ZINC PHOSPHATING WITH INTEGRATED SUBSEQUENT PASSIVATION

[0001] This invention relates to processes for the phosphatizing of metal surfaces using aqueous, acid phosphate solutions containing zinc ions, manganese ions, phosphate ions and up to 0.5 g/l of organic polymers. This invention also relates to the use of such processes as pre-treatment of metal surfaces for a subsequent coating, in particular an electrocoating or a powder coating. The process may be used for the treatment of surfaces of steel, zinc-coated steel or steel coated with zinc alloy, of aluminum, aluminum-magnesium alloys, aluminized steel or steel coated with aluminum alloy, and avoids the passivating rinse required hitherto.

[0002] The phosphatizing of metals seeks to produce on the metal surfaces firmly adhering metal phosphate layers, which alone already improve the resistance to corrosion and in combination with paints or other organic coatings contribute to a considerable increase in the paint adhesion and in the resistance to loss of the paint during corrosive stress. Such phosphatizing processes have been known for a long time. The low-zinc phosphatizing processes, wherein the phosphatizing solutions have comparatively low contents of zinc ions of, for example, from 0.5 to 2 g/l, are particularly suitable for pretreatment prior to coating, in particular electrocoating. An important factor in these low-zinc phosphatizing baths is the weight ratio of phosphate ions to zinc ions, which is generally above 8 and may be up to 30.

[0003] It has become apparent that phosphate layers having distinctly improved properties of corrosion protection and paint adhesion may be formed by the concomitant use of other polyvalent cations in the zinc phosphatizing baths. By way of example, low-zinc processes with an addition of, for example, from 0.5 to 1.5 g/l of manganese ions and, for example, from 0.3 to 2.0 g/l of nickel ions find wide application as so-called trication processes for the pretreatment of metal surfaces for coating, for example, for the cathodic electrocoating of car bodywork.

[0004] As nickel and the alternatively used cobalt are classified as critical from the aspects both of toxicology and of waste water technology, there is a need for phosphatizing processes which have a level of performance similar to that of the trication processes, but which function with considerably lower concentrations of nickel and/or cobalt in the baths and preferably without these two metals.

[0005] From DE-A 20 49 350 a phosphatizing solution is known which contains as essential constituents from 3 to 20 g/l of phosphate ions, from 0.5 to 3 g/l of zinc ions, from 0.003 to 0.7 g/l of cobalt ions or from 0.003 to 0.04 g/l of copper ions or preferably from 0.05 to 3 g/l of nickel ions, from 1 to 8 g/l of magnesium ions, from 0.01 to 0.25 g/l of nitrite ions and from 0.1 to 3 g/l of fluoride ions and/or from 2 to 30 g/l of chloride ions. Hence, this process is a zinc-magnesium phosphatizing, with the phosphatizing solution containing in addition the ions of one of the metals cobalt, copper or, preferably, nickel. Such a zinc-magnesium phosphatizing has not succeeded in gaining technical acceptance.

[0006] EP-B 18 841 describes a zinc phosphatizing solution accelerated by chlorate/nitrate, containing inter alia from 0.4 to 1 g/l of zinc ions, from 5 to 40 g/l of phosphate ions, as well as, optionally, at least 0.2 g/l, preferably from 0.2 to 2 g/l, of one or more ions selected from nickel, cobalt, calcium and manganese. The optional manganese, nickel or cobalt content is therefore at least 0.2 g/l. Nickel contents of 0.53 g/l and 1.33 g/l are given in the Examples.

[0007] EP-A 459 541 describes phosphatizing solutions which are substantially free from nickel and which contain, in addition to zinc and phosphate, from 0.2 to 4 g/l of manganese and from 1 to 30 mg/l of copper. DE-A 42 10 513 discloses nickel-free phosphatizing solutions which contain in addition to zinc and phosphate, from 0.5 to 25 mg/l of copper ions and, as an accelerator, hydroxylamine. These phosphatizing solutions optionally-contain-in addition-from 0.15 to 5 g/l of manganese.

[0008] German Patent Application DE 196 06 017.6 describes a phosphatizing solution reduced in heavy metals, which contains from 0.2 to 3 g/l of zinc ions, from 1 to 150 mg/l of manganese ions and from 1 to 30 mg/l of copper ions. This phosphatizing solution may optionally contain up to 50 mg/l of nickel ions and up to 100 mg/l of cobalt ions. Lithium ions in quantities of between 0.2 and 1.5 g/l are another optional constituent.

