CN115917055A

CN115917055A - Method for producing functional coatings on magnesium

Info

Publication number: CN115917055A
Application number: CN202180028668.5A
Authority: CN
Inventors: 侯峰岩; 克里斯托弗·威廉·古德
Original assignee: Cirrus Materials Science Ltd
Current assignee: Cirrus Materials Science Ltd
Priority date: 2020-04-24
Filing date: 2021-04-23
Publication date: 2023-04-04
Also published as: JP2023523703A; KR20230007331A; EP4139507A1; WO2021215940A1; CA3175977A1; US20220389604A1; TW202142744A; EP4139507A4; MX2022012873A; AU2021260429A1

Abstract

In an exemplary embodiment, a method for producing a coating is provided. The method includes placing a magnesium substrate in an anodizing bath, applying a voltage for a first amount of time to form a microporous anodized layer having a thickness of 1 to 50 micrometers on the magnesium substrate, placing the magnesium substrate with the microporous anodized layer in a plating bath, wherein the plating bath comprises a metal and a complexing agent and has a pH of 8 to 14, applying a first current to the plating bath for a second amount of time to form an interlock layer on the microporous anodized layer, and applying a second current to the plating bath for a third amount of time to form a coating on the interlock layer.

Description

Method for producing functional coatings on magnesium

Background

Magnesium and its alloys are widely used in automotive, structural and aerospace applications; however, without a suitable functional coating, many alloys suffer from environmental degradation due to corrosion. Many methods have been developed to protect magnesium surfaces, including anodization, plating, and chemical films. However, due to the reactivity of magnesium, direct plating on surfaces requires many processes that typically involve toxic chemicals, anodization is an energy intensive process, and plating on anodized films requires expensive or toxic catalysts.

The protection of surfaces of magnesium and its alloys has been well studied and patented, and more than 400 patents have been granted. Methods include conversion coating, plating, organic coating (including paint), surface treatment (e.g., thermal spraying or hot dip coating), and combinations of these methods.

Conversion coatings include chromating, phosphating, and anodizing. Anodizing magnesium, including the Dow17 process (US 2313754A) created by Dow Chemicals in the forties of the twentieth century and the HAE process (US 2723952A) created in the fifties of the twentieth century, requires the use of toxic chromates and fluorides. Micro-arc oxidation (MAO) processes were developed and patented at the end of the nineties of the twentieth century. These methods require high power and in some cases hazardous chemicals (US 6919012B 1).

Magnesium has been coated using electrodeposition and electroless coating methods as described in US20030008471 and WO 2015015524A. Such coatings rely on complex pretreatment processes to obtain an adherent coating, and the plating bath must be specially formulated to minimize displacement reactions. Some pretreatments include chromium and zinc processes, which are either complex or require hazardous chemicals.

More recently, a combination of anodization and a plating coating has been developed. Such coatings are deposited from electroless plating baths and the coating requires the presence of a catalyst to initiate the deposition because the anodization layer isolates the substrate from the plating bath (US 20090223829 A1). Despite the effective isolation of the substrate, galvanic corrosion can affect the plated MAO coating, as described in "A novel palladium-free surface activation process for electrolytic deposition on micro-arc oxidation film of AZ91D Mg alloy", journal of Alloys and Compounds Volume 623, 25February 2015.

U.S. patent publication No. 2018/0051388 describes a method of plating a coating by an anodization layer. However, this method does not produce the result of a magnesium coating.

Disclosure of Invention

According to aspects illustrated herein, a method for producing a coating on magnesium is provided. One disclosed feature of an embodiment is a method including placing a magnesium substrate in an anodizing bath, applying a voltage for a first amount of time to form a microporous anodized layer having a thickness of 1 to 50 micrometers on the magnesium substrate, placing the magnesium substrate having the microporous anodized layer in a plating bath, wherein the plating bath comprises a metal and a complexing agent and has a pH of 8 to 14, applying a first current to the plating bath for a second amount of time to form an interlocked layer on the microporous anodized layer, and applying a second current to the plating bath for a third amount of time to form a coating on the interlocked layer.

Another disclosed feature of an embodiment is a method for producing a coating on magnesium. The method comprises pretreating a magnesium substrate, cleaning the magnesium substrate with deionized water; forming a microporous anodization layer on the magnesium substrate in an anodizing bath, wherein a voltage is applied to the anodizing bath for a first amount of time to form the microporous anodization layer; rinsing the magnesium substrate with the microporous anodization layer; forming an interlock layer on the microporous anodization layer in a plating bath, wherein the plating bath comprises a metal and a complexing agent and has a pH of 8 to 14; wherein a first current is applied to the plating bath for a second amount of time to form an interlock layer; and forming a coating on the interlock layer in the plating bath, wherein a second current is applied to the plating bath for a third amount of time to form the coating.

Another disclosed feature of an embodiment is a method for producing a coating on magnesium. The method includes placing a magnesium alloy substrate in an anodizing bath; applying and maintaining a peak voltage of less than 75 volts for about 10 minutes to form a microporous anodization layer having a thickness of 5 to 15 micrometers on the magnesium alloy substrate; placing the magnesium alloy substrate with the microporous anodized layer in a plating bath, wherein the plating bath comprises a metal and a complexing agent and has a pH of 9 to 12; applying from 0 amperes per square decimeter (A/dm) for a second period of time to the plating bath ² ) Increased to 0.01A/dm ² To 0.5A/dm ² To form an interlock layer on the microporous anodization layer; and applying greater than the first current and 0.1A/dm to the plating bath for a third amount of time ² To 2A/dm ² To form a coating on the interlock layer.

