Patent Publication Number: US-2021172067-A1

Title: Production method for magnesium-containing metal material provided with coating

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 16/623,803 submitted Dec. 18, 2019, which is the U.S. National Stage of International Application No. PCT/JP2018/023448 filed Jun. 20, 2018, which claims the priority benefit of JP Application No. 2017-122643 filed Jun. 22, 2017, the entire respective disclosures of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a magnesium material or a magnesium alloy material, on a surface of which coatings having corrosion resistance are formed (hereinafter, the magnesium material, the magnesium alloy material or the like is referred to as “magnesium-containing metal material”). 
     BACKGROUND ART 
     Magnesium-containing metal materials have a low specific gravity and, because of their lightweight nature, it has been examined to apply magnesium-containing metal materials as structural materials of airplanes, automobiles, bicycles, home electric appliances, medical instruments, fishing gears and the like. However, since magnesium-containing metal materials are highly corrosive, it is necessary to improve their corrosion resistance by performing some sort of surface treatment. 
     Conventionally, a variety of methods have been developed as surface treatments for improving the corrosion resistance. For example, Patent Document 1 proposes a surface treatment method where a magnesium or magnesium alloy base material molded into a prescribed shape is immersed in an aqueous solution in which phosphate ions and non-chlorinated calcium ions are dissolved in a supersaturated state, whereby a bioabsorbable coating containing apatite crystals as a main component is deposited on the surface of the base material. 
     PRIOR ART DOCUMENT 
     Patent Document 
     [Patent Document 1] JP 2010-148682A 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     The coating obtained by the surface treatment method disclosed in Patent Document 1 does not have a sufficient corrosion resistance. Therefore, an object of the present invention is to provide a magnesium-containing metal material and the like that include coatings having excellent corrosion resistance on a surface. 
     Means for Solving the Problems 
     The present inventors intensively studied to solve the above-described problem and consequently discovered that a magnesium-containing metal material which includes, on a surface thereof, a magnesium hydroxide-containing coating, a dibasic calcium phosphate-containing coating and a hydroxyapatite and/or hydroxyapatite carbonate-containing coating in the order mentioned, has excellent corrosion resistance, thereby completing the present invention. 
     That is, the present invention encompasses, for example: 
     (1) A magnesium-containing metal material with coating, including: a magnesium hydroxide-containing first coating on a surface of a magnesium-containing metal material composed of magnesium or a magnesium alloy; a hydroxyapatite and/or hydroxyapatite carbonate-containing third coating over the first coating; and a dibasic calcium phosphate-containing second coating between the first coating and the third coating, and 
     (2) The magnesium-containing metal material with coating according to (1), wherein the second coating is a coating containing monetite and/or brushite. 
     Effects of the Invention 
     According to the present invention, a magnesium-containing metal material that includes coatings having excellent corrosion resistance on a surface can be provided. 
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     A magnesium-containing metal material with coating according to one embodiment of the present invention, and a method of producing the same will now be described in detail. 
     &lt;Magnesium-Containing Metal Material with Coating&gt; 
     The magnesium-containing metal material with coating according to one embodiment of the present invention includes: a magnesium hydroxide-containing first coating on a surface of a magnesium-containing metal material; a hydroxyapatite and/or hydroxyapatite carbonate-containing third coating over the first coating; and a dibasic calcium phosphate-containing second coating between the first coating and the third coating. This magnesium-containing metal material with coating has excellent corrosion resistance. 
     (Magnesium-Containing Metal Material) 
     The magnesium-containing metal material to be treated with the above-described coatings is a metal material that contains magnesium as a main component, such as a magnesium material or a magnesium alloy material. In the case of a magnesium alloy material composed of two metal components, the magnesium alloy material is satisfactory as long as it contains magnesium in an amount of not less than 50% by weight, preferably in an amount of not less than 80% by weight. Further, in the case of a magnesium alloy material composed of three or more metal components, the magnesium alloy material is satisfactory as long as it contains magnesium in the largest amount among the metal components. Examples of the type of the magnesium alloy material include AZ91, AM60, ZK51, ZK61, AZ31, AZ61, and ZK60. 
     (Magnesium Hydroxide-Containing Coating) 
     The first coating according to the present embodiment is not particularly restricted as long as it contains magnesium hydroxide, and the first coating can further contain a metal component, such as aluminum, zinc, or zirconium. The magnesium hydroxide can be crystalline magnesium hydroxide and/or amorphous magnesium hydroxide; however, the first coating preferably contains crystalline magnesium hydroxide. When the first coating contains both crystalline magnesium hydroxide and amorphous magnesium hydroxide, the content ratio thereof is not particularly restricted. It is noted here that the presence or absence of these components in the coating can be verified by X-ray diffractometry (XRD). 
