Patent Application
20090071573
The invention relates to a phosphating solution and a process for the phosphating of metallic surfaces with aqueous, acidic phosphating solutions that comprise zinc ions and phosphate ions as well as accelerators in free or bonded states, as well as their application as a pre-treatment of metal surfaces for subsequent coating, in particular an electro deposition. The process may be used to treat surfaces made from steel, galvanized or alloy-galvanized steel, aluminum, aluminized or alloy-aluminized steel.
The object of phosphating metals is to produce on the metal surface strongly adhering metal phosphate layers which in themselves improve corrosion resistance and, in conjunction with lacquers and other organic coatings, contribute towards a substantial increase in lacquer adhesion and resistance to corrosive delamination. Such phosphating processes have been known for a long time. Low-zinc phosphating processes, in which the phosphating solutions have relatively low contents of zinc ions of 0.5 to 2 g/l, are particularly suitable for pre-treatment prior to lacquer coating.
It has been found that phosphate layers having distinctly improved corrosion protection and lacquer adhesion properties may be formed by also using polyvalent cations other than zinc in the phosphating baths. For example, low-zinc processes with the addition of, for example, 0.5 to 1.5 g/l of manganese ions and, for example, 0.3 to 2.0 g/l of nickel ions are widely used as the so-called tri-cation process for preparing metal surfaces for lacquer coating, for example for cathodic electrocoating of automotive bodywork.
Generally, phosphating solutions comprise accelerators. They accelerate the layer formation, since they have a “depolarizing” effect in that they oxidize the elementary hydrogen that results from the pickling reaction to form water. However, certain accelerators, such as for example hydroxylamine, can also influence the form of the resulting phosphate crystals. Oxidizing accelerators also have the effect of oxidizing iron (II) ions resulting from the pickling reaction on steel surfaces to the trivalent state, so that they precipitate out as iron (III) phosphate.
A process for zinc phosphating is known from EP 414296, in which a combination of nitrate and hydrogen peroxide is employed 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 cannot be precipitated with calcium hydroxide in the neutral or alkaline range. In this case, H2O2 in amounts of 0.02 to 0.2 g/l, can be comprised as the accelerator. According to EP 866888, zinc phosphate solutions that comprise 0.005 to 0.5 g/l H2O2 together with 0.01 to 10 g/l formate find use.
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 in dip, spray or spray-dip processes, wherein the zinc phosphating solution exhibits a maximum nitrate ion content of 0.5 g/l and is free of manganese-, nickel- and cobalt ions and that it comprises:
0.3 to 2 g/l zinc ions,
5 to 40 g/l phosphate ions
as well as one or more of the following accelerators:
0.1 to 10 g/l hydroxylamine in free, ionic or complexed form,
0.3 to 5 g/l chlorate ions,
0.05 to 2 g/l m-nitrobenzene sulfonate ions,
0.05 to 2 g/l m-nitrobenzoate ions,
0.05 to 2 g/l p-nitrophenol
0.005 to 0.15 g/l hydrogen peroxide in free or bound form,
0.01 to 10 g/l of a reducing sugar.
This document further discloses that the phosphating solution, when it comprises hydroxylamine as the sole accelerator, should then preferably additionally comprise one or more aliphatic hydroxycarboxylic acids containing 3 to 6 carbon 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.
An admixture of hydroxycarboxylic acids to phosphating solutions is also mentioned in other locations. For example, EP 154367 describes a zinc phosphating solution that comprises nitrobenzene sulfonate as the accelerator and that can additionally comprise citrate or tartrate. EP 287133 discloses a zinc phosphating solution that comprises 5 to 30 g/l nitrate as the essential accelerator. Preferably, it further comprises 0.5 to 5 g/l iron (II), thereby excluding the presence of an oxidizing accelerator like H2O2. This phosphating solution can further comprise up to 3 g/l tartaric acid or citric acid. A phosphating solution is known from EP 433118 which comprises nitrate ions, iron (II)- or iron (III) ions as well as at least one organic chelating agent. This chelating agent can be a polyhydroxycarboxylic acid, such as for example tartaric acid or citric acid.
