METHOD FOR INCREASING THE CORROSION RESISTANCE OF A COMPONENT FORMED OF A MAGNESIUM-BASED ALLOY AGAINST GALVANIC CORROSION, AND CORROSION-RESISTANT COMPONENT OBTAINABLE BY SAID METHOD

Abstract

The invention relates to a method for increasing a corrosion resistance of a component formed with a magnesium-based alloy against galvanic corrosion, in particular micro-galvanic corrosion. According to the invention, an increase in a corrosion resistance against galvanic corrosion is achieved in a simple manner in that a surface layer of the component having a predefined thickness, which surface layer is formed with the magnesium-based alloy, is heated in order to configure the surface layer with a homogenized solid solution phase, whereupon the surface layer is cooled such that the surface layer is formed with a supersaturated solid solution phase. The invention furthermore relates to a corrosion-resistant component which is obtainable by a method of this type.

Claims

1. A method for increasing a corrosion resistance of a component formed with a magnesium-based alloy against galvanic, in particular micro-galvanic, corrosion, wherein a surface layer of the component having a predefined thickness, which surface layer is formed with the magnesium-based alloy, is heated in order to configure the surface layer with a homogenized solid solution phase, whereupon the surface layer is cooled such that the surface layer is formed with a supersaturated solid solution phase.

2. The method according to claim 1, wherein the surface layer is maximally heated up to a liquidus temperature of the magnesium-based alloy, in particular maximally up to 0.9 times a liquidus temperature of the magnesium-based alloy.

3. The method according to claim 1, wherein the surface layer is cooled at a cooling rate of more than 10 K/s.

4. The method according to claim 1, wherein the thickness of the surface layer is set to less than approximately 5 mm.

5. The method according to claim 1, wherein the surface layer is heated using an electric arc in particular a welding arc, or by induction.

6. The method according to claim 1, wherein the thickness of the surface layer is set by the power supplied for heating the surface layer.

7. The method according to claim 1, wherein a cooling of the surface layer is carried out with a gas flow or with a liquid bath.

8. The method according to claim 1, wherein the magnesium-based alloy contains aluminum as the second-largest amount in addition to magnesium as the main amount.

9. A corrosion-resistant component, formed with a magnesium-based alloy, which corrosion-resistant component is obtainable in particular by a method according to claim 1, wherein the corrosion-resistant component comprises a surface layer having a defined thickness as well as an inner region adjoining the surface layer, which surface layer and inner region are formed with the magnesium-based alloy, wherein the surface layer is formed with a supersaturated solid solution phase and the surface layer and inner region have a different phase structure.

10. The corrosion-resistant component according to claim 9, wherein the thickness of the surface layer is less than approximately 5 mm.

11. The corrosion-resistant component according to claim 1, wherein the magnesium-based alloy contains aluminum as the second-largest amount in addition to magnesium as the main amount.

12. The method according to claim 1, wherein the surface layer is cooled at a cooling rate of more than 20 K/s.

13. The method according to claim 1, wherein the thickness of the surface layer is set to between 0.1 mm and 3.0 mm.

14. The method according to claim 1, wherein the surface layer is heated using a welding arc or by induction.

15. The corrosion-resistant component according to claim 9, wherein the thickness of the surface layer is between 0.1 mm and 3.0 mm.

Description

[0032] Additional features, advantages and effects follow from the exemplary embodiments described below. The drawings which are thereby referenced show the following:

[0033] FIG. 1 A scanning electron microscope image of a surface of a component formed from an AZ91 alloy with galvanic corrosion on the surface;

[0034] FIG. 2a and FIG. 2b Schematic illustrations of the component from FIG. 1 in a cross section without galvanic corrosion and with galvanic corrosion;

[0035] FIG. 3 through FIG. 5 Photographic images of components formed from an AZ91 alloy after a period of 48 hours in a 5% NaCl solution, both untreated and also after a treatment with a method according to the invention;

[0036] FIG. 6 through FIG. 8 Stereomicroscopic images of the components from FIG. 3 through FIG. 5 at different magnifications.

