EP1230423B1 - Method for applying manganese phosphate layers - Google Patents

Abstract

The invention relates to a method for applying manganese phosphate layers on iron or steel surfaces. The aim of the invention is to produce a manganese phosphate layer with a minimum thickness of 2 mu m and an average surface roughness (Rz) of 2.5 mu m measured after drying. To this end, a phosphatizing solution is used that contains 0.2 to 4 g/l ferrous ions, 10 to 25 g/l manganese ions, 25 to 50 g/l phosphate ions (calculated as P2O5), 3 to 35 g/l nitrate ions, 0.5 to 5 g/l nitroguanidine, and that has 7 to 24 free acid points, 50 to 140 total acid points and an S value of 0.2 to 1. The phosphatization solution preferably contains 0.5 to 2 g/l nitroguanidine and maximally 2.5 g/l ferrous ions and additionally 0.2 to 4 g/l nickel ions or 0.2 to 4 g/l magnesium ions. The inventive method is especially applied to work pieces that are subject to a sliding friction.

Description

The invention relates to a method for applying manganese phosphate layers on iron or steel surfaces with manganese, phosphate, ferrous ions, as well as Nitroguanidine containing phosphating solutions and its application Workpieces subject to sliding friction.

For various applications, e.g. the reduction of friction sliding metal surfaces or facilitating cold working of Metals, manganese phosphate layers have become because of their high mechanical Resistance well proven. With the initially used However, manganese phosphate solutions were obtained relatively thick, coarsely crystalline Layers that are particularly disadvantageous when fine mechanical parts should be treated. With the aim of thin fine crystalline Manganese phosphate layers are therefore numerous proposals been submitted. For example, it is known by the addition of condensed phosphates to obtain a refinement of the phosphate layer. However, manganese phosphate phosphating solutions generally become used at high temperatures, so that as a result of high temperatures considerable hydrolysis, the effectiveness of the condensed phosphates quickly decreases or continuously condensed phosphate must be replenished.

Another way is described in German Auslegeschrift 1109 484 to to reach fine-grained phosphate layers. Thereafter, nitrate-containing Phosphate solutions are used, where the amount of nitrate, the amount of phosphate exceeds. The solutions should have a ratio of nitrate to phosphate of about (1.5 to 4.5): 1. It turned out, however, that in many cases the intended Effect is not achieved.

Furthermore, a method is known in which deliberately with an excessive proportion of Free acid of the phosphating solution is worked to very thin To produce layers (DE-C-1246356). However, these layers are because of their low basis weight only for special cases practically applicable.

Finally, it is known that a Phosphatierungslösung based on manganese phosphate or manganese iron phosphate, in which the concentrations of manganese, iron (II), phosphate and nitrate ions are within certain limits, more free P ₂ O ₅ in relation to Total P ₂ O ₅ supply as the phosphatization equivalent in the working phosphating corresponds. By the above measure should be achieved as an advantage that a significant reduction of the sludge formed in the phosphating and a reduction of the production of a certain amount of coating required chemicals is achieved (DE-B-22 13781).

The known method has in common that manganese phosphate layers with considerable roughness depths arise. The reason is that the Beizangriff with manganese phosphate systems from the beginning is strong and after extremely short exposure time leads to a punctiform metal removal. In contrast, the layer formation continues - compared with zinc phosphate systems - comparatively delayed. Strong pickling attack and delayed stratification are visually characterized by a strong gas evolution of longer duration, the so-called Gas time, recognizable.

The object of the invention is to provide a method that Manganese phosphate layers with the lowest possible roughness leads, whose layer thickness nevertheless lies in the middle to high range.

The object is achieved by the method of the aforementioned type according to the invention is designed such that the workpieces for the purpose of forming a manganese phosphate layer with a minimum thickness of 2.5 microns and an average maximum surface roughness (R _z ) of 2.5 microns - measured after drying - contacted with a phosphating solution at a temperature of at least 75 ° C, the 0.2 to 4 g / l iron (II) ions 10 to 25 g / l manganese ions 25 to 50 g / l phosphate ions (calc. as P ₂ O ₅ ) 3 to 35 g / l nitrate ions 0.5 to 5 g / l nitroguanidine contains 7 to 24 points of free acid, 50 to 140 points total acid and has an S value of 0.2 to 1.

The average roughness depth is defined according to DIN 4768, sheet 1 and represents the arithmetic mean of the individual roughness depths of five adjacent, equally long Einzelmeßstrecken accordingly R z = 0.2 (Z 1 + Z 2 + Z 3 + Z 4 + Z 5 ) represents.