     The thickness of the first coating is not particularly restricted. The thickness of the first coating is usually 0.1 μm or greater and can be 1 μm or greater, but is usually 100 μm or less and can be 30 μm or less, or 20 μm or less. The coating thickness can be determined by observing a cross-sectional shape of the coating under a scanning electron microscope (SEM). 
     (Dibasic Calcium Phosphate-Containing Coating) 
     The second coating according to the present embodiment is not particularly restricted as long as it contains dibasic calcium phosphate. Dibasic calcium phosphate crystals contained in this coating can be monetite and/or brushite. The coating preferably contains brushite. When the coating contains both monetite and brushite, the content ratio thereof is not particularly restricted. It is noted here that the presence or absence of a dibasic calcium phosphate crystal in the coating can be verified by X-ray diffractometry (XRD). 
     In the second coating, the average primary particle size of the crystalline particles of dibasic calcium phosphate is not particularly restricted; however, it is usually 0.7 μm or larger and can be 3 μm or larger, but is 100 μm or smaller and can be 30 μm or smaller, or 10 μm or smaller. The average primary particle size can be determined by observation under a scanning electron microscope. Specifically, a maximum diameter and a minimum diameter are measured for each of randomly selected 100 crystalline dibasic calcium phosphate particles and, from the thus obtained 200 data, an average value of the particle sizes is calculated as the average primary particle size. 
     The thickness of the second coating is not particularly restricted. The thickness of the second coating is usually 0.01 μm or greater and can be 1 μm or greater, or 2 μm or greater, but is usually 100 μm or less and can be 25 μm or less, or 20 μm or less. The coating thickness can be determined by observing a cross-sectional shape of the coating under a scanning electron microscope (SEM). 
     (Hydroxyapatite and/or Hydroxyapatite Carbonate-Containing Coating) 
     The third coating according to the present embodiment is not particularly restricted as long as it contains hydroxyapatite and/or hydroxyapatite carbonate. When the third coating contains both hydroxyapatite and hydroxyapatite carbonate, the content ratio thereof is not particularly restricted. It is noted here that the presence or absence of hydroxyapatite and/or hydroxyapatite carbonate in the coating can be verified by X-ray diffractometry (XRD). 
     &lt;Method of Producing Magnesium-Containing Metal Material with Coating&gt; 
     The magnesium-containing metal material with coating according to the present embodiment can be produced by, for example, a method including: the first step of forming the first coating on a surface of a magnesium-containing metal material; the second step of forming the second coating over the first coating; and the third step of forming the third coating by converting some or all of dibasic calcium phosphate crystals on the surface of the second coating into hydroxyapatite and/or hydroxyapatite carbonate. The magnesium-containing metal material with coating according to the present embodiment can also be produced by a method that includes the step of forming the third coating on the second coating after the above-described second step. 
     (First Step) 
     Examples of a method of forming the first coating on a surface of a magnesium-containing metal material include, but not limited to: known methods such as a steam treatment method (also referred to as, for example, “hydrothermal treatment” or “magnesium hydroxide coating-forming treatment”). When the first step is performed by a steam treatment method, the duration of the treatment with steam can be usually 1 minute or longer, 30 minutes or longer, or 60 minutes or longer, but usually 1,440 minutes or shorter, 600 minutes or shorter, or 300 minutes or shorter. 
     (Second Step) 
     Examples of a method of forming the second coating over the first coating include, but not limited to: a chemical conversion treatment method of bringing an aqueous solution containing phosphate ions and calcium ions (chemical conversion treatment agent) into contact with the surface of the magnesium-containing metal material having the first coating. Examples of a supply source of the phosphate ions include phosphoric acid and water-soluble phosphates. Examples of a supply source of the calcium ions include calcium hydroxide, calcium carbonate, and calcium nitrate. 
     The calcium ion concentration and the phosphate ion concentration of the chemical conversion treatment agent are not particularly restricted as long as a chemical conversion containing dibasic calcium phosphate crystals can be formed on the magnesium-containing metal material. The phosphate ion concentration is usually 500 ppm or higher and can be 1,000 ppm or higher, but is usually 20,000 ppm or lower and can be 10,000 ppm or lower. The calcium ion concentration is usually 100 ppm or higher and can be 500 ppm or higher, but is usually 10,000 ppm or lower and can be 5,000 ppm or lower. 
     The pH of the chemical conversion treatment agent is usually 2.0 or higher and can be 3.0 or higher, or 4.0 or higher, but is usually 5.0 or lower and can be 4.5 or lower. A pH modifier used for adjusting the pH of the chemical conversion treatment agent is not particularly restricted, and an acid component such as nitric acid, phosphoric acid or sulfuric acid, or an alkali component such as sodium hydroxide, sodium carbonate, aqueous ammonia or ammonium bicarbonate can be used. 