[0009] German Patent Application DE 195 38 778.3 describes the control of the layer weight in phosphate layers by the use of hydroxylamine as accelerator. The use of hydroxylamine and/or derivatives thereof to influence the shape of the phosphate crystals is known from a number of published patents. EP-A 315 059 mentions, as a particular effect of the use of hydroxylamine in phosphatizing baths, the fact that, on steel, the phosphate crystals still form in a desired columnar or nodular shape if the zinc concentration in the phosphatizing bath exceeds the range conventional for low-zinc processes. Hence it becomes possible to operate the phosphatizing baths at zinc concentrations of up to 2 g/l and using weight ratios of phosphate to zinc as low as 3.7. More details regarding advantageous combinations of cations in these phosphatizing baths are not provided, but nickel is used in all cases in the Examples given in the patent. Nitrates and nitric acid are also used in the Examples, although the description advises against the presence of nitrate in larger quantities. The required hydroxylamine concentration is given as 0.5 to 50 g/l, preferably 1 to 10 g/l. The maximum concentration of hydroxylammonium sulfate in the Examples is 5 g/l, from which the calculated hydroxylamine content is 2.08 g/l. (Hydroxylammonium sulfate contains 41.5 wt. % of hydroxylamine.) The phosphatizing solution is applied to the steel surfaces by spraying. The document does not mention the problems involved in a dipping process, which lead to phosphate layers having distinctly higher layer weights, which are undesirable as a foundation for a subsequent coating.

[0010] WO 93/03198 discloses the use of hydroxylamine as accelerator in tricationphosphatizing baths having zinc contents of between 0.5 and 2 g/l and nickel and manganese contents each of from 0.2 to 1.5 g/l, with definite weight ratios between zinc and the other divalent cations also having to be maintained. These baths also contain from 1 to 2.5 g/l of a “hydroxylamine accelerator”, which according to the description means salts of hydroxylamine, preferably hydroxylamine sulfate. If this stated quantity is calculated as free hydroxylamine, then hydroxylamine contents of between 0.42 and 1.04 g/l are provided.

[0011] As a rule, to improve the corrosion protection produced by the phosphate layer, a so-called “passivating rinse”, also termed post-passivation, is carried out in practice. Treatment baths containing chromic acid are still widely used for this purpose. For reasons of industrial safety and environmental protection there is a tendency to replace these chromium-containing passivating baths by chromium-free treatment baths. To this end, for example, organic reactive bath solutions containing complexing substituted poly(vinylphenols) are known. Such compounds are described, for example, in DE-C 31 46 265. Particularly effective polymers of this type contain amine substituents and may be obtained by a Mannich reaction of poly(vinylphenols) with aldehydes and organic amines. Such polymers are described, for example, in EP-B 91 166, EP-B 319 016 and EP-B 319 017. Polymers of this type are also used for the present purposes and so these four disclosures are incorporated herein by reference. The passivating rinse solutions may also contain polymers having amino groups, the amino group being joined directly to the polymer chain without any intervening aromatic ring. Polymers of this type, which may likewise be used according to the present invention, are described in DE-A 44 09 306.

[0012] In combination with a passivating rinse, the low-zinc phosphatizing baths used at present meet the corrosion protection standards set for automobile manufacture. This order of procedure has the disadvantage, however, that the passivating rinse is a separate treatment step which prolongs the production time and increases the space requirement of the pre-treatment line.

[0013] An object of the present invention is to provide a phosphatizing solution which satisfies the corrosion protection standards in the automobile industry and in the case of which the passivating rinse may be omitted. Hence the space requirement of the pretreatment line is decreased and the production time may be shortened.

[0014] The addition of polyacrylic acids to phosphatizing solutions is already known from the literature. An example which may be mentioned is the article by J. I. Wragg, J. E. Chamberlain, L. Chann, H. W. White, T. Sugama and S. Manalis: “Characterization of Polyacrylic Acid Modified Zinc Phosphate Crystal Conversion Coatings”, Journal of Applied Polymer Science, Vol. 50, 917-928 (1993). Here, however, model zinc phosphatizing solutions which were investigated clearly differ from those currently used in practice. They have higher contents of zinc; moreover, the widely used manganese and the accelerators generally used at present are absent. They are not, therefore, a model for the low-zinc phosphatizing solutions used according to the present invention.

[0015] The above object is fulfilled by a process for the phosphatizing of metal surfaces composed of steel, zinc-coated steel or steel coated with zinc alloy, and/or of aluminum, wherein the metal surfaces, by means of spraying or dipping for a period of between 3 seconds and 8 minutes, are contacted with a zinc-containing phosphatizing solution, characterized in that the phosphatizing solution contains:

[0016] 0.2 to 3 g/l of zinc ions;

[0017] 3 to 50 g/l of phosphate ions, calculated as PO4;

[0018] 0.001 to 4 g/l of manganese ions;

[0019] 0.001 to 0.5 g/l of one or more polymers selected from polyethers, polycarboxylates, polymeric phosphonic acids, polymeric phosphinocarboxylic acids and nitrogen-containing organic polymers;

[0020] and

[0021] one or more accelerators selected from:

[0022] 0.3 to 4 g/l of chlorate ions;

[0023] 0.01 to 0.2 g/l of nitrite ions;

[0024] 0.05 to 2 g/l of m-nitrobenzene-sulfonate ions;

[0025] 0.05 to 2 g/l of m-nitrobenzoate ions;

[0026] 0.05 to 2 g/l of p-nitrophenol;

[0027] 0.005 to 0.15 g/l of hydrogen peroxide in free or bound form;

[0028] 0.1 to 10 g/l of hydroxylamine in free or bound form;

[0029] 0.1 to 10 g/l of a reducing sugar.

