METHOD OF SURFACE TREATMENT FOR MAGNESIUM ALLOYS AND MAGNESIUM ALLOY MEMBERS TREATED FOR THIS
FORM BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION This invention relates to a novel surface treatment method that can be used to provide excellent corrosion resistance and excellent paint adhesion to the surface of magnesium alloys. This invention also relates to magnesium alloy members treated by the surface treatment method. 2. BACKGROUND OF THE TECHNIQUE Many of the metallic members (eg, aluminum alloys, steel, magnesium alloys) that are used to make automobiles, two-wheeled vehicles, and electronic consumer equipment must have a high corrosion resistance. and have an attractive appearance. As a consequence, the metallic members subjected to these requirements must, before their final use, pass through several surface treatments, typically followed by the application of paint. At least one of the purposes of the surface treatment is to remove contaminants, for example, cutting oil, which may remain on the substrate surface and form a coating on the surface.
dense, relatively thin conversion on the surface thus providing improved resistance to corrosion and better paint adhesion. With the problems of global environmental protection becoming a matter of increasing importance, an active effort has been made recently to use magnesium alloys, which represent the lightest of metals in widespread use and have excellent recycling capabilities. For example, in the automotive sector, magnesium alloys have begun to be used in components manufactured before other materials such as steel or aluminum alloy; The purpose of this use is to reduce the weight of the vehicle in order to improve fuel efficiency. In the area of consumer electronics, there is a movement for the conversion of the plastics used until now to the highly recyclable magnesium alloys - the focus here is on the box and enclosures for notebook-type computers and portable telephones. Most of these magnesium alloy members are formed through casting methods known as casting the metal mold and tixovated. In these methods of emptying, the casting alloy, in melted or semi-melted condition, is introduced into the mold or given at high speeds and high pressures. As a result, these emptying methods offer
excellent dimensional accuracy and productivity. According to the particular product, the formation can also be effected by forging or pressing using sweet grades of magnesium alloy sheets. The magnesium alloy members under consideration, such as aluminum alloys and steels, are first subjected to a surface treatment and then painted. Since magnesium alloys are among the most active of the highly used metals, they are easily susceptible to corrosion, the formation of a uniform, thin, and dense conversion coating in the surface treatment process is even more critical for alloys of magnesium than for aluminum alloys and steels. Regardless of this, it is extremely difficult to form a uniform, fine, and dense conversion coating on magnesium alloys, due to the chemical heterogeneity of the surfaces of the magnesium alloys. The chemical heterogeneity of magnesium alloy surfaces will therefore be considered in greater detail below. Magnesium alloys used for automobiles, two-wheeled vehicles and consumer electronic equipment generally contain large amounts of alloying components, for example, aluminum and zinc or manganese, in order to improve properties such as formability and mechanical strength or ductility.
For example, AZ91, which is the most commonly used magnesium alloy for castings, contains 8% aluminum and 1% zinc as alloying components. In the case of surface treatment processes where chemical reactions are used to form conversion coatings, the behavior of these alloy components in the substrate often has a greater influence on the capacity for surface treatment. While it is considered desirable that these alloying components assume a uniform and microfine distribution in the material in order to cause the formation of a thin and dense conversion coating, the alloying components, (e.g., aluminum, zinc) in members of Magnesium formed by casting in metal molds or thixomoldeo frequently do not assume a uniform distribution in the material and present segregation. This segregation in the magnesium alloys will be considered here with additional details. Segregation generally refers to the phenomena in which the impurities and / or alloying components in a metal assume a non-uniform distribution. The occurrence of segregation during the solidification of the alloy is the most frequent case. For example, in terms of regions in contact with the die from the outlet during casting in metallic mold or thixomoldeo, the purity tends to be higher when solidification occurs
from the beginning (proximity of the spill), while the impurity and concentration of alloy components tend to be higher in the regions that subsequently solidify (near the gate). As for the central areas of thick sections of the product, alloy components can segregate in these areas in very high concentrations, since the solidification occurs here at the end; but also, due to the pressurization during the solidification after the injection in the processes of casting in metallic and thixomoldeo molds, a liquid phase in which alloy components are segregated in high concentrations can slide in the solid phases and finally filter towards the surface due to capillary phenomena. These types of segregations are known as macro segregation. On the other hand, when the metallographic structure of the magnesium alloy is taken into account, it is found that the alloy consists of a phase a (alpha phase) composed of high purity magnesium and a b phase (beta phase) composed of intermetallic compounds containing alloying components, for example, Mgl7A112. This phase b is frequently segregated in the grain boundaries instead of assuming a uniform distribution in the material. This type of segregation is known as micro segregation. The macro segregation presents several behaviors according to