Drawings

FIG. 1 is a flow chart of an exemplary method for producing a coating on magnesium;

FIG. 2 is an exemplary graph illustrating anodization voltage for an exemplary anodized region of the present disclosure;

3A-3B illustrate exemplary effects of bath composition on anodization voltage and surface morphology of the present disclosure;

fig. 4 is an image of an exemplary hybrid silver coating on an AZ80 substrate of the present disclosure;

fig. 5 illustrates an image of a cross-section of an exemplary hybrid silver coating on an AZ80 substrate of the present disclosure;

fig. 6 illustrates an image of a cross-section of an exemplary hybrid copper coating on a ZK60 substrate of the present disclosure;

fig. 7 illustrates an image of an exemplary hybrid copper coating on a ZK60 substrate of the present disclosure;

fig. 8 illustrates an image of an exemplary surface and cross-section of a zinc-nickel coating on an AZ80 substrate of the present disclosure;

fig. 9 illustrates an SEM image of an exemplary anodized film using the anodizing bath of the present disclosure;

fig. 10 illustrates an optical image of an exemplary surface and cross-section of a hybrid Zn-Ni coating using the anodizing bath of the present disclosure;

fig. 11 illustrates an image of an exemplary cross-section of an electroless Ni-P coating on an AZ80 substrate of the present disclosure;

FIG. 12 illustrates a graph of anodizing voltage for a bath of a hybrid Zn-Ni coating of the present disclosure; and

fig. 13 illustrates an optical image of the surface of a Zn-Ni interlocking layer produced with an anodized layer of the disclosure.

Detailed Description

Embodiments described herein provide a method of forming a functional coating on a magnesium substrate. As noted above, previous attempts at magnesium coating have failed or are undesirable for a variety of reasons. For example, previous methods may use various processes that typically involve toxic chemicals, may use energy intensive anodization processes, and may be relatively expensive.

The present disclosure provides a method of forming a plating layer on a magnesium alloy substrate that does not use toxic chemicals, is less energy consuming, and is less expensive than previous methods. In an example, the process mechanically or chemically polishes and/or degreases the substrate. The substrate may be pretreated. A1 to 30 micron film may be anodized on a substrate in an anodizing bath comprising sodium hydroxide, potassium hydroxide, disodium metasilicate, sodium tetraborate, sodium carbonate, organic additives, other additives for producing a porous anodized layer, or any combination thereof.

Furthermore, a first plating layer (including an anodized film) of 2 to 50 microns may be deposited by electrolysis or autocatalysis, with the voltage profile for electrodeposition being employed to ensure that the anodized structure is completely filled and sealed. In addition, the first plating layer may form a surface on which other coating layers may be deposited.

The method can also deposit a second functional coating of 0 to 100 microns on the first layer. The second functional coating may be multilayered. The thickness of the mixed magnesium coating may be about 10 to 100 microns.

FIG. 1 illustrates an exemplary method 100 for producing a coating on magnesium. In one embodiment, the method 100 may be performed by various devices or tools in a processing facility under the control of a processor or controller.

At block 102, the method 100 begins. At block 104, the method 100 may pre-process the substrate. In one embodiment, the substrate may be a magnesium-based substrate, which may be a wrought or cast alloy of magnesium. Examples of such magnesium substrates may include AZ80 or ZK60. In one embodiment, the substrate may be any suitable magnesium alloy.

In one embodiment, the pre-treatment may comprise one or more processes. The process may include: the method comprises the steps of chemically treating the substrate in a concentrated nitric acid bath or a dilute sulfuric acid bath, mechanically roughening the substrate by emery paper or sand blasting, and/or cleaning the substrate in an alkaline bath comprising 10 to 20 grams per liter (g/L) of sodium carbonate and 15 to 20g/L of sodium phosphate, 10 to 20g/L of sodium silicate, and 1 to 3g/L of a commercially available OP-10 surfactant at 60 to 80 degrees celsius (° c) for 3 to 15 minutes.

Mechanically roughening the surface may result in enhanced adhesion between the anodized layer and the substrate. The adhesion may be further enhanced in the presence of stretching forces generated in the subsequently deposited functional surface layer. Mechanical roughening may be achieved by using a suitable grade of emery paper (up to 1200 mesh). In one embodiment, the grit blasting may produce a suitable surface on which to create the anodized layer.

At block 106, the method 100 may clean the substrate. The substrate may be cleaned prior to being anodized. The substrate may be cleaned by rinsing in deionized water (DI). In one embodiment, the substrate may be ultrasonically cleaned in a solution of ethanol or acetone. When cleaning the substrate, the cleaning step should prevent any oxide layer from being generated on the surface. In other words, cleaning the substrate should not allow for the creation of a new oxide layer on the surface.

At block 108, the method 100 selects an anodizing bath based on the substrate. For example, the composition of the anodizing bath may be selected according to the composition of the magnesium substrate. The bath composition may be selected from 30-100g/L sodium hydroxide, 30-100g/L potassium hydroxide, 0-60g/L sodium tetraborate, 0-100g/L disodium metasilicate, 0-50g/L sodium carbonate, 0-30g/L phosphate, 0-100g/L sodium aluminate, 0-0.05g/L triethanolamine, 0-20g/L citric acid, 0-20g/L sodium citrate, and/or 0-20ml/L hydrogen peroxide.

