paint adhesion.

Claims

1. A composition useful in the formation of a protective coating on a metal surface and which is substantially free of chlorate and chloride and which contains 0.8 to 2.5 parts zinc, 10 to 25 parts phosphate, 1.5 to 10 parts nitrate, 0.1 to 1.2 parts nickel, 0.5 to 1.5 parts silicofluoride and/or borofluoride and 0.25 to 2 parts nitrobenzene sulphonate (meaured as sodium salt) and 0 to 0.7 parts manganese, all parts being parts by weight.

2. - A composition useful in the formation of a protective coating on a metal surface and that contains 0.8 to 2.5 parts zinc, 10 to 25 parts phosphate, 1.5 to 10 parts nitrate, 0.1 to 1.2 parts nickel, 0.5 to 1.5 parts silicofluoride, 0.25 to 2 parts nitrobenzene sulphonate measured as sodium salt and 0.05 to 0.7 parts manganese, all parts being by weight.

3. A composition useful in the formation of a protective coating on a metal surface and that contains 1.4 to 2.5 parts zinc, 10 to 25 parts phosphate, 1.5 to 10 parts nitrate, 0.1 to 1.2 parts nickel, 0.5 to 1.5 parts silicofluoride, 0.25 to 2 parts nitrobenzene sulphonate measured as sodium salt and 0 to 0.7 parts manganese, all parts being by weight.

4. A composition useful in the formation of a protective coating on a metal surface and that contains 0.8 to 2.5 parts zinc, 10 to 25 parts phosphate, 1.5 to 10 parts nitrate, 0.1 to 1.2 parts nickel, 0.5 to 1.5 parts silicofluoride, 0.25 to 2 parts nitrobenzene sulphonate measured as sodium salt and 0 to 0.7 parts manganese and wherein the ratio Zn: P04 is 1:6 to 1:10.

5. A composition according to any of claims 2 to 4 that is substantially free of chlorate and chloride.

6. A composition according to any preceding claim containing 1.4 to 2 parts zinc.

7. A composition according to any preceding claim containing manganese in an amount of 0.1 to 0.25 parts.

8. A composition according to any preceding claim containing 1 2 to 1 8 parts phosphate, 3 to 6 parts nitrate and 0.15 to 3.5 parts nickel and 0.5 to 1.5 parts nitrobenzene sulphonate.

9. A composition according to any preceding claim containing up to 1.5 parts tartrate.

10. A composition according to any preceding claim having a ratio Zn: P04 of from 1:4 to 1:30.

11. A composition according to claim 8 having a ratio Zn: P04 of 1:6 to 1:13.

1 2. A composition according to any preceding claim containing 0.5 to 1.5 parts silicofluoride.

1 3. A composition according to any preceding claim in the form of a working solution containing sufficient water to provide 1000 parts.

1 4. A composition according to claim 1 3 having a total acid value of from 10 to 40 points and a free acid content of from 0.5 to 5 points and a free acid:total acid ratio of 1:10 to 1:30.

1 5. A composition according to claim 1 substantially as herein described with reference to the example.

16. A process of forming a protective coating on a metal surface comprising contacting the surface with a working solution according to claim 1 3 or claim 1 4 at a temperature of 25 to 55,C.

1 7. A process according to claim 1 6 in which the metal surface is a steel surface.

1 8. A process according to claim 1 6 or claim 1 7 in which the temperature of the solution during contact is below 40"C.

19. A process according to claim 16 substantially as herein described with reference to the example.