Electroceramic Coating for Magnesium Alloys

This invention relates to articles having magnesium-containing metal surfaces with an electroceramic coating chemically bonded to the metal surfaces and to articles having a composite coating comprising first sectors of electroceramic coating and second sectors comprising organic and/or inorganic components different from the electroceramic coating. The invention further relates to processes of making and using the articles.

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Description
FIELD OF THE INVENTION

This invention relates to articles having magnesium-containing metal surfaces with an electroceramic coating chemically bonded to the metal surfaces and to articles having a composite coating comprising first sectors of electroceramic coating and second sectors comprising organic and/or inorganic components different from the electroceramic coating. The invention further relates to processes of making and using the articles.

BACKGROUND OF THE INVENTION

The light weight (˜1.74 gm/cm3 density) and strength of magnesium and magnesium alloys makes products fashioned therefrom highly desirable for use in manufacturing parts, for example, electronic devices, including handheld electronic devices; motor vehicles; aircraft and other products where low density is beneficial.

One of the most significant disadvantages of magnesium and magnesium alloys is susceptibility to corrosion. Exposure to oxygen, moisture and other environmental agents, such as human fingerprint constituents, causes magnesium and magnesium alloy surfaces to corrode. This corrosion is both unsightly and reduces strength.

One method used to improve corrosion resistance of metal surfaces is anodization, see for example U.S. Pat. No. 4,978,432, U.S. Pat. No. 4,978,432 and U.S. Pat. No. 5,264,113. In anodization, a metal (M) surface is electrochemically oxidized to form metal oxides (MOx) from the metal surface thereby creating a coating layer. Although anodization of magnesium and magnesium alloys generating MgO affords some protection against corrosion, improvements in corrosion performance are desirable. As discussed in U.S. Pat. No. 5,683,522, conventional anodization often fails to form a protective layer on the entire surface of a complex workpiece. Anodized coatings have been found to contain cracks, some down to the metal surface, at sharp corners. Further, adhesion of paint to anodized magnesium surfaces is often insufficient and improvements are needed.

Plasma Electrolytic Oxidation (PEO), also known as Micro Arc Oxidation (MAO), Spark Anodizing and Microplasma Oxidation, referred to herein collectively as “PEO”, is a process in which the surfaces of certain metals, e.g. aluminum and magnesium, are converted into oxide coatings using high-voltage alternating current applied to a metal part submerged in an electrolytic bath. PEO is characterized by intense sparking due to micro-arc discharges which take place in this process thereby breaking down initially deposited oxide layers. The discharges leave “craters” on the surface of the growing coating having an average diameter of more than one micron after 1 min and more than two microns after 30 min. The surface roughness also increases as the PEO coating increases in thickness. Molten oxides are generated due to extremely high temperatures and pressures developed near plasma discharge areas. are

PEO processing of magnesium and its alloys oxidizes the magnesium producing a coating that contains crystalline magnesium oxide (60-80 vol. %) with minor amounts of magnesium silicate and/or magnesium phosphate, depending on the content of the PEO bath. PEO processes have disadvantages including weaker throwing power that can result in thin coatings on inner or less accessible surface areas of the substrate. Due to the voltage and amperage required to generate the “glow” or “sparking” needed for PEO, the process has electricity consumption greater than processes which do not require micro-arc discharges. The resulting oxide layer produced using PEO consists of two sub-layers, the outer layer is a brittle sub-layer with a porosity of more than 15%, which is removed by an additional polishing step. Removal of the outer layer has the disadvantages of additional processing and, often, manual labor, as well as loss of dimensional integrity of the article and challenges in uniformly polishing complex articles or those having non-uniform coating layers due to throw-power limitations of PEO.

A disadvantage of show surfaces of coated magnesium, e.g. casings for electronic devices, is their susceptibility to marring, scratching and scuffing. This drawback causes increased costs to manufacturers through increased scrap rate and efforts to address poor surface mar resistance, such as different and/or additional layers of coating some requiring polishing or other additional process steps.

Difficulties in consistently producing corrosion resistant, uniformly deposited corrosion resistant coatings on magnesium-containing articles have arisen when magnesium test substrates are replaced with more economical industrial magnesium materials having higher amounts of alloying metals and/or surface contaminants. Coating defects and coating failure upon Mg alloys, for example AZ91, are often due to non-uniform coating growth, which is caused by microstructural heterogeneity of the substrate. The coating process to form uniform coatings providing reliable corrosion resistance is complicated on heterogeneous and alloy rich Mg alloys such as AZ91, which possess wide microstructural heterogeneity and unequal distribution of Al within different phases. For example AZ91 (nominally Mg with 9 wt. % Al-1 wt. % Zn), has three main phases namely primary α (i.e. matrix), eutectic α (i.e. Al-rich α) and β phase (Mg17A112 intermetallic), since the substrate is electrochemically heterogeneous each constituent reacts differently in the an electrolytic coating bath leading to non-uniform coating growth which tends to degrade the corrosion resistance of coatings. Studies on anodization of AZ91 D alloy showed that uneven coatings were achieved because of the inhomogeneous composition and microstructure of the alloy substrate. The anodized coating on a phase had fewer pores and was more continuous, whereas upon β phase, the coating had many pores and large elongated defects.

The metal content and surface contamination of industrial grade magnesium alloys varies considerably based on alloy type, raw material and production conditions, and even the vendor. Many of these variables are outside the control of the product manufacturer and lead to variation in coating uniformity and reduced corrosion resistance. It is desirable to provide a process for uniformly coating Mg alloys and coated Mg alloy articles with improved corrosion resistance.

SUMMARY OF THE INVENTION

At least some of the drawbacks described above are reduced by the invention described herein. It is an object of the invention to provide a magnesium containing article having a uniform layer of an inorganic-based, desirably electrolytically deposited, coating chemically bonded to a magnesium alloy surface of the article. The inorganic-based coating may have additional layers deposited thereon, may form a composite coating comprising the inorganic-based coating and a second component distributed throughout at least a portion of the inorganic-based coating and/or the coating on the magnesium containing article may comprise a reaction product the inorganic-based coating and a second component.

It is an object of the invention to provide a method of improving corrosion resistance of magnesium containing metal substrates comprising:

    • A) providing an alkaline electrolyte comprised of water, a source of hydroxide ion, and one or more additional components selected from the group consisting of: water-soluble inorganic fluorides, water-soluble organic fluorides, water-dispersible inorganic fluorides, and water-dispersible organic fluorides and mixtures thereof;
    • B) providing a cathode in contact with the electrolyte;
    • C) placing a magnesium containing article having at least one bare metallic magnesium or magnesium alloy surface in contact with the electrolyte and electrically connected thereto such that the surface acts as an anode;
    • D) passing a current between the anode and cathode through the electrolyte solution for a time effective to generate a first layer of an inorganic-based coating chemically bonded directly to the surface;
    • E) removing the article having the first layer of an inorganic-based coating from the electrolyte and optionally drying it;
    • F) optionally post-treating the article having the first layer of an inorganic-based coating by:
      • i. infusing the first layer of an inorganic-based coating with a second component that is different from the inorganic-based coating thereby distributing the second component throughout at least a portion of the inorganic-based coating and/or
      • ii. contacting the first layer of an inorganic-based coating with a polymeric composition thereby forming a second layer comprising organic polymer chains and/or inorganic polymer chains; and
    • G) optionally applying a layer of paint after the post-treating step.

It is a further object of the invention to provide a method that is performed in the absence of any step prior to step D) that deposits silicate and/or fluoride on the magnesium surface.

It is an object of the invention to provide a method wherein, prior to generating the first layer, from 0.5 to 50 g/m2 of metal is removed from the bare metallic magnesium or magnesium alloy surface.

It is an object of the invention to provide a method comprising controlling temperature and concentration of the electrolyte and time and waveform of the current in step D) such that the inorganic-based coating is 1-20 microns in thickness and comprises carbon, oxygen, fluoride, magnesium and aluminum. It is a further object of the invention to provide a method wherein forming the first layer in step D) utilizes less than 10 kWh per square meter of the magnesium containing surface coated.

  • It is an object of the invention to provide a method of electrolytically depositing an inorganic-based coating comprising a first sub-layer directly bonded to the bare metallic magnesium or magnesium alloy surface at a first interface, the first sub-layer comprising at least 70 wt. % of a combined mass of fluorine and magnesium, and a positive amount of oxygen present in an amount of less than about 25 wt. %; and a second sub-layer integrally connected to the first sub-layer, the second sub-layer comprising external surfaces at the outer boundary of the inorganic-based coating, and internal surfaces defined by pores in the second sub-layer lying interior to the outer boundary of the inorganic-based coating and in communication therewith, the second sub-layer having a composition wherein:

first sub-layer Mg wt. %>second sub-layer Mg wt. %

first sub-layer F wt. %>second sub-layer F wt. %

first sub-layer O wt. %<second sub-layer O wt. %.

