WO2000044557A9

WO2000044557A9 - Surface treatment for magnesium alloys

Info

Publication number: WO2000044557A9
Application number: PCT/US2000/002368
Authority: WO
Inventors: Jun Kawaguchi
Original assignee: Henkel Corp; Jun Kawaguchi
Priority date: 1999-01-28
Filing date: 2000-01-28
Publication date: 2001-09-07
Also published as: WO2000044557A1; JP2000219975A

Abstract

An article of manufacture made of magnesium alloy can be provided with a surface having excellent corrosion resistance, paint adhesion, and aesthetic properties by coating the magnesium alloy surface with discontinuously spaced chromium metal microparticles and these particles and the remainder of the alloy surface with a continuous coating that comprises hydrated trivalent chromium oxide. Such a surface can be formed by subjecting the magnesium alloy surface to cathodic electrolysis in an acidic aqueous solution that has a pH from 0.5 to 3.0 and contains at least hexavalent chromium containing ions in a concentration from 2 to 100 g/l.

Description

Description SURFACE TREATMENT FOR MAGNESIUM ALLOYS

FIELD AND BACKGROUND OF THE INVENTION

This invention relates to surface-treated magnesium alloys that exhibit an excellent paint adherence and excellent aesthetics. This invention also relates to a surface treatment method for obtaining said surface-treated magnesium alloys. The demand for magnesium alloys has been steadily increasing in recent years due to its use for the housings and casings of personal information devices such as notebook computers, portable telephones, digital cameras, and digital video cameras. The demand for magnesium alloys in these applications is driven by the fact that magnesium alloys afford a better electromagnetic shielding performance and heat dissipating performance than the heretofore used plastics and do so without an increase in weight. Moreover, since the recycling behavior of a material is becoming increasingly important with the recent emphasis on environmental issues, one can expect that magnesium alloys will enter into use in a variety of fields in addition to the aforementioned personal information devices, However, while magnesium alloys have excellent properties from a material standpoint, magnesium itself has a lower redox potential than other metals and hence exhibits a very high chemical activity in humid environments at ordinary temperatures and pressures and is easily corroded in such environments. This drawback has been a hindrance to the commercial and industrial diversification of the applications of magnesium alloys.

This drawback makes the provision of a corrosion-inhibiting surface treatment practically essential for the commercial and industrial use of magnesium alloys. In addition, magnesium alloys in and of themselves may have to exhibit an aesthetic or decorative quality when used in a visible position, e.g., for the housings and casings of the per- sonal information devices described above. As a consequence, magnesium alloys are, for example, usually painted when a colored appearance is required and plated when a metallic texture becomes necessary.

As with other high-volume metals such as iron, steel, and aluminum, when magnesium alloys are to be painted or plated, they are first subjected to a chemical pretreat- ment in order to impart corrosion resistance and paint adherence to them. Since magnesium alloys have a high degree of chemical activity, the results of this chemical pre- treatment have a major influence on overall performance and chemical pretreatment is therefore typically more complicated for magnesium alloys than for the other high- volume metals.

For example, the mainstream pretreatment procedure in the case of painting is a conversion treatment in which a chromate coating is formed on a magnesium alloy surface by dipping in an aqueous solution whose main component is chromic acid. This procedure is typified by the Dow 7, Dow 20, and Dow 22 methods. Other technologies have recently appeared due to the desire created by environmental concerns for chromium-free surface treatments. These include dipping in an aqueous solution whose main component is a permanganate salt or manganate salt (Japanese Laid Open (Kokai or Unexamined) Patent Application Number Hei 10-219473 (219,473/1998)) and dipping in a nickel-free aqueous solution of a manganese-modified zinc phosphate (Japanese Laid Open (Kokai or Unexamined) Patent Application Number Hei 7-204577 (204,577/1995)).

When a magnesium alloy is to be plated, the pretreatment executed prior to the intended plating usually comprises the following steps in the sequence given: acid rinse; activation by treatment with, for example, a phosphoric acid-hydrofluoric acid mixed acid; substitutional zinc plating; and finally strike plating with copper or nickel.

