WO2000070123A1

WO2000070123A1 - Process for the surface treatment of magnesium alloys

Info

Publication number: WO2000070123A1
Application number: PCT/US2000/006715
Authority: WO
Inventors: Kenichirou Oshita; Masaru Shishido; Jun Kawaguchi
Original assignee: Henkel Corporation
Priority date: 1999-05-12
Filing date: 2000-05-12
Publication date: 2000-11-23
Also published as: KR20000077242A; JP3783995B2; AU4796800A; JP2000328261A; CN1288073A

Abstract

Excellent corrosion resistance and paint adherence and low surface electrical resistance can be imparted to magnesium alloys that have been molded by, for example, die casting or thixomolding, by first etching the surface with an acidic aqueous solution having a pH of 1 through 5 and preferably containing a carboxylic acid and then desmutting the resulting surface by contact with an alkaline aqueous solution that has a pH of 7 through 14 and contains an organophosphorus compound. Prior to this etching, the magnesium alloy surface is preferably degreased using an alkaline aqueous solution containing inorganic salt and surfactant or using an acidic aqueous solution containing acid and surfactant. The corrosion resistance and paint adhesion may be further increased by forming a conversion coating over the surface produced by desmutting.

Description

Description PROCESS FOR THE SURFACE TREATMENT OF MAGNESIUM ALLOYS

FIELD OF THE INVENTION

This invention relates to a novel surface treatment method for the purpose of imparting an excellent corrosion resistance, excellent paint adherence, and low surface electrical resistance to magnesium alloy surfaces. This invention can be applied with particularly good effect to magnesium alloy products that have been molded by the casting techniques known as die casting and thixomolding. The release agent used during casting as well as the alloying components (e.g., aluminum, zinc) typically segregate at the surface of magnesium alloy molded by the aforementioned casting techniques, thereby rendering the surface refractory to cleaning by surface treatment. The surface treatment method of this invention is particularly effective for cleaning such surfaces and thereby imparting thereto an excellent corrosion resistance, excellent paint adherence, and low surface electrical resistance.

Magnesium alloys, due to their low specific gravity, high strength, and excellent recyclability, are widely employed for products in the automotive and consumer electron- ics sectors. At the same time, however, magnesium alloys are the least noble of the common structural metals and hence are highly susceptible to corrosion. As a result, they are generally used after the formation thereon of a corrosion-resistant coating by conversion treatment. Mainly corrosion resistance and paint adherence are required in the case of automotive component applications, while application for the casings and en- closures of consumer electronic equipment (e.g., notebook personal computers, portable telephones) requires, in addition to corrosion resistance and paint adherence, a low surface electrical resistance after the surface treatment in order to avoid loss of the excellent electromagnetic shielding characteristics that magnesium alloys exhibit.

Many of the magnesium alloy products under consideration are molded by the casting techniques already mentioned above that are known as die casting and thixomolding. In these casting techniques, the molten or semi-molten magnesium alloy is molded by introduction at high velocity and high pressure into a die. Since a water- based or emulsion-type release agent is typically coated on the die surface in each casting cycle, the release agent ends up tenaciously bonded on the surface of the magnesium alloy product after molding. This release agent becomes quite refractory to cleaning by surface treatment as a consequence of its modification by the heat (approximately 660 °C) of the molten magnesium alloy and because a portion of the release agent is drawn into the material. The magnesium alloys processed by die casting and thixomolding typically contain aluminum and/or zinc as alloying component in order to improve the casting behavior and mechanical strength. The most widely used magnesium alloy, Type AZ91 D, contains 9 % aluminum and 1 % zinc as alloying components. Multicomponent alloys containing such alloying components can form solid phases with different concentrations and/or distributions of the alloying components as a function of the solidification rate, and the solidification rate of the outermost layer of magnesium alloy, which is quenched by the die, frequently varies substantially as a function of position. In particular, the alloying component concentration and crystallization morphology of the outermost layer often differ substantially between the position corresponding to the die inlet (known as the "gate side") and the side opposite the inlet (known as the "overflow side"). The status of the outermost layer also varies with product shape and casting conditions.

Thus, the most important point in the surface treatment of magnesium alloys is usually removal of the outermost layer containing the release agent and alloy segregation layer in order to form the cleanest possible surface prior to the conversion treatment process. An unsatisfactory cleaning prevents the execution of a uniform conversion treatment, which in turn impairs the manifestation of an excellent corrosion resistance and paint adherence and prevents the production of a low surface electrical resistance.