It is an object of the invention to provide a method wherein the post-treating step F) is present as a step of contacting a matrix of the first layer of inorganic-based coating with a second component different from the inorganic-based coating; distributing the second component throughout at least a portion of the matrix; and depositing a second layer that is different from the inorganic-based coating and is adhered to at least external surfaces of the inorganic-based coating,

It is an object of the invention to provide a method wherein step F) i) is present and comprises a step of introducing at least one vanadium containing composition as the second component to the second sub-layer of inorganic-based coating, contacting at least the external surfaces and desirably at least some of the internal surfaces of the second sub-layer, whereby the second component forms a thin film in contact with the external surfaces of the inorganic-based coating and lining at least a portion of the pores in the inorganic-based coating.

It is an object of the invention to provide a method wherein the infusing step comprises reacting the vanadium containing composition and elements of the inorganic-based coating to thereby form a portion of the second component, which is different from the inorganic-based coating and the vanadium containing composition.

It is an object of the invention to provide a method wherein step F) ii) is present and comprises contacting the first layer of an inorganic-based coating with a polymeric composition thereby forming a second layer comprising organic polymer chains and/or inorganic polymer chains; and optionally applying a layer of paint after the post-treating step.

It is an object of the invention to provide a magnesium-containing article comprising at least one metallic magnesium or magnesium alloy surface coated according to the methods disclosed herein. In one embodiment, a magnesium-containing article is provided comprising at least one metallic magnesium or magnesium alloy surface coated with a first layer of an inorganic-based coating chemically bonded directly to said surface wherein the inorganic-based coating has a bilayer structure. The bilayer structure may comprise: a first sub-layer directly bonded to the bare metallic magnesium or magnesium alloy surface at a first interface, said first sub-layer comprising at least 70 wt. % of a combined mass of fluorine and magnesium, and a positive amount of oxygen present in an average amount of less than about 20 wt. %; and a second sub-layer integrally connected to the first sub-layer, said second sub-layer comprising external surfaces at the outer boundary of the inorganic-based coating, and internal surfaces defined by pores in the second sub-layer lying interior to the outer boundary of the inorganic-based coating and in communication therewith, said second sub-layer comprising carbon, oxygen, fluoride, magnesium and aluminum, said oxygen present in the inorganic-based coating second sub-layer in an average amount of greater than about 25 wt. %

It is also an object of the invention to provide a magnesium-containing article having a composite coating comprising: a matrix formed by a first layer of an inorganic-based coating chemically bound directly to at least one metallic magnesium or magnesium alloy surface, said matrix having pores and internal surfaces defined by pores, at least some of said pores being in communication with an external surface of the first layer and forming openings therein; and a second component, different from the inorganic-based coating, distributed throughout at least a portion of the matrix comprising the pores, said second component being in contact with at least some of the internal surfaces and external surfaces. The article may further comprise a second layer that is different from the inorganic-based coating and is adhered to at least external surfaces of the inorganic-based coating.

Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, or defining ingredient parameters used herein are to be understood as modified in all instances by the term “about”. Throughout the description, unless expressly stated to the contrary: percent, “parts of”, and ratio values are by weight or mass; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description or of generation in situ within the composition by chemical reaction(s) between one or more newly added constituents and one or more constituents already present in the composition when the other constituents are added; specification of constituents in ionic form additionally implies the presence of sufficient counterions to produce electrical neutrality for the composition as a whole and for any substance added to the composition; any counterions thus implicitly specified preferably are selected from among other constituents explicitly specified in ionic form, to the extent possible; otherwise, such counterions may be freely selected, except for avoiding counterions that act adversely to an object of the invention; molecular weight (MW) is weight average molecular weight; the word “mole” means “gram mole”, and the word itself and all of its grammatical variations may be used for any chemical species defined by all of the types and numbers of atoms present in it, irrespective of whether the species is ionic, neutral, unstable, hypothetical or in fact a stable neutral substance with well-defined molecules; and the terms “solution”, “soluble”, “homogeneous”, and the like are to be understood as including not only true equilibrium solutions or homogeneity but also dispersions that show no visually detectable tendency toward phase separation over a period of observation of at least 100, or preferably at least 1000, hours during which the material is mechanically undisturbed and the temperature of the material is maintained at ambient room temperatures (18 to 25° C.).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electron micrograph of a cross-section of a panel of AZ-31 coated according to Example 1, prior to post-treating, at 2500× magnification. The line with white arrows indicates a distance of 3.08 micron between the end points.

FIG. 2 is a graph of elemental composition, in weight percent, of an inorganic-based electrolytically deposited coating according to the invention showing varying chemical composition of a coating of the invention, as a function of distance from the magnesium alloy surface.

 DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Articles according to the invention include magnesium-containing articles having a coating, which may be an electrolytically deposited coating, chemically bonded to one or more metal surfaces of the magnesium-containing article. Such articles are useful as for example, parts for motor vehicles, aircraft, and electronic devices, including handheld electronic devices, and other products where the light weight and strength of magnesium is desired. The articles generally have at least one metal surface, which comprises magnesium metal, and chemically bonded directly to that surface an inorganic-based coating. In some embodiments, the inorganic-based coating is post treated to improve corrosion resistance.

At least a portion of the article has a metal surface that contains not less than 50% by weight, more preferably not less than 70% by weight, magnesium. The term “magnesium-containing article”, as used in the specification and the claims, means an article having at least one surface that may be in whole or in part metallic magnesium or a magnesium alloy. The body of the article may be formed of metallic magnesium or a magnesium alloy or may be formed of other materials, e.g. metals other than magnesium, polymeric materials, refractory materials, such as ceramics, that have a layer of magnesium or magnesium alloy on at least one surface. The other materials may be other metals different from magnesium, non-metallic materials or combinations thereof, such as composites or assemblies. The article may comprise at least one surface of metallic magnesium or a magnesium alloy comprising, in order of increasing preference, at least about 51, 60, 65, 70, 75, 80, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98 or 99 wt. % magnesium.

Chemically bonded to at least one magnesium surface of the magnesium-containing article is a first layer comprising an inorganic-based coating. An inorganic-based coating may include some organic material, but contains a greater mass of inorganic material than of organic molecules. The inorganic material may act as a matrix in which any organic constituent may be distributed. Desirably the inorganic-based coating may be applied by an electrolytic deposition process as described herein. In one embodiment, the inorganic-based coating contains magnesium, fluorine, oxygen, at least one alloying element from the Mg substrate and at least one metal from the bath.

In some embodiments, despite the absence of organic or other carbonaceous components added to the electrolyte, the inorganic-based coating may comprise carbon. Both the carbon and alloying elements, if present, may be dispersed in an insulating ceramic layer. Even with inclusion of carbon and alloying elements in the inorganic-based coating, a uniform thickness is generated which provides uniform paint and adhesive bonding, as well as corrosion resistance, which is improved as compared to the bare surface of the magnesium containing substrate. This feature of the invention is beneficial in reducing scrap rate where substrates and the inorganic-based coatings deposited thereon achieve good coating quality even in the presence of carbon and alloying elements in the inorganic-based coating. In one embodiment, the inorganic-based coating comprises C, O, F, Al, Mg, and alkali metal. Desirably the alkali metal comprises less than 50, 40, 30, 20, 10, 5 or 1% Na.

The inorganic-based coating comprises magnesium, which may be present in a total amount ranging from, in order of increasing preference, at least about 10, 12, 14, 16, 18, or 20 atomic % and in order of increasing preference, not more than 45, 40, 35, 33, 30, 28, 26, 24 or 22 atomic %. The inorganic-based coating comprises fluorine, which may be present in a total amount ranging from, in order of increasing preference, at least about 15, 20, 22, 24, 26, 28, 30, 32, 34, 36, or 38 atomic % and in order of increasing preference, not more than 60, 55, 50, 45 or 40 atomic %. The inorganic-based coating comprises oxygen, which may be present in a total amount ranging from, in order of increasing preference, at least about 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, or 20 atomic % and in order of increasing preference, not more than 33, 30, 28, 26, 24 or 22 atomic %.