The preceding surface treatment technologies are not, however, universally applicable due to the extremely diversified surface states presented by magnesium alloys as a consequence of the different, application-specific, alloy compositions and forming methods used. For example, the above-described plating technology is reportedly unsatisfactory for AZ91 stock (a magnesium alloy containing 9 % Al and 1 % Zn), which is currently the alloy in greatest demand. As a countermeasure to this problem, Japanese Published (Kokoku or Examined) Patent Application Number Hei 2-25430 (25,430/1990) teaches a pretreatment comprising chemical etching, then neutralization, and finally electroless nickel plating.

As an additional consideration, when the surface condition of the magnesium alloy(s) that will actually be treated is not chemically uniform or homogeneous, a satisfactory conversion performance cannot be obtained by using the pre-painting pretreatments described above. When this problem occurs, the so-called "pretreatment step to the main pretreatment" also becomes critical. Procedures disclosed for this pretreatment step include, for example, the execution of a uniform etch using an acid rinse solution containing persulfate salt (Japanese Laid Open (Kokai or Unexamined) Patent Application Number Sho 53-102231 (102,231/1978)) and the generation of a clean surface by first carrying out an acid rinse followed by desmutting using a chelating agent (Japanese Laid Open (Kokai or Unexamined) Patent Application Number Hei 6- 220663 (220,663/1994)).

Plating and painting are discussed above as the existing technologies for imparting an aesthetic or decorative appearance to the surface of Magnesium alloys, and, as may be readily expected, plating is generally the more expensive of the two. In addition, plating technology requires the formation of a strike-plated nickel or copper interlayer, but the use of such materials makes recycling impossible, because the solid-dissolution of nickel or copper into magnesium alloys causes a substantial deterioration in their corrosion resistance.

One approach that may be considered for solving these problems through the use of painting would be to preliminarily impart a specular or mirror gloss to the magnesium alloys by a mechanical process such as polishing and to then execute a clear coat thereon. An essential prerequisite in this case would be that the glossy appearance of a magnesium alloy surface will remain essentially unaltered by the pretreatment process implemented prior to the clear coating. However, in the case of the prior-art technologies discussed hereinabove, for example, the chromate-based conversion systems and permanganate-based conversion systems produce a yellow appearance, while the phosphate-based conversion systems produce a iusterless gray appearance. Thus, a suitable surface treatment method that will not alter the glossy appearance of a magnesium alloy surface has remained undiscovered to date. It is therefore one object of this invention to provide a colorless and transparent surface treatment method that provides an excellent adherence and corrosion resistance on magnesium alloy surfaces, including those that are specularly glossy, and that does so without any adverse effects on the gloss or other aesthetic properties when the surface treatment is followed by a clear coating on a specularly glossy surface. An alternative object of the invention is to provide magnesium alloy objects on which this type of surface treatment method has been executed. Another alternative object is to provide a surface treatment method that is effective in preparing a larger group of magnesium alloys for subsequent painting with colored paints, including such paints that are pigmented and therefore not transparent; at present, only a relatively small group of magnesium alloys can be effectively prepared to receive a high quality, aesthetically pleasing coating of a colored paint. BRIEF SUMMARY OF THE INVENTION

It has been discovered that a conventional chromic acid-based treatment bath, which produces a yellow colored coating when contacted with a magnesium alloy surface without any passage of electric current from an external source through the magnesium alloy surface being coated, can serve instead as electrolyte in a process in which carefully controlled electric current is passed in a cathodic direction through the magnesium alloy surface being treated. The coating produced by such a process is a colorless and transparent coating and has good corrosion protective and paint adherence properties. The main components of this transparent coating are believed to be metallic chromium and trivalent chromium. BRIEF DESCRIPTION OF THE DRAWINGS

The drawing figures are all cross-sections. Figure 1 shows a flat, mirror-polished magnesium alloy specimen, with the polished surface on top. Figure 2 shows a cross- section of the same specimen after its polished surface has been subjected to cathodic electrolysis according to the invention, and Figure 3 shows a cross-section of the same specimen after the electrolyzed surface has been further coated with a clear coating to produce a preferred embodiment of an article of manufacture according to the invention. DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS A method according to this invention for treating the surface of a magnesium alloy characteristically comprises subjecting the magnesium alloy surface to cathodic electrolysis in an acidic aqueous solution that has a pH value between 0.5 and 3.0 and that contains at least hexavalent-chromium-containing ions in a concentration of 2 to 100 grams of hexavalent chromium per liter, this unit of concentration being freely used here- inafter for any other constituent as well as for hexavalent chromium and being hereinafter usually abbreviated as "g/r. This acidic aqueous solution also preferably contains up to 2.0 parts by weight of sulfuric acid per thousand parts by weight of total solution, this unit of concentration being freely used hereinafter for any other constituent as well as for hexavalent chromium and being hereinafter usually abbreviated as "ppt", and/or up to 0.40 ppt, measured as its stoichiometric equivalent as fluorine, of fluorine- containing compound(s).

