Method and composition for selective anodisation

Abstract

The invention relates to a composition for selective anodization comprising the substances amidosulfuric acid, magnesium sulfate and concentrated sulfuric acid as a base electrolyte and additionally sodium stannate and / or molybdenum oxide.

Description

[0001] The invention relates to a composition according to the preamble of claim 1 and to a method for selective anodization according to the preamble of claim 7.

[0002] The process of selective anodization is an electrochemical surface coating process. The anodization is carried out - as in Figure 1shown schematically - in tools specially adapted for the respective component (2). This tool, including the component surface (1) and the correspondingly connected peripherals, represents the actual coating cell (6). The tool simultaneously takes on the function of the cathode (3) and the seal, i.e. surface areas not to be anodized are sealed or masked, thus preventing the electrolyte from entering. As a result, no layer is formed in these areas, but only the area surrounded by the electrolyte is selectively anodized. Sealing elements are not shown for the sake of clarity. Depending on the component geometry of the surface to be anodized, as well as for functional reasons and ease of handling, the tool can consist of several sections, which are moved into position along the travel paths indicated by double arrows to seal and produce the closed coating cell.The tool simultaneously transports the electrolyte (supply and removal) and introduces the current required for anodization using a suitable power source (connected via the anode or cathode). The electrolyte flow is generated from the electrolyte tank (5) via a pump (4) and maintained for the duration of the anodization. Depending on the coating requirements, the electrolyte is additionally heated or cooled.

[0003] This selective anodization is known. For example, DE 101 40 934 A1 describes an analogous device and method for the galvanic surface treatment of workpieces, comprising a closed process chamber for accommodating a workpiece, which has at least one supply opening for supplying process fluid into the process chamber and at least one discharge opening for discharging process fluid. At least one electrode connectable to a power source is provided, and the workpiece, as a counterelectrode, can be connected to a power source of opposite polarity. Furthermore, the device comprises means for generating a flow of the process fluid through the process chamber along a treatment surface of the workpiece to be treated. A plurality of supply openings and a plurality of discharge openings are arranged spaced apart from the treatment surface, and a discharge opening and a supply opening are arranged adjacent to one another.FR 2 574 095 A1 also describes such a device.

[0004] System and procedure.

[0005] FR 2 490 685 A1 discloses a composition for selective anodization containing amidosulfuric acid, sulfuric acid and magnesium sulfate.

[0006] The use of the process described here offers advantages over conventional anodization in immersion baths: High sustainability through partially targeted functionalization; Low material usage (e.g., plant engineering, racks, chemicals); Short coating time; Low layer roughness; No pretreatment or post-treatment; Closed-loop system; Component-specific process monitoring;

[0007] However, previous systems often use electrolytes (compositions) containing chromic acid or chromium trioxide to achieve good coating results.

[0008] For example, EP 1 219 464 A1 describes the use of chromic acid in paragraph

[0264] .

[0009] However, the use of compositions containing chromium trioxide is no longer desirable due to the health risks and its use within the framework of the European Chemicals Regulation is only possible after approval of a corresponding authorisation application, which justifies the safe handling and the lack of alternatives to this use.

[0010] The invention is based on the object of providing a composition for selective anodization that enables properties of the layer produced on the substrate that are equal to or better than those achievable with a composition containing chromium trioxide. Furthermore, the invention is based on the object of providing a corresponding process for selective anodization using such a composition. In particular, the layers should be as smooth as possible, i.e., have a low roughness, despite the omission of chromium trioxide, while maintaining short coating times.

[0011] This object is achieved by a composition having the features of claim 1 and a method for selective anodization having the features of claim 7. Advantageous embodiments of the invention are specified in dependent claims 2 to 6 and 8 to 12.

[0012] According to the invention, an improved composition for selective anodization is provided which, despite omitting chromium trioxide, produces (hard) anodic layers which have a low roughness while maintaining short coating times.

[0013] Specifically, the present invention includes an electrolyte used for the aforementioned selective high-speed anodization. A particular challenge was the substitution of chromium trioxide. It was necessary to combine a (hard) anodic layer (> 5 µm, > 250 HV 0.01) with the lowest possible roughening and the shortest possible coating time. To achieve this, a base electrolyte was developed that is characterized by rapid layer growth even at relatively low voltages. The inventors believe that the reasons for this are, on the one hand, the dependence of roughening and pore size on the applied voltage and electrolyte temperature, and, on the other hand, the direct correlation of roughening with coating time. In summary, this means that the shorter the coating time, the smoother the resulting layer (at the same layer thickness).Depending on the electrolyte composition and conductivity, there are also advantages with higher voltages in combination with lower electrolyte temperatures, whereby lower temperatures in direct comparison also result in longer coating times.