     A method of bringing the chemical conversion treatment agent into contact is not particularly restricted, and examples thereof include a spray treatment method, an immersion treatment method, an electrolytic treatment method, and a pouring method. A contact temperature of the chemical conversion treatment agent is not particularly restricted, and it is usually 10° C. or higher and can be 40° C. or higher, or 70° C. or higher, but is usually 100° C. or lower and can be 90° C. or lower. 
     A contact time of the chemical conversion treatment agent is also not particularly restricted, and it is usually 1 minute or longer and can be 3 minutes or longer, or 5 minutes or longer, but is usually 60 minutes or shorter and can be 30 minutes or shorter, or 15 minutes or shorter. 
     &lt;Third Step&gt; 
     Examples of a method of forming the third coating by converting some or all of dibasic calcium phosphate crystals on the surface of the second coating into hydroxyapatite and/or hydroxyapatite carbonate include, but not limited to: a method of bringing an aqueous alkaline solution into contact with the surface of the second coating. An alkali component contained in the aqueous alkaline solution is not particularly restricted, and examples thereof include lithium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide, lithium carbonate, sodium carbonate, potassium carbonate, ammonium carbonate, sodium bicarbonate, potassium bicarbonate, ammonium bicarbonate, potassium sodium carbonate, and calcium carbonate. These alkali components can be used singly, or in combination of two or more thereof. Further, as the aqueous alkaline solution, an aqueous solution containing lithium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide or the like, in which carbon dioxide gas has been dissolved, can be used as well. 
     The alkali component concentration in the aqueous alkaline solution is not particularly restricted; however, it is usually 0.01 g/L or higher and can be 1 g/L or higher, but is usually 2,000 g/L or lower and can be 500 g/L or lower. The pH of the aqueous alkaline solution is usually 7.5 or higher and can be 8.0 or higher. 
     A method of bringing the aqueous alkaline solution into contact is not particularly restricted, and examples thereof include a coating treatment method, a spray treatment method, an immersion treatment method, and a pouring method. When the magnesium-containing metal material having the first and the second coatings is immersed in an aqueous alkaline solution containing lithium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide or the like, this immersion treatment can be performed while blowing carbon dioxide gas into the aqueous alkaline solution. 
     A contact temperature of the aqueous alkaline solution is not particularly restricted, and it is usually 10° C. or higher and can be 30° C. or higher, but is usually 140° C. or lower and can be 100° C. or lower. A contact time of the aqueous alkaline solution is also not particularly restricted, and it is usually 1 second or longer and can be 1 minute or longer, but is usually 360 minutes or shorter and can be 30 minutes or shorter. 
     In the method of producing the magnesium-containing metal material with coating according to the present embodiment, the step of forming the third coating on the second coating can be performed after the above-described second step, in place of the above-described third step. A method of forming the third coating is not particularly restricted as long as it is a known method, and examples thereof include a method of electrically heating the magnesium-containing metal material in a calcium phosphate solution, and a method of irradiating the magnesium-containing metal material with hydroxyapatite powder using an ultrafine particle beam. 
     Further, in the above-described method of producing the magnesium-containing metal material with coating, prior to the first step, a pretreatment step, such as solvent washing, alkaline washing, degreasing, acid pickling, etching, desmutting, paper polishing or lapping, can be performed on the surface of the magnesium-containing metal material, or two or more of such pretreatment steps can be performed sequentially. By these pretreatment steps, an oxide on the magnesium-containing metal material as well as oil, dirt and the like adhering to the magnesium-containing metal material can be removed to clean the surface. 
     Moreover, in the above-described method of producing the magnesium-containing metal material with coating, after the first step but prior to the second step, the surface conditioning step can be performed using a surface conditioner so as to efficiently form the second coating. 
     The surface conditioning step is the step of bringing a surface conditioner into contact with the magnesium-containing metal material having the first coating. Examples of a method of bringing the surface conditioner into contact include a spray coating method, a dip coating method, a roll coating method, a curtain coating method, a spin coating method, and appropriate combinations of these methods. 
     A contact temperature of the surface conditioner is the temperature of the surface conditioner or the temperature of the magnesium-containing metal material having the first coating, which is usually 0° C. or higher and can be 10° C. or higher, but is usually 40° C. or lower and can be 30° C. or lower. 
     A contact time of the surface conditioner is usually 1 second or longer and can be 5 seconds or longer, or 10 seconds or longer, but is usually 10 minutes or shorter and can be 5 minutes or shorter, 3 minutes or shorter, or 1 minute or shorter. 
     It is noted here that the washing step with water can be performed after the pretreatment step, the first step, the surface conditioning step, the second step, the third step and/or the like. If necessary, the drying step can also be performed as appropriate after each washing step. 
     &lt;Surface Conditioner&gt; 
     The above-described surface conditioner contains dibasic calcium phosphate particles having a specific particle size. 