[0030] The zinc concentration is preferably between about 0.3 and about 2 g/l, particularly between about 0.8 and about 1.6 g/l. Zinc contents above 1.6 g/l, for example, between 2 and 3 g/l, are of only slight advantage to the process and on the other hand, may increase the amount of sludge produced in the phosphatizing bath. Such zinc contents may be established in an operating phosphatizing bath if, during the phosphatizing of zinc-coated surfaces, additional zinc enters the phosphatizing bath as a result of corrosion by acid. Nickel ions and/or cobalt ions within the concentration range of about 1 to about 50 mg/l for nickel and about 5 to about 100 mg/l for cobalt, in combination with as low as possible a nitrate content, of no more than about 0.5 g/l, improve corrosion protection and paint adhesion compared with that of phosphatizing baths containing no nickel or cobalt or having a nitrate content of more than 0.5 g/l. Hence a favorable compromise is reached between the performance of the phosphatizing baths, on the one hand, and the requirements of waste water technology regarding the treatment of the rinse water, on the other.

[0031] In phosphatizing baths reduced in heavy metals, the manganese content may be within the range of about 0.001 to about 0.2 g/l. Otherwise, manganese contents of about 0.5 to about 1.5 g/l are usual.

[0032] From German Patent Application 195 00 927.4, it is known that lithium ions within the quantitative range of about 0.2 to about 1.5 g/l improve the corrosion protection attainable using zinc phosphatizing baths. Lithium contents within the quantitative range of 0.2 up to about 1.5 g/l, particularly of about 0.4 to about 1 g/l, also have a favorable effect on the corrosion protection attained in the phosphatizing process with integrated post-passivation according to the present invention. In addition to the cations mentioned above, which become incorporated into the phosphate layer or at least have a favorable effect on the crystal growth of the phosphate layer, the phosphatizing baths generally contain sodium ions, potassium ions and/or ammonium ions for the adjustment of the free acid. The concept of free acid is familiar to the person skilled in the art of phosphatizing. The method chosen in this document for determining the free acid and the total acid is given in the Examples. Free acid and total acid are important control variables for phosphatizing baths, as they have a large influence on the layer weight. Values for the free acid of between 0 and 1.5 points in phosphatizing of parts and, in the case of continuous phosphatizing, up to 2.5 points, and values for the total acid of between about 15 and about 30 points are within the usual technical range and are suitable in accordance with the present invention.

[0033] In phosphatizing baths which are to be suitable for different substrates it has become conventional to add free fluoride and/or fluoride bound in complex compounds in quantities of up to 2.5 g/l of total fluoride, up to 1 g/l thereof being free fluoride. The presence of these quantities of fluoride is also of advantage in the phosphatizing baths according to the present invention. In the absence of fluoride, the aluminum content of the bath is not to exceed 3 mg/l. In the presence of fluoride, owing to the formation of complexes, higher Al contents are tolerated, provided that the concentration of the uncomplexed Al does not exceed 3 mg/l. The use of fluoride-containing baths is therefore advantageous when the surfaces being phosphated consist at least partly of aluminum or contain aluminum. In these cases, it is beneficial not to use fluoride bound in complex compounds, but to use only free fluoride, preferably in concentrations of 0.5 to 1.0 g/l.

[0034] For the phosphatizing of zinc surfaces it is not absolutely necessary that the phosphatizing baths contain so-called accelerators. For the phosphatizing of steel surfaces, however, it is necessary that the phosphatizing solution contain one or more accelerators. These accelerators are common in the prior art as components of zinc phosphatizing baths. They are understood as including substances which, by being reduced themselves, chemically bind the hydrogen formed as a result of the attack by the acid on the metal surface. Oxidizing accelerators also have the effect of oxidizing to the trivalent state the iron(II) ions released by corrosive attack on steel surfaces, so that they may be precipitated as iron(III) phosphate.

[0035] The phosphatizing baths according to the present invention may contain as accelerators one or more of the following components:

[0036] 0.3 to 4 g/l of chlorate ions;

[0037] 0.01 to 0.2 g/l of nitrite ions;

[0038] 0.05 to 2 g/l of m-nitrobenzene-sulfonate ions;

[0039] 0.05 to 2 g/l of m-nitrobenzoate ions;

[0040] 0.05 to 2 g/l of p-nitrophenol;

[0041] 0.005 to 0.15 g/l of hydrogen peroxide in free or bound form;

[0042] 0.1 to 10 g/l of hydroxylamine in free or bound form;

[0043] 0.1 to 10 g/l of a reducing sugar.

[0044] In the phosphatizing of zinc-coated steel, it is necessary that the phosphatizing solution contain as little nitrate as possible. Nitrate concentrations of 0.5 g/l are not to be exceeded, as at higher nitrate concentrations there is the danger of a so-called “white specking”. By this is meant white, crater-like voids in the phosphate layer. Moreover, the paint adhesion on zinc-coated surfaces is impaired.