The sequences outlined below have generally been used as surface treatments for magnesium alloys. The degreasing step effects removal of light organic contaminants such as machine oils and cutting oils; the acid etch step effects removal of light organic contaminants such as machine oils and cuttings oils as well as the outermost layer containing segregated alloying constituents and release agent; and the conversion treatment step functions to impart corrosion resistance and paint adherence by forming a chromic acid chromate or manganese phosphate coating. The degreasing step can be omitted in the case of products that carry little organic contaminant (e.g., machine oil, cutting oil) and products that have been preliminarily shot-blasted or mechanically polished. The problems of the prior art will be discussed with reference to the following sequences 1 through 3. Sequence 1 : degreasing - water rinse - conversion treatment - water rinse - purified water rinse - drying. Sequence 2: degreasing - water rinse - acid etch - water rinse - conversion treatment → water rinse - purified water rinse - drying.

Sequence 3: degreasing - water rinse - acid etch - water rinse - desmutting - water rinse - conversion treatment - water rinse - purified water rinse - drying. Surfactant plus alkali builder (e.g., sodium hydroxide, sodium phosphate) mixtures and surfactant plus acid (e.g., sulfuric acid, nitric acid, tartaric acid) mixtures have been used in the degreasing step in these sequences. The acid etch step has typically used a mineral acid (e.g., sulfuric acid, nitric acid, phosphoric acid) or an organic acid (e.g., citric acid, oxalic acid, tartaric acid). However, when degreasing and acid etching are carried out to the extent of complete removal of the release agent and alloy segregation layer present on the magnesium alloy surface, a smut of the alloying components remains on the surface. This is believed to occur by selective etching of the magnesium matrix in these steps. A fine, dense, and uniform coating cannot be formed, and hence excellent properties cannot be obtained, when the conversion treatment is run in the presence of this residual smut. As a consequence, only a weak etch insufficient to produce smut can be used in Sequences 1 and 2 as described above. This makes it very difficult if not impossible to completely remove the part of the outer surface of the substrate that is contaminated with release agent and includes the alloy segregation layer.

Japanese Laid Open (Kokai or Unexamined) Patent Application Number Hei 6- 220663 (220,663/1994) teaches an example of the above-identified Sequence 3. This example comprises (i) etching in an acid etch step using, e.g., sulfuric acid, phosphoric acid, tartaric acid, or the like, until complete removal of the part of the outer surface of the substrate that is contaminated with release agent and includes the alloy segregation layer and (ii) executing a desmutting step in which the smut produced by the etch is itself removed. The desmutting step uses a treatment bath that contains ethylenediamine tet- raacetic acid and that has been adjusted to pH 12 to 13, using an alkalinizing agent. While this method can effect removal of the outermost layer containing the release agent and alloy segregation layer, when used to form a clean surface prior to conversion treatment this method also results in the formation of a hydroxide coating in the water rinse step that precedes conversion treatment. This hydroxide coating, being a porous structure, impedes the formation of a fine, dense conversion coating in the ensuing conversion treatment step and thereby degrades the corrosion resistance and paint adherence.

Japanese Laid Open (Kokai or Unexamined) Patent Application Number Hei 2- 25430 (25,430/1990) teaches a procedure that is used as a pre-plating treatment. In this procedure, an etch with a pyrophosphate-based treatment bath is followed by desmutting with a hydrofluoric acid-based treatment bath. This procedure produces a thin magnesium fluoride coating after desmutting, and this coating has the ability to inhibit formation of the hydroxide coating by the ensuing water rinse. This procedure, however, suffers from the following problems: When this procedure is followed by conversion treatment and painting, the resulting paint adherence is poor; the surface electrical resistance after conversion treatment is also high; and fluoride-containing treatment baths are polluting to the working environment and hazardous to the human body.

