DE3712339A1 - METHOD FOR PHOSPHATIZING BEFORE ELECTROPLATING - Google Patents

Description

The invention relates to a method for phosphating of workpieces made of steel or partially galvanized steel as a pretreatment for electrocoating, in which the cleaned and rinsed workpieces first a weak aqueous titanium phosphate activated alkaline solution and then with a aqueous acidic, containing zinc phosphate Phosphating solution to be treated by immersion and its application to the pretreatment for the cathodic electro dip painting.

It is known to acidic steel with aqueous Phosphating solutions based Treat zinc / iron (II) / nitrate / phosphate in diving and then paint. For the application of this Process in connection with electrocoating however, there were significant shortcomings. So she leads fluctuating phosphate layer thickness of the known methods for training unevenly thicker Electrocoat layers, some with streaks, Runners and craters are interspersed. Furthermore, the Corrosion protection often does not meet the requirements.

The object of the invention is a method for Phosphating of steel or partially galvanized steel to provide the evenly covering Supplies phosphate layers and particularly good as Pretreatment for electrocoating is suitable.  

The task is solved by the procedure of the beginning mentioned type according to the invention is designed that the workpieces at one Temperature from 40 to 60 ° C with a phosphating solution in contact that

1.8 to 5 g / l Zn
0.1 to 7 g / l Fe (II)
8 to 25 g / l P₂O₅
5 to 30 g / l NO₃

contains and in which the ratio of free acid to Total acid is set to 0.04 to 0.07.

The method according to the invention is used to treat Steel, e.g. B. from cold-rolled strip and sheet of soft, unalloyed steels and high-strength cold-rolled Thin sheets made of phosphorous steels, micro-alloyed Steels and dual-phase steels. The galvanizing layers on galvanized steel include e.g. B. the on the Melt dip paths generated, based on Zn, Zn + Fe, Zn + Al, Zn + Al + Si as well as electrolytically obtained based on Zn, Zn + Ni, Zn + Fe.

The method according to the invention can be applied to workpieces different types and shapes, d. H. Flatware, Deep-drawn parts, welding, folding, flanging and Adhesive structures can be applied. In the case of hollow bodies To achieve a good treatment of the inner surfaces must for adequate ventilation and outflow of liquid be taken care of. Typical workpieces of complex shape and with A wide variety of materials are automobile bodies.  

The workpieces are first made in the usual way, for example with alkaline degreasing agents, cleaned and then rinsed in water. Here the treatment closes in an aqueous, light alkaline solution, in the activating Titanium phosphate is finely dispersed.

Phosphating takes place in the temperature range from 40 to 60 ° C. Below this temperature the Phosphating process too slow to cover Form phosphate layers in reasonable times. Above 60 ° C the energy losses rise sharply and it there is an increasing risk of disruptive drying and incrustations.

The phosphating process according to the invention belongs in the Group of procedures on the "iron side" and draws by comparatively low sludge formation out. At the start of the baths, the iron (II) concentration can less than 0.1 g / l. After a few throughputs it arrives however due to the pickling removal taking place on the steel quickly in the area of the invention.

Compliance with the essentials of the invention Concentration ranges for Zn, Fe (II), P₂O₅ and NO₃ is the prerequisite for this, optimal for the subsequent Suitable electrocoating suitable phosphate layers produce. When falling below 1.8 g / l Zn z. B. only incompletely covering phosphate layers Steel produced. Above 5 g / l Zn the Phosphate layers too thick to still be flawless Allow painting. Above 7 g / l Fe (II) increases  the quality of the phosphate layers with regard to the subsequent electro-dip treatment significantly. Below The phosphate content of 8 g / l P₂O₅ is enough for one proper phosphating no longer occurs during a Exceeding 25 g / l P₂O₅ no more brings more technical advantages. At The baths lack NO₃ concentrations below 5 g / l the necessary acceleration of the Phosphate layer formation. Above 30 g / l NO₃ finds no further evaluable increase in Stratification rate takes place more.

Of great importance for the implementation of the The method according to the invention is the ratio of suitors Acid to total acid. Valuable go below 0.04 Components of the phosphating solution by increasing Sludge formation lost. The above takes 0.07 Phosphating rate drops sharply. The setting of the optimal ratio is determined by a Concentration adjustment of the bathroom components, optionally with the use of further cations, e.g. B. of Na, K, NH₄, or other anions, e.g. B. from Cl, SO₄, achieved.