The above-referenced cathodic electrolysis is preferably run at a current density of 0.1 to 20 amperes per square decimeter of surface being coated, this unit being hereinafter usually abbreviated as "A/dm²" and independently is run for a sufficient time to pass through the surface being coated a quantity of electricity that is from 10 to 1 ,000 coulombs per square decimeter.

The surface of the magnesium alloy to be coated by a method according to the invention preferably is preliminarily polished prior to cathodic electrolysis of the magnesium alloys. Independently, a clear coating is preferably provided after said cathodic elec- trolysis has been carried out. A surface-treated magnesium alloy according to this invention, which is normally but not necessarily produced by a method according to the invention as described herein, has chromium metal microparticles with diameters no greater than 15.0 micrometres (hereinafter usually abbreviated as "μm") discontinuously spaced over its surface at a surface density not exceeding 100,000 microparticles per square millimeter ("per square millimeter" being hereinafter usually abbreviated as "/mm²") of treated surface and is, together with said discontinuously spaced chromium microparticles, additionally coated over its entire surface with a continuous coating layer that comprises hydrated trivalent chromium oxide and is from 5 to 100 nanometers (hereinafter usually abbreviated as "nm") in thickness. The total amount of chromium contained in the chromium metal microparticles and continuous coating layer is preferably from 1 to 300 milligrams per square meter of surface coated, this unit of surface coating mass per unit area being hereinafter usually abbreviated as "mg/m²". The surface of the magnesium alloys is preferably subjected to a preliminary polishing prior to formation of the aforesaid coating layer. In addition and independently, the outermost layer of the coated magnesium alloy objects is preferably a clear coating layer.

The chromium metal microparticles in the coating of a magnesium alloy object according to this invention are believed to contribute to the post-painting corrosion resistance, while the continuous coating layer is believed to contribute to paint film adherence. A chromium metal microparticle size in excess of 15.0 μm and/or a microparticle surface density in excess of 100,000 microparticles/mm², while still giving a good corrosion resistance, may be aesthetically objectionable because the surface takes on a gray to blackish brown color under such circumstances. This will, however, be unproblematic when a colored paint will ultimately be applied. When the surface- treated magnesium alloys will be used only in relatively weakly corrosive environments, as is the case for personal information devices, the critical property is paint adherence rather than corrosion resistance. Under these conditions, then, the lower limit on the chromium metal microparticle size and the lower limit on the microparticle frequency are not critical, and in extreme cases it will be unproblematic if the number and/or size of the chromium metal microparticles is almost imperceptible. Certainly as few as 2,000 chromium microparticles per square decimeter are satisfactory in such instances, while in order to optimize practical benefits at a reasonable cost, the surface density of chromium microparticles in all instances preferably is not more than, with increasing preference in the order given, 90,000, 80,000, 70,000, 60,000, or 50,000 particles per square decimeter. On the other hand, a continuous coating layer that comprises, preferably consists essentially of, or more preferably consists of, hydrated trivalent chromium oxide is essential to the embodiments of this invention in which a material object with a specifically characterized surface is claimed. A continuous coating layer with a thickness below 5 nm is usually unable to uniformly coat the surface of the magnesium alloys and hence will be unable to provide an acceptable paint adherence. Thicknesses in excess of 100 nm, while not necessarily technically problematic, are uneconomical since the additional thickness provides no additional improvement in adherence. However, much greater thicknesses must be avoided, because they can be accompanied by cracking and/or a deterioration in the strength of the continuous coating layer itself. The technical limit on the thickness is thus about 300 nm. To provide maximum practical advantage at reasonable cost, the thickness of this coating layer preferably is at least, with increasing preference in the order given, 3, 5, 7, or 9 nm and independently preferably is not more than, with increasing preference in the order given, 75, 50, 35, 25, 23, 21 , 19, or 17 nm. The total amount of chromium in the chromium metal microparticles and continuous coating layer is preferably 1 to 300 mg/m² expressed as the weight per unit surface area. This parameter can be easily measured by, for example, x-ray fluorescence analysis, when it is desired to manage the weight per unit surface area as in the case of the commercial or industrial utilization of this invention. The type of highly aesthetic mag- nesium alloy that is one alternative principal object of this invention can be obtained by forming the subject surface treatment coating on an already mirror finish-polished surface of a magnesium alloy and then forming a clear coating layer as the top layer. Images of the surface of surface-treated magnesium alloys according to the present invention are provided in Figures 1 to 3. To obtain optimal practical properties at a reasonable cost, the total amount of chromium atoms in the chromium metal microparticles and continuous coating layer combined preferably is at least, with increasing preference in the order given, 2.0, 3.0, 4.0, 5.0, 6.0, or 6.9 and independently preferably is not more than, with increasing preference in the order given, 250, 200, 175, 150, 125, or 100 mg/m². There are no narrow restrictions on the magnesium alloys to which this invention may be applied. This invention can be applied to almost all magnesium alloys in current use, i.e., Mg-AI-Zn alloys, Mg-Mn alloys, and Mg-Zn alloys.