In one embodiment, the composition of the bath may be selected according to the composition of the substrate. In one embodiment, the magnesium substrate may be a ZK60 alloy, and the anodizing bath may comprise 70g/L sodium hydroxide, 60g/L sodium tetraborate, 60g/L disodium metasilicate, and/or 30g/L sodium carbonate. The Zk60 alloy is an alloy containing zinc and zirconium.

In one embodiment, the substrate may be AZ80 and the bath may comprise 70g/L sodium hydroxide, 60g/L sodium tetraborate, 30g/L sodium carbonate and/or 10g/L citric acid. AZ80 is an alloy containing aluminum and zinc.

In one embodiment, the substrate may be AZ80 or ZK60 and the bath may comprise 70g/L sodium hydroxide, 60g/L disodium metasilicate, 12g/L sodium citrate, 6mL/L hydrogen peroxide, and/or 0.01 moles (M) to 0.05M sodium chloride. In one embodiment, about 0.01M sodium chloride may be included in the bath.

The presence of aluminum in the alloy has been found to be an important factor in the choice of bath chemistry. The exact bath composition may depend on the magnesium alloy chemistry.

At block 110, the method 100 places the substrate in a bath comprising at least one of sodium hydroxide or disodium metasilicate to produce an anodized layer. In one embodiment, the anodizing bath may be in a heating and/or cooling apparatus to maintain a stable solution temperature. In one embodiment, the anodizing bath may include a stainless steel counter electrode. In one embodiment, a Direct Current (DC) power supply may provide voltage and current to perform anodization. In one embodiment, a pulsed DC power supply may provide the anodization power.

In one embodiment, the anodizing bath may be operated at 18 ℃ to 30 ℃. In one embodiment, the anodizing bath may be maintained at a temperature of about 20 ℃.

In one embodiment, a constant current anodization current may be employed. In one embodiment, the constant current may be maintained at 0.5 to 6 amperes per square decimeter (A/dm) ² ). In one embodiment, the current may be limited to 1A/dm ² 。

The peak anodization voltage may determine the structure of the anodization layer. The desired structure may be microporous. In one embodiment, the peak anodization voltage may be maintained at 60 volts (V) to 110V. In one embodiment, the peak anodization voltage may be kept below 75V.

The formation of the microporous layer provides two benefits. First, the microporous layer confines the free magnesium surface. Second, a sufficiently thick microporous layer (e.g., at least 2 microns or 4-20 microns) provides a strong bond between the anodized layer and the metal layer, which provides excellent adhesion of the metal layer to the magnesium substrate.

In one embodiment, the available magnesium accessible surface percentage may be 1% to 25% magnesium-based bottom surface area, 2% to 15% substrate surface area, or 4% to 10% substrate surface area. In one embodiment, about 8% of the substrate surface may be accessible by the anodization layer.

In one embodiment, organic agents in the anodizing bath may be used to control the peak voltage. In one embodiment, the organic agent may be citric acid. Citric acid is a macromolecule that is adsorbed on the surface of a substrate to limit conductivity.

In one embodiment, the organic reagent may be hydrogen peroxide. In one embodiment, the organic agent may be sodium citrate. In one embodiment, the organic agent may be triethanolamine. In one embodiment, any combination of the above listed organic agents may be used, or other organic agents that may provide similar effects may be used.

In one embodiment, the amount of organic agent in the bath may be selected to prevent the peak anodization voltage from exceeding 75V. In one embodiment, the amount of citric acid used may be about 10g/L. In one embodiment, the amount of hydrogen peroxide may be about 6g/L. In one embodiment, the amount of sodium citrate may be 12g/L. In one embodiment, the amount of triethanolamine may be 20ml/L.

The thickness of the anodized film in the present disclosure may be 1 to 50 micrometers. However, the thickness may also be 4 to 20 micrometers. In one embodiment, the thickness may be 5 to 15 microns.

Anodizing was carried out for 10 minutes under the above conditions to obtain an anodized film of about 6 μm. The anodized layer becomes a critical layer of the magnesium hybrid coating system, allowing the subsequently deposited layer to interlock strongly with the anodized layer to provide adhesion over conventional plating solutions.

At block 112, the method 100 rinses the substrate. The anodized layer of the substrate may be rinsed in deionized water or ultrasonically cleaned in ethanol.

At block 114, the method 100 selects an interlock layer plating bath. For example, a suitable plating bath for depositing a metal layer in the interlock layer may be selected. The nature of the plating bath may determine whether metal deposition is beneficial for dissolution of the magnesium substrate and spalling of the anodized coating.

In one embodiment, both the pH of the plating bath and the complexing agent (anionic species) in the plating bath are characteristics that determine whether the deposition was successful. The pH is a characteristic as shown in the reaction:

MgO+H ₂ O→Mg(OH) ₂ ，

the minimum amount of anodized layer dissolves at a pH of 8 to 14. In one embodiment, the pH of the plating bath may be 9 to 12. In one embodiment, the pH of the plating bath may be about 10.

Bath complexing agents (anionic species) can control the solubility product (Ksp) of magnesium. Minimizing Ksp may be desirable. In one embodiment, the anionic species includes a complexing agent to slow the displacement reaction between the metal ions in the plating bath and the magnesium-based substrate. Since any target metal ion is more inert than magnesium, the following reactions occur at the exposed surface:

Mg+M ⁺⁺ →Mg ⁺⁺ +M，

wherein M is the metal in the plating bath. Due to the high negative voltage of magnesium (e.g., -1.75V) and the relatively low voltage of the target metal coating (e.g., ni-0.3V), this reaction can be extreme, resulting in dissolution of the substrate and spalling of the anodized layer.