[0014] The base electrolyte used in the invention consists of sulfuric acid, magnesium sulfate and amidosulfonic acid, with which sufficient layer thicknesses were obtained in a short time, which had appealing, but not always sufficient, roughness values.

[0015] The base electrolyte preferably contains concentrated sulfuric acid in a range of 5 to 150 g / L, magnesium sulfate hydrate from 5 to 200 g / L and amidosulfonic acid from 5 to 200 g / L.

[0016] The inventors have discovered that, in addition to the coating time and the applied voltage (the resulting current density), the solubility of trivalent aluminum in the anodizing medium also plays a decisive role in the resulting roughness. The direct transfer of Al 3+< into the solution reduces the anodic current efficiency with respect to aluminum oxide formation, thereby prolonging the coating time, with a correspondingly negative impact on the roughness. In addition, the redissolution of already formed aluminum oxide, which is naturally favored when using a medium that dissolves Al 3+< well, enlarges the pores in the layer, resulting in a corresponding roughening of the layer. One way to reduce the solubility of Al(III) is to intercept it as it attempts to escape from the substrate and instead force it to be incorporated into the layer.The inventors assume that this is, with some probability, precisely how chromic acid works, since the anodized layers produced from chromic acid electrolytes contain chromium, which in turn is assumed to be incorporated into the layer as aluminum chromate. Conversely, this means that the chromic acid captures Al(III) species escaping from the substrate and contributes the latter to the layer structure in the form of a heteropolyoxometalate. Since chromic acid itself has a tendency to form oligomers, compounds exhibiting a reactivity similar to that of chromic acid were selected as possible chromium substitutes.According to the inventors' assumption, alternatives are metal oxides which preferably have high (but not too high and thus preventing the formation of polyoxometalates) nuclear charges, i.e. early and middle transition metal oxides (preferably groups 5 and 6), but also main group metal and semimetal oxides in their high oxidation states.

[0017] Specifically, these include the oxides of the high oxidation states of metals that are in close proximity to chromium, i.e., vanadium(V) oxide and its derived vanadates and polyvanadates. Furthermore, the heavier congeners of Groups 5 and 6, such as molybdenum(VI) oxide, molybdates and polymolybdates, niobium(V) oxide and tantalum(V) oxide and their derivatives, tungsten(VI) oxide, tungstates, and polytungstates.

[0018] Similar to transition metals, the nuclear charge also plays a crucial role in the formation of polyoxometalates among main group metals. Accordingly, the inventors believe that oxides of elements such as gallium, germanium, indium, tin, lead, and bismuth are suitable for potential use in acidic anodizing electrolytes, although lead was not considered for obvious reasons.

[0019] It has now been shown that these classes of substances actually have a very beneficial influence on the roughness of the (hard) anodic layer without negatively affecting the layer hardness and the layer thickness growth (coating time).

[0020] In particular, molybdenum oxides and stannates have been found to be particularly suitable.

[0021] In the specific case, both MoO 3 (with and without crystal water), Na 2 MoO 4 *2H 2 O, and Na 2 SnO 3 *3H 2 O (correctly Na 2 [Sn(OH) 6 ]) show a pronounced reduction in surface roughness with the same anodized layer thickness and hardness as well as a comparable coating time.

[0022] Since sodium stannate (Na 2 [Sn(OH) 6 ]) has a pronounced tendency to precipitate as β-stannic acid (hydrated tin(IV) oxide) in non-alkaline aqueous solution, the stannate was previously converted into the corresponding carboxylate in a dicarboxylic acid solution, such as oxalic acid, malonic acid or succinic acid, without isolating the reaction product.

[0023] Each of the above-mentioned compounds led to the described reduction in roughness on its own, but it was shown that the effects of molybdenum(IV) oxide and sodium stannate add up, so that the corresponding combination in particular delivers results that are equivalent to the chromic acid-containing processes.