The subject matter of WO 94/13856 is zinc phosphating solutions, particularly for strip processes, which exhibit a relatively high content of free acid (for the definition: see below) of 2 to 6 points. These phosphating solutions comprise water-soluble organic acids, whose dissociation constant lies between the dissociation constants of the first and second step of phosphoric acid. By way of example, a whole range of suitable acids are mentioned, among others citric acid. In addition, the phosphating solution can comprise an oxidizing agent selected from nitrite, chlorate, bromate, hydroxylamine, organic aromatic nitro compounds as well as hydrogen peroxide or peroxy compounds. The concentration of the organic acids should be in the range 0.008 to 0.15 mol/l, the concentration of hydrogen peroxide in the range 0.01 to 0.1 g/l. Neither hydrogen peroxide nor citric acid was used in the examples.
Phosphating solutions that in accordance with EP 414296 exhibit minor contents of hydrogen peroxide are difficult to control in practice, as the actual H2O2 concentration has to be very exactly measured and adjusted. High concentrations of H2O2 have the disadvantage that the H2O2 undergoes self-decomposition in the heavy metal ion-containing phosphating solutions, without developing its accelerating action. The thus increased consumption of H2O2 is economically disadvantageous.
Therefore, on the one hand, higher concentrations of H2O2 as the accelerator are preferred, as in practice these are easier to adjust than concentrations below about 20 mg/l. On the other hand however, too great a self-decomposition of H2O2 must be prevented and a sufficiently good acceleration action guaranteed. This is evidenced in the formation of complete, but fine crystalline metal phosphate layers. The layer weight of the metal phosphate layer on steel, for example, should be in the range 1 to 3, preferably in the range 1.5 to 2.5 g/m2. The present invention constitutes a compromise between these different requirements.
In a first aspect, the present invention relates to an acidic, aqueous phosphating solution, comprising
Chelating carboxylic acids are understood to mean carboxylic acids with at least two functional groups (including the carboxyl groups) that possess atoms with at least one free electron pair. Complexes with suitable metal ions, particularly transition metal cations, can be formed through the electron pairs of these functional groups. Chelate complexes result if at least two such functional groups of the same carboxylic acid coordinate the same metal cation, such that a cyclic structure is formed that incorporates the metal cation. Preferably, these rings possess five to seven atoms, including the metal cation.
The aliphatic chelating carboxylic acids preferably possess at least two carboxyl groups as well as at least one hydroxyl group that is not part of a carboxyl group. They can be selected from tartronic acid, malic acid, tartaric acid and citric acid, for example.
Whether the carboxylic acids in the phosphating solution exist as free acids or as acid anions depends on the acid constant of the particular carboxylic acid and on the pH of the phosphating solution. Generally, a chemical equilibrium between free carboxylic acid and carboxylic acid anions will be reached. The abovementioned concentration data are to be understood as the total concentration, i.e. as the sum of the concentrations of the free carboxylic acids and their anions.
Typical parameters for controlling phosphating baths known to the person skilled in the art are the free acid and total acid contents. The term “free acid” is commonly used by the person skilled in the field of phosphating. The method of determination (=definition) chosen in this paper for “free acid” as well as for total acid is given in the examples. In the context of the present invention, the free acid content is limited to a maximum value of one point. Values of free acid between about 0.3 and 1 point and of total acid between about 15 and about 35 points are suitable in the context of this invention.
When the otherwise inventive phosphating solution has a higher free acid content than one point, there is an increasing danger of rust formation on freshly phosphated steel surfaces when these are slowly dried in air. This can be the case for example if after a dip-phosphating the freshly phosphated parts are slowly dried or further transported for a long time or if the unit stops while freshly phosphated parts stand in air. The inventive limitation of the free acid reduces this danger and thereby significantly increases operating reliability.
The phosphating solution preferably comprises 20 to 25 mg/l hydrogen peroxide or an equivalent amount of a substance that splits off hydrogen peroxide as a favorable compromise between acceleration, controllability and decomposition losses. An equivalent amount will be understood by those of skill in the art to mean the amount of a substance that is the source of the hydrogen peroxide that provides the desired amount of free form H2O2.
