US20220042176A1 - Advanced anticorrosion coatings on lightweight magnesium alloys by atmospheric co2 plasma treatment - Google Patents

Advanced anticorrosion coatings on lightweight magnesium alloys by atmospheric co2 plasma treatment Download PDF

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US20220042176A1
US20220042176A1 US17/395,606 US202117395606A US2022042176A1 US 20220042176 A1 US20220042176 A1 US 20220042176A1 US 202117395606 A US202117395606 A US 202117395606A US 2022042176 A1 US2022042176 A1 US 2022042176A1
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mgo
plasma
magnesium
mgco
atmospheric
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Gyoung Gug JANG
Donovan N. Leonard
Ji Heon Jun
Michael P. Brady
Michael Z. Hu
Peter Yancey
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UT Battelle LLC
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UT Battelle LLC
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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C22/82—After-treatment
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/10—Oxidising
    • C23C8/16—Oxidising using oxygen-containing compounds, e.g. water, carbon dioxide
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/02—Pretreatment of the material to be coated
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/36—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases using ionised gases, e.g. ionitriding
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C22/05—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions
    • C23C22/68—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous solutions with pH between 6 and 8

Definitions

  • the present invention relates to the plasma treatment of a magnesium substrate for the prevention of corrosion.
  • Magnesium is among the lightest and the lowest cost structural materials with excellent mechanical properties for various fields, including ground/air/marine transportation, electronic components, medical implants, hydrogen storage, and battery electrodes. Particularly in automotive applications, the replacement of structural steels with magnesium alloys enables a 60% weight reduction for existing vehicle components, which can improve fuel efficiency and decrease gas emissions. While approximately 350 lbs. of magnesium alloy parts have been approved for use in chassis, interiors, bodies and powertrain components, there are still only an average of 11-14 lbs. of magnesium components used in each North American vehicle. One of the main issues is the poor corrosion resistance of magnesium alloys. Generally, magnesium alloys have poor corrosion resistance due to their high chemical reactivity and the lack of a protective passive oxide film. In air, the surface of magnesium naturally forms Mg(OH) 2 and MgO layers, which are porous and unprotective, especially in humid environments.
  • the method includes providing a magnesium substrate including a native surface layer of nanoporous MgO and Mg(OH) 2 .
  • the method further includes reacting activated CO 2 gas molecules with the native surface layer by performing an atmospheric CO 2 plasma treatment (CO 2 -AP) at room temperature to convert at least a portion of the native surface layer of nanoporous MgO and Mg(OH) 2 into a nano- to micro-structured MgO/MgCO 3 coating.
  • the method includes reacting activated CO 2 gas molecules with the native surface layer in successive sweeps of a plasma plume.
  • the resulting nano- to micro-structured MgO/MgCO 3 coating can include a thickness of between 0.1 ⁇ m and 10 ⁇ m, inclusive, further optionally between 0.5 ⁇ m and 5 ⁇ m, inclusive, comprising MgCO 3 and MgO uniformly dispersed therein.
  • the magnesium substrate is prepared by forming a surface film of nanoporous MgO and Mg(OH) 2 .
  • the step of preparing the magnesium substrate optionally includes forming the surface film via salt water immersion (e.g., aqueous NaHCO 3 ) followed by distilled water immersion of the magnesium substrate.
  • FIG. 1 is an illustration of a method for preventing corrosion of a magnesium substrate using a CO 2 plasma treatment.
  • FIG. 2 is a graph depicting the results of hydrogen collection tests performed on magnesium substrates subjected to a variety of CO 2 plasma treatment methods.
  • FIG. 3 includes STEM images a) through h) depicting a micro-structured MgO/MgCO 3 layer on magnesium substrates generated by a CO 2 plasma treatment.
  • FIG. 4 is a material characterization depicting the formation of a micro-structured MgO/MgCO 3 layer on a magnesium substrate generated by a CO 2 plasma treatment.
  • the current embodiment generally relates to a method for preventing corrosion of a magnesium substrate using a CO 2 plasma treatment.
