Pretreatment of aluminum surfaces with zirconium and molybdenum containing compositions

Abstract

The invention relates to a method for pre-treating a workpiece having an aluminum or aluminum alloy surface for non-machining forming and / or connecting to an identical pre-treated or optionally pre-coated workpiece or to an optionally pre-treated part made of steel and / or galvanized steel and / or alloyed galvanized steel by means of a welding or bonding connecting method, and the workpiece is used for a subsequent anti-corrosion treatment by a phosphating method, by a chromium-free conversion treatment, by a primer application method or by painting. The invention relates to a method for producing a workpiece, said workpiece being a) pickled with an acidic aqueous solution comprising a mineral acid by means of a dip-coating or spray-coating process, b) rinsed with water, and c) contacted by means of a dip-coating or spray-coating application with an acidic aqueous solution which is free of chromate and comprises Zr in the form of complex fluoride and Mo in the form of molybdate and which has a Zr: Mo weight ratio (calculated as Zr / Mo metal) of 15: 1 to 3.5: 1, the invention relates to a method for producing a Zr-Mo-Mo-Zr-Mo-Mo-Mo-Zr-Mo-Mo-Mo-Zr-Mo-Mo-Mo-Zr-Mo-Mo-Mo-Zr-Mo-Mo-Mo-Zr-Mo-Mo-Mo-Zr-Mo-Mo-Mo-Zr-Mo-Mo-Mo-Mo-Zr-Mo-Mo-Mo-Mo-Zr-Mo-Mo-Mo-Mo-Zr-Mo-Mo-Mo-Mo-Zr-Mo-Mo-

Description

[0001] This application is a divisional application of patent application No. 201680051452.X, filed on September 14, 2016, entitled "Pretreatment of aluminum surfaces with compositions containing zirconium and molybdenum".

[0002] The present invention relates to a method for pretreating a workpiece having an aluminum or aluminum alloy surface, which workpiece is intended for non-cutting forming and / or joining to a similarly pretreated or optionally precoated workpiece or to an optionally pretreated part consisting of steel and / or zinc-plated steel and / or alloy zinc-plated steel by welding or adhesive bonding and which workpiece is also intended for subsequent anticorrosion treatment by phosphatizing, by chromium-free conversion treatment, by applying a primer or by surface coating.

[0003] For the chemical surface treatment of metals, for example as a preparation for the application of surface coatings, adhesives and polymers, methods are known in which in a first step the metal surface is cleaned, in a second step it is rinsed with water and finally in a third step it is wetted with an aqueous solution to form a chemical conversion coating and the liquid film is dried. In this way, when the composition of the treatment liquid and the reaction conditions are suitably selected, a non-metallic thin coating which definitely improves the surface quality can be formed on the metal. Thus, for example, coatings consisting of surface coating compositions, adhesives and polymers, optionally in the form of films, can exhibit significantly better adhesion and significantly improved anticorrosion properties when they are applied to metals which have been pretreated in this way.

[0004] Methods of the type described above use, for example, aqueous solutions which contain hexavalent chromium, trivalent chromium, alkali metal ions and silicon dioxide in specific proportions and produce coatings for electrical insulation, for anticorrosion and as adhesion substrates for surface coatings, etc. (DE-B 17 69 582).

[0005] Due to the presence of hexavalent chromium, these methods all have the disadvantage that precautions need to be taken, in particular for the application of coating compositions and for the handling of the coated metals.

[0006] In order to avoid the disadvantages associated with the use of solutions containing hexavalent chromium, a different class of methods provides for the application of conversion coatings, in particular to aluminum surfaces, which are treated using compositions based on zirconium and / or titanium fluorine anions (US-A-4148 670, FR-A-942 789, EP-A-106 389, EP-A-825 280).

[0007] In recent years, the use of workpieces having an aluminum or aluminum alloy surface in vehicle construction has become increasingly important. The reasons for this are the reduction in the weight of the vehicle and the advantageous recyclability of such workpieces. At this point, it is customary to manufacture vehicles by joining together a plurality of parts which are usually previously shaped without cutting and consist of workpieces having an aluminum or aluminum alloy surface or having a steel and / or zinc-plated steel and / or alloy zinc-plated steel surface. The most important joining or assembly forms to date are welding or adhesive bonding.

[0008] Usually, an anticorrosion treatment is carried out after the joining of the parts, which can include phosphating, chromium-free conversion, application of a primer or surface coating, depending on the nature of the joined parts.