In one embodiment, the complexing agent, particularly an anionic species or an organic species, may be selected to form a stable chelate with the metal ions in the plating bath. Examples of anionic species that may be selected as complexing agents include cyanides, pyrophosphates, hydroxides, and the like. It should be noted that the listed anionic species are provided as examples, and that other complexing agents may work with the target metal ions, such as certain organic species, glycerol, citric acid, lactic acid, malic acid, pyrrole, and EDTA. It will be appreciated that other methods may be employed to select a complexing agent that works well with the target metal ion.

In one embodiment, the metal in the plating bath may be silver and the complexing agent may be cyanide. The bath may contain silver cyanide, potassium cyanide and potassium hydroxide and have a pH of 12.5. In one embodiment, the metal may be copper, the complexing agent may be pyrophosphate, and the bath may be copper pyrophosphate (which is commercially available from Atotech Corporation) comprising copper pyrophosphate, potassium pyrophosphate, ammonia, and proprietary ingredients. The pH may be 9. In one embodiment, the bath may be a commercially available Zn — Ni bath (commercially available from Atotech Corporation), the complexing agent may be a proprietary complexing agent, and the pH may be 13.

In one embodiment, high pH autocatalytic baths are also suitable. For example, a high pH may include a pH of 8 to 11. The autocatalytic bath may comprise an electroless nickel bath comprising 25g/L nickel sulfate, 25g/L sodium hypophosphite, 50g/L sodium pyrophosphate, and 1g/L thiourea. Here, pyrophosphate may be a complexing agent, and thiourea is used to moderate the reaction rate. The autocatalytic bath may have a pH of 11, and ammonia may be added to achieve a pH of 11.

At block 116, the method 100 places the substrate in an autocatalytic or electrodeposition bath to plate the substrate. For example, the substrate may be placed in an electrodeposition bath following a plating current profile for a predetermined period of time. For example, the current may be gradually increased according to the plating current distribution.

In one embodiment, the first electrodeposited coating is applied onto the anodized film from a bath selected from a range of possible baths as described herein. Controlling an electrical parameter related to the first electrodeposited coating, wherein the first plating current or current profile is applied for a first plating period comprising a first plating phase, and the second plating current is applied for a second plating period comprising a second plating phase. The first electrodeposited layer forms an interlocking layer that completely fills the holes in the anodized layer to securely lock the first electroplated layer to the surface of the anodized layer.

In one embodiment, the substrate is placed in a high pH autocatalytic bath, such as an electroless nickel phosphorous bath or an electroless nickel boron bath, at a temperature that depends on the bath chemistry. The substrate may be placed in the bath for a period of time selected to ensure that the pores of the anodized layer are at least completely filled. In one embodiment, the substrate may be placed in a bath for a period of time to ensure that the entire anodized layer is encapsulated in the electroless coating.

At block 118, the method 100 plates at a second current for a second period of time. For example, the plating current may be increased to the recommended bath plating current and held constant at the recommended bath plating current to produce a plating layer (e.g., a metal coating or layer) having a desired initial coating thickness. The second current may be greater than the first current.

For example, once the hole is filled to a particular level (e.g., less than completely filled, greater than completely filled, etc.), the second plating stage is initiated. During the second phase, the current may remain the same as during the first plating phase, or the current may be immediately increased to the recommended bath plating current. The second plating time period is selected to be sufficient to ensure complete coverage of the anodized film, to form a desired plating thickness, to form a desired surface morphology, and/or to achieve other desired characteristics of the first electrodeposited layer. In one embodiment, the thickness of the second plating state may be 5 to 50 micrometers. At block 120, the method 100 ends.

In one embodiment, the first electrodeposited layer may be 15-50 microns thick. In one embodiment, this thickness may be achieved if the first electrodeposited layer is the only electrodeposited layer that provides all of the functional attributes of the plated surface.

In one embodiment, a silver cyanide bath from technical Corporation is used, and the first plating current distribution is from 0A/dm ² Raised to 0.01 to 0.5A/dm ² . In one example, the first plating current profile may be 0.2A/dm over a first plating time period of 20 to 60 minutes ² . In one example, the first plating period may be 40 minutes.

The current and time are selected to ensure that the porous anodized layer is filled with silver, and depend on the thickness of the anodized layer and the low current deposition rate of the silver bath. The current is then increased to 0.1 to 2A/dm ² (e.g., 1A/dm in one example) ² ) For a second plating period of 10 to 30 minutes (e.g., 20 minutes in one example) to form a total coating thickness of 25 microns.

In one embodiment, a pyrogenic copper (pyro-copper) bath, commercially available from Atotech Corporation, may be used to deposit the interlock layer. In the case of using a pyrometallurgical copper bath, the plating current distribution was from 0A/dm ² Raised to 0.01 to 1A/dm ² . In one example, the plating current profile may be raised to 0.2A/dm ² . The plating current may be increased over a period of 10 to 70 minutes. In one example, the plating current may be increased over a period of 20 minutes. The current and time may be selected to ensure that the porous anodized layer is filled with copper. The plating current profile and time may depend on the thickness of the anodized layer and the low current deposition rate of the copper bath. The current is then raised to 0.1 to 3A/dm ² . In one example, the current may be raised to 1A/dm ² . The plating current may be increased for a second plating period of 5 to 90 minutes. In one embodiment, the second plating time period may be about 70 minutes to form a total coating thickness of 20 microns.