  • the method includes providing or preparing a magnesium substrate including a native surface layer or surface film of nanoporous MgO and Mg(OH) 2 and reacting activated CO 2 gas molecules with the native surface layer or surface film by performing an atmospheric CO 2 plasma treatment at room temperature to convert at least a portion of the native surface layer or surface film of nanoporous MgO and Mg(OH) 2 into a nano- to micro-structured MgO/MgCO 3 coating.
  • the method for preventing corrosion of magnesium generally includes providing a magnesium substrate including a native surface layer of nanoporous MgO and Mg(OH) 2 .
  • the magnesium substrate includes magnesium or a magnesium alloy.
  • the magnesium alloy can further include aluminum, zinc, manganese, silicon, copper, rare earths, carbon, iron, or zirconium.
  • a “native” surface layer is a section of the magnesium substrate that naturally forms from the reaction of water in ambient air with the magnesium or the magnesium within the magnesium substrate, i.e., a surface layer solely resulting from the exposure of the magnesium substrate to ambient air at standard temperature and pressure.
  • the method includes actively forming a surface film of nanoporous MgO and Mg(OH) 2 .
  • the magnesium substrate is contacted with water, either in a vapor or a liquid state, and the magnesium reacts with the water to form the surface film of nanoporous MgO and Mg(OH) 2 .
  • the water is distilled water and the surface film is formed via the immersion of the magnesium substrate into deionized water.
  • the magnesium substrate is first contacted with salt water (e.g., aqueous NaHCO 3 ) to shorten the pretreatment immersion in distilled water from 24 hours to less than 10 minutes.
  • salt water e.g., aqueous NaHCO 3
  • the method for preventing corrosion of magnesium includes the step of reacting activated CO 2 gas molecules with the native surface layer or the surface film of the magnesium substrate by performing an atmospheric CO 2 plasma treatment at room temperature to convert at least a portion of the native surface layer of nanoporous MgO and Mg(OH) 2 into a nano- to micro-structured MgO/MgCO 3 coating.
  • the atmospheric CO 2 plasma treatment is open to the air.
  • the atmospheric CO 2 plasma treatment is carried out in a CO 2 enriched cover gas chamber to maximize MgCO 3 formation.
  • the atmospheric CO 2 plasma treatment of the native surface layer or surface film is performed manually.
  • the atmospheric CO 2 plasma treatment of the native surface layer or surface film is performed by an automated system.
  • the automated system is conducted as a batch process.
  • the automated system is conducted as a continuous process.
  • the treatment time is 10 to 60 minutes, alternatively 15 to 45 minutes, alternatively around 30 minutes.
  • the method also includes generating a CO 2 plasma at atmospheric pressure.
  • a CO 2 gas feedstock is provided in a cover gas chamber at atmospheric pressure and at room temperature, and the CO 2 plasma is generated by applying an electrical field to the CO 2 gas feedstock.
  • the electrical field is generated by at least two electrodes that are spaced apart from each other.
  • the electrodes produce a discharge voltage of from 1 to 10,000 V, alternatively 200 to 1000 V.
  • the method also includes flowing the CO 2 plasma from a nozzle exit as a plasma plume. Generally, the plasma plume is substantially oriented toward the magnesium substrate.
  • the method optionally includes exposing the native surface layer or the surface film to the plasma plume.
  • the step of reacting activated CO 2 gas molecules with the native surface layer or surface film is performed in successive sweeps of a plasma plume. In some embodiments, less than 30 sweeps are made, alternatively less than 10, alternatively less than 5.
  • the nano- to micro-structured MgO/MgCO 3 coating can include a thickness of between 0.1 ⁇ m and 10 ⁇ m, inclusive, alternatively between 0.5 ⁇ m and 5 ⁇ m, inclusive.
  • the MgO/MgCO 3 coating includes MgCO 3 and MgO uniformly dispersed therein.
  • the MgO/MgCO 3 coating is distinct from an MgO coating.
  • the MgO coating is between the MgO/MgCO 3 coating and a magnesium body.