factors such as the cooling rate and pressurization conditions during emptying. The same happens in the case of micro segregations. As a consequence, even in the case of the same alloy composition, the degree of segregation and the metallographic structure vary depending on the shape and region of the member and as a function of the emptying conditions. This in turn makes the surface chemically heterogeneous and this strongly affects the ability to form a thin, dense, and uniform conversion coating. Methods for the surface treatment of magnesium alloys have typically used one of the following three treatment sequences. (Treatment sequence 1) degreased - > rinse with water - > conversion treatment - > rinse with water - > rinse with pure water - > dried (Treatment sequence 2) degreased - > rinse with water - > chemical attack - > rinse with water - > conversion treatment - > rinse with water - > rinse with pure water - > dried (Treatment sequence 3) degreased - > rinse with
water - > chemical attack - > rinse with water - > dirt removal - > rinse with water - > conversion treatment - > rinse with water - > rinse with pure water - > Drying The main purpose of the degreasing process employed in each of the treatment sequences mentioned above is the removal of light organic contaminants such as machine oils and cutting oils. The main purpose of the chemical etching process is to dissolve and remove the most superficial layers, including, in addition to the light organic contaminants (machine oils, cutting oils), mold release agent, alloy segregation layer, and a layer of hydroxide. The main purpose of the soil removal process is to remove the dirt, that is, the erosion products produced by the chemical attack that remain on the surface after the chemical attack process, and remove concentrated alloy components on the surface without attack. chemical. The main purpose of the conversion treatment process is to form a conversion coating on the surface. The conversion coating can be, for example, a
chromic acid chromate system or a manganese phosphate system and works mainly to improve the corrosion resistance and adhesion of the paint film. The selection of a particular treatment sequence is carried out based on factors such as the required performance of the treatment surface and the magnitude of the surface contamination. For example, treatment sequence 2 or treatment sequence 3 is used for a member carrying a relatively large amount of mold release agent, while treatment sequence 1 is used when the member is only relatively charged weak with mold release agent. Numerous inventions and much information have been provided to date in relation to the surface treatment methods that we are discussing. The conversion treatment baths may themselves be broadly classified into treatment baths consisting of hexavalent chromium (chromate types) and treatment baths that do not contain hexavalent chromium (non-chromate types). Within the sphere of treatment baths containing hexavalent chromium, the treatment baths developed by Dow Chemical (United States of America) are widely known and have achieved commercialization. These baths
Treatment includes a treatment bath with chromic acid (the Dow 1 method), a treatment bath with dicromic acid (the Dow 7 method), a treatment bath with alkaline dicromic acid (the Dow 9 method), and a treatment bath of manganese chromate (the Dow 22 method). These relatively processing baths are not influenced by surface variations and offer excellent corrosion resistance and excellent paint film adhesion. However, the extravalent chromium present in these treatment baths can be harmful to humans. Sometimes the appearance of a treatment bath without hexavalent chromium is desired. The appearance of a surface treatment method using such a bath can also sometimes be desired to provide surfaces with still higher levels of corrosion resistance and paint film adhesion, and doing so with a still smaller influence of the variations Of the surface. Numerous inventions have appeared within the realms of conversion treatment baths that do not contain hexavalent chromium and some examples are given below. Japanese Patent Application Laid-Open (Koko uo Examined) No. Hei 5-58073 (58,073 / 1993), entitled "Anticorrosion treatment method for members made of magnesium alloy", [Teach anti-corrosion treatment for members made of magnesium alloy], teach the formation of a
corrosion-resistant protective coating by application to a member of an erosive bath containing at least one element selected from the group consisting of nitric acid, sulfuric acid, and phosphoric acid. The Japanese Open Patent Application (Kokai or unexamined) No. Hei 9-228062 (228,062 / 1997), entitled "Method for treating metal surfaces", teaches the use of at least a metal organ compound selected from metal alkoxides, metal acetylacetonates, and metal carboxylates, and at least one film-forming aid selected from acids, bases, and salts thereof, and organic compounds containing the hydroxide, carboxyl group , or amino. The Japanese Open Patent Application (Kokai or not examined) No. Hei 9-241861 (241, 861/1997), entitled "Method for treating the surface of magnesium alloy components and magnesium alloy components whose surface has been treated by said method" [Method for the surface treatment of magnesium alloy components and magnesium alloy components whose surface has been treated by this method], teaches the formation of a sparingly soluble salt of magnesium and organic acid on magnesium surfaces by surface reaction of a magnesium alloy member with the aqueous solution of an organic acid or the soluble salt of an organic acid. Application
Patent Open Patent (Kokai or Not Examined) No. Hei 9-24338 (24,338 / 1997), entitled "Method for forming the highly corrosion-resistant paint films on magnesium alloys" [Method for forming paint films highly resistant to corrosion in magnesium alloys], teaches the treatment with an aqueous solution containing zinc ion, manganese ion, phosphate ion, a fluorine compound, a film-forming aid, nickel ion, cobalt ion, and copper ion, each in concentrations specific. The Japanese Open Patent Application (Kokai or unexamined) No. Hei 8-35073
(35.073 / 1996), entitled "Method for modifying the surface of magnesium base metal moldings", teaches the treatment of molded metal parts based on magnesium with a solution aqueous containing at least one water soluble salt of permanganic acid or manganic acid. However, all these conversion treatment baths are strongly influenced by variations in the substrate and none of them offer stable performance. There are also inventions regarding the degreaser used in the degreasing process and the chemical attack agent used in the chemical attack process. For example, Japanese Open Patent Application (Kokai or unexamined) No. Sho 53-102231 (102,231 / 1998), entitled "Acid Rinse Bath for Magnesium and Magnesium Alloys".
