PROTECTIVE SURFACE COATING FOR MAGNESIUM ALLOY MATERIAL

Description

STATEMENT REGARDING PRIOR DISCLOSURE BY THE INVENTORS

Aspects of the present disclosure are described in “Effects of in-situ incorporation of h-BN nanosheets, low-melting nanoclays and corrosion-inhibiting stannate on plasma electrolytic oxidized AZ31B Mg alloy” Applied Surface Science, 669 (2024) 160476, which is incorporated herein by reference in its entirety.

BACKGROUND

Technical Field

The present disclosure is directed towards anti-corrosive surface coating techniques, and more particularly, related to a surface oxide layer for a magnesium alloy material.

Description of Related Art

The “background” description provided herein is to present the context of the disclosure generally. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present invention.

Metals are a vital component of the modern society and the growth of various industries including the transportation industry, the civil engineering industry, and the oil and gas industries, due to their exceptional mechanical and thermal properties. However, many metals and metal alloys face significant challenges, such as poor resistance to corrosion and wear, which restricts potential applications of the metal alloys. One such widely used metal alloy is magnesium metal alloy, known for lightweight and strong structural construction, which is often used in aerospace and biomedical applications. However, prevalent commercial application of magnesium alloys is restricted by weak aqueous corrosion resistance owing to the formation of a less protective surface oxide layer with an exterior porous hydroxide layer. Similarly, one reason for the degradation of metals stems from prolonged exposure to an aggressive, damp, and corrosive environment. While corrosion is inevitable, its impact may be mitigated by employing protective coatings that exhibit a robust barrier effect, active protection assisted by incorporated corrosion inhibitors, and improved surface hydrophobicity, which provides moisture repellency.

Numerous coating approaches have been developed for alloys primarily aiming at enhancing corrosion resistance and surface hardness of the alloys and subsequently addressing the inherent vulnerabilities of the alloy. Traditionally, the metal surface coatings may include, but are not limited to, chemical conversion coating, anodizing, electroless/electroplating, organic coating, laser surface treatment, physical vapor deposition, chemical vapor deposition, and thermal/cold spray. Most of the aforementioned coatings use toxic or harmful chemicals to produce a coating on the substrate and require extensive care and control during operation. Anodizing forms a protective oxide layer but may suffer from uneven thickness and poor adhesion under mechanical stress. Thermal spraying allows for the application of various coatings but may introduce defects and porosity, compromising the protective qualities. Electroplating may provide a dense coating but often leads to issues with uniformity and may not adequately protect against corrosion in an aggressive environment. These drawbacks underscore the challenges in achieving reliable and effective coatings for alloys in demanding applications.

Presently, research has been focused on various aspects of alloy coatings, aiming to enhance coating thickness, adhesion, mechanical properties, and corrosion resistance. The goals of such research include optimizing coating processes to achieve uniform and thicker layers that provide better protection against environmental degradation. Plasma electrolytic oxidation (PEO) is a process used to enhance the surface properties of metals such as magnesium and aluminum. The aforementioned metals are distinctive owing to wear resistance, corrosion resistance, thermal stability, and chemical stability. PEO surface coating technology is often used for the light metals and their alloys including aluminium, magnesium and titanium. It may be applied to other metals as well, such as zirconium, tantalum, niobium and hafnium, and also cobalt. The PEO surface coating process has been researched recently and may be considered a better approach for biomedical, electronic, aerospace, and automobile applications, providing comparative advancements over conventional surface treatment processes. PEO is one of the preferred approaches for magnesium alloy surface coatings in order for the alloy to develop a highly corrosion-resistant electrochemical conversion layer. PEO relies on local plasma discharges at the metal/electrolyte interface at high voltages e.g. ranging from 150 volts (V) to 450 V for magnesium alloys, which converts the metal into a thick and firmly bound protective barrier oxide layer via electro, thermo, and plasma-chemical reactions. Plasma-assisted deposition often forms crystalline microscale or nanoscale surface oxides, substantially enhancing mechanical properties, wear, and corrosion resistance. A PEO layer may provide about three to five times more hardness and more than 10 times the corrosion resistance of a traditional hard anodized coating.

Typically, the PEO layer on magnesium alloys is characterized by an external layer with pores and cavities, a drawback concerning their long-term protective barrier effect. Further, PEO is effective in creating a durable and adherent oxide layer, but may require precise control over the process parameters to avoid excessive roughness or cracking. The in-situ integration of nano/micro-particles into the PEO layer, facilitated by optimized electrolyte additives, is an attractive approach for producing oxide layers with better protective capability. This potential for creating enhanced composite oxide layers is a promising aspect of PEO technology. Furthermore, it may be challenging to obtain uniform incorporation of particles due to various factors, including potential particles agglomeration, on magnesium alloy surfaces. The high melting point of hard ceramic nanomaterials is a challenging factor hindering their reactive integration into the PEO layer. Hence, a need arises for better anti-corrosive surface layer technologies, that may be more efficient, effective, long-lasting, and cost-effective.

Accordingly, it is one object of the present disclosure to provide a magnesium alloy material with an effective surface oxide layer deposited via efficient PEO technique that may circumvent the above stated drawbacks, such as, low longevity, low stability, and high costs, of traditional methods known in the art.

SUMMARY

In an exemplary embodiment, a magnesium alloy material is described. The magnesium alloy material includes a bulk portion, and a surface oxide layer formed on the bulk portion and including magnesium oxide, hexagonal boron nitride nanosheets, nanoclay and stannate. The bulk portion includes, based on a total weight of the bulk portion, 91.0 percent by weight (wt. %) to 97.5 wt. % of magnesium (Mg), 2.0 wt. % to 4.0 wt. % of aluminium (Al), 0 wt. % to 0.1 wt. % of copper (Cu), 0 wt. % to 0.01 wt. % of iron (Fe), 0.01 wt. % to 2.0 wt. % of manganese (Mn), 0 wt. % to 0.01 wt. % of nickel (Ni), 0 wt. % to 0.5 wt. % of silicon (Si) and 0.3 wt. % to 2.0 wt. % of zinc (Zn).

In some embodiments, the surface oxide layer includes, based on a total weight of the surface oxide layer, 50 wt. % to 70 wt. % of Mg, 15 wt. % to 30 wt. % of O, 0.5 wt. % to 1.5 wt. % of Al, 0.01 wt. % to 0.2 wt. % of Mn, 3 wt. % to 10 wt. % of Si, 0.1 wt. % to 1.5 wt. % of Zn, 5 wt. % to 15 wt. % of carbon (C), and 0.3 wt. % to 2.0 wt. % of tin (Sn).

In some embodiments, the surface oxide layer includes a bottom portion formed on the bulk portion, a middle portion formed on the bottom portion, and a top portion formed on the middle portion, and Sn is distributed non-uniformly in the bottom portion, the middle portion and the top portion of the surface oxide layer. The hexagonal boron nitride nanosheets are distributed non-uniformly in the bottom portion, the middle portion and the top portion of the surface oxide layer, and the nanoclay is distributed non-uniformly in the bottom portion, the middle portion and the top portion of the surface oxide layer.

In some embodiments, Sn is present at the highest concentration in the bottom portion of the surface oxide layer, the hexagonal boron nitride nanosheets are present at a highest concentration in the top portion of the surface oxide layer, and the nanoclay is present at a highest concentration in the top portion of the surface oxide layer.

In some embodiments, Sn is present at an average concentration of 0.62 wt. % in the surface oxide layer based on the total weight of the surface oxide layer and Sn is present at the highest concentration of 1.45 wt. % in the bottom portion of the surface oxide layer, based on a total weight of the bottom portion of the surface oxide layer.

In some embodiments, the bulk portion includes, based on the total weight of the bulk portion 94.0 wt. % to 97.0 wt. % of Mg, 2.5 wt. % to 3.5 wt. % of Al, 0.01 wt. % of Cu, 0.003 wt. % of Fe, 0.2 wt. % to 1.0 wt. % of Mn, 0.001 wt. % of Ni, 0.08 wt. % of Si and 0.6 wt. % to 1.4 wt. % of Zn.

In some embodiments, the surface oxide layer includes based on the total weight of the surface oxide layer 55.76 wt. % of Mg, 24.91 wt. % of O, 0.83 wt. % of Al, 0.04 wt. % of Mn, 5.80 wt. % of Si, 0.72 wt. % of Zn, 11.32 wt. % of C, and 0.62 wt. % of Sn.

In some embodiments, the hexagonal boron nitride nanosheets have an average radius of 300 nanometers (nm) to 1000 nm.

In some embodiments, the hexagonal boron nitride nanosheets are at least partially hydroxylated.

In some embodiments, the nanoclay includes montmorillonite that is surface-modified by octadecylamine and aminopropyl triethoxysilane.

In some embodiments, the surface oxide layer includes pores having an average size of 0.5 micrometers (μm) to 2.0 μm and voids having an average size of 1 μm to 4 μm.

In some embodiments, the surface oxide layer has a thickness of 50 μm to 200 μm.

In some embodiments, a corrosion current density of the magnesium alloy material is less than 0.6% of a corrosion current density of a first comparative example that has an identical composition of the bulk portion of the magnesium alloy material, and the corrosion current density of the magnesium alloy material is less than 5% of a corrosion current density of a second comparative example that is the same as the magnesium alloy material except having no hexagonal boron nitride nanosheets, nanoclay or stannate.

In some embodiments, a corrosion rate of the magnesium alloy material less than 0.6% of a corrosion rate of a first comparative example that has an identical composition of the bulk portion of the magnesium alloy material, and the corrosion rate of the magnesium alloy material is less than 4% of a corrosion rate of a second comparative example that is the same as the magnesium alloy material except having no hexagonal boron nitride nanosheets, nanoclay or stannate.

In some embodiments, the magnesium alloy material has a water contact angle (WCA) of 70° to 85°, which is larger than that of a comparative example that is the same as the magnesium alloy material except having no hexagonal boron nitride nanosheets, nanoclay or stannate.

In some embodiments, the surface oxide layer includes no crystallized magnesium stannate hydroxide (MgSn(OH)₆).

In some embodiments, the surface oxide layer is formed on the bulk portion by at least a plasma electrolytic oxidation process including pulsing a voltage of a direct current between a first value of 180V to 220V and a second value of 80V to 120V at an interval of 10 to 60 seconds for a first period of 12.5 to 17.5 minutes between a magnesium alloy anode and a cathode in the presence of an electrolyte solution including sodium metasilicate, calcium fluoride, potassium hydroxide, the hexagonal boron nitride nanosheets, the nanoclay, the stannate and water and maintaining the voltage of the direct current at the first value for a second period of 7.5 to 12.5 minutes.