[0009] To ensure that parts having an aluminum or aluminum alloy surface are joined to one another without defects or to parts consisting of steel and / or zinc-plated steel or alloy zinc-plated steel without defects, it must be ensured that the aluminum or aluminum alloy surface is free of oxides of aluminum or of possible alloying components of aluminum. Since the aluminum surface is covered after a very short time with a fresh oxide layer, an etching treatment which would appear attractive in this respect does not lead to the achievement of this object.

[0010] To solve the above-mentioned problems, the method of EP-B-700 452 provides for the pretreatment of the surface of aluminum or its alloys by contacting it with an aqueous solution of complex fluorides of the elements boron, silicon, titanium, zirconium or hafnium, each or mixtures of one another, with a total concentration of fluoride anions of 100 to 4000 mg / 1 and with a pH of 0.3 to 3.5, and then carrying out a second permanent anticorrosion treatment. Between the pretreatment and the permanent anticorrosion conversion treatment, the parts consisting of aluminum or its alloys are subjected to a non-cutting and / or cutting shaping process and / or are joined together by adhesive bonding and / or welding or to parts consisting of steel and / or zinc-plated steel and / or alloy zinc-plated steel. The application of a solution optionally containing a polymer of specific properties can be carried out by spraying, dipping or a non-rinse method, wherein in the case of a non-rinse method, the amount of wet film is preferably 2 to 10 ml / m 2 metal surface, preferably 4 to 6 ml / m 2 metal surface. Regardless of the manner of application of the solution, it is advantageous to carry out the drying at a temperature of 40 to 85 °C. In terms of cleaning, the parts consisting of aluminum or its alloys are cleaned under acidic or alkaline conditions before the first conversion treatment, wherein a further cleaning step and an intermediate rinsing with water and / or an activation rinsing bath are carried out before the permanent anticorrosion treatment.

[0011] In terms of the data for the concentration of fluoride anions in the solution to be applied and the amount of wet film, for the case of the application of a fluorine-containing titanate solution, an application amount of 0.06 to 11.73 mg / m 2 , preferably 0.12 to 7.04 mg / m 2(in each case reported as titanium metal) and, for the case of applying a fluorine-containing zirconate solution, in an amount of 0.09 to 17.78 mg / m 2 , preferably 0.18 to 7.04 mg / m 2 (in each case reported as zirconium metal).

[0012] Previous investigations of the present inventive concept have shown that many of the process possibilities described in EP-B-700 452 lead to not very advantageous results, in particular with regard to the temporary corrosion protection obtained from the first chemical conversion treatment and the volume resistance which is very important for the production of welded joints.

[0013] Good adhesive bonding can be obtained by the method disclosed in US 6,020,030 A, in which an organic phosphorus compound is used to pretreat the aluminum substrate. However, the bath of this method is susceptible to contamination with aluminum ions, so that the aluminum ions have to be removed by means of ion exchange resins.

[0014] It is an object of the present invention to provide a method for pretreating a workpiece having an aluminum or aluminum alloy surface, which workpiece is to be joined, in particular by adhesive bonding, to a similarly pretreated or optionally precoated workpiece or to a part consisting of steel and / or zinc-plated steel and / or alloy zinc-plated steel, in particular to a similarly pretreated workpiece, and which is subsequently also permanently corrosion-protected by phosphating and / or by a chromium-free conversion treatment or by applying a primer and / or by surface coating, in particular by phosphating and chromium-free conversion treatment or by surface coating, which generally leads to a workpiece having a sufficiently low volume resistance as well as good phosphatability and bondability.

[0015] Furthermore, the method according to the invention should preferably lead to good temporary corrosion protection values and be essentially insensitive to contamination of the bath with aluminum ions.

[0016] Said object is achieved firstly by a method of the type mentioned at the outset, which is arranged in such a way that the workpiece

[0017] a) is pickled by dip coating or spraying with an acidic aqueous solution comprising an inorganic acid,

[0018] b) is rinsed with water, and

[0019] c) is contacted by dip coating or spraying application with an acidic aqueous solution which is chromium-free and comprises Zr in the form of complex fluoride and Mo in the form of molybdate and has a Zr:Mo weight ratio (calculated as Zr / Mo metal) of 15:1 to 3.5:1, so that, after subsequent drying, in each case 2 to 15 mg / m 2of Zr and Mo, wherein the solution comprises 100-800 mg / l of Zr and 30-100 mg / l of Mo (calculated as Zr / Mo metal) and has a pH of 2.5-4.5.