The claimed maximum value of 2.5 μm refers only to the roughness of the Manganese phosphate layer and leaves the depth of the untreated metal surface unconsidered.

The above-mentioned total score is in a conventional manner determined by adding 10 ml of the phosphating solution after dilution with water About 50 ml using phenolphthalein as indicator until Color change from colorless to red are filtered. The number of this 0.1 ml of sodium hydroxide solution used gives the total number of points. Others for the Titration suitable indicators are thymolphthalein and ortho-cresolphthalein.

Similarly, the free acid points are determined using dimethyl yellow as an indicator and titrating to pink to yellow to pink. Previously, interfering metal ions are removed by addition of hexacyanoferrate (II) or hexacyanocobaltate (III) ions. The S value is defined as the ratio of free P ₂ O ₅ to total P ₂ O ₅ . (For details, see W. Rausch, "The phosphation of metals", Eugen G. Leuze Verlag, Stuttgart 1974, pages 273 et seq, executed)

Although it is known from GB-A-510684, manganese phosphate layers with the aid of Phosphating solutions, among many others Oxidizing agents may also contain nitroguanidine. However, from the information to the score of free acid and total acid calculable, that the Phosphating solutions significantly lower concentrations containing phosphating components and - according to the The aim of the known processes was to increase the corrosion resistance of metals improve - create layers with a very low coating weight. a contains some kind of reference to the surface roughness of the phosphate layer Patent specification not.

The studies carried out to design the present invention have shown that when using nitrate as an accelerator, due to the autocatalytic nitrite forms, or at, conventional high phosphating temperatures Use of nitrite or chlorate because of the lack of iron (II) - content Layer formation is disturbed or layers with only a very small layer weight or very small layer thickness are formed. In contrast, the Use of nitroguanidine the iron (II) concentration below certain To keep limits without an undesirable strong reduction of the for the Training a high quality layer required iron (II) - content he follows.

In support of the oxidation of iron (II) can be added to the phosphating solution oxygen-containing gas, for example compressed air, are blown. Also can Iron (II) oxidizing substances, preferably potassium permanganate added become. However, it is important to ensure that an iron (II) - concentration of 0.2 g / l under no circumstances. Otherwise, the desired coating weight will not achieved.

A preferred embodiment of the invention provides, the workpieces with a Phosphating solution containing 0.5 to 2 g / l nitroguanidine. In particular cost reasons are decisive for this.

Furthermore, it is advantageous to increase the concentration of iron (II) ions in the Adjust phosphating solution to a maximum of 2.5 g / l. This ensures that Even with difficult to phosphatierenden workpieces with security fine crystalline Layers with low roughness occur.

If workpieces with steel surfaces are to be phosphated, see one further advantageous embodiment of the invention before, the phosphating solution Complexing agent for complexing the alloy components of the steel add. Such an alloying ingredient is especially chromium. When Complexing agents are, for example, tartaric acid, but especially citric acid suitable. Due to the addition of complexing agents, the components of the Steel, which may affect the quality of the coating, intercepted.

A further advantageous embodiment of the invention is to bring the workpieces with a phosphating in contact, in addition
0.2 to 4 g / l nickel ions
or
0.2 to 4 g / l magnesium ions.

By these additions, a homogenization of the pickling attack on the treated metal surface and thus requires a stronger adhesion of the Achieved phosphate layer. In addition, the appearance of the phosphate layer is due the generally desired darkening improved. The content of Magnesium ions also reduces the consumption of the whole Chemical consumption.

Finally, it is expedient the workpieces with a phosphating in To bring into contact in order to dull the free acid at least part Manganese ions are supplemented by manganese carbonate.

The contact of the workpieces with the phosphating solution is preferably carried out at a temperature in the range of 75 to 95 ° C.

The workpieces can be mixed with the phosphating solution in any way Contact is preferred, the immersion treatment is preferred. As treatment duration are generally 1 to 15 minutes appropriate.

As a rule, it is necessary to clean the workpieces before phosphating. Acid, neutral or alkaline cleaners are used for this purpose. In general, between Thoroughly rinse with water to clean the workpieces and phosphating. Especially after treatment with alkalis and acids, the workpieces should pre-rinsed in an aqueous slurry of finely divided manganese phosphate to be in the subsequent phosphating training especially to promote uniform fine-crystalline layers.

With the aid of the process according to the invention, phosphate layers having a coating weight of generally from 5 to 30 g / m ^{2 are} achieved.

