METHOD FOR PRODUCING A CORROSION-RESISTANT ALUMINUM-SILICON ALLOY CASTING, SUCH CORROSION-RESISTANT ALUMINUM-SILICON ALLOY CASTING AND ITS USE

Abstract

The present invention is related to the field of metal surface preparation by anodizing processes and refers to a method for producing a corrosion-resistant aluminum-silicon alloy casting and more particularly to the optimization of the anodizing cast aluminum parts with high silicon content, by using a multiple step anodizing cycle. Moreover, the present invention refers to a corrosion-resistant aluminum-silicon alloy casting and its use.

Claims

1-18. (canceled)

19. A method for producing a corrosion-resistant aluminum-silicon alloy casting comprising: (a) providing an aluminum-silicon alloy casting and (b) growing a corrosion-protection layer at least partially on the surface of the aluminum-silicon alloy casting with a multi-step anodizing process comprising: (b1) a first step of pre-anodization for oxidizing aluminum at the surface of the casting at a voltage of 1 to 40 V; and (b2) a second step of anodization for oxidizing aluminum and silicon at the surface of the casting at a voltage of 25 to 50 V, wherein the voltage of the second step (b2) is higher than the voltage of the first step (b1) and wherein the first step (b1) and the second step (b2) are conducted in an acidic bath with different organic additives.

20. The method according to claim 19, wherein the voltage applied during the first step (b1) is from 5 to 30 V and/or the voltage applied during the second step (b2) is from 25 to 40 V.

21. The method according to claim 19, wherein the first step (b1) is conducted at a temperature from 1 to 50° C. and/or the second step (b2) is conducted at a temperature from 1 to 50° C.

22. The method according to claim 19, wherein the two steps are conducted in an acidic bath comprising sulfuric acid.

23. The method according to claim 22, wherein the concentration of sulfuric acid is from 50 to 250 g/L.

24. The method according to claim 19, wherein the organic additives are selected from the group consisting of oxalic acid, tartaric acid, glycolic acid, ethylene glycol, and combinations thereof.

25. The method according to claim 19, wherein the first step (b1) of pre-anodization is preceded by at least one of the following pre-treatment steps: (a) a desmutting step in which the aluminum-silicon alloy casting is exposed to an acid, (b) an acidic pre-treatment step in which the aluminum-silicon alloy casting is exposed to an acid, and/or (c) a degreasing step in which the aluminum-silicon alloy casting is exposed to a cleaning agent.

26. The method according to claim 19, wherein the second step (b2) is followed by a sealing process.

27. The method according to claim 26, wherein the sealing process is selected from one of the following processes: (a) hot sealing in which the anodized aluminum alloy is exposed to water with a temperature of 90 to 100° C. and/or surface active agents to remove smut; (b) medium temperature sealing in which the anodized aluminum alloy is exposed to any organic agents or metal salts to improve the sealing quality such as nickel acetate or magnesium acetate; and/or (c) cold sealing with a first sealing step in which the anodized aluminum alloy casting is exposed to a metal salt selected from the group consisting of a nickel salt, a magnesium salt, a chromium salt, and a zirconium salt, and at least one surfactant and a second aging step in which the anodized aluminum alloy is exposed to deionized water and/or at least one surfactant to remove any smut formed on the surface.

28. A corrosion-resistant aluminum-silicon alloy casting having an aluminum oxide film with an average thickness from 4 to 90 μm as corrosion protection layer, wherein the surface of the substrate is substantially free of zero spots, wherein the coverage and zero spot measurements are determined according to the norm TL 212 Issue 2016-12 from Volkswagen, wherein the coverage rate of the surface is determined by a percentage of the examined measurement length and wherein the zero-point width in the microsection must not exceed 60 μm.

29. The corrosion-resistant casting according to claim 28, wherein the aluminum oxide film has an average thickness of 5 to 90 μm.

30. The corrosion-resistant casting according to claim 28, wherein the aluminum oxide film has a ratio between the average highest coating thickness and the average lowest coating thickness of 8:1, wherein the ratio is calculated by taking an image of a cross section of 300 μm by SEM 250X, determining three points with the highest coating thickness and three points with the lowest coating thickness and measuring their thickness, and calculating the average highest coating thickness and the average lowest coating thickness.