[0020] In the application by dip coating or spray coating, it has been found that the application of a solution comprising only fluorozirconates or only molybdates, the application of a solution with a zirconium: molybdenum weight ratio (calculated as Zr / Mo metal) of more than 15:1 to 3.5:1, the application of a solution resulting in a layer weight of zirconium and molybdenum of more than 2 to 15 mg / m 2 do not achieve satisfactory results for the purposes of the present application.

[0021] The acidic aqueous solution applied by dip coating or spray coating in step c) preferably comprises Zr and Mo in a weight ratio of 15:1 to 5:1, particularly preferably 13:1 to 7:1, very particularly preferably 11 :1 to 9:1.

[0022] The dip coating or spray coating application preferably results in a layer weight of Zr and Mo of 2-12 mg / m 2 , particularly preferably 2-10 mg / m 2 , very particularly preferably 2-8 mg / m 2 after drying in each case.

[0023] The aqueous solution used for the application by dip coating or spray coating preferably comprises 250-700 mg / l of Zr and 30-80 mg / l of Mo, particularly preferably 400-600 mg / l of Zr and 40-60 mg / l of Mo, very particularly preferably 475-525 mg / l of Zr and 45-55 mg / l of Mo.

[0024] Furthermore, the aqueous solution used for the application by dip coating or spray coating preferably has a pH of 3.1 -4.3, particularly preferably 3.6-4.0, and also preferably has a temperature of 20-50 °C, particularly preferably 20-30 °C.

[0025] Secondly, the object is achieved by a method of the type mentioned at the outset, which is provided according to the application in such a way that the workpiece

[0026] a) is pickled by dip coating or spray coating with an acidic aqueous solution comprising an inorganic acid,

[0027] b) is rinsed with water, and

[0028] c) is contacted by a roll coating process with an acidic aqueous solution which is chromium-free and comprises Zr in the form of a complex fluoride and Mo in the form of a molybdate and has a Zr:Mo weight ratio (calculated as Zr / Mo metal) of 2:1 to 1 :2, so that after subsequent drying a layer weight of 2-15 mg / m 2of Zr and Mo, wherein the solution comprises 0.4-7.5 g / l of Zr and 0.4-7.5 g / l of Mo (calculated as Zr / Mo metal) and has a pH of 1.0-3.0.

[0029] In the case of application by the roll coating method, it has been found that the application of solutions comprising only fluorozirconates or only molybdates, the application of solutions with a zirconium: molybdenum weight ratio (calculated as Zr / Mo metal) of more than 2:1 to 1 :2 by weight, the application of solutions resulting in layer weights of zirconium and molybdenum of more than 2-15 mg / m 2 of Zr and Mo, respectively, do not achieve satisfactory results for the purposes of the present application.

[0030] The acidic aqueous solution used in step c) for application by the roll coating method preferably comprises Zr and Mo in a weight ratio of 1.7:1 to 1 :1.7, particularly preferably 1.4:1 to 1 :1.4, very particularly preferably 1.1 :1 to 1 :1.1.

[0031] The application by the roll coating method preferably results in a layer weight of Zr and Mo of 2-12 mg / m 2 , particularly preferably 2-10 mg / m 2 , very particularly preferably 2-8 mg / m 2 , after drying in each case.

[0032] The aqueous solution used for application by the roll coating method preferably comprises 1.0-6.0 g / l of Zr and 1.0-6.0 g / l of Mo, particularly preferably 2.0-4.0 g / l of Zr and 2.0-4.0 g / l of Mo, very particularly preferably 2.8-3.2 g / l of Zr and 2.8-3.2 g / l of Mo.

[0033] Furthermore, the aqueous solution used for application by the roll coating method preferably has a pH of 1.4-2.7, particularly preferably 1.8-2.5.

[0034] It is of great importance for the defined layer weight of zirconium and molybdenum (2-15 mg / m 2 in each case) that different acidic aqueous solutions are used in step c) depending on the application method (i.e. application by spraying or dipping or application by the roll coating method) in terms of the concentration and the proportion of the fluorozirconate and molybdate anions and the pH.

[0035] The acidic aqueous solution in step c) can be provided by prior dilution of a suitable concentrate, preferably dilution by a factor of 1 :30 to 1 :100, more preferably by a factor of about 1 :50, preferably with water and optionally setting the pH value.