[0024] 0.1 to 100 g / L sodium stannate, preferably pre-dissolved in 5 to 100 g / L dicarboxylic acid, and / or 0.1 to 100 g / L molybdenum(VI) oxide are added to the base electrolyte, although it should be noted that higher concentrations of the latter may only dissolve completely during anodizing.

[0025] The mentioned base electrolyte and chemical compounds were operated under high-speed anodization conditions.

[0026] According to the invention, the current density is in the range from 10 A / dm 2 to 500 A / dm 2 , the temperature is between -5 °C and 55 °C, and the flow rate is in the range from 0.1 m 3 / h to 15 m 3 / h. The coating time is between 5 and 60 s. The applied voltage is between 10 V and 120 V, which can be DC or pulsed (unipolar).

[0027] Using the process and electrolyte composition mentioned above, it is possible to produce layers by anodization, especially aluminum alloys and also aluminum casting alloys with high silicon contents.

[0028] The resulting wear and corrosion protection layers have only a slightly higher roughness compared to the non-anodized surface.

[0029] Depending on the alloy and surface condition before anodization, a roughness of Ra < 0.8 can be achieved with the described process (even with high silicon contents of e.g. 12 wt%).

[0030] The achieved roughness of the layers produced with the composition according to the invention depends on the initial roughness before anodization and depending on the aluminum alloy to be anodized, preferably in the range Ra <0.8, particularly preferably <0.6, most preferably <0.4.

[0031] The invention describes a process for the selective anodization of aluminum surfaces. Specifically, a process is described that is free of chromium trioxide (chromium(VI) oxide) without sacrificing the advantages of the conventional chromium(VI) oxide-containing process.

[0032] The advantages of the invention include, in summary, the unrestricted possibility of selective anodization in a coating cell with a short coating time (< 60 s) as well as the required layer properties such as layer thickness (> 5 µm), hardness (> 250 HV 0.01 ) and roughness Ra < 0.8.

[0033] The following examples serve to illustrate the invention.

[0034] The following examples of high-speed anodization serve to demonstrate the use of the electrolyte compositions according to the invention and thus the possible replacement of chromium trioxide-containing electrolytes without compromising the technically required minimum layer characteristics.

[0035] The selective anodization was carried out according to the setup as initially described and in Figure 1presented by varying the alloys described below, as well as the electrolyte compositions and process parameters according to the invention. The comparative examples were also carried out in the same way. Round flat samples of the respective alloy were used as samples. The measurements of the various roughness parameters served to determine the roughening caused by anodization and were carried out before and after anodization using a tactile testing device from Perthen or Mahr in accordance with DIN EN ISO 4287.

[0036] The coating hardness was measured according to the Vickers method using a Matsuzawa MMT-X 7B device in accordance with DIN EN ISO 4516, DIN EN ISO 4545-1, and DIN EN ISO 6507-1:2018. The coating thickness was determined on a cross-section using a Polyvar Met microscope in accordance with DIN ISO 1463. Comparative example 1: Anodization of EN AC-Al Si12 with chromium trioxide-containing electrolyte

[0037] The following reference values ​​are available for the material mentioned, using a sulfuric acid-based electrolyte containing chromium trioxide. Anodization was carried out using the following parameters: Flow rate: 7.3 m 3 / h Temperature: 26 °C Voltage: 46 V Current density: 340 A / dm 2 (at the beginning), 40 A / dm 2 (at the end) Anodization time: 48 s Table 1: Coating properties with chromium trioxide-containing electrolyte, Comparative Example 1 Hardness HV 0.010 Layer thickness in µm roughness Ra Rz Rpk Rmax >300 (360) approx. 12 previously afterward previously afterward previously afterward previously afterward 0,037 0,609 0,321 3,396 0,055 0,431 0,401 4,226 Comparative example 2: Anodization of EN AC-Al Si12 with chromium trioxide-free electrolyte

[0038] The following reference values ​​are available for the material mentioned, using a sulfuric acid-based and chromium trioxide-free electrolyte (the base electrolyte of the electrolytes according to the invention). Anodization was carried out using the following parameters: Flow rate: 3.0 m 3 / h Temperature: 10 °C Voltage: 40 V (unipolar, pulsed at 10 Hz [60 ms pulse / 40 ms pause]) Current density: 393 A / dm 2 (at the beginning), 21 A / dm 2 (at the end) Anodization time: 6 s) Table 2: Electrolyte composition for high-speed anodization, chromium trioxide-free electrolyte, Comparative Example 2 Designation Comparison example 2 Quantity in g / l Amidosulfuric acid 100,00 Magnesium sulfate heptahydrate 100,00 Sulfuric acid conc. 38,5 Table 3: Coating properties with chromium trioxide-free electrolyte, comparative example 2 Hardness HV 0.010 Layer thickness in µm roughness Ra Rz Rpk Rmax >300 (360) 14,5 previously afterward previously afterward previously afterward previously afterward 0,06 2,29 0,50 10,62 0,66 12,54 Comparative example 3: Anodization of EN AW-6061 with chromium trioxide-containing electrolyte

