CA1332910C

CA1332910C - Process of phosphating before electroimmersion painting

Info

Publication number: CA1332910C
Application number: CA000560326A
Authority: CA
Inventors: Gerhard Muller; Werner Rausch
Original assignee: Metallgesellschaft AG
Current assignee: GEA Group AG
Priority date: 1987-04-11
Filing date: 1988-03-02
Publication date: 1994-11-08
Anticipated expiration: 2011-11-08
Also published as: GB2204067A; ES2030493T3; JPS64277A; JP2674075B2; EP0287133B1; DE3869455D1; GB2204067B; BR8801679A; EP0287133A1; AU1442788A; DE3712339A1; GB8808478D0

Abstract

In a process of phosphating workpieces made of steel or partly galvanized steel in preparation for electro-immersion painting the cleaned and rinsed workpieces are first activated with a weakly alkaline aqueous solution which contains titanium phosphate and are subsequently dipped into an acid aqueous phosphating solution at a temperature from 40 to 60°C. The solution contains:

1.8 to 5 g/l Zn 0.1 to 7 g/l Fe(II) 8 to 25 g/l P2O5 to 30 g/l NO3 and in the solution, the controlled ratio of free acid to total acid is between 0.04 and 0.07. The phosphating solution preferably contains 3 g/l zinc and 0.5 to 5 g/l iron (II) and additionally contains up to 3 g/l manganese.
Other suitable components of the phosphating solution are Co, Ni, hydroxylamine, fluorides, tartaric acid, citric acid, and m-nitrobenzene sulfonate.

Description

The present invention relates to a process of phosphating workpieces made of steel or partly galvanized steel in preparation for electro-immersion painting wherein the cleaned and rinsed workpieces are first activated with a weakly alkaline aqueous solution which contains titanium phosphate and are subsequently dipped into an acid aqueous phosphating solution which contains zinc phosphate and to the use of that process for preparing the workpieces for cathodic electro-immersion painting.
It is known that steel can be dipped into acid aqueous phosphating solutions which contain zinc/iron(II)/nitrate/phosphate and the steel can subsequently be painted. But the use of said processes in conjunction with electro-immersion painting has revealed substantial disadvantages. For instance, the varying thickness of the phosphate layer formed in the known processes has the result that the coatings applied by electro-immersion painting vary in thickness and in part exhibit surface waves, tears and craters. Besides, the corrosion resistance often fails to meet the requirements.
It is an object of the invention to provide for phosphating steel or partly galvanized steel a process which results in uniformly covering phosphate layers and is particularly suitable as a preparation for electrol-immersion painting.
According to the present invention, there isprovided a process of phosphating workpieces made of steel or partly galvanized steel in preparation for electro-immersion painting, wherein said workpieces:
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- are cleaned and rinsed, then - they are activated with a slightly alkaline aqueous solution which contains titanium phosphate, then - they are dipped into an acid aqueous phosphating solution which contains zinc phosphate at a temperature from 40 to 60C, the phosphating solution containing:
1.8 to 5 g/l Zn 0.1 to 7 g/l Fe (II) 8 to 25 g/l P205 5 to 30 g/l N03 and having a ratio of free acid to total acid between 0.04 and 0.07.
According to the present invention there is also provided a process of phosphating a surface of a workpiece made of steel or partly galvanized steel in preparation for electro-immersion painting including:
- dipping the activated surface with an acid aqueous phosphating solution at a temperature of from 40 to 60C, the phosphating solution consisting essentially of:
1.8 to 5 g/l Zn 0.1 to 7 g/l Fe (II) 8 to 25 g/l P205 5 to 30 g/l NO3 and having a ratio of free acid to total acid of from 0.04 to 0.07; and - maintaining the content of divalent iron in the range of 0.1 to 7 g/l in that surplus divalent iron that has entered the phosphating solution is precipitated as iron (III) phosphate by means of at least one of an oxygen-containig gas, a chlorate compound and a peroxide compound.
According to the present invention there is also provided a phosphating solution including:

- 2a -1.8 to 5 g/l Zn 0.1 to 7 g/l Fe (II) 8 to 25 g/l P2O5 to 30 g/l NO3 and having a ratio of free acid to total acid of from 0.04 to 0.07.
According to the present invention, there is also provided the use of the above process in the preparation of cathodic immersion painting.
The process in accordance with the present invention is used to treat steel, such as cold-rolled strip and sheet made of soft, unalloyed steels and cold-rolled sheets having a higher strength and made of phosphorus-enriched steel, micro-alloyed steel and dual-phase steels.
Zinc layers on galvanized steel may comprise, e.g., layers consisting of Zn, Zn+Fe, Zn+Al, Zn+Al+Si which have been applied by hot dipping and layers consisting of Zn, Zn+Ni, Zn+Fe and applied by electrodeposition.
The process in accordance with the invention may be used with workpieces of different kinds and shapes, such as flat material, deep-drawn parts, welded, seamed and adhesively joined structures. For an effective treatment of the inside surfaces of hollow /
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-1~32910 bodies, an adequate venting and an adequate draining of the liquid must be ensured. Typical workpieces having complex shapes and consisting of different materials are automobile bodies.
The workpieces are initially cleaned in conven-tional manner, e.g., with alkaline degreasing agents, and are subsequently rinsed in water. This is succeeded by a treatment in a slightly alkaline aqueous solution containing activating titanium phosphate in a fine dispersion.
lo Phosphating is effected in a temperature range from 40 to 60C. At lower temperatures the phosphating process will be too slow for the formation of covering phos-phate layers in reasonable times. Above 60C the energy losses rise steeply and there is an increasing risk of a formation of disturbing dry deposits or crusts.
The phosphating process in accordance with the invention belongs to the processes which are carried out on the "iron side" and for this reason distinguishes by a comparatively small formation of sludge. The initial iron(II) concentration of the baths may be less than 0.1 g/l. After a few passes said concentration will quickly rise into the range called for by the invention owing to the removal of material from the steel by the pickling action of the phosphating solution.
The maintenance of the concentrations of Zn, Fe(II), P2O5 and NO3 in the ranges which are essential for the invention is a requisite for a formation of optimum phosphate layers for the succeeding electro-immersion painting. For instance, phosphate layers providing only an incomplete coverage on steel will be formed if the zinc content is less than 1.8 g/l. With zinc contents in excess of 5 g/l the phosphate layers are too thick for a satisfactory painting. If the Fe(II) content exceeds 7 g/l X

the quality of the phosphate layers for the succeeding electro-immersion painting will distinctly decrease. If the P205 content is less than 8 g/l, the phosphate content will no longer be sufficient for a proper phosphating. A P205 content in excess of 25 g/l will no afford additional technological advantages. If the content of N03 is less than 5 g/l, the baths will no accelerate the formation of the phosphate layer to the required degree. An N03 content above 30 g/l will not result in a useful further increase of the rate at which the layer is formed.
The ratio of free acid to total acid is of high significance in the process in accordance with the invention. If that ratio is below 0.04, the increasing formation of sludge will result in a loss of valuable components of hte phosphating solution. If /
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13~2~10 the ratio exceeds 0.07, the phosphating rate will strongly decrease. An optimum ratio can be adjusted by a control of the concentrations of the components of the bath and by an optional addition of further cations, such as Na, K, NH4, or of further anions, such as Cl, SO4.
Particularly good results will be produced in the subsequent electro-immersion painting if, in a preferred embodiment of the invention, the workpieces are contacted with a phosphating solution which contains up to 3 g/l zinc and preferably 0.5 to 5 g/l iron(II).
The layer being formed can be modified by an addition of other divalent cations, e.g., from the group consisting of Ca, Co, Cu, Mg, Mn, Ni, to the phosphating solution.
The workpieces obtained as above described may be preferably contacted with a phosphating solution which additionally contains up to 3 g/l manganese and/or up to 3 g/l magnesium. Cobalt may be further added preferably in an amount of up to 0.3 g/l and nickel preferably in an amount of up to 0.15 g/l. The presence of cobalt in an amount in excess of 0.3 g/l and/or of nickel in an amount in excess of 0.15 g/l may result in a formation of streaky phosphate layers on steel.
In another suitable embodiment of the invention the workpieces obtained as above described may be contacted with a phosphating solution which contains hydroxylamine in an amount of up to 3 g/l, preferably at least 0.3 gtl. In that case the concentration of nickel can be increased to 0,5 g/l if at the same time at least 0,3 g/l hydroxylamine is present. Hydroxylamine will accelerate the phosphating process.
In order to increase the aggressiveness of the phosphating solution and the phosphating rate and to C

