Method for forming a magnesium alloy coating

Abstract

The pH level needs to be adjusted frequently. [Solution] In a film formation method in which a magnesium alloy that has undergone pretreatment such as degreasing is subjected to electrolytic treatment in an electrolyte to form a film, the electrolyte is an aqueous solution to which alkaline sodium hydroxide and acidic phosphoric acid are added, the power supply for the electrolytic treatment is a positive-negative polarity reversal power supply that periodically reverses the output of positive and negative, and the electrolytic treatment is performed using a power supply that uses both DC and AC or switches between DC and AC to output the electrolytic treatment, the electrolyte is circulated through a circulation path during the electrolytic treatment, the pH value of the electrolyte is measured in this circulation path and the pH value is adjusted, thereby enabling continuous film formation with a homogeneous electrolyte at all times.

Description

Technical Field

[0001] The present invention relates to a method for forming a film on a magnesium alloy, in which an insulating film made of a magnesium compound is thickly formed on the magnesium alloy. In particular, in the technology for forming a film with high corrosion resistance and the technology for improving the film formation efficiency (formation rate), the present invention relates to a method for forming a film on a magnesium alloy that can always maintain the quality of the electrolytic solution uniformly.

Background Art

[0002] Conventionally, in order to cope with the weight reduction of various structural materials, the use of magnesium, which is lighter than aluminum, has been increasing. Magnesium is not only the lightest among practical metals with a specific gravity of 1.7, but is also very excellent in terms of formability, vibration absorption, dent resistance, etc., and requires little energy for recycling and is suitable for recycling, so it has recently attracted particular attention. However, generally an acidic bath is used for plating treatment. Magnesium is strongly alkaline but weak in acid and easily dissolves in an acidic bath. Therefore, a method for plating a magnesium alloy mainly composed of magnesium with sufficient corrosion resistance has not been put into practical use. The adhesion between the magnesium alloy and paint is poor, and it is difficult to directly apply paint on the magnesium alloy.

[0003] On the other hand, as variously specified in JIS H8651-1995, the surface of the magnesium alloy substrate is subjected to chemical conversion treatment with chemicals, or a magnesium oxide film is formed on the surface of the substrate by anodic oxidation. After improving the adhesion between the surface of the substrate and the paint by either method, painting is performed for anticorrosion treatment. However, the anticorrosion treatment method by directly painting on the substrate surface has a problem that the paint film applied to the substrate surface is easily worn and peeled off, and the corrosion resistance cannot be maintained. Furthermore, focusing on the fact that magnesium oxide has electrical insulating properties, an attempt was made to create a thick insulating magnesium oxide film on the surface of the magnesium alloy substrate by anodizing, and to perform corrosion protection solely through film formation. However, with the JIS anodizing method, although it is possible to create a thin film necessary as a base for painting, it is extremely difficult to create a uniform and comprehensive film. Moreover, even if the film formation process time is extended, because the magnesium alloy substrate is the positive electrode, aluminum, zinc, copper, etc. in the magnesium alloy casting chemical components dissolve along with the film formation, causing the surface of the substrate to become extremely rough and preventing the film from growing thickly, thus making it impossible to form a thick film.

[0004] Therefore, the applicant has developed a method for forming a film on a magnesium alloy that has undergone pretreatment such as degreasing, by electrolytic treatment in an electrolyte solution, wherein the electrolyte solution is an aqueous solution to which sodium hydroxide and phosphoric acid have been added, and the power supply for the electrolytic treatment is a positive-negative polarity reversal power supply that periodically reverses the output of positive and negative, and the electrolytic treatment is performed using a power supply that uses both DC and AC or switches between DC and AC output (see, for example, Patent Document 1).