rinsing for magnesium and magnesium alloys] teaches the addition of at least one acid selected from sulfuric acid, hydrochloric acid, nitric acid and oxalic acid to an aqueous solution containing a specific amount of persulfate salt. The Japanese Open Patent Application (Kokai or unexamined) No. Hei 6-220663 (220,663 / 1994), entitled "Method for removing from magnesium alloy surfaces" [Method for removing dirt from surfaces of magnesium alloys], teaches a dirt removal treatment that removes the remaining dirt on the surface after the magnesium alloy has been rinsed with acid. This dirt removal treatment is carried out using an aqueous alkaline solution containing a specific amount of ethylenediaminetetaacetic acid. The object of the above inventions is to remove the mold release agent, the oxide film, and the alloy segregation layer by etching the magnesium alloy surface. Regardless of the above, it is still difficult to use these methods to produce a fine, dense, and uniform conversion treatment on magnesium surfaces and therefore it remains difficult to obtain excellent corrosion resistance and excellent film film adhesion using these methods
COMPENDIUM OF THE INVENTION The present invention is therefore focused on the resolution of the problems identified above in the prior art. More specifically, an object of the present invention is to provide a surface treatment method for magnesium alloys wherein the method has the ability to form fine, dense, and uniform conversion coatings on surfaces of magnesium alloys bearing release agent. of mold, an oxide layer, and a segregation layer of alloying components (for example, aluminum, zinc) and in this base has the ability to provide excellent corrosion resistance and excellent paint film adhesion. A further object of the present invention is to provide members of magnesium alloys having the surface treated by the surface treatment method of the present invention. It has been unexpectedly discovered that these problems of the prior art can be solved and a thin, dense, uniform conversion coating can be formed on magnesium alloy surfaces by first contacting the magnesium alloy surface in a process of chemical attack with an aqueous solution containing a compound of phosphoric acid type and then executing a conversion treatment on the surface of the
magnesium. This etching process results in the formation of a magnesium phosphate coating while dissolving and removing the mold release agent, oxide layer, and segregation layer of the alloy components. The present invention is based on this discovery. More specifically, the method for treating the surface of the magnesium alloys comprises degreasing a surface of the magnesium alloy, etching the surface of the magnesium alloy, and forming a conversion coating on the surface of the magnesium alloy. the magnesium alloy, wherein the etching step comprises subjecting the surface of the magnesium alloy to an aqueous solution containing a compound of the phosphoric acid type to form a magnesium phosphate coating on the surface of the alloy of magnesium having a coating weight of 10 to 2,000 mg / m2, measured as phosphorus. After forming or molding by casting process (for example, casting in a metallic mold or thixomolding), press-working, or forging, the surface of the resulting magnesium alloy member will generally be chemically heterogeneous. This heterogeneity is caused, for example, by the presence on the surface of mold release agent coated in the die during casting,
by segregation on the surface of the components of the alloy (for example, aluminum and zinc or manganese), and by the growth on the surface of a thick oxide film caused by the reaction with atmospheric oxygen. This chemical heterogeneity makes it generally difficult to form a thin, dense and uniform conversion coating. The surface treatment method of the present invention is particularly effective in inducing the formation on such surfaces of a thin, dense and uniform conversion coating and as a consequence to induce the appearance of excellent corrosion resistance and excellent adhesion of paint film. on these surfaces. The compound of phosphoric acid type in the aqueous solution containing compound of phosphoric acid type used in the aforementioned chemical etching process preferably includes at least one component selected from the group consisting of orthophosphoric acid, phosphoric acid, pyrophosphoric acid, tripolyphosphoric acid, and alkali metal salts of the acids mentioned above. This aqueous solution preferably has a concentration of compound of phosphoric acid type within a range of 1 to 200 g / L and pH within a range of 1 to 12. The conversion treatment bath in the conversion treatment process of The present invention is preferably an aqueous acidic solution with a pH of 2 to 6 which
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it contains at least one orthophosphoric acid with the radons 1 metal ion selected from the group consisting of Zn, Mn, and Ca, or is preferably an aqueous solution with a pH of 2 to 6 containing an oxo-acid compound of at least 1 metal selected from the group consisting of Mn, Mo, W, Ta, Re, Nb, and V and at least 1 fluorine compound selected from the group consisting of hydrofluoric acid, fiuosilic acid, fiuozirconic acid and acid fluotitanico. Magnesium alloy members according to this invention characteristically comprise magnesium alloy members whose surface has been treated by the method of the present invention described above for the surface treatment of magnesium alloys. DETAILED DESCRIPTION OF THE PREFERRED MODALITY (S) (S)
The method for the surface treatment of magnesium alloys of the present invention (in some cases simply referred to below as the surface treatment method) will be explained in more detail below. The surface treatment method of the present invention is applied to magnesium alloy members. The type of magnesium alloy is not a critical factor and the magnesium alloy can include, without necessarily being limited to, casting alloys, for example,