  • Magnesium substrates treated with the atmospheric CO 2 plasma treatment were discovered to possess an excellent corrosion rate mitigation effect.
  • magnesium substrates treated with an atmospheric CO 2 plasma treatment demonstrated superhydrophobicity and possessed minimal water dispersion when compared to untreated magnesium substrates.
  • the treated magnesium substrates also exhibited a thin layer of air on a surface of the magnesium substrate when immersed in an aqueous salt solution.
  • the atmospheric CO 2 plasma treatment was discovered to facilitate the chemical and/or physical adsorption of acidic CO 2 molecules on a surface of the magnesium substrate and the formation of various carbonate surface species (e.g. potentially including bicarbonate, bidentate, and/or unidentate carbonates).
  • the present invention is further described below in connection with a laboratory example.
  • the laboratory example simulated a highly aggressive environment to evaluate corrosion resistance for long-term exposure.
  • a magnesium substrate was first immersed in a 3.5 wt % NaCl saltwater solution for 25 hours.
  • a hydrogen collection test was then performed on the treated magnesium substrate, with the results shown in FIG. 2 (“Comparative Example”). Three treated samples were then evaluated against the comparative sample.
  • the first treated sample included a magnesium substrate that was immersed in distilled water to form a surface film of nanoporous MgO and Mg(OH) 2 .
  • a CO 2 plasma plume was generated at atmospheric pressure, and the surface film was exposed to a single pass of the plasma plume.
  • the plasma plume (generated by 100% CO 2 gas) passed through the sample surface with 0.38 cm height, 762 cm/min, and 0.06 cm spacing at 70 SLPM (standard liters per minute) of flow rate.
  • the magnesium substrate was then immersed in a 3.5 wt % NaCl saltwater solution for 25 hours.
  • a hydrogen collection corrosion test was then performed on the treated magnesium substrate. The results are shown in FIG. 2 (“Example 1”).
  • the second treated sample included a magnesium substrate that was immersed in distilled water to form a surface film of nanoporous MgO and Mg(OH) 2 .
  • a CO 2 plasma plume was generated at atmospheric pressure, and the surface film was exposed to five passes of the plasma plume.
  • the magnesium substrate was then immersed in a 3.5 wt % NaCl saltwater solution for 25 hours.
  • a hydrogen collection corrosion test was then performed on the treated magnesium substrate. The results are shown in FIG. 2 (“Example 2”).
  • a third treated sample included a magnesium substrate that was exposed to the open air such that a native surface layer was formed.
  • a CO 2 plasma plume was generated at atmospheric pressure, and the native surface layer was exposed to a single pass of the plasma plume.
  • the magnesium substrate was then immersed in a 3.5 wt % NaCl saltwater solution for 25 hours.
  • the results are shown in FIG. 2 (“Example 3”).
  • STEM scanning transmission electron microscope
  • high-magnification cross-sectional HAADF-STEM images (a, e) of the treated Mg (five-times swing) along with corresponding energy-dispersive X-ray spectroscopy elemental mapping (b-d, f-h) show the distribution of Mg, C, and O in the treated layer (C: 1.5 atomic %, O: 53.0 atomic %, and Mg: 26.5 atomic % in the zoomed-in image).
  • X-ray diffraction (XRD) of the MgO/MgCO 3 coating confirmed that the phase content of the resultant layer was primarily MgO and MgCO 3 .
  • XRD patterns of untreated Mg were measured at two angles (0° and 90°) to avoid the crystalline directional effect.

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Abstract

An improved method for preventing corrosion of magnesium is provided. The method includes providing a magnesium substrate including a native surface layer of nanoporous MgO and Mg(OH)2. The method includes generating a CO2 plasma at atmospheric pressure, flowing the CO2 plasma from a nozzle exit as a plasma plume, and exposing the surface film to the plasma plume. The method further includes reacting activated CO2 gas molecules with the native surface layer by performing an atmospheric CO2 plasma treatment at room temperature to convert at least a portion of the native surface layer of nanoporous MgO and Mg(OH)2 into a nano-structured to micro-structured MgO/MgCO3 coating.

