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.

Description

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.

The invention furthermore relates to a corrosion-resistant component, formed with a magnesium-based alloy, which corrosion-resistant component is obtainable in particular by a method of this type.

Magnesium-based alloys (Mg-based alloys) constitute a frequently used structural material for producing components, for example by die casting. A disadvantage of Mg-based alloys, however, is the poor corrosion resistance thereof, in particular against galvanic corrosion. This applies in particular for electrolytic environments with moderate to low pH values, such as salt water for example. A corrosion or a corrosion behavior of typical Mg-based alloys is thereby in particular dependent on different corrosion potentials of different metallic phases of the Mg-based alloy. In the case of AZ alloys, for example AZ91 (Mg-based alloy comprising 9 wt % Al, 1 wt % Zn, remainder Mg), a corrosion rate is often determined, among other things, by an intermetallic Mg₁₇Al₁₂ phase (β phase), which compared to an Mg solid solution phase (α phase, also referred to as Mg(α) phase) or an Mg solid solution matrix has a cathodic effect and results in a corrosive decomposition of the Mg solid solution phase. Precipitation phases or impurities can also lead to differing corrosion potentials in the Mg-based alloy and thereby promote corrosion processes. Micro-galvanic or phase-dependent corrosion processes of this type are often a limitation for a practical use of components formed with or from Mg-based alloys.

For this reason, various methods were developed in order to counteract galvanic corrosion, or to inhibit it to the greatest possible extent, in components made of Mg-based alloys. These include, on the one hand, measures for improving a corrosion resistance of the Mg-based alloy itself, for example by providing high purity grades of the Mg-based alloy or the composition thereof, through a homogenization of the Mg-based alloy by heat treating the entire component, and/or through a targeted alloying with other elements, in particular rare earth metals, in order to achieve a stable and dense oxide layer on a surface of a component formed with the Mg-basis alloy. On the other hand, coating methods and surface treatments are also known which envisage that a surface of a component formed with the Mg-based alloy is provided with a layer such that a barrier is formed between an inner region of the Mg-based alloy and an electrolytic environment and galvanic processes are thereby inhibited. This includes, for example, chemical treatments such as chromate coating, electrochemical treatments such as galvanizing, or an application of coating materials to a surface of the component. However, methods of this type are normally associated with a great deal of effort, both in terms of a component preparation as well as a component coating.

This is addressed by the invention. The object of the invention is to specify a method of the type named at the outset with which an increase in a corrosion resistance of a component formed with an Mg-based alloy is enabled in a simple and feasible manner.

A further object is to specify a corrosion-resistant component of the type named at the outset, which component has a high corrosion resistance to galvanic corrosion.

The object is attained according to the invention in that, in a method of the type named at the outset, 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 basis for the invention is the idea of protecting a component formed with or from an Mg-based alloy against corrosion not by applying an additional layer to a surface of the component or by chemically altering a surface of the component, but rather by modifying a phase structure of a surface layer that is formed with or from the magnesium-based alloy, that is, an outer layer of the component itself. Since only the phase structure of the component's surface layer is modified, the remaining phase structure, or the micro-structural composition of the component or of the Mg-based alloy of which the component is formed, remains unchanged so that mechanical properties of the component are virtually unaffected. For this purpose, it is provided that the surface layer of the component is formed with or from a supersaturated solid solution phase or phase structure, in particular a homogenized supersaturated solid solution or phase structure, and a corrosion potential of the surface layer is thereby reduced. The surface layer thus forms a barrier or protective layer against external galvanic corrosion exposure. This is achieved by heating the surface layer such that the surface layer is homogenized, that is, phases of the surface layer are disintegrated, and the surface layer is thus formed with a or from a homogenized solid solution phase. The surface layer is then cooled, typically cooled in an intensified manner, in particular quenched, whereby a formation of precipitates in particular is severely inhibited or prevented, so that the surface layer is formed with or from a supersaturated solid solution phase. The surface layer thereby has a certain thickness, typically of maximally a few millimeters, whereby a remaining micro-structural composition or phase structure of the component is virtually unaffected and mechanical properties of the component are therefore preserved without any changes.