[0005] This method allows for the suppression of magnesium dissolution through the action of sodium hydroxide, and the promotion of magnesium oxidation at the positive electrode during electrolysis through the action of phosphoric acid, thereby enabling the electrolytic generation of magnesium oxide. By repeatedly applying magnesium oxide at the positive electrode and smoothing the treated surface at the negative electrode through an action similar to electrolytic polishing, a film mainly composed of magnesium oxide is grown, producing a film with extremely high corrosion resistance. This dramatically improves the durability of magnesium alloys and expands their applications. Furthermore, since corrosion protection and insulation can be achieved solely by the magnesium oxide coating, there is no need to apply paint as a surface treatment, thus eliminating problems such as paint peeling. Furthermore, when used as a base for paint or decorative coatings such as vacuum plating, magnesium alloys exhibit tremendous effectiveness as a corrosion-resistant base. This means that magnesium alloys can be applied to products used under harsh conditions. For example, by using magnesium alloys in the interior and exterior parts of automobiles, it is possible to lighten the vehicle and contribute to energy conservation, environmental protection, and the prevention of global warming. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 3673477 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, in the conventional technology described above, the pH value of the electrolyte is extremely important, but the pH value of the electrolyte fluctuates after one or several film formation processes. Therefore, there were issues that needed to be resolved, such as the need to pause the electrolytic treatment process each time and adjust the pH value. [Means for solving the problem]

[0008] In view of the problem in the conventional technology described above, where pH value adjustment work has to be performed by pausing the electrolytic treatment process, the present invention provides a method for forming a film on a magnesium alloy that has undergone pretreatment such as degreasing, by electrolytic treatment in an electrolyte, wherein the electrolyte is an aqueous solution to which alkaline sodium hydroxide and acidic phosphoric acid are added, and the power supply for the electrolytic treatment is a positive-negative polarity reversal power supply that periodically reverses the output of positive and negative, and the electrolytic treatment is performed using a power supply that uses both DC and AC or switches between DC and AC, wherein the electrolytic treatment is performed by circulating the electrolyte through a circulation path, analyzing the sodium hydroxide concentration in the electrolyte in this circulation path and adjusting it to a set concentration, and measuring the pH value of the electrolyte and adjusting it to a set pH value, thereby enabling continuous film formation with a homogeneous electrolyte even during operation, and solving the above problem. [Effects of the Invention]

[0009] In short, the present invention provides a method for forming a film on a magnesium alloy that has undergone pretreatment such as degreasing by electrolytic treatment in an electrolyte solution, wherein the electrolyte solution is an aqueous solution to which alkaline sodium hydroxide and acidic phosphoric acid are added, and the power supply for the electrolytic treatment is a positive-negative polarity reversal power supply that periodically reverses the output of positive and negative, and the electrolytic treatment is performed using a power supply that uses both DC and AC, or switches between DC and AC. Alternatively, the present invention provides a method for forming a film on a magnesium alloy with a magnesium content of 92% or more that has undergone pretreatment such as degreasing by electrolytic treatment in an electrolyte solution, wherein the electrolyte solution is an alkaline aqueous solution mainly to which alkaline sodium hydroxide is added, and the power supply for the electrolytic treatment is a positive-negative polarity reversal power supply that periodically reverses the output of positive and negative, or uses both DC and AC, or switches between DC and AC. In a film formation method in which electrolytic treatment is performed using a power supply that outputs both DC and AC, or a power supply that switches between DC and AC, the electrolyte is circulated through a circulation path to perform the electrolytic treatment, and the pH value of the electrolyte is measured and adjusted in this circulation path. In a film formation method in which the electrolyte is circulated through a circulation path to perform the electrolytic treatment, the sodium hydroxide concentration in the electrolyte is analyzed and adjusted to a set concentration in this circulation path, and the pH value of the electrolyte is measured and adjusted to a set pH value, so that the freezing of the electrolyte can be completely prevented, especially when the system is shut down at low temperatures in winter, and because the sodium hydroxide concentration and the set pH value can be adjusted on the circulation path side during the electrolytic treatment, a homogeneous electrolyte can always be used for continuous film formation.