The inorganic-based coating may comprise carbon, which may be present in a total amount ranging from, in order of increasing preference, at least about 3, 4, 5, 6, 7, 8, 9, 10 atomic % and in order of increasing preference, not more than 33, 30, 28, 26, 14 or 12 atomic %. The inorganic-based coating may comprise alloying metals from the magnesium-containing article; alkaline earth metals, different from magnesium; and/or alkali metals, which may be present in a total amount ranging from, in order of increasing preference, at least about 1, 2, 3, 4, or 5 atomic % and in order of increasing preference, not more than 14, 13, 12, 10, 8 or 6 atomic %. In some embodiments, more than 50 wt. % of the total amount of these constituents present in the inorganic-based coating may be localized near the external surface of the inorganic-based coating, meaning the inorganic-based coating surface that is not in direct contact with a metallic surface of the magnesium-containing article.

The inorganic-based coating may comprise fluorine and magnesium in atomic ratios of fluorine to magnesium of about 0.25:1, 0.3:1, 0.4:1, 0.5:1, 0.75:1, 1:1, 1.25: 1, 1.5:1, 1.75:1, 2:1, 2.25:1, 2:5, 2.75:1, 3;1, 3.25:1, 3.5:1, or 3.75;1.

The ratio of oxygen to fluorine in the inorganic-based coating may exhibit a gradient wherein amount of oxygen relative to amount of fluorine increases as a function of distance from the magnesium-containing article's metal surface. In one embodiment, the ratio may range from about 0.1:1 up to about 1:1.

The inorganic-based coating may have a bi-layer morphology, as shown in FIG. 1 and FIG. 2. FIG. 1 shows an electron micrograph of a cross-section of a magnesium alloy panel coated according to Example 1, prior to application of a post-treatment, at 2500× magnification. The inorganic-based coating has a bilayer structure, despite being deposited in a single processing step: a first sub-layer 1 directly bonded to the metal surface 2 and having an interface 5 with the metal surface (first interface); and a second sub-layer 3 in direct contact with the first sub-layer and spaced away from the metal surface by the first sub-layer lying there between. The second sub-layer is directly bonded with the first sub-layer at an interface 6 with the first sub-layer (second interface). The second sub-layer of the inorganic-based coating comprises pores 4, and has internal surfaces 7 and external surfaces 8. The internal surfaces are defined by pores in the second sub-layer and lie interior to the outer boundary 9 of the inorganic-based coating, which comprises the external surfaces of the second sub-layer. The white arrowed line in FIG. 1, extending from the metal surface to the outer boundary of the inorganic-based coating, represents a thickness of about 3 microns for the inorganic-based coating.

The external surfaces of the second sub-layer, lie in a boundary between inorganic-based coating and an external environment or a secondary layer applied to the outer boundary and are not in direct contact with a metallic surface of the magnesium-containing article. The first sub-layer may have few or no pores and has a more dense composition than the second sub-layer. Any pores present in the first sub-layer are desirably not contiguous between the metallic surface of the article and the external surface of the inorganic-based coating layer, and optionally smaller than the pores of the second sub-layer. Some of the pores of the second sub-layer are open pores in communication with the external surface. In some embodiments, the second sub-layer may comprise open and closed cell pore structure. Pore size may range from about 0.1 microns to 5 microns and may make up as much as 50% or more of the volume of the deposited coating. The electrolytically applied inorganic-based coating may have a surface area that is about 75 -150× that of the uncoated substrate surface.

FIG. 2 is a graph of an elemental depth profile taken of inorganic-based coatings according to the invention using glow discharge optical emission spectroscopy (GDOES). Amounts of various elements are shown in weight percent at particular distances from the metal surface. FIGS. 1 and 2 show that the first sub-layer and the second sub-layer are different in morphology and elemental content. Composition of the first sub-layer may vary somewhat depending upon the Mg alloy used, and may comprise 50, 60, 70, 80 or 90 wt. % of a combined mass of fluorine and magnesium, and may additionally comprise about 1 to about 20 wt. % oxygen. Composition of the second sub-layer, as compared to the first sub-layer: the second sub-layer may have a weight percent of fluorine that is less than the weight percent of fluorine found in the first sub-layer; the second sub-layer may have a weight percent of Mg that is less than the weight percent of Mg found in the first sub-layer; and the second sub-layer may have a weight percent of oxygen that is greater than the weight percent of oxygen found in the first sub-layer.

At least a portion of the inorganic-based coating has an amorphous structure. Physical morphology of the inorganic-based coating may comprise non-crystalline compounds of magnesium and one or more of elements. In one embodiment, the inorganic-based coating shows amorphous structure by X-ray crystallography (XRD). Desirably, the inorganic-based coating may be a hard (Vickers 400-900 by nanoindentation), amorphous coating comprising non-stoichiometric magnesium compounds. Nonstoichiometric glasses of Mg and F, with or without oxygen may be present. In one embodiment the inorganic-based coating is an inorganic composition comprising Mg, O & F, including stoichiometric and non-stoichiometric compounds of said elements with each other. In another embodiment, the inorganic composition comprises crystalline and non-crystalline compounds comprising magnesium, with more than 50 atomic percent of the composition comprising non-crystalline compounds.

Coating thickness of the inorganic-based electrolytically deposited coating may range from 0.1 microns to about 50 microns, desirably 1-20 microns depending upon the desired use of the coated article. Coating thickness of the inorganic-based electrolytically deposited coating desirably is at least, in increasing order of preference 0.5, 1, 3, 5, 7, 9, 10 or 11 microns thick, and no more than, if only for economic reasons, in increasing order of preference, 50, 30, 25, 20, 15, 14, 13, or 12 microns thick. As a decorative layer, the coating may range from 2-5 microns. In one embodiment, the coating thickness ranges from 3 to 8.5 microns.

The Examples show that electrolytically applied inorganic-based coatings according to the invention perform better than commercially available conversion coatings for magnesium in unpainted and painted corrosion testing, as well as providing improved corrosion resistance when compared to PEO coatings on magnesium alloys typically used in the automotive industry, e.g. magnesium casting alloys and forged alloys. The electrolytically applied inorganic-based coatings perform better than commercially available conversion coatings for magnesium in unpainted and painted corrosion testing, as well as providing improved corrosion resistance when compared to PEO coatings on magnesium alloys typically used in the automotive industry, e.g. magnesium casting alloys and forged alloys.

In one embodiment, magnesium-containing article may have a composite coating wherein the inorganic-based coating may act as a matrix. This embodiment may include a coating comprising:

    • A) a matrix of a first layer of an inorganic-based coating chemically bonded directly to a magnesium containing surface and
    • B) a second component that is different from the inorganic-based coating and distributed throughout at least a portion of the matrix.

In a further embodiment, the coating on the magnesium containing article may comprise:

    • A) a first layer of inorganic-based coating chemically bonded directly to a magnesium containing surface,
    • B) a second component, e.g. a vanadium post-treatment, that is different from the inorganic-based coating and distributed throughout at least a portion of the inorganic-based coating and
    • C) second layer that is different from the inorganic-based coating and is adhered to at least external surfaces of the inorganic-based coating,

In one embodiment of the invention, the second component may have the same composition as the second layer. In another embodiment of the invention, the second component may be different from both A) and C). In one embodiment, the second component and/or the second layer may form reaction products with elements in the inorganic-based coating. In one embodiment, the inorganic-based coating has a layer of paint deposited thereon, which may comprise the second layer or may be in addition to the second layer.

For a variety of reasons, it is preferred that inorganic-based coatings according to the invention, and aqueous compositions for depositing the inorganic-based coatings, as defined above, may be substantially free from many ingredients used in compositions for similar purposes in the prior art. Specifically, it is increasingly preferred in the order given, independently for each preferably minimized ingredient listed below, that aqueous compositions according to the invention, when directly contacted with metal in a process according to this invention, contain no more than 1.0, 0.5, 0.35, 0.10, 0.08, 0.04, 0.02, 0.01, 0.001, or 0.0002 percent, more preferably said numerical values in grams per liter, of each of the following constituents: chromium, cyanide, nitrite ions; organic materials, e.g. organic surfactants; amines, e.g. hydroxylamines; ammonia and ammonium cations; silicon, e.g. siloxanes, organosiloxanes, silanes, silicate; phosphorus; rare earth metals; sodium; sulfur, e.g. sulfate; permanganate; perchlorate; borate and/or free chloride. Also it is increasingly preferred in the order given, independently for each preferably minimized ingredient listed below, that as-deposited inorganic-based coatings and inorganic secondary layers according to the invention, contain no more than 1.0, 0.5, 0.35, 0.10, 0.08, 0.04, 0.02, 0.01, 0.001, or 0.0002 percent, more preferably said numerical values in parts per thousand (ppt), of each of the following constituents: chromium, cyanide, nitrite ions; organic materials, e.g. organic surfactants; amines, e.g. hydroxylamines; ammonia and ammonium cations; silicon, e.g. siloxanes, organosiloxanes, silanes, silicate; phosphorus; rare earth metals; sodium; sulfur, e.g. sulfate; permanganate; perchlorate; borate and/or free chloride.