The phosphate layers produced by the invention can be known per se Painted or provided with plastic coatings. Combined with Corrosion protection oils serve to increase the rust resistance. Of the However, the main application of the method according to the invention lies in the Treatment of workpieces subjected to sliding friction. These are, for example, axles, transmission parts and pistons of Internal combustion engines and compressors.

With the aid of the method according to the invention, it is possible to use manganese phosphate layers to produce with medium to high layer thicknesses, which nevertheless only a very small have average roughness. It is about 30 to 50% lower than previously usually obtained values. Due to the low roughness depth is in workpieces, which are exposed to a sliding friction, the frictional resistance considerably reduced. The shortening of the so-called gas time to about half of the date The usual shows that the duration of the pickling attack of the phosphating solution and so that the metal removal from the workpiece is significantly reduced. It will assumed that the content of the phosphating solution of nitroguanidine to a certain passivation of the metal surface leads, but a reduced Beizangriff allows or causes an earlier onset layer formation.

The invention will be explained in more detail with reference to the following example.

Example:

Steel tappets were initially made with a strongly alkaline, aqueous Degreased cleaner in diving, then rinsed with water, then in a Pre-rinsed slurry of finely divided manganese phosphate and finally in a phosphating solution of 80 ° C for ten minutes in the dive phosphated.

The phosphating solution contained 11.8 g / l manganese, 0.5 g / l nickel, 1 g / l iron (II), 36 g / l phosphate (calculated as P ₂ O ₅ ), 4.6 g / l nitrate and 0.36 g / l citrate (calculated as citric acid).

The total number of dots of the phosphating solution was 80, the number of points of Free acid 11 (measured with 60 g of concentrate per 1 l of water). To determine the Total Acid Score and Free Acid Score is based on the above References.

For the preparation of the phosphating a concentrate, the 6.45 wt .-% manganese, 0.28 wt .-% nickel, 0.05 wt .-% iron (II), 19.8 wt .-% P ₂ O _{5 was used} , 2.5% by weight of nitrate and 0.2% by weight of citric acid, in an amount of 183 g, which was made up to one liter with demineralized water.

Fine-crystalline phosphate layers having a coating weight of 7 g / m ² , corresponding to a layer thickness of 3 to 4 μm, and an average roughness R _z of 1.3 to 2.4 μm were obtained. The gas time was 2 to 3 minutes.

In a comparative experiment was under identical conditions with the above phosphating, but no nitroguanidine, worked. Phosphate layers were formed which, while also finely crystalline, had an average roughness R _z of 5 to 6 μm. The coating weight was 6 g / m ² . The gas time was 6 to 10 minutes.

Claims (7)

1. Method for applying manganese phosphate layers to iron or steel surfaces using phosphating solutions containing manganese, phosphate or iron(II) ions as well as nitroguanidine, characterised in that in order to form a manganese phosphate layer having a minimum thickness of 2 µm and an averaged maximum roughness depth (R_z) of 2.5 µm measured after drying, the workpieces are brought into contact at a temperature of at least 75°C with a phosphating solution containing 0.2 to 4 g/l of iron(II) ions 10 to 25 g/l of manganese ions 25 to 50 g/l of phosphate ions (calc. as P₂O₅) 3 to 35 g/l of nitrate ions 0.5 to 5 g/l of nitroguanidine that has 7 to 24 points of free acid, 50 to 140 points of total acid, as well as an S value of 0.2 to 1.
2. Method according to claim 1, characterised in that the workpieces are brought into contact with a phosphating solution that contains 0.5 to 2 g/l of nitroguanidine.
3. Method according to claim 1 or 2, characterised in that the workpieces are brought into contact with a phosphating solution that contains at most 2.5 g/l of iron(II) ions.
4. Method according to claim 1, 2 or 3, characterised in that the workpieces are brought into contact with a phosphating solution that in the case of the treatment of steel contains a complex-forming agent for the alloying constituents of the steel, preferably citric acid.
5. Method according to one or more of claims 1 to 4, characterised in that the workpieces are brought into contact with a phosphating solution that additionally contains
      0.2 to 4 g/l of nickel ions
   or
      0.2 to 4 g/l of magnesium ions.
6. Method according to one or more of claims 1 to 5, characterised in that the workpieces are brought into contact with a phosphating solution in which a proportion of the manganese ions are replaced by manganese carbonate in order to neutralise the free acid.
7. Use of the method according to one or more of claims 1 to 6 for workpieces that are subjected to a sliding friction, such as axles, gear mechanism parts and engine pistons.