GB2196024A

GB2196024A - Process for producing phosphate coatings

Info

Publication number: GB2196024A
Application number: GB08720788A
Authority: GB
Inventors: Eng Dieter Hauffe
Original assignee: Pyrene Chemical Services Ltd
Current assignee: Pyrene Chemical Services Ltd
Priority date: 1986-09-05
Filing date: 1987-09-04
Publication date: 1988-04-20
Also published as: DE3630246A1; EP0258922A1; GB8720788D0; ZA876589B; JPS63145784A

Description

GB2196024A 1

SPECIFICATION

Process for producing phosphate coatings The invention relates to a process for producing phosphate coatings on surfaces comprising zinc 5 or zinc alloys by flooding or immersion treatment with accelerated zindphosphate phosphating solutions.

A phosphating solution for the phosphating of surfaces comprising zinc or zinc alloys generally contains zinc and phosphate as essential components determining the development of the layer.

Cations such as copper, calcium, manganese, alkali metal and, especially, nickel can be included 10 in the solution to modify the formation of the layer. Oxidising agents, for example from the group comprising nitrate, chiorate, nitrite, peroxide, organic nitro compounds are used to acceler ate layer formation. Other possible additives include fluoride, complex fluoride, chloride, organic polyhydroxycarboxylic acid, complex phosphates and surface-active agents. The solutions can be applied by immersion, spray/immersion or spray processes at temperatures which are usually 15 from 30 to 70'C and for treatment times of, for example, from 0.5 to 5 min (EP-A 69 950).

Although the resultant phosphate coating is substantially uniform it is often observed that it contains small whitish spots and microscopic examination has shown these to be corroded pits with crystals heaped round the edge. We believe that during contact with the phosphating solution the zinc surface is covered substantially completely with a protective phosphate layer as 20 the phosphating time increases but that individual points, having a diameter typically of from 0.1 to lmm, remain at which the corrosive effect of the phosphating solution persists. The reason for this is not yet known. A large supply of zinc ions originates from these corroded pits and leads to the precipitation of tertiary zinc phosphate at the rim of the pits. This grows like the rim of a crater and, during subsequent painting, the spots cause small mounds in the film of 25 paint and necessitate expensive grinding work. The above-mentioned phenomenon occurs not only with surfaces composed completely of zinc or zinc alloys but also with compound metals whose surfaces consist only partly of zinc or zinc alloy.

When the zinc surface is formed by electroplating on to steel, it is proposed in EP-A 137540 to avoid this spot formation by use of a particular type of sulphuric acid electrolyte. However 30 this does not help avoid spot formation on surfaces that have already been galvanised or that are surfaces of solid zinc or of solid zinc alloy articles.

It is accordingly an object of the invention to modify the actual phosphating process so as to minimise or avoid spot formation on any zinc or zinc alloy surfaces.

According to the invention a phosphate coating is formed on surfaces comprising zinc or a 35 zinc alloy by flooding or immersion treatment with an accelerated zinc phosphate coating solu- tion and the accelerator comprises either (a) chlorate and the solution is kept substantially free from nitrate and nitrite, or (b) nitrate and/or nitrite and the solution is kept substantially free from chlorate and chloride. Often the coating solution contains nickel.

Although it is often implied in the literature (e.g., EP-A 69950, 39093, 15436, 121274 and 40 109110) that a single accelerator can be used, the same literature usually recommends mixtures of accelerators. The particular process of the invention, and the fact that it minimises or avoids the problems of spot formation, has not been proposed.

The zinc phosphate solutions used in the invention are aqueous acidic solutions based on zinc phosphate and contain primary zinc phosphate, optionally alkali phosphate, and a certain quantity 45 of free acid adapted to the respective bath concentraion, type of application and bath tempera ture. The pH of the solutions is generally between about 2.0 and 3.9, depending on the processing conditions.

The solutions can be based on the so-called low zinc technology as well as by the normal zinc technology. This means that the surfaces can be brought into contact with a phosphating 50 solution in which the ratio by weight of Zn to P205 is adjusted to 1:(8 to 85) or with a phosphating solution in which the ratio by weight of Zn to P105 is adjusted to l:(0.3 to <8).

In the first case, if the surfaces consist partly of steel, phosphate layers with a high proportion of phosphophyllite to hopeite are formed on steel. These coatings give excellent corrosion protection and are very suitable as preparation for painting. The best protective properties are 55 achieved in conjunction with cathodic electro-immersion painting.

The second case results in phosphating processes which are distinguished by a higher phos- phating speed than the low zinc process. The phosphate layers produced by them have good properties for a wide range of applications in the spheres of corrosion protection and non-cutting cold-forming. 60 The phosphating solutions can also contain further cations known in the phosphating art, for example Co, Cu, Mn, Ca, Mg, Fe, Na, K, Li, NH,, etc. The invention is of particular value when the solution contains Ni. The concentrations of the most important of these ions should be up to 59/1, preferably 0.1 to 2g/1 in the case of Mn and Ca, up to 8g/1 preferably 0.1 to 5g/1 in the case of Fe(II). It may be necessary or desirable to use further anions different from phosphate, 65 2. 1 GB2196024A 2 for example S04, F, 13174, SW., c itrate, tartrate, etc., for achieving special technical effects and for adjustment of the acid ratio.