AZ91, AM60, ZK51 or ZK61, or sweet alloys such as AZ31, AZ61, or ZK60. Any type of forming technique can be employed to form the magnesium alloy member. Suitable techniques include, but are not necessarily limited to, casting in metal mold, thixomolding, press molding, and forging. The method for the surface treatment of magnesium alloys of the present invention is based on the processing sequence 2 described above, ie, degreasing - > rinse with water - > chemical attack - > rinse with water - > conversion treatment - > rinse with water - > rinse with pure water - > drying However, the degreasing, chemical attack and conversion treatment processes are the critical processes in the method of the present invention, around which the other processes must be properly configured as necessary or desired. In addition, when the surface of the magnesium alloy workpiece is relatively importantly loaded with mold release agent or when the oxide film layer has grown to become relatively thick, the following treatment sequence is applied to the surface of the magnesium alloy. it is particularly preferred to obtain stable coating properties. Treatment sequence 4: Degreasing - > rinse with water - > chemical attack
(aqueous acid solution) - > rinse with water - > dirt removal - > rinse with water - > chemical attack - > rinse with water - > conversion treatment - > rinse with water - > rinse with pure water - > Drying The chemical attack process (acidic aqueous solution) present in the treatment sequence 4 is different from the chemical attack mentioned above with reference to the present invention to the extent that the former is simple chemical attack with the aqueous acidic solution. The first will be known below as "chemical attack process (aqueous acid solution)" to distinguish it from the chemical attack mentioned above. The various processes will be described below in the preferred order of execution. The degreasing process The surface of the magnesium alloy must be degreased before the chemical attack process; this degreasing helps to provide the formation of a thin, dense and uniform magnesium phosphate coating in the subsequent chemical attack process. The degreasing treatment in the degreasing process is carried out by contacting the magnesium alloy workpiece with a degreasing bath. Methods for contacting the alloy workpiece
Magnesium with the degreasing bath may be for example, an immersion method or a spray method as is known in the art. Any method can be used in this invention, and the concept of contact in each of the processes described below should be considered similarly. The composition of the degreasing bath used in the degreasing process is not critical to the extent that the bath can remove organic contaminants. However, preferably, an aqueous alkaline solution containing surfactant is employed in the degreasing bath. The alkaline element in said degreasing bath can be, for example, an alkali metal hydroxide, phosphate, silicate, or carbonate. The surfactant may be a nonionic, cationic, or anionic surfactant. A queiation agent can be added in order to improve degreasing efficiency. The temperature and the contact time interval between the degreasing bath and the magnesium alloy are not critical factors. Contact in a range of 35 to 70 ° C for 2 to 10 minutes is preferred depending on the magnitude of the surface contamination of the magnesium alloy. The concentration of the degreasing bath should be selected in such a way that it is appropriate taking into account factors such as the extinction of the contamination of the magnesium alloy surface and the components in
the degreasing bath. When the surface of the magnesium alloy workpiece is relatively strongly contaminated with mold release agent and / or when the oxide film layer has grown to a relatively large thickness, the surface of the magnesium alloy may be subjected to a cleaning treatment with shot blast followed by degreasing as a substitute for the treatment sequence 4 provided above. Shot blasting can physically remove contaminants that remain on the magnesium alloy surface. Degreasing with a degreasing bath may be omitted only when cleaning with shot blasting is applied. However, degreasing with a degreasing bath is preferably carried out even when the pellet cleaning has been carried out since the magnesium reaction surface subjected to pellet cleaning will continue to carry material subjected to abrasion by cleaning with Shots of pellets and oily material contained there. For the purposes of the present invention, the degreasing concept also includes a cleaning treatment with shot blasting. Due, . the degreasing process designated as an essential process in the present invention includes the following 3 variants: jet cleaning treatment
of pellets only, treatment with degreasing in a degreasing bath alone and degreased in a degreasing bath after cleaning treatment with pellets of pellets. The process of chemical attack (aqueous acid solution) Co or already mentioned, a chemical attack process (aqueous acid solution) is performed in a desirable manner before the chemical attack (see below) when the surface of the alloy workpiece Magnesium is charged relatively strongly with a mold release agent or when the oxide film layer has become relatively thick. This chemical attack process
(aqueous acid solution) can chemically remove contaminants that remain on the magnesium alloy surface and can produce a totally clean surface. The etching treatment in this etching process (aqueous acid solution) is carried out by contacting an aqueous acid solution and the magnesium alloy workpiece. The nature of the aqueous acid solution is not a critical factor in that it can effectively dissolve and remove contaminants on the magnesium alloy surface; however, the use of sulfuric acid, nitric acid, hydrochloric acid, tartaric acid, or oxalic acid is preferred. Conditions such as concentration and