[0045] The use of nitrite as accelerator leads to technically satisfactory results, particularly on steel surfaces. For reasons of industrial safety (danger of the evolution of nitrous gases), it is, however, advisable to dispense with nitrite as accelerator. For the phosphatizing of zinc-coated surfaces, this is also advisable on technical grounds, as nitrate may be formed from nitrite and this, as explained above, may lead to the problem of white specking and to lowered paint adhesion on zinc.

[0046] Particularly preferred accelerators are hydrogen peroxide for reasons of environmental acceptability and hydroxylamine for the technical reasons involving the possibility of simplified formulations for make-up solutions. The joint use of these two accelerators is not advisable, however, as hydroxylamine is decomposed by hydrogen peroxide. If hydrogen peroxide in free or bound form is used as accelerator, concentrations of from 0.005 to 0.02 g/l of hydrogen peroxide are particularly preferred. Hydrogen peroxide may be added as such to the phosphatizing solution. It is also possible, however, to add hydrogen peroxide in bound form as compounds which yield hydrogen peroxide as a result of hydrolysis reactions in the phosphatizing bath. Examples of such compounds are persalts, such as perborates, percarbonates, peroxosulfates or peroxodisulfates. Other suitable sources of hydrogen peroxide are ionic peroxides, such as alkali metal peroxides. A preferred embodiment of the present invention involves the use of a combination of chlorate ions and hydrogen peroxide in phosphatizing by a dipping process. In this embodiment, the concentration of chlorate may be, for example, 2 to 4 g/l and the concentration of hydrogen peroxide may be 10 to 50 ppm.

[0047] The use of reducing sugars as accelerator is known from U.S. Pat. No. 5,378,292. According to the present invention, they may be used in quantities of between about 0.01 and about 10 g/l, preferably of between about 0.5 and about 2.5 g/l. Examples of such sugars are galactose, mannose and, in particular, glucose (dextrose).

[0048] Another preferred embodiment of the present invention involves the use of hydroxylamine as accelerator. Hydroxylamine may be used as a free base, as a hydroxylamine complex, as an oxime, which is a condensation product of hydroxylamine and a ketone, or in the form of hydroxylammonium salts. If free hydroxylamine is added to the phosphatizing bath or to a phosphatizing bath concentrate, it will be present largely in the form of hydroxylammonium cations owing to the acid character of these solutions. If it is used in the form of hydroxylammonium salt, the sulfates and phosphates are particularly suitable. In the case of the phosphates, the acid salts are preferred owing to the better solubility thereof. Hydroxylamine or the compounds thereof are added to the phosphatizing bath in quantities such that the calculated concentration of the free hydroxylamine is between 0.1 and 10 g/l, preferably between 0.3 and 5 g/l. Here it is preferred that the phosphatizing baths contain hydroxylamine as the only accelerator, possibly together with at most 0.5 g/l of nitrate. Accordingly, in a preferred embodiment, phosphatizing baths are used which contain none of the other known accelerators, such as nitrite, oxo anions of halogens, peroxides or nitrobenzene-sulfonate. A positive side effect is that hydroxylamine concentrations above about 1.5 g/l lower the risk of rust formation on inadequately flooded areas of the structural parts being phosphatized.

[0049] During the application of the phosphatizing process to steel surfaces, iron passes into solution in the form of iron(II) ions. If the present phosphatizing baths do not contain substances which oxidize iron(II), the divalent iron is converted into the trivalent state solely as a result of atmospheric oxidation, so that it may precipitate as iron(III) phosphate. This is the case, for example, when hydroxylamine is used. Consequently, the iron(II) contents which may build up in the phosphatizing baths are significantly greater than those in baths containing oxidizing agents. In this case, iron(II) concentrations of up to 50 ppm are normal, with values of up to 500 ppm also being possible for a short period in the course of production. These iron(II) concentrations are not detrimental to the phosphatizing process according to the present invention. When hard water is used, the phosphatizing baths may in addition contain the hardness-producing cations Mg(II) and Ca(II) in a total concentration of up to 7 mmol/l. Mg(II) or Ca(II) may also be added to the phosphatizing bath in quantities of up to 2.5 g/l.

[0050] The weight ratio of phosphate ions to zinc ions in the phosphatizing baths may vary within wide limits, provided it is between 3.7 and 30. A weight ratio of between 10 and 20 is particularly preferred. For the purpose of stating the phosphate concentration, the total phosphorus content of the phosphatizing bath is regarded as being present in the form of phosphate ions PO43−. In the calculation of the weight ratios, therefore, no account is taken of the fact that, at the pH of phosphatizing baths, which are generally in the range of about 3 to about 3.6, only a very small part of the phosphate is actually present in the form of the triply negatively charged anions. At these pH values, it is more probable that the phosphate exists mainly as a singly negatively charged dihydrogen phosphate anion, together with smaller quantities of undissociated phosphoric acid and doubly negatively charged hydrogen phosphate anions.