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optimize the formation of a layer on aluminum-containing zinc surfaces, a further desirable embodiment of the invention comprises contacting the workpieces obtained as above described with a phosphating solution which further contains up to 3 g/l SiF6 and/or up to 3 g/l BF4 and/or up to 1.5 g/l F.
Tartaric acid and/or citric acid preferably in an amount of up to 3 g/l may be added to the solution to reduce the weight of the phosphate layers per unit of area and to further accelerate the formation of the layer in the workpieces.
It will also be possible to contact the workpieces with a phosphating solution which further contains up to 0.5 g/l, preferably 0.05 to 0.35 g/l of m-nitrobenzene sulfonate. The presence of m-nitrobenzene sulfonate strongly accelerates the phosphating process and distinctly decreases the thickness of the phosphate layer. To prevent a shift of the phosphating baths from the iron side to the nitrite side by an autocatalytic formation of nitrite, nitrite-destroying additives, such as urea or amidosulfonic acid, are preferably added to the baths.
In order to avoid a rise of the concentration of iron(II) above the desired value it is desirable to oxidize part of the iron(II) which has been dissolved by the pickling action, whereby iron(III) is formed and is precipitated as difficultly soluble iron(III) phosphate sludge. In preferred embodiments of the invention, said oxidation may be effected in that the phosphating solution is contacted with oxygen-cont~;n;ng gas and in that chlorate and/or peroxide compounds are added.
The content of free acid in the phosphating baths may be decreased by an addition of, e.g., alkali hydroxide and alkali carbonates.
Another particularly desirable way is the use of ~ 7 - 1332~10 zinc oxide, zinc carbonate and/or manganese carbonate, whereby additional layer-forming cations are introduced into the phosphating solution.
Besides, the phosphating process in accordance with the invention may be modified in that the dip into the phosphating solution is preceded and/or succeeded by a spraying with the phosphating solution. The dip times usually are in the range from 2 to 5 minutes and the preceding and/or succeeding spraying may have a duration from a few seconds to about 0.5 minute.
In a desirable embodiment the phosphating process in accordance with the invention is performed to produce phosphate layers having a weight of 1 to 5 g/m2. This will result in an optimum corrosion protection in /
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-combination with a formation of paint coatings having a high bond strength when flexed.
The layers formed in the process in accordance with the invention will constitute an effective base coating for paints applied by anodic and cathodic electro-immersion.
Particularly desirable results will be produced when the process is carried out in preparation for a cathodic electro-immersion painting which is intended to produce paint films in thicknesses in the range from about 15 to 40 o ~m. The paint layers applied by electro-immersion may constitute a base paint for additiona paint layers or a paint monolayer.
The invention will be explained more in detail and by way of example with reference to the following Examples.
Example Sheets made of body steel and galvanized steel were degreased with an alkaline degreasing agent, rinsed in water, activated by being dipped at 40C for 1 minute into an aqueous suspension consisting of about 50 mg/1 titanium phosphate in an aqueous solution of 1 g/l disodium phosphate and 0.25 g/l tetrasodium pyrophosphate and were subsequently dip-phosphated at 55C in the phosphating solutions 1 to 6 stated in the Table.
It was found that the minimum phosphating time was at least 2 to 3 minutes on steel and less than 1 minute on galvanized steel. The minimum phosphating time is the time which is required for the treatment in the phosphating bath for a formation of a phosphate layer which has a visually uniform coverage.
The weight of the layer was between 3.6 and 4.3 g/m2 on steel and between 2.2 and 3.0 g/m2 on zinc.
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The free acid (total acid) is defined as the quantity of n/10 NaOH in ml which is required to neutralize a bath sample of 10 ml against dimethyl yellow ~phenolphtalein). The ratio of free acid to total acid was (0.054 to 0.063) : 1.
The phosphating was succeeded by a rinsing with water, a passivating with a chromium-containing after-rinsing solution and an afterrinsing with deionized water.
This was followed by electro-immersion painting.
Uniform paint layers were obtained, which without and with additional paint layers exhibited a very strong adhesion to the metallic substrate and an excellent resis-tance to corrosion. That quality was at least equivalent to that which was obtained by the known low-zinc phosphating processes with acceleration by chlorate and/or nitrite in baths which contained virtuaIly no iron(II).
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TAB~E

EXample 1 2 3 4 5 6 Zn (g/l) 2 2 2 4 4 4 Fe(II) (g/l) 1 2.5 5 1 2.5 5 Ni (g/l) 0.10.1 0.1 0.1 0.1 o.