The temperature of the aqueous acid solution at the time of contact with the magnesium alloy surface is also not critical and should be selected as appropriate taking into account such factors as the magnitude of the magnesium alloy surface contamination and the components of the aqueous acid solution. The process of removing dirt It is desirable when a chemical attack treatment has been carried out through the chemical attack process described above (acidic aqueous solution), that the chemical attack treatment has been followed by a process of removing dirt with the object to remove the societies that remain on the surface of the magnesium alloy. The removal of dirt in this process of removal of dirt is done by contacting the magnesium alloy workpiece with a dirt removal bath. The nature of this dirt removal bath is not critical insofar as it can effectively remove the remaining dirt on the magnesium alloy surface. The dirt removal bath, for example, may be an aqueous sodium hydroxide solution adjusted to a pH of at least 12 or may be a strongly alkaline aqueous solution containing a chelating component. Representative examples of this last type are
disclosed in Japanese Patent Application Open (Kokai or unexamined) No. 6-220663 (220,663 / 1994), entitled "Method for removing from magnesium alloy surfaces" [Method for removing dirt from magnesium alloy surfaces]. Conditions such as the concentration of the temperature of the dirt removal bath as well as the contact time with the magnesium alloy surface are not critical and should be appropriately selected taking into account factors such as the amount of dirt bound on the surface of magnesium alloy and the components of the dirt removal bath. The Chemical Attack Process In the chemical etching process, the magnesium alloy workpiece is contacted with an aqueous solution containing a compound of the phosphoric acid type. This induces the formation of a magnesium phosphate film while at the same time cleaning the surface of the magnesium alloy. Thus, the treatment by the etching process according to the present invention differs from the treatments with an aqueous acid solution generally labeled as chemical etching insofar as the former is also contemplated to result in the formation of a coating. A treatment through the chemical etching process of the present invention produces a phosphate coating of
magnesium while dissolving and removing at the same time the mold release agent, the oxide layer, and the segregation layer of alloy component that remain on the magnesium alloy surface. The deposition weight of the magnesium phosphate coating that is formed on the magnesium alloy surface should be 10 to 2,000 mg / m2, measured as phosphorus and is preferably 50 to 1,000 mg / m2, measured as phosphorus. The substrate can not be satisfactorily covered by the magnesium phosphate coating having a magnesium phosphate coating deposit weight of less than 10 mg / m2, measured as phosphorus, which creates a potential for lower corrosion resistance and Less adhesion of paint film. At the other end of the range, the coating becomes uneven if the coating has a magnesium phosphate deposit weight greater than 2,000 mg / m2, measured as phosphorus, which may again cause a decreased resistance to corrosion and adhesion. minor of the painting film. Since, as described above, the surface of the magnesium alloys is chemically heterogeneous, the magnesium phosphate coating will be more easily formed in the chemically active regions of the magnesium alloy surface, more specifically, this coating will be more easily in regions where
which aluminum alloy and zinc components have been segregated at relatively high concentrations and in the region that do not have a relatively coarse oxide coating. The present compound of phosphoric acid type in the aqueous solution containing compound of phosphoric acid type (abbreviated below as aqueous solution containing PAC) preferably comprises at least one element selected from the group consisting of orthophosphoric acid, phosphonic acids, pyrophosphonic acid, tripolyphosphoric acid, and the alkali metal salts of the above acids. The concentration of the aqueous solution containing PAC varies according to the type of compound of phosphoric acid type, but the concentration of the compound of phosphoric acid type will preferably be from 1 to 200 g / L. It is essentially impossible to obtain the specified phosphor deposit when the concentration of the phosphoric acid compound is below 1 g / L, while the coating may become uneven when the concentration exceeds 200 g / L. The aqueous solution containing PAC of the present invention preferably has a pH within a range of 1 to 12. A pH of less than 1 can produce an excessive etching and uneven coating. A pH in the strongly alkaline region (range above pH 12) produces a
chemical attack deficient and does not generate the specified phosphor deposit and therefore does not produce good corrosion resistance or good paint film adhesion. In addition, at the same time that the etching process induces the formation of a magnesium phosphate coating on the magnesium alloy surface, it also works through the surface attack, to dissolve and remove mold removal agent, layer of oxide, and the segregation layer of alloy components. As a consequence, the weak ability of a strongly alkaline treatment bath to chemically attack surfaces of magnesium alloys also prevents a complete decrease and removal of the mold release agent, oxide layer and alloy component segregation layer, and in that sense can again be a factor that has negative influences on the corrosion resistance and adhesion of a paint film. The aqueous solution containing PAC of the present invention preferably has a pH within a range of 1 to 10 and more preferably within a range of 1 to 7. The pH of the aqueous solution containing PAC is determined directly by the compound of the particular phosphoric acid type selected and its concentration. However, small pH adjustments can be adjusted through the proper addition of a base component to adjust to the side