[0051] The organic polymers used according to the present invention preferably have molecular weights (which may be determined, for example, by gel permeation chromatography) of about 500 to about 50,000, in particular from about 800 to about 20,000.

[0052] The phosphatizing baths preferably contain the organic polymers in a concentration of between about 0.01 and about 0.1 g/l. At lower concentrations, the required passivating effect diminishes. Higher concentrations do not increase the effect substantially and therefore become increasingly uneconomic.

[0053] The polymers which may be used according to the present invention may be members of various chemical types. Common to them, however, is that they carry oxygen atoms and/or nitrogen atoms either in the polymer chain or in the side groups. The simplest polymers of this type are polyalkylene glycols, for example, polyethylene glycol or polypropylene glycol, which preferably have a molecular weight of from 500 to 10,000. Polymeric carboxylic acids, such as homo- or co-polymers of acrylic acid, methacrylic acid and maleic acid, are likewise suitable, as also are polymeric phosphonic acids or polymeric phosphinocarboxylic acids. An example which may be given is a polyphosphinocarboxylic acid which may be regarded as acrylic acid-sodium hypophosphite copolymer and is available on the market as “Belciene® 500” from the FMC Corporation, Great Britain.

[0054] The organic polymers may also be selected from homo- or co-polymeric compounds containing amino groups and containing or consisting of structural units corresponding to the general formula (I):

[0055] and hydrolysis products thereof, wherein R1 and R2 are the same or different and may represent hydrogen or alkyl having 1 to 6 carbon atoms, for example methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, t-butyl, amyl, n-hexyl, isohexyl or Diclohexyl1. 1This word has not been translated, because it has been determined that the original a mistake and therefore has no proper translation.

[0056] A comprehensive list of such polymers may be found in DE-A 44 09 306, which disclosure is incorporated herein by reference. Specific examples are hydrolysis products of homo- and co-polymers of N-vinyl-formamide, N-vinyl-N-methyl-formamide, N-vinyl-acetamide, N-vinyl-N-methylacetamide, N-vinyl-N-ethylacetamide, N-vinyl-propion-amide and N-vinyl-N-methyl-propionamide, with N-vinylformamide being preferred, as it is very readily hydrolysable. Suitable comonomers are monoethylenically unsaturated carboxylic acids having 3 to 8 carbon atoms, as well as the water-soluble salts of these monomers.

[0057] The organic polymers may also be selected from poly-4-vinylphenol compounds corresponding to the general formula (II):

[0058] wherein:

[0059] n represents a number between 5 and 100,

[0060] x independently represents hydrogen and/or CRR1OH groups, wherein R and R1 represent hydrogen, aliphatic and/or aromatic groups having 1 to 12 carbon atoms.

[0061] These polymers are described as separate rinsing solutions in DE-C 31 46 265. According to this specification, poly-4-vinylphenol compounds of the type wherein at least one x represents CH2OH are particularly suitable. A method for the preparation thereof is given in the above-mentioned document.

[0062] Particularly preferred is the use of organic polymers selected from homo- or copolymeric compounds containing amino groups and including at least one polymer selected from the group consisting of (a), (b), (c) or (d), wherein:

[0063] (a) comprises a polymeric material which has at least one unit corresponding to the formula:

[0064] wherein:

[0065] R1 to R3 for each of the units are independently selected from the group consisting of hydrogen, an alkyl group having 1 to 5 carbon atoms or an aryl group having 6 to 18 carbon atoms;

[0066] Y1 to Y4 for each of the units are independently selected from the group consisting of hydrogen, —CR11R5OR6, —CH2Cl or an alkyl or aryl group having 1 to 18 carbon atoms or Z below:

[0067] but at least a fraction of Y1, Y2, Y3 or Y4 of the homo- or co-polymeric compound or material must be Z;

[0068] R5 to R12 for each of the units are independently selected from the group consisting of hydrogen, an alkyl, aryl, hydroxyalkyl, aminoalkyl, mercaptoalkyl or phosphoalkyl group;

[0069] R12 may also represent —O(−1) or —OH;

[0070] W1 for each of the units is independently selected from the group consisting of hydrogen, an acyl group, an acetyl group, a benzoyl group; 3-allyloxy-2-hydroxy-propyl; 3-benzyloxy-2-hydroxy-propyl; 3-butoxy-2-hydroxy-propyl; 3-alkyloxy-2-hydroxy-propyl; 2-hydroxy-octyl; 2-hydroxy-alkyl; 2-hydroxy-2-phenylethyl; 2-hydroxy-2-alkyl-phenylethyl; benzyl; methyl; ethyl; propyl; alkyl; allyl; alkylbenzyl; haloalkyl; haloalkenyl; 2-chloropropenyl; sodium; potassium; tetraarylammonium; tetraalkylammonium; tetraalkylphosphonium; tetraarylphosphonium or a condensation product of ethylene oxide, propylene oxide or a mixture or a copolymer of the same;

[0071] (b) comprises:

[0072] a polymeric material having at least one unit corresponding to the formula:

[0073] wherein:

[0074] R1 to R2 for each of the units are independently selected from the group consisting of hydrogen, an alkyl group having 1 to 5 carbon atoms or an aryl group having 6 to 18 carbon atoms;

[0075] Y1 to Y3 for each of the units are independently selected from the group consisting of hydrogen, —CR4R5OR6, —CH2Cl or an alkyl or aryl group having 1 to 18 carbon atoms or Z:

[0076] but at least a fraction of Y1, Y2, or Y3 of the end product must be Z; R4 to R12 for each of the units are independently selected from the group consisting of hydrogen, an alkyl, aryl, hydroxyalkyl, aminoalkyl, mercaptoalkyl or phosphoalkyl group; R12 may also be —O(−) or OH;

[0077] W2 for each of the units is independently selected from the group consisting of hydrogen, an acyl group, an acetyl group, a benzoyl group; 3-allyloxy-2-hydroxy-propyl; 3-benzyloxy-2-hydroxy-propyl; 3-alkyl-benzyloxy-2-hydroxy-propyl; 3-phenoxy-2-hydroxy-propyl; 3-alkyl-phenoxy-2-hydroxy-propyl; 3-butoxy-2-hydroxy-propyl; 3-alkyloxy-2-hydroxy-propyl; 2-hydroxy-octyl; 2-hydroxy-alkyl; 2-hydroxy-2-phenylethyl; 2-hydroxy-2-alkyl-phenylethyl; benzyl; methyl; ethyl; propyl; alkyl; allyl; alkyl-benzyl; haloalkyl; haloalkenyl; 2-chloro-propenyl or a condensation product of ethylene oxide, propylene oxide or a mixture of the same;

[0078] (c) comprises:

[0079] a copolymeric material, wherein at least a part of the copolymer has the structure:

[0080] and at least a fraction of the said part is polymerized with one or more monomers which for each unit are independently selected from the group consisting of acrylonitrile, methacrylonitrile, methyl acrylate, methyl methacrylate, vinyl acetate, vinyl methyl ketone, isopropenyl methyl ketone, acrylic acid, methacrylic acid, acrylamide, methacrylamide, n-amyl methacrylate, styrene, m-bromostyrene, bromostyrene, pyridine, diallyl-dimethylammonium salts, 1,3-butadiene, n-butyl acrylate, t-butylaminoethyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, n-butyl vinyl ether, t-butyl vinyl ether, m-chlorostyrene, o-chlorostyrene, p-chlorostyrene, n-decyl methacrylate, N,N-diallyl-melamine, N,N-di-n-butylacrylamide, di-n-butyl itaconate, di-n-butyl maleate, diethylaminoethylmethacrylate, diethylene glycol monovinyl ether, diethyl fumarate, diethyl itaconate, diethylvinyl phosphate, vinyl-phosphonic acid, diisobutyl maleate, diisopropyl itaconate, diisopropyl maleate, dimethyl fumarate, dimethyl itaconate, dimethyl maleate, di-n-nonyl fumarate, di-n-nonyl maleate, dioctyl fumarate, di-n-octyl itaconate, di-n-propyl itaconate, N-dodecyl vinyl ether, acidic ethyl fumarate, acidic ethyl maleate, ethyl acrylate, ethyl cinnamate, N-ethylmethacrylamide, ethyl methacrylate, ethyl vinyl ether, 5-ethyl-2-vinyl-pyridine, 5-ethyl-2-vinyl-pyridine-1 oxide, glycidyl acrylate, glycidyl methacrylate, n-hexylmethacrylate, 2-hydroxy-ethyl methacrylate, 2-hydroxy-propyl methacrylate, isobutyl methacrylate, isobutyl vinyl ether, isoprene, isopropyl methacrylate, isopropyl vinyl ether, itaconic acid, lauryl methacrylate, N-methylolacrylamide, N-methylol-methacrylamide, N-isobutoxy-methylacrylamide, N-isobutoxy-methylmethacrylamide, N-alkyloxy-methylacrylamide, N-alkyloxy-methylmethacrylamide, N-vinyl-caprolactam, N-methyl-methacrylamide, α-methyl-styrene, m-methyl-styrene, o-methylstyrene, p-methyl-styrene, 2-methyl-5-vinyl-pyridine, n-propyl methacrylate, sodium p-styrene sulfonate, stearyl methacrylate, styrene, p-styrene sulfonic acid, p-styrene sulfonamide, vinyl bromide, 9-vinyl-carbazole, vinyl chloride, vinylidene chloride, 1-vinyl-naphthalene, 2-vinylnaphthalene, 2-vinyl-pyridine, 4-vinyl-pyridine, 2-vinyl-pyridine N-oxide, 4-vinyl-pyrimidine, N-vinyl-pyrrolidone; and W1, Y1 to Y4 and R1 to R3 are as described under (a); (d) comprises a condensation polymer of the polymeric materials (a), (b) or (c), a condensable form of (a), (b), (c) or a mixture of the same being condensed with a second compound selected from the group consisting of phenols, tannins, novolak resins, lignin compounds, together with aldehydes, ketones or mixtures thereof, in order to prepare a condensation resin product, the condensation resin product forming, by the addition of “Z” to at least a part of itself by further reaction of the resin product with 1) an aldehyde or ketone, (2) a secondary amine, a final adduct which may react with an acid.