Almost all structural magnesium alloy components of current importance are fabricated using die casting technology or alternatively in recent years using thixomolding technology. Often present at the surface of such components are residues of the organic release agent used during the forming operation and a quenching solidification layer, i.e., a layer into which at least some alloying metals have been concentrated to higher levels than in the bulk of the component. The quenching solidification layer is about 5 μm thick. The surface of any magnesium alloy coated in a method of this invention is therefore preferably polished at least to a degree sufficient to remove these layers. When a clear coat will be applied, polishing is preferably performed to a degree sufficient to give a mirror or specular gloss. The cathodic electrolysis treatment of this invention is preferably applied immediately after this polishing step.

This polishing step is, however, not necessary or even always preferred when the usual colored paints will be applied to the surface produced by a process according to this invention. Such heretofore known pretreatment steps as degreasing, acid rinsing, and surface conditioning may nevertheless be, and usually preferably are, implemented in such instances, because the removal of the quenching solidification layer and organic release agent is still desirable. The cathodic electrolysis treatment of this invention can then be applied after these steps.

The cathodic electrolysis treatment of this invention must be run in an acidic aqueous solution that has a pH from 0.5 to 3.0 and that contains at least hexavalent chromium-containing ions in a concentration of 2 to 100 g/l, measured as their stoichio- metric equivalent as hexavalent chromium. Coating formation by the electrolysis treatment usually is quite difficult at a hexavalent chromium concentration below 2 g/l. Concentrations in excess of 100 g/l are not specifically problematic, but are economically undesirable since the excess beyond 100 g/l provides no additional improvement in coating-formation efficiency. Since the coating-formation efficiency gradually declines as the pH declines, pH values below 0.5 are undesirable. Higher pH values in excess of 3.0 are undesirable due to a sharp drop off in coating-formation efficiency. For optimization of practical benefits at a reasonable cost, the pH value preferably is at least, with increasing preference in the order given, 0.60, 0.70, 0.75, or 0.80 and independently preferably is not more than, with increasing preference in the order given, 2.5, 2.0, 1.5, 1.3, 1.10, 1.00, 0.95, or 0.90, and independently the concentration of hexavalent chromium in the acidic aqueous solution used as the electrolyte during cathodic electrolysis in a process according to the invention preferable is at least, with increasing preference in the order given, 3.0, 3.5, 4.0, 4.5, or 4.9 g/l and independently preferably is not more than, with increasing preference in the order given, 75, 50, 40, 35, 30, 25, or 21 g/l. The source of the hexavalent chromium ions for the aqueous chromic acid solu- tion under consideration is not critical. From a cost standpoint it will be advantageous to dilute chromic acid with water. The pH can be adjusted using aqueous ammonia or an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide. A portion of the chromic acid may therefore be converted to an alkali metal salt or ammonium salt of dichromic acid in order to obtain the particular desired pH.