The amount of the chosen accelerator can be conventional, typically above lg/L Any accelera- tor that to be substantially absent is preferably totally absent but it is sometimes satisfactory to tolerate small quantities, preferably below 0.29/1. Blends of accelerator, especially with nitroben- 5 zene sulphonate, can be used advantageously. Replenishment and other process conditions must be conducted so as to maintain the desired composition of the treatment solution.

The contact of the surface with the phosphating solution is by flooding or immersion. contact time is usually between 1 and 10 min. Shorter treatment times, for example of from 2 to 30 see. are also adequate if zinc or zinc alloys are treated alone. The temperature of the phosphat- 10 ing solution is from about 30 to WC. The lower the temperature, the longer the contact time usually is.

The surfaces may be given conventional pre-treatments or post-treatments. For instance the pre-treatments include degreasing and rust removal, pre-rinsing to activate phosphate layer formation. The post-treatments include rinses with solutions based on Cr(V1) and/or Cr(III), or 15 treatment with impregnating agents such as rust-proofing oils, waxes and synthetic resins.

The invention is described in more detail by the Examples.

Examples

Steel sheets (A) electrolytically galvanised on both sides as well as compound metals com- 20 posed of steel sheet and galvanised steel sheet (B) were treated by the following operations:

1. Steam degreasing.

2. Cleaning with an alkaline cleaner at WC for 9.5 minutes immersion.

3. Rinsing with cold water by immersion.

4. Activation with a titanium phosphate solution for 1 minute by immersion. 25 5. Immersion phosphating at 60C with a stock solution containing 1.2 g/[ Zn 9/1 P201 0.03 g/1 Fe(I11) 30 3-6 g/1 Na and with 2.1 points free acid, 26 points total acid and an acid value of 0.1.

6. Rinsing with cold water by immersion.

7. Drying in an oven for 15 minutes. 35 The sock phosphating solution was modified by addition of N03, C1031 Cl, nickel and nitroben- zene sulphonate (NBS), and resultant spot formation are shown in the following Table. In this L=Iarge (at least lmm diameter) and S=small (below lmm diameter.) 3 GB2196024A 3 TABLE

5.Example Ni NO 3 C10 3 cl NBS Number of 5 (g/1) (g/1) (g/1) (g/1) (g/1) SpotS/C1M2 B A 10 1 0 0.1 0 0 0 0 0 2 0 0.1 0 1 0 0 0 3 0 5 0 0 0 0 0 15 4 0 5 0 1 0 0 0 0 0.1 3 0 0 0 0 6 0 0.1 3 1 0 0 0 20 7 0.8 0.1 0 0 0 0 0 8 0.8 0.1 0 1 0 0 0 9 0.8 5 0 0 0 0 0 25 0.8 5 0 1 0 100L 100S 11 0.8 0.1 3 0 0 0 0 12 0.8 0.1 3 1 0 0 0 30 13 0.8 0.1 3 1 1 0 0 14 0.8 1 3 1 1 15L los 35 The foregoing tests show that the formation of spots is avoided if attention is paid to substantial freedom from NO, when using chlorate-containing phosphating solutions (Examples 11 to 13) and substantial freedom from chforate or chloride when using nitrate-containing 40 phosphating solutions (Examples 9 and 10).

Whenusing both Chlorate and nitrate (Example 14) or nitrate and chloride (Example 10) containing phosphating solutions, on the other hand, considerable spot formation takes place.

Examples 13 and 14 show, in particular, that spot formation is not caused by the nitrobenzene sulphonate content. 45 Finally, comparison of Examples 1 to 6 with the other Examples shows that the problem of spot formation is much greater when the phosphating solution contains nickel and can be overcome in the invention.

Claims

CLAIMS 50

1. A process for producing a phosphate coating on surfaces of zinc or a zinc alloy by flooding or immersion treatment with an accelerated zinc phosphate phosphating solution and in which the accelerator comprises either (a) chlorate and the solution is kept substantially free from nitrate and nitrite or (b) nitrate and/or nitrite and the solution is kept substantially free from chlorate and chloride. 55

2. A process according to claim 1 in which the solution contains nickel.

3. A process according to claim 1 -or claim 2 in which the solution contains nitrobenzene- sulphonate as further accelerator.

4. A process according to any preceding claim in which the surfaces are subsequently painted. 60

5. A process according to claim 4 in which the painting is by electro- immersion painting and the ratio by weight of Zn:P20. in the phosphating solutions is 1:(8 to 85).

6. A process according to claim 5 in which the painting is by cathodic electro-immersion painting.

7. A process according to any preceding claim in which the accelerator comprises chlorate 65 4 GB2196024A 4 and the solution is maintained substantially free of nitrate an nitrite.

8. A process according to any of claims 1 to -6 in which the accelerator comprises nitrate and/or nitrite and the solution is maintained substantially free of chlorate and chloride.

Published 1988 at The Patent Office, State House, 66/71 High Holborn, London WC1R 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Burgess & Son (Abingdon) Ltd. Con. 1/87.