For an efficient homogenization, it is provided that the surface layer is maximally heated up to a liquidus temperature of the magnesium-based alloy, preferably maximally up to 0.9 times a liquidus temperature of the magnesium-based alloy. Heating to a temperature between 0.6 and 0.9 times the liquidus temperature has proven suitable for this purpose. A pronounced homogeneity and particularly consistently formed thickness of the surface layer are achieved if the surface layer is heated to a temperature between 0.7 and 0.8 times the liquidus temperature. A heating of the surface layer to a liquidus temperature of the magnesium-based alloy, or in particular higher than said temperature, has proven disadvantageous in terms of a consistency in the thickness of the surface layer. When the surface layer is heated to a temperature greater than a liquidus temperature of the magnesium-based alloy, that is, to a fusing of the magnesium-based alloy, selective evaporation processes also often occur which can cause a change in the elemental composition of an outer layer of the component. Particularly in view of a pronounced homogeneity of the surface layer that is to be achieved and an especially high corrosion resistance associated therewith, a heating of the surface layer to a temperature greater than the liquidus temperature of the magnesium-based alloy is to be avoided.

A high corrosion resistance is achieved if the surface layer is cooled at a cooling rate of more than 10 K/s, preferably more than 20 K/s. In this manner, diffusion processes in the Mg-based alloy can be efficiently inhibited and a high degree of homogenization of the supersaturated solid solution phase is achieved. This holds especially true when the surface layer is cooled at a cooling rate of more than 30 K/s.

It is beneficial if the thickness of the surface layer is set to less than approximately 5 mm, preferably between 0.1 mm and 3.0 mm. A thickness of this type has proven feasible for efficiently minimizing corrosion processes. In principle, the thickness of the surface layer can be chosen such that it is adapted to the intended application of the component. Even setting the thickness of the surface layer to approximately 0.1 mm has been shown to be sufficient for highly minimizing corrosion processes. For typical application conditions, particularly of structural components, it has proven especially suitable if the thickness of the surface layer is set to between 0.1 mm and 3.0 mm, preferably between 0.2 mm and 1.5 mm. For a use of a component in a corrosion-prone environment, however, it can also be expedient if the thickness of the surface layer is set to between 1.5 mm and 3.0 mm.

A simple application is achieved if the surface layer is heated using an electric arc, in particular a welding arc, or by induction. Specifically an electric arc, and with particularly workable effect a welding arc, has proven beneficial for heating up a surface layer in a targeted, and in particular localized, manner. In principle, typical methods known to a person skilled in the art for heating up a material surface or surface layer can be used, such as electrical heating elements for example. A heating-up by induction has proven very suitable. Here, eddy currents are typically produced in the surface layer using an alternating magnetic field, whereby the surface layer heats up as a result of the electrical resistance thereof. It is also advantageous in this case that a penetration depth of the eddy currents in the surface layer can be well controlled, whereby the thickness of the surface layer that is being heated up can be set in a precise manner. Typical heating-up methods used as part of welding processes have proven to be very easy-to-use methods of heating-up, for example a heating-up using an electric arc, using a laser beam, using a combustion gas, using electron beams and/or using current flux over an electrical resistance of the surface layer.

Expediently, the surface layer is heated up with the use of inert gas or shielding gas in order to protect the heated-up surface against undesired ambient influences, in particular chemical reactions with the surrounding environment such as oxidation. For this purpose, inert gas or shielding gas such as argon, helium, or nitrogen, for example, can be guided onto a surface of the surface layer.

It has proven effective that the thickness of the surface layer be set using the power supplied for heating the surface layer. The necessary thickness of the surface layer, which is typically predefined depending on a component size and/or an eventual intended application of the component, can be set in this manner.

Depending on a heating-up method used and/or a specific composition of the Mg-based alloy, it can be sufficient if a heat source is merely switched off or a heating is merely stopped in order to achieve a sufficiently rapid cooling, in particular through a heat transfer of the component, to produce a supersaturated solid solution phase. Thus, if the surface layer is heated up using an electric arc, for example, thermal energy can be supplied in a quick and spatially limited manner, wherein when the electric arc is switched off or the heating-up is stopped, a heat transfer of the component or component material is often sufficient to cool a heated-up region of the surface layer such that a supersaturated solid solution phase is formed.

It is beneficial if the surface is cooled in an intensified manner in order to ensure a reliable configuration of the surface layer with a supersaturated solid solution phase. Here, intensified cooling means a cooling with an additional measure which increases a cooling rate of the surface layer, in particular in comparison with a cooling of the surface layer by itself after the heating-up is stopped.