The aqueous chromic acid solution under consideration preferably contains sul- furic acid up to 2.0 ppt and/or a fluorine compound up to 0.40 ppt calculated as fluorine. The use of these additives serves to increase the surface density of deposition of the above-described chromium metal microparticles and in this manner provides additional improvements in the corrosion resistance of the surface-treated magnesium alloys of this invention. However, the addition of more than 2.0 ppt of sulfuric acid has the contrary effect of inhibiting deposition of the chromium metal microparticles. In the case of the fluorine compound, additions in excess of 0.40 ppt provide no additional improvement in activity and at the same time can produce a deterioration in the appearance of the surface by corroding the surface of the magnesium alloys. For optimization of practical benefits at a reasonable cost, all of the following preferences apply, each independently: the sulfuric acid concentration is not more than, with increasing preference in the order given, 1.5, 1.0, 0.8, 0.6, or 0.4 ppt; - if sulfuric acid is present in the solution used in a process according to the invention, its concentration in ppt preferably has a ratio to the concentration of hexavalent chromium in g/l that is at least, with increasing preference in the order given, 0.05:1.00, 0.08:1.00, 0.11 :1.00, 0.14:1.00, 0.17:1.00, or 0.20:1.00 and independently preferably is not more than, with increasing preference in the order given, 1.0:1.00, 0.8:1.00, 0.6:1.00, 0.4:1.00, 0.30:1.00, or 0.25:1.00; if fluorine-containing anions are present, their concentration measured as its stoi- chiometric equivalent as fluorine preferably is at least, with increasing preference in the order given, 0.02, 0.04, 0.06, or 0.08 ppt and independently preferably is not more than, with increasing preference in the order given, 0.30, 0.25, 0.20, 0.15, or 0.10 ppt.

The fluorine compound under consideration can be added in the form of hydrofluoric acid, fluorosilicic acid, fluorozirconic acid, or fluorotitanic acid or their alkali metal salts or ammonium salts. Among these, fluorozirconic acid and its salts are most preferred. Cathodic electrolysis is run in the subject aqueous chromic acid solution at a cur- rent density of 0.1 to 20 A/dm² and for a sufficient time to pass a quantity of electricity of 10 to 1 ,000 coulombs/dm² during the total cathodic coating formation process. The current efficiency for producing the desired type of coating declines at a current density below 0.1 A/dm². Current densities in excess of 20 A/dm² are not specifically problemat- ic, but are industrially impractical due to the corresponding extremely short electrolysis time necessary to pass the appropriate quantity of electricity. To optimize practical benefit at a reasonable cost, the current density in a method according to the invention preferably is at least, with increasing preference in the order given, 0.5, 1.0, 1.5, 2.0, 2.5, or 2.9 A/dm² and independently preferably is not more than, with increasing preference in the order given, 75, 50, 30, 20, 15, 13, 11 , or 9.0 A/dm². A quantity of electricity below 10 coulombs/dm² does not usually produce the required amounts of chromium metal microparticles and continuous coating layer, while exceeding 1 ,000 coulombs/dm² produces excessive amounts of chromium metal microparticles and continuous coating layer. To optimize practical benefits at a reasonable cost, the number of coulombs passed through a surface being coated by electrolysis in a method according to the invention preferably is not more than, with increasing preference in the order given, 900, 800, 700, 600, or 500 coulombs/dm² and independently preferably is not less than, with increasing preference in the order given, 20, 30, 40, 50, or 59 coulombs/dm². At one of the preferred current densities, a preferred number of coulombs/dm² can be passed through a surface being electrolyzed in a method according to the invention within a time from 10 to 200, or more preferably from 20 to 60, seconds.

The cathodic electrolysis under consideration is optimally run at a temperature in the range from 20 to 50 °C. Overly low temperatures can result in the appearance of a yellow color, which is believed to be due to the presence of hexavalent chromium, in the continuous coating. Overly high temperatures can result in the formation of a continuous coating even in the absence of cathodic electrolysis; this is undesirable again because hexavalent chromium is believed to be present in such electroless continuous coatings. To optimize practical benefit at a reasonable cost, the temperature in a method according to the invention preferably is at least, with increasing preference in the order given, 25, 30, 32, 34, 36, or 38 °C and independently preferably is not more than, with increasing preference in the order given, 48, 46, 44, or 42 °C.

As the cathodic electrolysis treatment continues, trivalent chromium ions accumulate in the aqueous chromic acid solution used in surface treatment according to this invention, but no particular difficulties are raised by this accumulation. Since the surface-treated magnesium alloys of this invention are free of hexavalent chromium, they essentially will not pollute the environment during their commercial circulation. However, the use of an aqueous chromic acid solution in the cathodic electrolysis step necessitates a thorough water rinse after surface treatment. In the absence of a thorough water rinse, trace amounts of hexavalent chromium can remain in the surface layer.