[0039] The following reference values ​​are available for the material mentioned, using a sulfuric acid-based electrolyte containing chromium trioxide. Anodization was carried out using the following parameters: Flow rate: 2.5 m 3 / h Temperature: 26 °C Voltage: 34 V Current density: 183 A / dm 2 (at the beginning), 25 A / dm 2 (at the end) Anodization time: 34 s Table 4: Coating properties with chromium trioxide-containing electrolyte, comparative example 3 Hardness HV 0.010 Layer thickness in µm roughness Ra Rz Rpk Rmax >400 (451) 7,7 previously afterward previously afterward previously afterward previously afterward 0,087 0,134 0,548 0,898 0,077 0,128 0,637 1,49 Comparative example 4: Anodization of EN AW-6061 with chromium trioxide-free electrolyte

[0040] The following reference values ​​are available for the material mentioned, using a sulfuric acid-based, chromium trioxide-free electrolyte. Anodization was carried out using the following parameters: Flow rate: 3.0 m 3 / h Temperature: 10 °C Voltage: 55 V (unipolar, pulsed at 10 Hz [60 ms pulse / 40 ms pause]) Current density: 432 A / dm 2 (at the beginning), 51 A / dm 2 (at the end) Anodization time: 5 s) Table 5: Electrolyte composition for high-speed anodization, chromium trioxide-free electrolyte, Comparative Example 4 Designation Comparison example 4 Quantity in g / l Amidosulfuric acid 100,00 Magnesium sulfate heptahydrate 100,00 Sulfuric acid conc. 38,5 Table 6: Coating properties with chromium trioxide-free electrolyte, comparative example 4 Hardness HV 0.010 Layer thickness in µm roughness Ra Rz Rpk Rmax 250 18,0 previously afterward previously afterward previously afterward previously afterward 0,07 0,39 0,64 2,80 0,15 0,37 1,13 3,47 Invention Example 1: Anodization of EN AC-Al Si12 with inventive electrolyte without chromium trioxide

[0041] The following reference values ​​are available for the material mentioned above using the electrolyte according to the invention. Anodization was carried out using the following parameters: Flow rate: 6.0 m 3 / h Temperature: 10 °C Voltage: 55 V (unipolar, pulsed at 10 Hz [60 ms pulse / 40 ms pause]) Current density: 129 A / dm 2 (at the beginning), 3.2 A / dm 2 (at the end) Anodization time: 40 s Table 7: Electrolyte composition for high-speed anodization, inventive electrolyte without chromium trioxide, Invention Example 1 Designation Invention Example 1 Quantity in g / l Amidosulfuric acid 50,00 Magnesium sulfate heptahydrate 50,00 Sulfuric acid conc. 19,25 Oxalic acid dihydrate 50,00 Sodium stannate trihydrate 30,00 Molybdenum oxide 20,00 Table 8: Layer properties with inventive electrolyte without chromium trioxide, Invention Example 1 Hardness HV 0.010 Layer thickness in µm roughness Ra Rz Rpk Rmax >300 (415) 12,25 previously afterward previously afterward previously afterward previously afterward 0,307 0,493 0,59 3,272 - - 0,824 4,665

[0042] According to the object of the invention, the smallest possible increase in roughness (roughening) by the resulting anodization layer is desirable. Smaller resulting roughness values ​​after anodization are therefore better. It can be seen that the simultaneous use of sodium stannate and molybdenum oxide (Inventive Example 1) achieves roughness values ​​at least as those also possible with chromium trioxide-containing compositions (Comparative Example 1). The Ra value is considered the key benchmark for this.

[0043] The micrographs in Figure 2 illustrate the characteristics of the layer and the low roughness according to Invention Example 1. Figure 2A documents a metallographic section of the anodic layer according to invention example 1 at 500x magnification. Figure 2B also represents a metallographic section of the layer of Invention Example 1 at 1000x magnification.