This invention has as its object a surface treatment process that can produce a highly corrosion-resistant, strongly paint-adherent, and low surface electrical resistance conversion coating on magnesium alloy surfaces, while overcoming at least some of the problems noted above with prior art processes. BRIEF SUMMARY OF THE INVENTION

The present invention involves executing an acid etch on the magnesium alloy surface, in order to dissolve and remove the release agent-containing outermost layer; and thereafter effecting contact of the thus treated surface with an alkaline bath containing an organophosphorus compound chelating agent in order to selectively dissolve the alloying component smut left on the surface by the acid etch while at the same time producing a thin phosphorus containing film that inhibits hydroxide coating growth during the ensuing water rinse and constitutes a conversion coating treatment. DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

A process according to the invention for treating a magnesium alloy surface com- prises, preferably consists essentially of, or more preferably consists of, at least the following operations in addition to rinsing with water:

(I) etching the magnesium alloy surface by contacting said surface with an acidic etching liquid having a pH from 1 through 5; and

(II) bringing the surface of the magnesium alloy as modified by etching in operation (I) into contact with an alkaline aqueous solution that has a pH from 7 through 14 and that contains an organophosphorus compound. Operation (I) as described immediately above may be briefly denoted hereinafter as "etching" and operation (II) as described immediately above may be briefly denoted hereinafter as "desmutting". Preferably, before beginning operation (I) as described above, the magnesium alloy surface is degreased in order to remove organic contaminants such as machine oil and cutting oil and is only thereafter brought into contact with the acidic etching liquid. The degreasing liquid used for this purpose preferably contains surfactant and independently preferably has a pH value from 9 to 14 if alkaline or a pH value from 0 to 6 if acidic. The aforesaid acid etching liquid preferably contains at least one carboxylic acid compound selected from the group consisting of glycolic acid, citric acid, tartaric acid, malic acid, oxalic acid, malonic acid, formic acid, acetic acid, lactic acid, glutaric acid, propionic acid, butyric acid, benzoic acid, and phthalic acid. The concentration of these organic acids is not critical, but in order to obtain a reasonably fast etching rate at a reasonable cost, the total concentration of these organic acids preferably is at least, with increasing preference in the order given, 2, 5, 8, 11 , 13, 15, 17, or 19 grams of these organic acids per liter of etching liquid, the unit of concentration being freely applied hereinafter to any component in a liquid and being hereinafter usually abbreviated as "g/l"; independently, this concentration of these organic acids preferably is not more than, with increasing preference in the order given, 100, 75, 50, 45, 40, 37, 34, 31 , or 29 g/l.

The aforesaid organophosphorus compound in a desmutting treatment liquid used in a process according to the invention preferably is at least one selection from the group consisting of hydroxyethanediphosphonic acid, aminotrimethylenephosphonic acid, ethylenediaminetetramethylenephosphonic acid, diethylenetriaminepentamethyl- enephosphonic acid, hydroxyliminobismethylenephosphonic acid, and hexamethylene diaminetetramethylenephosphonic acid. In addition, the aqueous solution used in the desmutting step under consideration preferably also contains at least one inorganic phosphoric acid selected from the group consisting of orthophosphoric acid, pyrophosphoric acid, and tripolyphosphoric acid. Inasmuch as the desmutting liquid is preferably strongly alkaline as indicated later, these acids will probably be present predominantly in the form of one or more of their salts in the actual desmutting liquid, but the stoichiometric equivalent as acid is used for any quantitative description of preferred amounts of these acids even if the acids have been completely or partially neutralized.

Preferably, when maximized corrosion resistance is desired, desmutting operation (II) as described above is followed by forming a conversion coating on the surface as treated at the end of desmutting operation (II). More preferably the conversion coating is formed by contact with either:

(A) an acidic aqueous solution that has a pH from 2 through 6 and that contains orthophosphoric acid and at least one metal ion selected from the group consisting of Zn, Fe, Mn, Mg, and Ca; or

(B) an acidic aqueous solution that has a pH of 2 through 6 and that contains:

(1 ) at least one fluorine compound selected from the group consisting of hydrofluoric acid, hexafluorosilicic acid, hexafluorozirconic acid, and hexa- fluorotitanic acid; and (2) at least one metal oxyacid compound selected from the group consisting of the oxyacids of Cr, Mo, W, Re, and V. The necessary acid etching operation in this invention is believed to completely dissolve and remove the part of the treated surface containing any release agent and alloy segregation layer. Although the thickness of the outermost layer containing any release agent and alloy segregation layer will vary with the product shape, position on the product, and casting conditions, this thickness generally is from 5 to 10 micrometres, hereinafter usually abbreviated as "μm", inward from the exterior surface. Etching conditions capable of removing a depth from 10 to 15 μm from the original surface of the treated substrate are therefore preferably established. Failure to achieve an etch of at least 5 μm usually will leave at least some release agent and alloy segregation layer, resulting in a decline in the corrosion resistance and paint adherence and in an increase in the surface electrical resistance. The etching conditions can be adjusted through the parameters of concentration, temperature, and time.