In some embodiments, the first value is 200V, the second value is 100V, the interval is 30 seconds, the first period is 15 minutes, and the second period is 10 minutes.

In some embodiments, the electrolyte solution includes 25 grams per liter (g/L) of the sodium metasilicate, 5 g/L of the calcium fluoride, 5 g/L of the potassium hydroxide, 0.5 g/L of the hexagonal boron nitride nanosheets, 0.5 g/L of the nanoclay and 2 g/L of the stannate.

In some embodiments, the electrolyte solution has a pH of about 12.5, the electrolyte solution is maintained at a temperature of 15° C. to 25° C. during the plasma electrolytic oxidation process, the cathode includes stainless steel, and an area ratio of the magnesium alloy anode and the cathode is from 1:2 to 1:10.

The foregoing general description of the illustrative embodiments and the following detailed description thereof are merely exemplary aspects of the teachings of this disclosure, and are not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of this disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings.

FIG. 1 illustrates a schematic flow of the mechanism of corrosion protection for a surface of a magnesium (Mg) alloy, according to certain embodiments.

FIG. 2A shows a scanning electron microscope (SEM) image of boron nitride (h-BN) nanosheets, according to certain embodiments.

FIG. 2B shows an SEM image of nanoclay, according to certain embodiments.

FIG. 3A shows a two-dimensional/three-dimensional (2D/3D) optical profilometry image of a Mg alloy after plasma electrolytic oxidation (PEO) coating, according to certain embodiments.

FIG. 3B shows a 2D/3D optical profilometry image of a Mg alloy after boron nitride (BN)-incorporated PEO coating (B-PEO), according to certain embodiments.

FIG. 3C shows a 2D/3D optical profilometry image of a Mg alloy after nanoclay (NC)-incorporated PEO coating (C-PEO), according to certain embodiments.

FIG. 3D shows a 2D/3D optical profilometry image of a Mg alloy after BN- and NC-incorporated PEO coating (BC-PEO), according to certain embodiments.

FIG. 3E shows a 2D/3D optical profilometry image of a Mg alloy after BN-, NC- and ST-incorporated PEO coating (BCS-PEO), according to certain embodiments.

FIG. 4A shows a 3D atomic force microscopy (AFM) image of a PEO coated sample, according to certain embodiments.

FIG. 4B shows a 3D AFM image of a B-PEO coated sample, according to certain embodiments.

FIG. 4C shows a 3D AFM image of a C-PEO coated sample, according to certain embodiments.

FIG. 4D shows another 3D AFM image of a C-PEO coated sample, according to certain embodiments.

FIG. 4E shows a 3D AFM image of a BC-PEO coated sample, according to certain embodiments.

FIG. 4F shows a 3D AFM image of a BCS-PEO coated sample, according to certain embodiments.

FIG. 5A shows an SEM image of a PEO sample, according to certain embodiments.

FIG. 5B shows an SEM image of B-PEO, according to certain embodiments.

FIG. 5C shows an SEM images of C-PEO, according to certain embodiments.

FIG. 5D shows an SEM image of BC-PEO, according to certain embodiments.

FIG. 5E shows an SEM images of BCS-PEO, according to certain embodiments.

FIG. 5F shows another SEM image of the PEO sample of FIG. 5A, according to certain embodiments.

FIG. 5G shows another SEM image of the B-PEO sample of FIG. 5B, according to certain embodiments.

FIG. 5H shows another SEM image of the C-PEO sample of FIG. 5C, according to certain embodiments.

FIG. 5I shows another SEM image of the BCS-PEO sample of FIG. 5E, according to certain embodiments.

FIG. 6A is an energy dispersive spectroscopy (EDS) line scan analysis of boron (B), silicon (Si) and carbon (C) across the surface of a B-PEO sample along with line scan analysis of boron provided separately, according to certain embodiments.

FIG. 6B is an EDS line scan analysis of B, Si and C across the surface of a BC-PEO sample along with line scan analysis of boron provided separately, according to certain embodiments.

FIG. 7 shows EDS line scan analyses of a BCS-PEO sample, conducted across the layer thickness, according to certain embodiments.

FIG. 8 is a graph depicting X-ray diffraction (XRD) analysis of a PEO sample, a B-PEO sample, a C-PEO sample, and a BCS-PEO sample, according to certain embodiments.

FIG. 9A is a Fourier transform infrared spectroscopy (FTIR) analysis results of a PEO coated sample, according to certain embodiments.

FIG. 9B is a FTIR analysis result of a B-PEO coated sample, according to certain embodiments.

FIG. 9C is a FTIR analysis result of a C-PEO coated sample, according to certain embodiments.

FIG. 9D is a FTIR analysis result of a BCS-PEO coated sample, according to certain embodiments.

FIG. 10A depicts water contact angle (WCA) of a PEO coated sample, according to certain embodiments.

FIG. 10B depicts WCA of a B-PEO coated sample, according to certain embodiments.

FIG. 10C depicts WCA of a C-PEO coated sample, according to certain embodiments.

FIG. 10D depicts WCA of a BC-PEO coated sample, according to certain embodiments.

FIG. 10E depicts WCA of a BCS-PEO coated sample, according to certain embodiments.

FIG. 11A is a bar graph depicting the elastic moduli of a PEO coated sample, a B-PEO coated sample, a C-PEO coated sample, a BC-PEO coated sample, and a BCS-PEO coated sample, according to certain embodiments.

FIG. 11B is a bar graph depicting nano-hardness of a PEO coated sample, a B-PEO coated sample, a C-PEO coated sample, a BC-PEO coated sample, and a BCS-PEO coated sample, according to certain embodiments.

FIG. 11C is a critical load micrograph a PEO coated sample, a B-PEO coated sample, a C-PEO coated sample, a BC-PEO coated sample, and a BCS-PEO coated sample, according to certain embodiments.

FIG. 11D is an optical micrograph of a scratch test conducted on a PEO coated sample, a B-PEO coated sample, a C-PEO coated sample, a BC-PEO coated sample, and a BCS-PEO coated sample, according to certain embodiments.

FIG. 12 shows a potentiodynamic plot of bare Mg alloy, PEO coated Mg alloy, B-PEO coated Mg alloy, C-PEO coated Mg alloy, and BCS-PEO coated Mg alloy, according to certain embodiments.

FIG. 13A shows electrochemical impedance spectroscopy (EIS) conducted in 3.5 wt. % NaCl and a resultant Nyquist plot for bare Mg alloy, PEO coated Mg alloy, B-PEO coated Mg alloy, C-PEO coated Mg alloy, and BCS-PEO coated Mg alloy, according to certain embodiments.

FIG. 13B shows EIS conducted in 3.5 wt. % NaCl and another resultant Nyquist plot for bare Mg alloy, PEO coated Mg alloy, B-PEO coated Mg alloy, C-PEO coated Mg alloy, and BCS-PEO coated Mg alloy, according to certain embodiments.

FIG. 13C depicts a set of Bode plots for bare Mg alloy, PEO coated Mg alloy, B-PEO coated Mg alloy, C-PEO coated Mg alloy, and BCS-PEO coated Mg alloy, according to certain embodiments.

FIG. 13D depicts another set of Bode plots for bare Mg alloy, PEO coated Mg alloy, B-PEO coated Mg alloy, C-PEO coated Mg alloy, and BCS-PEO coated Mg alloy, according to certain embodiments.

FIG. 13E depicts an equivalent circuit used for curve fitting of a bare Mg alloy, according to certain embodiments.

FIG. 13F depicts an equivalent circuit used for curve fitting of a PEO coated Mg alloy, according to certain embodiments.

FIG. 14A shows a scanning electrochemical microscopy (SECM) image taken during continuous immersion (for 1 hour) of a bare Mg alloy sample in 0.1 M NaCl, according to certain embodiments.

FIG. 14B shows an SECM image taken during continuous immersion (for 8 hours) of a bare Mg alloy sample in 0.1 M NaCl, according to certain embodiments.

FIG. 14C shows an SECM image taken during continuous immersion (for 24 hours) of a bare Mg alloy sample in 0.1 M NaCl, according to certain embodiments.

FIG. 14D shows an SECM image taken during continuous immersion (for 1 hour) of a PEO coated Mg alloy sample in 0.1 M NaCl, according to certain embodiments.

FIG. 14E shows an SECM image taken during continuous immersion (for 8 hours) of a PEO coated Mg alloy sample in 0.1 M NaCl, according to certain embodiments.

FIG. 14F shows an SECM image taken during continuous immersion (for 24 hours) of a PEO coated Mg alloy sample in 0.1 M NaCl, according to certain embodiments.

FIG. 14G shows an SECM image taken during continuous immersion (for 1 hour) of a BCS-PEO coated Mg alloy sample in 0.1 M NaCl, according to certain embodiments.

FIG. 14H shows an SECM image taken during continuous immersion (for 8 hours) of a BCS-PEO coated Mg alloy sample in 0.1 M NaCl, according to certain embodiments.

FIG. 14I shows an SECM image taken during continuous immersion (for 24 hours) of a BCS-PEO coated Mg alloy sample in 0.1 M NaCl, according to certain embodiments.

FIG. 15A shows an SECM image taken during continuous immersion (for 1 hour) of a B-PEO coated Mg alloy sample in 0.1 M NaCl, according to certain embodiments.

FIG. 15B shows an SECM image taken during continuous immersion (for 8 hours) of a B-PEO coated Mg alloy sample in 0.1 M NaCl, according to certain embodiments.

FIG. 15C shows an SECM image taken during continuous immersion (for 24 hours) of a B-PEO coated Mg alloy sample in 0.1 M NaCl, according to certain embodiments.

FIG. 15D shows an SECM image taken during continuous immersion (for 1 hour) of a C-PEO coated Mg alloy sample in 0.1 M NaCl, according to certain embodiments.

FIG. 15E shows an SECM image taken during continuous immersion (for 8 hours) of a C-PEO coated Mg alloy sample in 0.1 M NaCl, according to certain embodiments.

FIG. 15F shows an SECM image taken during continuous immersion (for 24 hours) of a C-PEO coated Mg alloy sample in 0.1 M NaCl, according to certain embodiments.

FIG. 15G shows an SECM image taken during continuous immersion (for 1 hour) of a BC-PEO coated Mg alloy sample in 0.1 M NaCl, according to certain embodiments.

FIG. 15H shows an SECM image taken during continuous immersion (for 8 hours) of a BC-PEO coated Mg alloy sample in 0.1 M NaCl, according to certain embodiments.

FIG. 15I shows an SECM image taken during continuous immersion (for 24 hours) of a BC-PEO coated Mg alloy sample in 0.1 M NaCl, according to certain embodiments.