Description

    CROSS-REFERENCE TO RELATED APPLICATIONS
  • This application claims the benefit of U.S. Provisional Application 63/062,455, filed Aug. 7, 2020, the disclosure of which is incorporated by reference in its entirety.
    • STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT
  • This invention was made with government support under Contract No. DE-AC05-00OR22725 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
    • FIELD OF THE INVENTION
  • The present invention relates to the plasma treatment of a magnesium substrate for the prevention of corrosion.
    • BACKGROUND OF THE INVENTION
  • Magnesium is among the lightest and the lowest cost structural materials with excellent mechanical properties for various fields, including ground/air/marine transportation, electronic components, medical implants, hydrogen storage, and battery electrodes. Particularly in automotive applications, the replacement of structural steels with magnesium alloys enables a 60% weight reduction for existing vehicle components, which can improve fuel efficiency and decrease gas emissions. While approximately 350 lbs. of magnesium alloy parts have been approved for use in chassis, interiors, bodies and powertrain components, there are still only an average of 11-14 lbs. of magnesium components used in each North American vehicle. One of the main issues is the poor corrosion resistance of magnesium alloys. Generally, magnesium alloys have poor corrosion resistance due to their high chemical reactivity and the lack of a protective passive oxide film. In air, the surface of magnesium naturally forms Mg(OH)2 and MgO layers, which are porous and unprotective, especially in humid environments.
    • SUMMARY OF THE INVENTION
  • An improved method for preventing corrosion of magnesium is provided. The method includes providing a magnesium substrate including a native surface layer of nanoporous MgO and Mg(OH)2. The method further includes reacting activated CO2 gas molecules with the native surface layer by performing an atmospheric CO2 plasma treatment (CO2-AP) at room temperature to convert at least a portion of the native surface layer of nanoporous MgO and Mg(OH)2 into a nano- to micro-structured MgO/MgCO3 coating. In some embodiments, the method includes reacting activated CO2 gas molecules with the native surface layer in successive sweeps of a plasma plume. The resulting nano- to micro-structured MgO/MgCO3 coating can include a thickness of between 0.1 μm and 10 μm, inclusive, further optionally between 0.5 μm and 5 μm, inclusive, comprising MgCO3 and MgO uniformly dispersed therein. In other embodiments, the magnesium substrate is prepared by forming a surface film of nanoporous MgO and Mg(OH)2. The step of preparing the magnesium substrate optionally includes forming the surface film via salt water immersion (e.g., aqueous NaHCO3) followed by distilled water immersion of the magnesium substrate.
  • These and other features of the invention will be more fully understood and appreciated by reference to the description of the embodiments and the drawings.
    • BRIEF DESCRIPTION OF THE DRAWINGS
  • FIG. 1 is an illustration of a method for preventing corrosion of a magnesium substrate using a CO2 plasma treatment.
  • FIG. 2 is a graph depicting the results of hydrogen collection tests performed on magnesium substrates subjected to a variety of CO2 plasma treatment methods.
  • FIG. 3 includes STEM images a) through h) depicting a micro-structured MgO/MgCO3 layer on magnesium substrates generated by a CO2 plasma treatment.
  • FIG. 4 is a material characterization depicting the formation of a micro-structured MgO/MgCO3 layer on a magnesium substrate generated by a CO2 plasma treatment.
    •  DETAILED DESCRIPTION OF THE CURRENT EMBODIMENT
  • As discussed herein, the current embodiment generally relates to a method for preventing corrosion of a magnesium substrate using a CO2 plasma treatment. The method includes providing or preparing a magnesium substrate including a native surface layer or surface film of nanoporous MgO and Mg(OH)2 and reacting activated CO2 gas molecules with the native surface layer or surface film by performing an atmospheric CO2 plasma treatment at room temperature to convert at least a portion of the native surface layer or surface film of nanoporous MgO and Mg(OH)2 into a nano- to micro-structured MgO/MgCO3 coating.
