EP1929070A1

EP1929070A1 - Phosphatising solution with hydrogen peroxide and chelating carboxylic acids

Info

Publication number: EP1929070A1
Application number: EP06776869A
Authority: EP
Inventors: Jan-Willem Brouwer; Jerzy-Tadeusz Wawrzyniak; Jens KRÖMER; Maximilian SCHÖNHERR
Original assignee: Henkel AG and Co KGaA
Current assignee: Henkel AG and Co KGaA
Priority date: 2005-09-30
Filing date: 2006-08-16
Publication date: 2008-06-11
Also published as: CN101278075A; CN101278075B; RU2008116542A; US20090071573A1; RU2428518C2; WO2007039015A1; ZA200802778B; DE102005047424A1

Abstract

An acid, aqueous phosphatising solution containing 0.2 to 3 g/l zinc(II) ions, 3 to 50 g/l phosphate ions, calculated as PO43-, 15 to 50 mg/l hydrogen peroxide or an equivalent amount of a hydrogen peroxide-cleaving substance, 0.5 to 1.5 g/l one or more aliphatic hydroxycarboxylic acids with 3-6 C atoms, has a free acid content of maximum one point; in a process for phosphatising metal surfaces made of steel, zinc-coated steel or alloyed zinc-coated steel and/or aluminium, the metal surfaces are brought into contact with this phosphatising solution by spraying or dipping, or by a combination of the same, for a period of 3 seconds to 8 minutes.

Description

7:08:06

"Phosphating solution with hydrogen peroxide and chelating carboxylic acids"

The invention relates to a phosphating solution and a process for phosphating metal surfaces with aqueous, acidic phosphating solutions containing zinc and phosphate ions and accelerators in free or bound form, and their use as a pretreatment of the metal surfaces for subsequent painting, in particular an electrodeposition coating. The method is applicable for the treatment of steel, galvanized or alloy galvanized steel, aluminum, aluminized or alloy aluminised steel surfaces.

The purpose of phosphating metals is to produce metal-phosphate layers on the metal surface that are inherently resistant to corrosion and, in combination with paints and other organic coatings, contribute significantly to adhesion and resistance to corrosion undercutting. Such phosphating processes have long been known in the art. For the pretreatment prior to painting, in particular the low-zinc phosphating, in which the phosphating relatively low levels of zinc ions of z. B. have 0.5 to 2 g / l.

It has been shown that the co-use of polyvalent cations other than zinc in the Phosphatierbädem phosphate layers can be formed with significantly improved corrosion protection and paint adhesion properties. For example, find low-zinc process with the addition of z. B. 0.5 to 1, 5 g / l manganese ions and z. B. 0.3 to 2.0 g / l nickel ions as a so-called Trikation- method for the preparation of metal surfaces for painting, for example, for the cathodic electrocoating of car bodies, wide application. - -

Phosphating solutions usually contain so-called "accelerators" which, on the one hand, accelerate film formation by "depolarizing" them by oxidizing the elementary hydrogen formed during the pickling attack to metal, although certain accelerators, such as hydroxylamine, can also alter the shape Oxidizing accelerators also cause iron (II) ions formed in the pickling reaction on steel surfaces to be oxidized to the trivalent state, so that they precipitate as iron (III) phosphate.

From EP 414296 a process for zinc phosphating is known in which a combination of nitrate and hydrogen peroxide is used as the accelerator. The maximum peroxide concentration should be 17 mg / l. DE 4243214 describes a phosphating process based on magnesium phosphate, which should be free of those inorganic substances that are not precipitated with calcium hydroxide in the neutral or alkaline range. H ₂ O _{2 may be present} in amounts of 0.02 to 0.2 g / l as the accelerator. According to EP 866888, zinc phosphating solutions containing 0.005 to 0.5 g / l of H ₂ O ₂ together with 0.01 to 10 g / l of formate can be used.