This acid etch step can also remove organic contaminants such as machine oil and cutting oil at the same time as removal of the outermost layer containing the release agent and alloy segregation layer. However, the implementation of a degreasing step prior to this acid etch step is preferred, because this enables the acidic etch liquid to be used effectively for a longer time without replacement. The composition of the degreasing liquid is not particularly critical as long as the degreasing liquid has the ability to remove organic contaminants. In a preferred embodiment, the degreasing liquid contains surfactant and uses base or acid. Usable as the base are, for example, the hydroxides, phosphates, silicates, and carbonates of alkali metals; usable as the acid are, for example, sulfuric acid, nitric acid, and tartaric acid. The surfactant can be a nonionic surfactant, anionic surfactant, or cationic surfactant. The additional presence of a chelating agent is preferred in order to improve the degreasing efficiency. The time and temperature of contact by the degreasing liquid with the magnesium alloy are not critical, but contact at 35 to 70 °C for 2 to 10 minutes is usually preferred.

This acid etch preferably is followed by a thorough rinse with water and then contact with an alkaline liquid that contains an organophosphorus compound chelating agent. This latter step serves to selectively dissolve and remove the alloying component smut left on the surface by the acid etch step. Since the main component of this smut is aluminum present as an alloying component in the magnesium alloy being treated in a process according to the invention, phosphonic acid compounds, with their known ability to preferentially chelate aluminum are effectively used as the chelating agent under consideration. The concentration of the phosphonic acid compound is not critical, but it preferably is at least, with increasing preference in the order given, 5, 10, 15, 20, 23, 25, 27, or 29 g l and independently preferably is not more than, with increasing preference in the order given, 100, 90, 80, 70, 60, 50, 45, 40, 37, 35, 33, or 31 g/l. An alkaline pH is preferred for inducing a preferential smut dissolution, because the aforementioned chelating agents function most effectively in the alkaline region and because dissolution of mag- nesium is retarded in the alkaline region. The pH must be from 7 through 14, and preferably is from 9 through 13. A simple solution of phosphonic acid(s) in water can be adjusted into this pH range by addition of at least one pH regulator, which preferably (primarily for economy and convenience) is selected from the group consisting of hydroxides, phosphates, and carbonates. Contact between etched magnesium alloy and a desmutting treatment liquid as described above results in the formation of a thin phosphorus compound film on the magnesium alloy surface concomitantly with smut removal. This thin phosphorus compound film has the ability to inhibit hydroxide film growth in the ensuing water rinse step without negatively impacting the paint adherence or surface electrical resistance and for this reason supports the formation of a fine, dense conversion coating. While the time and temperature of contact between the subject treatment liquid and the magnesium alloy are not critical, contact at 60 to 90 °C for 1 to 10 minutes is preferred. An even finer and denser phosphorus compound thin film is formed on the magnesium alloy surface when the desmutting liquid also contains at least one inorganic phosphoric acid or salt thereof, the acid preferably being selected from the group consisting of orthophosphoric acid, pyrophosphoric acid, and tripolyphosphoric acid. In order to increase the effectiveness of the water rinse in actual surface treatment lines, the operator of the treatment line may choose to increase the temperature of the water rinse and/or the length of the rinse time. However, these measures also lead to even more favorable conditions for hydroxide film growth. The addition of an inorganic phosphoric acid as described above is effective for restraining hydroxide film growth even under these more favorable conditions. While the inorganic phosphoric acid concentration is not critical, this concentration (which for a salt is measured as its stoichiometric equivalent as the corresponding acid) preferably is at least, with increasing preference in the order given, 0.1 , 0.3, 1.0, 2.0, 3.0, 4.0, 5.0, or 6.0 g/l and independently preferably is not more than, with increasing preference in the order given, 30, 25, 20, 17, 14, 11 , 9, or 7 g ι.