     The surface conditioner according to the present embodiment can also contain a solvent and a component(s) other than the dibasic calcium phosphate particles as long as the effects of the present invention are exerted, or the surface conditioner can consist of only a solvent and the dibasic calcium phosphate particles. 
     (Dibasic Calcium Phosphate Particles) 
     Dibasic calcium phosphate is also called “calcium monohydrogen phosphate”. Dibasic calcium phosphate exists in the forms of an anhydrate (CaHPO 4 ) and a dihydrate (CaHPO 4 .2H 2 O), and the anhydrate and the dihydrate are called “monetite” and “brushite”, respectively. 
     The surface conditioner according to the present embodiment can contain only either of monetite and brushite, or can contain both of monetite and brushite. When the surface conditioner contains both of monetite and brushite, their content ratio is not particularly restricted. 
     The dibasic calcium phosphate can be crystalline dibasic calcium phosphate or amorphous dibasic calcium phosphate; however, crystalline dibasic calcium phosphate is usually used. 
     As the dibasic calcium phosphate, a commercially available product can be used, or the dibasic calcium phosphate can be produced from a phosphoric acid material and a calcium material. With regard to a method of producing dibasic calcium phosphate, for example, dibasic calcium phosphate can be obtained by allowing an aqueous phosphoric acid solution to react with a calcium material such as calcium carbonate or calcium hydroxide, and adjusting the pH of the resultant to be 4 to 5. In this process, monetite is obtained by controlling the reaction temperature to be at least 80° C. or higher, while brushite is obtained by controlling the reaction temperature to be at least 60° C. or lower. It is noted here, however, that monetite can be obtained even when the reaction temperature is lower than 80° C., and brushite can be obtained even when the reaction temperature is higher than 60° C. 
     The dibasic calcium phosphate particles usually have a D 50  of 0.1 μm or larger, and can have a D 50  of 0.2 μm or larger, or 0.3 μm or larger. The upper limit of the D 50  is usually 0.8 μm or smaller, and can be 0.6 μm or smaller, or 0.5 μm or smaller. 
     Further, the dibasic calcium phosphate particles usually have a D 90  of 0.15 μm or larger, and can have a D 90  of 0.2 μm or larger, or 0.3 μm or larger. The upper limit of the D 90  is usually 1.5 μm or smaller, and can be 1.2 μm or smaller, or 1.0 μm or smaller. 
     The D 50  and the D 90  represent the particle sizes at 50% by volume and 90% by volume, respectively, on a cumulative curve of particles that is determined by taking a total volume of the dibasic calcium phosphate particles in the surface conditioner as 100%. The particle size distribution of the dibasic calcium phosphate particles in the surface conditioner can be determined by, for example, analyzing the intensity of light scattering from the particles irradiated with a laser beam and a diffraction image produced on a focal plane by collecting the light through a lens. Further, from the thus obtained particle size distribution, the particles sizes at 50% by volume and 90% by volume can be determined. 
     The size of the dibasic calcium phosphate particles can be adjusted by a conventional method, such as a wet pulverization method. More specifically, the size of the dibasic calcium phosphate particles can be adjusted by pulverizing a mixture of water, a dispersant and the dibasic calcium phosphate particles using a bead mill. The mass concentration of the dibasic calcium phosphate particles in the mixture is not particularly restricted; however, it is preferably 5 to 50% by weight. 
     (Dispersant) 
     Examples of the dispersant include: monosaccharides, polysaccharides, and derivatives thereof; normal phosphoric acid, polyphosphoric acids, and salts thereof, as well as organic phosphonic acid compounds and salts thereof; water-soluble polymer compounds composed of a vinyl acetate polymer or a derivative thereof, or a copolymer of vinyl acetate and a monomer copolymerizable with vinyl acetate; and polymers and copolymers that are obtained by polymerizing at least one monomer selected from monomers represented by Formula: H 2 C═C(R 1 )—COOR 2  (wherein, R 1  represents H or CH 3 , and R 2  represents H, a C 1 -C 5  alkyl group or a C 1 -C 5  hydroxyalkyl group) and α,β-unsaturated carboxylic acid monomers with 50% by weight or less of a monomer copolymerizable with the above-described monomers. 
     Examples of basic constituent saccharides of the above-described monosaccharides, polysaccharides and derivatives thereof include fructose, tagatose, psicose, thulbose, erythrose, threose, ribose, arabinose, xylose, lyxose, allose, altrose, glucose, mannose, gulose, idose, galactose, and talose. 
     As monosaccharides, the above-exemplified basic constitutent saccharides themselves can be used and, as polysaccharides, homopolysaccharides or heteropolysaccharides of the above-exemplified basic constituent saccharides can be used. Further, as derivatives thereof, it is possible to use monosaccharides obtained by esterifying hydroxy groups of the basic constituent saccharides with substituents such as NO 2 , CH 3 , C 2 H 4 OH, CH 2 CH(OH)CH 3  and CH 2 COOH, and homopolysaccharides and heteropolysaccharides whose structures contain a monosaccharide substituted with any of the above-described substituents. Moreover, several kinds of these monosaccharides, polysaccharides and derivatives thereof can be used in combination. 
     In the classification of saccharides, saccharides are sometimes classified into monosaccharides, oligosaccharides and polysaccharides based on the degree of hydrolysis; however, in the present invention, saccharides that yield two or more monosaccharides through hydrolysis are defined as polysaccharides, and saccharides that are themselves not hydrolysable any further are defined as monosaccharides. 
