EP0414301A1 - Process for obtaining phosphate coatings on metal surfaces - Google Patents

Abstract

In a process for the production of phosphate coatings on metal surfaces by means of aqueous zinc phosphate solutions containing iron(II) and nitrate ions, it is possible to work in the absence of waste water if the metal surfaces are brought into contact with an aqueous phosphating solution which contains   0.4 to 30 g/l of Zn,   4 to 30 g/l of P2O5,   5 to 50 g/l of NO3,   not more than 10 g/l of Fe(II) and   not more than 0.3 g/l of Fe(III),   in which the weight ratio   free P2O5 : total P2O5 is (0.04 to 0.50) : 1,   which is supplemented with Zn, NO3 and P2O5 in a weight ratio of   Zn : NO3 : P2O5 = (0.60 to 0.30) : (0.2 to 0.4) : 1 and in which the Fe(II) content is established only by oxidation with nitrate, nitrite formed therefrom, optionally together with oxygen-containing gas, H2O2 and/or nitrous gases. A rinsing bath cascade comprising at least two rinsing baths is located downstream of the phosphating bath, water which has a low salt content, preferably salt-free water, is fed into the last rinsing bath, viewed in the direction of workpiece flow, the water overflow is passed into the preceding rinsing bath in each case or into the phosphating bath, and water having a low salt content or salt-free water is removed from the phosphating bath at least in an amount such that the said phosphating bath can hold the phosphate-enriched rinsing water from the cascade.

Description

The invention relates to a wastewater-free process for producing phosphate coatings on metal surfaces by means of aqueous zinc phosphate solutions containing iron (II) and nitrate ions.

The process of producing phosphate coatings by means of aqueous zinc phosphate solutions is widely used in the metalworking industry. The phosphate layers produced on the treated metal surfaces with this process are used in particular to facilitate sliding and non-cutting cold forming, as well as for corrosion protection and as a paint primer.

Such phosphating baths usually have a pH between about 1.8 and 3.8 and contain zinc and phosphate ions as process-determining components. In addition to the cation zinc, other cations, e.g. Ammonium, calcium, cobalt, iron, potassium, copper, sodium, magnesium, manganese, may be present. To accelerate the formation of the phosphate layer, the phosphating baths are generally oxidizing agents, e.g. Bromate, chlorate, nitrate, nitrite, organic nitro compounds, perborate, persulfate, hydrogen peroxide, added. Oxygen-containing gas can also be used for the oxidation of iron (II) to iron (III). In order to optimize the layer formation on certain materials, additives such as Fluoride, silicon fluoride, boron fluoride, citrate and tartrate. Due to the large number of individual components and their possible combinations, there are a large number of different phosphating bath compositions.

The phosphating baths are usually brought into contact with the workpiece surfaces to be treated in immersion, flooding or spraying. During the contact period between a few seconds up to half an hour and more, crystalline phosphate layers are firmly integrated by chemical reaction with the metal. Since residues of the phosphating solution remaining on the surface usually interfere with further processing, the water is thoroughly rinsed after the phosphating. In order to avoid harmful accumulation of the ingredients of the phosphating bath in the rinsing baths, these are operated with an inflow of fresh water and an overflow of contaminated rinsing water. The contaminated rinsing water contains environmentally hazardous substances and therefore requires special treatment before it can be discharged into the sewage system or a drain.

Since the necessary treatment of used rinse water and its discarding is a disadvantage for the application of phosphating processes, e.g. DE-C-23 27 304 propose to apply a zinc phosphating process, the solutions of which are so composed that practically all components with Ca (OH) ₂ can be precipitated. In this way, the rinse water treatment is made considerably easier and at the same time achieved as an advantage that it can be reused for the process with good quality. The disadvantage here, however, is that the demand for feasibility limits the freedom to adapt the composition of the phosphating bath to practical requirements.

According to F. Wilhelm (Metallfläche, 33 (1979) 8, 301 to 307)), considerations have also been made to provide a cascade rinse after the zinc phosphating and to drive the water saving so far that the supplementary losses for the zinc phosphating zone are covered with the rinsing water could become. However, according to the author, this is not feasible for procedural and economic reasons.

