Method and composition for selective anodization

Abstract

A composition for selective anodization, comprising the substances amidosulphuric acid, magnesium sulphate and concentrated sulphuric acid as a base electrolyte and additionally sodium stannate and/or molybdenum oxide. A corresponding method of selectively anodizing a substrate or workpiece includes providing a substrate having a surface which is to be selectively coated, where the substrate is arranged in a tool and forms a coating cell, selectively bathing the surface with the composition for selective anodization, and applying an electric current between substrate (anode) and tool (cathode) for selective anodization of the surface.

Claims

1. A composition for selective anodization, said composition comprising: amidosulphuric acid, magnesium sulphate and concentrated sulphuric acid as a base electrolyte, and additionally sodium stannate and/or molybdenum oxide, wherein the composition is free of chromium trioxide.

2. The composition as claimed in claim 1, wherein 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. The composition as claimed in claim 2, wherein 0.1 to 100 g/L sodium stannate is contained.

4. The composition as claimed in claim 3, further comprising dicarboxylic acid, and wherein the sodium stannate is contained pre-dissolved in 5 to 100 g/L of the dicarboxylic acid.

5. The composition as claimed in claim 4, wherein 0.1 to 100 g/L molybdenum(VI) oxide is contained.

6. The composition as claimed in claim 5, wherein 50 g/L amidosulphuric acid, 50 g/L magnesium sulphate heptahydrate, 19.25 g/L concentrated sulphuric acid, 30 g/L sodium stannate, 50 g/L oxalic acid dihydrate and 20 g/L molybdenum(VI) oxide are contained.

7. The composition as claimed in claim 1, wherein 0.1 to 100 g/L sodium stannate is contained.

8. The composition as claimed in claim 7, further comprising dicarboxylic acid, and wherein the sodium stannate is contained pre-dissolved in 5 to 100 g/L of the dicarboxylic acid.

9. The composition as claimed in claim 1, wherein 0.1 to 100 g/L molybdenum(VI) oxide is contained.

10. The composition as claimed in claim 1, wherein 50 g/L amidosulphuric acid, 50 g/L magnesium sulphate heptahydrate, 19.25 g/L concentrated sulphuric acid, 30 g/L sodium stannate, 50 g/L oxalic acid dihydrate and 20 g/L molybdenum(VI) oxide are contained.

11. A method for selective anodization of a substrate, comprising: providing a substrate having a surface which is to be selectively coated, wherein the substrate is arranged in a tool and forms a coating cell; selectively bathing the surface with a composition for selective anodization as claimed in claim 1; and applying an electric current between substrate and tool for selective anodization of the surface.

12. The method as claimed in claim 11, wherein the current density is between 10-500 A/dm2.

13. The method as claimed in claim 12, wherein the temperature of the composition is between ?5 and +55 degrees Celsius.

14. The method as claimed in claim 13, wherein a flow rate of the composition in the chamber is between 0.1 and 15 m.sup.3/h.

15. The method as claimed in claim 14, wherein said applying an electric current comprises applying an electric current for between 5 and 60 s.

16. The method as claimed in claim 15, wherein said applying an electric current comprises applying a voltage between 10 V and 120 V, and wherein it can be present as a DC voltage or a pulsed voltage.

17. The method as claimed in claim 11, wherein the temperature of the composition is between ?5 and +55 degrees Celsius.

18. The method as claimed in claim 11, wherein a flow rate of the composition in the chamber is between 0.1 and 15 10 m.sup.3/h.

19. The method as claimed in claim 11, wherein said applying an electric current comprises applying an electric current for between 5 and 60 s.

20. The method as claimed in claim 11, wherein said applying an electric current comprises applying a voltage between 10 V and 120 V, and wherein it can be present as a DC voltage or a pulsed voltage.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic illustration of a tool for selective anodization;

(2) FIGS. 2A and 2B are micrographs illustrating characteristics of the layer and low roughness of an example in accordance with the present invention; and

(3) FIGS. 3A and 3B are micrographs illustrating the surface characteristics and low roughness of another example in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) The following examples are used for the purpose of explaining the invention. The following examples of high-speed anodization serve to demonstrate the use of the electrolyte compositions in accordance with the invention and thus the possible replacement of chromium trioxide-containing electrolytes without losing the technically required minimum layer features.

(5) The selective anodization was effected according to the structure as described initially and shown in FIG. 1, with variation in the alloys described hereinafter as well as the inventive electrolyte compositions and process parameters. The comparative examples were also performed in the same way. Round flat samples of the respective alloy were used as samples. The measurement of the different roughness characteristic variables served to determine the roughening caused by the anodization and was performed before and after anodization by means of a tactile testing device from the company Perthen or Mahr in accordance with DIN EN ISO 4287.