Inorganic-based coatings can be produced by a variety of processes capable of generating hard, amorphous coatings chemically bonded to magnesium-containing metals. In one embodiment, the inorganic-based coating may be formed using electrolytic deposition according to the inventive process described herein.

Processes according to the invention are directed to methods of improving corrosion resistance on magnesium containing substrates comprising:

    • providing an alkaline electrolyte comprised of water, a source of hydroxide ion, and one or more additional components selected from the group consisting of: water-soluble inorganic fluorides, water-soluble organic fluorides, water-dispersible inorganic fluorides, and water-dispersible organic fluorides and mixtures thereof;
    • providing a cathode electrically connected to, desirably in physical contact with the electrolyte;
    • placing a magnesium containing article in contact with the electrolyte and electrically connected thereto such that the magnesium containing article acts as an anode;
    • passing a current between the anode and cathode through the electrolyte solution for a time effective to form a first layer of an inorganic-based coating chemically bonded directly to the magnesium containing surface;
    • removing the article having the first layer of an inorganic-based coating from the electrolyte and optionally drying it;
    • optionally post-treating the article having the first layer of an inorganic-based coating by:
      • infusing the article having the first layer of an inorganic-based coating with a second component that is different from the inorganic-based coating thereby distributing the second component throughout at least a portion of the inorganic-based coating and/or
      • contacting the article having the first layer of an inorganic-based coating with a polymeric composition thereby forming a second layer comprising organic polymer chains and/or inorganic polymer chains; and
  • optionally applying a layer of paint after the post-treating step.

Depending on condition of the surface of the magnesium containing surface to be coated, the process may comprise optional steps of: cleaning, etching, deoxidizing and desmutting with or without intervening steps of rinsing with water. Where utilized, a rinse water may be counterflowed into a preceding bath. Prior to contacting the magnesium containing article with the electrolyte, a step 5) of masking or closing off portions of the article to limit or prevent contact with the electrolyte may be performed. For example, masking may be applied to magnesium containing portions of the article where no coating is desired or may be applied to protect article components or surfaces that might be damaged by the electrolyte, likewise hollow portions of an article, e.g. the lumen of a pipe, may be closed off or plugged to prevent electrolyte contact of interior surfaces.

Desirably between the steps of removing the coated article from the electrolyte and the infusing step the inorganic-based coating is not physically or chemically removed or etched. Specifically, no more than 1000, 500, 100, 50, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 or 0.5 mg/m2 of the inorganic-based coating may be removed from the article. Preferably none of the deposited inorganic-based coating is removed.

As discussed above, there is no specific limitation on the article to be subjected to processing in accordance with the present invention, provided that the surface to be electrolytically coated has sufficient magnesium metal or other light metal in combination with magnesium, desirably in the zero oxidation state, to permit coating generation and the non-magnesiferous surfaces are not negatively affected by the treatments. Masking of selected surfaces to prevent contact with electrolyte can be accomplished by methods known in the art. The electrolytic treatment is advantageously applicable to magnesium-base alloys containing one or more other elements such as Al, Zn, Mn, Zr, Si and rare earth metals.

If electrolytic deposition is to be used, the magnesium containing surfaces to be coated are contacted with an electrolyte, desirably an aqueous electrolyte comprising dissolved fluoride ions and free of phosphorus. The electrolyte may have a pH of 10 or more, desirably greater than 10, 11, 12 or 13. In carrying out the electrolytic deposition, an electrolyte is employed which may be maintained at a temperature between about 5° C. and about 90° C., desirably from about 20° to about 45° C. A magnesium or magnesium alloy surface is contacted with, desirably immersed in, an aqueous electrolyte and electrolyzed as the anode in the circuit. One such process comprises immersing at least a portion of the article in the electrolyte, which is preferably contained within a bath, tank or other such container. A second article that is cathodic relative to the anode is also placed in the electrolyte. Alternatively, the electrolyte is placed in a container which is itself cathodic relative to the article (anode). Voltage is applied across the anode and cathode for a time sufficient to form an inorganic-based electrolytic coating. The time required to produce a coating in an electrolytic process according to the invention may range, in increasing order of preference, from about 30, 60, 90, 120 seconds, up to about 150, 180, 210, 240, 300 seconds. Longer deposition times may be utilized but are considered commercially undesirable. Electrolytic processing time can be varied to maximize efficiency by reducing time to Vmax and to control coating weight.

Alternating current, direct current or a combination may be used to apply the desired voltage, e.g. straight DC, pulsed DC, AC waveforms or combinations thereof. In one embodiment, pulsed DC current is used. Desirably a period of at least 0.1, 0.5, 1.0, 3.0, 5.0, 7.0, 9.0, or 10 millisecond and not more than 50, 45, 40, 35, 30, 25, 20, or 15 millisecond may be used, which period may be held constant or may be varied during the immersion period. Waveforms may be rectangular, including square; sinusoidal; triangular, sawtooth; and combinations thereof, such as by way of non-limiting example a modified rectangle having at least one vertical leg that is not perpendicular to the horizontal portion of the rectangular wave.

Peak voltage potential desirably may be, in increasing order of preference, up to about 800, 700, 600, 500, 400 volts, and may desirably is at least in increasing order of preference 140, 150, 160, 170, 180, 200, 300 volts. In one embodiment peak voltage can range from 120-200 volts.

Average voltage may be in increasing order of preference at least 50, 70, 90, 100, 120, 130, 140, or 150 volts and independently preferably may be less than 600, 550, 500, 450, 400, 350, 300, 250, 200 or 180 volts. In one embodiment, average voltage can range from about 120-300 volts. In another embodiment, average voltage may be selected to be in a higher range of 310-500 volts.

Voltage is applied across the electrodes until a coating of the desired thickness is formed on the surface of the article. Generally, higher voltages result in increased overall coating thickness and sparking may ensue. Higher voltages may be used within the scope of the invention provided that the substrate is not damaged and coating formation is not negatively affected.

The electrolyte for the process may be an aqueous alkaline composition comprising a source of fluorine and a source of hydroxide ions. The source of hydroxide may be inorganic or organic, provided that it can be dissolved or dispersed in the aqueous electrolyte and does not interfere with the deposition of the inorganic-based coating. Suitable examples include NaOH and KOH, with KOH being preferred. The source of fluoride may be inorganic or organic, provided that it can be dissolved or dispersed in the aqueous electrolyte and does not interfere with the deposition of the inorganic-based coating. Suitable examples include at least one of alkali metal fluorides, certain alkaline earth metal fluorides and ammonium bifluoride. In one embodiment, the electrolyte may comprise KF and KOH. Desirably, free alkalinity is measured and maintained at approximately 10-25 ml titrant using the following alkalinity titration: Pipet 50 mls (volumetric pipet) of bath into a beaker and titrate with phenolphthalein indicator until a clear endpoint is reached using 0.10 M HCl titrant. In one embodiment, the process alkalinity is controlled to be at least in order of increasing preference 7, 8, 9,10, 11, 12, 13, or 14 ml and is not more than 24, 23, 22, 21, 20, 19, 18, 17, 16 or 15 ml.

The above-described coating process provides improved energy efficiency by lower electrical consumption compared to PEO/MAO processes. The inventive process generally requires less than 20%, 15% or 10% of the electricity consumption (in kWh) to apply an electroceramic coating equal in thickness to a PEO coating per unit surface area. In one embodiment, the inventive process utilizes in increasing order of preference, no more than 10, 9, 8, 7, 6, 5, 4, 3, 2, 1.5, 1 kWh/m2 and energy consumption may be as low as in increasing order of preference 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 kWh/m2. The electroceramic coating process also has lower cooling requirements for the electrolyte compared to PEO/MAO, generally less than 20%, 15% or 10% cooling of the electrolyte needed, which is in part due to the lack of spark generation.

Prior to electrolytic coating, magnesium-containing surfaces may be subjected to one or more of cleaning, etching, deoxidizing and desmutting steps, with or without rinsing steps. Cleaning may be alkaline cleaning and a cleaner may be used to etch the surfaces. A suitable cleaner for this purpose is Parco Cleaner 305, an alkaline cleaner commercially available from Henkel Corporation. Desirably, the magnesium-containing surfaces may be etched by at least in increasing order of preference 1, 3, 5, 7, 10, or 15 g/m2 and independently preferably, at least for economy, not more than 20, 25, 30, 35, 40, 45 or 50 g/m2. Etching can be accomplished using commercially available etchants and/or deoxidizers for magnesium. Depending on the magnesium or magnesium alloy composition and cleanliness, a desmutting step may also be included in processing. Suitable desmutters include acids such as carboxylic acids, e.g. hydroxyacetic acid, alone or in combination with chelators and nitrates. If any of the above-described steps is utilized, the magnesium-containing surfaces are typically rinsed as a final step to reduce introduction of the prior steps' chemistries into the electrolyte.