WO 97/16581 discloses a process for phosphating steel, galvanized or alloy galvanized steel and / or aluminum or its alloys by treatment with a zinc phosphating solution by dip, spray or spray dipping, characterized in that the zinc phosphating solution has a maximum nitrate ion content of 0, 5 g / l and is free of manganese, nickel and cobalt ions and that they contain 0.3 to 2 g / l of zinc ions,

5 to 40 g / l phosphate ions and one or more of the following accelerators:

0.1 to 10 g / l of hydroxylamine in free, ionic or complexed

Shape,

0.3 to 5 g / l chlorate ions, 0.05 to 2 g / l m-nitrobenzenesulfonate ions, - -

0.05 to 2 g / l m-nitrobenzoate ion

0.05 to 2 g / l p-nitrophenol

0.005 to 0.15 g / l of hydrogen peroxide in free or bound form,

0.01 to 10 g / l of a reducing sugar.

Furthermore, this document discloses that the phos- phating solution, if it contains hydroxylamine as the sole accelerator, should preferably additionally contain one or more aliphatic hydroxycarboxylic acids having 3 to 6 C atoms in a total amount of 0.5 to 1.5 g / l. These hydroxycarboxylic acids are preferably selected from lactic acid, glycolic acid, tartronic acid, malic acid, tartaric acid and citric acid.

Addition of hydroxycarboxylic acids to phosphating solutions is also mentioned elsewhere. For example, EP 154367 describes a zinc phosphating solution which contains nitrobenzenesulfonate as an accelerator and which may further contain citrate or titrate. EP 287133 discloses a zinc phosphating solution containing as essential accelerator 5 to 30 g / l of nitrate. Preferably, it further contains 0.5 to 5 g / l of iron (II), precluding the presence of an oxidizing accelerator such as H ₂ O ₂ . This phosphating solution may additionally contain up to 3 g / l of tartaric acid or citric acid. From EP 433118 a phosphating solution is known, which contains nitrate ions, iron (II) - or iron (III) ions and at least one organic Chelatkomplexbildner. This complexing agent may be a polyhydroxycarboxylic acid such as tartaric acid or citric acid.

WO 94/13856 deals with zinc phosphating solutions, in particular for belt processes, which have a relatively high content of free acid (for definition: see below) of 2 to 6 points. These phosphating solutions contain water-soluble organic acids whose dissociation constant is between the dissociation constants of the first and second stages of phosphoric acid. By way of example, a whole series of corresponding acids are listed, including citric acid. Furthermore, the phosphating solution can be an oxidizing agent selected from nitrite, chlorate, bromate, hydroxylamine, organic aromatic nitro compounds and hydrogen peroxide or peroxide compounds. The concentration of the organic acids should be in the range of 0.008 to 0.15 mol / l, the concentration of hydrogen peroxide in the range of 0.01 to 0.1 g / l. In the embodiments, neither hydrogen peroxide nor citric acid are used.

Phosphating solutions, which according to EP 414296 have low contents of hydrogen peroxide, are difficult to control in practice, since the actual H ₂ O ₂ concentration has to be measured and adjusted very precisely. Higher concentrations of H ₂ O ₂ have the disadvantage that this self-decomposes in the heavy metal ion-containing phosphating without developing its accelerating effect. The resulting increased consumption of H ₂ O ₂ is economically disadvantageous.

Accordingly, one would prefer on the one hand higher concentrations of H ₂ O ₂ as an accelerator, since they are easier to adjust in practice than concentrations below about 20 mg / l. On the other hand, one has to prevent excessive self-decomposition of H ₂ O ₂ and guarantee a sufficiently good accelerator effect. This manifests itself in the formation of closed but finely crystalline metal phosphate layers. The layer weight of the metal phosphate layer should be, for example, on steel in the range of 1 to 3, preferably in the range of 1, 5 to 2.5 g / m ² . The present invention represents a compromise between these different requirements.