A clear coating (or colored coating) is preferably executed after the above-described surface treatment procedure. This clear coating can employ the usual colorless and transparent clear paints or can employ a colored clear paint loaded with moderate amounts of pigment. This will permit the production of various types of decorative de- signs or finishes.

The chromium metal microparticles in the coating are believed to be responsible in this invention for the corrosion resistance, while the trivalent continuous coating layer is believed to be responsible for paint adherence. More particularly, since the continuous coating layer under consideration exhibits an excellent adherence for almost all organic resins as well as ceramics, it can be used as an underpaint treatment for paints and also as an underlayer treatment or priming treatment for laminates and adhesives. In addition, because the use of the polishing step renders unnecessary the complex chemical pretreatment sequences seen in the prior art for the purpose of surface cleaning, a general-purpose surface treatment method is obtained that is also advantageous in terms of cost.

This invention and its benefits may be further appreciated by consideration of the following working and comparative examples.

Type AZ91 D magnesium alloy coupons with dimensions of 100 mm * 70 mm * 2 mm were used as the substrate material. A mirror or specular condition was imparted to the coupon surface in a preliminary polishing step. This was followed by a gentle water rinse to remove the polish and yield the test coupon in a state ready for cathodic electrolysis. EXAMPLE 1

Test coupons were subjected to cathodic electrolysis using an anode of Type SUS304 stainless steel. This electrolysis was run in an aqueous chromic acid solution containing hexavalent chromium in a concentration of 20 g/l and having a pH adjusted to 0.8 by the addition of aqueous ammonia. The cathodic electrolysis was run for 60 seconds at a constant current density of 8 A dm² and at a temperature of 40 °C. After a thorough rinse with deionized water followed by drying, absolutely no modification was seen in the appearance of the material. X-ray fluorescence analysis of the chromium add-on to the surface of a test coupon from Example 1 gave a value of approximately 15 milligrams of chromium per square meter of surface treated, this unit of add-on mass per unit area (also called "coating weight") being hereinafter usually abbreviated as "mg/m². Analysis of the chemical state of the surface by X-ray photoelectron spectroscopy (hereinafter usually abbreviated as "XPS") indicated trivalent chromium in a thickness of about 10 nm from the outermost surface boundary and also a micro amount of chromium metal in the underlying layer. In analysis of the coupon surface by an electron power microanalyzer (hereinafter usually abbreviated as "EPMA"), microparticles with diameters of 1 to 6 μm were indicated in the electron secondary image at a surface density of approximately 5,000 per mm². Elemental analysis confirmed them to be chromium.

MAGICRON™ acrylic clear paint from Kansai Paint Co., Ltd. was spray-painted to a film thickness of 20 μm on test coupons from Example 1. This was followed by baking for 20 minutes at 150 °C. A 100-mesh grid was laid out by introducing cuts (10 * 10, orthogonal, with a spacing of 1 mm between cuts) with a sharp cutter so as to reach the basis metal. Absolutely no peeling was observed when this grid was peeled with adhesive tape (primary adhesion test). In addition, an identical painted test coupon was immersed for 4 hours in boiling deionized water and then subjected to the Crosshatch evaluation that had been carried out in the primary adhesion test. Again absolutely no peel- ing was observed (this test in which the coating is tested after immersion in boiling water is denoted below as the secondary adhesion test).

Cross-sections of a specimen in various stages of a process according to the invention as described in this Example 1 are shown in the three drawing figures, in all of which the same features are indicated with the same reference numbers. Figure 1 shows the simple cross-section of the substrate 1. Figure 2 shows the cross-section after electrolysis but before clear coating, with continuous coating 2 and discontinuously spaced chromium metal microparticles 3. Figure 3 shows the cross section after coating with clear acrylic paint 4. EXAMPLE 2 Mirror-polished test coupons as used in Example 1 were subjected to cathodic electrolysis using the same type of anode as in Example 1. This electrolysis was run in an aqueous solution prepared by adding 0.40 ppt of sulfuric acid and 0.100 ppt of fluorozirconic acid to an aqueous chromic acid solution containing 20 g/l of hexavalent chromium ions and adjusting the pH to 0.8 with aqueous ammonia. The cathodic elec- trolysis was run for 20 seconds at a constant current density of 3 A/dm² and at a temper- ature of 40 °C. Electrolysis was followed by a thorough rinse with deionized water and drying. A white cloudiness, considered to be insignificant, was observed in the surface of the resulting material.