[0044] Further examples of high-speed anodization with electrolytes according to the invention for other materials are listed below. Corresponding comparative values ​​with a chromium trioxide-containing or chromium trioxide-free base electrolyte can be found in the comparative examples above. Invention Example 2: Anodization of EN AW-6061 with inventive electrolyte without chromium trioxide

[0045] The following reference values ​​are available for the material mentioned above using the inventive electrolyte without chromium trioxide. The electrolyte compositions of Invention Example 2 and the three variants, as well as the associated anodization parameters, can be found in the following tables. Table 9: Electrolyte compositions for high-speed anodization, inventive electrolyte without chromium trioxide, Invention Example 2 (in 3 variants) Designation Invention Example 2 Variant 1 Variant 2 Variant 3 Quantity in g / l Quantity in g / l Quantity in g / l Amidosulfuric acid 50,00 50,00 50,0 Magnesium sulfate heptahydrate 50,00 50,00 50,0 Sulfuric acid conc. 19,25 19,25 19,25 Oxalic acid dihydrate 50,00 50,00 50,00 Natriu mstan nat-tri hyd rat 30,00 10 Molybdenum oxide 20,00 50,00 Table 10: Test parameters with the inventive electrolyte without chromium trioxide, Invention Example 2 (in 3 variants) Variant (V) Flow rate in m 3 / h Temperature in °C Voltage in V (unipolar) Current density at the beginning in A / dm 2< Current density at the end in A / dm 2< Time in s 1 1,4 10 60 [at 10 Hz (60 ms pulse / 40 ms pause)] 181 23 49 2 1,2 10 20 [at 10 Hz (60 ms pulse / 40 ms pause)] 83 41 14 3 3,0 2 55 [at 10 Hz (60 ms pulse / 40 ms pause)] 451 87 8 Table 11: Layer properties with inventive electrolyte without chromium trioxide, Invention Example 2 (in 3 variants) V Hardness HV 0.010 Layer thickness in µm roughness Ra Rz Rpk Rmax previously afterward previously afterward previously afterward previously afterward 1 >400 (486) 24,1 0,083 0,179 0,711 1,369 0,145 0,277 1,089 1,615 2 383 7,8 0,088 0,192 0,724 1,532 0,167 0,295 1,217 1,666 3 320 16 0,074 0,187 0,640 1,451 0,153 0,319 1,130 1,750

[0046] It can be seen that the roughness values ​​for variant 1 are lower than for variant 2 and / or variant 3. This confirms the additive effect of the additives sodium stannate and molybdenum oxide, which leads to a better result compared to the respective variant with sodium stannate or molybdenum oxide alone. However, the inventive compositions with either sodium stannate or molybdenum oxide alone are always significantly better than the pure base electrolyte (Comparative Example 4). The partially different layer thicknesses and / or initial roughnesses of the samples from Inventive Example 2 and Comparative Example 4 must be taken into account when considering the absolute values, but do not change the function of the additives mentioned and the described relationships.

[0047] The micrographs in Figure 3 illustrate the surface quality and the low roughness according to invention example 2, variant 1. In Figure 3AIn the metallographic section, the anodic layer according to invention example 2 can be seen, V1 at 500x magnification. Figure 3B documents the anodic layer in the metallographic section of invention example 2, V1 at 1000x magnification. Invention Example 3: Anodization of EN AC-AlSi4 with inventive electrolyte without chromium trioxide