In one embodiment, a zinc-nickel bath commercially available from Atotech Corporation may be used to deposit the interlock layer. In the example using a zinc-nickel bath, the plating current distribution is from 0A/dm ² Raised to 0.01 to 0.5A/dm ² . In one example, the plating current profile may be raised to 0.2A/dm ² . The plating current profile may be increased over a first plating period of 20 to 90 minutes. In one embodiment, the first plating time period is 50 minutes. The plating current and time are selected to ensure that the porous anodized layer is filled with Zn-Ni. The plating current profile and time may depend on the thickness of the anodized layer and the low current deposition rate of the Zn-Ni bath. The current is then raised to 0.1 to 2A/dm ² . In one embodiment, the current may be raised to 1A/dm ² . The plating current may be increased for a second plating period of 10 to 90 minutes. In one embodiment, the second plating period is 50 minutes. The plating current may be increased during the second plating period to form a total coating thickness of 50 microns.

In one embodiment, autocatalytic deposition from a high pH electroless nickel phosphorous bath is used in a single process. Here, the anodized pores may be completely filled with Ni — P, and a layer of 8 μm may be deposited on the deposited anodized layer in a time period of 10 to 180 minutes. In one embodiment, the time period is 30 minutes. The electroless nickel phosphorous bath may comprise 25g/L nickel sulfate and 25g/L sodium hypophosphite. 50g/L sodium pyrophosphate was used as a complexing agent, and 1mg/L thiourea was used to moderate the reaction rate. The bath pH was maintained at 11 ± 0.5 using ammonia and operated at a temperature of 40 to 75 degrees celsius. In one embodiment, the bath is operated at 60 degrees celsius to minimize dissolution of the substrate.

In one embodiment, the first metal layer securely interlocks with the anodization layer and provides a solid foundation for the deposition of other metal layers. The strength of the interlock can be measured by a number of methods known in the art. In one embodiment, the interlocking strength is measured according to the American Society for Testing and Materials (ASTM) D3359 standard by completely cutting the anodized layer and the first metal layer in a grid pattern with I nes on the center of 1 millimeter (mm) and evaluating adhesion using a tape. In one embodiment, the adhesion is evaluated as 4B-5B. In one example, the adhesion was evaluated as 5B.

In one embodiment, the first electrolytically or autocatalytically deposited layer may provide the first functional component of the overall coating system. In particular, the first deposited layer may provide corrosion protection and a low conductivity path to the substrate. In this case, the first electrodeposited layer may have a conductivity of <0.1 milliohms (m Ω) when measured using the procedure specified in Mil DTL 81706.

In one embodiment, the first deposited layer may be deposited from a commercial bath, such as those set forth above, to which a sol of ceramic phase is added in the manner described in U.S. application No. 13/381,487, which is incorporated herein in its entirety, to provide enhanced functional attributes to the coated surface.

In one embodiment, additional electrolytically or autocatalytically deposited layers may be applied over the first electrodeposited layer to provide additional functional aspects of the coating. Such one or more layers may enhance the appearance, hardness, abrasion resistance, electrical conductivity, etc. of the coating system.

In one embodiment, the multilayer coating system is deposited to provide corrosion protection to the surface due to the high Open Circuit Potential (OCP) (-1.75V) of magnesium. Here, the composition of the coating system is designed to direct corrosion away from the surface by appropriate selection and sequencing of the coating materials of each layer to have a specific OCP.

In one embodiment, zn-Ni is selected for the interlock layer because it is capable of forming a dense coating with an OCP of about-1.3V. The thick Zn-Ni layer of 10 to 30 microns provides a barrier layer that directs corrosion away from the substrate. The second layer of semi-bright nickel was selected based on hardness, with an OCP of about-0.3V and about 6 microns deposited as a sacrificial coating. Finally, the decorative or functional surface layer is selected to have an OCP higher than that of semi-bright nickel.

In one embodiment, a final layer may be deposited, which may be a bright Ni layer with an OCP of-0.4V. It should be noted that many metals may be selected as the final layer, as long as the corrosion is directed to the sacrificial layer by the OCP potential of each layer. Such a multi-layer hybrid coating system can provide neutral salt spray test performance according to ASTM specification B117 for over 150 hours.

Examples

The following examples indicate exemplary operating conditions and provide examples of producing functional coatings on magnesium substrates as described in the present disclosure. However, these examples should not be considered as limiting the scope of the disclosure. The examples were chosen to illustrate aspects of anodizing bath development, metal interlock layer characteristics, and producing a coating stack that provides good corrosion protection for magnesium-based substrates.

Example 1 development of a magnesium anodized layer for hybrid coatings

Anodization was performed on AZ80 and ZK60 magnesium alloy substrates. Each substrate was cut into 2cm by 3cm by 1cm. Electrical connection to the substrate was made by drilling a 4mm hole in one edge and taping (taping) it and screwing a threaded aluminum wire into the hole. The connection points are electrically protected by coating the top edge with epoxy and taping the aluminum wire.

The substrate was pretreated by manually abrading the surface to remove the native oxide layer. A series of 400 to 1000 grit sandpaper was used. The substrate was washed in deionized water to remove residue. No other pretreatment was used because the alkaline anodizing bath was sufficient to remove any oil and grease and the sanded surface provided adhesion to the anodized layer.

It has been found that a low power anodization system from a non-toxic bath can produce a relatively thick porous anodization layer suitable for depositing a metal coating by electrolytic or autocatalytic deposition. The structure of the anodization layer, like the hybrid coating on aluminum described in the previous disclosure, can determine whether a successful coating is produced.