31. The corrosion-resistant casting according to claim 28, wherein the surface of the substrate is completely free of zero spots.

32. The corrosion-resistant casting according to claim 28, wherein the corrosion-resistant aluminium-silicon alloy casting aluminum oxide film has L, a, b values of 49 to 65 for L, −0.7 to −0.1 for a, and 1.7 to 4 for b.

33. The corrosion-resistant casting according to claim 28, wherein the aluminum oxide film has a maximum pure silicon concentration of 5 wt.-%.

34. The corrosion-resistant casting according to claim 28, wherein the casting comprises from 0.5 wt.-% to 70 wt.-% silicon.

35. The corrosion-resistant casting according to claim 28, wherein the casting comprises further a metals selected from the group consisting of magnesium, iron, manganese, titanium, copper, chromium, zinc, tin, nickel, lead, silver, beryllium, bismuth, lithium, cadmium, zirconium, vanadium, scandium, and combinations thereof.

36. The corrosion-resistant casting according claim 28, wherein the casting comprises an AlSi.sub.7Mg alloy, an AlSi.sub.10 alloy, an AlSi.sub.12(Fe) alloy, or a combination thereof.

37. A corrosion-resistant aluminum-silicon alloy casting produced by the method of claim 19.

38. An automotive part, aerospace part, or an appliance part prepared from the corrosion-resistant aluminum-silicon alloy casting of claim 28.

Description

[0066] With reference to the following figures and examples, the subject-matter according to the present invention is intended to be explained in more detail without wishing to restrict said subject-matter to the specific embodiments shown here.

[0067] FIG. 1 shows OM 200 X cross section images of a surface produced by a classical anodization process (FIG. 1a) and a surface obtained by using double a step anodization process (FIG. 1b). On the surface of sample of the sample FIG. 1 a), which was produced by classical anodization process, zero spot can be seen. However, on the surface of the sample of FIG. 1 b), which was obtained by using double step anodization, homogeneous and zero spot free anodic oxide layer is seen. High silicon concentration may prevent anodic oxide formation locally. The zone that is not covered by the oxide layer with zero spots should not affect coating properties, which is only possible with minimum 85% oxide coverage.

[0068] FIG. 2 shows SEM SEI 500 X cross section images of a sample A as control group (FIG. 2a), a sample B which was anodized with sulfuric acid and proprietary organic anodizing additive (FIG. 2b), a sample C which was pretreated for 4 minutes and anodized with sulfuric acid and proprietary organic anodizing additive (FIG. 2c) and a sample D which was pretreated for 10 minutes and anodized with sulfuric acid and proprietary organic anodizing additive (FIG. 2d).

[0069] FIG. 3 shows SEM SEI 500 X surface images of a sample A as control group (FIG. 3a), a sample B which was anodized with sulfuric acid and proprietary organic anodizing additive (FIG. 3b), a sample C pretreated for 4 minutes and anodized with sulfuric acid and proprietary organic anodizing additive (FIG. 3c) and a sample D pretreated for 10 minutes and anodized with sulfuric acid and proprietary organic anodizing additive (FIG. 3d).

[0070] FIG. 4 shows NSS results of Samples A, B, C and D after subjecting to NSS for 480 hours according to ISO 9227

EXAMPLES

[0071] 1. Sample Preparation

[0072] The cast aluminum alloys AlSi.sub.7Mg, AlSi.sub.10 and AlSi.sub.12(Fe) samples were cut to size 5×5 inches and degreased by using standard propriety chemicals available in the industry. The first set of samples were anodized using direct current in an acid based bath with different organic additives.

[0073] Degreasing is conducted in Alumal Clean 118 L containing mainly surface active agents for cleaning at 40 g/L. Acidic pretreatment is conducted with e.g pure phosphoric acid at 100% (concentrated). Desmuting is conducted in Nitric acid in 250 g/I. The acidic bath for anodization is composed of Sulfuric acid at a concentration of 200 g/I and the organic additives Alumal Elox 557 in concentration of 30 g/L.

[0074] After anodizing only the samples chosen for the NSS test were colored black at 66° C. for 15 minutes. The samples for surface investigation studies were put directly to nickel fluoride at a concentration of 6 g/Land a pH=5.9 cold seal process followed by a warm rinse bath with deionized water with a conductivity of 25 micro Siemens. The results have been repeated 3 times to show the repeatability.

