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

Abstract

The present invention relates to the field of metal surface preparation by anodizing processes, and to a method for producing a corrosion-resistant aluminum-silicon alloy casting, and more particularly to the optimization of the anodizing of high-silicon aluminum castings by using a multi-stage anodizing cycle. The present invention also relates to a corrosion-resistant aluminum-silicon alloy casting and its use.

Claims

1. A method for producing a corrosion-resistant aluminum-silicon alloy casting comprising the following steps: a) providing an aluminum-silicon alloy casting; and b) developing a protective layer against corrosion, at least partially, on the surface of the aluminum-silicon alloy casting by a multi-stage anodizing process having b1) a first pre-anodizing stage for oxidizing aluminum on the surface of the casting at a voltage of 1 to 40 V; b2) a second anodizing stage for oxidizing aluminum and silicon on the surface of the casting at a voltage of 25 to 50 V, wherein the voltage of the second anodizing stage b2) is higher than the voltage of the first pre-anodizing stage b1), and wherein the first stage b1) and the second stage b2) are carried out in an acid bath with different organic additives.

2. Method according to claim 1, characterized in that the voltage applied during the first stage b1) is from 5 to 30 V, preferably from 10 and 20 V and / or the voltage applied during the second stage b2) is from 25 to 40 V.

3. Method according to any one of claim 1 or 2, characterized in that the first step b1) is carried out at a temperature of 1 to 50 °C, preferably at a temperature of 5 to 30 °C and more preferably at a temperature of 10 to 20 °C, and / or the second step b2) is carried out at a temperature of 1 to 50 °C, preferably at a temperature of 5 to 30 °C and more preferably at a temperature of 10 to 20 °C.

4. Method according to any one of claims 1 to 3, characterized in that these two steps are carried out in an acid bath, preferably comprising sulfuric acid, wherein the concentration of sulfuric acid is preferably from 50 to 250 g / l, more preferably from 100 to 200 g / l.

5. Method according to claim 4, characterized in that the acid bath comprises organic additives, preferably selected from the group LnzRnn / Lznz / e / YiAi consisting of oxalic acid, tartaric acid, glycolic acid, ethylene glycol and combinations thereof.

6. A method according to any one of claims 1 to 5, characterized in that the first pre-anodizing step b1) is preceded by at least one of the following pretreatment steps: a) a desquashing step, in which the aluminum-silicon alloy casting is exposed to an acid, preferably an acid selected from the group consisting of nitric acid, phosphoric acid, sulfuric acid, fluoride containing acidic media and any organic acid, their combinations not being limited to any catalyst such as hydrogen peroxide or ferric persulfate or sulfate, b) an acid pretreatment step, in which the aluminum-silicon alloy casting is exposed to an acid, c) a degreasing step, in which the aluminum-silicon alloy casting is exposed to a cleaning agent.

7. A method according to any one of claims 1 to 6, characterized in that the second anodizing stage b2) is followed by a sealing process, which is preferably selected from one of the following processes: a) hot sealing, in which the anodized aluminum alloy is exposed to water at a temperature of 90 to 100 °C and / or surfactants to remove soot; b) medium-temperature sealing, in which the anodized aluminum alloy is exposed to organic agents or metallic salts to improve the quality of the seal, such as nickel acetate or magnesium acetate;c) cold sealing with a first sealing stage, in which the anodized aluminum alloy casting is exposed to a metal salt selected from the group of a nickel salt and / or a magnesium salt and / or a chromium salt and / or a zirconium salt, preferably a nickel fluoride and / or nickel acetate and / or trivalent chromium and / or a zirconium salt and / or magnesium acetate and / or lithium hydroxide, more preferably a nickel fluoride and / or nickel acetate, and at least one surfactant, and a second aging stage, LnzRnn / Lznz / e / YiAi, in which the anodized aluminum alloy is exposed to deionized water and / or at least one surfactant to remove any soot formed on the surface.

8. Corrosion-resistant aluminum-silicon alloy casting having an aluminum oxide film with an average thickness of 4 to 90 pm as a corrosion protection layer, characterized in that the substrate surface is essentially free of zero points, meaning that the surface coverage by the oxide is greater than 88%, wherein the coverage and measurement of the zero points are determined in accordance with Volkswagen standard TL 212 No. 2016-12, wherein the surface coverage rate is determined by a percentage of the measured length examined and wherein the width of the zero point in the microsection should not exceed 60 pm.

9. Corrosion-resistant casting according to claim 8, characterized in that the aluminum oxide film has an average thickness of 5 to 90 pm, preferably 10 to 50 pm.

10. Corrosion-resistant casting according to claim 8 or 9, characterized in that the aluminum oxide film has a ratio between the highest average coating thickness and the lowest average coating thickness of 8:1, preferably a ratio of 6:1, more preferably a ratio of 4:1, wherein the ratio is calculated by taking an image of a 300 pm cross-section using 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 highest average coating thickness and the lowest average coating thickness.

11. Corrosion-resistant casting according to any one of claims 8 to 10, characterized in that the substrate surface is completely free of zero points.

12. Corrosion-resistant casting according to any one of claims 8 to 11, characterized in that the aluminum oxide film of the corrosion-resistant aluminum-silicon alloy casting LnzRnn / Lznz / e / YiAi has L, a and b values ​​of 49 to 65 for L, -0.7 to -0.1 for a and 1.7 to 4 for b, preferably 52 to 60 for L, -0.5 to -0.3 for a and 1.8 to 3.8 for b.

13. Corrosion-resistant casting according to any one of claims 8 to 12, characterized in that the aluminum oxide film has a maximum concentration of pure silicon of 5% by weight, preferably from 0.5 to 2% by weight.

14. Corrosion-resistant casting according to any one of claims 8 to 13, characterized in that the casting comprises from 0.5% by weight to 70% by weight of silicon, preferably from 5% to 20% by weight, more preferably from 6% to 15% by weight.

15. Corrosion-resistant casting according to any one of claims 8 to 14, characterized in that the casting comprises other 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.

16. Corrosion-resistant casting according to any one of claims 8 to 15, characterized in that the casting comprises or consists of an alloy of AIS1zMg, an alloy of AIS1o, an alloy of AIS1i2(Fe) and combinations thereof.

17. Corrosion-resistant casting according to any one of claims 8 to 16 and obtainable by the method of any one of claims 1 to 7.

18. Use of the corrosion-resistant aluminum-silicon alloy casting of any one of claims 8 to 16 in the automotive, aerospace and household appliance industries, in particular for fuse boxes, electronic parts, frames and brake calipers.