The object of the invention is to provide a process for producing phosphate coatings on metals, in particular steel, galvanized steel, alloy-galvanized steel, aluminized steel and aluminum, with the aid of zinc phosphate solutions containing iron (II) and nitrate ions, which works free of waste water and the known ones , especially the aforementioned, does not have disadvantages.

The object is achieved by designing the method of the type mentioned at the outset in accordance with the invention in such a way that the metal surfaces are brought into contact with a phosphating solution which
0.4 to 30 g / l Zn
4 to 30 g / l P₂O₅
5 to 50 g / l NO₃
maximum 10 g / l Fe (II) and
maximum 0.3 g / l Fe (III)
contains in which the weight ratio
Free P₂O₅: total P₂O₅ = (0.04 to 0.50): 1, which is supplemented with Zn, NO₃ and P₂O₅ in a weight ratio of
Zn: NO₃: P₂O₅ = (0.80 to 0.30): (0.17 to 0.4): 1
preferably (0.60 to 0.40): (0.20 to 0.35): 1
and in which the Fe (II) content is adjusted only by oxidation with nitrate, nitrite formed therefrom, optionally together with oxygen-containing gas, H₂O₂ and / or nitrous gases, and in that the phosphating bath is followed by a rinsing bath cascade consisting of at least two rinsing baths, low in salt, preferably salt-free water in the - seen in the workpiece flow - feeds the last rinsing bath, directs the water overflow into the preceding rinsing bath or the phosphating bath and draws at least as little salt-free or salt-free water from the phosphating bath that it the rinse water enriched with phosphate can absorb from the cascade.

The term "wastewater-free" in the sense of the invention means that no water from the rinsing baths enters the sewage system or the receiving water with the intended effect of counteracting an accumulation of undesirable chemicals in the phosphating bath.

The method according to the invention is particularly suitable for the surface treatment of iron and steel, low-alloy steel, galvanized steel, alloy-galvanized, i.e. e.g. steel coated with ZnAl, ZnFe and ZnNi, aluminized steel, aluminum and its alloys.

The phosphating solutions contain Zn, P₂O₅ and NO₃ as basic components. Other cations and / or anions can also be present. Phosphating solutions in which certain concentrations of bromate, chlorate, organic nitro compounds, perborate and / or persulfate have to be maintained during the work by supplementing them are not suitable for the wastewater-free procedure according to the invention. Likewise unsuitable are those to which alkali nitrite must be added as an accelerator from time to time or continuously.

According to a preferred embodiment of the invention, the metal surfaces are brought into contact with a phosphating solution which additionally
up to 10 g / l Mg
up to 20 g / l approx
up to 20 g / l Mn
up to 20 g / l Ni
up to 10 g / l Co
up to 0.02 g / l Cu
up to 20 g / l Na and / or K and / or NH₄
up to 8 g / l SiF₆
up to 8 g / l BF₄
up to 5 g / 1 F
up to 10 g / l Cl
contains.

According to further advantageous developments of the method according to the invention, the metal surfaces should be brought into contact with a phosphating solution in which the weight ratio
(Mg + Ca + Mn + Ni + Co): Zn equal / less than 4: 1
and that of the components Mg, Ca, Mn, Fe, Ni, Co and Cu according to the molar ratio
(Mg + Ca + Mn + Fe + Ni + Co + Cu): Zn equal to / less than 2 is added.

Of the aforementioned cations which are optionally present in the phosphating baths, Fe (II) is usually not added as a chemical, but it accumulates during the throughput of iron and steel as a result of the pickling attack, provided that it has not been converted into the trivalent form by oxidizing agents and as iron ( III) -phosphate is precipitated.

Fe (III) in the baths serves, among other things, to stabilize the phosphating balance. By using Mg and / or Ca and / or Mn, phosphate coatings are obtained which, in addition to Zn and optionally Fe (II), also contain these cations. Mixed phosphates of this type are notable for increased alkali resistance and are therefore particularly suitable as a primer for paints. But they have also proven themselves as a lubricant carrier in non-cutting cold forming. Ni and / or Co are preferably used to reduce the aggressiveness of the baths Increase steel and improve the phosphating of zinc surfaces. Small amounts of copper have an accelerating effect. Alkaline cations and / or ammonium serve primarily to set the desired acid ratio. The anions F, BF₄ and SiF₆ generally increase the rate of phosphating and are advantageous for the treatment of aluminum-containing zinc surfaces. The presence of free fluoride (F⁻) is essential for the crystalline phosphating of aluminum and its alloys. Cl can be used to adjust the electronic neutrality of the baths and in special cases also to increase their aggressiveness. The thickness or weight per unit area of the phosphate layers produced can be influenced by adding, for example, polyhydroxyarboxylic acid, such as tartaric acid and / or citric acid.