[0036] It is also important for achieving the objects pursued by the present application that the workpieces are pickled by dip coating or spray coating with an aqueous acidic solution containing an inorganic acid. For example, alkaline cleaning leads to the formation of zirconium / molybdenum layers having a poor volume resistance.

[0037] The workpieces pretreated according to the present application can be joined to surfaces of aluminum or its alloys of similarly pretreated or optionally precoated (e.g. phosphatized) workpieces. If it is intended to join to parts consisting of steel and / or galvanized steel and / or alloyed galvanized steel, these parts can have bare or precoated surfaces. A suitable precoating can for example be a phosphate layer or a conductive primer layer having a layer weight of not more than 2 g / m2. 2

[0038] If the workpieces are oily, a cleaning / degreasing step should be carried out prior to the pickling process or the pickling process should be carried out simultaneously with the cleaning / degreasing. The latter can be achieved by adding a surfactant to the pickling solution.

[0039] As a method of phosphatizing, the use of zinc phosphate-based solutions, in particular methods corresponding to the low-zinc technology or alkali metal phosphates, is promising. The solutions can be modified by the addition of small amounts of other polyvalent cations such as calcium, magnesium, nickel, copper or manganese.

[0040] For chromium-free conversion treatments, in particular the use of acidic solutions of titanium, zirconium, hafnium or silicon fluorocomplexes, the solutions optionally contain an amount of organic polymers.

[0041] These acidic solutions can additionally contain at least one organosilane and / or at least one hydrolysis product thereof and / or at least one condensation product thereof.

[0042] The at least one organosilane preferably has at least one amino group. Particularly preferred is bis(trimethoxysilylpropyl)amine or an organosilane which can be hydrolyzed to aminopropylsilanol and / or 2-aminoethyl-3-aminopropylsilanol.

[0043] As a primer, a reactive primer or an adhesive can be applied.

[0044] The workpiece pretreatment according to the present application ensures a satisfactory temporary corrosion protection over a longer storage time. During this time, no adverse effects on the weldability (in particular for resistance welding) or on the adhesive bonding occur. Furthermore, it ensures a substantially uniform volume resistance over all surface areas of the workpiece with regard to the weldability.

[0045] For the purposes of the present application, the workpieces are strips, sheets and individual parts such as profiles.

[0046] ​The application of the solution according to step c) can be carried out by spraying or by dip coating, in each case with or without rinsing with water. In the case of application without rinsing with water, it is advantageous to remove the excess treatment solution by means of a doctor blade roll.

[0047] In the treatment of sheet or strip, it is particularly advantageous to apply the treatment solution by means of a roll coating process. This allows the desired wet film thickness setting to be defined in one operation.

[0048] After the solution application of the type described above, the workpiece is dried or the solution is evaporated. A target temperature of 30 to 90°C is particularly advantageous.

[0049] To supplement the treatment liquid, it is common practice to use a concentrate which is diluted with low-salt water, preferably deionized water, to the concentration to be set in each case. In order to avoid the introduction of alkali metal ions, it is particularly advantageous to introduce the required fluoride anions of zirconium by means of a free acid and, if necessary, to set the corresponding pH value by means of the addition of ammonia. On the other hand, the molybdate is advantageously introduced in the form of ammonium heptamolybdate and / or sodium heptamolybdate, preferably in the form of ammonium heptamolybdate, particularly preferably in the form of ammonium heptamolybdate x 7 H2O.

[0050] For the purposes of the present application, the term "molybdate" also includes the protonated form, for example in particular molybdic acid.

[0051] The pickling of the workpiece (process step a)) is carried out using an acidic aqueous solution which comprises an inorganic acid. It can be carried out electrolytically or chemically. In the case of electrolytic pickling, phosphoric acid is particularly suitable as inorganic acid. The chemical route pickling, which is generally preferred on account of the simpler mode of operation in terms of the apparatus, can be carried out using nitric acid or nitric acid / hydrofluoric acid. In a preferred embodiment of the present application, the workpiece is pickled by means of spraying or dip coating using a solution which comprises a surfactant, sulfuric acid and a compound selected from the group consisting of hydrofluoric acid, phosphoric acid and iron(III) sulfate, preferably hydrofluoric acid, wherein a solution comprising 3 to 8 g / l of sulfuric acid, 50 to 150 mg / l of uncomplexed free fluoride and 1 to 3 g / l of a non-ionic surfactant is found to be particularly suitable. Adducts of ethylene oxide with fatty alcohols and, for example, abietic acid are particularly suitable as non-ionic surfactants.