P205 (g/l 16 16 16 16 16 16 3~0~; (g/l 11 11 11 11 11 11 Tartaric acid (g/l) BF4 (g/l~ 1.41.4 1.4 1.4 1.4 1.4 Na ~he quantity required to obtain the following data by titration Free acid (F ) 1.62.0 2.0 1.7 1.9 2.2 Total acid (TA) 28.132.0 31-5 31.5 34.2 36.0 ~otal acld ~T~43 0.0570.063 0.060 0.0540.056 0.060 ~inimum phosphating time on steel (min.~ 3 3 3 2 2 Weight of lay~r on steel (g/m ) 3.64.0 3.6 3.6 4.3 4.1 Appearance of layer Gray, finely crystalline and uniform on steel throughout ~inimum phos~hating time onZinc (min.) Generally less than 1 Weight of la~er on zinc (g/m ) 2.2 2.6 2.2 2.5 3.0 2.9 Appearance of layer on Gray, finely crystalline, and uniform zinc ~throughout~

Claims

1. A process of phosphating workpieces made of steel or partly galvanized steel in preparation for electro-immersion painting, wherein said workpieces:
- are cleaned and rinsed, then - they are activated with a slightly alkaline aqueous solution which contains titanium phosphate, then - they are dipped into an acid aqueous phosphating solution which contains zinc phosphate at a temperature from 40 to 60°C, the phosphating solution containing:
1.8 to 5 g/l Zn 0.1 to 7 g/l Fe (II) 8 to 25 g/l P2O5 to 30 g/l NO3 and having a ratio of free acid to total acid between 0.04 and 0.07.

2. A process according to claim 1, characterized in that the workpieces are contacted with a phosphating solution which contains up to 3 g/l zinc.

3. A process according to claim 2, characterized in that the workpieces are contacted with a phosphating solution which contains from 0.5 to 5 g/l iron(II).

4. A process according to claim 1, 2 or 3, wherein the workpieces are contacted with a phosphating solution which additionally contains up to 3 g/l manganese.

5. A process according to claim 4, wherein the workpieces are contacted with a phosphating solution which additionally contains up to 3 g/l magnesium.

6. A process according to claim 5, wherein the workpieces are contacted with a phosphating solution which additionally contains up to 0.3 g/l cobalt and/or up to 0.15 g/l nickel.

7. A process according to claim 1, 2 or 3, wherein the workpieces are contacted with a phosphating solution which contains hydroxylamine in an amount of up to 3 g/l.

8. A process according to claim 1, 2 or 3, wherein the workpieces are contacted with a phosphating solution which contains hydroxylamine in an amount from 0.3 g/l to 3 g/l.

9. A process according to claim 8, wherein the workpieces are contacted with a phosphating solution which contains up to 0.5 g/l nickel.

10. A process according to claim 9, wherein the workpieces are contacted with a phosphating solution which contains up to 3 g/l SiF6 and/or up to 3 g/l BF4 and/or up to 1.5 g/l F.

11. A process according to claim 10, wherein the workpieces are contacted with a phosphating solution which contains up to 3 g/l tartaric acid and/or citric acid.

12. A process according to claim 11, wherein the workpieces are contacted with a phosphating solution which contains m-nitrobenzene sulfonate in an amount of up to 0.5 g/l.

13. A process according to claim 11, wherein the workpieces are contacted with a phosphating solution which contains m-nitrobenzene sulfonate in an amount from 0.05 to 0.35 g/l.

14. A process according to claim 13, wherein the workpieces are contacted with a phosphating solution which contains nitrite-destroying substances.

15. A process according to claim 14, wherein said nitrite-destroying substances are chosen from urea and amido-sulfonic acid.

16. A process according to claim 15, wherein the workpieces are contacted with a phosphating solution in which the content of divalent iron is adjusted in that surplus divalent iron that has entered the phosphating solution is precipitated as iron(III) phosphate by means of oxygen-containing gases, and by adding chlorate and/or peroxide compounds.

17. A process according to claim 16, wherein the workpieces are contacted with a phosphating solution in which the content of free acid is adjusted by an addition of zinc oxide, zinc carbonate and/or manganese carbonate.

18. A process according to claim 17, wherein the workpieces are contacted with sprayed phosphating solution before and/or after the workpieces are dipped into the phosphating solution.

19. A process according to claim 16, 17 or 18, wherein the workpieces are contacted with a phosphating solution so as to apply phosphate layers having a weight from 1 to 5 g/m2.