[0075] Finally an alternative acidic pretreatment was developed to improve the aluminum oxide film properties.

[0076] The aluminum oxide film was characterized with Optical Microscopy (OM) and Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM/EDS) and spectrophotometry and XPS. The corrosion resistance was examined by using Natural Salt Spray (NSS).

[0077] The L, a, b values were measured on a Shimadzu UV-2600 Spectrometer and the measurement wavelength was comprised between 220 and 1400 nm. Then the software COL-UVPC Color Measurement Software calculates the color values of the measured object from the spectra obtained by the spectrophotometer.

[0078] To show the negative effect of the silicon intermetallics standard anodized samples were investigated with OM under polarized light, and SEM/EDS. For cross section examination the samples were cut with precision cutter, polished and finally molded with cold resin. For cross section SEM studies the prepared samples were also sputtered with Au for at least 20 seconds to prevent any charge build up. Finally NSS was applied all the black dyed parts according to ISO 9227:2017 standard for a maximum of 480 hours and the first initiation of the corrosion as well as fading of the color was reported.

[0079] The different conditions used with the samples have been listed in Table 1 to 3 below.

TABLE-US-00001 TABLE 1 Process sequence for the Samples of the examples according to the invention (AlSi7Mg Alloy) AlSi7Mg Alloy Sample Degreasing Acidic Pretreatement Desmutting Pre-anodization A 15 min at 65° C. — — — B 15 min at 65° C. — — — C 15 min at 65° C. — — 5 min at 16 V at 15-18° C. D 15 min at 65° C. 4 min at 91° C. 2 min at 35° C. 5 min at 16 V at 15-18° C. E 15 min at 65° C. 4 min at 91° C. 2 min at 35° C. 5 min at 16 V at 15-18° C. F 5 min at 55-60° C. — 1 min at 35° C. 5 min at 16 V at 15-18° C. G 5 min at 55-60° C. 7 min at 85-90° C. 1 min at 35° C. 5 min at 16 V at 15-18° C. H 5 min at 55-60° C. 3 min at 85-90° C. 1 min at 35° C. 5 min at 16 V at 15-18° C. I 5 min at 55-60° C. 5 min at 85-90° C. 1 min at 35° C. 5 min at 16 V at 15-18° C. J 5 min at 55-60° C. 10 min at 85-90° C. 1 min at 35° C. 5 min at 16 V at 15-18° C. K 5 min at 55-60° C. 10 min at 85-90° C. 1 min at 35° C. 5 min at 20 V at 15-18° C. L 5 min at 55-60° C. 10 min at 85-90° C. 1 min at 35° C. 5 min at 20 V at 15-18° C. M 5 min at 55-60° C. 10 min at 85-90° C. 1 min at 35° C. 5 min at 20 V at 15-18° C. Sample Anodization Sealing Thickness A 10 min at 30 V at 15-18° C. Cold seal 15 min at 35° C.  3 μm Warm rinse 15 min at 66° C. B 30 min at 30 V at 15-18° C. Cold seal 15 min at 35° C. 20 μm Warm rinse 15 min at 66° C. C 15 min at 30 V at 15-18° C. Cold seal 15 min at 35° C.  9 μm Warm rinse 15 min at 66° C. D 15 min at 30 V at 15-18° C. Cold seal 15 min at 35° C. 30 μm Warm rinse 15 min at 66° C. E 15 min at 30 V at 15-18° C. Cold seal 15 min at 35° C. 45 μm Warm rinse 15 min at 66° C. F 10 min at 30 V at 15-18° C. Cold seal 20 min at 35° C. 22 μm Warm rinse 10 min at 70-80° C. G 5 min at 30 V at 15-18° C. Cold seal 20 min at 35° C. 23 μm Warm rinse 10 min at 70-80° C. H 10 min at 30 V at 15-18° C. Cold seal 20 min at 35° C. 36 μm Warm rinse 10 min at 70-80° C. I 20 min at 30 V at 15-18° C. Cold seal 20 min at 35° C. 48 μm Warm rinse 10 min at 70-80° C. J 5 min at 30 V at 15-18° C. Cold seal 20 min at 35° C. 23 μm Warm rinse 10 min at 70-80° C. K 5 min at 30 V at 15-18° C. Cold seal 20 min at 35° C. 20 μm Warm rinse 10 min at 70-80° C. L 10 min at 30 V at 15-18° C. Cold seal 20 min at 35° C. 35 μm Warm rinse 10 min at 70-80° C. M 20 min at 30 V at 15-18° C. Cold seal 20 min at 35° C. 50 μm Warm rinse 10 min at 70-80° C.