[0052] The measurement of the free fluoride is carried out using a fluoride-sensitive electrode, the calibration of which is carried out using a solution having the same pH value as the solution to be tested.

[0053] In order to obtain a layer having optimum electrical resistance in the subsequent treatment of step c), the pickling process should be carried out so as to obtain a layer thickness of approximately 0.1 to 0.6 g / m 2 Metal removal from the service workpiece.

[0054] Corresponding to step b), rinsing with water after the pickling of the workpiece is preferably carried out in a plurality of rinsing steps, it being particularly advantageous for the rinsing water to be conveyed in a cascade manner counter-current to the workpiece. At this point, the last rinsing step should be carried out using deionized water. The treatment according to step c) after the pickling and rinsing steps prevents the re-growth of an oxide layer on the workpiece having an aluminum or aluminum alloy surface.

[0055] In an advantageous embodiment of the application, the solution used in step c) additionally comprises at least one polymer selected from the group consisting of poly(meth)acrylic acid, (meth)acrylic acid copolymers, polyvinylphosphonic acid, vinylphosphonic acid copolymers and maleic acid copolymers.

[0056] It is preferred to use (meth)acrylic acid-maleic acid copolymers as (meth)acrylic acid copolymers and vinylphosphonic acid-acrylic acid copolymers as vinylphosphonic acid copolymers. Particularly suitable polymers are polyacrylic acid and acrylic acid copolymers, in the case of the latter especially acrylic acid-maleic acid copolymers.

[0057] The poly(meth)acrylic acid used preferably has a number-average molecular weight (Mw) of 4000 to 300 000 g / mol, particularly preferably 50000 to 250000 g / mol, very particularly preferably 100 000 to 250 000 g / mol.

[0058] The (meth)acrylic acid copolymers used preferably have a number-average molecular weight (Mw) of 4000 to 100 000 g / mol, particularly preferably 60000 to 80000 g / mol.

[0059] The polyvinylphosphonic acid used or the vinylphosphonic acid copolymers used preferably have a number-average molecular weight (Mw) of 4000 to 70000 g / mol, particularly preferably 10000 to 30000 g / mol.

[0060] The concentration of the at least one polymer is 100 to 600 mg / l, preferably 100 to 400 mg / l, particularly preferably 135 to 290 mg / l, very particularly preferably 170 to 180 mg / l. The use of the at least one polymer makes it possible to obtain a layer weight of Zr and Mo within the target range in each case in the range from 2 to 15 mg / m 2 The layer weight of Zr and Mo within the target range, and relatively independently of the spraying time, preferably substantially independently of the spraying time. This is advantageous, inter alia, because similar layer weights can be obtained even at different strip speeds. On the other hand, no adverse effects of the volume resistance caused by the polymer content were observed.

[0061] In case of a subsequent treatment by non-cutting forming after step c), a further advantageous embodiment of the present application provides for applying a lubricant to the workpiece. Such a lubricant is in particular a forming oil based on mineral oil, which can be completely synthetic or of natural origin, or a dry lubricant based on polyethylene / polyacrylate.

[0062] It is generally useful to insert a cleaning step and a water rinsing step before the permanent corrosion protection treatment. In case a primer or surface coating is envisaged, a prior drying is desirable. If a chromium-free conversion treatment is subsequently carried out, the dip or spray application can also be carried out in wet-on-wet fashion, i.e. without prior drying. In case of application by roll coating, an intermediate drying is indispensable. In case of a subsequent phosphating treatment, which can also be carried out in wet-on-wet fashion, it is advantageous to provide an activation treatment, for example using an activator containing titanium and phosphates.

[0063] The method of the present application generally imparts to the workpiece a layer which can be formed defect-free and / or bonded with adhesive or which can be welded to layers defect-free and problem-free due to a uniform low volume resistance on the surface of the workpiece. Furthermore, the workpiece is very well suited for a subsequent permanent corrosion protection treatment.

[0064] The present application will be explained in more detail by means of the following examples, which are not to be interpreted as constituting a restriction.

[0065] Example:

[0066] In addition to the metal sheet cleaned under alkaline conditions (comparative example VB2 in Table 1), sheets consisting of aluminium alloys of the grades AA 6111 and AA 5754 were first degreased and pickled by dip coating or spray coating at a temperature of 50°C. The pickling solution contained 6 g / l sulphuric acid (100% strength), 100 mg / l hydrofluoric acid (100% strength) and 2 g / l of a non-ionic surfactant consisting of ethoxylated fatty alcohol and ethoxylated rosin acid in a weight ratio of 1 : 1.