Additional processing steps may be used after deposition of the inorganic-based coating, such as rinsing with water, alkaline solutions, acid solutions and combinations of such steps. In some embodiments, the process may include steps of applying at least one post-treatment, which may be dispersed in the inorganic-based coating, may form reaction products therewith, and/or may form an additional layer and combinations thereof. The additional layer may be an inorganic layer, an organic layer or a layer that comprises inorganic and organic components. Advantageously, any post-treatments, including for example additional layers described herein, are durably bound to the inorganic-based coating; while other removable layers for masking during manufacture or for shipping after coating may be applied.

The porous structure of the electrolytically deposited inorganic-based coatings on the magnesium containing article was a particular challenge for post-treatments that are not pore closing due to the significant surface area present on the internal surfaces of inorganic-based coatings. Surface area of inorganic-based coatings according to the invention is generally 75 to 100 times that of the original metal surface, by BET measurement. Such surface area is typically not found in conventional conversion coatings, such as for example oxide coatings of Zr, Ti, Co, and the like. A vanadium-containing post-treatment step was surprisingly found to be a suitable method for introducing a second component for additional corrosion protection, in processes according to the invention, despite other post-treatments useful for anodized layers having little or no positive effect on corrosion resistance. For example, conventional post-treatments for anodized magnesium, including nickel based salts and lithium salts were found to provide insufficient unpainted corrosion resistance. In contrast, post-treatment of the inorganic-based coating with a vanadium-containing composition provided improvements in corrosion resistance. The vanadium containing post-treatment step may be used immediately after deposition of the inorganic-based coating, which may be dried. Articles having the inorganic-based coating deposited thereon via electrolytic deposition may be rinsed for 1-30 seconds and then contacted with the vanadium containing composition.

Vanadium can be present in the post-treatment in various oxidation states such as III, IV, and V. Sources of vanadium ions in the post-treatment can include metallic V, organic and inorganic V-containing materials, for example V-containing minerals and compounds. Suitable compounds of V include by way of non-limiting example oxides, acids and their salts, and V- containing organic materials, e.g. vanadyl acetylacetonate, vanadyl 3-ethylacetylacetonate, vanadium (V) oxytrialkoxides, bis(cyclopentadienyl) vanadium(II), phenylalkoxyovanadium compounds, bis[(2-methyl-4-oxo-pyran-3-yl)oxy]-oxo-vanadium and the like. Many decavanadate salts have been characterized: NH4+, Ca2+, Ba2+, Sr2+, and group I decavanadate salts may be prepared by the acid-base reaction between V2O5 and the oxide, hydroxide, carbonate, or hydrogen carbonate of the desired positive ion. Suitable decavanadate post-treatments include: Vanadium acetyl acetonate, (NH4)6[V10O28].6H2O, K6[V10O28].9H2O, K6[V10O28].10H2O, Ca3[V1028.16H2O, K2Mg2[V10O28 ].16H2O, K2Zn2[V10O28].16H2O, Cs2Mg2[V10O28.16H2O, Cs4Na2[V10O28].10H2O, K4Na2[V10O28].16H2O, Sr3[V10O28].22H2O, Ba3[V10O28].19H2O, [(C6H5)4P]H3V10O28.4CH3CN and sodium ammonium decavanadate (nominally (NH4)4Na2[V10O28]). Suitable vanadium containing compositions according to this invention comprise, consist essentially of, or preferably consist of, water and vanadate ions, particularly decavanadate ions. The concentration of vanadium atoms present in vanadate ions in the composition according to this invention preferably is, with increasing preference in the order given, at least 0.0005, 0.001, 0.002, 0.004, 0.007, 0.012, 0.020, 0.030, 0.040, 0.050, 0.055, 0.060, 0.065, 0.068, 0.070, or 0.071 M and independently preferably is, with increasing preference in the order given, primarily for reasons of economy, not more than 1.0, 0.5, 0.30, 0.20, 0.15, 0.12, 0.10, 0.090, 0.080, 0.077, 0.074, or 0.072 M. The temperature of such a post-treating composition, during contact with the inorganic-based coating on the magnesium containing article as described above preferably is, with increasing preference in the order given, at least 30° C., 35° C., 40° C., 45° C., 48° C., 51° C., 53° C., 55° C., 56° C., 57° C., 58° C. or 59° C. and independently preferably is, with increasing preference in the order given, not more than 90° C., 80° C., 75° C., 72° C., 69° C., 67° C., 65° C., 63° C., 62° C. or 61° C. At 60° C., the time of contact between the vanadium containing composition and the inorganic-based coating on the magnesium containing article as described above preferably is, with increasing preference in the order given, not less than 0.5, 1.0, 2.0, 2.5, 3.0, 3.5, 4.0, 4.3, 4.6, or 4.9 min and independently preferably is, with increasing preference in the order given, primarily for reasons of economy, not greater than 60, 30, 15, 12, 10, 8, 7.0, 6.5, 6.0, 5.7, 5.4, or 5.1 min.

At least one vanadium containing composition is desirably introduced to the second sub-layer of inorganic-based coating, contacting at least the external surfaces and desirably at least some of the internal surfaces thereof. The second component may comprise the vanadium containing composition and/or may comprise reaction products of the vanadium containing composition and elements of the inorganic-based coating. In one embodiment, the vanadium containing composition reacts with elements of the inorganic-based coating to thereby form a second component, which is different from the inorganic-based coating at least in that the second component comprises vanadium. The second component may form a thin film in contact with the external surfaces of the inorganic-based coating and lining at least a portion of the pores in the inorganic-based coating.

In some embodiments, the vanadium containing compositions may also contact internal surfaces of the inorganic-based coating and/or react with elements of the internal surfaces rendering the inorganic-based coating more resistant to corrosion producing agents reaching the magnesium containing surface. Vanadium may further infiltrate the inorganic-based coating such that vanadium is distributed throughout at least a portion of the inorganic-based coating. Analysis of inorganic-based coatings according to the invention that have been contacted with a vanadium containing composition showed the presence of vanadium in the inorganic-based coating matrix. Depth of penetration of vanadium second components into the inorganic-based coating matrix may include up to 100, 90, 80, 70, 65, 60, 55 or 50% of total thickness of the porous second sub-layer of the inorganic-based coating, said total thickness being measured from the second interface to the external surface of the inorganic-based coating.

In some embodiments, the vanadium containing composition may be reactive with elements in the inorganic-based coating. Contacting the inorganic-based coating with a vanadium-containing composition provides improved corrosion resistance and does not cover up the pores in external surfaces of the inorganic-based coating. This is beneficial if a subsequent paint step is to be used because the pores provide anchoring sites for adhering paint to the surface.

Another post-treatment step which may be employed is depositing an additional layer comprising a polymer, preferably this may be done using a thermosetting resin which may or may not react with the inorganic-based coating. Average thickness of the polymeric second layer, as measured from an external surface of the inorganic-based coating to an outer surface of the second layer, may range from, in order of increasing preference, at least about 0.1, 0.25, 0.5, 0.75, 1, 2, 3, 4, or 5 microns and in order of increasing preference, not more than about 14, 12, 10, 8 or 6 microns. In contrast, typical paint thicknesses are at least 25 microns thick. Use of either a thin polymeric layer, as described above, or a paint, generally covers the pores in the external surfaces of the inorganic-based coating, the pores providing improved adhesion of the polymer or paint and surprisingly resulting in a uniform surface.

Desirably, polymers forming the second layer may comprise organic polymer chains or inorganic polymer chains. Examples of polymers suitable for an additional layer include by way of non-limiting example, silicone, epoxy, phenolic, acrylic, polyurethane, polyester, and polyimide. In one embodiment, organic polymers selected from epoxy, phenolic and polyimide are utilized. Preferred polymers forming additional layers include phenol-formaldehyde-based polymers and copolymers generated from, for example novolac resins, which have a formaldehyde to phenol molar ratio of less than one, and resole resins having a formaldehyde to phenol molar ratio of greater than one. Such polyphenol polymers can be made as is known in the art for example according to U.S. Pat. No. 5,891,952. Novolac resins are desirably used in combination with a crosslinking agent to facilitate curing. In one embodiment, a resole resin having a formaldehyde to phenol molar ratio of about 1.5 is utilized to form a polymer additional layer on the inorganic-based coating. Phenolic resins useful in forming polymeric layers desirably have molecular weights of about 1000 to about 5000 g/mole, preferably 2000 to 4000 g/mole.