The coordination of the type and amount of anions and cations in the phosphating solutions used to carry out the process according to the invention is carried out in such a way that the ratio of free P₂O₅ to total P₂O₅ (0.04 to 0.50): 1, with higher (lower ) Bath temperatures and / or concentrations in the phosphating solution each have to be chosen to be higher (lower) ratios.

In the interest of a good layer formation, the concentration of Fe (II) should not exceed that of zinc, while the sum of the concentrations of Mg + Ca + Mn + Ni + Co should not exceed four times the zinc concentration.

Since bath losses due to mechanical discharge do not occur in the method according to the invention and thus their compensating effect ceases, the correct selection of the supplement is of particular importance. For this reason, the weight ratio for Zn: NO₃: P₂O₅ at of the supplement within the narrow limits of (0.60 to 0.30): (0.2 to 0.4): 1 hold. In addition, if the addition is intended, the molar ratio of (Mg + Ca + Mn + Fe + Ni + Co + Cu): Zn should be 2: 1.

The supplement is particularly effective if, according to a further preferred embodiment of the invention, the metal surfaces are brought into contact with a phosphating solution, which by adding phosphate with a ratio of free P₂O₅ to total P₂O₅ when supplementing (- 0.4 to + 0.5): 1 is added. In the aforementioned definition of the ratio of free P to total P₂O₅ the minus sign means that there is no free P₂O₅, but rather part of the phosphate is in the secondary phosphate stage. The value minus 0.19 means, for example, that 19% of the total P₂O₅ are present as secondary phosphate.

According to another definition, the phosphate components in the supplement are in a range which is 40% secondary and 60% primary phosphate (calc. As P₂O₅ on the one hand and 50% primary phosphate and 50% free phosphoric acid (calc. As P₂O₅) on the other hand is limited.

If the ratio of free P₂O₅ to total P₂O₅ in the supplement is greater than or equal to approximately 0.2: 1, the supplementary components are usually added in the form of an acidic aqueous chemical concentrate. Since liquid supplement concentrates with a ratio of free P₂O₅ to total P₂O₅ less than 0.2: 1 are not stable, the supplement is then carried out with at least two separate concentrates. The rate of addition is expediently chosen such that the composition of the phosphating solution is at least largely constant even with fluctuating throughput, ie fluctuating consumption fades. Special portions of the necessary supplement can also be added to the bath separately from the actual supplement concentrate. An example is the addition of zinc oxide or zinc carbonate, which on the one hand increases the zinc concentration and on the other hand a correction of the ratio of free P₂O₅ to total P₂O₅ is possible.

Only NO₃, optionally together with oxygen-containing gas, H₂O₂ and / or nitrous gases are used as oxidation accelerators in the process according to the invention. In the case of baths moving autocatalytically on the nitrite side, i.e. those with a weight ratio of NO₃ to P₂O₅ of greater than 2: 1, a small amount of nitrite, about 0.05 to 0.15 g / l, for example as zinc nitrite or calcium nitrite is preferably added at the start of work. The nitrite generation from the nitrate can also be initiated by brief phosphating of zinc, zinc granules or zinc dust or by initially lower throughput density of steel. Alkali nitrite should only be used in exceptional cases when starting the bath, otherwise alkali will accumulate to a disruptive extent. Due to the absence of excess nitrite or H₂O₅ accumulates in the baths (Fe (II) when iron and steel are treated. Intensive contact of the solution with oxygen-containing gas, such as air, and / or H₂O₂ can enrich the iron via the Interference limit can be avoided.