As already mentioned in the introduction, it is usual in the field of zinc phosphating that the phosphating solution further comprises one or more cations that are incorporated into the crystalline phosphate layer. Accordingly, it is also preferred in the context of the invention that the phosphating solution additionally comprises 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),
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 of this, the phosphating solution is poor in nickel or free of nickel. The positive action of the nickel ions on the paint adhesion and corrosion protection is then assumed by the ecologically less risky copper ions. This embodiment is characterized in that the phosphating solution comprises 0.1 to 4 g/l manganese (II) ions, 0.002 to 0.2 g/l copper ions and not more than 0.05 g/l nickel ions.
However, in the context of the present invention, one can also stay with the mature “tri-cation technology”. In this embodiment, the phosphating solution comprises 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 particularly 0.5 to 1.5 g/l.
Apart from the cited cations that are incorporated into the metal phosphate layer, phosphating baths generally comprise sodium-, potassium- and/or ammonium ions. Alkaline reacting compounds of these cations are frequently added to the phosphating solution to adjust the “free acid”.
The weight ratio of phosphate ions to zinc ions in the phosphating baths can vary widely in so far as it is the range between 3.7 and 30. A weight ratio between 10 and 20 is particularly preferred.
For phosphating zinc-containing surfaces, it has proven favorable to limit the nitrate content of the phosphating baths to maximum 0.5 g/l. In this way the problem of “speckling” is repressed and the corrosion protection improved. Phosphating baths for zinc surfaces that comprise less than 0.05 g/l and especially no nitrate are then particularly preferred. However, for phosphating steel, nitrate contents of up to 2 g/l can be advantageous.
In phosphating baths, which are intended to be suitable for different substrates, one may add free and/or complexed fluoride in quantities of up to 2.5 g/l total fluoride, up to 750 mg/l of which as free fluoride, each calculated as F−. The presence of such quantities of fluoride is advantageous for the inventive phosphating baths. In the absence of fluoride, the aluminum content of the bath should not exceed 3 mg/l. In the presence of fluoride, thanks to complexation, higher Al contents may be tolerated, provided that the concentration of non-complexed Al does not exceed 3 mg/l.
It is possible to add the hydrogen peroxide as such i.e. in free form or also in bound form, for example as ionic peroxide or in the form of peroxy compounds, such as for example peroxydisulfuric acid, Caro's acid or also peroxyphosphoric acid. Sodium perborate also is a further carrier for hydrogen peroxide in bound form.
In principle, the phosphating solution could be made up at the point of use by dissolving the individual components in water to the application concentrations. However, in practice this rarely occurs. It is much more usual to provide concentrates for initial use and for the replenishment of a phosphating solution. The make-up concentrate is then diluted at the point of use with water to the application concentration, wherein the content of free acid and/or the pH generally have to be adjusted to the application range. Ranges for the free acid content have already been given above. The pH is then generally between 2.7 and 3.6. The replenishment concentrates are used in order to keep the active substances in a phosphating solution in the prescribed range during operation.
Accordingly, a further aspect of the present invention also relates to an aqueous concentrate that after dilution with water by a factor between 10 and 100 and adjustment if needed of the free acid content to a value of maximum one point, adjustment of the pH to a working range between 2.7 and 3.6 as well as adjustment if needed of the concentration of H2O2 or of a substance that splits off hydrogen peroxide to the prescribed range, results in an above described phosphating solution.
Phosphating bath concentrates are generally adjusted to be strongly acidic on the grounds of stability, such that the free acid content after dilution with water is initially significantly above the desired working range. The value of free acid is lowered to the required range by adding an alkaline substance such as, for example caustic soda or a sodium carbonate solution.
A separate addition of H2O2 or a substance that splits off H2O2, as sources of H2O2, is generally required, as these accelerators are not sufficiently stable in concentrated form in the amounts required for a phosphating bath concentrate. This means that the inventive concentrate comprises the active principles of the phosphating solution except for H2O2 or a substance that splits off H2O2.