FIG. 2 shows a mixture of sodium hydroxide at 70g/L, sodium tetraborate at 60g/L, disodium metasilicate at 60g/L, and sodium tetraborate at 30g/Lgraph of anodization voltage versus time when an AZ80 magnesium substrate was anodized in a bath of g/L sodium carbonate and 12g/L citric acid. The bath temperature was 19 ℃ and was 1A/dm ² Constant current anodization was performed. Upon application of power, the voltage quickly climbs to 53.1V and remains below 54V with little observable change on the surface (201) due to the formation of the oxide barrier. The voltage then rises rapidly, with many small arcs appearing covering the surface, and anodization can be observed on the surface (202). After about 150 seconds, the voltage stabilizes at 80V when the surface is covered by the anodization layer (204). The growth of the anodization layer is associated with the occurrence of a large arc over the entire surface (203) and a slow increase in voltage to 86V. After 10 minutes, an anodization (204) of 15-20 microns will form. The total Ampere Hour (AH) for forming this coating was 0.133AH/dm ² And the power is-12 Wh/dm ² I.e., two orders of magnitude less than that required for the MAO process.

It was also found that the structure of the anodization layer was controlled by the peak anodization voltage, as shown in graph 302 shown in fig. 3A. Here, the graph 302 shows the use of different bath compositions at 1A/dm ² Constant current anodization at bottom the anodization voltage of the anodization varied with time. Table 301 shows the two bath compositions and substrate types. The surface 303 shown in fig. 3B produces a high final voltage (92.3V) during anodization growth and a large sparse arc and is characterized by surface features with large pores that do not plate well. The surface 304, also shown in fig. 3B, produces a lower final voltage (73.3V) and has small surface features and many small holes, which provide a satisfactory interlocking layer for the plating process.

Example 2 electrodeposition of silver in an interlocking layer

The hybrid coating comprising the 15 micron anodized interlocking layer together with the silver first functional layer provides uniform coverage and provides a good substrate for deposition of other functional layers.

Anodizing was performed on the AZ80 magnesium alloy substrate cut into 2cm by 3cm by 1cm. Electrical connection to the substrate was achieved by drilling a 4mm hole in one edge and taping it and screwing a threaded aluminum wire into the hole. The connection points are electrically protected by coating the top edge with epoxy and taping the aluminum wire.

The substrate was pretreated by manually abrading the surface to remove the native oxide layer. A series of 400 to 1000 grit sandpaper was used. The substrate was washed in deionized water to remove residue.

The substrate was placed in an anodizing bath containing 70g/L sodium hydroxide, 65g/L sodium tetraborate and 30g/L sodium carbonate at 22 ℃ and used at 1A/dm ² Constant current anodization was continued for 10 minutes. An anodization layer of 6 micron structure was formed.

The substrate was rinsed in deionized water and placed in a silver cyanide bath at pH 12. Silver cyanide baths are available from technical Corporation and comprise silver cyanide, potassium cyanide and proprietary ingredients. As previously described, DC plating was performed over a total period of 60 minutes following the current profile to ensure hole filling. The current was 0.036A/dm in 40 minutes ² Then 0.18A/dm in 20 minutes ² . A 25 micron uniform silver layer was formed as shown in fig. 4.

Fig. 5 shows an optical cross-section 502 and a Scanning Electron Microscope (SEM) cross-section 501 of a silver interlock layer formed on a substrate 503. The coating thickness was about 25 microns. SEM cross section 501 clearly shows that silver 506 penetrates the anodization layer 504.

Example 3 electrodeposition of copper in an interlock layer

The hybrid coating comprising the 8 micron anodized interlock layer along with the copper first functional layer provides uniform coverage and provides a good substrate for deposition of other functional layers.

Anodization was performed on a ZK60 magnesium alloy substrate cut into 2cm × 3cm × 1cm. Electrical connection to the substrate was achieved by drilling a 4mm hole in one edge and taping it and screwing a threaded aluminum wire into the hole. The connection points are electrically protected by coating the top edge with epoxy and taping the aluminum wire.

The substrate is pretreated by hand grinding the surface to remove the native oxide layer. A series of 400 to 1000 grit sandpaper was used. The substrate was washed in deionized water to remove residue.

At 22 deg.C, mixingThe bottom was placed in an anodizing bath containing 70g/L sodium hydroxide, 60g/L sodium tetraborate, 60g/L disodium silicate, 30g/L sodium carbonate and 12g/L citric acid, and used at 1A/dm ² Constant current anodization was continued for 10 minutes. An 8 micron structured anodization layer was formed.

The substrate was rinsed in deionized water and placed in a pyrogenic copper bath having a pH of about 10. The pyrometallurgical copper bath was purchased from Atotech Corporation and comprised copper pyrophosphate, potassium pyrophosphate, citric acid and proprietary ingredients. As previously described, DC plating was performed over a total period of 48 minutes following the current profile to ensure hole filling. The current increased to 0.15A/dm in 28 minutes ² Then 0.15A/dm in 10 minutes ² Then 0.3A/dm over 10 minutes ² Is constant current. A copper layer of about 40 microns was formed as shown in fig. 7 and disclosed below.

Fig. 6 shows an optical cross-section 602 and SEM cross-section 601 of a copper interlock layer on a substrate 603. The coating thickness was about 40 microns. Anodization was performed with an unconstrained voltage, resulting in an anodized surface with large pores. This surface is not an ideal surface for plating the interlock layer. As can be seen from the SEM cross-section 601, the copper 606 penetrates the anodization layer 604; however, copper is discontinuous and is characterized by discontinuous nodules having a diameter of about 100 microns.