Execution of the sequence through the desmutting step yields a surface that is clean and that evidences some degree of corrosion resistance. A conversion coating treatment is optionally performed in order to impart better paint adherence and an even higher corrosion resistance. The conversion treatment liquid can be an acidic liquid with a pH of 2 through 6 that contains orthophosphoπc acid and at least one type of metal ions selected from the group consisting of Zn, Fe, Mn, Mg, and Ca The main component of the resulting coating is a phosphate of the selected metal or metals in the conversion treatment liquid The conversion coating weight can be adjusted through the 5 parameters of treatment liquid concentration, treatment temperature, and treatment time to yield a conversion coating suited to the expected conditions of use, which are generally known in the extensive conversion coating art The orthophosphoπc acid content and the metal ions content in the liquid are also not critical, but are preferably from 0 1 to 50 g/l Narrower preferences for specific applications may be taken from ιo prior conversion coating art

Also usable as the conversion treatment liquid are acidic aqueous solutions with a pH of 2 through 6 that contain at least one fluorine compound selected from the group consisting of hydrofluoric acid, hexafluorosilicic acid, hexafluorozirconic acid, and hexa- fluorotitanic acid and at least one metal oxyacid compound selected from the group con- 5 sisting of the oxyacids of Cr, Mo, W, Re, and V Again, the coating weight is not critical but is desirably adjusted in correspondence to the required properties The coating weight can be adjusted through the parameters of treatment liquid concentration, treatment temperature, and treatment time The fluorine compound content and the metal oxyacid compound content in the liquid are also not critical, but are preferably from 0 1 o to 50 g/l Narrower preferences for particular intended uses can be taken from prior conversion coating art

Execution of a surface treatment described in the foregoing preferably is followed by a thorough water rinse, a rinse with purified water, and finally drying The drying conditions are not critical Painting can then be carried out if such is desired The paint can 5 be a solvent-based or water-based system

EXAMPLES AND COMPARISON EXAMPLES

Several working examples of the surface treatment method of this invention are provided below and are not to be regarded as limiting the invention except to whatever extent their conditions may be incorporated into the appended claims The effectiveness o of these working examples will be made clear by comparison with comparative examples TEST MATERIAL

Die castings of Type AZ91 D magnesium alloy with dimensions of 100 millimeters (hereinafter usually abbreviated as "mm") x 100 mm x 1 mm 5 EVALUATION OF THE EFFICACY OF RELEASE AGENT REMOVAL

The extent of release agent removal was evaluated by measuring the residual total organic carbon on the surface of the test specimen using a total organic carbon instrument for use with solids (TOC5000-A/SSM5000-A from Shimadzu) The test specimen was cut to 10 x 30 mm, treated through the conversion treatment step, and dried The release agent remaining on the surface of the test specimen was then subjected to combustion at 600 °C for 10 minutes, and the residual total organic carbon was determined using the infrared absorbance of the evolved carbon dioxide In general, lower values for the residual total organic carbon are indicative of a better efficacy of release agent removal and result in a better corrosion resistance and paint adherence and a lower surface electrical resistance EVALUATION OF THE POST-CONVERSION TREATMENT CORROSION RESISTANCE

The conversion-treated test specimen was submitted to salt-spray testing with 5 % aqueous sodium chloride solution according to Japanese Industrial Standard Z-2371 for 240 hours, after which the corroded surface area was determined visually and reported on the following scale: + + + ^• less than 5 % corroded area,

+ + : corroded area is at least 5 %, but less than 10 %;

+ : corroded area is at least 10 %, but less than 20 %; x : corroded area is at least 20 %

EVALUATION OF THE POST-PAINTING CORROSION RESISTANCE The painted test specimen was subjected to the above-described salt-spray test for 240 hours, after which the corrosion blister width from a cross cut was measured EVALUATION OF THE PAINT ADHERENCE

The conversion-treated test specimen was spray-painted with a solvent-based epoxy system paint system (two coat/one bake, total thickness 50 μm, dried for 20 min- utes at 150 °C, and then submitted to evaluation of the paint adherence This evaluation was carried out using an Elcometertest instrument (from Cotek) that measures the bonding force when the paint film is forcibly peeled off In general, higher values in this test are indicative of a better paint adherence EVALUATION OF THE SURFACE ELECTRICAL RESISTANCE The surface electrical resistance was measured with a Rolester MP surface electrical resistance meter from Mitsubishi Kagaku, Model No. MCP-T350, MCP-TP01 two- point probe

Example 1 Treatment Sequence: degreasing - water rinse - acid etch - water rinse - desmutting - water rinse - conversion coating - water rinse - purified water rinse - drying

- painting - drying. Treatment Liquid Compositions and Treatment Conditions:

Degreasing: a solution in water of FINE CLEANER® 4442 concentrate from Nihon

Parkerizing Co., Ltd., 30 g/l, 60 °C, 5.0 minutes, dipping; Water Rinse: 25 °C, 30 seconds, dipping; Acid Etching: 28 g/l of citric acid in water, 25 °C, 5.0 minutes, dipping;