[0010] Although the electrolyte is less likely to freeze if it is in circulation when the electrolytic treatment is stopped at low temperatures in winter, the risk of freezing cannot be eliminated. Therefore, in the magnesium alloy film formation method described in claim 1 or 2, the electrolyte can be heated in the circulation path, so the electrolyte can be circulated while being warmed when the operation is stopped, which has a great practical effect, as it completely prevents the electrolyte from freezing. [Modes for carrying out the invention]

[0011] The present invention provides a method for producing a highly corrosion-resistant coating on a magnesium alloy. This method involves first performing an immersion degreasing step to remove oily components adhering to the substrate surface, followed by a water washing step to remove the degreasing solution adhering to the substrate surface. Next, an electrolytic treatment step is performed using an electrolyte solution containing sodium hydroxide and phosphoric acid, under a positive-negative polarity reversal power supply described later. This involves anodic electrolysis (anodic oxidation) where the magnesium alloy substrate is the positive electrode, and cathode electrolysis where it is the negative electrode, to uniformly and completely produce an insulating coating mainly composed of magnesium oxide. Subsequently, a water washing step is performed to remove the electrolyte adhering to the coating surface, followed by a hot air drying step (this method is referred to as the Magma Color SL method).

[0012] This coating method is not limited in any way to the composition of the magnesium alloy and can be applied to various types of magnesium alloys, such as die-cast materials like AZ91D and wrought materials like AZ31 and AZ61. The main component of the resulting coating is magnesium oxide (MgO).

[0013] Magnesium oxide has electrical insulating properties and is also extremely chemically stable. Even when subjected to oxidation, the oxidation reaction does not proceed further, so coatings with magnesium oxide as the main component have extremely high corrosion resistance.

[0014] The electrolyte in the Magma Color SL method described above is an aqueous solution to which sodium hydroxide and phosphoric acid have been added. For example, the optimal pH value is determined by the amount of phosphoric acid added relative to 100 g / l of sodium hydroxide. This electrolyte is circulated through a circulation system. During this circulation, the concentration of sodium hydroxide in the electrolyte is analyzed, and any deficiencies are added. The pH value of the electrolyte is also measured, and any deficient phosphoric acid is replenished to adjust the pH value. Sodium hydroxide inhibits the dissolution of magnesium and affects the efficiency (rate) of film formation, while phosphoric acid, due to its strong oxidizing power, promotes the oxidation of magnesium at the positive electrode during electrolysis, thereby electrolytically generating magnesium oxide and also affecting the efficiency of film formation.

[0015] The electrolysis described above is performed using a power supply that periodically reverses the positive and negative outputs, such as a high-speed current reversing power supply or an AC / DC switching power supply, and this reversal ratio can be changed in various ways.

[0016] Next, in the AC / DC switching power supply, an AC / DC switching power supply that switches between DC and AC, or an AC / DC combined or AC combined power supply that uses both DC and AC, is used, and these are referred to as AC / DC switching power supplies. For electrolysis, power supplies other than the high-speed reversing power supply or AC / DC switching power supply mentioned above, such as a single-phase incomplete rectified wave power supply, can also be used, and the AC may be three-phase instead of single-phase.

[0017] Next, we will explain other examples of film formation methods. The method for generating a coating in a short time involves, for magnesium alloys with a magnesium content of 92% or more, first performing an immersion degreasing step to remove oily components adhering to the substrate surface, a water washing step to remove the degreasing solution adhering to the substrate surface, an etching step to remove impurities from the substrate surface, a water washing step to remove the etching solution adhering to the substrate surface, a surface adjustment step to neutralize any etching solution remaining after water washing to prevent acidic liquids from mixing into the electrolyte, and a water washing step to remove any surface adjustment solution adhering to the substrate surface. Next, an electrolytic treatment step is performed using an alkaline electrolyte with added sodium hydroxide and a positive / negative polarity reversal power supply to uniformly and completely generate an insulating coating mainly composed of magnesium hydroxide. After that, a water washing step to remove the electrolyte adhering to the coating surface and a hot air drying step are performed (this method is called the Magma Color SH method).

[0018] This coating method is applicable to magnesium alloys with a magnesium content of 92% or more, such as wrought materials like AZ31 and AZ61.

[0019] The main component of the generated film is magnesium hydroxide (Mg(OH)2), and magnesium hydroxide has electrical insulation properties.