TABLE-US-00002 TABLE 2 Process sequence for the samples of the examples according to the invention (AlSi10 alloy) AlSi10 alloy Sample Degreasing Acidic Pre-treatement Desmutting Pre-anodization N 5 min at 55-60° C. — — — O 5 min at 55-60° C. 3 min at 85-90° C. 1 min at 35° C. 5 min at 16 V at 15-18° C. P 5 min at 55-60° C. 5 min at 85-90° C. 1 min at 35° C. 5 min at 16 V at 15-18° C. R 15 min at 55-60° C. 3 min at 85-90° C. 1 min at 35° C. 5 min at 16 V at 15-18° C. S 15 min at 55-60° C. 5 min at 85-90° C. 1 min at 35° C. 5 min at 16 V at 15-18° C. T 5 min at 55-60° C. 5 min at 85-90° C. 1 min at 35° C. 5 min at 20 V at 15-18° C. U 5 min at 55-60° C. 5 min at 85-90° C. 1 min at 35° C. 5 min at 20 V at 15-18° C. Sample Anodization Sealing Thickness N 40 min at 30 V at 15-18° C. Cold seal 20 min at 35° C. 20 Warm rinse 10 min at 70-80° C. O 10 min at 30 V at 15-18° C. Cold seal 20 min at 35° C. 24 μm Warm rinse 10 min at 70-80° C. P 20 min at 30 V at 15-18° C. Cold seal 20 min at 35° C. 29 μm Warm rinse 10 min at 70-80° C. R 10 min at 30 V at 15-18° C. Cold seal 20 min at 35° C. 22 Warm rinse 10 min at 70-80° C. S 20 min at 30 V at 15-18° C. Cold seal 20 min at 35° C. 26 Warm rinse 10 min at 70-80° C. T 10 min at 30 V at 15-18° C. Cold seal 20 min at 35° C. 25 Warm rinse 10 min at 70-80° C. U 20 min at 30 V at 15-18° C. Cold seal 20 min at 35° C. 35 Warm rinse 10 min at 70-80° C.

TABLE-US-00003 TABLE 3 Process sequence for the samples of the examples according to the invention (AlSi12(Fe) alloy) AISi12(Fe) alloy Sample Degreasing Acidic Pre-treatement Desmutting Pre-anodization V 5 min at 55-60° C. — — — W 5 min at 55-60° C. — 1 min at 35° C. 5 min at 16 V at 15-18° C. X 5 min at 55-60° C. — 1 min at 35° C. 5 min at 16 V at 15-18° C. Y 5 min at 55-60° C. 3 min at 85-90° C. 1 min at 35° C. 5 min at 16 V at 15-18° C. Z 5 min at 55-60° C. 5 min at 85-90° C. 1 min at 35° C. 5 min at 16 V at 15-18° C. AA 5 min at 55-60° C. 5 min at 85-90° C. 1 min at 35° C. 5 min at 16 V at 15-18° C. BB 5 min at 55-60° C. 10 min at 85-90° C. 1 min at 35° C. 5 min at 16 V at 15-18° C. Sample Anodization Sealing Thickness V 50 min at 30 V at 15-18° C. Cold seal 20 min at 35° C. 15 μm Warm rinse 10 min at 70-80° C. W 10 min at 30 V at 15-18° C. Cold seal 20 min at 35° C. 14 μm Warm rinse 10 min at 70-80° C. X 20 min at 30 V at 15-18° C. Cold seal 20 min at 35° C. 19 μm Warm rinse 10 min at 70-80° C. Y 10 min at 30 V at 15-18° C. Cold seal 20 min at 35° C. 15 μm Warm rinse 10 min at 70-80° C. Z 20 min at 30 V at 15-18° C. Cold seal 20 min at 35° C. 18 μm Warm rinse 10 min at 70-80° C. AA 20 min at 20 V at 15-18° C. Cold seal 20 min at 35° C. 13 μm Warm rinse 10 min at 70-80° C. BB 5 min at 30 V at 15-18° C. Cold seal 20 min at 35° C. 11 μm Warm rinse 10 min at 70-80° C.