(6) The layer hardness was measured according to Vickers with a device from the company Matsuzawa MMT-X 7B in accordance with DIN EN ISO 4516, DIN EN ISO 4545-1 and DIN EN ISO 6507-1:2018. The layer thickness was determined on the cross-section polish 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

(7) The following reference values are given for said material with a sulphuric acid-based and chromium trioxide-containing electrolyte. The anodization was effected with the following parameters: Flow rate: 7.3 m.sup.3/h Temperature: 26? C. Voltage: 46 V Current density: 340 A/dm.sup.2 (start), 40 A/dm.sup.2 (end) Anodization duration: 48 s)

(8) TABLE-US-00001 TABLE 1 Layer properties with chromium trioxide-containing electrolyte, comparative example 1 Layer Roughness Hardness thickness Ra Rz Rpk Rmax HV 0.010 in ?m Before After Before After Before After Before After >300 (360) ca. 12 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

(9) The following reference values are given for said material with a sulphuric acid-based and chromium trioxide-free electrolyte (base electrolyte of the electrolytes in accordance with the invention). The anodization was effected with the following parameters: Flow rate: 3.0 m.sup.3/h Temperature: 10? C. Voltage: 40 V (unipolar, pulsed with 10 Hz [60 ms pulse/40 ms pause]) Current density: 393 A/dm.sup.2 (start), 21 A/dm.sup.2 (end) Anodization duration: 6 s)

(10) TABLE-US-00002 TABLE 2 Electrolyte composition for high-speed anodization, chromium trioxide-free electrolyte, comparative example 2 Comparative example 2 Designation Quantity in g/l Amidosulphuric acid 100.00 Magnesium sulphate heptahydrate 100.00 Sulphuric acid conc. 38.5

(11) TABLE-US-00003 TABLE 3 Layer properties with chromium trioxide-free electrolyte, comparative example 2 Layer Roughness Hardness thickness Ra Rz Rpk Rmax HV 0.010 in ?m Before After Before After Before After Before After >300 (360) 14.5 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

(12) The following reference values are given for said material with a sulphuric acid-based and chromium trioxide-containing electrolyte. The anodization was effected with the following parameters: Flow rate: 2.5 m.sup.3/h Temperature: 26? C. Voltage: 34 V Current density: 183 A/dm.sup.2 (start), 25 A/dm.sup.2 (end) Anodization duration: 34 s)

(13) TABLE-US-00004 TABLE 4 Layer properties with chromium trioxide-containing electrolyte, comparative example 3 Layer Roughness Hardness thickness Ra Rz Rpk Rmax HV 0.010 in ?m Before After Before After Before After Before After >400 (451) 7.7 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

(14) The following reference values are given for said material with a sulphuric acid-based and chromium trioxide-free electrolyte. The anodization was effected with the following parameters: Flow rate: 3.0 m.sup.3/h Temperature: 10? C. Voltage: 55 V (unipolar, pulsed with 10 Hz [60 ms pulse/40 ms pause]) Current density: 432 A/dm.sup.2 (start), 51 A/dm.sup.2 (end) Anodization duration: 5 s)

(15) TABLE-US-00005 TABLE 5 Electrolyte composition for high-speed anodization, chromium trioxide-free electrolyte, comparative example 4 Comparative example 4 Designation Quantity in g/l Amidosulphuric acid 100.00 Magnesium sulphate heptahydrate 100.00 Sulphuric acid conc. 38.5

(16) TABLE-US-00006 TABLE 6 Layer properties with chromium trioxide-free electrolyte, comparative example 4 Layer Roughness Hardness thickness Ra Rz Rpk Rmax HV 0.010 in ?m Before After Before After Before After Before After 250 18.0 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

(17) The following reference values are given for said material with electrolyte in accordance with the invention. The anodization was effected with the following parameters: Flow rate: 6.0 m.sup.3/h Temperature: 10? C. Voltage: 55 V (unipolar, pulsed with 10 Hz [60 ms pulse/40 ms pause]) Current density: 129 A/dm.sup.2 (start), 3.2 A/dm.sup.2 (end) Anodization duration: 40 s)

(18) TABLE-US-00007 TABLE 7 Electrolyte composition for high-speed anodization, inventive electrolyte without chromium trioxide, invention example 1 Invention example 1 Designation Quantity in g/l Amidosulphuric acid 50.00 Magnesium sulphate heptahydrate 50.00 Sulphuric acid conc. 19.25 Oxalic acid dihydrate 50.00 Sodium stannate trihydrate 30.00 Molybdenum oxide 20.00