[0048] The following reference values ​​are available for the material mentioned above using the inventive electrolyte without chromium trioxide. The electrolyte composition of Invention Example 3 and the associated anodization parameters can be found in the following tables. Table 12: Electrolyte composition for high-speed anodization, inventive electrolyte without chromium trioxide, Invention Example 3 Designation Invention Example 3 Quantity in g / l Amidosulfuric acid 50,00 Magnesium sulfate heptahydrate 50,00 Sulfuric acid conc. 19,25 Oxalic acid dihydrate 50,00 Sodium stannate trihydrate 30,00 Molybdenum oxide 20,00 Table 13: Test parameters with the inventive electrolyte without chromium trioxide, Invention Example 3 (in 4 variants) Variant (V) Flow rate in m 3 / h Temperature in °C Voltage in V Current density at the beginning in A / dm 2< Current density at the end in A / dm 2< Time in s 1 1,2 10 25 17 12 12 2 1,2 10 30 35 29 7 3 1,2 40 20 21 17 13 4 1,2 40 25 44 39 7 Table 14: Layer properties with inventive electrolyte without chromium trioxide, Invention Example 3 (in 4 variants) V Hardness HV 0.010 Layer thickness in µm roughness Ra Rz Rpk Rmax previously afterward previously afterward previously afterward previously afterward 1 380 - 390 6 0,071 0,795 1,009 4,212 0,131 0,411 1,581 4,456 2 380 - 390 6 0,096 0,569 1,281 3,795 0,124 0,321 1,737 4,067 3 380 - 390 6 0,098 0,990 1,610 5,092 0,172 0,475 2,097 5,329 4 380 - 390 6 0,105 0,712 1,498 4,233 0,138 0,335 1,854 4,531 Invention Example 4: Anodization of EN AC-AlSi4 with inventive electrolyte without chromium trioxide

[0049] The following reference values ​​are available for the material mentioned above using the inventive electrolyte without chromium trioxide. The electrolyte composition of Invention Example 4 and the associated anodization parameters can be found in the following tables. Table 15: Electrolyte composition for high-speed anodization, inventive electrolyte without chromium trioxide, Invention Example 4 Designation Invention Example 4 Quantity in g / l Amidosulfuric acid 50,00 Magnesium sulfate heptahydrate 50,00 Sulfuric acid conc. 19,25 Oxalic acid dihydrate 50,00 Sodium stannate trihydrate 10,00 Molybdenum oxide 20,00 Table 16: Test parameters with inventive electrolyte without chromium trioxide, Invention Example 4 (in 4 variants) Variant (V) Flow rate in m 3 / h Temperature in °C Voltage in V Current density at the beginning in A / dm 2< Current density at the end in A / dm 2< Time in s 1 1,2 10 20 18 10 15 2 1,2 10 25 25 21 9 3 1,2 10 25 29 18 9 4 1,2 40 20 33 28 7 Table 17: Layer properties with inventive electrolyte without chromium trioxide, Invention Example 4 (in 4 variants) V Hardness HV 0.010 Layer thickness in µm roughness Ra Rz Rpk Rmax previously afterward previously afterward previously afterward previously afterward 1 430 6,3 0,101 0,783 1,297 4,524 0,129 0,378 1,878 4,851 2 430 6,3 0,090 0,808 1,360 4,503 0,132 0,461 1,921 4,722 3 430 6,3 0,091 0,665 1,323 3,985 0,121 0,378 1,735 4,349 4 430 6,3 0,097 0,844 1,388 4,298 0,127 0,400 1,668 4,532

[0050] Based on the invention examples 3 and 4, it becomes clear that the invention can also be used with variation of the composition of the electrolyte according to the invention (amounts of sodium stannate, molybdenum oxide or combinations thereof) and variation of the anodization parameters (temperature, voltage, current density) without having any loss in anodization time, layer thickness, layer hardness or roughness parameters. Invention Example 5: Anodization of EN AW-6061 with inventive electrolyte without chromium trioxide

[0051] The following reference values ​​are available for the material mentioned above using the electrolyte according to the invention. The electrolyte compositions of the two variants can be found in the table below, and anodization was carried out for both variants using the following parameters: Flow rate: 1.4 m 3 / h Temperature: 10°C Voltage: 60 V (unipolar, pulsed at 10 Hz [60 ms pulse / 40 ms pause]) Current density: 180 A / dm 2 (at the beginning), 25 A / dm 2 (at the end) Anodization time: 50 s Table 18: Electrolyte compositions for high-speed anodization, inventive electrolyte without chromium trioxide, Invention Example 5 (in 2 variants) Designation Invention Example 5 Variant 1 Variant 2 Quantity in g / l Quantity in g / l Amidosulfuric acid 50,00 50,00 Magnesium sulfate heptahydrate 50,00 50,00 Sulfuric acid conc. 19,25 19,25 Oxalic acid dihydrate 80,00 Malonic acid 80,00 Sodium stannate trihydrate 30,00 30,00 Molybdenum oxide 20,00 20,00 Table 19: Layer properties with inventive electrolyte without chromium trioxide, Invention Example 5 (in 2 variants) V Hardness HV 0.010 Layer thickness in µm roughness Ra Rz Rpk Rmax previously afterward previously afterward previously afterward previously afterward 1 >400 (481) 22,3 0,087 0,175 0,698 1,312 0,135 0,262 1,007 1,584 2 >400 (473) 21,9 0,092 0,181 0,732 1,420 0,121 0,301 1,113 1,638 Invention Example 6: Anodization of EN AW-6061 with inventive electrolyte without chromium trioxide