FIG. 16A is an optical microscopy image of a PEO coated Mg alloy sample after continuous immersion in 0.1 M NaCl for 24 hours, according to certain embodiments.

FIG. 16B is an optical microscopy image of a B-PEO coated Mg alloy sample after continuous immersion in 0.1 M NaCl for 24 hours, according to certain embodiments.

FIG. 16C is an optical microscopy image of a C-PEO coated Mg alloy sample after continuous immersion in 0.1 M NaCl for 24 hours, according to certain embodiments.

FIG. 16D is an optical microscopy image of a BCS-PEO coated Mg alloy sample after continuous immersion in 0.1 M NaCl for 24 hours, according to certain embodiments.

FIG. 16E is an SEM image of a bare Mg alloy sample after continuous immersion in 0.1 M NaCl for 24 hours, according to certain embodiments, according to certain embodiments.

FIG. 16F is an SEM image of a BCS-PEO coated Mg alloy sample after continuous immersion in 0.1 M NaCl for 24 hours, according to certain embodiments.

DETAILED DESCRIPTION

In the drawings, like reference numerals designate identical or corresponding parts throughout the several views. Further, as used herein, the words “a,” “an” and the like generally carry a meaning of “one or more,” unless stated otherwise.

Furthermore, the terms “approximately,” “approximate,” “about,” and similar terms generally refer to ranges that include the identified value within a margin of 20%, 10%, or preferably 5%, and any values therebetween.

As used herein, the term “Plasma Electrolytic Oxidation” (PEO) refers to an electrochemical surface treatment process where a voltage is applied to a metal substrate immersed in an electrolyte, resulting in localized plasma discharges. These discharges can facilitate the formation of a thick, dense, and ceramic-like oxide layer on the metal surface through electrochemical, thermal, and plasma-chemical reactions. PEO can significantly enhance the surface properties of metals, such as wear resistance, corrosion protection, and hardness, making it ideal for applications in harsh environments.

As used herein, the term “alloy” refers to a material composed of two or more elements, at least one of which is a metal. Alloys are typically designed to enhance specific properties such as strength, corrosion resistance, ductility, or thermal and electrical conductivity compared to their individual components. The elements in an alloy can form solid solutions, mixtures, or compounds, and the resulting material often exhibits superior performance characteristics for various industrial applications.

As used herein, the term “cathode” refers to an electrode in an electrochemical cell or system where a reduction reaction occur, meaning it is the site where electrons are gained by ions or molecules. In electrolysis, the cathode is typically the negatively charged electrode, attracting positively charged cations from the electrolyte. In a battery or galvanic cell, it is the positively charged electrode during discharge, where the reduction process happens. The cathode plays a crucial role in various electrochemical processes, including corrosion protection, electroplating, and energy storage.

As used herein, the term “bulk portion” refers to the main mass or substantial body of a material, distinguishing it from surface layers or coatings. This portion typically represents the majority of the material's volume and possesses inherent properties that define its mechanical, thermal, and electrical characteristics. In the context of alloys and composites, the bulk portion is crucial for determining overall performance, as it provides the foundational structure and functionality upon which surface treatments, such as coatings or oxidation layers, can enhance specific attributes like corrosion resistance or wear protection.

As used herein, the term “surface oxide layer” refers to a thin, protective film of oxide that forms on the outer surface of a metal or alloy due to its reaction with oxygen or other oxidizing agents in the environment. This layer can be a barrier to further oxidation and corrosion, thus enhancing the material's durability and longevity. The surface oxide layer's composition, thickness, and structure can vary depending on the material type, environmental conditions, and processing methods. Its properties play a role in determining the overall performance of the material in various applications, including its resistance to wear, adhesion of coatings, and aesthetic qualities.

As used herein, the term “corrosion” refers to the chemical or electrochemical degradation of a material, typically a metal, resulting from its interaction with environmental factors such as moisture, oxygen, acids, or salts. This process leads to the gradual deterioration of the material's properties, including its strength, appearance, and structural integrity. Corrosion can manifest in various forms, including uniform attack, pitting, galvanic corrosion, and stress corrosion cracking, each affecting the material differently. Understanding and mitigating corrosion is crucial in extending the lifespan and performance of materials in various applications, particularly in industries such as construction, transportation, and manufacturing.

As used herein, the term “corrosion rate” refers to the measure of the speed at which a material, typically a metal, undergoes degradation due to corrosion processes. It is often quantified as the mass loss of the material over time, commonly expressed in units such as millimeters per year (mm/year) or grams per square meter per day (g/m²/day). The corrosion rate can be influenced by various factors, including environmental conditions (such as temperature, humidity, and exposure to corrosive agents), the properties of the material itself, and protective measures implemented. Understanding the corrosion rate is essential for assessing the durability and lifespan of materials in specific applications, allowing for better material selection, protective coating design, and maintenance strategies.

Aspects of this disclosure are directed to a magnesium alloy material. In this disclosure of magnesium alloy materials, Plasma Electrolytic Oxidation (PEO) offers significant advantages by forming a hard, durable, and corrosion-resistant oxide layer on the surface. This process enhances the alloy's mechanical properties, wear resistance, and protection against environmental degradation. PEO also allows for the incorporation of functional nanoparticles, further improving the material's performance in demanding applications such as automotive, aerospace, and biomedical industries, where lightweight and high-strength materials are needed.

A magnesium alloy material is described. The magnesium alloy material includes a bulk portion; and a surface oxide layer formed on the bulk portion. The bulk portion can be made from lightweight magnesium alloy, commonly used in industries like automotive and aerospace. Magnesium's reactivity, especially in wet environments, makes it prone to corrosion. To address this, a surface oxide layer is formed through oxidation, acting as a protective barrier. This oxide layer enhances corrosion resistance and mechanical durability by providing a harder outer shell.

In some embodiments, the bulk portion of the magnesium alloy may optionally include, aluminum alloy, titanium alloy, stainless steel, carbon steel, copper, brass, nickel alloy, bronze, zinc alloy, composite materials, silicon carbide, tool steel, inconel, cobalt-chromium alloy, beryllium, cast iron, tungsten, superalloys, niobium alloy, and platinum-iridium alloy.

In some embodiments, the surface oxide layer includes magnesium oxide, hexagonal boron nitride nanosheets, nanoclay and stannate. The surface oxide layer may additionally include, but is not limited to, aluminum oxide (Al₂O₃), titanium dioxide (TiO₂), zinc oxide (ZnO), iron oxide (Fe₂O₃), silicon dioxide (SiO₂), chromium oxide (Cr₂O₃), nickel oxide (NiO), copper oxide (CuO), hafnium oxide (HfO₂), zirconium oxide (ZrO₂), tantalum oxide (Ta₂O₅), niobium oxide (Nb₂O₅), beryllium oxide (BeO), vanadium oxide (V₂O₅), manganese oxide (MnO₂), tin oxide (SnO₂), cobalt oxide (CoO), lead oxide (PbO), and antimony oxide (Sb₂O₃). In preferred embodiment.

In some embodiments, the surface oxide layer may be formed on the bulk portion by various processes such as anodizing, thermal oxidation, chemical vapor deposition, physical vapor deposition, oxidative annealing, hot-dip galvanization, electrochemical deposition, and thermal spraying, which applies molten or semi-molten materials onto a surface. Techniques like sol-gel coating, laser oxidation, and sputtering are also effective for oxide formation. Additionally, atomic layer deposition (ALD), cathodic oxidation, and ion beam-assisted deposition (IBAD) are used for controlled oxide growth. Further processes include hydrothermal oxidation, high-temperature oxidation, electroless plating, and spin-coating oxidation. In preferred embodiments, the surface oxide layer is formed on the bulk portion by the plasma electrolytic oxidation process (PEO).

PEO is an advanced electrochemical technique in which a voltage is applied to a metal (e.g. a magnesium alloy) submerged in an electrolyte. This leads to localized plasma discharges at the metal/electrolyte interface. The extreme conditions from these discharges—high temperature and pressure—promote electrochemical, thermal, and plasma-chemical reactions that convert the metal surface into an oxide layer.

A PEO process includes pulsing a voltage of a direct current between a first value and a second value at an interval for a first period between a magnesium alloy anode and a cathode in the presence of an electrolyte solution. The PEO process may further include maintaining the voltage of the direct current at the first value for a second period. In some embodiments, the first value ranges from 180-220V, e.g. 180V, 190V, 200V, 210V, 185V, 195V, 205V, 220V or any values therebetween. In a preferred embodiment, the first value of voltage is 200V. In some embodiments, the second value ranges from 80-120 V, e.g. 80 V, 90 V, 100 V, 110 V, 120 V or any values therebetween. In a preferred embodiment, the second value is 100V. In some embodiments, the interval may range from 10 to 60 seconds, e.g. 10 seconds, 20 seconds, 30 seconds, 40 seconds, 50 seconds, 60 seconds or any values therebetween. In a preferred embodiment, the interval is 30 seconds. In some embodiments, the first period ranges from 12.5 to 17.5 minutes, e.g. 12.5 minutes, 13.5 minutes, 14.5 minutes, 15.5 minutes, 16.5 minutes, 17.5 minutes or any values therebetween. In a preferred embodiment, the first period is 15 minutes. In some embodiments, the second period ranges from 7.5 to 12.5 minutes, e.g. 7.5 minutes, 8.5 minutes, 9.5 minutes, 10.5 minutes, 11.5 minutes, 12.5 minutes or any values therebetween. In a preferred embodiment, the second period is 10 minutes.

In some embodiments, the cathode in PEO may include, but is not limited to, stainless steel, graphite, copper, nickel, platinum, aluminum, titanium, zinc, silver, lead, tin, nickel-chromium, copper-nickel, bronze, carbon fiber, carbon nanotubes, molybdenum, tantalum, gold, palladium, rhodium, and iridium. In a preferred embodiment, the cathode includes stainless steel. When the cathode includes stainless steel, it benefits from the material's excellent corrosion resistance, durability, and good electrical conductivity, making it suitable for various electrochemical processes. Stainless steel is highly resistant to oxidation and corrosion, especially in harsh or aqueous environments, which prolongs the lifespan of the cathode and ensures stable performance.

The anode in PEO includes a magnesium alloy. In some embodiment, an area ratio of the magnesium alloy anode and the cathode ranges from 1:2 to 1:10, e.g. 1.2, 1:4, 1:6, 1:8, 1:9, 1:10 or any values therebetween. In a preferred embodiment, an area ratio of the magnesium alloy anode and the cathode is 1:5.