[0037] FIG. 1 shows a scanning electron microscope image of a surface of a component formed from an AZ91 alloy (Mg—Al9%-Zn1%, in wt %), after the component was exposed to a 5% NaCl solution for a period of 72 hours. Visible is a massive galvanic corrosion of the surface, wherein the corrosion can be explained, particularly with phase dependency, as the result of different corrosion potentials of an Mg solid solution phase, referred to as an Mg α phase 1 or α phase, and an Mg.sub.17Al.sub.12 phase, called β phase 2. The β phase 2 has a cathodic effect relative to the Mg α phase 1 and causes a corrosive disintegration of the Mg α phase 1. This is illustrated schematically in FIG. 2a and FIG. 2b. FIG. 2a shows the component from FIG. 1 in a cross section without galvanic corrosion; FIG. 2b shows the component from FIG. 1 in a cross section with visible galvanic corrosion at a surface of the illustrated component. It is visibly illustrated in FIG. 2b that the Mg α phase 1 was disintegrated at the surface of the component, whereas the β phase 2 remains at the surface as a partially exposed structure.

[0038] To inhibit a corrosive attack of this nature, it is provided according to the invention that a surface layer of the component is heated such that the surface layer is formed with or from a homogenized solid solution phase, whereupon the surface layer is cooled in an intensified manner or is quenched, so that the surface layer is formed with or from a supersaturated solid solution phase. A supersaturated solid solution phase of this type has a reduced corrosion potential and protects the component in that the surface layer covers the component in the function of a barrier layer or protective layer. With the surface layer, a phase-dependent corrosion attack which acts externally on the surface of the component is inhibited. The surface layer thereby has a predefined thickness, typically approximately 0.1 mm to 1.5 mm, depending on the eventual intended application of the component. Since only the phase structure of the surface layer is altered by the method according to the invention, the remaining phase structure or micro-structure of the component remains unchanged, so that mechanical properties of the component are hardly affected by the method according to the invention.

[0039] Over the course of experimental procedures, components formed from AZ91 were treated using a method according to the invention and subsequently exposed to a 5% NaCl solution in order to compare a corrosion behavior of the components in particular with untreated components formed from AZ91 as a reference.

[0040] For this purpose, a surface layer of the components was heated up by means of an electric arc of a tungsten inert gas welding device and subsequently cooled in an intensified manner. A cooling was carried out using different cooling rates, among other things using cooling with an airflow or using cooling with a water bath.

[0041] FIG. 3 through FIG. 5 show photographic images of different components formed from AZ91 after said components were exposed to a 5% NaCl solution for a period of 48 hours. The components shown in FIG. 4 and FIG. 5 were treated beforehand with the aforementioned method according to the invention, wherein the component from FIG. 4, or the surface layer thereof, was cooled with an airflow and the component from FIG. 5, or the surface layer thereof, was cooled with a water bath. FIG. 3 shows a component made from a typical untreated AZ91 alloy. It can be seen that the untreated component shown in FIG. 3 exhibits massive corrosion damage on the surface thereof. The components from FIG. 4 and FIG. 5, however, exhibit virtually no corrosive damage.

[0042] In FIG. 6 through FIG. 8, stereomicroscopic images of the surfaces of the components shown in FIG. 3 through FIG. 5 are depicted at different magnifications. Each image is shown at a 7×, 12.5×, and 20× magnification. FIG. 6 thereby depicts the surface of the untreated component; FIG. 7 depicts the component treated according to the invention, the surface layer of which was cooled with an airflow; and FIG. 8 depicts the component treated according to the invention, the surface layer of which was cooled with a water bath. It is clearly visible that the components treated using a method according to the invention exhibit hardly any corrosion damage on their surface, whereas the untreated component exhibits significant corrosive damage on its surface.

[0043] A method according to the invention renders it possible to increase a corrosion resistance of a component formed with an Mg-based alloy, in particular an Mg-based alloy with aluminum, against galvanic corrosion. This can be carried out with little effort and in a simple manner in particular in that a surface layer of the component is homogenized by heating and is subsequently cooled such that the surface layer is formed with a supersaturated solid solution phase. In this manner, the surface layer forms a protective barrier against galvanically corrosive external influences. The surface layer is thereby embodied with a predefined thickness, depending on the intended application planned for the component, so that a remaining structural composition of the component is virtually unaffected and mechanical properties of the component are not altered or negatively affected. A corrosion-resistant component can thus be obtained in a simple and feasible manner, which component has a high corrosion resistance against galvanic, in particular micro-galvanic, corrosion.