[0067] The pickling process was carried out in such a way that in the case of the alloy AA 5754, the material removed during pickling was 0.05-0.2 g / m 2 and in the case of the alloy AA 6111, 0.05-0.4 g / m 2 . The treatment time required for this purpose was 5-20 seconds.

[0068] The workpiece was subsequently rinsed thoroughly with water, in the last step with deionised water. The volume resistance measured on the individual sheets was about 60 μΩ in the case of the AA 5754 alloy and about 13 μΩ in the case of the alloy AA 6111.

[0069] Subsequently, treatment with a hexafluorozirconic acid and / or molybdate solution, which in each case optionally comprises a polymer or copolymer, and the data on the concentration of zirconium and / or molybdenum and (co)polymer, the pH and the temperature applied are shown in Table 1, was carried out, the time for spray application being 6 seconds. If necessary, the pH was adjusted using an ammonia solution. The excess treatment solution was removed by means of a doctor blade, and the treated surface was subsequently dried.

[0070] The polymers A-D in Table 1 are as follows:

[0071] A: polyacrylic acid with MW = ca. 60 000 g / mol in colloidal solution,

[0072] B: acrylic acid-maleic acid copolymer with MW = ca. 70 000 g / mol,

[0073] C: polyacrylic acid with MW = ca. 250 000 g / mol,

[0074] D: vinylphosphonic acid-acrylic acid copolymer with MW = 4000-70 000 g / mol.

[0075] Table 1:

[0076]

[0077] Table 2:

[0078]

[0079] Table 3:

[0080]

[0081] n.m. = not measured

[0082] Table 2, column 2 shows the layer weights of Zr and Mo in mg / m 2 The measurement of the layer weights was carried out by X-ray fluorescence (XRF) analysis.

[0083] In addition, the volume resistances in μΩ obtained in each case in the measurements on the metal sheets are shown in Table 2, column 3. The measurement of the volume resistances was carried out immediately after drying / evaporation (in each case, first line "0 d") and after 30 days of storage (in each case, second line "30 d"). This was carried out in accordance with sheet 2929 of the Deutscher Verbandes für Schweiβen und verwandte Verfahren e.V. (DSV) (September 2001) using a copper electrode with a diameter of 20 mm.

[0084] The corresponding phosphatability and the results of the improved APGE test (in cycles survived) which will be described below are given in table 3, columns 2 and 3.

[0085] The adhesion was determined by means of the improved APGE (Arizona Proving Ground Equivalent) test. For this, in each case two test panels (each 56.25 x 25 x 0.25 mm) were coated with an industrial dry lubricant and bonded by means of a suitable industrial adhesive. Then, six pairs of the test panels were screwed together at their respective ends to form a chain which was subjected to a tensile stress of 2400 N. For each week of the test, the following conditioning program was used:

[0086] 1. immersion in a 5 wt.-% NaCI solution in distilled water for 15 minutes,

[0087] 2. drying in a dry environment for 105 minutes,

[0088] 3. 22 hours at controlled temperature and atmospheric humidity: 50°C and 90% relative atmospheric humidity,

[0089] 4. repetition of steps 1-3 for 4 times,

[0090] 5. 48 hours at 50°C and 90% relative atmospheric humidity.

[0091] One sequence of steps 1-3 represents a cycle by definition. Each case was considered to have survived one cycle when the adhesive bond between all test panels of the chain was strong. The test was considered to have passed overall when at least 45 cycles were survived.

[0092] The phosphatability was determined by means of scanning electron micrographs. Here, "+" in table 3 means a closed, fine crystalline phosphate layer, "o" means a closed, coarsened phosphate layer (crystals with > 20 pm edge length), "-" means a non-closed to non-existent phosphate layer.

[0093] The measured values in tables 2 and 3 lead to the following conclusions: In comparative example VB2, the treatment of the aluminum sheet with a basic cleaner to determine the necessity of the acidic pickling treatment in step a) leads to a sheet material with a poor volume resistance (18 μΩ for AA 6111 and 26 μΩ for AA 5754).

[0094] From comparative example VB1 it can be seen that the Zr layer weight of the layer obtained in the treatment according to step c) is too high (37 mg / m 2 in the case of AA 6111 26 mg / m 2), which leads to very high volume resistances, especially after 30 days of storage (100 μΩ in the case of AA 5754 and 38 μΩ in the case of AA 6111).

