CN113454256A

CN113454256A - Method for improving the corrosion resistance of a part formed with a magnesium-based alloy against galvanic corrosion and corrosion-resistant part obtainable by this method

Info

Publication number: CN113454256A
Application number: CN201980092296.5A
Authority: CN
Inventors: S·弗兰克; S·格内格; A·贝茨
Original assignee: LKR Leichtmetallkompetenzzentrum Ranshofen GmbH
Current assignee: LKR Leichtmetallkompetenzzentrum Ranshofen GmbH
Priority date: 2018-12-18
Filing date: 2019-12-11
Publication date: 2021-09-28
Also published as: AT521500A4; US20220081778A1; EP3899076A1; AT521500B1; EP3899076C0; EP3899076B1; WO2020124112A1

Abstract

The present invention relates to a method for improving the galvanic corrosion resistance, in particular the corrosion resistance to micro-galvanic corrosion, of components formed from magnesium based alloys. According to the present invention, an improvement in corrosion resistance against galvanic corrosion is obtained in a simple manner, wherein a surface layer of the component has a predetermined thickness, the surface layer formed with the magnesium-based alloy is heated to configure the surface layer with a homogenized solid solution phase, and subsequently the surface layer is cooled so that the surface layer is formed with a supersaturated solid solution phase. The invention also relates to a corrosion resistant component obtainable by a method of this type.

Description

Method for improving the corrosion resistance of a part formed with a magnesium-based alloy against galvanic corrosion and corrosion-resistant part obtainable by this method

Technical Field

The present invention relates to a method for improving the galvanic corrosion resistance, in particular the corrosion resistance to micro-galvanic corrosion, of components formed from magnesium based alloys.

The invention also relates to a corrosion resistant component formed from a magnesium based alloy, in particular obtainable by such a method.

Background

Magnesium-based alloys (Mg-based alloys) constitute a commonly used structural material for producing components, for example by die casting. However, one disadvantage of Mg-based alloys is their poor corrosion resistance, especially poor galvanic corrosion resistance. This is particularly useful in electrolysis environments with moderate to low pH values, such as brine for example. The corrosion or corrosion behavior of typical Mg-based alloys is thus dependent inter alia on the different corrosion potentials of the different metal phases of the Mg-based alloy. In the case of AZ alloys, such as AZ91 (Mg-based alloys comprising 9 wt.% Al, 1 wt.% Zn, and the remainder Mg), the corrosion rate is generally determined by the intermetallic Mg, among other factors₁₇Al₁₂The phase (β phase) determines that it has a cathodic effect as compared with a Mg solid solution phase (α phase, also referred to as Mg (α) phase) or a Mg solid solution matrix, and causes corrosive decomposition of the Mg solid solution phase. Precipitated phases or impurities may also lead to different corrosion potentials in Mg-based alloys and thereby promote the corrosion process. This type of micro-galvanic corrosion process or dependently dependent corrosion process is generally a limitation in practical applications for components formed with or from Mg-based alloys.

For this reason, various methods have been developed to counteract or to the greatest possible extent prevent galvanic corrosion in parts made of Mg-based alloys. These methods comprise, on the one hand, measures for improving the corrosion resistance of the Mg-based alloy itself, for example by providing a high purity grade of the Mg-based alloy or its components, by homogenization of the Mg-based alloy throughout the component by heat treatment, and/or by targeted alloying with other elements, in particular rare earth metals, in order to obtain a stable and dense oxide layer on the surface of the component formed with the Mg-based alloy. On the other hand, coating methods and surface treatments are also known, which are intended to provide the surface of a component formed with Mg-based alloys with a layer such that a barrier is formed between the internal regions of the Mg-based alloys and thereby inhibit the electrolytic environment and galvanic processes. This includes, for example, chemical treatment (such as chromate coating), electrochemical treatment (such as galvanization), or applying a coating material to the surface of the component. However, this type of process is often associated with a great deal of effort, both in terms of component preparation and component coating.

Disclosure of Invention

This is solved by the present invention. The object of the invention is to specify a method of the type mentioned at the outset with which the corrosion resistance of components formed from Mg-based alloys is improved in a simple and feasible manner.

Another object is to specify a corrosion-resistant component of the type mentioned at the outset which has a high resistance to galvanic corrosion.