A phosphate cascade with at least two rinse baths follows the phosphating. The principle of the rinsing bath cascade is that fresh water is only supplied to the last rinsing bath and a corresponding overflow into the previous baths is triggered. In this way, a rinse water flow manufactured, which is opposite to the flow of the workpieces. Depending on the inflow of fresh water, the fluid transfer to the workpieces, the number of rinsing baths in the cascade and the concentration of the phosphating solution, there are different impurity concentrations in the individual rinsing baths (see Table 1). Table 1 - Equilibrium concentration in cascade rinsing in 1 to 6 steps - Concentration in the bath in front of the cascade (g / l): 50 - Liquid transfer to the parts (ml / sqm): 30 - Liquid counterflow, based on part area (ml / sqm): 200 Calculated concentrations of the individual baths (g / l) bath 1 cascade 2 cascade 3-cascade 4-cascade 5-cascade 6-cascade 1 6,522 7,356 7,478 7,497 7,500 7,500 2nd --- 0.959 1,100 1,121 1,124 1,125 3rd --- --- 0.144 0.165 0.168 0.169 4th --- --- --- 0.022 0.025 0.025 5 --- --- --- --- 0.003 0.004 6 --- --- --- --- --- 0,000

In the process according to the invention, at least as much low-salt or salt-free water is withdrawn from the phosphating bath by a suitable process that it can absorb the phosphate-rich overflow from the cascade.

The characteristics of the cascade (number of stages, liquid countercurrent, liquid transfer to the parts) must be selected so that the degree of purity of the last rinsing bath is sufficient for the technical requirements of the further treatments. The effectiveness of a rinsing bath cascade can be increased even more by not overflowing directly from one bath into the previous one, but in such a way that the parts running out of the previous bath are sprayed off first and only then is it introduced into the rinsing bath itself.

Further preferred embodiments of the method according to the invention consist in extracting the salt-free or low-salt water from the phosphating bath by means of single-stage or multi-stage evaporation, reverse osmosis or electrodialysis and returning it to the rinsing bath cascade as fresh water.

A further preferred embodiment provides for the phosphate-containing rinsing water from the rinsing bath cascade to be concentrated, in particular by evaporation, electrodialysis or reverse osmosis, before they are introduced into the phosphating bath.

During the phosphating process, a bath sludge is obtained, which is separated from the system continuously or from time to time, for example by sedimentation, filtration and the like. 50 to 90% of the phosphating solution adheres to this wet sludge. According to a further preferred embodiment of the invention, which serves to reduce the consumption of chemicals and further reduce the amount of wastewater, this phosphate sludge is washed with water after it has been separated off and introduced into the rinsing bath cascade or directly into the phosphating bath.

The phosphate sludge can be washed in several stages, possibly in the manner of a cascade, with rinsing water from the individual rinsing baths.

It is particularly advantageous to wash the phosphate sludge in several stages with the water from the rinsing bath cascade and then to introduce the washing water into the rinsing bath cascade or directly into the phosphating bath.

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

example 1

Bare steel sheets were degreased by immersion in an aqueous cleaner and then rinsed with water. The samples prepared in this way were phosphated in an aqueous solution of the following composition at 90 ° C. for 10 min:
21.6 g / l P₂O₅
28.6 g / l Zn
0.028 g / l Ni
42.2 g / l NO₃
Free P₂O₅ = 7.8
Total P₂O₅ = 21.6
Free P₂O₅ / total P₂O₅ = 0.36
Score: 80

A 3-stage rinse bath cascade followed the phosphating. During the material throughput, 0.2 l / m² of treated steel surface were evaporated from the phosphating bath. In the last rinsing bath of the cascade, 0.2 l of salt-free water per m² of treated surface were added. The resulting overflow reached rinsing bath 2, rinsing bath 1 and finally the phosphating bath.

The phosphating bath was supplemented to a constant score with a concentrate of the following composition:
25% P₂O₅
6.25% NO₃
12.5% Zn
0.03% Ni
Free P₂O₅: total P₂O₅ = 0.2
Zn: NO₃: P₂O₅ = 0.5: 0.25: 1

Air was stirred into the phosphating bath during the throughput, thereby limiting the Fe (II) concentration to a maximum of 5 g / l.

• Spülbad 1: 12 Pkt.
• Spülbad 2: 1,8 Pkt.
• Spülbad 3: 0,2 Pkt.

In the steady state after larger material throughput, the following scores were obtained in the rinsing baths:

• Rinsing bath 1:12 pts.
• Rinsing bath 2: 1.8 pts.
• Rinsing bath 3: 0.2 pts.