alkaline of an acid component to adjust the acid side. The base component can be exemplified by sodium hydroxide, sodium carbonate, tertiary sodium phosphate, and ammonia, while the acid component can be exemplified by phosphoric acid, nitric acid, sulfuric acid, tartaric acid, and oxalic acid. The temperature and contact time between the aqueous solution containing PAC and the magnesium alloy workpiece vary depending on the species of the magnesium alloy workpiece and the nature, concentration and pH of the aqueous solution containing PAC itself; however, in all cases, the contact temperature and the contact time must be selected to produce the phosphor deposit specified above. The conversion treatment process Once the surface of the magnesium alloy workpiece has been cleaned and coated with a magnesium phosphate coating in the etching process, said surface is subjected to a conversion treatment process and subjected to a conversion treatment. The surface of the magnesium alloy workpiece is usually thoroughly rinsed with water between the etching process and the conversion treatment process. The conversion treatment is carried out by contacting the magnesium alloy workpiece with
a conversion treatment bath. There are no particular limitations with regard to the conversion treatment bath used here, and the conversion treatment baths known in the art for application to magnesium or magnesium alloy - including what are known as system conversion treatment baths. of chromate - can be used for the purposes of the present invention. Of course, chromate system conversion treatment baths contain environmentally suspect hexavalent chromium, and hexavalent chromium free conversion treatment baths are preferred. The conversion treatment bath in this process is preferably selected from two types (1) and (2) described below. (1) Acid aqueous solutions having a pH of 2 to 6 and containing at least orthophosphoric acid and at least 1 metal ion selected from the group consisting of Zn, Mn, and Ca. The concentration of orthophosphoric acid in the The conversion treatment bath of type (1) is preferably within a range of 10 to 100 g / L and more preferably within a range of 30 to 70 g / L. The concentration of metal ion in the conversion treatment bath of type (1) is preferably within a range of 1 to 10 g / L and more preferably within a range of 3 to 7 g / L:
The desired weight of the conversion coating deposit in the case of the conversion treatment with a conversion treatment bath type (1) depends on the species of metal ion, but preferably lies within a range of 30 to 300 mg / m2, measured as the metal ion, and more preferably is within a range of 50 to 200 mg / m2, measured as metal ion. (2) Acid aqueous solutions having a pH of 2 to 6 and containing an oxo acid compound of at least 1 metal selected from the group consisting of Mn, Mo, W, Ta, Re, Nb, and V and at least one fluorine compound selected from the group consisting of hydrochloric acid, fluosilicic acid, fluocirconic acid, and fluotitanic acid. The concentration of fluorine compound in a conversion treatment bath of type (2) is preferably within a range of 20 to 1,000 mg / L and more preferably within a range of 50 to 500 mg / L. The concentration of the metal oxoacid compound is preferably within a range of 0.5 to 10 g / L and more preferably within a range of 1 to 7 g / L. The desired conversion coating deposit weight in the case of the conversion treatment with a treatment bath of type (2) will depend on the species of metal ion, but preferably lies in a range of 10 to 300
mg / m2, measured as the metal ion and more preferably within a range of 30 to 200 mg / m2, measured as the metal ion. The temperature of the conversion treatment bath in the conversion treatment process and the contact time between the conversion treatment bath and the magnesium alloy workpiece should be selected as appropriate, taking into account factors such as composition of concentration of the conversion treatment bath, the surface condition of the magnesium alloy workpiece and the weight of the desired conversion coating deposit. The coating of magnesium phosphate formed on the surface of the magnesium alloy formation in the etching process has a low chemical activity, and consequently, almost no conversion coating is formed in this magnesium phosphate coating during the process of conversion treatment. The formation of the conversion coating occurs, however, in regions either not covered or not fully covered by the magnesium phosphate coating. In addition, the low chemical activity of the magnesium phosphate coating formed by the etching process causes this coating to act as a conversion coating to improve corrosion resistance and
adhesion of the paint film. Thus, - and the conversion treatment in the subsequent conversion treatment process of these regions not fully covered by the magnesium phosphate coating in the etching process - allows the formation of a thin, dense, uniform conversion coating (ie, a composite coating) on the chemically heterogeneous surface of the magnesium alloys. The overall result is the generation of excellent corrosion resistance and excellent paint film adhesion. The various processes of rinsing with water A process of rinsing with water is preferably placed between each of the above processes, in order to avoid disturbances (for example, deterioration of a treatment bath due to the introduction there of another bath of water). treatment) caused by the entrainment of the treatment bath in an upstream process (degreasing bath, acidic aqueous solution, dirt removal bath, aqueous solution containing PAC, or conversion treatment bath) in a subsequent process. The rinsing with water in each rinsing process with water is carried out by contacting the magnesium alloy workpiece with water. There are no specific limitations regarding the amount of rinsing with water (for example, contact time,