  • The method for preventing corrosion of magnesium generally includes providing a magnesium substrate including a native surface layer of nanoporous MgO and Mg(OH)2. The magnesium substrate includes magnesium or a magnesium alloy. The magnesium alloy can further include aluminum, zinc, manganese, silicon, copper, rare earths, carbon, iron, or zirconium. As used herein, a “native” surface layer is a section of the magnesium substrate that naturally forms from the reaction of water in ambient air with the magnesium or the magnesium within the magnesium substrate, i.e., a surface layer solely resulting from the exposure of the magnesium substrate to ambient air at standard temperature and pressure.
  • In other embodiments, the method includes actively forming a surface film of nanoporous MgO and Mg(OH)2. The magnesium substrate is contacted with water, either in a vapor or a liquid state, and the magnesium reacts with the water to form the surface film of nanoporous MgO and Mg(OH)2. In such embodiments, the water is distilled water and the surface film is formed via the immersion of the magnesium substrate into deionized water. In other embodiments, the magnesium substrate is first contacted with salt water (e.g., aqueous NaHCO3) to shorten the pretreatment immersion in distilled water from 24 hours to less than 10 minutes.
  • The method for preventing corrosion of magnesium includes the step of reacting activated CO2 gas molecules with the native surface layer or the surface film of the magnesium substrate by performing an atmospheric CO2 plasma treatment at room temperature to convert at least a portion of the native surface layer of nanoporous MgO and Mg(OH)2 into a nano- to micro-structured MgO/MgCO3 coating. The atmospheric CO2 plasma treatment is open to the air. Generally, the atmospheric CO2 plasma treatment is carried out in a CO2 enriched cover gas chamber to maximize MgCO3 formation. In some embodiments, the atmospheric CO2 plasma treatment of the native surface layer or surface film is performed manually. In alternative embodiments, the atmospheric CO2 plasma treatment of the native surface layer or surface film is performed by an automated system. In such embodiments, the automated system is conducted as a batch process. In certain embodiments, the automated system is conducted as a continuous process. Generally, the treatment time is 10 to 60 minutes, alternatively 15 to 45 minutes, alternatively around 30 minutes.
  • The method also includes generating a CO2 plasma at atmospheric pressure. A CO2 gas feedstock is provided in a cover gas chamber at atmospheric pressure and at room temperature, and the CO2 plasma is generated by applying an electrical field to the CO2 gas feedstock. The electrical field is generated by at least two electrodes that are spaced apart from each other. The electrodes produce a discharge voltage of from 1 to 10,000 V, alternatively 200 to 1000 V. The method also includes flowing the CO2 plasma from a nozzle exit as a plasma plume. Generally, the plasma plume is substantially oriented toward the magnesium substrate. The method optionally includes exposing the native surface layer or the surface film to the plasma plume. In certain embodiments, the step of reacting activated CO2 gas molecules with the native surface layer or surface film is performed in successive sweeps of a plasma plume. In some embodiments, less than 30 sweeps are made, alternatively less than 10, alternatively less than 5.
  • The nano- to micro-structured MgO/MgCO3 coating can include a thickness of between 0.1 μm and 10 μm, inclusive, alternatively between 0.5 μm and 5 μm, inclusive. The MgO/MgCO3 coating includes MgCO3 and MgO uniformly dispersed therein. In other embodiments, the MgO/MgCO3 coating is distinct from an MgO coating. In these embodiments, the MgO coating is between the MgO/MgCO3 coating and a magnesium body.
  • Magnesium substrates treated with the atmospheric CO2 plasma treatment were discovered to possess an excellent corrosion rate mitigation effect. In laboratory testing, magnesium substrates treated with an atmospheric CO2 plasma treatment demonstrated superhydrophobicity and possessed minimal water dispersion when compared to untreated magnesium substrates. The treated magnesium substrates also exhibited a thin layer of air on a surface of the magnesium substrate when immersed in an aqueous salt solution. Additionally, the atmospheric CO2 plasma treatment was discovered to facilitate the chemical and/or physical adsorption of acidic CO2 molecules on a surface of the magnesium substrate and the formation of various carbonate surface species (e.g. potentially including bicarbonate, bidentate, and/or unidentate carbonates).