20. A process according to claim 1, wherein the workpieces are contacted with a phosphating solution which additionally contains up to 3 g/l magnesium.

21. A process according to claim 1, wherein the workpieces are contacted with a phosphating solution which additionally contains up to 0.3 g/l cobalt and/or up to 0.15 g/l nickel.

22. A process according to claim 20, wherein the workpieces are contacted with a phosphating solution which contains hydroxylamine in an amount of up to 3 g/l.

23. A process according to claim 1, wherein the workpieces are contacted with a phosphating solution which contains hydroxylamine in an amount of at least 0.3 g/l.

24. A process according to claim 23, wherein the workpieces are contacted with a phosphating solution which contains up to 0.5 g/l nickel.

25. A process according to claim 1, 23 or 24, wherein the workpieces are contacted with a phosphating solution which contains up to 3 g/l SiF6 and/or up to 3 g/l BF4 and/or up to 1.5 g/l F.

26. A process according to claim 1, wherein the workpieces are contacted with a phosphating solution which contains up to 3 g/l tartaric acid and/or citric acid.

27. A process according to claim 1, wherein the workpieces are contacted with a phosphating solution which contains m-nitrobenzene sulfonate in an amount of up to 0.5 g/l.

28. A process according to claim 1, 26 or 27, wherein the workpieces are contacted with a phosphating solution which contains m-nitrobenzene sulfonate in an amount from 0.05 to.35 g/l.

29. A process according to claim 1, wherein the workpieces are contacted with a phosphating solution which contains nitrite-destroying substances.

30. A process according to claim 1, wherein said nitrite-destroying substances are chosen from urea and amido-sulfonic acid.

31. A process according to claim 1, 29 or 30, wherein the workpieces are contacted with a phosphating solution in which the content of divalent iron is adjusted in that surplus divalent iron that has entered the phosphating solution is precipitated as iron (III) phosphate by means of oxygen-containing gases and by adding chlorate and/or peroxide compounds.

32. A process according to claim 1, wherein the workpieces are contacted with a phosphating solution in which the content of free acid is adjusted by an addition of zinc oxide, zinc carbonate and/or manganese carbonate.

33. A process according to claim 1, wherein the workpieces are contacted with sprayed phosphating solution before and/or after the workpieces are dipped into the phosphating solution.

34. A process according to claim 1, 32 or 33, wherein the workpieces are contacted with a phosphating solution so as to apply phosphate layers having a weight from 1 to 5 g/m .

35. The use of a process according to claim 1, 2, 3, 5, 6, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 20, 21, 22, 23, 24, 26, 27, 29, 30, 32 or 33, in preparation for cathodic electro-immersion painting.

36. A process of phosphating a surface of a workpiece made of steel or partly galvanized steel in preparation for electro-immersion painting including:
- dipping the activated surface with an acid aqueous phosphating solution at a temperature of from 40° to 60°C, the phosphating solution consisting essentially of:
1.8 to 5 g/l Zn 0.1 to 7 g/l Fe (II) 8 to 25 g/l P2O5 to 30 g/l NO3 and having a ratio of free acid to total acid of from 0.04 to 0.07; and - maintaining the content of divalent iron in the range of 0.1 to 7 g/l in that surplus divalent iron that has entered the phosphating solution is precipitated as iron (III) phosphate by means of at least one of an oxygen-containing gas, a chlorate compound and a peroxide compound.

37. A process according to claim 36, wherein the workpiece surface is contacted with a phosphating solution so as to apply phosphate layers having a weight of from 1 to 5 g/m2.

38. A process according to claim 36, wherein prior to the dipping, the workpiece surface is cleansed, rinsed and activated with a slightly alkaline aqueous solution which contains titanium phosphate.

39. A process according to claim 1, further comprising the step of applying by cathodic electro-immersion painting a coat of paint on said workpiece.

40. A process according to claim 36, wherein the workpiece is sprayed with the phosphating solution after the dipping.

41. A process according to claim 36, wherein the phosphating solution contains at least 0.3 g/l hydroxylamine and nickel in an amount of 0 < Ni? 0.5 g/l.

42. A phosphating solution including:
1.8 to 5 g/l Zn 0.1 to 7 g/l Fe (II) 8 to 25 g/l P2O5 to 30 g/l NO3 and having a ratio of free acid to total acid of from 0.04 to 0.07.