In some embodiments, an electrolyte solution may include, but is not limited to, sodium chloride, sodium sulfate, potassium chloride, potassium nitrate, ammonium chloride, sodium bicarbonate, sodium carbonate, phosphoric acid, sulfuric acid, hydrochloric acid, acetic acid, citric acid, oxalic acid, boric acid, lithium chloride, lithium hydroxide, zinc sulfate, magnesium sulfate, copper sulfate, ammonium sulfate, ammonium nitrate, sodium perchlorate, sodium thiosulfate, tetraethylammonium bromide, ethanolamine, ethylene glycol, triethylamine, potassium permanganate, sodium acetate, and calcium chloride. In a preferred embodiment, an electrolyte solution includes sodium metasilicate, calcium fluoride, potassium hydroxide, hexagonal boron nitride nanosheets, the nano clay, stannate, and water.

In some embodiments, the concentration of sodium metasilicate in the electrolyte solution ranges from 10-50 g/L e.g. 10 g/L, 15 g/L, 20 g/L, 30 g/L, 35 g/L, 40 g/L, and 45 g/L, 50 g/L or any values therebetween. In a preferred embodiment, the concentration of sodium metasilicate in the electrolyte solution is 25 g/L. In some embodiments, the concentration of calcium fluoride in the electrolyte solution ranges from 1-15 g/L e.g. 1 g/L, 3 g/L, 7 g/L, 9 g/L, 11 g/L, 15 g/L or any values therebetween. In a preferred embodiment, the concentration of calcium fluoride in the electrolyte solution is 5 g/L. In some embodiments, the concentration of potassium hydroxide in the electrolyte solution ranges from 1-15 g/L e.g. 1 g/L, 3 g/L, 7 g/L, 9 g/L, 11 g/L, 15 g/L or any values therebetween. In a preferred embodiment, the concentration of potassium hydroxide in the electrolyte solution is 5 g/L. In some embodiment, the concentration of the hexagonal boron nitride nanosheets in an electrolyte solution ranging from 0.1-1.5 g/L, e.g. 0.1 g/L, 0.3 g/L, 0.7 g/L, 0.9 g/L, 1.1 g/L, 1.5 g/L or any values therebetween. In a preferred embodiment, the concentration of the hexagonal boron nitride nanosheets in the electrolyte solution is 0.5 g/L. In some embodiment, the concentration of the nanoclay in the electrolyte solution ranges from 0.1-1.5 g/L, e.g. 0.1 g/L, 0.3 g/L, 0.7 g/L, 0.9 g/L, 1.1 g/L, 1.5 g/L or any values therebetween. In a preferred embodiment, the concentration of the stannate in the electrolyte solution is 0.5 g/L. In some embodiment, the concentration of potassium hydroxide in the electrolyte solution ranges from 0.1 g/L to 10 g/L e.g. 0.1 g/L, 0.5 g/L, 1.0 g/L, 3 g/L, 5 g/L, 10 g/L or any values therebetween. In a preferred embodiment, the concentration of stannate in the electrolyte solution is 2 g/L.

In some embodiments, the electrolyte solution may have a pH value ranging from 10 to 15 e.g. 10, 11, 12, 13, 14, 15 or any values therebetween. In a preferred embodiment, the electrolyte solution has a pH of 12.5. In some embodiments, the pH of the electrolyte solution may be maintained by various bases such as sodium hydroxide (NaOH), calcium hydroxide (Ca(OH)₂), ammonium hydroxide (NH₄OH), lithium hydroxide (LiOH), magnesium hydroxide (Mg(OH)₂), barium hydroxide (Ba(OH)₂), strontium hydroxide (Sr(OH)₂), sodium carbonate (Na₂CO₃), potassium carbonate (K₂CO₃), sodium bicarbonate (NaHCO₃), tetramethylammonium hydroxide (TMAH), calcium carbonate (CaCO₃), sodium phosphate (Na₃PO₄), tris(hydroxymethyl)aminomethane (TRIS), sodium silicate (Na₂SiO₃), sodium metaborate (NaBO₂), ammonium carbonate ((NH₄)₂CO₃), potassium phosphate (K₃PO₄), sodium aluminate (NaAlO₂), and ammonium acetate (NH₄OAc). In a preferred embodiment, the pH of the electrolyte solution is maintained by potassium hydroxide (KOH). For example, the concentration of potassium hydroxide in the electrolyte solution can range from 1-15 g/L, e.g. 1 g/L, 3 g/L, 7 g/L, 9 g/L, 11 g/L, 15 g/L or any values therebetween. In a preferred embodiment, the concentration of potassium hydroxide in the electrolyte solution is 5 g/L.

In some embodiments, the electrolyte solution may be maintained at a temperature ranging from 15-25° C., e.g. 15° C., 17° C., 19° C., 21° C., 23° C., 25° C. or any values therebetween, during the PEO process. In a preferred embodiment, the electrolyte solution is maintained at a temperature of ˜20±5° C. Note that the temperature does not have to be constant during the PEO process and may fluctuate between 15° C. and 25° C.

In some embodiments, the bulk portion of the Mg alloy includes, based on the total weight of the bulk portion, 90.0-98.0 wt. % (e.g. 90.0 wt. %, 92.0 wt. %, 94.0 wt. %, 95.0 wt. %, 96.0 wt. %, 97.0 wt. %, 97.5 wt. %, 98.0 wt. % or any values therebetween) of Mg. In a preferred embodiment, the bulk portion includes, based on the total weight of the bulk portion, 94.0-97.0 wt. % of Mg.

In some embodiments, the bulk portion includes, based on the total weight of the bulk portion, 1.0-5.0 wt. % (e.g. 1.0 wt. %, 2.0 wt. %, 3.0 wt. %, 4.0 wt. %, 5 wt. % or any values therebetween) of Al. In a preferred embodiment, the bulk portion includes, based on the total weight of the bulk portion, 2.5-3.5 wt. % of Al.

In some embodiments, the bulk portion includes, based on the total weight of the bulk portion, 0-0.5 wt. % (e.g. 0 wt. %, 0.001 wt. %, 0.005 wt. %, 0.01 wt. %, 0.03 wt. %, 0.05 wt. %, 0.1 wt. %, 0.5 wt. % or any values therebetween) of Cu. In a preferred embodiment, the bulk portion includes, based on the total weight of the bulk portion, 0.01 wt. % of Cu.

In some embodiments, the bulk portion includes, based on the total weight of the bulk portion, 0-0.5 wt. % (e.g. 0 wt. %, 0.0001 wt. %, 0.0005 wt. %, 0.001 wt. %, 0.002 wt. %, 0.005 wt. %, 0.01 wt. %, 0.05 wt. %, 0.1 wt. %, 0.5 wt. % or any values therebetween) of Fe. In a preferred embodiment, the bulk portion includes, based on the total weight of the bulk portion, 0.003 wt. % of Fe.

In some embodiments, the bulk portion includes, based on the total weight of the bulk portion, 0.001-5.0 wt. % (e.g. 0.001 wt. %, 0.005 wt. %, 0.01 wt. %, 0.05 wt. %, 0.1 wt. %, 0.3 wt. %, 0.5 wt. %, 1.0 wt. %, 3.0 wt. %, 5.0 wt. % or any values therebetween) of Mn. In a preferred embodiment, the bulk portion includes, based on the total weight of the bulk portion, 0.2-1.0 wt. % of Mn.

In some embodiments, the bulk portion includes, based on the total weight of the bulk portion, 0-0.5 wt. % (e.g. 0 wt. %, 0.0001 wt. %, 0.0005 wt. %, 0.001 wt. %, 0.002 wt. %, 0.005 wt. %, 0.01 wt. %, 0.05 wt. %, 0.1 wt. %, 0.5 wt. % or any values therebetween) of Ni. In a preferred embodiment, the bulk portion includes, based on the total weight of the bulk portion, 0.001 wt. % of Ni.

In some embodiments, the bulk portion includes, based on the total weight of the bulk portion, 0-1 wt. % (e.g. 0 wt. %, 0.001 wt. %, 0.005 wt. %, 0.01 wt. %, 0.03 wt. %, 0.05 wt. %, 0.1 wt. %, 0.2 wt. %, 0.5 wt. %, 1 wt. %, or any values therebetween) of Si. In a preferred embodiment, the bulk portion includes, based on the total weight of the bulk portion, 0.08 wt. % of Si.

In some embodiments, the bulk portion includes, based on the total weight of the bulk portion, 0.01-5.0 wt. % (e.g. 0.01 wt. %, 0.05 wt. %, 0.1 wt. %, 0.3 wt. %, 0.5 wt. %, 1.0 wt. %, 3.0 wt. %, 5.0 wt. % or any values therebetween) of Zn. In a preferred embodiment, the bulk portion includes, based on the total weight of the bulk portion, 0.6-1.4 wt. % of Zn.

In some embodiments, the bulk portion includes 0-2 wt. % (e.g. 0 wt. %, 0.01 wt. %, 0.5 wt. %, 1 wt. %, 1.5 wt. %, 2 wt. % or any values therebetween) of C and 0-0.1 wt. % (e.g. 0 wt. %, 0.01 wt. %, 0.05 wt. %, 0.1 wt. % or any values therebetween) of Sn. Preferably, the bulk portion includes no carbon and no tin.

In some embodiments, the surface oxide layer of the Mg alloy includes, based on the total weight of the surface oxide layer, 50-70 wt. % (e.g. 50 wt. %, 55 wt. %, 60 wt. %, 65 wt. %, 68 wt. %, 70 wt. % or any values therebetween) of Mg. In a preferred embodiment, the surface oxide layer includes, based on the total weight of the bulk portion, 55.76 wt. % of Mg.

In some embodiments, the surface oxide layer includes, based on the total weight of the surface oxide layer, 15-30 wt. % (e.g. 15 wt. %, 20 wt. %, 22 wt. %, 25 wt. %, 27 wt. %, 30 wt. % or any values therebetween) of O. In a preferred embodiment, the surface oxide layer includes, based on the total weight of the bulk portion, 24.91 wt. % of O.

In some embodiments, the surface oxide layer includes, based on the total weight of the surface oxide layer, 0.5-1.5 wt. % (e.g. 0.5 wt. %, 0.7 wt. %, 0.9 wt. %, 1.1 wt. %, 1.3 wt. %, 1.5 wt. % or any values therebetween) of Al. In a preferred embodiment, the surface oxide layer includes, based on the total weight of the bulk portion, 0.83 wt. % of Al.

In some embodiments, the surface oxide layer includes, based on the total weight of the surface oxide layer, 0.01-0.2 wt. % (e.g. 0.01 wt. %, 0.03 wt. %, 0.06 wt. %, 0.1 wt. %, 0.15 wt. %, 0.2 wt. % or any values therebetween) of Mn. In a preferred embodiment, the surface oxide layer includes, based on the total weight of the surface oxide layer, 0.04 wt. % of Mn.