  • The present invention is further described below in connection with a laboratory example. The laboratory example simulated a highly aggressive environment to evaluate corrosion resistance for long-term exposure. As a comparative sample, a magnesium substrate was first immersed in a 3.5 wt % NaCl saltwater solution for 25 hours. A hydrogen collection test was then performed on the treated magnesium substrate, with the results shown in FIG. 2 (“Comparative Example”). Three treated samples were then evaluated against the comparative sample.
  • The first treated sample included a magnesium substrate that was immersed in distilled water to form a surface film of nanoporous MgO and Mg(OH)2. A CO2 plasma plume was generated at atmospheric pressure, and the surface film was exposed to a single pass of the plasma plume. The plasma plume (generated by 100% CO2 gas) passed through the sample surface with 0.38 cm height, 762 cm/min, and 0.06 cm spacing at 70 SLPM (standard liters per minute) of flow rate. The magnesium substrate was then immersed in a 3.5 wt % NaCl saltwater solution for 25 hours. A hydrogen collection corrosion test was then performed on the treated magnesium substrate. The results are shown in FIG. 2 (“Example 1”). The second treated sample included a magnesium substrate that was immersed in distilled water to form a surface film of nanoporous MgO and Mg(OH)2. A CO2 plasma plume was generated at atmospheric pressure, and the surface film was exposed to five passes of the plasma plume. The magnesium substrate was then immersed in a 3.5 wt % NaCl saltwater solution for 25 hours. A hydrogen collection corrosion test was then performed on the treated magnesium substrate. The results are shown in FIG. 2 (“Example 2”). A third treated sample included a magnesium substrate that was exposed to the open air such that a native surface layer was formed. A CO2 plasma plume was generated at atmospheric pressure, and the native surface layer was exposed to a single pass of the plasma plume. The magnesium substrate was then immersed in a 3.5 wt % NaCl saltwater solution for 25 hours. A hydrogen collection corrosion as was then performed on the treated magnesium substrate. The results are shown in FIG. 2 (“Example 3”).
  • As shown in FIG. 3, scanning transmission electron microscope (STEM) images illustrate the MgO/MgCO3 coating on magnesium substrates generated by CO2 plasma treatment. This figure shows that chemically converted nanoparticle agglomerations are formed pillars on a dense layer that is formed on the Mg surface. The elemental mapping of the structured layer with ˜1-2 μm thickness indicates that three major elements (Mg, 0, and C) are well-dispersed throughout the layer, verifying the coexistence of MgO and MgCO3. In particular, high-magnification cross-sectional HAADF-STEM images (a, e) of the treated Mg (five-times swing) along with corresponding energy-dispersive X-ray spectroscopy elemental mapping (b-d, f-h) show the distribution of Mg, C, and O in the treated layer (C: 1.5 atomic %, O: 53.0 atomic %, and Mg: 26.5 atomic % in the zoomed-in image). As also shown in FIG. 4, X-ray diffraction (XRD) of the MgO/MgCO3 coating confirmed that the phase content of the resultant layer was primarily MgO and MgCO3. XRD patterns of untreated Mg were measured at two angles (0° and 90°) to avoid the crystalline directional effect.
  • The results of the foregoing laboratory example demonstrated a greater than ten-fold increase in corrosion resistance of treated magnesium when compared to untreated magnesium. The excellent corrosion rate mitigation effect is believed to be associated with the non-wettable surface properties (super hydrophobicity), chemical composition, and crystalline changes from pure magnesium due to the atmospheric CO2 plasma treatment.