At least one of the above-described resins is desirably introduced to the first layer of inorganic-based coating, contacting at least the external surfaces thereof, and crosslinking to thereby form a polymeric layer on external surfaces of the inorganic-based coating. This polymeric second layer is different from the inorganic-based coating and is adhered to the inorganic-based coating.

In some embodiments, the resin may also contact internal surfaces of the inorganic-based coating and upon curing form a polymeric second component that is different from the inorganic-based coating and distributed throughout at least a portion of the inorganic-based coating. Analysis of inorganic-based coatings according to the invention that have been contacted with a resole resin having a formaldehyde to phenol molar ratio of 1.5 showed the polymeric components present in the inorganic-based coating matrix thereby forming a composite coating. Depth of penetration of polymeric second components into the inorganic-based coating matrix may range in increasing order of preference from 1, 2, 5, 10, 15, 20 or 25% and in increasing order of preference may be not more than 70, 65, 60, 55 or 50, 45, 40 or 35% of total thickness of the inorganic-based coating, said total thickness being measured from the first interface to the external surface of the inorganic-based coating.

In some embodiments, the resin may comprise functional groups reactive with elements in the inorganic-based coating, which may form bonds between the resin and the inorganic-based coating. For example, uncured novolac and resole resins comprise OH functional groups which may react with metals in the inorganic-based coating thereby linking the polymer to the inorganic-based coating.

Coated substrates according to the invention are useful in motor vehicles; aircraft and electronics where the combination of the inorganic-based coating and post-treatment layers can provide more corrosion protection than paint or anodizing alone, while ceramic-type hardness of the combination imparts additional toughness to external layers because sharp objects have greater difficulty in deforming a harder substrate prime layer than magnesium, which is relatively soft as compared to ceramic. Coatings according to the invention also can be beneficial in keeping the topcoat gloss and color readings relatively consistent by providing a relatively uniform paint base.

The process and coated articles of the invention provide a more uniform surface layer on magnesium alloys, by way of non-limiting example AZ91 B, AZ91 D and AZ31 B, and magnesium containing surfaces having contamination, which provides improved corrosion resistance.

EXAMPLES

Commercially available magnesium alloy test panels were utilized for all examples. The AZ-31 Mg alloy panels were about 93-97 wt. % Mg, the remainder being made up of Al, Zn, Mn, and less than 0.5 wt. % of other metal and metalloid impurities. The AZ-91 Mg alloy panels had less magnesium, about 87-91 wt. % Mg, with the remainder being made up of Al, Zn, Mn, and less than 1.2 wt. % of other metal and metalloid impurities.

The conditions for electrolytic coating process for the Examples, unless stated otherwise, were an electrolyte bath concentration of 40 grams/liter KF and 5 grams/liter of KOH, temperature was maintained between 20-22 ° C. and the panels and parts were processed for 3 minutes at a current set to reach 160 V (Vmax) within 90 seconds. The term Vmax refers to the time required for the power source for the electrolytic coating process to reach a maximum voltage set for a process run. After coating and removal from the electrolyte, the coated panels and parts were rinsed with deionized water.

All paints were cured per the manufacturer's directions, unless otherwise noted.

Cleaning Steps:

All AZ-31 panels were cleaned in 5% BONDERITE® C-AK 305, an alkaline cleaner commercially available from Henkel Corp., at 60 ° C. for 3 minutes; rinsed with DI water; deoxidized in 3% BONDERITE®C-IC HX-357 at 20-22 ° C. for 90 seconds, which was about a 30 g/m2 etch rate.

All AZ-91 panels were cleaned with Turco® 6849 alkaline cleaner for one minute; rinsed with DI water; deoxidized with a commercially available phosphate based deoxidizer at 20-22 ° C. for 60 seconds; desmutted in with a 25,000 KHz ultrasound bath of 1 gram per liter citric acid.

Example 1

Inorganic-Based Coating with Post-Treatment and No Paint

AZ-31 Mg alloy panels were immersed in an electrolyte bath containing 40 grams/liter KF and 5 grams/liter of KOH. The panels were electrolyzed as the anode using a 25 msec. on and 9 msec off square waveform for about 180 seconds generating an edge-covering, inorganic-based coating. The coated panels were removed from the electrolyte bath and rinsed with DI water for 300 sec. The coating was observed to have a uniform surface appearance and a thickness of 4 microns. The electrolytically deposited inorganic-based coating was not dried. Thereafter, each coated panel was immersed in an aqueous post-treatment containing one of the compositions recited in Table 1. The coated panels were subjected to 3 minutes of immersion time in the post-treatment tank.

The coated and post-treated panels were rinsed with DI water and were allowed to dry. The panels were then subjected to salt fog testing according to ASTM B-117 (2011) and the results after 24 and 168 hours exposure are shown in Table 1.

TABLE 1 Post-treatment study for AZ-31 alloy unpainted salt fog testing Temperature of Time of Exposure in Unpainted Salt Post-treatment Post-treatment Fog Test (ASTM B-117) Concentration (° F.) 2 hours 24 hours 168 hours 0.15 g/L SAVAN 140 Pass Pass Pass 0.15 g/L SAVAN 180 Pass Pass Pass 0.75 g/L SAVAN 140 Pass Pass Pass 0.75 g/L SAVAN 180 Pass Pass Pass  1.2 g/L SAVAN 140 Pass Pass Pass  1.2 g/L SAVAN 180 Pass Pass Pass Hot DI Water 186 Pass Fail Fail 11% Post-treatment 1 130 Pass Fail Fail Post-treatment 2 130 Pass Fail Fail Post-treatment 3 120 Fail Fail Fail Post-treatment containing SAVAN: sodium ammonium decavanadate, Post-treatment 1 was a commercially available calcium containing post-treatment, Post-treatment 2 was a benchmarking solution of 6.1 g/l calcium nitrate and Post-treatment 3 was a benchmarking solution of 0.60 g/l phosphoric acid.

In the above test, “Pass” means that no visible pitting was observed on the panels. This test showed that the post-treatment improved corrosion resistance of the coated panels and appeared to be effective over a range of temperatures and concentrations.

Example 2

Inorganic-Based Coating with Post-Treatment and No Paint

A new set of samples treated according to the procedure of Example 1 were prepared using test panels of a different Mg alloy (AZ-91) having higher levels of impurities. Some samples were post-treated with a second commercially available post-treatment instead of the SAVAN. All panels were tested according to the procedure of Example 1 and the test results are shown in Table 2.

TABLE 2 Post-treatment study for AZ-91 alloy unpainted salt fog testing Time of Exposure Temperature of in Unpainted Salt Post-treatment Post-treatment Fog Test (ASTM B-117) Concentration (° F.) 2 hours 24 hours 0.15 g/L SAVAN 140 Pass Pass 0.15 g/L SAVAN 180 Pass Pass 0.75 g/L SAVAN 140 Pass Fail 0.75 g/L SAVAN 180 Pass Fail  1.2 g/L SAVAN 140 Pass Fail  1.2 g/L SAVAN 180 Pass Fail Control: Hot DI Water 185 Pass Fail 11% Post-treatment 1 130 Pass Fail Post-treatment 2 130 Fail Fail Post-treatment 3 120 Fail Fail In the above test, “Pass” means that no visible pitting was observed on the panels.

Comparing salt spray resistance of the control water rinsed panels of Table 1 and Table 2 shows that Mg alloys having lesser amounts of Mg and/or greater amounts of alloying metals and impurities show corrosion sooner (24 hours) than Mg alloys with higher Mg metal concentrations (168 hours). The test results showed some improvement in corrosion resistance of the coated panels post-treated using a vanadium-containing post-treatment despite the Mg alloy having higher amounts of impurities. As compared to the commercially available post-treatment, the vanadium-containing post-treatment improved performance at some concentrations and was effective over a range of temperatures. Surprisingly, the AZ-91 panels treated with lower concentrations of the vanadium-containing post-treatment performed better in the salt spray test than panels treated with higher concentrations.

Example 3

Pretreatment Comparison in Painted Corrosion Performance

AZ-31 Mg alloy panels, were treated as described in the below Table. All panels had a bare 6061 aluminum skin bonded to the test panel with Terocal 5089 adhesive, commercially available from Henkel Corp. The dissimilar metals were used to set up galvanic reactions in the samples. The panels were scribed through the paint and underlying coatings down to the metal surface and then subjected to 504 hours of salt fog testing according to ASTM B-117. The results are shown in Table 3.