Fig. 7 shows an image 701 of a copper layer formed on a substrate as described above. Image 702 shows that the copper layer adhered well to the anodized layer, as confirmed by the cross-hatch adhesion test performed according to ASTM D3359, which passed without delamination (5B).

Image 703 shows a simple measurement of the conductivity of the coating up to the substrate, clearly indicating that copper is directly attached to magnesium.

Example 4 electrodeposition of Zinc Nickel in an interlocking layer with an improved anodizing bath

The hybrid coating comprising the 10 micron anodized interlocking layer together with the zinc-nickel first functional layer provides uniform coverage and provides a good substrate for deposition of other functional layers.

Anodizing was performed on the AZ80 magnesium alloy substrate cut into 2cm by 3cm by 1cm. The electrical connection to the substrate was made by drilling a 4mm hole in one edge and taping it and screwing a threaded aluminum wire into the hole. The connection points are electrically protected by coating the top edge with epoxy and taping the aluminum wire.

The substrate is pretreated by hand grinding the surface to remove the native oxide layer. A series of 400 to 1000 mesh sandpaper was used. The substrate was washed in deionized water to remove residue.

The substrate was placed in an anodizing bath containing 70g/L sodium hydroxide, 60g/L sodium tetraborate, 60g/L disodium silicate, 30g/L sodium carbonate and 12g/L citric acid at 20 ℃ and used at 1A/dm ² Constant current anodization was continued for 10 minutes. An anodization layer of 10 micron structure was formed.

The substrate was rinsed in deionized water and placed in a zinc nickel bath at pH 13.2. The zinc-nickel bath is available from Atotech Corporation and contains sodium hydroxide, zinc oxide, a complexing agent and several proprietary ingredients. As mentioned before, in compliance with 0-0.1A/dm ² DC plating was performed for a total period of 60 minutes of current distribution to ensure hole filling. The current is 0.1A/dm within 40 minutes ² Then 0.3A/dm in 20 minutes ² 。

Fig. 8 shows a sample image 801, an optical microscope surface image 802, and an optical microscope cross-sectional image 803 of a hybrid Zn-Ni coating formed on an anodized layer produced from the original bath.

Coatings

802 and 803 are very rough, with large nodules 804 visible on the surface. Nodules 804 are larger than is common with Zn-Ni coatings and are created by preferential coating growth, wherein the current path can be through the anodized film to the substrate. By controlling the anodizing bath chemistry, temperature and conductivity, the number and size of pores can be varied such that the Zn-Ni coating becomes uniform and continuous. The cross-sectional image 803 shows the substrate 805 and anodized film 806 at about 22 microns penetrated by the Zn-Ni interlocking layer 807.

When the Zn-Ni interlocking layer 807 is formed, it has been found that a bath containing sodium citrate and hydrogen peroxide produces improved results. ZK60 samples were pretreated as described above and contained 70g/L NaOH, 60g/L Na ₂ SiO ₃ 、12g/L citric acid, 6ml/L hydrogen peroxide and 0.01M to 0.05M NaCl. Fig. 12 shows a voltage time curve 1201 during anodization using this bath. It can be seen that, in contrast to the curve in fig. 3, the more controlled conductivity in the bath results in a flatter voltage curve, and the peak voltage is limited to below 75V. It has been found that the bath improves the density and uniformity of pores in the anodized layer, which works better with Zn-Ni coatings and other interlocking layer metals.

Fig. 9 shows

images

901, 902 and 903 of anodized samples produced using the new bath. Surface SEM 904 has a surface formed using a fresh bath. Surface SEM 904 shows a surface with a well defined pore structure, the improved surface morphology of which provides excellent gripping of the interlocking layer. The area SEM 905 and related EDS data 906 show that magnesium and oxygen are higher in the dark area near box 908 in the area SEM 905, indicating that the porous structure supports the conductivity of the substrate. The lighter area near box 910 in area SEM 905 and the last row of EDS table 906 show more oxygen and silicon, indicating a strong silica framework for depositing the interlock layer.

Fig. 13 also shows an optical image of the surface of a Zn-Ni interlocking layer that can be produced with the anodization layer of the present disclosure. Surface 1302 shows a dense nodular structure typical of a Zn-Ni coating. Fig. 10 shows surface and cross-sectional images of a hybrid Zn-Ni coating produced using a new bath. The sample image 1001 shows a fine uniform surface. The optical microscope images show finer and more uniform surface nodular structures than the example in fig. 8 formed using the original anodizing bath discussed above. Cross-sectional SEM image 1002 shows the Zn-Ni interlocking layer well integrated with the anodized structure and shows a more uniform Zn-Ni surface. SEM line scan 1003 and associated EDS data 1004 show the Zn-Ni coating penetrating the anodized layer. Dashed circle 1005 shows the coating composition as the Zn-Ni interlock penetrates the anodized layer. The addition of silicon, oxygen and magnesium is characteristic of anodized structures formed with fresh baths

Example 5 deposition of Ni-P in an interlock layer

The hybrid coating comprising the 4 micron anodized interlock layer along with the Ni-P first functional layer provides uniform coverage and provides a good substrate for deposition of other functional layers.

Anodizing was performed on an AZ80 magnesium alloy substrate cut into 2cm × 3cm × 1cm. The electrical connection to the substrate was made by drilling a 4mm hole in one edge and taping it and screwing a threaded aluminum wire into the hole. The connection points are electrically protected by coating the top edge with epoxy and taping the aluminum wire.