     In one embodiment of the present invention, the steric configurations (D-configuration/L-configuration) and the optical rotations (+/−) of monosaccharides and those of monosaccharides constituting oligosaccharides and polysaccharides are not particularly restricted, and the steric configurations and the optical rotations of monosaccharides constituting oligosaccharides and polysaccharides can be entirely or partially the same, or can be entirely different. Further, in order to improve the water solubility of monosaccharides, polysaccharides and derivatives thereof, sodium salts or ammonium salts of the above-described monosaccharides, polysaccharides and derivatives thereof can be used. Moreover, when it is difficult to solubilize a saccharide having the above-described structure in water, the saccharide can be dissolved in a water-compatible organic solvent in advance before being used. 
     The above-described normal phosphoric acid is orthophosphoric acid. Examples of the polyphosphoric acids that can be used include pyrophosphoric acid, triphosphoric acid, trimetaphosphoric acid, tetrametaphosphoric acid, hexametaphosphoric acid, and sodium salts and ammonium salts of these polyphosphoric acids. Examples of the organic phosphonic acid compounds that can be used include aminotrimethylene phosphonic acid, 1-hydroxyethylidene-1,1-diphosphonic acid, ethylenediamine tetramethylene phosphonic acid, diethylene triamine pentamethylene phosphonic acid, and sodium salts thereof. It is noted here that the above-described normal phosphoric acid, polyphosphoric acids and organic phosphonic acid compounds can be used singly, or in combination of two or more thereof. 
     Examples of the vinyl acetate polymer or derivative thereof that can be used include polyvinyl alcohols that are saponified products of vinyl acetate polymers; cyanoethylated polyvinyl alcohols obtained by cyanoethylating polyvinyl alcohols with acrylonitrile; formalized polyvinyl alcohols obtained by acetalizing polyvinyl alcohols with formalin; urethanized polyvinyl alcohols obtained by urethanizing polyvinyl alcohols with urea; and water-soluble polymer compounds obtained by introducing a carboxyl group, a sulfone group, an amide group or the like into polyvinyl alcohols. The term “water-soluble” used herein means a property that not less than 0.1 g of a substance dissolves in 100 g of water at 25° C., or a property that a mixture of the substance and water is transparent (the same applies hereinafter). Further, examples of the monomer copolymerizable with vinyl acetate that can be used in the present invention include acrylic acid, crotonic acid, and maleic anhydride. 
     The above-described vinyl acetate polymer or derivative thereof, or copolymer of vinyl acetate and a monomer copolymerizable with vinyl acetate is not restricted as long as it is soluble in water. Accordingly, the effects thereof are not influenced by its polymerization degree and functional group introduction rate. It is noted here that the above-described vinyl acetate polymer or derivative thereof, or copolymer can be used singly, or in combination of two or more thereof. 
     As the monomers represented by the above-described Formula, for example, methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, pentyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, pentyl methacrylate, hydroxymethyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl acrylate, hydroxypentyl acrylate, hydroxymethyl methacrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, hydroxybutyl methacrylate, and hydroxypentyl methacrylate can be used. 
     Further, as the α,β-unsaturated carboxylic acid monomers, for example, acrylic acid, methacrylic acid, and maleic acid can be used. As the monomer copolymerizable with the above-described monomers, for example, vinyl acetate, styrene, vinyl chloride, and vinyl sulfonic acid can be used. Alternatively, a polymer obtained by polymerizing one of the above-described monomers can be used. Further, a copolymer obtained by polymerizing some of the above-described monomers in combination can be used as well. 
     (Solvent) 
     The solvent is not particularly restricted as long as dibasic calcium phosphate can be preferably dispersed therein; however, water is usually used. An organic solvent can be added to water and, in that case, the content of the organic solvent with respect to a whole amount of the solvent is usually 10% by weight or less, and can be 5% by weight or less, or 3% by weight or less. 
     The type of the organic solvent is not particularly restricted, and examples thereof include alcohol-based organic solvents, hydrocarbon-based organic solvents, ketone-based organic solvents, and amide-based organic solvents. 
     The content of the dibasic calcium phosphate particles in the surface conditioner is usually not less than 0.05 g/L and can be not less than 0.1 g/L, in terms of solid concentration. The upper limit of the content is usually 20 g/L or less, and can be 10 g/L or less, or 5 g/L or less. As long as the content is within this range, the dispersibility of the dibasic calcium phosphate particles in the surface conditioner is favorable. 
     (Other Components) 
     The surface conditioner can contain a thickening agent, a dispersion stability improver, a pH modifier and the like as required. 
     The thickening agent can ensure the dispersibility of the dibasic calcium phosphate particles in the surface conditioner to inhibit caking caused by sedimentation of the dibasic calcium phosphate particles. The type of the thickening agent is not particularly restricted, and examples thereof include: natural polymers, such as proteins, natural rubbers, sugars (including sugar derivatives), alginates, and celluloses; synthetic polymers, such as amine-based resins, carboxylic acid-based resins, olefin-based resins, ester-based resins, urethane-based resins, PVAs, acrylic (methacrylic) resins, and copolymers obtained by copolymerizing two or more kinds of these resins in combination; various surfactants, such as nonionic surfactants, cationic surfactants, anionic surfactants, and amphoteric surfactants; and various coupling agents, such as silane coupling agents and titanium coupling agents. 