purity of water and temperature, number of stages in the water rinse, degree of dilution), and the conditions of rinsing with water should be selected in such a way that they are appropriate taking into account such factors as the concentration of the particular treatment bath and the amount of influence on when mixing in a downstream bath. The rinsing process with pure water The surface treatment according to this invention ends once the magnesium alloy has passed through the conversion treatment process. However, if the conversion treatment bath should remain on the surface, its concentration, during drying, could cause corrosion on the surface of the magnesium alloy or corrosion of the coating formed on the surface. In addition, the application of paint on a surface that still carries small amounts of contaminants can result in the appearance of defects such as lumps, shifts, and perforations on the painted surface. Finally, when the paint application operation is carried out by immersion, the impurities still present on the surface can be introduced into the paint bath with degradation resulting from the paint itself. It is for these reasons that the execution of a rinse with water
Pure is desirable for the purpose of replacing a conversion treatment bath that remains on the surface with pure water either free of impurities or containing only small amounts of impurities. The pure water used in this pure water rinse does not have to be pure water as such, and deionized water of the quality used as pure water in the paint industry will suffice for the present purposes. The drying process After the magnesium alloy has passed through the various processes described above (or has passed through a subset of these processes under the circumstances), it is desirable to dry it in order to evaporate the remaining moisture in the surface. Obviously, when applying paint with a water-based paint, drying is not essential, since, in this case, the application of paint is possible even with moisture on the surface. However, even in the case of application of paint with a water-based paint, a drying process is still desirable since the introduction of moisture into the paint can alter the concentration of the paint. The drying process itself is not critical and drying can be effected, for example, by spontaneous drying. However, drying in a furnace is preferably carried out using a convection heater or heater
infrared. A magnesium alloy member according to the present invention is obtained by employing the surface treatment of the magnesium alloy surfaces of the present invention in accordance with what is described above. The resulting magnesium alloy member in accordance with the present invention will exhibit excellent corrosion resistance without further processing, but may be painted as desired in order to obtain additional improvements in corrosion resistance or additional improvements in the characteristics aesthetics of the magnesium alloy member. There is no particular restriction as to the type of paint used in the painting operation, and either water based paints or solvent based paints can be used. There is also no particular restriction as to the paint application method, and any paint application method known in the art can be used, for example, spray painting, immersion, electrodeposition, etc. EXAMPLES Working examples of the surface treatment method of the present invention are given below, and the effectiveness of the working examples is illustrated by
comparison with comparative examples that are also offered below. The invention, however, is not limited to the following working examples. Small adjustments in the pH of the treatment baths were effected through the addition of adequate amounts of sodium hydroxide to adjust the alkaline side and phosphoric acid to adjust to the acidic side (excluding comparative example 6). The test materials The following three types of magnesium alloy sheet are used as test materials: • AZ91D (ASTM designation, cast in metal mold, 100 mm x 100 mm x 1 mm) • AM60B (ASTM designation, cast in mold metal, 100 mm x 100 mm x 1 mm) • AZ31C (ASTM designation, laminated sheet, 100 mm x 100 mm x 1 mm) (0064) Measurement of the coating tank 1. The magnesium phosphate coatings The weight of the deposit The coating of the magnesium phosphate coatings formed by the etching process was determined by measurement of the phosphorus deposit in the coating. The measurements were made using a commercial fluorescent x-ray diffraction instrument.
Multiple samples that have different known amounts of phosphorus deposit were measured in advance and the resulting intensity values (cps) were used to construct a work curve of intensity versus deposit amount. The samples produced in the working examples and comparative examples were measured under the same conditions, and the measured intensity values were converted into deposit amount using the work curve. The measurement samples were produced in the following working examples and comparative examples by treatment through the chemical etching process followed, without execution of the conversion treatment, by rinsing with water, drying, and cutting to the measurement size. Conversion coatings Since two types of conversion treatment baths as described above were used (a manganese system and a zircon system), the amount of coating deposit was determined by measuring the manganese deposit or zirconium deposit in the coating. Using a commercial fluorescent x-ray diffraction instrument, the measured intensity was
converted into a deposit amount using a work curve that has been preliminarily prepared in accordance with what is described above to measure the amount of phosphor deposit. The measurement samples were produced in the following working examples and comparative examples by executing the conversion treatment in the conversion treatment process followed by rinsing with water, drying, and cutting to the measurement size. Since manganese was also present as an alloying component in some of the test materials, the manganese deposit was obtained in such cases by subtracting the value measured before the conversion treatment from the measured value after the conversion treatment (the measurement value). of preconversion treatment was determined at the same time as the measurement of the phosphorus deposit, supra). Example 1 AZ91D was used as the test material and was subjected to surface treatment using the treatment sequence described below and the treatment bath compositions and treatment condition in each process in accordance with what is described below. Sequence of treatment: degreasing (alkaline degreasing) - »rinse with water -»