[0081] Methods for preparing such polymers are described in the publications EP-B 319 016 and EP-B 319 017 already cited. Polymers of this type may be obtained from The Henkel Corporation, Parker Amchen Division, USA, under the tradenames Parcolene® 95C, Deoxylyte® 90A, 95A, 95AT, 100NC and TD-1355—CW.

[0082] In this connection particularly preferred polymers are those wherein at least a fraction of the groups Z of the organic polymer possesses a polyhydroxy-alkylamine functionality which originates from the condensation of an amine or of ammonia with a ketose or aldose having 3 to 8 carbon atoms. The condensation products may, if desired, be reduced to amine.

[0083] Further examples of such polymers are condensation products of a polyvinylphenol with formaldehyde or paraformaldehyde and with a secondary organic amine. Here it is preferable to start from polyvinylphenols having a molecular weight of about 1,000 to about 10,000. Particularly preferred condensation products are those wherein the secondary organic amine is selected from methylethanolamine and N-methylglucamine.

[0084] Within the stated concentration ranges the organic polymers are stable in the phosphatizing baths and do not lead to precipitation. They also show no adverse effects on the layer formation and hence do not lead, for example, to the manifestation of passivation, which may inhibit the growth of the phosphate crystals, on the metal surface.

[0085] The organic polymers may also be selected from substituted polyalkylene derivatives containing the structural units:

[0086] wherein R1, R2, R3 may independently represent hydrogen or a methyl or ethyl group, x represents 1, 2, 3 or 4, and Y represents a substituent which contains at least one nitrogen atom and is incorporated in an alkylamino group or in a mono- or poly-nuclear saturated or unsaturated heterocyclic compound.

[0087] Here, those polymers are preferred wherein R1, R2, R3 each represent hydrogen. Preferably, x represents 1. Accordingly, substituted polyethylenes are particularly preferred polymers. Organic polymers which contain one or more of the following structural units are particularly preferred:

[0088] In a further preferred embodiment, the organic polymers are polymeric sugar derivatives containing amino groups. An example of these are chitosans, which may contain, for example, the following structural group:

[0089] It is valid for all organic polymers containing nitrogen that, at the pH of the phosphatizing solution, at least some of the nitrogen atoms are protonated and therefore carry a positive charge.

[0090] Phosphatizing baths are generally distributed in the form of aqueous concentrates, which are adjusted in situ to the concentration to be used by the addition of water. For reasons of stability, these concentrates may contain an excess of free phosphoric acid so that, on dilution of the bath concentration, the value of the free acid is initially too high and the pH is too low. The value of the free acid is lowered to the required range by the addition of alkalies such as sodium hydroxide, sodium carbonate or ammonia. It is also known that the content of free acid may increase with time while the phosphatizing bath is in use, as a result of the consumption of layer-forming cations and possibly as a result of decomposition reactions of the accelerator. In these cases, it is necessary to re-adjust the value of the free acid to the required range by adding alkali from time to time. This means that the contents of alkali metal or ammonium ions in the phosphatizing baths may fluctuate within wide limits and, in the course of the period that the phosphatizing baths are in use, the free acid tends to increase owing to dealkalization. The weight ratio of alkali metal ions and/or ammonium ions to, for example, zinc ions may accordingly be very low in the case of freshly prepared phosphatizing baths, for example, it may be <0.05 and in extreme cases may even be 0, while it generally rises over time as a result of bath maintenance procedures, so that the ratio becomes greater than 1 and the values may reach up to 10 and above. As a rule, low-zinc phosphatizing baths require additions of alkali metal ions or ammonium ions in order that the free acid may be adjusted to within the required range at the required weight ratio of PO43−:Zn of >8. Similar observations may also be made regarding the ratios of alkali metal ions and/or ammonium ions to other constituents of the bath, for example, to phosphate ions.

[0091] In the case of lithium-containing phosphatizing baths it is preferable to avoid using sodium compounds to adjust the free acid, as the beneficial effect of lithium on the corrosion protection is suppressed by excessively high sodium concentrations. In this case, basic lithium compounds are preferably used for the adjustment of the free acid. Alternatively, potassium compounds are also suitable.

[0092] In principle, the form in which the cations giving rise to or influencing the layer are introduced into the phosphatizing baths is unimportant. Nitrates, are however, to be avoided, so as not to exceed the preferred upper limit for the nitrate content. The metal ions are preferably used in the form of those compounds which do not introduce any foreign ions into the phosphatizing solution. For this reason it is most advantageous to use the metals in the form of the oxides or carbonates thereof. Lithium may also be used as sulfate.