     From the standpoint of maintaining favorable dispersibility, the content of the thickening agent in the surface conditioner is usually not less than 0.1% by weight, and can be not less than 0.5% by weight. The upper limit of the content is usually 20% by weight or less, and can be 10% by weight or less. 
     The dispersion stability improver is an agent that enhances the dispersion stability of the dibasic calcium phosphate particles in the surface conditioner. Examples of the dispersion stability improver include condensed alkali phosphates, such as sodium polyphosphate, potassium polyphosphate, sodium metaphosphate, potassium metaphosphate, sodium pyrophosphate, and potassium pyrophosphate. 
     The pH modifier is an agent that adjusts the pH of the surface conditioner to be in a prescribed range. The type of the pH modifier is not particularly restricted, and examples thereof include: phosphate-based alkaline additives, such as disodium phosphate hydrates, dipotassium phosphate, dimagnesium phosphate hydrates, and diammonium phosphate; and carbonate-based alkaline additives, such as sodium carbonate, potassium carbonate, basic magnesium carbonate hydrates, ammonium bicarbonate, and calcium carbonate. 
     The pH of the surface conditioner is usually adjusted to be 6 to 11. The lower limit thereof can be 7 or higher, while the upper limit can be 10 or lower, or 9 or lower. In the present specification, a pH indicates a value obtained by measuring the surface conditioner at 25° C. using a commercially available pH meter. 
     It is noted here that, in the surface conditioning step of the present embodiment, a known surface conditioner, such as an aqueous colloid solution of titanium phosphate or an aqueous solution containing phosphate ions and zinc ions that are used for a zinc phosphate coating treatment, can be used in place of the above-described surface conditioner. 
     &lt;Method of Producing Surface Conditioner&gt; 
     The surface conditioner according to the present embodiment can be produced by, for example, adjusting a mixture, which is obtained by mixing the dibasic calcium phosphate particles in a solvent along with, as required, a dispersant, to have a prescribed pH using a pH modifier, subsequently wet-pulverizing the thus pH-adjusted mixture, and then stirring the mixture and thereby dispersing the dibasic calcium phosphate particles. Alternatively, the surface conditioner according to the present embodiment can be produced by mixing the dibasic calcium phosphate particles, which have been previously adjusted to have a prescribed particle size, in a solvent along with, as required, a dispersant, and subsequently adjusting the resulting mixture to have a prescribed pH using a pH modifier. In the above-described methods, the dispersant and the pH modifier are mixed with the dibasic calcium phosphate particles in advance prior to wet pulverization; however, one of the dispersant and the pH modifier can be mixed with the dibasic calcium phosphate particles prior to the wet pulverization, and the other can be mixed with the dibasic calcium phosphate particles after the wet pulverization. Alternatively, the dispersant and the pH modifier can be mixed with the dibasic calcium phosphate particles after the wet pulverization. 
     An order of adding the materials to the solvent is not particularly restricted, and dibasic calcium phosphate, a dispersant and a pH modifier can be added together at once, or dibasic calcium phosphate can be added to a solvent to which only a dispersant has been added and a pH modifier can be subsequently added thereto as required. 
     Wet pulverization for adjusting the particle size can be performed using, for example, but not limited to, a bead mill. The pulverization time is not particularly restricted, and the wet pulverization can be performed until a desired particle size is attained. 
     EXAMPLES 
     The present invention will now be described in more detail by way of Examples thereof. 
     &lt;Magnesium Material&gt; 
     In the present Examples, a pure magnesium plate material having a purity of 99.9% or higher was used. 
     &lt;Production of Magnesium Material with Coating&gt; 
     (Pretreatment) 
     On a surface of the pure magnesium plate material, a degreasing treatment was performed by spraying thereto an alkali degreasing agent [an aqueous solution obtained by mixing FINE CLEANER MG110E (manufactured by Nihon Parkerizing Co., Ltd.) with water at a concentration of 30 g/L] at 65° C. for 120 seconds, followed by washing with water. Subsequently, the thus degreased surface of the pure magnesium plate material was physically polished with sandpaper while applying deionized water thereto, after which the plate material was washed with deionized water and dried with hot air. 
     On this pretreated pure magnesium plate material, the below-described steam treatment, surface conditioning treatment, chemical conversion treatment and apatite conversion treatment were sequentially performed, whereby test pieces of Examples 1 to 5 were each produced. 
     (Steam Treatment) 
     Using an autoclave, the pretreated pure magnesium plate material was subjected to a steam treatment at each temperature for each time period as shown in Table 1. Subsequently, the thus steam-treated pure magnesium plate material was taken out of the autoclave, washed with deionized water and then dried with hot air, whereby a pure magnesium plate material having a magnesium hydroxide-containing coating was produced. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Steam treatment  
                 Steam treatment  
               