A high corrosion resistance can be achieved if a cooling of the surface layer is carried out with a gas flow, in particular an airflow, or with a liquid bath, in particular a water bath. A pronounced homogeneity of the supersaturated solid solution phase can thus be ensured. Particularly with a liquid bath, primarily a water bath, in which the component or the surface layer is typically immersed at least partially for cooling, high cooling rates can be realized and an advantageously high homogeneity of the supersaturated solid solution phase can thus be achieved. A simple and less laborious procedure can be achieved when a cooling of the surface layer is carried out with an airflow or a water bath.

The method according to the invention is particularly suitable if the magnesium-based alloy contains aluminum as the second-largest amount in addition to magnesium as the main amount. This applies above all to a magnesium-based alloy comprising, in addition to magnesium as the main amount (in wt %),

more than 0.0% to 20% aluminum,

optionally more than 0.0% to 1% zinc,

magnesium and production-related impurities as a remainder.

If, in addition to aluminum and zinc according to the aforementioned content ranges, the magnesium-based alloy is also formed with manganese, preferably in an amount of more than 0.0 wt % to 0.5 wt %, a corrosion resistance can be further increased.

Particularly the class of known AZ alloys, referred to according to the customary abbreviated designation based on the ASTM standard, such as AZ31 (Mg—Al3%-Zn1%, in wt %), AZ61 (Mg—Al6%-Zn1%, in wt %) or AZ91 (Mg—Al9%-Zn1%, in wt %) for example, have proven to be very suitable for increasing a corrosion resistance according to the aforementioned method according to the invention.

The further object of the invention is attained with a corrosion-resistant component of the type named at the outset, which corrosion-resistant component is obtainable in particular by an aforementioned method, 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 or from 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. Because the surface layer is formed with or from a supersaturated solid solution phase, it constitutes a barrier or protective layer against external galvanic corrosion exposure and thus protects the inner region in particular. Typically, the surface layer thereby has a thickness of maximally just a few millimeters, whereby the mechanical properties of the corrosion-resistant component, which are often mainly determined by the phase structure of the inner region, are maintained virtually unchanged in comparison with a component that comprises no such surface layer.

A corrosion-resistant component of this type is obtainable in a simple and feasible manner in accordance with a method according to the invention. Of course, the corrosion-resistant component or the surface layer thereof or the magnesium-based alloy thereof can be embodied according to or analogously to the aforementioned features and embodiments and with the associated corresponding effects which are described within the scope of the method according to the invention for increasing a corrosion resistance of a component formed with a magnesium-based alloy or the surface layer thereof or the magnesium-based alloy thereof. With regard to further embodiments or forms according to the invention of the corrosion-resistant component or the surface layer thereof or the magnesium-based alloy thereof, as well as to the advantageous effects thereof, reference is thus hereby made to the preceding paragraphs in particular.

It is advantageously provided that the thickness of the surface layer is less than approximately 5 mm, preferably between 0.1 mm and 3.0 mm. Said thickness of the surface layer has proven to be feasible for efficiently minimizing corrosion processes. According to the forms and effects noted above, a thickness of the surface layer between 0.1 mm and 3.0 mm, preferably between 0.2 mm and 1.5 mm, has proven to be particularly suitable for highly minimizing corrosion processes. For a use of the corrosion-resistant component in a corrosion-prone environment, it can be expedient if the surface layer has a thickness between 1.5 mm and 3.0 mm.

A particularly high corrosion resistance can be achieved if the magnesium-based alloy contains aluminum as the second-largest amount in addition to magnesium as the main amount. This applies above all to a magnesium-based alloy comprising, in addition to magnesium as the main amount (in wt %),

more than 0.0% to 20% aluminum,

optionally more than 0.0% to 1% zinc,

magnesium and production-related impurities as a remainder.

With regard to other advantageous embodiments of the magnesium-based alloy of the corrosion-resistant component, reference is hereby made to the preceding paragraphs, which apply analogously to the corrosion-resistant component according to the invention or the magnesium-based alloy of the corrosion-resistant component.

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

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

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

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;

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

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 Mg₁₇Al₁₂ 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.

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.

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.

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.

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.

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.

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.

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.