[0095] In the case of comparative examples VB4 and VB5, the layer weights obtained are in the desired range, however, due to the zirconium / molybdenum ratio of 20:1 (see VB4) or 2:1 (see VB5) in the treatment solution of process step c), the volume resistances obtained are not acceptable, especially for AA 6111 (26 μΩ and 44 μΩ in the case of VB4 and 19 μΩ and 22 μΩ in the case of VB5).

[0096] In the case of comparative example VB3, in which the pH value was too low, i.e. 2.1, an undesired high Zr application weight (24 mg / m 2 and 18 mg / m 2 for AA 6111) and a very high volume resistance (40 μΩ and 79 μΩ for AA 5754 and 67 μM and 73 μΩ for AA 6111) were observed.

[0097] In contrast, examples B1-B7 show that when the essential conditions of the present application in terms of the type of pickling treatment, the Zr / Mo ratio, the layer weight obtained, the respective concentrations and the pH range of the treatment solution are met, layers with excellent volume resistance and good adhesion properties are obtained.

[0098] Furthermore, it can be seen from Table 3 that all examples B1-B7 passed the improved APGE test, i.e. survived at least 45 cycles in each case (in the case of AA 5754 as well as in the case of AA 6111), whereas this clearly does not apply to comparative example VB1 (survived only 10-30 cycles in each case). The improved APGE test was not carried out for the other comparative examples VB2-VB5, since poor results had already been obtained for the layer weight or the volume resistance (Table 2, see above).

[0099] In terms of phosphatability (see Table 3), all examples B1-B7 (except B7 for AA 5754) always showed closed, finely crystalline phosphate layers (“+”), whereas comparative examples VB2-VB5 only had unsealed to non-existent phosphate layers (“-”) in the case of AA 5754 and (except VB2) also showed significantly worse results in the case of AA 6611.

[0100] The test panels of examples B6 and B7, which were pretreated according to the present application, and the unpretreated bare test panel VB6 were also subjected to a multi-step corrosion protection treatment, which consisted of the following steps:

[0101] i) alkaline cleaning (60°C; 180 s)

[0102] ii) rinsing (tap water; RT, 60 s)

[0103] iii) activation (titanium phosphate; RT, 30 s)

[0104] iv) phosphating (trikationen; 53°C, 180 s)

[0105] v) rinsing (tap water; RT, 30 s)

[0106] vi) passivation (zirconium fluoride; RT, 45 s)

[0107] vii) post-rinsing (softened water; RT, 30 s)

[0108] viii) drying (convection oven; 100°C; 7 min)

[0109] After the subsequent electrophoretic coating and topcoat application, a filiform corrosion test (average value according to DIN EN ISO 4628-8) was carried out in accordance with DIN EN 3665 and a cyclic corrosion test (average value according to DIN EN ISO 4628-8) was carried out in accordance with VDA 621-415. The smaller the migration under the coating (in mm) measured, the better the corrosion protection. The results are shown in the following table.

[0110] Table 4:

[0111]

[0112] The corrosion protection in the case of B6 and B7 was in each case comparable to that in the case of VB6. Thus, the inventive pretreatment did not have a detrimental effect on the corrosion protection obtained subsequently by the corrosion protection treatment.

Claims

1. A method for pretreating a workpiece having an aluminum or aluminum alloy surface, which workpiece is used for non-cutting forming and / or joining to a similarly pretreated or optionally precoated workpiece or to an optionally pretreated part consisting of steel and / or galvanized steel and / or alloyed galvanized steel by welding or adhesive bonding and which workpiece is also used for a subsequent permanent corrosion protection by phosphating, by a chromium-free conversion treatment, by applying a primer or by a surface coating, wherein the workpiece is: a) pickled by dip coating or spraying with an acidic aqueous solution comprising an inorganic acid, b) rinsed with water, and c) treated with an acidic aqueous solution comprising Zr in complex fluoride form and Mo in molybdate form.

2. The method according to claim 1, wherein the acidic aqueous solution in step c) comprises Zr in complex fluoride form and Mo in molybdate form, and the Zr:Mo weight ratio (calculated as Zr / Mo metal) is 13:1 to 7:

1.