According to the invention, this object is achieved with a method of the type mentioned at the outset in that the surface layer of the component, which surface layer is formed from a magnesium-based alloy, is provided with a homogenized solid solution phase by heating the surface layer and is subsequently cooled so that the surface layer is formed with a supersaturated solid solution phase.

The invention is based on the idea of protecting a component formed with or from a Mg-based alloy against corrosion, not by applying an additional layer to the surface of the component or by chemically modifying the surface of the component, but by modifying the phase structure of a surface layer formed with or from a magnesium-based alloy, i.e. the outer layer of the component itself. Since only the phase structure of the surface layer of the component is modified, the remaining phase structure or microstructure composition of the component, or of the Mg-based alloy forming the component, remains unchanged, so that the mechanical properties of the component are substantially unaffected. For this purpose, a surface layer of the component is provided which is formed with or from a supersaturated solid solution phase or phase structure, in particular a homogenized supersaturated solid solution or phase structure, and the corrosion potential of the surface layer is thereby reduced. Thus, the surface layer forms a barrier or protective layer that prevents external galvanic corrosion exposure. This is achieved by heating the surface layer such that the surface layer is homogenized, i.e. the phases of the surface layer are decomposed and thus the surface layer is formed with or from a homogenized solid solution phase. The surface layer is then cooled, usually in a reinforced manner, in particular quenched, whereby in particular the formation of precipitates is severely inhibited or prevented, so that the surface layer is formed with or from a supersaturated solid solution phase. The surface layer thus has a certain thickness, typically a maximum of a few millimeters, whereby the remaining microstructure composition or phase structure of the component is substantially unaffected and the mechanical properties of the component are thus maintained without any change.

For effective homogenization, it is provided that the surface layer is heated up to the liquidus temperature of the magnesium based alloy, preferably up to 0.9 times the 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 significant homogeneity and a particularly uniformly formed thickness of the surface layer can be achieved if the surface layer is heated to a temperature between 0.7 and 0.8 times the liquidus temperature. Heating the surface layer to the liquidus temperature of the magnesium-based alloy, or in particular above said temperature, has proven to be disadvantageous in terms of consistency of the thickness of the surface layer. Selective evaporation processes also often occur when the surface layer is heated to a temperature above the liquidus temperature of the magnesium-based alloy (i.e., to the melting of the magnesium-based alloy), which may result in a change in the elemental composition of the outer layer of the component. In particular, heating of the surface layer to a temperature higher than the liquidus temperature of the magnesium-based alloy is to be avoided, in view of the remarkable homogeneity of the surface layer and the particularly high corrosion resistance associated therewith to be achieved.

High corrosion resistance is obtained if the surface layer is cooled at a cooling rate of more than 10K/s, preferably more than 20K/s. In this way, it is possible to effectively suppress the diffusion process in the Mg-based alloy, and to achieve high homogeneity of the supersaturated solid solution phase. This is especially true when the surface layer is cooled at a cooling rate of more than 30K/s.

It is advantageous if the thickness of the surface layer is set to be less than about 5mm, preferably between 0.1mm and 3.0 mm. Such thicknesses have proven to be feasible for effectively minimizing the corrosion process. In principle, the thickness of the surface layer may be chosen such that it is adapted to the intended application of the component. It has been shown that even setting the thickness of the surface layer to about 0.1mm is sufficient to greatly minimize the corrosion process. For the usual application conditions, in particular for structural components, it has proven to be particularly suitable if the surface layer thickness is set between 0.1mm and 3.0mm (preferably between 0.2mm and 1.5 mm). However, for the use of the component in a corrosion prone environment, it may also be advantageous if the surface layer thickness is set between 1.5mm and 3.0 mm.

A simple application is achieved if an electric arc, in particular a welding arc, is used, or the surface layer is heated by induction. In particular, it has proved advantageous for the arc, and the welding arc with particularly practical effects, to heat the temperature-increasing surface layer in a targeted and, in particular, localized manner. In principle, conventional methods known to the person skilled in the art for heating surfaces or surface layers of materials can be used, such as for example electrical heating elements. Heating by induction has proven to be very suitable. Here, an alternating magnetic field is generally used to generate eddy currents in the surface layer, whereby the surface layer heats up due to its electrical resistance. This is also advantageous in case the penetration depth of eddy currents in the surface layer can be well controlled, whereby the thickness of the heated surface layer can be set in a precise manner. The usual heating methods used as part of the welding process have proven to be very easy to use heating methods, e.g. using an electric arc, using a laser beam, using combustion gases, using an electron beam and/or using a current flux through the surface layer resistance.