TABLE 3 AZ-31 alloy no post-treat, painted Salt Fog Test (ASTM B-117) Pre-treatment Paint 504 Sample Pre-treatment layer Paint thickness hours Sample 1 Conversion 47.1 g/m2 Clear Topcoat 2 mils Fail (Comparative) Coating 1 Sample 2 Electro-ceramic 7.2 micron Clear Topcoat 2 mils Pass Coating Sample 3 Conversion 47.1 g/m2 Urethane 1.96 mils Fail (Comparative) Coating 1 Sample 4 Electro-ceramic 7.2 micron Urethane 2.39 mils Pass Coating Sample 5 Conversion 47.1 g/m2 Fluoro carbon 2.09 mils Fail (Comparative) Coating 1 polymer Sample 6 Electro-ceramic 7.2 micron Fluoro carbon 1.91 mils Pass Coating polymer Sample 7 Conversion 47.1 g/m2 Polyurethane 1.65 mils Fail (Comparative) Coating 1 Sample 8 Electro-ceramic 7.1 micron Polyurethane 1.65 mils Pass Coating Conversion Coating 1 was a commercially available chromium free conversion coating formulated for treating non-ferrous alloys applied at coating weights customary for these type products. Electroceramic coating was an electrolytically applied inorganic-based coating according to the invention having amorphous two layered structure. Paints were commercially available powder paint: Clear Topcoat acrylic (Akzo) cured at 350° F. for 25 minutes; JAVA Brown Fluoropolymer Interpon D3000, cured for 15 minutes at 400° F.; Urethane PCU 73101 silver (PPG) cured at 375° F. for 25 min.; and Polyurethane Silver (Cardinal) cured at 375° F. for 20 minutes.

Inspection of the samples of Table 3 showed that bond areas between the aluminum skin and the Mg alloy panel coated with the inorganic-based coating showed less than 1% corrosion despite surface corrosion of the uncoated aluminum skins, and no corrosion from the scribed lines. In the Comparative Examples, the aluminum skin and the scribed lines, as well as the underlying Mg alloy panel showed corrosion.

Example 4

Pretreatment Comparison in Painted Corrosion Performance

Magnesium automobile wheel rims were coated as described in Table 4. The rims were scribed as described for Example 1, and then subjected to 1008 hours of salt fog testing according to ASTM B-117 or to 300 hours GM4472 CASS corrosion testing. The CASS test (Copper Accelerated Salt Spray) is a variant on the salt spray test, with the difference being that the solution used is a mixture of sodium chloride, acetic acid and copper chloride (cupro-acetic mixture), the specifics of the test are available online at GM Matspec. The results are shown in Table 4.

TABLE 4 Mg alloy no post-treat, painted Salt Fog Test (ASTM B-117) Pre- Sample treatment Paint Test Results Sample 9 PEO Urethane 1008 hours Fail (Comparative) Coating ASTM B-117 Edge Salt Fog Corrosion Sample 10 Electro- Urethane 1008 hours Pass ceramic ASTM B-117 0-1% Coating Salt Fog corrosion Sample 11 Conversion Urethane 1008 hours Fail (Comparative) Coating 2 ASTM B-117 Paint Salt Fog delaminated Sample 12 Electro- Urethane CASS 300 hours Pass ceramic 0-1% Coating corrosion Sample 13 Conversion Urethane CASS 300 hours Fail (Comparative) Coating 2 Paint delaminated Sample 14 PEO Urethane CASS 300 hours Fail (Comparative) Scribe and lumen corroded PEO Coating was a crystalline MgO-based coating applied using a commercially available plasma electrolytic oxidation process. Conversion Coating 2 was a commercially available chromium free conversion coating formulated for treating Mg having a typical layer thickness of less than 1 μm. Electroceramic coating was an inorganic-based coating electrolytically applied according to the invention having amorphous regions and a two layered structure. Urethane paint utilized was PCU 73101 silver powder paint (PPG) cured at 375° F. for 25 min.

No post-treatments were used in Examples 3 and 4, which were painted with powder paint after pretreatment. Samples 2, 4, 6, 8, 10 & 12 of Examples 3 and 4, having an electrolytically deposited inorganic-based coating according to the invention out performed Comparative Examples, even without a post-treatment. The PEO showed extensive corrosion of the coated metal article in the hollow lumen designed for passage of a wheel stud through the wheel likely due to poor throwing power of the PEO process.

Example 5

Inorganic-Based Coating Process Variations in Painted Corrosion Performance

AZ-91 Mg alloy panels were used for this example and were cleaned as described above. Each of the panels was immersed in one of the electrolyte baths shown in the below table. Fluoride concentration measurement was made with a 101 D meter which measures the fluoride attack on a silicon wafer according to the manufacturer's instructions. The panels were electrolyzed as the anode using a 25 msec. on and 9 msec off square waveform for about 180 seconds. An edge-covering, inorganic-based coating having a uniform surface containing pores, resulted on each of the panels. The coated panels were removed from the electrolyte baths, rinsed with DI water for 240 sec. and allowed to dry. The panels were painted with a liquid paint cured according to manufacturer's specification. The Mg alloy panels coated with the inorganic-based coating and cured layer of paint were tested for corrosion resistance according to ASTM B-117 for 504 hours and were tested for cross-hatch adhesion according to ASTM 3359 method B and the results are shown in the Table 5 below.

TABLE 5 Thick- Salt Panel Ramp ness Fog Sample Alka- Fluoride Time Temp. (mi- B- ASTM Group linity (microamps) (sec). (° F.) crons) 117 3359 5.1 13.0 800 75 68 7.62 N 5 5.2 13.0 800 120 85 8.35 N 5 5.3 15.0 560 91 70 3.68 N 5 5.4 15.0 560 74 85 3.64 N 5 5.5 19.0 430 90 70 2.96 N 5 5.6 19.0 430 76 85 3.1 N 5 5.7 17.0 190 92 68 3.30 N 5 5.8 17.0 190 88 88 3.93 N 5 5.9 14.0 770 42 72 7.3 N 5 5.10 14.0 770 59 88 6.74 N 5 5.11 15.0 610 29 70 4.6 N 5 5.12 15.0 610 61 85 6.13 N 5 “N” means no corrosion visible at the scribe. ASTM 3359 scale is 0 to 5, 5: no removal or peeling, edges of the cuts are smooth and none of the squares of the lattice is detached.

Sample Groups 5.1 to 5.12 having an inorganic-based coating and a layer of paint showed excellent corrosion resistance and paint adhesion across a range of process parameters and coating thicknesses. The above table shows that by controlling the alkalinity, fluoride concentration and temperature of the electrolyte, the ramp time to Vmax can be and coating thickness can be controlled for a given contact time, current and waveform. Using these non-linear relationships Vmax can be reduced thereby increasing throughput of the process without adversely affecting corrosion resistance or paint adhesion.

TABLE 6 Atomic Percent Ex. C O F Al Mg K 5.2 11.20 4.45 43.56 0.72 27 13.07 5.3 15.28 4.46 42.1 2.13 24.59 11.4 5.4 10.9 10.3 39.05 3.11 33.94 2.7 5.5 10.7 7.86 39.34 3.17 35.24 3.64 5.7 8.75 11.32 35.46 4 39.29 1.18 5.8 10.45 11.08 35.1 3.68 38.75 0.94 5.9 5.87 14.7 24.14 6.89 43.78 4.62 Magnesium alloy panels coated according to Example 5, but not painted were analyzed by Energy Dispersive X-ray Spectrometry (EDS). Results showing approximate atomic percent are shown in Table 6 above.

Example 6

Inorganic-Based Coating with an Organic Second Layer

This experiment tested a new set of samples according to the procedure of Example 1 except that the panels were electrolyzed for a time sufficient to generate a uniform, edge-covering, inorganic-based coating, and an organic-based post-treatment was used instead of a post-treatment. The post-treatment used was a resole resin comprising phenol formaldehyde condensate with a degree of polymerization greater than 1.5.

After the inorganic-based coating was applied, the panel was allowed to dry. Thereafter the organic-based post-treatment was applied and dried for 20 minutes at 160 ° C. (320 ° F.). A first set of panels were provided with a post-treatment having a dried thickness of 6 microns, resulting in a total inorganic/organic coating thickness of 12 microns. A second set of panels were provided with a post-treatment having a dried thickness of 10 microns, resulting in a total inorganic/organic coating thickness of 16 microns. All panels were tested for corrosion resistance according to ASTM B-117. After 1000 hours of testing the panels showed no corrosion at the scribe and did not show any field or edge corrosion. These results show significant improvements to corrosion resistance from a composite coating having an inorganic-based layer bonded to the Mg substrate and an aromatic resin based post-treatment as compared to inorganic-based coatings on Mg with no post-treatment, see Ex. 3 & 4. To obtain similar performance with paint over conversion coating or anodized Mg, generally requires approx. 50-150 microns of total film build.