[0093] Phosphatizing baths according to the present invention are suitable for the phosphatizing of surfaces composed of steel, zinc-coated steel or steel coated with zinc alloy, aluminum, aluminized steel or steel coated with aluminum alloy, as well as of aluminum-magnesium alloys. Here the term “aluminum” includes the aluminum alloys common in technology such as AlMg0.5Si14. The aforesaid materials may, as is becoming increasingly common in automobile manufacture, also be juxtaposed.

[0094] In this connection, parts of the bodywork may also consist of already pre-treated material, as occurs, for example, in the Bonazink® process. Here, the substrate material is first of all chromed or phosphated and subsequently coated with an organic resin. The phosphatizing process according to the present invention then leads to a phosphatizing on damaged areas of this pre-treated layer or on untreated reverse sides.

[0095] The present process is suitable for application by dipping, spraying or spray/dipping. It may be used particularly in automobile manufacture, where treatment times of between 1 and 8 minutes, particularly of 2 to 5 minutes, are conventional. However, its use in continuous phosphatizing in steelworks, wherein the treatment times are between 3 and 12 seconds, is also possible. When continuous phosphatizing processes are used, it is advisable to adjust the bath concentrations in each case within the upper half of preferred range according to the present invention. For example, the zinc content may be from 1.5 to 2.5 g/l and the content of free acid from 1.5 to 2.5 points. Especially zinc-coated steel and electrolytically galvanized steel in particular are suitable as substrates for continuous phosphatizing.

[0096] As is also conventional in other known phosphatizing baths, suitable bath temperatures, irrespective of the field of application, are between 30 and 70° C., with the temperature range of between 45 and 60° C. being preferred.

[0097] The phosphatizing process according to the present invention is intended particularly for the treatment of the above-mentioned metal surfaces prior to coating, for example, prior to a cathodic electrocoating, such as is conventional in automobile manufacture. It is also suitable as a pre-treatment prior to a powder coating, such as is used, for example, for domestic appliances. The phosphatizing process should be seen as an individual step in the conventional industrial pre-treatment chain. In this chain, the steps involving cleaning and degreasing, intermediate rinsing and activation precede the phosphatizing process with the activation generally being carried out by means of activating agents containing titanium phosphate.

EXAMPLES

[0098] The phosphatizing processes according to the present invention and comparison processes were tested on steel sheets ST 1405, which are used in automobile manufacture. The dipping process carried out was the following procedure, conventional in the manufacture of car bodywork:

[0099] 1. Cleaning by means of an alkaline cleaner (Ridoline® 1501, Henkel KGaA), formulation 2% in tap water, 55° C., 4 minutes.

[0100] 2. Rinsing with tap water, room temperature, 1 minute.

[0101] 3. Activation by means of an activating agent containing titanium phosphate (Fixodine® 950, Henkel KGaA), formulation 0.1% in demineralized water, room temperature, 1 minute.

[0102] 4. Phosphatizing using phosphatizing baths of the following composition:

[0103] 1.0 g/l of Zn2+

[0104] 1.0 g/l of Mn2+

[0105] 0.1 g/l of Fe2+

[0106] 14 g/l of PO43−

[0107] 0.95 g/l of SiF62−

[0108] 0.2 g/l of F−

[0109] 1.7 g/l of (NH3OH)2SO4

[0110] Polymers as in the Table.

[0111] In addition to the cations listed above, the nitrate-free phosphatizing baths contained, if necessary, sodium ions for adjusting the free acid.

[0112] The number of points of the free acid was 0.9 and that of the total acid was 23; the pH was 3.35. The number of points of the free acid means the required consumption in ml of 0.1 N sodium hydroxide solution to titrate 10 ml of bath solution until a pH of 3.6 is attained. Similarly, the number of points of total acid indicates the consumption in ml to attain a pH of 8.2.

[0113] 5. Rinsing with demineralized water

[0114] 6. Blowing dry using compressed air.

[0115] The mass per unit of surface (“layer weight”) was determined by dissolving in 5 % chromic acid solution in accordance with DIN 50942. It is shown in the Table.

[0116] The phosphated specimen sheets were coated with a cathodic dipping paint from the firm BASF (FT 85-7042). The anticorrosive action was tested in an alternating climate test by VDA 621-415 over 9 cycles. The result is included in the Table as the loss of the paint at the scribe (half gap width).

1TABLEPhosphatizing baths and results of phosphatizingLoss ofPolymerLayer weightpaint (HalfNo.(concentration)(g/cm2)gap width)Comp. 1Without polymer3.71.9Ex. 1Polyethylene glycol3.21.7Molecular weight 1000, 10 ppmEx. 2As Example 1, 50 ppm3.91.5Ex. 3TD-1355-CW 3000*)3.51.5Ex. 4As Example 3, 50 ppm3.81.3Ex. 5TD-1355-CW 8600**)3.71.4Ex. 6As Example 5, 50 ppm3.21.1*)Mannich reaction product of polyvinyl-phenol (molecular weight 3000, determined by gel permeation chromatography) with paraformaldehyde and glucamine (Parker Amchen, USA) **)as above, molecular weight of the polyvinylphenol: 8600.

ZINC PHOSPHATING WITH INTEGRATED SUBSEQUENT PASSIVATION

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