X-ray fluorescence analysis of the chromium add-on to the surface of a test cou- pon from Example 2 gave a value of approximately 95 mg chromium per m². Analysis of the chemical state of the surface by XPS indicated trivalent chromium in a thickness of about 16 nm from the outermost surface boundary, while a clear peak for chromium metal was detected in the underlying layer. In analysis of the coupon surface by EPMA, microparticles with diameters of 1 to 3 μm were indicated in the electron secondary image at a surface density of approximately 40,000 per mm². Elemental analysis confirmed them to be chromium.

Test coupons from Example 2 were then painted as in Example 1 , which rendered the white cloudiness observed on the material surface after cathodic electrolysis almost imperceptible. Moreover, there was a complete absence of peeling in the pri- mary adhesion test and secondary adhesion tests carried out as in Example 1.

COMPARATIVE EXAMPLE 1

Mirror-polished test coupons as used in Example 1 were directly painted as described in Example 1 without having been first subjected to the cathodic electrolysis treatment. In this case, the peeling score was 3/100 (3 peeled of 100 squares) in the primary adhesion test and 55/100 in the secondary adhesion test (boiling water immersion).

COMPARATIVE EXAMPLE 2

A mirror-polished test coupon as described above was processed by the Dow 20 method by dipping for 30 seconds at ambient temperature in a mixed aqueous solu- tion of 15 g/l of sodium acid fluoride, 180 g/l of sodium dichromate, 10 g/l of sodium acetate, and 84 milliliters of concentrated aqueous nitric acid per liter of the mixed aqueous solution. Dipping was followed by a thorough rinse with deionized water and drying. The resulting test coupon had a non-glossy, yellowish-black appearance. EXAMPLE 3 Mirror-polished test coupons as used in Example 1 were subjected to cathodic electrolysis using the same type of anode as in Example 1. This electrolysis was run in an aqueous chromic acid solution containing hexavalent chromium in a concentration of 5 g/l and having a pH adjusted to 0.8 by the addition of aqueous ammonia. The cathodic electrolysis was run for 60 seconds at a constant current density of 3 A/dm² and at a temperature of 40 °C. After a thorough rinse with deionized water followed by drying, absolutely no modification was seen in the appearance of the material.

X-ray fluorescence analysis of the chromium add-on to the surface of a test coupon from Example 3 gave a value of approximately 7 mg chromium per m². Analysis of the chemical state of the surface by XPS indicated trivalent chromium in a thickness of about 10 nm from the outermost surface boundary and also a very small amount of chromium metal in the underlying layer. Observation of the coupon surface by scanning electron microscopy (hereinafter usually abbreviated as "SEM") showed the presence of microparticles with diameters of 1 to 12 μm at a surface density of approximately 2,000 per mm². Test coupons from Example 3 were spray-painted to a film thickness of 10 μm with a ceramic paint, CERA-STATTS™ 124CL-3 from Parker Kako Co., Ltd. This was followed by baking for 20 minutes at 200 °C. Absolutely no peeling of the paint film was observed after a test coupon had been immersed for 2 hours in 5 % salt water at 25 °C. This procedure was repeated until four test coupons had been evaluated. The produc- tion of minor rust spots was observed on three of the four test coupons. EXAMPLE 4

Mirror-polished test coupons as used in Example 1 were subjected to cathodic electrolysis using the same type of anode as in Example 1. This electrolysis was run in an aqueous solution prepared by adding 0.100 ppt of sulfuric acid and 0.080 ppt of fluorozirconic acid to an aqueous chromic acid solution containing 5 g/l of hexavalent chromium and adjusting the pH to 0.8 with aqueous ammonia. The cathodic electrolysis was run for 20 seconds at a constant current density of 3 A/dm² and at a temperature of 40 °C. Electrolysis was followed by a thorough rinse with deionized water and drying. A moderate white cloudiness was observed in the surface of the resulting material. X-ray fluorescence analysis of the chromium add-on to the surface of a test coupon from Example 4 gave a value of approximately 63 mg chromium per m². Analysis of the chemical state of the surface by XPS indicated trivalent chromium in a thickness of about 15 nm from the outermost surface boundary, while a clear peak for chromium metal was detected in the underlying layer. Observation of the coupon surface by SEM showed the presence of microparticles with diameters of 1 to 3 μm at a surface density of approximately 20,000 per mm².