3. A method for pretreating a workpiece having an aluminum or aluminum alloy surface, which workpiece is used for non-cutting forming and / or joining to a similarly pretreated or optionally precoated workpiece or to an optionally pretreated part consisting of steel and / or galvanized steel and / or alloyed galvanized steel by welding or adhesive bonding and which workpiece is also used for a subsequent permanent corrosion protection by phosphating, by a chromium-free conversion treatment, by applying a primer or by a surface coating, wherein the workpiece is: a) pickled by dip coating or spraying with an acidic aqueous solution comprising an inorganic acid, b) rinsed with water, and c) treated with an acidic aqueous solution comprising Zr in complex fluoride form and Mo in molybdate form. c) contacting with a chromium-free and acidic aqueous solution comprising Zr in the form of a complex fluoride and Mo in the form of a molybdate and a Zr:Mo weight ratio (calculated as Zr / Mo metal) of 15:1 to 3.5:1 by dip or spray application, resulting in a layer weight of 2 to 15 mg / m 2 of Zr and Mo in each case after subsequent drying, wherein the solution comprises 100 to 800 mg / 1 of Zr and 30 to 100 mg / 1 of Mo (calculated as Zr / Mo metal) and has a pH of 2.5 to 4.

5.

4. The method according to claim 3, wherein the acidic aqueous solution in step c) comprises Zr in complex fluoride form and Mo in molybdate form, and the Zr:Mo weight ratio (calculated as Zr / Mo metal) is 1.4:1 to 1 :1.

4.

5. The method according to any one of claims 1 to 4, wherein the acidic aqueous solution in step c) additionally comprises at least one polymer selected from the group consisting of poly(meth)acrylic acid, (meth)acrylic acid copolymers, polyvinyl phosphonic acid, vinyl phosphonic acid copolymers and maleic acid copolymers.

6. The method according to claim 5, wherein the at least one polymer is polyacrylic acid and / or acrylic acid-maleic acid copolymers.

7. The method according to any one of claims 1 to 6, wherein the workpiece is pickled with the aid of a solution comprising a surfactant, hydrofluoric acid and sulfuric acid. c) contacting with an acidic aqueous solution free of chromium and comprising Zr in the form of a complex fluoride and Mo in the form of a molybdate and a Zr:Mo weight ratio (calculated as Zr / Mo metal) of 2:1 to 1 :2 by a roll coating process, thereby resulting in a layer weight of 2 to 15 mg / m 2 of Zr and Mo in each case after subsequent drying, wherein the solution comprises 0.4 to 7.5 g / l of Zr and 0.4 to 7.5 g / l of Mo (calculated as Zr / Mo metal) and has a pH of 1.0 to 3.

0.

8. The method according to claim 7, wherein the workpiece is pickled with the aid of a solution comprising 3 to 8 g / l sulfuric acid, 50 to 150 mg / l free fluorides and 1 to 3 g / l non-ionic surfactant.

9. The method according to any one of claims 1 to 8, wherein in the case of pretreating a workpiece for non-cutting forming, a lubricant is applied prior to the forming process.

10. The method according to any one of claims 1 to 8, wherein a cleaning, water washing and optionally an activation treatment are carried out prior to the permanent corrosion protection. ​ ​ ​ ​ 11. An acidic aqueous solution for pretreating a workpiece having an aluminum or aluminum alloy surface, wherein the solution is chromium-free and comprises Zr in the form of a complex fluoride and Mo in the form of a molybdate, and the Zr:Mo weight ratio (calculated as Zr / Mo metal) is 15:1 to 3.5:1, wherein the solution comprises 100 to 800 mg / 1 of Zr and 30 to 100 mg / 1 of Mo (calculated as Zr / Mo metal) and has a pH of 2.5 to 4.

5.

12. A concentrate from which a solution according to claim 11 can be obtained by dilution with water or an aqueous solution and optional adjustment of the pH.

13. An acidic aqueous solution for pretreating a workpiece having an aluminum or aluminum alloy surface, wherein the solution is chromium-free and comprises Zr in the form of a complex fluoride and Mo in the form of a molybdate, and the Zr:Mo weight ratio (calculated as Zr / Mo metal) is 2:1 to 1:2, wherein the solution comprises 0.4 to 7.5 g / 1 of Zr and 0.4 to 7.5 g / 1 of Mo (calculated as Zr / Mo metal) and has a pH of 1.0 to 3.

0.

14. A concentrate from which a solution according to claim 13 can be obtained by dilution with water or an aqueous solution and optional adjustment of the pH.

15. A workpiece having an aluminum or aluminum alloy surface, wherein the workpiece has been pretreated by means of a method according to any one of claims 1 to 10.

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