The present invention relates, in a first aspect, to an acidic, aqueous phosphating solution containing from 0.2 to 3 g / l of zinc (II) ions,

3 to 50 g / l phosphate ions, calculated as PO4

15 to 50 mg / l of hydrogen peroxide or an equivalent amount of a

Hydrogen peroxide-releasing substance, a total of 0.3 to 1, 5 g / l, preferably 0.5 to 1 g / l of one or more aliphatic chelating carboxylic acids having 2 to 7, preferably - -

with 3 to 6 carbon atoms, which has a maximum free acid content of one point.

Chelating carboxylic acids are to be understood as meaning carboxylic acids having at least two functional groups (including the carboxyl group) which have atoms with at least one free electron pair. Complexes with metal ions suitable for this purpose, in particular with transition metal cations, can be formed via the lone pairs of electrons of these functional groups. Chelate complexes are formed when at least two such functional groups of the same carboxylic acid coordinate to the same metal cation, forming an annular structure including the metal cation. Preferably, these rings, including the metal cation, have five to seven atoms.

Preferably, the aliphatic chelating carboxylic acids have at least two carboxyl groups and at least one hydroxy group which is not part of a carboxyl group. They may be selected, for example, from tartronic acid, malic acid, tartaric acid and citric acid.

Whether the carboxylic acids are present in the phosphating solution as free acids or as acid anions depends on the acid constant of the carboxylic acid and on the pH of the phosphating solution. In general, a chemical balance will be established between free carboxylic acid and carboxylic acid anions. The above concentrations are by total concentration, i. as the sum of the concentrations of the free carboxylic acid and its anions.

The usual parameters for the control of phosphating baths are known to the person skilled in the art, the contents of "free acid" and of total acid The term "free acid" is familiar to the person skilled in the phosphating art. The determination method (= definition) of the "free acid" and of the total acid chosen in this document is given in the examples section In the context of the present invention, the "free acid" content is limited to a maximum of one point. - -

Values of the free acid between about 0.3 and 1.0 points and the total acid between about 15 and about 35 points are useful in this invention.

If the phosphating solution according to the present invention has a higher "free acid" content than a maximum of one point, there is an increasing risk of rusting on freshly phosphated steel surfaces when they are slowly drying in air, which may be the case, for example, after immersion phosphating the freshly phosphated parts are transported slowly or for a long time or when the system stops while freshly phosphated parts are in the air, the restriction of the "free acid" according to the invention reduces this danger and thereby increases the operational safety.

As a favorable compromise between acceleration effect, controllability and decomposition losses, the phosphating solution preferably contains 20 to 35 mg / l of hydrogen peroxide or an equivalent amount of a hydrogen peroxide-releasing substance.

As already explained in the introduction, it is customary in the field of zinc phosphating that the phosphating solution additionally contains one or more cations which are incorporated into the crystalline phosphate layer. Accordingly, it is also preferred in the context of the present invention for the phosphating solution additionally to contain one or more of the following cations:

0.1 to 4 g / l manganese (II),

0.2 to 2.5 g / l magnesium (II) _I

0.2 to 2.5 g / l calcium (II),

0.002 to 0.2 g / l copper (II),

0.1 to 2 g / l cobalt (II),

0.1 to 2.5 g / l nickel (II). - -

In a particular embodiment thereof, the phosphating solution is low in nickel or preferably nickel-free. The positive effect of nickel ions on paint adhesion and corrosion protection is then taken over by the ecologically less questionable copper ions. This embodiment is characterized in that the phosphating solution contains 0.1 to 4 g / l of manganese (II) ions, 0.002 to 0.2 g / l of copper ions and not more than 0.05 g / l of nickel ions.

However, in the context of the present invention, it is also possible to stick to the proven "trication technology." In this embodiment, the phosphating solution contains 0.1 to 4 g / l manganese (II) ions and 0.1 to 2.5 g / l nickel ions ,

The content of zinc ions is preferably 0.4 to 2 g / l and more preferably 0.5 to 1.5 g / l.