Particularly favorable results with regard to the following Electrocoating is achieved when appropriate a preferred embodiment of the invention Workpieces in contact with a phosphating solution brought, the maximum 3 g / l Zn and preferably 0.5 contains up to 5 g / l Fe (II).

Additional layers can be modified to modify the layer formation  divalent cations e.g. B. from the group Ca, Co, Cu, Mg, Mn, Ni, are added to the phosphating solution. The workpieces are preferably provided with a Phosphating solution contacted that additionally Manganese in an amount up to 3 g / l or magnesium in one Contains up to 3 g / l. When adding cobalt, up to 0.3 g / l and preferred from nickel to 0.15 g / l. At Can hold over 0.3 g / l Co or over 0.15 g / l Ni streaky phosphate layers are formed on steel.

Another expedient embodiment of the invention consists of the workpieces with a To bring phosphating solution in contact that up to 3 g / l, preferably at least 0.3 g / l, hydroxylamine contains. In this case, the concentration of Ni on 0.5 g / l can be increased if at the same time at least 0.3 g / l of hydroxylamine are present. Hydroxylamine works accelerating on the phosphating process.

To increase the aggressiveness of the phosphating solution, to increase the rate of phosphating and Optimization of the layer formation on aluminum-containing Zinc surfaces are advantageous according to another Embodiment of the invention, the workpieces with a Phosphating solution contacted up to 3 g / l SiF₆ and / or up to 3 g / l BF₄ and / or up to Contains 1.5 g / l F.

Additions of preferably up to 3 g / l tartaric acid and / or Citric acid serve to determine the basis weight of the Lower phosphate layers and layer formation accelerate further.  

It is also advantageous to use phosphating solutions use the up to 0.5 g / l, preferably 0.05 to Contain 0.35 g / l m-nitrobenzenesulfonate. m-Nitrobenzenesulfonate causes a strong acceleration the phosphating at the same time more clearly Reduction of the phosphate layer thickness.

To avoid that the phosphating baths from the Iron side through autocatalytic nitrite formation on the Come nitrite side, the baths are preferably nitrite destructive connections, e.g. B. urea or Amidosulfonic acid added.

To increase the iron (II) concentration above the It is to avoid desired concentration beyond expedient to part of the solution by pickling oxidized iron (II) to iron (III) and thus in the form of poorly soluble iron (III) phosphate sludge to fail. According to preferred embodiments of the For this oxidation, the contact of the invention Phosphating solution with oxygen-containing gas and the Add chlorate and / or peroxide compounds.

To reduce the free acidity in the phosphating baths can e.g. B. added alkali hydroxides and carbonates will. The use of is particularly advantageous Zinc oxide, zinc carbonate and / or manganese carbonate, with the additional layer-forming cations in the Phosphating solution are introduced.

The phosphating process according to the invention can also do so  be modified that diving into the A spraying with the phosphating solution Phosphating solution precedes and / or connected becomes. The diving times are usually in the range of 2 to 5 min while the pre- and / or post-injection is in progress from a few seconds to about 0.5 min.

According to an advantageous embodiment, the Phosphating process according to the invention performed such that Phosphate layers with a basis weight of 1 to 5 g / m² can be generated. This will make it more optimal Corrosion protection with good bending adhesion Paint coats connected.

Those trained with the method according to the invention Layers are used as a primer for anodic and cathodic Suitable for electro dipping. Particularly favorable results are achieved when the pretreatment procedure for the cathodic electrodeposition serves and here Paint film thicknesses in the range of about 15 to 45 microns achieved should be. The electrocoat layers can be used as Base coat for further layers of paint or as Single-layer paints are used.

The invention is illustrated by the following examples for example and explained in more detail.

example

Sheets made of body steel and galvanized steel were included degreased with an alkaline cleaner, rinsed in water,  with an aqueous suspension of about 50 mg / l Titanium phosphate in an aqueous solution of 1 g / l Disodium phosphate and 0.25 g / l tetrasodium pyrophosphate activated by diving for 1 min at 40 ° C and then in the phosphating solutions listed in the table 1 phosphated to 6 at 55 ° C while diving.

A minimum phosphating time of 2 to 3 min, on galvanized steel less than 1 min found. The minimum phosphating time is Treatment time in the phosphating bath defined that for Formation of a visually uniform covering Phosphate layer is necessary.