               
                   
                 Test piece 
                 temperature (° C.) 
                 time (min) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Example 1 
                 120 
                 60 
               
               
                   
                 Example 2 
                 125 
                 180 
               
               
                   
                 Example 3 
                 140 
                 60 
               
               
                   
                 Example 4 
                 120 
                 60 
               
               
                   
                 Example 5 
                 140 
                 60 
               
               
                   
                   
               
            
           
         
       
     
     &lt;Surface Conditioning Treatment&gt; 
     The thus obtained pure magnesium plate material having a magnesium hydroxide-containing coating was immersed in a surface conditioner at 25° C. for 30 seconds to perform a surface conditioning treatment. It is noted here that the surface conditioner was prepared as follows. 
     In 55 parts by weight of deionized water, 1 part by weight of carboxymethyl cellulose was dissolved. To this solution, 24 parts by weight of monetite or brushite was added, and the resulting mixture was stirred and then wet-pulverized using a DYNO-MILL pulverizer (1-mmφ alkali glass beads). The particle size distribution of solids in the thus pulverized mixture (a suspension having a solid concentration of 30%) was measured using a MICROTRAC analyzer UPA-EX150 manufactured by Nikkiso Co., Ltd. to determine the values of D 50  and D 90 . As a result, the D 50  and the D 90  were found to be 0.45 μm and 0.9 μm, respectively. 
     To the above-described suspension, sodium pyrophosphate and trisodium phosphate were added to final concentrations of 250 ppm and 200 ppm, respectively, whereby a surface conditioner was prepared. 
     &lt;Chemical Conversion Treatment&gt; 
     The pure magnesium plate material having a magnesium hydroxide-containing coating, which had been subjected to the above-described surface conditioning treatment, was immersed in a chemical conversion treatment agent at 50° C. for 5 minutes to perform a chemical conversion treatment. Subsequently, the thus treated pure magnesium plate material was washed with deionized water and dried with hot air, whereby a pure magnesium plate material having a dibasic calcium phosphate-containing coating formed on a magnesium hydroxide-containing coating was produced. It is noted here that the chemical conversion treatment agent was prepared as follows. 
     In deionized water, 75% phosphoric acid and calcium nitrate tetrahydrate were dissolved to final concentrations of 7 g/L and 12 g/L, respectively, and the pH of the resultant was adjusted to be 3.5 with sodium hydroxide, whereby a chemical conversion treatment agent was prepared. 
     &lt;Apatite Conversion Treatment&gt; 
     The thus obtained pure magnesium plate material having a magnesium hydroxide-containing coating and a dibasic calcium phosphate-containing coating was immersed in each aqueous alkaline solution at the respective treatment temperatures for the respective treatment times as shown in Table 2 to perform an alkali treatment. Subsequently, the thus alkali-treated pure magnesium plate material was washed with deionized water and dried with hot air, whereby a pure magnesium plate material with coating, in which dibasic calcium phosphate on the surface of the dibasic calcium phosphate-containing coating was partially or entirely substituted with hydroxyapatite and/or hydroxyapatite carbonate (pure magnesium plate material with coating which had a magnesium hydroxide-containing coating, dibasic calcium phosphate-containing coating, and a hydroxyapatite and/or hydroxyapatite carbonate-containing coating; each test piece of Examples 1 to 5) was produced. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Aqueous  
                   
                 Treat- 
               
               
                   
                   
                 alkaline solution  
                 Treatment 
                 ment 
               
               
                   
                 Aqueous alkaline 
                 concentration  
                 temperature 
                 time 
               