Desmutting: 30 g/l hydroxyethanephosphonic acid in water, pH 12, adjusted with

NaOH; 80 °C, 5.0 minutes, dipping; (Total Etch Depth from Alkaline Degreasing Through Desmutting: 8 μm); Conversion Coating: a solution in water of MAGBOND® P10 concentrate from Nihon Parkerizing Co., Ltd., 70 g/l, 43 °C, 5.0 minutes, dipping, add-on of manganese

= 75 milligrams per square meter of surface treated, this unit of add-on being hereinafter usually abbreviated as "mg/m²".

Example 2 Surface treatment was carried out using the sequence and treatment liquids described for Example 1 , except in this instance omitting the degreasing operation (and the water rinse immediately following degreasing). In this instance, the total etch depth from acid etching through desmutting was 7 μm.

Example 3 Surface treatment was carried out using the treatment sequence and treatment liquids as described for Example 1 , except for changing the etching treatment liquid to a solution in water of 20 g/l of tartaric acid. The total etch depth from alkaline degreasing through desmutting was 8 μm.

Example 4 Surface treatment was carried out using the same treatment liquids and treatment sequence as for Example 2, except that before acidic etching, the test material had been preliminarily subjected to shot-blasting. The total etch depth from alkaline degreasing through desmutting was 8 μm.

Example 5

Surface treatment was carried out by the treatment sequence and treatment liquids described for Example 1 , except that in the desmutting step, a solution in water of 30 g/l of diethylenetriaminepentamethylenephosphonic acid, pH adjusted as specified for Example 1 was used instead of the desmutting liquid used in Example 1. The total etch depth from alkaline degreasing through desmutting was 8 μm.

Example 6 Surface treatment was carried out using the same treatment sequence and treatment liquids as in Example 1 , except that every water rinse was lengthened to 5.0 min- utes duration.

Example 7 Surface treatment was run by the sequence of treatments and the same treatment liquids as described in Example 1 , except that the desmutting liquid for Example 7 contained 10 g/l of sodium pyrophosphate in addition to 30 g/l of hydroxyethanephos- phonic acid and the separate conversion coating operation and its immediately subsequent water rinse were eliminated.

Comparative Example 1 Surface treatment was carried out using the same treatment sequence and treat- ment liquids as in Example 1 , except for omitting both the acidic etching and the desmutting operations and their immediately subsequent water rinses.

Comparative Example 2 Surface treatment was carried out using the same treatment sequence and treatment liquids as in Example 1 , except that the desmutting operation and its immediately subsequent water rinse were eliminated and the contact time in the acidic etching operation was reduced to 1.0 minute. The total etch depth from degreasing through acid etching in this instance was only 2 μm.

Comparative Example 3 Surface treatment was carried out using the same treatment sequence and treat- ment liquids as for Example 1 , except that in this instance the desmutting treatment liquid was a solution in water of 100 g/l of ethylenediaminetetraacetic acid, adjusted to pH 12 with NaOH. The total etch depth from alkaline degreasing through desmutting in this instance was 10 μm. (This comparative example was a follow-up examination of Japanese Laid Open (Kokai or Unexamined) Patent Application Number Hei 6-220663). Comparative Example 4

This comparative example was a follow-up examination of Japanese Laid Open

(Kokai or Unexamined) Patent Application Number Hei 2-25430. The treatment sequence was: degreasing - water rinse - chemical etching - water rinse - fluoride treatment - water rinse - neutralization → water rinse - conversion treatment - water rinse - purified water rinse - drying - painting → drying.

Treatment Liquid Composition and Treatment Conditions (Those not described below were the same as for the operation with the same name in Example 1 ): Chemical etching: A solution in water that contained 150 g/l of potassium pyrophosphate trihydrate, 100 g/l of sodium nitrate, and 50 g/l of sodium sulfate was the etching liquid; 80 °C, 2 minutes, dipping;

Fluoride treatment: A solution in water that contained 200 milliliters of 85% by weight phosphoric acid in water per liter of solution and 100 g/l of ammonium acid fluoride was the treatment liquid; 25 °C, 1 .0 minute, dipping; Neutralization: A solution in water containing, per liter of solution, 25 grams of sodium dihydrogen phosphate monohydrate; 25 milliliters of concentrated aqueous ammonia, and 10 grams of diammonium monoacid citrate was used as the treatment liquid; 25 °C, 1.0 minute, dipping. In this comparative example, the total etch depth from alkaline degreasing through neutralization was 7 μm.