[0052] The following reference values ​​are available for the material mentioned above using the electrolyte according to the invention. The electrolyte compositions of the two variants can be found in the table below, and anodization was carried out for both variants using the following parameters: Flow rate: 1.2 m 3 / h Temperature: 10 °C Voltage: 20 V (unipolar, pulsed at 10 Hz [60 ms pulse / 40 ms pause]) Current density: 82 A / dm 2 (at the beginning), 40 A / dm 2 (at the end) Anodization time: 15 s Table 20: Electrolyte compositions for high-speed anodization, inventive electrolyte without chromium trioxide, Invention Example 6 (in 2 variants) Designation Invention Example 6 Variant 1 Variant 2 Quantity in g / l Quantity in g / l Amidosulfuric acid 50,00 50,00 Magnesium sulfate heptahydrate 50,00 50,00 Sulfuric acid conc. 19,25 19,25 Oxalic acid dihydrate 25,00 Malonic acid 25,00 Sodium stannate trihydrate 10,00 10,00 Table 21: Layer properties with inventive electrolyte without chromium trioxide, Invention Example 6 (in 2 variants) V Hardness HV 0.010 Layer thickness in µm roughness Ra Rz Rpk Rmax previously afterward previously afterward previously afterward previously afterward 1 383 7,8 0,091 0,192 0,695 1,489 0,143 0,290 1,196 1,801 2 375 8,3 0,105 0,210 0,774 1,558 0,160 0,288 1,097 1,744

Claims

1. Composition for selective anodisation, comprising the substances amidosulphuric acid, magnesium sulphate and concentrated sulphuric acid as a base electrolyte, characterised in that additionally sodium stannate and / or molybdenum oxide are contained.

2. Composition as claimed in claim 1, characterised in that concentrated sulphuric acid in a range from 5 to 150 g / L, magnesium sulphate hydrate in a range from 5 to 200 g / L and amidosulphonic acid in a range from 5 to 200 g / L are contained.

3. Composition as claimed in claim 1 or 2, characterised in that 0,1 to 100 g / L, preferably 5 to 50 g / L, particularly preferably 20 to 40 g / L sodium stannate is contained.

4. Composition as claimed in claim 3, characterised in that the sodium stannate is contained pre-dissolved in 5 to 100 g / L, preferably 20 to 80 g / L, particularly preferably 40 to 60 g / L of dicarboxylic acid.

5. Composition as claimed in any one of the preceding claims, characterised in that 0,1 to 100 g / L, preferably 5 to 50 g / L, particularly preferably 20 to 50 g / L molybdenum(VI) oxide is contained.

6. Composition as claimed in any one of the preceding claims, characterised in that 50 g / L amidosulphuric acid, 50 g / L magnesium sulphate heptahydrate and 19,25 g / L concentrated sulphuric acid and additionally 30 g / L sodium stannate as well as 50 g / L oxalic acid dihydrate and 20 g / L molybdenum(VI) oxide are contained.

7. Method for selective anodisation of a substrate, which comprises: - providing the substrate having a surface which is to be selectively coated, is arranged in a tool and forms a coating cell; - selectively bathing the surface with a composition for selective anodisation as claimed in any one of claims 1 to 6; and - applying an electric current between substrate and tool for selective anodisation of the surface.

8. Method as claimed in claim 7, characterised in that current density is between 10 - 500, preferably 50 - 500, particularly preferably 80 - 400 A / dm2.

9. Method as claimed in claim 7 or 8, characterised in that the temperature of the composition is between -5 and +55, preferably 0 and 30, particularly preferably 0 and 15 degrees Celsius.

10. Method as claimed in any one of claims 7 to 9, characterized in that the flow rate of the composition in the chamber is between 0,1 and 15, preferably 1 and 10 m3 / h.

11. Method as claimed in any one of claims 7 to 10, characterised in that the coating duration is between 5 and 60 s.

12. Method as claimed in any one of claims 7 to 11, characterised in that the applied voltage is between 10 V and 120 V, wherein it can be present as a DC voltage or a pulsed voltage.

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