               
                 Test piece 
                 solution 
                 (g/L) 
                 (° C.) 
                 (min) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Example 1 
                 sodium hydroxide 
                 40 
                 60 
                 1 
               
               
                 Example 2 
                 sodium carbonate 
                 50 
                 80 
                 5 
               
               
                 Example 3 
                 sodium carbonate 
                 15 
                 90 
                 5 
               
               
                 Example 4 
                 potassium carbonate 
                 100 
                 80 
                 5 
               
               
                 Example 5 
                 potassium hydroxide 
                 50 
                 60 
                 1 
               
               
                   
               
            
           
         
       
     
     As a Comparative Example, a pure magnesium plate material subjected to only the above-described pretreatment (test piece of Comparative Example 1) was prepared. In addition, a chemical conversion-treated pure magnesium plate material (test piece of Comparative Example 2) was prepared by immersing a pretreated pure magnesium plate material in a mixture, which was obtained by adjusting the pH of an aqueous solution containing 50 mM of Ca-EDTA and 50 mM of KH 2 PO 4  to be 6.4 with an addition of a 1/40-amount of a 1N—NaOH aqueous solution, at 95° C. for 8 hours. Moreover, a chemical conversion-treated pure magnesium plate material (test piece of Comparative Example 3) was also prepared by immersing a pretreated pure magnesium plate material in a mixture, which was obtained by adjusting the pH of an aqueous solution containing 50 mM of Ca-EDTA and 50 mM of KH 2 PO 4  to be 7.3 with an addition of a 1/20-amount of a 1N—NaOH aqueous solution, at 95° C. for 8 hours. 
     &lt;Identification of Coating Crystal System&gt; 
     The coatings formed on the surfaces of the test pieces of Examples 1 to 5 and Comparative Examples 1 to 3 were measured by an X-ray diffraction method, and the crystal systems thereof were identified. As a result, crystalline magnesium hydroxide and dibasic calcium phosphate crystals were detected from the test pieces of Examples 1 to 5. Hydroxyapatite crystals were detected from the test pieces of Examples 1 and 5, hydroxyapatite carbonate crystals were detected from the test pieces of Examples 2 and 4, and hydroxyapatite crystals and hydroxyapatite carbonate crystals were detected from the test piece of Example 3. In the test pieces of Examples 1 to 5, it was thus confirmed that a dibasic calcium phosphate-containing coating was formed on a magnesium hydroxide-containing coating, and a hydroxyapatite and/or hydroxyapatite carbonate-containing coating was formed on the dibasic calcium phosphate-containing coating. On the other hand, none of these crystals was detected from the test piece of Comparative Example 1, while crystalline magnesium hydroxide and hydroxyapatite crystals were detected from the test pieces of Comparative Examples 2 and 3. 
     &lt;Evaluation of Corrosion Resistance&gt; 
     The test pieces of Examples 1 to 5 and Comparative Examples 1 to 3 were immersed in an aqueous solution containing the ions shown in Table 3 at the respective concentrations, at 38° C. for 24 hours. Subsequently, the test pieces were washed with deionized water and dried with hot air, after which the test pieces were each irradiated with a light and the projected area was measured. Thereafter, the disappeared area was determined by comparing before and after the immersion treatment in the aqueous solution, and the corrosion resistance was evaluated based on the below-described criteria. The results thereof are shown in Table 4. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Contained ion species 
                 Ion concentration (mol/L) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Na +  ion 
                 0.14 
               
               
                   
                 K +  ion 
                 0.006 
               
               
                   
                 Ca 2+  ion 
                 0.0013 
               
               
                   
                 Mg 2+  ion 
                 0.0008 
               
               
                   
                 Cl −  ion 
                 0.14 
               
               
                   
                 HCO 3   2−  ion 
                 0.004 
               
               
                   
                 HPO 4   2−  ion 
                 0.0008 
               
               
                   
                   
               
            
           
         
       
     
     (Evaluation Criteria) 
     5: The disappeared area was 0. 
     4: The disappeared area was smaller than 5%. 
     3: The disappeared area was 5% or larger but smaller than 50%. 
     2: The disappeared area was 50% or larger but smaller than 90%. 
     1: The disappeared area was 90% or larger. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Test piece 
                 Corrosion resistance 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Example 1 
                 4 
               
               
                   
                 Example 2 
                 5 
               
               
                   
                 Example 3 
                 5 
               
               
                   
                 Example 4 
                 4 
               
               
                   
                 Example 5 
                 5 
               
               
                   
                 Comparative Example 1 
                 1 
               
               
                   
                 Comparative Example 2 
                 2 
               
               
                   
                 Comparative Example 3 
                 3 
               
               
                   
                   
               
            
           
         
       
     
     The present invention has been described above in detail referring to concrete examples thereof; however, it is obvious to those skilled in the art that various modifications and changes can be made without departing from the gist and the scope of the present invention.