Comparative Example 5 Surface treatment was carried out using the same treatment sequence as in Example 1 , except that in the desmutting operation the treatment liquid contained no chelating agent and instead was simply a solution of NaOH in water with a pH of 12. The total etch depth from alkaline degreasing through desmutting was 7 μm.

Comparative Example 6 In this comparative example, all operations and treatment liquids were the same as for Comparative Example 2, except that every water rinse was lengthened to 5.0 minutes duration.

Values obtained by the above noted tests of the magnesium alloy substrate surfaces prepared in each of these examples and comparative examples are shown in Table 1.

These results demonstrate that Examples 1 through 7 of this invention gave a better efficacy of release agent removal, corrosion resistance, and paint adherence as well as a lower surface electrical resistance than did Comparative Examples 1 through 6. In addition, no deterioration in properties was observed in Example 6 with its lengthened water rinse time. Therefore, surface treatment of die-cast or thixomolded magnesium alloy by the method of this invention produces a highly corrosion-resistant and strongly paint-adherent surface that also has a low surface electrical resistance. In addition, since a preferred process according to the invention completely dissolves and removes any release agent and alloy segregation layer and inhibits growth of any hydroxide coating during subsequent water rinsing, it substantially eliminates any influence from the casting conditions, product shape, and position on the product. Table 1

Claims

1. A process for treating a magnesium alloy surface to improve the corrosion resistance thereof, said process comprising the following operations in addition to rinsing with water: 5 (I) etching the magnesium alloy surface by contacting said surface with an acidic etching liquid having a pH from 1 through 5; and

(II) bringing the surface of the magnesium alloy as modified by etching in operation (I) into contact with an alkaline aqueous desmutting solution that has a pH from 7 through 14 and that contains at least one organophosphorus compound. o 2. A process according to claim 1 , wherein operation (I) of claim 1 is preceded by degreasing the magnesium alloy surface.

3. A process according to claim 2, wherein said degreasing is effected by contacting the magnesium alloy surface with at least one of: an alkaline aqueous solution that has a pH of 9 through 14 and that contains s inorganic salt and surfactant; and an acidic aqueous solution that has a pH of 0 through 6 and that contains acid and surfactant.

4. A process according to claim 3, wherein the acidic aqueous treatment solution used in operation (I) contains a total of at least 2.0 g/l of compounds selected from the o group consisting of glycolic acid, citric acid, tartaric acid, malic acid, oxalic acid, malonic acid, formic acid, acetic acid, lactic acid, glutaric acid, propionic acid, butyric acid, benzo- ic acid, and phthalic acid.

5. A process according to claim 2, wherein the acidic aqueous treatment solution used in operation (I) contains a total of at least 2.0 g/l of compounds selected from the 5 group consisting of glycolic acid, citric acid, tartaric acid, malic acid, oxalic acid, malonic acid, formic acid, acetic acid, lactic acid, glutaric acid, propionic acid, butyric acid, benzo- ic acid, and phthalic acid.

6. A process according to claim 1 , wherein the acidic aqueous treatment solution used in operation (I) contains a total of at least 2.0 g/l of compounds selected from the o group consisting of glycolic acid, citric acid, tartaric acid, malic acid, oxalic acid, malonic acid, formic acid, acetic acid, lactic acid, glutaric acid, propionic acid, butyric acid, benzo- ic acid, and phthalic acid.

7. A process according to claim 6, wherein said organophosphorus compound used in operation (II) is a phosphonic acid compound.

8. A process according to claim 7, wherein said desmutting liquid comprises: a total stoichiometric equivalent of at least 5.0 g/l of one or more compounds selected from the group consisting of hydroxyethanediphosphonic acid, aminotri- methylenephosphonic acid, ethylenediaminetetramethylenephosphonic acid, di- ethylenetriaminepentamethylenephosphonic acid, hydroxyliminobismethylene- phosphonic acid, and hexamethylenediaminetetramethylenephosphonic acid; and a total stoichiometric equivalent of at least 0.1 g/l of one or more acids selected from the group consisting of orthophosphoric, pyrophosphoric, and tripolyphos- phoric acids.

9. A process according to claim 5, wherein said organophosphorus compound used in operation (II) is a phosphonic acid compound.