(19) TABLE-US-00008 TABLE 8 Layer properties with inventive electrolyte without chromium trioxide, invention example 1 Layer Roughness Hardness thickness Ra Rz Rpk Rmax HV 0.010 in ?m Before After Before After Before After Before After >300 (415) 12.25 0.307 0.493 0.59 3.272 0.824 4.665

(20) In accordance with the object of the invention, it is desirable for the increase in roughness (roughening) caused by the resulting anodization layer to be as small as possible. Smaller resulting roughness values after anodization are consequently better. It can be seen that the simultaneous use of sodium stannate and molybdenum oxide (invention example 1) achieves at least roughness values as can also be achieved with chromium trioxide-containing compositions (comparative example 1). The Ra value is considered to be the important measure for this.

(21) The micrographs in FIGS. 2A and 2B illustrate the characteristics of the layer and the low roughness in accordance with invention example 1. FIG. 2A documents a metallographic polished section of the anodic layer in accordance with invention example 1 at 500? magnification. FIG. 2B likewise illustrates a metallographic polished section of the layer of invention example 1 at 1000? magnification.

(22) Further examples of high-speed anodization with electrolytes in accordance with the invention in other materials are specified below. Corresponding comparative values with a chromium trioxide-containing or chromium trioxide-free base electrolyte can be found in the above comparative examples.

Invention Example 2: Anodization of EN AW-6061 with Inventive Electrolyte without Chromium Trioxide

(23) The following reference values are given for said material with 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.

(24) TABLE-US-00009 TABLE 9 Electrolyte compositions for high-speed anodization, inventive electrolyte without chromium trioxide, invention example 2 (in 3 variants) Invention example 2 Variant 1 Variant 2 Variant 3 Designation Quantity in g/l Quantity in g/l Quantity in g/l Amidosulphuric acid 50.00 50.00 50.0 Magnesium sulphate 50.00 50.00 50.0 heptahydrate Sulphuric acid conc. 19.25 19.25 19.25 Oxalic acid dihydrate 50.00 50.00 50.00 Sodium stannate 30.00 10 trihydrate Molybdenum oxide 20.00 50.00

(25) TABLE-US-00010 TABLE 10 Test parameters with inventive electrolyte without chromium trioxide, invention example 2 (in 3 variants) Current Current density density Flow Voltage at start at end Variant rate in Temperature in V in in Time (V) m.sup.3/h in ? C. (unipolar) A/dm.sup.2 A/dm.sup.2 in s 1 1.4 10 60 181 23 49 [at 10 Hz (60 ms pulse/40 ms pause)] 2 1.2 10 20 83 41 14 [at 10 Hz (60 ms pulse/40 ms pause)] 3 3.0 2 55 451 87 8 [at 10 Hz (60 ms pulse/40 ms pause)]

(26) TABLE-US-00011 TABLE 11 Layer properties with inventive electrolyte without chromium trioxide, invention example 2 (in 3 variants) Layer Roughness Hardness thickness Ra Rz Rpk Rmax V HV 0.010 in ?m Before After Before After Before After Before After 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

(27) It can be seen that the roughness values in variant 1 are less than in variant 2 and/or variant 3. This confirms the additive effect of the additives sodium stannate and molybdenum oxide, which leads to the better result in comparison with the respective variant exclusively with sodium stannate or molybdenum oxide. However, the compositions in accordance with the invention either with sodium stannate or molybdenum oxide only are always significantly better than the pure base electrolyte (comparative example 4). The partly different layer thicknesses and/or initial roughnesses of the samples from invention example 2 and comparative example 4 must be taken into account when considering the absolute values but do not change anything about the function of said additives and the described relationships.

(28) The micrographs in FIGS. 3A and 3B illustrate the surface characteristics and the low roughness according to invention example 2, variant 1. FIG. 3A shows a metallographic polished section of the anodic layer in accordance with invention example 2, V1 at 500? magnification. FIG. 3B documents the anodic layer in the metallographic polished section of invention example 2, V1 at 1000? magnification.

Invention Example 3: Anodization of EN AC-AlSi4 with Inventive Electrolyte without Chromium Trioxide

(29) The following reference values are given for said material with 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.