Advantageously, the surface layer is heated using an inert gas and a protective gas in order to protect the heated surface layer from undesired environmental influences, in particular chemical reactions with the surrounding environment, such as oxidation. For this purpose, an inert gas or a protective gas (such as argon, helium or nitrogen) may for example be directed onto the surface of the surface layer.

It has proven to be effective that the thickness of the surface layer is set using the power supplied for heating the surface layer. The necessary thickness of the surface layer may be set in this way, which generally depends on the component dimensions and/or the final intended application of the component.

Depending on the heating method used and/or the specific composition of the Mg-based alloy, it may be sufficient if only the heating source is turned off or only the heating is stopped in order to achieve a sufficiently rapid cooling, in particular by heat conduction of the component to produce a supersaturated solid solution phase. Thus, if the surface layer is heated using an electric arc, for example, thermal energy can be supplied in a fast and space-limited manner, wherein the heat conduction of the component or the component material is sufficient for cooling the heating zone of the surface layer when the electric arc is switched off or the heating is stopped, so that a supersaturated solid solution phase is usually formed.

It is advantageous if the surface is cooled in a strengthened manner in order to ensure a reliable arrangement of the surface layer with supersaturated solid solution phases. Here, the intensive cooling means that cooling is performed with an additional measure to increase the cooling rate of the surface layer, in particular compared to cooling of the surface layer itself after heating is stopped.

High corrosion resistance can be obtained if the cooling of the surface layer is carried out with a gas flow, in particular an air flow, or with a liquid bath, in particular a water bath. It is therefore possible to ensure significant homogeneity of the supersaturated solid solution phase. In particular with liquid baths, mainly water baths, into which the component or the surface layer is usually at least partially immersed for cooling, high cooling rates can be achieved and thus advantageously high homogeneity of the supersaturated solid solution phase can be obtained. When the cooling of the surface layer is performed with a gas flow or a water bath, a simple and labor-saving procedure is obtained.

The method according to the invention is particularly suitable if the magnesium-based alloy comprises, as the second major component, in addition to magnesium as the main component, also aluminum. This applies above all to magnesium-based alloys, which, apart from magnesium as main component (in% by weight), comprise,

greater than 0.0% to 20% aluminum,

optionally greater than 0.0% to 1% zinc,

the balance being magnesium and production related impurities.

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

In particular the known class of AZ alloys, referred to according to the customary abbreviation names based on ASTM standards, such as for example AZ31(Mg-Al 3% -Zn 1%, in weight%), AZ61(Mg-Al 6% -Zn 1%, in weight%) or AZ91(Mg-Al 9% -Zn 1%, in weight%), has proved very suitable for improving the corrosion resistance according to the method of the invention according to the preceding paragraph.

Another object of the invention is achieved by a corrosion resistant component of the type mentioned at the beginning, in particular obtainable by the aforementioned method, wherein the corrosion resistant component comprises a surface layer having a defined thickness and an inner region adjoining the surface layer, wherein the surface layer and the inner region are formed from or by a magnesium based alloy, wherein the surface layer is formed with a supersaturated solid solution phase and the surface layer and the inner region have different phase structures. Since the surface layer is formed with or from a supersaturated solid solution phase, it constitutes a barrier or protective layer against exposure to external galvanic corrosion, and thus protects the internal region in particular. The surface layer thus typically has a thickness of only a few millimetres at the most, whereby the mechanical properties of the corrosion resistant component (which are usually mainly determined by the phase structure of the inner zone) remain substantially unchanged compared to a component not comprising such a surface layer.

According to the invention, a corrosion-resistant component of this type is obtainable in a simple and feasible manner according to the method described above. Of course, the corrosion resistant component or the surface layer thereof or the magnesium based alloy thereof may be implemented in accordance with or similar to the aforementioned features and embodiments and with associated corresponding effects (which are described within the scope of the method according to the invention) for improving the corrosion resistance of the component formed with the magnesium based alloy or the surface layer thereof or the magnesium based alloy thereof. With regard to further embodiments or forms of the corrosion resistant component or a surface layer thereof or a magnesium based alloy thereof, and the benefits thereof, reference is hereby made in particular to the preceding paragraphs.