10. A process according to claim 9, wherein said desmutting liquid comprises: a total stoichiometric equivalent of at least 5.0 g/l of one or more compounds selected from the group consisting of hydroxyethanediphosphonic acid, aminotri- methylenephosphonic acid, ethylenediaminetetramethylenephosphonic acid, diethylenetriaminepentamethylenephosphonic acid, hydroxyliminobismethylene- phosphonic acid, and hexamethylenediaminetetramethylenephosphonic acid; and - a total stoichiometric equivalent of at least 0.1 g/l of one or more acids selected from the group consisting of orthophosphoric, pyrophosphoric, and tripolyphos- phoric acids.

1 1. A process according to claim 4, wherein said organophosphorus compound used in operation (II) is a phosphonic acid compound.

12. A process according to claim 1 1 , wherein said desmutting liquid comprises: a total stoichiometric equivalent of at least 5.0 g/l of one or more compounds selected from the group consisting of hydroxyethanediphosphonic acid, aminotri- methylenephosphonic acid, ethylenediaminetetramethylenephosphonic acid, diethylenetriaminepentamethylenephosphonic acid, hydroxyliminobismethylene- phosphonic acid, and hexamethylenediaminetetramethylenephosphonic acid; and a total stoichiometric equivalent of at least 0.1 g/l of one or more acids selected from the group consisting of orthophosphoric, pyrophosphoric, and tripolyphos- phoric acids.

13. A process according to claim 3, wherein said organophosphorus compound used in operation (II) is a phosphonic acid compound.

14. A process according to claim 13, wherein said desmutting liquid comprises: a total stoichiometric equivalent of at least 5.0 g/l of one or more compounds se- 5 lected from the group consisting of hydroxyethanediphosphonic acid, aminotri- methylenephosphonic acid, ethylenediaminetetramethylenephosphonic acid, di- ethylenetriaminepentamethylenephosphonic acid, hydroxyliminobismethylene- phosphonic acid, and hexamethylenediaminetetramethylenephosphonic acid; and o - a total stoichiometric equivalent of at least 0.1 g/l of one or more acids selected from the group consisting of orthophosphoric, pyrophosphoric, and tripolyphos- phoric acids.

15. A process according to claim 2, wherein said organophosphorus compound used in operation (II) is a phosphonic acid compound. s

16. A process according to claim 15, wherein said desmutting liquid comprises: a total stoichiometric equivalent of at least 5.0 g/l of one or more compounds selected from the group consisting of hydroxyethanediphosphonic acid, aminotri- methylenephosphonic acid, ethylenediaminetetramethylenephosphonic acid, diethylenetriaminepentamethylenephosphonic acid, hydroxyliminobismethylene- 0 phosphonic acid, and hexamethylenediaminetetramethylenephosphonic acid; and a total stoichiometric equivalent of at least 0.1 g/l of one or more acids selected from the group consisting of orthophosphoric, pyrophosphoric, and tripolyphos- phoric acids. 5

17. A process according to claim 1 , wherein said organophosphorus compound used in operation (II) is a phosphonic acid compound.

18. A process according to claim 17, wherein said desmutting liquid comprises: a total stoichiometric equivalent of at least 5.0 g/l of one or more compounds selected from the group consisting of hydroxyethanediphosphonic acid, aminotri- o methylenephosphonic acid, ethylenediaminetetramethylenephosphonic acid, diethylenetriaminepentamethylenephosphonic acid, hydroxyliminobismethylene- phosphonic acid, and hexamethylenediaminetetramethylenephosphonic acid; and a total stoichiometric equivalent of at least 0.1 g/l of one or more acids selected from the group consisting of orthophosphoric, pyrophosphoric, and tripolyphos- phoric acids.

19. A process according to any one of claims 1 through 18, wherein said magnesium alloy surface has been degreased before beginning operation (I) and said process comprises an operation (III) after operation (II) has been completed, operation (III) being either:

(A) an acidic aqueous solution that has a pH from 2 through 6 and that contains orthophosphoric acid and at least one metal ion selected from the group consisting of Zn, Fe, Mn, Mg, and Ca; or (B) an acidic aqueous solution that has a pH of 2 through 6 and that contains:

(1 ) at least one fluorine compound selected from the group consisting of hydrofluoric acid, hexafluorosilicic acid, hexafluorozirconic acid, and hexa- fluorotitanic acid; and

(2) at least one metal oxyacid compound selected from the group consisting of the oxyacids of Cr, Mo, W, Re, and V.