In some embodiments, the surface oxide layer includes, based on the total weight of the surface oxide layer, 3-10 wt. % (e.g. 3 wt. %, 4 wt. %, 5 wt. %, 6 wt. %, 7 wt. %, 8 wt. %, 9 wt. %, 10 wt. % or any values therebetween) of Si. In a preferred embodiment, the surface oxide layer includes, based on the total weight of the surface oxide layer, 5.80 wt. % of Si.

In some embodiments, the surface oxide layer includes, based on the total weight of the surface oxide layer, 0.1-1.5 wt. % (e.g. 0.1 wt. %, 0.2 wt. %, 0.3 wt. %, 0.5 wt. %, 0.7 wt. %, 0.9 wt. %, 1.1 wt. %, 1.3 wt. %, 1.5 wt. % or any values therebetween) of Zn. In a preferred embodiment, the surface oxide layer includes, based on the total weight of the surface oxide layer, 0.72 wt. % of Zn.

In some embodiments, the surface oxide layer includes, based on the total weight of the surface oxide layer, 5-15 wt. % (e.g. 5 wt. %, 7 wt. %, 9 wt. %, 11 wt. %, 12 wt. %, 13 wt. %, 15 wt. % or any values therebetween) of C. In a preferred embodiment, the surface oxide layer includes, based on the total weight of the surface oxide layer is 11.32 wt. % of C.

In some embodiments, the surface oxide layer includes, based on the total weight of the surface oxide layer, 0.3-2.0 wt. % (e.g. 0.3 wt. %, 0.5 wt. %, 0.7 wt. %, 0.9 wt. %, 1.1 wt. %, 1.3 wt. %, 1.5 wt. %, 2.0 wt. % or any values therebetween) of Sn. In a preferred embodiment, the surface oxide layer includes, based on the total weight of the surface oxide layer, 0.62 wt. % of Sn.

The surface oxide layer includes a bottom portion formed on the bulk portion, a middle portion formed on the bottom portion, and a top portion formed on the middle portion. The bottom portion is directly formed on and is in contact with the bulk portion. This layer is typically dense and serves as the primary bonding interface. The middle portion, built on top of the bottom layer, is often thicker and contributes to the overall mechanical strength and durability of the oxide coating. Finally, the top portion, formed on the middle layer, is usually more porous and serves as the outermost protective layer, enhancing corrosion resistance and wear protection. Together, these portions create a multi-layered structure that significantly improves the material's performance in harsh environments.

In the surface oxide layer, tin (Sn) is distributed unevenly across the bottom, middle, and top portions. The concentration of Sn is highest in the bottom portion, which promotes adhesion between the oxide layer and the bulk material, ensuring structural integrity and reducing delamination. Additionally, Sn contributes to the corrosion resistance of the layer, acting as a sacrificial element that can protect the underlying metal from environmental degradation. As the Sn distribution moves upwards, its concentration decreases in the middle portion and is lowest in the top portion. This gradient in Sn concentration likely contributes to the enhanced protective properties of the oxide layer, ensuring a robust, corrosion-resistant foundation at the bottom while maintaining overall coating integrity.

In some embodiments, Sn is present at an average concentration of 0.3-2.0 wt. % (e.g. 0.3 wt. %, 0.5 wt. %, 0.7 wt. %, 0.9 wt. %, 1.1 wt. %, 1.3 wt. %, 1.5 wt. %, 2.0 wt. % or any values therebetween) in the surface oxide layer based on the total weight of the surface oxide layer. Preferably, Sn is present at an average concentration of 0.62 wt. % in the surface oxide layer based on the total weight of the surface oxide layer.

In some embodiments, Sn is present at the highest concentration of 1.0-3.0 wt. % (e.g. 1.0 wt. %, 1.5 wt. %, 2.0 wt. %, 2.5 wt. %, 3.0 wt. % or any values therebetween) in the bottom portion of the surface oxide layer, based on the total weight of the bottom portion of the surface oxide layer. Preferably, Sn is present at the highest concentration of 1.45 wt. % in the bottom portion of the surface oxide layer, based on the total weight of the bottom portion of the surface oxide layer.

The hexagonal boron nitride (h-BN) nanosheets are distributed non-uniformly in the bottom portion, the middle portion, and the top portion of the surface oxide layer, with the highest concentration found in the top portion. This arrangement is beneficial because the top layer is the most exposed to environmental factors, such as moisture and mechanical wear. The presence of h-BN nanosheets in this region enhances the layer's mechanical properties, providing superior hardness and wear resistance while also improving its thermal stability. Also, h-BN's excellent chemical inertness contributes to increased corrosion resistance, helping protect the underlying magnesium alloy.

In some embodiments, the hexagonal boron nitride nanosheets may have an average radius ranging from 300 to 1000 nm e.g. 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 900 nm, 1000 nm or any values therebetween.

The hexagonal boron nitride (h-BN) nanosheets can be at least partially hydroxylated, meaning some of their surface boron atoms have been modified with hydroxyl (—OH) groups. This modification enhances the nanosheets' compatibility with the surrounding matrix in the surface oxide layer, improving their dispersion and integration within the coating. The presence of hydroxyl groups increases the hydrophilicity of the h-BN, facilitating better interaction with the electrolyte during the plasma electrolytic oxidation (PEO) process. This leads to a more uniform distribution of h-BN within the oxide layer and enhances adhesion between the nanosheets and the matrix, resulting in a stronger protective coating.

The nanoclay is distributed non-uniformly in the bottom portion, the middle portion, and the top portion of the surface oxide layer, with the highest concentration in the top portion. This positioning enhances the coating's protective qualities by reducing porosity and increasing density in the area most exposed to environmental factors. The presence of nanoclay in the top layer also improves mechanical strength and flexibility, allowing the coating to withstand wear and stress better, thereby effectively protecting the underlying material.

In some embodiments, the nanoclay includes montmorillonite. The montmorillonite may be surface-modified by an agent that includes, but is not limited to, cetyltrimethylammonium bromide, dodecylamine, dimethyldioctadecylammonium chloride, stearic acid, 3-aminopropyltriethoxysilane, hexadecylamine, polyethyleneimine, tetraethoxysilane, benzyltrimethylammonium chloride, oleylamine, ethylenediamine, triethanolamine, glycidoxypropyltrimethoxysilane, trimethylamine, octyltrimethoxysilane, phenyltrimethoxysilane, butylamine, octylamine, methyltriethoxysilane, and dodecyltrimethoxysilane. In a preferred embodiment, the nanoclay include montmorillonite that is surface-modified by octadecylamine and aminopropyl triethoxysilane. This modification enhances the nanoclay's compatibility with the surrounding matrix in the surface oxide layer, improving dispersion and interfacial adhesion.

The surface oxide layer may or may not include crystallized MgSn(OH)₆. In a preferred embodiment, the surface oxide layer does not include crystallized MgSn(OH)₆.

In some embodiments, the surface oxide layer includes pores that may have an average size ranging from 0.5-2.0 μm (e.g. 0.5 μm, 0.7 μm, 0.9 μm, 1.0 μm, 1.5 μm, 2.0 μm or any values therebetween) and voids that may have an average size of 1-4 μm (e.g. 1 μm, 2 μm, 3 μm, 1.5 μm, 2.5 μm, 4 μm or any values therebetween).

In some embodiments, the surface oxide layer may have a thickness ranging from 50-200 μm, e.g. 50 μm, 70 μm, 90 μm, 110 μm, 130 μm, 150 μm, 170 μm, 200 μm or any values therebetween. In a preferred embodiment, the thickness of the surface oxide layer is 80-120 μm.

In some embodiments, a corrosion current density of the magnesium alloy material may be less than 0.6% (e.g. 0.599%, 0.5%, 0.4%, 0.3%, 0.2%, 0.1% or any values therebetween) of a corrosion current density of a first comparative example (e.g. a bare magnesium alloy) that has an identical composition of the bulk portion of the magnesium alloy material. In some embodiments, a corrosion rate of the magnesium alloy material may be less than 0.6% (e.g. 0.599%, 0.5%, 0.4%, 0.3%, 0.2%, 0.1% or any values therebetween) of a corrosion rate of the first comparative example.

In some embodiments, the corrosion current density of the magnesium alloy material may be less than 5% (e.g. 4.99%, 4%, 3%, 2%, or 1% or any values therebetween) of a corrosion current density of a second comparative example that is the same as the magnesium alloy material except having no hexagonal boron nitride nanosheets, nanoclay or stannate. In some embodiments, the corrosion rate of the magnesium alloy material may be less than 4% (e.g. 3.99%, 3%, 2%, 1%, or any values therebetween) of a corrosion rate of the second comparative example.

In some embodiments, the magnesium alloy material may have a water contact angle (WCA) of 70-85°, e.g. 70°, 72°, 74°, 76°, 78°, 80°, 82°, 83°, 85°, or any values therebetween, which is larger than that of a comparative example that is the same as the magnesium alloy material except having no hexagonal boron nitride nanosheets, nanoclay or stannate. In a preferred embodiment, the magnesium alloy material has a water contact angle (WCA) of 780 which is larger than that of the comparative example.

EXAMPLES

The following examples demonstrate a magnesium alloy material having a bulk portion and a surface oxide layer formed on the bulk portion to provide corrosion resistance to the magnesium alloy, as depicted in FIG. 1. The examples in the present disclosure are provided solely for illustration and are not to be construed as limitations of the present disclosure, as many variations are possible without departing from the spirit and scope of the present disclosure.

Example 1: Materials

The magnesium (Mg) alloy used for the following studies includes 2.5 percent by weight (wt. %) to 3.5 wt. % of aluminum (Al), 0.01 wt. % of copper (Cu), 0.003 wt. % of iron (Fe), 0.2 wt. % to 1.0 wt. % of manganese (Mn), 0.001 wt. % of nickel (Ni), 0.08 wt. % of silicon (Si), 0.6 wt. % to 1.4 wt. % of zinc (Zn), and 94.0 wt. % to 97.0 wt. % of magnesium (Mg). The Mg alloy samples had average dimensions of about (3.0×1.0×0.3) cm³. The Mg alloy samples were polished with varying grit silicon carbide (SiC) papers, with a grit size ranging from 400 grit to 1000 grit (P2500). Further, the Mg alloy samples were degreased with acetone, ultrasonically cleaned in ethanol, and blow dried with air.