Test coupons from Example 4 were then painted as in Example 3, which almost completely extinguished the white cloudiness observed in the material surface after cathodic electrolysis. The salt water immersion test described in Example 3 was also car- ried out. In this case, abnormalities such as paint film exfoliation and rusting were com- pletely absent. COMPARATIVE EXAMPLE 3

Mirror-polished test coupons as used in Example 1 were directly painted as described in Example 3 without having first been subjected to the cathodic electrolysis. These were then submitted to the salt water immersion test described in Example 3, which was repeated to provide four test coupons for evaluation. Paint film exfoliation occurred on one test coupon with significant corrosion also occurring in the region exposed by the exfoliation. In the case of the other three test coupons, no peeling was seen, but rust spots were observed over the entire surface of the test coupons. ADVANTAGEOUS EFFECTS OF THE INVENTION

As has been described above, since the surface treatment method of this invention essentially does not modify the appearance of the surface of magnesium alloys, in its application as an underpaint treatment for clear coating it can provide a broad range of decorative and aesthetic possibilities. In addition, since the type of pretreatment — such as mechanical polishing or already known chemical procedures — used with the method of this invention is not critical, the method of this invention can itself be applied as a general pretreatment for painting operations and therefore constitutes a general-purpose surface treatment that is independent of the type of magnesium alloys. While the method of this invention does employ an aqueous hexavalent chromium-containing solution, the absence of hexavalent chromium in the coating layer on the treated magnesium alloys makes this a surface treatment that imposes little load on the environment.

Claims

1. An article of manufacture having an underlying surface of at least one magnesium alloy, said underlying surface being discontinuously coated with chromium metal microparticles that have diameters no greater than 15.0 μm and have a density over the underlying surface that does not exceed 100,000 microparticles/mm², said microparticles and any parts of said underlying surface that are not covered by any of said microparticles both being covered with a continuous coating that has a thickness of 5 to 100 nm and comprises hydrated trivalent chromium oxide.

2. An article of manufacture according to claim 1 , in which the total amount of chromium contained in the chromium metal microparticles and the continuous coating is from 1 to 300 mg/m².

3. An article of manufacture according to claim 2, wherein the surface of the magnesium alloy has been subjected to a preliminary polishing prior to formation of said continuous coating layer over said surface of the magnesium alloy.

4. An article of manufacture according to claim 1 , wherein the surface of the magnesium alloy has been subjected to a preliminary polishing prior to formation of said continuous coating layer over said surface of the magnesium alloy.

5. An article of manufacture according to any of claims 1 through 4 that has a clear coating layer as its outermost layer.

6. An article of manufacture according to claim 5, wherein: the total amount of chromium contained in the chromium metal microparticles and the continuous coating is from 6.9 to 100 mg/m²; the density of the chromium microparticles over the surface is from 2,000 to

50,000 particles/dm²; and - the thickness of the continuous coating is from 9 to 17 nm.

7. A method for treating a surface of a magnesium alloy to form a coating thereon, said method comprising subjecting the surface of a magnesium alloy to cathodic electrolysis in an acidic aqueous solution that has a pH from 0.5 to 3.0 and comprises ions containing hexavalent chromium in a concentration of 2 to 100 g/l.

8. A method according to claim 7, wherein the acidic aqueous solution additionally comprises at least one of: sulfuric acid in a concentration that is not greater than 2.0 ppt; and at least one type of fluorine containing anions, all of said anions collectively having a stoichiometric equivalent concentration as fluorine that is not more than

0.40 ppt.

9. A method according to claim 8, wherein during said cathodic electrolysis there is a current density of 0.1 to 20 A/dm² and a quantity of electricity of 10 to 1 ,000 coulombs/dm² is passed through the surface being treated.

10. A method according to claim 7, wherein during said cathodic electrolysis there is a current density of 0.1 to 20 A/dm² and a quantity of electricity of 10 to 1 ,000 coulombs/dm² is passed through the surface being treated.

11. A method according to any of claims 7 through 10, wherein the surface of the magnesium alloy has been polished prior to said cathodic electrolysis and a clear coating is additionally applied over the coating produced by said cathodic electrolysis.