(30) TABLE-US-00012 TABLE 12 Electrolyte composition for high-speed anodization, inventive electrolyte without chromium trioxide, invention example 3 Invention example 3 Designation Quantity in g/l Amidosulphuric acid 50.00 Magnesium sulphate heptahydrate 50.00 Sulphuric acid conc. 19.25 Oxalic acid dihydrate 50.00 Sodium stannate trihydrate 30.00 Molybdenum oxide 20.00

(31) TABLE-US-00013 TABLE 13 Test parameters with inventive electrolyte without chromium trioxide, invention example 3 (in 4 variants) Current Current density density Variant Flow rate Temperature Voltage at start at end Time (V) in m.sup.3/h in ? C. in V in A/dm.sup.2 in A/dm.sup.2 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

(32) TABLE-US-00014 TABLE 14 Layer properties with inventive electrolyte without chromium trioxide, invention example 3 (in 4 variants) Layer Roughness Hardness thickness Ra Rz Rpk Rmax V HV 0.010 in ?m Before After Before After Before After Before After 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

(33) The following reference values are given for said material with inventive electrolyte without chromium trioxide. The electrolyte composition of invention example 4 and the associated anodization parameters can be found in the following tables.

(34) TABLE-US-00015 TABLE 15 Electrolyte composition for high-speed anodization, inventive electrolyte without chromium trioxide, invention example 4 Invention example 4 Designation Quantity in g/l Amidosulphuric acid 50.00 Magnesium sulphate heptahydrate 50.00 Sulphuric acid conc. 19.25 Oxalic acid dihydrate 50.00 Sodium stannate trihydrate 10.00 Molybdenum oxide 20.00

(35) TABLE-US-00016 TABLE 16 Test parameters with inventive electrolyte without chromium trioxide, invention example 4 (in 4 variants) Current Current density density Variant Flow rate Temperature Voltage at start at end Time (V) in m.sup.3/h in ? C. in V in A/dm.sup.2 in A/dm.sup.2 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

(36) TABLE-US-00017 TABLE 17 Layer properties with inventive electrolyte without chromium trioxide, invention example 4 (in 4 variants) Layer Roughness Hardness thickness Ra Rz Rpk Rmax V HV 0.010 in ?m Before After Before After Before After Before After 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

(37) It becomes apparent from invention examples 3 and 4 that the invention can also be used with a variation of the composition of the electrolyte in accordance with the invention (amounts of sodium stannate, molybdenum oxide or combinations thereof) and a variation of the anodization parameters (temperature, voltage, current density) without having losses in anodization duration, layer thickness, layer hardness or roughness parameters.

Invention example 5: Anodization of EN AW-6061 with inventive electrolyte without chromium trioxide

(38) The following reference values are given for said material with electrolyte in accordance with the invention. The electrolyte compositions of the two variants can be found in the table below and the anodization was effected for both variants at the following parameters: Flow rate: 1.4 m.sup.3/h Temperature: 10? C. Voltage: 60 V (unipolar, pulsed with 10 Hz [60 ms pulse/40 ms pause]) Current density: 180 A/dn.sup.2 (start), 25 A/dn.sup.2 (end) Anodization duration: 50 s)

(39) TABLE-US-00018 TABLE 18 Electrolyte compositions for high-speed anodization, inventive electrolyte without chromium trioxide, invention example 5 (in 2 variants) Invention example 5 Variant 1 Variant 2 Designation Quantity in g/l Quantity in g/l Amidosulphuric acid 50.00 50.00 Magnesium sulphate heptahydrate 50.00 50.00 Sulphuric 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

(40) TABLE-US-00019 TABLE 19 Layer properties with inventive electrolyte without chromium trioxide, invention example 5 (in 2 variants) Layer Roughness Hardness thickness Ra Rz Rpk Rmax V HV 0.010 in ?m Before After Before After Before After Before After 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

(41) The following reference values are given for said material with electrolyte in accordance with the invention. The electrolyte compositions of the two variants can be found in the table below and the anodization was effected for both variants at the following parameters: Flow rate: 1.2 m.sup.3/h Temperature: 10? C. Voltage: 20 V (unipolar, pulsed with 10 Hz [60 ms pulse/40 ms pause]) Current density: 82 A/dm.sup.2 (start), 40 A/dm.sup.2 (end) Anodization duration: 15 s)

(42) TABLE-US-00020 TABLE 20 Electrolyte compositions for high-speed anodization, inventive electrolyte without chromium trioxide, invention example 6 (in 2 variants) Invention example 6 Variant 1 Variant 2 Designation Quantity in g/l Quantity in g/l Amidosulphuric acid 50.00 50.00 Magnesium sulphate heptahydrate 50.00 50.00 Sulphuric acid conc. 19.25 19.25 Oxalic acid dihydrate 25.00 Malonic acid 25.00 Sodium stannate trihydrate 10.00 10.00

(43) TABLE-US-00021 TABLE 21 Layer properties with inventive electrolyte without chromium trioxide, invention example 6 (in 2 variants) Layer Roughness Hardness thickness Ra Rz Rpk Rmax V HV 0.010 in ?m Before After Before After Before After Before After 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