It is advantageous to provide the surface layer with a thickness of less than about 5mm, preferably between 0.1mm and 3.0 mm. Said thickness of the surface layer has proven to be feasible for effectively minimizing the corrosion process. According to the forms and effects described above, a thickness of the surface layer of between 0.1mm and 3.0mm (preferably between 0.2mm and 1.5 mm) has proved to be particularly suitable for highly minimizing the corrosion process. For the use of corrosion resistant components in corrosion prone environments, it may be advantageous if the surface layer has a thickness between 1.5mm and 3.0 mm.

Particularly high corrosion resistance can be obtained if the magnesium-based alloy contains aluminum as the second largest component in addition to magnesium as the main component. This applies above all to magnesium-based alloys, which, apart from magnesium as main component (in% by weight), comprise,

greater than 0.0% to 20% aluminum,

optionally from greater than 0.0% to 1% zinc,

the balance being magnesium and production related impurities.

With regard to other advantageous embodiments of the magnesium-based alloy of the corrosion resistant component, reference is made to the preceding paragraphs herein, which are similarly applicable to the corrosion resistant component or the magnesium-based alloy of the corrosion resistant component according to the present invention.

Drawings

Additional features, advantages and effects are derived from the exemplary embodiments described below. The drawings referred to herein show the following figures:

FIG. 1 is a scanning electron microscope image of a surface of a component formed from AZ91 alloy, the surface having galvanic corrosion;

FIGS. 2a and 2b are schematic cross-sectional views of the component of FIG. 1, with no galvanic corrosion and galvanic corrosion;

photographic images of the parts of figures 3 to 5 formed from AZ91 alloy in a 5% NaCl solution for a period of 48 hours, both untreated and after treatment with the method according to the invention;

fig. 6 to 8 stereomicroscope images of the components of fig. 3 to 5 at different magnifications.

Detailed Description

Fig. 1 shows a scanning electron microscope image of the surface of a part formed from an AZ91 alloy (Mg-Al 9% -Zn 1%, in weight%), after the part has been exposed to a 5% NaCl solution for a period of 72 hours. There is a large amount of galvanic corrosion seen on the surface, where corrosion can be explained particularly by phase dependence as a Mg solid solution phase (known as magnesium alpha phase 1 or alpha phase, and Mg₁₇Al₁₂Phase, referred to as beta phase 2). The β phase 2 has a cathode effect with respect to the Mg α phase 1, and causes corrosive decomposition of the Mg α phase 1. This is illustrated in fig. 2a and 2 b. FIG. 2a shows a cross-section of the component of FIG. 1 without galvanic corrosion; FIG. 2b shows a cross-section of the component of FIG. 1 illustrating galvanic corrosion visible on the surface of the component. It is clearly illustrated in fig. 2b that the Mg α phase 1 is decomposed at the surface of the component, while the β phase 2 remains at the surface as a partially exposed structure.

In order to suppress such natural corrosion attacks, it is provided according to the invention to heat the surface layer of the component such that the surface layer is formed with or from a homogenized solid solution phase and subsequently to cool the surface layer or to quench the surface layer in a reinforced manner such that the surface layer is formed with or from a supersaturated solid solution phase. Supersaturated solid solution phases of this type have a reduced corrosion potential and protect the component in such a way that the surface layer covering component acts as a barrier or protective layer. The surface layer is used to inhibit dependent corrosion attack that occurs outside the surface of the component. The surface layer thus has a predetermined thickness, typically about 0.1mm to 1.5mm, depending on the final intended application of the component. Since only the phase structure of the surface layer is changed by the method according to the invention, the remaining phase structure or microstructure of the component remains unchanged, so that the mechanical properties of the component are hardly affected by the method according to the invention.

During the course of the experimental procedure, the parts formed from AZ91 were treated using the method according to the invention and subsequently exposed to a 5% NaCl solution in order to compare the corrosion behavior of the parts, in particular with reference to untreated parts formed from AZ 91.