In addition to the cations mentioned, which are incorporated into the metal phosphate layer, phosphating baths usually contain sodium, potassium and / or ammonium ions. Alkaline compounds of these cations are often added to phosphating solutions to adjust the "free acid".

The weight ratio of phosphate ions to zinc ions in the phosphating baths can vary within wide limits, provided it is in the range between 3.7 and 30. A weight ratio between 10 and 20 is particularly preferred.

In the phosphating of zinc-containing surfaces, it has proved to be beneficial to limit the nitrate content of the phosphating to a maximum of 0.5 g / l. As a result, the problem of so-called specks is suppressed and the corrosion protection improved. Phosphating baths which contain less than 0.05 g / l and in particular no nitrate are particularly preferred. In the phosphatization of steel, however, nitrate contents of up to 2 g / l can be favorable.

In the case of phosphating baths which are to be suitable for different substrates, it has become customary to calculate free and / or complexed fluoride in amounts of up to 2.5 g / l of total fluoride, of which up to 750 mg / l of free fluoride, in each case - -

F ^' , too. The presence of such amounts of fluoride is also advantageous for the phosphating baths according to the invention. In the absence of fluoride, the aluminum content of the bath should not exceed 3 mg / l. In the presence of fluoride, higher Al contents are tolerated as a result of complex formation, provided that the concentration of the non-complexed Al does not exceed 3 mg / l.

As such, hydrogen peroxide can be d. H. in free form, or else in bound form, for example as ionic peroxide or in the form of peroxo compounds such as, for example, peroxodisulfuric acid, Caro's acid or peroxo phosphoric acid. Another carrier for hydrogen peroxide in bound form is sodium perborate.

In principle, the phosphating solution at the place of use could be assembled by dissolving the individual components in water to the application concentration. However, this does not happen in practice. Rather, it is customary to provide concentrates for the first batch and for supplementing a phosphating solution. The batch concentrate is then diluted at the point of use with water to the use concentration, usually the content of free acid and / or the pH must be adjusted to the application. Free acid content ranges have already been indicated above. The pH is then usually between 2.7 and 3.6. Supplementary concentrates are used to maintain the active ingredients in a phosphating solution during operation in the desired range.

Therefore, in another aspect, the present invention also relates to an aqueous concentrate which after diluting with water by a factor of between 10 and 100 and optionally adjusting the level of free acid to a maximum of one point, the pH to a range between 2.7 and 3.6 and optionally adjusting the concentration of H ₂ O ₂ or a hydrogen peroxide-releasing substance to the desired range results in a phosphating solution described above. - -

Phosphatizing bath concentrates are generally rendered strongly acidic for reasons of stability, so that the content of free acid after dilution with water is initially significantly above the desired working range. By adding an alkaline substance such as caustic soda or sodium carbonate solution, the value of the free acid is lowered to the desired range.

A separate addition of H ₂ O ₂ or a H ₂ O ₂ - releasing substance is usually required because these accelerators are not sufficiently stable in the concentration required for a Phosphatierbad- concentrate in such a concentrate. This means that the concentrate according to the invention contains the active ingredients of the phosphating solution with the exception of H ₂ O ₂ or a H ₂ O ₂ - releasing substance.

Finally, in a further aspect, the present invention relates to a process for phosphating metal surfaces of steel, galvanized or alloy galvanized steel and / or aluminum, by spraying or dipping the metal surfaces or by a combination thereof for a time between 3 seconds and 8 Minutes with a phosphating solution described above in contact.

Here, the temperature of the phosphating solution is in the range of about 30 to about 70 and in particular from about 40 to about 60 ⁰ C. In particular, temperatures in the range of 50 to 55 ° C are set in practice.

The method according to the invention is suitable for phosphating surfaces of steel, galvanized or alloy-galvanized steel, aluminum, aluminized or alloy-aluminized steel. The materials mentioned can - as is increasingly common in the automotive industry - also exist side by side. In this case, parts of the body can also consist of pre-treated material, as it arises, for example, after the Granocoat ^ process. Here, the base material is first pretreated and then coated with a weldable coating of an organic resin. The - -

The phosphating process according to the invention then leads to phosphating at damaged areas of this pretreatment layer or on untreated backsides.