The stationary composition of the phosphating solution was as follows:
20.5 to 23 g / l P₂O₅
22 to 24 g / l Zn
4 to 5 g / l Fe (II)
41 to 43 g / l NO₃
Free P₂O₅: total P₂O₅ = 0.32 to 0.46

The experiment shows that it is possible with the method according to the invention
to produce flawless phosphate layers,
- to keep the concentration of the phosphating solution stationary,
- to work free of contaminated rinse water and
- operate the last rinsing bath with a lower salt concentration (0.2 pt., corresponding to 0.23 g / l salt).

Example 2

Table 2 lists various phosphating bath compositions suitable for carrying out the process according to the invention and supplementary concentrates suitable therefor.

Claims (10)

1. Waste water-free process for the production of phosphate coatings on metal surfaces by means of aqueous iron (II) and nitrate ions containing zinc phosphate solutions, characterized in that the metal surfaces are brought into contact with a phosphating solution which
0.4 to 30 g / l Zn
4 to 30 g / l P₂O₅
5 to 50 g / l NO₃
maximum 10 g / l Fe (II) and
maximum 0.3 g / l Fe (III)
contains in which the weight ratio
Free P₂O₅: total P₂O₅ = (0.04 to 0.50): 1
is, which is supplemented with Zn, NO₃ and P₂O₅ in the weight ratio of
Zn: NO₃: P₂O₅ = (0.80 to 0.30): (0.17 to 0.4): 1
preferably (0.60 to 0.40): (0.20 to 0.35): 1
and in which the Fe (II) content is adjusted only by oxidation with nitrate, nitrite formed therefrom, optionally together with oxygen-containing gas, H₂O₂ and / or nitrous gases, and in that the phosphating bath is followed by a rinsing bath cascade consisting of at least two rinsing baths, low in salt, preferably salt-free water in the - seen in the workpiece flow - feeds the last rinsing bath, directs the water overflow into the preceding rinsing bath or the phosphating bath and removes at least as little salt-free or salt-free water from the phosphating bath that it absorbs the rinsing water enriched with phosphate from the cascade can. 2. The method according to claim 1, characterized in that the metal surfaces are brought into contact with a phosphating solution, the additional
up to 10 g / l Mg
up to 20 g / l approx
up to 20 g / l Mn
up to 20 g / l Ni
up to 10 g / l Co
up to 0.02 g / l Cu
up to 20 g / l Na and / or K and / or NH₄
up to 8 g / l SiF₆
up to 8 g / l BF₄
up to 5 g / l F
up to 10 g / l Cl
contains. 3. The method according to claim 1 or 2, characterized in that the metal surfaces are brought into contact with a phosphating solution in which the ratio of
FeII: Zn equal / less than 1: 1
and that of
(Mg + Ca + Mn + Ni + Co): Zn equal / less than 4: 1
is. 4. The method according to claim 1, 2 or 3, characterized in that the metal surfaces are brought into contact with a phosphating solution which, in terms of the components Mg, Ca, Mn, Ni, Fe, Co and / or Cu, according to the molar ratio
(Mg + Ca + Mn + Fe + Ni + Co + Cu): Zn equal / less than 2: 1
is added. 5. The method according to one or more of claims 1 to 4, characterized in that the metal surfaces are brought into contact with a phosphating solution which by adding phosphate with a ratio of free P₂O₅ to total P₂O₅ in the addition of (- 0, 4 to + 0.5): 1 is added. 6. The method according to one or more of claims 1 to 5, characterized in that one removes the low-salt or salt-free water obtained from the phosphating bath by single or multi-stage evaporation, by reverse osmosis and / or by electrodialysis. 7. The method according to one or more of claims 1 to 6, characterized in that the low-salt or salt-free water obtained from the phosphating bath is fed to the rinsing bath cascade as fresh water. 8. The method according to one or more of claims 1 to 7, characterized in that the active substances for the phosphating of the rinsing bath cascade are concentrated, in particular by evaporation, electrodialysis or reverse osmosis, before they are introduced into the phosphating bath. 9. The method according to one or more claims 1 to 8, characterized in that the sludge obtained in the phosphating bath is washed with water after its separation and the washing water is introduced into the rinsing bath cascade or directly into the phosphating bath. 10. The method according to claim 9, characterized in that the phosphate sludge is washed in multiple stages with water from the rinsing bath cascade.