For this purpose, the surface layer of the component is heated by means of an arc of a tungsten inert gas welding device and subsequently cooled in an intensive manner. Cooling is performed using different cooling rates, using air flow cooling or using a water bath, among others.

Figures 3 to 5 show photographic images of different parts formed from AZ91 after exposure of the parts to a 5% NaCl solution for a period of 48 hours. The component shown in fig. 4 and 5 is pretreated with the aforementioned method according to the invention, wherein the component of fig. 4 or its surface is cooled with a gas flow and the component of fig. 5 or its surface is cooled with a water bath. Figure 3 shows a component made from a conventional untreated AZ91 alloy. It can be seen that the untreated component shown in fig. 3 exhibits a large amount of corrosion damage on its surface. However, the components in fig. 4 and 5 exhibit virtually no galvanic damage.

In fig. 6 to 8, stereomicroscope images of the surfaces of the components shown in fig. 3 to 5 are depicted at different magnifications. Each image is displayed at 7 x, 12.5 x and 20 x magnification, respectively. FIG. 6 thus depicts the surface of an untreated part; FIG. 7 depicts a part treated according to the present invention with a surface layer cooled with a gas stream; and figure 8 depicts a part treated according to the invention with its surface layer cooled with a water bath. It is clearly visible that the parts treated using the method according to the invention exhibit hardly any corrosion damage on their surface, whereas the untreated parts exhibit significant corrosion damage on their surface.

The method according to the invention makes it possible to improve the corrosion resistance of parts formed with Mg-based alloys, in particular the corrosion resistance against galvanic corrosion of Mg-based alloys with aluminium. This can be done without any effort and in a simple manner, in particular wherein the surface layer of the component is homogenized by heating and subsequently cooled such that the surface layer forms a solid solution phase with supersaturation. In this way, the surface layer forms a protective barrier against external influences of galvanic corrosion. Thus, the surface layer has a predetermined thickness, depending on the intended application for the component, such that the remaining structural composition of the component is not substantially affected and the mechanical properties of the component are not altered or negatively affected. Thus, a corrosion resistant component can be obtained in a simple and feasible manner, which component has a high corrosion resistance against galvanic corrosion, in particular against galvanic corrosion.

Claims

1. A method for improving the galvanic corrosion resistance, in particular the corrosion resistance against micro-galvanic corrosion, of a component formed with a magnesium based alloy, characterized in that a surface layer of the component having a predetermined thickness is heated such that the surface layer is provided with a homogenized solid solution phase, the surface layer being formed with the magnesium based alloy, followed by cooling the surface layer such that the surface layer is formed with a supersaturated solid solution phase.

2. A method according to claim 1, characterised in that said surface layer is heated up to the liquidus temperature of said magnesium based alloy, in particular up to 0.9 times the liquidus temperature of said magnesium based alloy.

3. Method according to claim 1 or 2, characterized in that the surface layer is cooled at a cooling rate of more than 10K/s, preferably more than 20K/s.

4. A method according to any one of claims 1-3, characterised in that the thickness of the surface layer is set to less than about 5mm, preferably between 0.1mm and 3.0 mm.

5. Method according to any one of claims 1 to 4, characterized in that the surface layer is heated using an electric arc, in particular a welding arc, or by induction.

6. The method according to any one of claims 1 to 5, characterized in that the thickness of the surface layer is set by supplying power for heating the surface layer.

7. Method according to any of claims 1-6, characterized in that the cooling of the surface layer is carried out with a gas flow or with a liquid bath.

8. A method according to any one of claims 1 to 7, characterized in that said magnesium-based alloy comprises aluminium as the second largest constituent in addition to magnesium as the main constituent.

9. A corrosion resistant component formed with a magnesium based alloy, in particular obtainable by the method according to any of claims 1-8, characterized in that the corrosion resistant component comprises a surface layer having a defined thickness, and an inner zone adjoining the surface layer, the surface layer and the inner zone being formed with the magnesium based alloy, wherein the surface layer is formed with a supersaturated solid solution phase and the surface layer and the inner zone have different phase structures.

10. The corrosion resistant component of claim 9 wherein said thickness of said surface layer is less than about 5mm, preferably between 0.1mm and 3.0 mm.

11. The corrosion resistant member according to claim 9 or 10, wherein said magnesium-based alloy contains aluminum as a second major component in addition to magnesium as a major component.