  • The above description is that of current embodiments of the invention. Various alterations and changes can be made without departing from the spirit and broader aspects of the invention as defined in the appended claims, which are to be interpreted in accordance with the principles of patent law including the doctrine of equivalents. This disclosure is presented for illustrative purposes and should not be interpreted as an exhaustive description of all embodiments of the invention or to limit the scope of the claims to the specific elements illustrated or described in connection with these embodiments. For example, and without limitation, any individual element(s) of the described invention may be replaced by alternative elements that provide substantially similar functionality or otherwise provide adequate operation. This includes, for example, presently known alternative elements, such as those that might be currently known to one skilled in the art, and alternative elements that may be developed in the future, such as those that one skilled in the art might, upon development, recognize as an alternative. Further, the disclosed embodiments include a plurality of features that are described in concert and that might cooperatively provide a collection of benefits. The present invention is not limited to only those embodiments that include all of these features or that provide all of the stated benefits, except to the extent otherwise expressly set forth in the issued claims. Any reference to claim elements in the singular, for example, using the articles “a,” “an,” “the” or “said,” is not to be construed as limiting the element to the singular.

Claims (15)

1. A method for preventing corrosion of magnesium, the method comprising:
providing a magnesium substrate including a native surface layer of nanoporous MgO and Mg(OH)2; and
reacting activated CO2 gas molecules with the native surface layer by performing an atmospheric CO2 plasma treatment at room temperature to convert at least a portion of the native surface layer of nanoporous MgO and Mg(OH)2 into a nano- to micro-structured MgO/MgCO3 coating.
2. The method of claim 1, wherein reacting activated CO2 gas molecules with the native surface layer is performed in successive sweeps of a plasma plume.
3. The method of claim 1, wherein the atmospheric CO2 plasma treatment includes generating a CO2 plasma at atmospheric pressure, flowing the CO2 plasma from a nozzle exit as a plasma plume, and exposing the native surface layer to the plasma plume.
4. The method of claim 3, wherein generating the CO2 plasma includes applying an electrical field to a CO2 gas feedstock.
5. The method of claim 1, wherein the nano- to micro-structured MgO/MgCO3 coating comprises a thickness of between 0.1 μm and 10 μm, inclusive.
6. The method of claim 1, wherein the MgO/MgCO3 coating comprises MgCO3 with MgO uniformly dispersed therein.
7. The method of claim 1, wherein the magnesium substrate comprises a magnesium alloy.
8. A method for preventing corrosion of magnesium, the method comprising:
preparing a magnesium substrate by forming a surface film of nanoporous MgO and Mg(OH)2; and
reacting activated CO2 gas molecules with the surface film by performing an atmospheric CO2 plasma treatment at room temperature to convert the surface film of nanoporous MgO and Mg(OH)2 into a nano-structured to micro-structured MgO/MgCO3 coating.
9. The method of claim 8, wherein forming the surface film includes distilled water immersion of the magnesium substrate.
10. The method of claim 8, wherein the nano- to micro-structured MgO/MgCO3 coating comprises a thickness of between 0.1 μm and 10 μm, inclusive.
11. The method of claim 8, wherein reacting activated CO2 gas molecules with the surface film is performed in successive sweeps of a plasma plume.
12. The method of claim 8, wherein the atmospheric CO2 plasma treatment includes generating a CO2 plasma at atmospheric pressure, flowing the CO2 plasma from a nozzle exit as a plasma plume, and exposing the surface film to the plasma plume.
13. The method of claim 13, wherein generating the CO2 plasma includes applying an electrical field to a CO2 gas feedstock.
14. The method of claim 8, wherein the nano-structured to micro-structured MgO/MgCO3 coating comprises MgCO3 with MgO uniformly dispersed therein.
15. The method of claim 8, wherein the magnesium substrate comprises a magnesium alloy.
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Citations (2)

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CN106381460A (en) * 2016-09-07 2017-02-08 西安交通大学 Anti-corrosion method of magnesium and magnesium alloy and application of MgCO3 layer as anti-corrosion layer
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WO2017087991A1 (en) * 2015-11-22 2017-05-26 Atmospheric Plasma Solutions, Inc. Method and device for promoting adhesion of metallic surfaces
CN106381460A (en) * 2016-09-07 2017-02-08 西安交通大学 Anti-corrosion method of magnesium and magnesium alloy and application of MgCO3 layer as anti-corrosion layer

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