Example 7

Enemy Consumption Testing

Approximately 3 square meters of surface area of magnesium castings were coated with a selected thickness of inorganic-based coating according the invention and electrical consumption was measured to be 2.81 Kilowatt hours (kWh), which is approximately 1 kWh/m2. 3.2 square meters of surface area of wrought magnesium required only about 1.5 kWh, which is about 0.46 kWh/m2. This energy consumption was approximately 20 times less energy consumption than required for generating the same thickness using conventional PEO processes.

Example 8

Bare Inorganic-Based Coating Performance

A new set of samples were provided with an electrolytic coating according to the procedure of Example 1. The unpainted panels were subjected to tests as set forth in the table below, which also shows test results.

Test Results Paint Adhesion (crosshatch) 5A ASTM 3359 method B Thermal Shock Resistance to 5A Delamination Adhesive Bonding >200% increase in shear strength as compared to bare Mg alloy Reverse Impact Resistance Impact force was increased until substrate fracture with no delamination of coating Vickers Hardness 400-900 Vickers Solvent Resistance >100 hours resistance to a boiling 70:30 ethylene glycol to water mixture Thermal Shock Testing comprises baking panels at 550° C. for 60 minutes, removing panels from the oven, immersing the panels in ice water (0° C.) without a cooling step and testing for adhesion using ASTM 3359 method B (crosshatch). Adhesive Bonding was tested by creating specimens from uncoated Mg alloy panels and from panels coated according to Example 8. Each specimen had a lap joint with 1″ overlapping epoxy structural adhesive and 1″ wide shear specimens. Force was applied at a controlled rate to each specimen until the bond at the lapjoint failed and the maximum force was recorded. Reverse Impact Resistance was tested according to ASTM D2794. Vickers hardness measurement was by nanoindentation and appears to be affected by the underlying alloy.

The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and do come within the scope of the invention. Accordingly, the scope of legal protection afforded this invention can only be determined by studying the following claims.

Claims

1. A method of improving corrosion resistance of magnesium containing metal substrates comprising:

A) providing an alkaline electrolyte comprised of water, a source of hydroxide ion, and one or more additional components selected from the group consisting of: water-soluble inorganic fluorides, water-soluble organic fluorides, water-dispersible inorganic fluorides, and water-dispersible organic fluorides and mixtures thereof;
B) providing a cathode in contact with the electrolyte;
C) placing a magnesium containing article having at least one bare metallic magnesium or magnesium alloy surface in contact with the electrolyte and electrically connected thereto such that said surface acts as an anode;
D) passing a current between the anode and cathode through the electrolyte solution for a time effective to generate a first layer of an inorganic-based coating chemically bonded directly to said surface;
E) removing the article having the first layer of an inorganic-based coating from the electrolyte and optionally drying it;
F) optionally post-treating the article having the first layer of an inorganic-based coating by: i. infusing the first layer of an inorganic-based coating with a second component that is different from the inorganic-based coating thereby distributing the second component throughout at least a portion of the inorganic-based coating and/or ii. contacting the first layer of an inorganic-based coating with a polymeric composition thereby forming a second layer comprising organic polymer chains and/or inorganic polymer chains; and
G) optionally applying a layer of paint after the post-treating step.

2. The method of claim 1 wherein said method is performed in the absence of any step prior to step D) that deposits silicate and/or fluoride on the magnesium surface.

3. The method of claim 2 further comprising performing at least one step selected from cleaning, etching, deoxidizing, desmutting, and combinations thereof prior to placing the magnesium containing article in contact with the electrolyte such that prior to generating the first layer, from 0.5 to 50 g/m2 of metal is removed from the bare metallic magnesium or magnesium alloy surface.

4. The method of claim 1 comprising masking portions of the magnesium containing article prior to placing the magnesium containing article in contact with the electrolyte.

5. The method of claim 1 comprising controlling temperature and concentration of the electrolyte and time and waveform of the current in step D) to thereby produce the inorganic-based coating at a thickness of 1-20 microns and comprises carbon, oxygen, fluoride, magnesium and aluminum.

6. The method of claim 5 wherein forming the first layer in step D) utilizes less than 10 kWh per square meter of the magnesium containing surface coated.

7. The method of claim 1 wherein after step E), no more than 10 mg/m2 of the inorganic-based coating is removed.

8. The method of claim 1 wherein said current is pulsed direct current having an average voltage in a range of 50 to 600 volts

9. The method of claim 5 wherein the oxygen has a ratio to the fluorine in the inorganic-based coating that exhibits a concentration gradient wherein amount of oxygen relative to amount of fluorine increases as a function of distance from the magnesium-containing article's metal surface.

10. The method of claim 5 wherein the inorganic-based coating deposited in step D) has a bilayer structure, comprising:

a. a first sub-layer directly bonded to the bare metallic magnesium or magnesium alloy surface at a first interface, said first sub-layer comprising at least 70 wt. % of a combined mass of fluorine and magnesium, and a positive amount of oxygen present in an amount of less than about 25 wt. %;
b. a second sub-layer integrally connected to the first sub-layer, said second sub-layer comprising external surfaces at the outer boundary of the inorganic-based coating, and internal surfaces defined by pores in the second sub-layer lying interior to the outer boundary of the inorganic-based coating and in communication therewith, said second sub-layer having a composition wherein: first sub-layer Mg wt. % >second sub-layer Mg wt. % first sub-layer F wt. % >second sub-layer F wt. % first sub-layer O wt. % <second sub-layer O wt. %.

11. The method of claim 1 wherein the post-treating step F) is present as a step of contacting a matrix of the first layer of inorganic-based coating with a second component different from the inorganic-based coating; distributing the second component throughout at least a portion of the matrix; and depositing a second layer that is different from the inorganic-based coating and is adhered to at least external surfaces of the inorganic-based coating,

12. The method of claim 10 wherein step F) i) is present and comprises a step of introducing at least one vanadium containing composition as the second component to the second sub-layer of inorganic-based coating, contacting at least the external surfaces and desirably at least some of the internal surfaces of the second sub-layer, whereby said second component forms a thin film in contact with the external surfaces of the inorganic-based coating and lining at least a portion of the pores in the inorganic-based coating.

13. The method of claim 12 wherein the infusing step comprises reacting the vanadium containing composition and elements of the inorganic-based coating to thereby form a portion of the second component, which is different from the inorganic-based coating and the vanadium containing composition.

14. The method of claim 1 wherein step F) ii) is present and comprises contacting the first layer of an inorganic-based coating with a polymeric composition thereby forming a second layer comprising organic polymer chains and/or inorganic polymer chains; and optionally applying a layer of paint after the post-treating step.

15. A magnesium-containing article comprising at least one metallic magnesium or magnesium alloy surface coated according to claim 1.

16. A magnesium-containing article comprising at least one metallic magnesium or magnesium alloy surface coated with a first layer of an inorganic-based coating chemically bonded directly to said surface wherein the inorganic-based coating has a bilayer structure, comprising:

a. a first sub-layer directly bonded to the bare metallic magnesium or magnesium alloy surface at a first interface, said first sub-layer comprising at least 70 wt. % of a combined mass of fluorine and magnesium, and a positive amount of oxygen present in an average amount of less than about 20 wt. %;
b. a second sub-layer integrally connected to the first sub-layer, said second sub-layer comprising external surfaces at the outer boundary of the inorganic-based coating, and internal surfaces defined by pores in the second sub-layer lying interior to the outer boundary of the inorganic-based coating and in communication therewith, said second sub-layer comprising carbon, oxygen, fluoride, magnesium and aluminum, said oxygen present in the inorganic-based coating second sub-layer in an average amount of greater than about 25 wt. %

17. A magnesium-containing article having a composite coating comprising:

a. a matrix formed by a first layer of an inorganic-based coating chemically bound directly to at least one metallic magnesium or magnesium alloy surface, said matrix having pores and internal surfaces defined by pores, at least some of said pores being in communication with an external surface of the first layer and forming openings therein; and
b. a second component, different from the inorganic-based coating, distributed throughout at least a portion of the matrix comprising the pores, said second component being in contact with at least some of the internal surfaces and external surfaces.

18. The magnesium-containing article of claim 17 further comprising a second layer that is different from the inorganic-based coating and is adhered to at least external surfaces of the inorganic-based coating.

Patent History
Publication number: 20170121841
Type: Application
Filed: Jan 13, 2017
Publication Date: May 4, 2017
Inventors: Shawn E. Dolan (Sterling Heights, MI), Kirk Kramer (West Bloomfield, MI), Lisa K. Salet (Sterling Heights, MI), Michael A. Murphy, Jr. (Sterling Heights, MI)
Application Number: 15/405,774
Classifications
International Classification: C25D 11/30 (20060101); C25D 11/02 (20060101);