The method can be used in particular in the automotive industry, where treatment times are usually between 1 and 8 minutes. It is intended in particular for the treatment of said metal surfaces before painting, in particular before a cathodic electrodeposition coating, as is customary in the automotive industry. The phosphating process can be seen as a partial step of the technically customary pretreatment chain. In this chain, phosphating is usually preceded by the steps of cleaning / degreasing, intermediate rinsing and activating, wherein the activation is usually carried out with titanium phosphate-containing activating agents. However, the activation can also be carried out with a suspension of finely divided (<5 μm) particulate phosphates of divalent or trivalent metals in an alkali metal phosphate solution. This activation method is described for example in EP 1368508.

The phosphating according to the invention may, optionally after an intermediate rinse, be followed by a passivating after-treatment. For such a passivating aftertreatment, chromic acid-containing treatment baths were widely used. For reasons of labor and environmental protection as well as disposal reasons, however, there is a tendency to replace these chromium-containing passivation baths with chrome-free treatment baths. For this purpose, purely inorganic baths, in particular based on zirconium compounds, or organic baths, for example based on poly (vinylphenols), known. Between this post-passivation and the usually subsequent electrocoating, an intermediate rinse with demineralized water is generally carried out.

As the following experimental results show, the effectiveness of H ₂ O ₂ as an accelerator in the immersion phosphating process for steel is not sufficient. On steel, no perfect closed phosphate layers are produced. Addition of an aliphatic chelating carboxylic acid, here for example citric acid, improves the accelerator effect - -

considerably. Starting at an H ₂ O ₂ concentration of 15 mg / l, layer weights in the particularly desired range below 2.5 g / m ² are obtained. Rust formation is not observed. The addition of hydroxycarboxylic acid with its complex-forming properties thus not only stabilizes the H ₂ O ₂ , but also supports its accelerator effect.

embodiments

The phosphating methods according to the invention and comparison methods were tested on cold-rolled steel sheets, such as those used in the automotive industry. The following process, which is customary in bodywork, was carried out as a dipping process:

1. Clean with an alkaline cleaner (Ridoline ^ 1562, Henkel KGaA), approach 4% in city water, 6O ⁰ C, 5 minutes of dipping.

2. Rinse with city water, room temperature, 1 minute.

3. Activate with a titanium phosphate-containing activator (Fixodine ^ 950, Henkel KGaA), batch 0.5% in demineralized water, room temperature, 1 minute dipping.

4. Phosphate (3 minutes dipping) with phosphating baths according to Table 1. Temperature 52 ° C. In addition to the cations listed in Table 1, the phosphating baths contained only sodium ions to adjust the free acid. Free acid score is taken to mean the consumption in ml of 0.1 N sodium hydroxide to titrate 10 ml of bath solution to a pH of 3.6. Analogously, the total acid score indicates consumption in ml up to a pH of 8.5.

5. Rinse with demineralized water.

6. Drying in the air

The basis weight ("layer weight" = SG) was determined by peeling in 5 greasy chromic acid solution according to DIN 50942. - -

Table 1: Phosphating parameters and results Composition of phosphating bath:

Zn: 1, 1 g / l

Mn: 0.6 g / l

Ni: 0.8 g / l

PO ₄ ^{3 "} : 17 g / l (all phosphates and free phosphoric acid calculated as PO ₄ ^3" )

NO ₃ ^" : 0.5 g / l

SiF ₆ ^{2 "} : 1, 0 g / l

Accelerator and citric acid according to the following table

Free acidity: 0.7 points or (as a comparison) 1, 2 points

Total acidity: 25 points

H 06807 T-IS

Coating weight determination and phosphate layer assessment at different process settings:

- -

^* 'FS =' free acid '; ng = not closed. / OK = fine.