Example 2: Plasma Electrolytic Oxidation (PEO) Set-Up

PEO was conducted using a two-electrode system including a Mg alloy anode and a stainless-steel cathode. The anode-to-cathode area ratio was about 1:5. The electrolyte solution used for PEO was a mixture of sodium metasilicate (Na₂SiO₃, 25 g/L, Sigma-Aldrich), calcium fluoride (CaF₂ anhydrous, 5 g/L, Acros Organics) and potassium hydroxide (KOH, 5 g/L, Sigma-Aldrich) in distilled water. The mixture solution was sonicated using an ultrasonication probe for 1 hour (h) before use. The electrolyte solution had a pH of about 12.5. PEO was conducted in a 1-liter beaker with 500 mL of the electrolyte solution, which was kept in an ice bath to preserve the electrolyte temperature at around 20±5° C. A magnetic stirrer was used to provide mild agitation at around 100 rpm, to the electrolyte solution. A direct current power supply (Agilent HP 6448B) was used to provide potential difference for 25 minutes (min). During the first 15 min, the voltage was pulsed between 200 V and 100 V at fixed intervals of 30 sec, in order to regulate the initial intense spark discharge. In the next 10 min, the voltage was maintained at 200V. After PEO, the sample was taken out, washed with flowing water and ethanol, further blow dried with air.

Example 3: In-Situ Incorporation of Nanomaterials to PEO

For in-situ incorporation, a pre-determined volume of the stock solution of the h-BN, organo-modified nanoclay (NC) and/or sodium stannate (ST) was added and dispersed in the PEO electrolyte so that the concentration of the BN and NC in the electrolyte solution was maintained at 0.5 g/L. The stock solutions (10 g/L) of BN and NC in distilled water had been subjected to ultrasonication for 1 h to achieve uniform dispersion of particles beforehand. The ST concentration in the PEO bath was 2 g/L. The surface-modified nanoclay (montmorillonite) contained octadecylamine and aminopropyl triethoxysilane (Sigma-Aldrich, 682632). h-BN ultrafine powder (Micro-Lubrol) was freely hydroxylated. FIGS. 2A-2B show SEM images of the nanoparticles of BN and NC used in the present disclosure.

Example 4: Sample Nomenclature

The nomenclature for samples of boron nitride nanosheets, organo-modified nanoclay and stannate are respectively denoted as BN, NC and ST, as further listed in Table 1. The Mg alloy samples coated by PEO, BN-incorporated PEO, NC-incorporated PEO, BN- and NC-incorporated PEO, and BN-, NC- and ST-incorporated PEO are respectively named as PEO, B-PEO, C-PEO, BC-PEO and BCS-PEO.

Example 5: Characterization of the Developed Coatings

The surface topography of the Mg alloy after PEO coating (PEO samples) was examined by an optical profilometer (Profilm3D) and an atomic force microscope (AFM), (MFP-3D Infinity) to evaluate the effect of particle addition at the microscale and nanoscale. The optical (non-contact) profilometer utilizes interferometric techniques and light, and the topography of the surface corresponds to the generated interference pattern. The AFM was operated in contact mode to give a more accurate surface representation by applying a constant force between the cantilever and the sample surface. The surface topography of the sample corresponds to the deflection of the cantilever as it scans over the surface of the sample. SEM (JSM-6610LV), operating at 20 kilo electron volt (keV) accelerating voltage, was used to examine the surface morphology and coating thickness of the sample. Elemental analysis by energy dispersive spectroscopy (EDS) was carried out simultaneously. The phase composition of the oxide layers was studied by X-ray diffraction (XRD) (Rigaku) using Cu Kα1 with a scattering angle of 20°-80° at a 2θ step of 0.02°. Fourier transform infrared (FTIR) spectroscopic evaluations were performed at 4000 cm⁻¹ to 600 cm⁻¹, using a spectrometer (Thermo Scientific).

Example 6: Mechanical Properties and Adhesion of the PEO Coatings

Nanoindentation tests were carried out to assess mechanical durability of the prepared coatings by selecting the lowest possible load without the influence of the base substrate. The nanoindentation hardness was performed on a mechanical surface testing platform (STEP). A Berkovich diamond indenter was used with a normal load of 50 mega Newton (mN) for 10 s of dwelling time. The loading and unloading rates were kept constant at 100 mN/min. According to the Oliver and Pharr model, hardness and elastic modulus were calculated from the load-displacement curve using indentation software. Experiments were conducted at five repetitions, and the average of the three matching results was selected.

The coating adhesion was assessed using an indentation instrumentation tester (Anton Paar NHT3) equipped with scratch-testing capabilities (Scratch software). Evaluations were carried out using a Rockwell C indenter with a tip diameter of 200 μm. The indenter was moved linearly at a speed of 5 mm/min across the coated surface, while the applied load was incrementally increased from 0.03 N to 30 N at a rate of 15 N/min over a distance of 5 mm. During the test, penetration depth, acoustic emission, frictional force and the coefficient of friction were obtained. The critical loads were subsequently determined and analyzed through a combined evaluation of acoustic emission signals and optical micrographs of the scratch track.

Example 7: Corrosion Test on PEO-Coated Mg Alloy Samples

Electrochemical corrosion evaluations of the PEO-coated Mg alloy samples were conducted using a Gamry potentiostat (Reference 3000™) with a three-electrode cell set-up. The Mg alloy sample, Ag/AgCl electrode and graphite rod were respectively used as working, reference and counter electrodes. The area of the exposed sample was 0.5 cm². The electrolyte was freely aerated with 3.5 wt. % NaCl. The sample was kept in the electrolyte for 15 min to stabilize the electrode potential. Electrochemical impedance spectroscopy (EIS) was performed in the frequency region of 1 kHz to 0.05 Hz with 10 mV amplitude and 10 points acquired per decade. To quantify the obtained data, equivalent circuit fitting was performed using the Echem analyst software. Potentiodynamic polarization (PDP) tests were conducted in the potential range of ±250 mV (vs open circuit potential) at 1 mV/sec. The corrosion parameters were obtained via Tafel fitting of PDP plots. During Tafel fitting analysis, the cathodic polarization region was mainly selected to estimate corrosion current density (i_corr). The anodic region was not considered due to the negative difference effect. The Stern-Geary equation [R_p=βa×βc/2.303 (i_corr)(βa+βc)] was used to calculate the polarization resistance (R_p), where “βa“and” βc” correspond to Tafel constants.

Example 8: Scanning Electrochemical Microscopy (SECM) Evaluations for Self-Healing

SECM was conducted in freely aerated 0.1 M NaCl using an electrochemical workstation (M370 BioLogic), with a polarizable ultra-microelectrode (UME) probe of platinum (Pt) tip of 10 m, enclosed in a glass tube. A lower chloride concentration was used to avoid the instigation of rapid localized corrosion and to observe the relative variation of surface film in the presence and absence of the added particles and inhibitor. The distance between the SECM probe and the sample surface was fixed through a Z-approach curve test. The scanning was performed in a (500×500) m² region, and the changes in the monitored faradaic current at the tip were recorded. UME tip potential of 0.0 V vs SCE was applied to analyze the hydrogen evolution reaction at the Mg surface. As the hydrogen evolution is diffusion-controlled, the monitored current at the UME tip can provide direct information on the local hydrogen concentration. Measurements were conducted at regular intervals, and the data corresponding to 1, 8 and 24 h of immersion are presented. After 24 h of continuous immersion in the 0.1 M NaCl, the samples were further examined via an optical microscope (GX53, Olympus) and SEM (FEI Quanta 250) to study the corrosion propagation.

Example 9: Results

A relatively low voltage of 200 V was used for surface analysis of the Mg alloy. Further, the voltage was pulsed between 200 V and 100 V during the first 15 min to avoid the continuous spark discharge to develop a surface with reduced pores and cracks and then kept at 200 V for 10 min. Based on preliminary studies, the overall PEO time was fixed at 25 min to attain a desirable layer thickness. As observed, the spark discharge intensity was reduced as the oxidation continued, particle addition inhibited spark discharge as it may intervene in the interface processes. The PEO samples appeared light yellowish. Adding BN, NC, or ST does not affect the physical appearance observed by naked eyes.

FIGS. 3A-3E depict the surface profilometer images of the samples investigated in the present disclosure. FIGS. 3A-3E show the advantage of the incorporation of the particles. The uncovered base substrate areas or the coating valley areas/defects, as shown in FIG. 3A, are covered with the in-situ incorporation of BN or NC. Due to relatively large-sized plate morphology of BN, shown in FIG. 3A, the BN-incorporated surface provides a rougher outlook with more extensive valleys, as depicted in FIG. 3B. The NC-incorporated PEO yields a smoother surface appearance with improved surface coverage, as depicted in FIG. 3C. The combined incorporation may be a preferred approach, as shown in FIG. 3D. Incorporating ST does not result in significant variation, even though a fraction of the surface increased, indicating that the inhibitor has a certain extent of interference with the PEO process, as shown in FIG. 3E. The corresponding RMS roughness values in FIGS. 3A-3E were 0.2564 μm, 0.2891 μm, 0.2802 μm, 0.2821 μm, and 0.2623 μm, respectively.

FIGS. 4A-4F respectively show 3D atomic force microscopy (AFM) images of a PEO coated sample, a B-PEO coated sample, a C-PEO coated sample, a C-PEO coated sample, a BC-PEO coated sample and a BCS-PEO coated sample, according to certain embodiments. AFM studies supported a more uniform surface appearance at the nanoscale upon NC incorporation, as shown in FIG. 4C, suggesting that NC is beneficial for covering the surface more effectively. As can be seen from FIG. 4C, AFM imaging of the same sample at a different location provided a mildly rough appearance. The low-melting NC may deliver relatively smoother local surface regions with better surface coverage. As can be seen from FIG. 4B, the BN incorporation resulted in a rougher surface, attributed to the larger plate-sized morphology. FIGS. 4B-4F depict AFM images of the modified samples, revealing a better surface morphology than the bare PEO of FIG. 4A. The beneficial effect of the combined incorporation is evident from FIG. 4D. The RMS roughness values of the corresponding samples in FIGS. 4A-4F were 72.359 nm, 103.754 nm, 50.604 nm, 55.820 nm, 61.817 nm, and 52.461 nm.