The substrate was placed in an anodizing bath containing 70g/L sodium hydroxide, 60g/L sodium tetraborate and 30g/L sodium carbonate at 20 ℃ and used at 1A/dm ² Constant current anodization was continued for 10 minutes. An anodized layer of 4 micron structure was formed.

The substrate was rinsed and ultrasonically cleaned in an alcohol bath for 1-5 minutes.

The cleaned anodized samples were placed in an alkaline nickel phosphorus bath containing 25g/L nickel sulfate and 25g/L sodium hypophosphite. 50g/L sodium pyrophosphate was used as a complexing agent, and 1mg/L thiourea was used to moderate the reaction rate. The bath was heated to 70 ℃ and the pH of the bath was adjusted to 11 using ammonia. A plating time of 3 hours was sufficient to fill the pores and deposit a uniform layer on the anodized surface.

Fig. 11 depicts SEM images of the resulting coating. Image 1102 of the surface shows nodular Ni-P surface morphology. Here, the nodule 1106 shown in the image 1102 does not completely cover the surface, and there are some voids 1107. By carefully controlling the chemistry, temperature and conductivity of the anodizing bath, the density of pores in the anodized layer can be optimized to ensure that the Ni-P coating is continuous. Image 1101 shows an SEM image of the coating cross-section. Here, the substrate 1103 is anodized to a thickness of about 3-5 microns, and the Ni-P coating is 10-20 microns. Ni-P significantly penetrates anodized layer 1104.

It will be appreciated that variations of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.

Claims

1. The method comprises the following steps:

placing a magnesium-based substrate in an anodizing bath;

applying a voltage for a first amount of time to form a microporous anodization layer having a thickness of 1 to 50 microns on the magnesium substrate;

placing the magnesium substrate with the microporous anodized layer in a plating bath, wherein the plating bath comprises a metal and a complexing agent and has a pH of 8 to 14;

applying a first current to the plating bath for a second amount of time to form an interlock layer on the microporous anodization layer; and

applying a second current to the plating bath for a third amount of time to form a coating on the interlock layer.

2. The method of claim 1, wherein the magnesium substrate comprises a magnesium alloy substrate.

3. The method of claim 1, wherein the anodizing bath comprises at least one of sodium hydroxide or disodium metasilicate.

4. The method of claim 1, wherein the voltage is applied by a pulsed dc power supply.

5. The method of claim 1, wherein the pulsed dc power supply applies a constant current of 0.5 to 6 amps per square decimeter at a temperature of 18 degrees celsius to 30 degrees celsius.

6. The method of claim 1, wherein the voltage has a peak voltage of less than 75 volts.

7. The method of claim 6, wherein the anodizing bath contains an organic agent to control the peak voltage.

8. The method of claim 1, wherein the complexing agent comprises at least one of cyanide, pyrophosphate, or hydroxide.

9. The method of claim 1, wherein the first current follows a plating current profile to allow the interlock layer to fill pores in the microporous anodization layer.

10. The method of claim 1, wherein the second current is a constant current greater than the first current.

11. The method of claim 1, wherein the coating has a thickness of 5 to 50 microns.

12. The method comprises the following steps:

pretreating a magnesium substrate;

cleaning the magnesium substrate with deionized water;

forming a microporous anodization layer on the magnesium substrate in an anodization bath, wherein a voltage is applied to the anodization bath for a first amount of time to form the microporous anodization layer;

rinsing the magnesium substrate with the microporous anodization layer;

forming an interlock layer on the microporous anodization layer in an autocatalytic plating bath for a second amount of time, wherein the plating bath comprises a metal and a complexing agent and has a pH of 8 to 14; and

forming a coating on the interlocking layer in the autocatalytic plating bath, wherein a current is applied to the plating bath for a third amount of time to form the coating.

13. The method of claim 12, wherein the pre-treatment comprises at least one of treating the magnesium substrate in an acid bath, mechanically roughening the magnesium substrate, or cleaning the magnesium substrate in a base bath.

14. The method of claim 12, wherein the anodizing bath comprises at least one of sodium hydroxide or disodium metasilicate.

15. The method of claim 12, wherein the voltage has a peak voltage of less than 75 volts.

16. The method of claim 15, wherein the anodizing bath comprises citric acid to control the peak voltage.

17. The method of claim 12, wherein the second amount of time is sufficient to fill pores of the anodization layer.

18. The method of claim 12, wherein the autocatalytic plating bath comprises a nickel phosphorous bath or a nickel boron bath.

19. The method of claim 18, wherein the second current is 0.1 amperes per square decimeter (a/dm) ² ) To 2A/dm ² 。

20. The method comprises the following steps:

placing a magnesium alloy substrate in an anodizing bath;

applying and maintaining a peak voltage of less than 75 volts over a first time period of about 10 minutes to form a microporous anodization layer having a thickness of 5 to 15 micrometers on the magnesium alloy substrate;

placing the magnesium alloy substrate with the microporous anodized layer in a plating bath, wherein the plating bath comprises a metal and a complexing agent and has a pH of 9 to 12;

applying from 0 amperes per square decimeter (A/dm) for a second period of time to the plating bath ² ) Increased to 0.01A/dm ² To 0.5A/dm ² To form an interlock layer on the microporous anodization layer; and

applying greater than that to the plating bath for a third period of timeThe first current is 0.1A/dm ² To 2A/dm ² To form a coating on the interlock layer.