Finally, a further aspect of the present invention relates to a process for the phosphating of metal surfaces made of steel, galvanized or alloy galvanized steel and/or of aluminum, in which the metal surfaces are brought into contact with an above described phosphating solution by spraying or dipping or by a combination thereof for a period between 3 seconds and 8 minutes.
For this 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° C. In practice, the temperatures are especially adjusted to the range 50 to 55° C.
The inventive process is suitable for phosphating surfaces made of steel, galvanized or alloy-galvanized steel, aluminum, aluminized or alloy-aluminized steel. The cited materials—as is increasingly common in the automotive construction industry—can also be present side by side. Here, bodywork parts can also consist of already pre-treated material, as is the case for example in the Granocoat® process. In this connection, the base material is first pre-treated and then coated with a weldable coating of an organic resin. The inventive phosphating process then leads to a phosphating of damaged spots of this pre-treatment layer or of the untreated reverse sides.
The process can be employed particularly in the automotive construction industry, where treatment times between 1 and 8 minutes are typical. It has been conceived for the treatment of the cited metal surfaces prior to lacquering, especially before a cathodic electrodepositioning, as is typical in the automotive construction industry. The phosphating process should be regarded as a partial step of the industrially conventional pre-treatment chain. In this chain, the steps cleaning/degreasing, intermediate rinsing and activation are usually upstream of the phosphating, wherein the activation usually occurs with titanium phosphate-containing activators. However, the activation can also be effected with a suspension of finely divided (<5 μm) particulate phosphates of divalent or trivalent metals in an alkali metal phosphate solution. This activation process is described, for example in EP 1368508.
The inventive phosphating can be followed, optionally after an intermediate rinse, by a post-passivation treatment. Treatment baths containing chromium salts were widely used for this purpose. However, for reasons of occupational hygiene and environmental protection as well as for disposal, there is the tendency to replace these chromium-containing passivation baths with chromium-free treatment baths. For this, purely inorganic baths, in particular based on zirconium compounds, or also organic baths, for example based on polyvinylphenols, are known. Generally, an intermediate rinse with totally deionized water is carried out between this post-passivation and the typically subsequent electro deposition coating.
As the following experimental results demonstrate, the efficiency of H2O2 as the accelerator in the dip phosphating process is inadequate for steel. Flawless, complete phosphate layers are not produced on steel. The acceleration is considerably improved by the addition of an aliphatic chelate-forming carboxylic acid, here, for example, citric acid. Layer weights in the particularly desired range below 2.5 g/m2 are obtained with an H2O2 concentration of 15 mg/l and above. Rust formation is not observed. The addition of the hydroxycarboxylic acid with its complexing characteristics therefore stabilizes not only the H2O2 but also simultaneously supports the acceleration activity of the peroxide.
The inventive phosphating processes as well as the comparative processes were tested on cold-rolled steel sheet as used in the automotive construction industry. To this end, the following dipping procedure, as is conventional in vehicle body production, was performed:
Composition of the phosphating bath was as follows:
Accelerators and citric acid according to Table 1.
Determination of layer weight and assessment of the phosphate layer for various process adjustments was made. The mass on the surface (“layer weight”=LW) was determined according to DIN 50942 by dissolution in a 5% conc. chromic acid solution.
Comparative examples 16 and 17 demonstrate that an adequate phosphating result is obtained by the use of nitrite or hydroxylamine as the accelerator even without the addition of a chelating carboxylic acid. However, if one wants to use H2O2 as the accelerator, for example from ecological grounds, then at a free acid content of maximum one point, an adequate phosphating result is obtained only with the addition of the chelating carboxylic acid. On increasing the free acid content to 1.2 points, then rust formation occurs even with a combination of citric acid/H2O2 (see exp. 13 to 15).
The difference in acceleration is clearly seen by comparing
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2005 047 424.1 | Sep 2005 | DE | national |
This application is a continuation under 35 USC Sections 365(c) and 120 of International Application No. PCT/EP2006/008063, filed Aug. 16, 2006 and published as WO2007/039015, which claims priority from German Application No. 10 2005 047 424.1 filed Sep. 30, 2005, which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2006/008063 | Aug 2006 | US |
| Child | 12055984 | US |