FIGS. 5A-5I respectively show SEM images of a PEO sample, B-PEO, C-PEO, BC-PEO, BCS-PEO, the PEO sample of FIG. 5A, the B-PEO sample of FIG. 5B, the C-PEO sample of FIG. 5C and the BCS-PEO sample of FIG. 5E, according to certain embodiments. Surface and lateral view SEM images are provided in FIGS. 5A-5D. The bare PEO surface shows the typical porous microstructure with pore sizes ranging from about 100 nm to 1000 nm, shown in FIG. 5A. PEO pore size is a function of the discharge density and the process time. The relatively smaller pores may be attributed to the voltage pulsing and the avoidance of continuous intense sparking. BN addition resulted in a more compact surface with distorted and elongated pores, shown in FIG. 5B, whereas the NC addition reduced the pore density and local pore-less areas are seen, shown in FIG. 5C. A similar pattern was seen with the combined addition, shown in FIG. 5D, supporting that the NC addition provides a smoother surface with reduced pore density. Other than the micropores, bigger discharge voids are also observed, The ST addition marginally affected the surface morphology, shown in FIG. 5E. EDS analysis showed that the particles seen over the surface were rich in Ca and F, suggesting that the white particles are CaF₂ from the electrolyte, which might have come from the marginal undissolved fraction of anhydrous CaF₂ used for the electrolyte preparation, Results showed that the addition of the particles marginally affects the surface roughness and microstructure of the PEO coatings. The reactive incorporation of low-melting NC may happen.

The coating thickness varied with the addition of particles. The thickness was not uniform throughout, as such, some areas were thicker, whereas some were less thick. The average thickness of the PEO, B-PEO, C-PEO, BC-PEO and BCS-PEO samples were in the range of 70 μm, 80 μm, 100 μm, 90 μm, and 80 μm, with an error factor of 10 μm, in that order, as shown in FIGS. 5F-5I. Results showed a marginal thickness enhancement with the particle incorporation. The coating thickness is expected to vary with the type and the amount of added particles in the electrolytes. When the extent of particles exceeds a threshold limit, the PEO process kinetics is hindered, reducing the coating thickness. On the other hand, the electrophoretic deposition (EPD)-assisted deposition of particles may enhance the coating thickness. The charged particles in the electrolyte can electrophoretically deposited during the PEO. This suggests that properly controlling the particle content and the PEO deposition parameters may yield high coating thickness. The uneven coating thickness in different areas, as described in the present disclosure may be due to the existence of polishing scratches or irregular corrosion instigated.

FIGS. 6A, 6B and 7 respectively show energy dispersive spectroscopy (EDS) line scan analyses of a B-PEO sample, a BC-PEO sample and a BCS-PEO sample, according to certain embodiments. The incorporation of BN and NC may be confirmed via EDS line scan analysis across the surface, as shown in FIGS. 6A-6B, and coating cross-section, as shown in FIG. 7. The Si in the line analysis may be from the silicate electrolyte and the incorporated NC. On comparing the Si content of B-PEO, C-PEO and BC-PEO, a greater extent of Si content in the C-PEO and BC-PEO can be associated with the incorporated NC in the PEO layer. The distribution peaks of C also showed a marginal increase. The additional carbon may come from the incorporated organo-modified NC. On comparing the B distribution in B-PEO and BC-PEO (FIGS. 6A-6B), a better and homogeneous distribution of B is evident in the latter, supporting that the presence of low-melting NC assisted in more effective incorporation of BN in the PEO layer. The EDS spectrum recorded across the coating thickness confirms the presence of B, Si and Sn within the coating, as shown in FIG. 7.

The EDS elemental composition (e.g. an average composition across the surface oxide layer) in wt. % of BCS-PEO from selected area analysis across coating thickness was O (24.91), Mg (55.76), Si (5.80), C (11.32), Mn (0.04), Zn (0.72), Al (0.83) and Sn (0.62). A corresponding EDS composition in wt. % obtained from the interface (e.g. the aforementioned bottom portion of the surface oxide layer) of BCS-PEO was O (24.06), Mg (56.02), Si (5.87), C (10.42), Mn (0.53), Zn (0.74), Al (0.91) and Sn (1.45), confirming that ST concentration was more on the surface (e.g. the aforementioned bottom portion of the surface oxide layer) and may exist as an adsorbed layer over the bulk portion of the Mg alloy. An elemental composition (in wt. %) measured on the surface of the bare PEO was O (22.90), Mg (63.28), Si (3.52), C (7.44), Mn (0.73), Zn (0.91), and Al (1.22). The C in the composition could originate from the impurity in the Mg alloy, organic modifications, organic compounds used in synthesizing NC and BN, or natural organic contaminations FIG. 8 illustrates the X-ray diffraction analysis of PEO, B-PEO, C-PEO, and BCS-PEO coated samples. For the PEO coated sample, irrespective of the incorporation of the particles, revealed only peaks corresponding to Mg oxides and the Mg substrate. The incorporated BN was not detected in XRD, due to the smaller volume fraction below the detection limit. An expected magnesium silicate (Mg₂SiO₄) phase was also not detected. The absence of Mg₂SiO₄ peaks in the XRD plots may be due to the voltage pulsing during the initial PEO period, or the peaks may overlap with that of the Mg substrate. On comparing the different XRD plots, a notable observation is that the MgO (200) peak intensity continuously increased, and the Mg substrate peaks continuously suppressed, indicating a much-improved surface coverage of the pores and defects of the PEO layer with the incorporated NC and ST.

FIGS. 9A-9D respectively show Fourier transform infrared spectroscopy (FTIR) analysis results of a PEO coated sample, a B-PEO coated sample, a C-PEO coated sample and a BCS-PEO coated sample, according to certain embodiments. With pure PEO, the peaks were not evident in the as-obtained spectra. A y-axis expanded plot presents analogous peaks observed with BN, NC and ST-incorporated samples. No significant difference between the spectra was noted. Peaks corresponding to BN or ST were also not distinguished. The broad band at 3390 cm⁻¹ and 1640 cm⁻¹ relate to H—O—H stretching and bending vibrations of adsorbed water, commonly reported in the PEO coatings on Mg, which is attributed to the hydrophilic nature of Mg oxides. The Si—O—Si bands appeared at 1050 cm⁻¹, corresponding to the incorporated silicate. The low peak at 1450 cm⁻¹ may be correlated with CO₃²⁻, as hydroxylated MgO may react with atmospheric CO₂, forming magnesium hydroxyl carbonate. The peak at 600 cm⁻¹ can be credited to Mg—O stretching vibrations. Peaks at 2930 cm⁻¹ and 2850 cm⁻¹ may be due to CH₃ and CH₂ stretching peaks. Such peaks may originate from organic modifications, organic compounds used in synthesizing NC and BN, or natural organic contamination, The bands at 2359 cm⁻¹ and 2340 cm⁻¹ are also linked with the hydrocarbon chain (C—H), and those at 1542 cm⁻¹ may be due to the C═C stretching vibration. FTIR spectra confirms the presence of silicate in the PEO layer, inferring that significant silicate was incorporated and may exist as amorphous magnesium silicates or adsorbed silicates.

FIGS. 10A-10E respectively show water contact angles (WCAs) of a PEO coated sample, a B-PEO coated sample, a C-PEO coated sample, a BC-PEO coated sample and a BCS-PEO coated sample, according to certain embodiments. The PEO coated surface is hydrophilic with a WCA of about 29°, as shown in FIGS. 10A-10E. In general, hydrophilicity is associated with the porous and defective oxide surface. The WCA becomes about 50° upon incorporation of 13N and NC. Increment of angle may be related to the coverage of the defective oxide surface with the particles and the associated variation in surface roughness. The WCA is further increased to about 78° upon ST incorporation, as depicted in FIGS. 10A-10E. The variation shows that the inhibitor effectively developed a surface layer in addition to the possible incorporation within the PEO layer.

FIGS. 11A-IID respectively show bar graphs depicting the elastic moduli, nano-hardness, critical load and scratch test results of (a) a PEO coated sample, (b) a B-PEO coated sample, (c) a C-PEO coated sample, (d) a BC-PEO coated sample, and (e) a BCS-PEO coated sample, according to certain embodiments. Mechanical properties and bonding strength depicted in FIGS. 11A-11B showing variation of the elastic modulus and nano hardness of the different PEO samples. The hardness and moduli of all the modified samples are better than those of the bare PEO. BN incorporation yielded the highest values. Further incorporation of NC or ST resulted in a marginal reduction. These results suggest particles or inhibitor incorporation in the PEO layer positively impacts the mechanical properties. The enhancement in mechanical properties may be correlated to the improvement in the morphology of the modified PEO layer.

The adhesive force between the coating and substrate was determined by measuring the critical load at which the coating failed, as such, the load for which the oxide layer continuously delaminated. The adhesive strength is highest for C-PEO, at about 14.9 N, while that of bare PEO is 12.1 N, as shown in FIG. 11C. B-PEO, BC-PEO, and BCS-PEO exhibited comparable adhesive strengths with no significant variation. The results suggest that NC incorporation improved the coating adhesion compared to BN. The ST incorporation does not negatively affect the adhesion. The presence of BN may inhibit spark discharge, thereby impacting coating quality and reducing adhesion, whereas NC favors adhesion through low melting point assisted sintering. The adhesive strength of all the samples is better than the bare PEO. The micrograph corresponding to the scratch test are also provided in FIG. 11D.

FIG. 12 illustrates the PDP plots of (a) bare Mg alloy, (b) PEO, (c) B-PEO, (d) C-PEO, and (e) BCS-PEO. The extracted corrosion parameters, as listed in Table 2, show the advantages of particle and inhibitor incorporation in enhancing corrosion resistance. The PEO layer reduced the corrosion current (icon) and corrosion rate (CR) of bare Mg alloy by about 7 times. The addition of BN has a better performance than NC. The BN, NC and ST incorporated PEO coating (BCS-PEO) reduced the CR by about 150 times compared to bare Mg alloy and about 15 times compared to the PEO sample. The improvement is associated with the morphology variation due to the incorporation of the particles in the PEO layer and the additional protection offered by the stannate. The marginal increment in coating thickness can have a role. With the modified coatings, both cathodic (hydrogen evolution) and anodic (Mg oxidation) polarization plots are moved to the lower current region, signifying the reduced reaction kinetics. A short anodic passive region appears with the subsequent rapid passive film breakdown. The passive region formed on BN-incorporated samples is more durable. The rapid film breakdown is associated with the localized corrosion instigation at the coating defects. After the test, one or two visible black spots were seen on the sample surface. The corrosion potential (E_corr) variation was insignificant among the different PEO samples. The additives significantly altered the coating morphology and adherence, improving corrosion resistance due to an enhanced barrier effect over electrolyte penetration. The polarization resistance (R_p) values calculated for the particles and inhibitor-incorporated PEO samples were significantly higher.