[0020] The electrolyte in the above-described Magma Color SH method is mainly an aqueous solution obtained by adding sodium hydroxide. This electrolyte is basically circulated through a circulation path composed of a delivery path and a feed path for the electrolyte connected to an electrolytic cell and a pump to which the delivery path and the feed path are connected. The electrolyte is analyzed in this circulation path, the pH value is measured, and it is adjusted to a set pH value as necessary.

[0021] Regarding the optimum conditions for the current density and the inversion ratio of the plasma plus / minus polarity inversion power source and the high-speed current inversion power source used in the electrolysis treatment step, they are the same as those of the above-described Magma Color SL method.

[0022] Next, the operation of the method for forming a film on a magnesium alloy according to the present invention will be described. In the film formation method of the present application, electrolysis is performed using an electrolyte and a high-speed current inversion power source or the like. In such a power source, the plus electrode and the minus electrode are inverted and repeated at high speed, or the plus electrode time in a specific time zone is sandwiched and repeated in the same manner. When the base material (magnesium alloy) is the plus electrode, magnesium on the surface of the base material changes to magnesium oxide or magnesium hydroxide by an electrolytic reaction to form a film, and aluminum, zinc, copper, etc. in the casting chemical composition of the magnesium alloy dissolve, and a large number of minute recesses are generated on the surface of the base material. A film is further formed on the surface of the base material roughened into a concavo-convex state by such minute recesses, and the treatment surface is roughened. On the other hand, when it is the minus electrode, the film on the minute protrusions of the roughened treatment surface is reduced by the action of electrolytic reduction, and the reduced minute protrusions are separated from the treatment surface, and the treatment surface is smoothed by an action similar to electrolytic polishing. In the film formation and smoothing action due to the inversion of the plus electrode and the minus electrode, a smooth film is generated and grows thicker by repeating the inversion of the plus electrode and the minus electrode.

[0023] Furthermore, if the electrolytic treatment process is suspended for an extended period, such as during business closures, especially during the cold winter months, the electrolyte will freeze if left unattended, causing problems when the electrolytic treatment process is restarted. However, by using a heater permanently installed in the circulation path or a heater added to the circulation path when in use, the electrolyte can be heated to an appropriate temperature while circulating during the suspension of the electrolytic treatment process, preventing the electrolyte from freezing and allowing the electrolytic treatment process to be restarted without any problems.

Claims

1. In a method for forming a film on a magnesium alloy that has undergone pretreatment such as degreasing, a film is formed by electrolytic treatment in an electrolyte solution, the electrolyte solution being an aqueous solution to which alkaline sodium hydroxide and acidic phosphoric acid are added, the power supply for the electrolytic treatment is a positive-negative polarity reversal power supply that periodically reverses the output of positive and negative, and the electrolytic treatment is performed using a power supply that either uses DC and AC in combination or switches between DC and AC output, A method for forming a magnesium alloy film, characterized in that the electrolyte is circulated through a circulation path to perform electrolytic treatment, the sodium hydroxide concentration in the electrolyte is analyzed and adjusted to a set concentration in this circulation path, and the pH value of the electrolyte is measured and adjusted to a set pH value.

2. In a method for forming a film on a magnesium alloy with a magnesium content of 92% or more that has undergone pretreatment such as degreasing, the film is formed by electrolytic treatment in an electrolyte, the electrolyte is mainly an alkaline aqueous solution with added alkaline sodium hydroxide, and the power supply for the electrolytic treatment is a positive-negative polarity reversal power supply that periodically reverses the output of positive and negative, or a power supply that uses both DC and AC or switches between DC and AC, the electrolytic treatment is performed using a power supply that uses both DC and AC or switches between DC and AC, A method for forming a magnesium alloy film, characterized in that the electrolyte is circulated through a circulation path to perform electrolytic treatment, the sodium hydroxide concentration in the electrolyte is analyzed and adjusted to a set concentration in this circulation path, and the pH value of the electrolyte is measured and adjusted to a set pH value.

3. In the method for forming a magnesium alloy film according to claim 1 or 2, A method for forming a magnesium alloy film, characterized in that the electrolyte can be heated in the aforementioned circulation path.