Comparative Examples 16 and 17 show that when using nitrite or hydroxylamine as accelerator, a satisfactory phosphating result is obtained even without the addition of a chelating carboxylic acid. However, if it is desired, for example for ecological reasons, to use H2O2 as the accelerator, a satisfactory phosphating result is obtained with a free acid content of at most one point only with the addition of the chelating carboxylic acid. If the content of "free acid" is increased to 1, 2 points, rust formation also occurs with a combination of citric acid / H ₂ O ₂ (compare 13 to 15).

The difference in the accelerator effect is also evident in scanning electron micrographs: Figure 1 shows scanning electron micrographs of a phosphate layer according to Comparative Example 6, Figure 2 Scanning electron micrographs of a phosphate layer according to Example 1. In the latter case, significantly smaller and more compact phosphate crystals are obtained in a closed phosphate layer ,

- -

Captions:

Illustration 1 :

Scanning electron micrographs of a phosphate layer of Comparative Example 6

Figure 2:

Scanning electron micrographs of a phosphate layer according to Example 1

Claims

- Patent claims

1. Acid, aqueous phosphating solution containing 0.2 to 3 g / l of zinc (II) ions,

3 to 50 g / l phosphate ions, calculated as PO4

15 to 50 mg / l of hydrogen peroxide or an equivalent amount of hydrogen peroxide-releasing substance,

0.3 to 1, 5 g / l of one or more aliphatic chelating carboxylic acids having 2 to 7 carbon atoms, which has a free acid content of not more than one point.

2. phosphating solution according to claim 1, characterized in that the aliphatic chelating carboxylic acid has at least two carboxyl groups and at least one hydroxyl group which is not part of a carboxyl group.

3. phosphating solution according to claim 2, characterized in that the aliphatic chelating carboxylic acid is selected from tartronic acid, malic acid, tartaric acid and citric acid.

4. phosphating according to one or more of claims 1 to 3, characterized in that it contains 20 to 35 mg / l of hydrogen peroxide or an equivalent amount of a hydrogen peroxide-releasing substance.

- -

5. phosphating solution according to one or more of claims 1 to 4, characterized in that it additionally contains one or more of the following cations:

0.1 to 4 g / l manganese (II),

0.2 to 2.5 g / l magnesium (II),

0.2 to 2.5 g / l calcium (II),

From 0.002 to 0.2 g / l of copper (II),

0.1 to 2 g / l cobalt (II),

0.1 to 2.5 g / l nickel (II).

6. phosphating solution according to claim 5, characterized in that it contains 0.1 to 4 g / l manganese ions, 0.002 to 0.2 g / l copper ions and not more than 0.05 g / l nickel ions.

7. phosphating solution according to claim 5, characterized in that it contains 0.1 to 4 g / l manganese ions and 0.1 to 2.5 g / l nickel ions.

8. phosphating according to one or more of claims 1 to 7, characterized in that it contains 0.4 to 2 g / l, preferably 0.5 to 1, 5 g / l zinc ions.

9. phosphating according to one or more of claims 1 to 8, characterized in that they additionally free and / or complex fluoride in amounts up to 2.5 g / l of total fluoride, of which up to 750 mg / l of free fluoride, each calculated as F ^" , contains.

10. Aqueous concentrate which, after dilution with water, has a factor of between 10 and 100 and where appropriate adjusts the acidity to a maximum of one point, the pH to a working range between 2.7 and 3.6 and adjusting the concentration of H ₂ O ₂ or a H ₂ O ₂ -sabspenden substance a phosphating solution after one or - -

several of claims 1 to 9 results.

11. A method of phosphating metal surfaces of steel, galvanized or alloy galvanized steel and / or aluminum, comprising spraying or dipping the metal surfaces or by a combination thereof for a time between 3 seconds and 8 minutes with a phosphating solution according to one or more of of claims 1 to 9 brings into contact.