FIGS. 13A-13D illustrate the EIS plots of (a) bare Mg alloy, (b) PEO, (c) B-PEO, (d) C-PEO, and (e) BCS-PEO, and FIGS. 13E and 13F respectively illustrate the equivalent circuits (ECs), used for EIS data fitting of a bare Mg alloy and a PEO coated Mg alloy. The fit parameters are provided in Table 3. The Bode phase shift plot of bare Mg alloy, as depicted in FIG. 13C, shows a prominent one-time constant and a low-frequency inductance tail. The inductance loop is evident in the corresponding Nyquist plot, shown in FIG. 13B. The capacitive loop with the bare Mg alloy is allied to the charge transfer process, whereas the inductive loop is credited to the dissolution and pitting corrosion. The phase shift plot of PEO coated Mg alloy, shown in FIG. 13C, describes two distinct one-time constants corresponding to the coated surface and the substrate. The larger phase angles at high frequencies correspond to the oxide layer thickening due to PEO. A low-frequency inductance tail at low frequency is apparent in the Nyquist and Bode plots of FIGS. 13A-13D. The improved corrosion resistance of the PEO sample over the bare Mg alloy is obvious from the large diameter of the Nyquist plot of the former, shown in FIG. 13B. As can be seen from FIG. 13D, the particles and inhibitor incorporated PEO samples do not show well-defined two-time constants, compared to bare PEO. However, a diffusive behavior is shown, indicating that most of the porous structure of PEO was covered with the added particles. The higher and broader intermediate frequency phase angles suggest that incorporated coatings of the particle are uniform and denser, and the interface electrochemical events are primarily controlled by adsorbed outer layer of the particle. The larger diameters of the Nyquist plot shown in FIG. 13B and greater low-frequency impedance modulus in the Bode magnitude plot shown in FIG. 13C, describe the much-enhanced barrier protection of the particles and inhibitor incorporated PEO layer. The absence of the inductive loop is noteworthy. The low-frequency impedance in the Bode magnitude plots of particles incorporated PEO coatings is greater than 10⁵ Ohm·cm². The improved corrosion resistance in the present disclosure has significant contribution from the barrier effect of the incorporated BN and NC.

In the first and the second ECs for bare alloy, shown in FIGS. 13E-13F, R_S and R₁ denote solution resistance and charge transfer resistance, respectively. R₂ corresponds to pitting corrosion with an inductance (L). In the ECs of PEO coated alloys, shown in FIG. 13F, the time constant Q₁ and R₁ (high-frequency response) correlate to the outer porous PEO layer, and Q₂ and R₂ (middle-frequency response) correlate to the barrier PEO layer. Q₃ and R₃ correspond to the low-frequency response and owe to the electrochemical corrosion reaction at the coating/substrate interface due to electrolyte penetration through the coating defects. R₁, R₂ and R₃ of the second circuit signify the pore resistance, barrier layer resistance and charge transfer resistance, respectively. In general, R₁ signifies the coating resistance to the ionic pathways through the porous PEO film, and R₂ elucidates the resistance of the barrier PEO film to obstruct the penetration of aggressive species to reach the metallic surface. Further, R₃ is the resistance to the interface charge transfer reactions.

Low chi-square (χ²) values supported the goodness of the fitting, as listed in Table 3. The higher charge transfer resistance (R₃) of the BCS-PEO coating signifies that the coating/substrate interface acts as an excellent barrier to corrosion, indicating the role of ST in preventing the interface electron transfer and reducing the susceptibility/propagation of corrosion. Analogous to reported works, the resistance of the inner barrier layer (R₂) is higher than that of the outer porous layer (R₁). The highest R₂ is also recorded with the BCS-PEO sample. The highest R₁ value of BC-PEO suggests a better blocking of the porous outer layer by the BN and NC. A higher Q₃ of the C-PEO sample suggests that NC deposition may absorb water. However, the corresponding values for BC-PEO and BCS-PEO are low. For better corrosion protection, using clay as a component is desirable.

FIGS. 14A-14I respectively show scanning electrochemical microscopy (SECM) images taken during continuous immersion in 0.1 M NaCl for 1 hour of a bare Mg alloy sample, for 8 hours of a bare Mg alloy sample, for 24 hours of a bare Mg alloy sample, for 1 hour of a PEO coated Mg alloy sample, for 8 hours of a PEO coated Mg alloy sample, for 24 hours of a PEO coated Mg alloy sample, for 1 hour of a BCS-PEO coated Mg alloy sample, for 8 hours of a BCS-PEO coated Mg alloy sample and for 24 hours of a BCS-PEO coated Mg alloy sample, according to certain embodiments. FIGS. 14A-14I illustrate SECM images of bare Mg alloy, PEO and BCS-PEO samples, measured at regular intervals during direct immersion in 0.1 M NaCl. In general, SECM is effective for evaluating surfaces immersed in liquids. In the present disclosure, SECM studies were conducted at a relatively low concentration of NaCl to examine the film-repairing effect of the incorporated ST. The variation presented in FIGS. 14A-14I provides an idea of the extent of H₂ evolution on the surface of the Mg alloy samples. H₂ evolution is higher in regions mimicking a linear projection, while the H₂ evolution is lower in the regions that are relatively flat when compared to the linear projection regions. Further, sharp projections in FIGS. 14A-14C also depict a relatively neutral H₂ evolution. According to the present disclosure, the bare Mg alloy showed higher H₂ release by showing a higher SECM tip current and uneven surface mapping, due to the inhomogeneous H₂ evolution from the adjacent locations, indicating severe corrosion occurring at the Mg surface, which increased with the exposure period, shown in FIG. 14A-14C.

The coated surfaces of both PEO and BCS-PEO revealed a more homogeneous surface, as shown in FIGS. 14A-14C. As can be seen from FIG. 14G-14I, the entire surface looks flat (passive) with the addition of ST system, except for a few projections (H₂ evolution), and the trend continues with time signifying better corrosion protection. After 24 hours of continuous immersion, the surface of the bare Mg alloy was visibly severely attacked, however, it was covered with a black surface layer, as observed during experiments. The formed black layer of MgO/Mg(OH)₂ may suppress hydrogen generation in local areas even for the bare alloy. The appearance of black areas on the bare Mg alloy after 24 hours of immersion may be attributed to hydrogen generation. In addition, FIGS. 15A-15I illustrate an analogous variation for B-PEO, C-PEO and BC-PEO samples. In other words, as compared to the homogenous surfaces shown in FIGS. 14A-14C, the samples depict an analogous variation in FIGS. 15A-15I. In particular, FIGS. 15A-15C are descriptive of B-PEO sample after durations of 1 hour, 8 hours, and 24 hours, respectively. FIGS. 15D-15F are descriptive of C-PEO sample after durations of 1 hour, 8 hours, and 24 hours, respectively. FIGS. 15G-15I are descriptive of BC-PEO sample after durations of 1 hour, 8 hours, and 24 hours, respectively.

The SECM samples, after 24 hours of immersion in 0.1 M NaCl, were taken out and cleaned with ethanol, and the surface morphology was examined by optical microscopy, and the results are provided in FIGS. 16A-16F. FIGS. 16A-16F respectively show optical microscopy images of a PEO coated Mg alloy sample after continuous immersion in 0.1 M NaCl for 24 hours, a B-PEO coated Mg alloy sample after continuous immersion in 0.1 M NaCl for 24 hours, a C-PEO coated Mg alloy sample after continuous immersion in 0.1 M NaCl for 24 hours, a BCS-PEO coated Mg alloy sample after continuous immersion in 0.1 M NaCl for 24 hours, a bare Mg alloy sample after continuous immersion in 0.1 M NaCl for 24 hours, and a BCS-PEO coated Mg alloy sample after continuous immersion in 0.1 M NaCl for 24 hours, according to certain embodiments.

As can be seen from FIG. 16A, large pits depict complete damage of the PEO sample. The bare Mg alloy surface was attacked intensively with even larger pits. As can be seen from FIGS. 16B-16C, the incorporation of BN and NC had a positive impact in reducing the aggressiveness of the electrolyte. The combined addition of BN and NC also yielded a similar image. The ST addition significantly enhanced the scenario, indicating that stannate may provide additional corrosion protection and resist corrosion progression by effectively healing and suppressing the damages, as shown in FIG. 16D. The linear crevices in the optical images of FIG. 16D correspond to corrosion-affected areas. The analogous regions in the SEM image of FIG. 16F show a similar morphology to the corroded bare Mg alloy surface of FIG. 16E. The corrosion may have been initiated and propagated via coating defects or regions with fewer particles or inhibitors.

Typically, the PEO coating forms under a generated plasma environment at a very high temperature which is generally greater than 1000° C., where the substrate locally melts and reacts with the electrolyte components, followed by a fast solidification of the reaction products and the associated crater (discharge channel) formations. The formation mechanism is complex and associated with multiple chemical, thermal, and electrochemical reactions, including the formation and dissolution of the oxide film and gas evolution. The above mentioned parameters may vary with the electrode and electrolyte parameters. The incorporation of particles is expected to include oxidation-deposition under a combined PEO-EPD process. Under intense sparking, the coating constituents begin to melt, specifically, the low-melting clay, which may raise the melt volume and prompt the melt pool to flow back to the discharge channels, yielding bigger diameter pores and areas with filled pores. Low melting point particles with high chemical stability can quickly achieve inert incorporation.

The morphology and coating thickness may lead to better corrosion resistance. The primary factor contributing to the enhancement of corrosion resistance in the present disclosure is the improved morphology of the PEO layer with the incorporated particles (improved surface coverage and pore blocking) and the added active protection by the ST. Moreover, the present disclosure includes voltage pulsing between 100 and 200 V. The pulsing approach may have helped to quench the developed spark discharges momentarily, however, the coating growth continued due to the anodic oxidation and EPD, which may be beneficial to create a more compact oxide layer with fewer deeper cracks and pores. Immediate surface passivation by the inhibitor may happen as soon as the sample comes into contact with the electrolyte. The inhibitor adsorption may be enhanced during the PEO process, and the ST may disperse homogeneously throughout the PEO layer, as was evident from the EDS line scan analysis. The loaded corrosion inhibitors may migrate to the damaged areas and prevent rapid progress of the corrosion, as shown in the scheme depicted in FIG. 1. The migration is evident in the optical/SEM images after the SECM evaluations. The pores of the PEO layer may be loaded with ST, providing a type of self-healing that improves corrosion resistance. It may be suggested that increasing the protection performance of an ST coated panel with immersion time may provide the conversion of the tin-oxide film formed from a less protective oxide (II) to a more stable one, which is oxide (VI). In addition to the protection offered by the incorporated BN, NC and ST, the better corrosion protection performance of the PEO coating of the present disclosure has contributions from the silicate-rich surface precipitation/passivation layer as well as from the CaF₂ in the electrolyte.

Numerous modifications and variations of the present disclosure are possible in light of the above teachings. Therefore, it is to be understood that within the scope of the appended claims, the invention may be practiced other than as specifically described herein.