The layer weight for steel was between 3.6 and 4.3 g / m², for zinc between 2.2 and 3.0 g / m².

Free acid (total acid) is defined as the amount to ml n / 10 NaOH, which is used to neutralize 10 ml bath sample against dimethyl yellow (phenolphthalein) is required. The Free acid to total acid ratio was (0.054 to 0.063): 1.

After the phosphating was rinsed with water, with passivation solution containing chrome passivated, with deionized water and then cathodic electrocoated.

There were even layers of paint, without and with further paint build-up a very good adhesion to the metallic surface and an excellent Showed corrosion protection. This quality was at least  the same as that with the known Low zinc phosphating process, operated with chlorate and / or nitrite acceleration in practical Iron (II) -free baths were obtained.  

table

Claims (16)

1. A process for phosphating workpieces made of steel or partially galvanized steel as a pretreatment for electrocoating, in which the cleaned and rinsed workpieces are first activated with an aqueous, weakly alkaline solution containing titanium phosphate and then treated with an aqueous acidic, phosphate-containing phosphate solution containing zinc are characterized in that the workpieces are brought into contact with a phosphating solution containing 1.8 to 5 g / l Zn at a temperature of 40 to 60 ° C
0.1 to 7 g / l Fe (II)
8 to 25 g / l P₂O₅
Contains 5 to 30 g / l NO₃ and in which the ratio of free acid to total acid is set to 0.04 to 0.07. 2. The method according to claim 1, characterized in that the workpieces with a phosphating solution Brings contact, the maximum 3 g / l zinc and preferably contains 0.5 to 5 g / l of iron (II). 3. The method according to claim 1 or 2, characterized characterized in that the workpieces with a Contacting phosphating solution that additionally contains manganese in an amount of up to 3 g / l.   4. The method according to claim 1, 2 or 3, characterized characterized in that the workpieces with a Contacting phosphating solution that additionally Mangesium in an amount up to 3 g / l contains. 5. The method according to claim 1, 2, 3 or 4, characterized characterized in that the workpieces with a Contacting phosphating solution that additionally up to 3 g / l cobalt and / or up to 0.15 g / l Contains nickel. 6. The method according to one or more of claims 1 to 5, characterized in that the workpieces with a phosphating solution in contact, which up to at 3 g / l, preferably at least 0.3 g / l, Contains hydroxylamine. 7. The method according to one or more of claims 1 to 6, characterized in that the workpieces with in contact with a phosphating solution, the Nickel content in the presence of at least 0.3 g / l Hydroxylamine adjusted to a maximum of 0.5 g / l nickel is. 8. The method according to one or more of claims 1 to 7, characterized in that the workpieces with a phosphating solution in contact, which up to to 3 g / l SiF₆ and / or up to 3 g / l BF₄ and / or contains up to 1.5 g / l F. 9. The method according to one or more of claims 1 to 8, characterized in that the workpieces with  a phosphating solution in contact, which up to contains 3 g / l tartaric acid and / or citric acid. 10. The method according to one or more of claims 1 to 9, characterized in that the workpieces with a phosphating solution in contact, which up to to 0.5 g / l, preferably 0.05 to 0.35 g / l contains m-nitrobenzenesulfonate. 11. The method according to one or more of claims 1 to 10, characterized in that the workpieces with in contact with a phosphating solution which Nitrite-destroying substances such as urea or Contains sulfamic acid. 12. The method according to one or more of claims 1 to 11, characterized in that the workpieces with in contact with a phosphating solution, the Bivalent iron content by precipitation excess in the phosphating solution transitioned divalent iron in the form of Iron III phosphate using oxygen-containing gases and / or chlorate and / or peroxide compounds is set. 13. The method according to one or more of claims 1 to 12, characterized in that the workpieces with in contact with a phosphating solution, the Free acid content by adding zinc oxide, Zinc carboant and / or manganese carbonate adjusted has been.   14. The method according to one or more of claims 1 to 13, characterized in that the workpieces in front and / or after the diving treatment with the In contact with the phosphating solution in the syringe. 15. The method according to one or more of claims 1 to 14, characterized in that the workpieces with in contact with a phosphating solution in such a way that phosphate layers with a basis weight of 1 up to 5 g / m² can be applied. 16. Use of the method according to one or more of the Claims 1 to 15 on the pretreatment for the cathodic electrodeposition treatment.