[0080] 2. Sample Characterization

[0081] The sample A is used as the control sample in comparison to samples B, C and D. The properties of aluminum oxide film were investigated by using SEM cross section and surface analysis as presented in FIGS. 2 and 3.

[0082] FIGS. 2 a) and 3 a) are taken from a classical anodizing done which belongs to Sample A. The detrimental effect of the Si due to their relatively inert nature the aluminum oxide film growth on the high silicon containing zones are dampened thus causing discontinuous and very thin (up to 0.15 to 0.2 mils) oxide layer.

[0083] By using a two-step anodizing process, the second sample set Sample B was produced with the same parameters as the control group Sample A. As it can be seen from the FIG. 2 b) compared to the FIG. 2 a), the oxide growth has a higher thickness up to 0.47 mils. Furthermore, the increased thickness also counter acts with the inhibiting effect of the silicon intermetallics as can be seen from the surface SEM image in FIG. 3 b), resulting in a continuous aluminum oxide film, where the silicon secondary phases are trapped in/on the oxide film.

[0084] The pretreatment allows to further improve the oxide layer thickness from 0.98 mils up to 1.37 mils with a denser coating on the surface as can be seen from the FIG. 2 c) and d). The surface images also reveal an enhanced continuity of the layer with the silicon particles mostly embedded into the aluminum oxide film showing much less cracks than the Sample B in FIG. 3 b). Comparing the images from FIGS. 2 and 3 of Sample C and D where the pretreatment time is increased from 4 minutes to 10 minutes no significant improvement on the layer thickness and/or integrity have been observed. However looking at the surface images from the FIG. 3 it can be said that due to the brightening effect of the pretreatment the 10 minutes option has a smoother appearance.

[0085] The samples B, F, J, K, L, N, O, R, V, W, Y were used to measure the L, a, b values of the aluminum casting obtained by the process. Those values can be found on the Table 4 and Table 5 below with the color obtained for each sample.

TABLE-US-00004 TABLE 4 L, a, b values for sample J, K, L, N, O, R, V, W and Y Sample Code L a b Sample J 56.26 −0.48 3.74 Sample K 53.55 −0.39 3.26 Sample L 49.66 −0.45 3.82 Sample N 51.05 −0.43 3.91 Sample O 57.12 −0.39 3.12 Sample R 58.24 −0.40 3.15 Sample V 51.02 −0.34 3.33 Sample W 53.06 −0.31 3.27 Sample Y 53.86 −0.37 3.87

TABLE-US-00005 TABLE 5 L, a, b values for gold colored samples (sample B and F) Sample Code L a b Sample B 52.13 2.12 33.78 Sample F 67.66 2.06 34.61

[0086] The final color of the cast aluminum oxide layer depends on base metal gloss and color. Clear anodic oxide color is the primary condition of obtaining durable and aesthetically appealing final color. Color comparison of classically anodized and double anodized samples are given in Table 5. Samples were anodized by using different anodization parameters and colored in the inorganic gold dye. By using double anodization, more vivid and clear color can be obtained.

[0087] 3. Determination of Corrosion Resistance

[0088] In order to determine the contribution of different surface treatments on the corrosion resistance samples were subjected to NSS tests. To be able to see the corrosion spots more clearly and also observe the effect of this test on the color fade, the samples were dye colored in black.

[0089] The results from the NSS test (shown in Table 6) are in agreement with the SEM observations, the best corrosion behavior was achieved for the Samples C and D as expected. For sample B, although no corrosion sign was detected, presence of color change indicated the important role of oxide film thickness on color integrity.

TABLE-US-00006 TABLE 6 NSS results of Sample A, B, C and D after 480 hours according to ISO 9227 Sample Code Result/Comment Sample A Base metal corrosion, and color change was observed Sample B No base metal corrosion, color change was observed Sample C No base metal corrosion and no color change was observed Sample D No base metal corrosion and no color change was observed

[0090] Due to the good performance of Sample C and D after 480 hours of NSS, larger area samples have been produced to repeat the test to see where the first sign of corrosion and/or color fading will start.