chemical attack - »rinse with water - > conversion - »rinse with water -» rinse with pure water - »drying. Treatment bath compositions and treatment conditions in each process • degreasing process (alkaline degreasing): FINECLEANER (registered trademark) MG101 from Nihon Parkerizing Co., Ltd, 30 g / L, 60 ° C, 5 minutes, immersion. • the chemical attack process: orthophosphoric acid, 30 g / L (adjusted to pH 2.5), 25 ° C, 2 minutes, immersion, phosphorus deposit: 200 mg / m2 • the conversion treatment process: MAGBOND (registered trademark) ) P20, a manganese system from Nihon Parkerizing Co., Ltd., 200 g / L, 43 ° C, 3 minutes, immersion, manganese deposit: 75 mg / m2 • water rinses between each process: tap water 25 ° C, 30 seconds, immersion • rinse with pure water: deionized water (electrical conductivity = 2 μS), distribution of flow over the entire surface • drying: drying in convection oven for 10 minutes at 120 ° C Example 2
The surface treatment was carried out as in Example 1, except that the treatment bath composition and the treatment conditions in the chemical etching process changed in accordance with that described below in comparison with the composition and the treatment conditions in Example 1. Likewise the chemical attack process: sodium orthophosphate, 30 g / L (adjusted to pH 9.5), 60 ° C, 5 minutes, immersion, phosphorus deposit: 130 mg / m2 Example 3 The surface treatment was carried out as in Example 1, except that the treatment bath composition and the treatment conditions in the chemical etching process changed in accordance with that described below in comparison with the composition and conditions of example 1. • the process of chemical attack: orthophosphoric acid, 30 g / L (adjusted to pH 2.5), 25 ° C, 6 minutes, immersion, phosphorus deposit: 500 mg / m2 Example 4 The surface treatment was carried out as in example 1, except that the treatment bath composition and the treatment conditions in the etching process changed in accordance with what is described below in
comparison with the treatment bath composition and the treatment conditions of example 1. • the etching process: orthophosphoric acid, 100 g / L (adjusted to pH 2.5), 25 ° C, 6 minutes, immersion, phosphor deposit : 1500 mg / m2 Example 5 The surface treatment was carried out as in Example 1, with the exception that the treatment bath composition and the treatment conditions in the chemical attack process were changed in accordance with what is described below in comparison with the treatment bath composition and the treatment conditions of example 1. • the etching process: orthophosphoric acid, 30 g / L (adjusted to pH 2.5), 25 ° C, 15 seconds, immersion, phosphorus deposit: 12 mg / m2 Example 6 The surface treatment was carried out as in Example 1, except that the treatment bath composition and the treatment conditions in the conversion treatment process were ca mies according to what is described below in comparison with the treatment bath composition and the treatment conditions of example 1.
• the conversion treatment process: MAGBOND (registered trademark) M30, a zirconium system from Nihon Parkerizing Co., Ltd., 50 g / L, 60 ° C, 1 minute, immersion, zirconium deposit: 50 mg / m2 Example 7 The surface treatment was carried out as in example 1 with the following changes: the test material was changed to AM60B; the deposit of phosphorus in the chemical attack process was adjusted to 180 mg / m2; and the manganese deposit in the conversion treatment process was adjusted to 70 mg / m2. EXAMPLE 8 The surface treatment was carried out as in Example 1 with the following changes: the test material was changed to AZ31C; the deposit of phosphorus in the chemical attack process was adjusted to 110 mg / m2; and the manganese deposit in the conversion treatment process was adjusted to 30 mg / m2. Comparative Example 1 The surface treatment was carried out as in Example 1, but in this case omitting the etching process from the treatment sequence described for Example 1. Comparative Example 2 The surface treatment was carried out as in the example
7, but in this case omitting the etching process from the treatment sequence described for example 7. Comparative Example 3 The surface treatment was carried out as in example 8, but in this case omitting the chemical etching process from the treatment sequence described for example 8. Comparative Example 4 The surface treatment was carried out as in example 6, but in this case omitting the etching process from the treatment sequence described for example 6. Comparative Example 5 The treatment surface was carried out as in Example 1, except that the treatment bath composition and the treatment conditions in the chemical etching process changed in accordance with that described below the treatment bath composition and the treatment conditions of example 1. • the chemical attack process: orthophosphoric acid, 30 g / L (adjusted to pH 2.5), 25 ° C, 10 seconds, immersion n, phosphorus deposit: 5 mg / m2 Comparative Example 6 The surface treatment was performed as in Example 1, except that the etching process described in Example 1 was changed to an etching process
(aqueous acidic solution) using the treatment bath composition and the treatment conditions described below. • the chemical attack process (aqueous acid solution): sulfuric acid, 20 g / L (adjusted to pH 2.5 with sodium hydroxide), 25 ° C, 30 seconds, immersion (the phosphor deposit was 0 mg / m2) Evaluation tests The following evaluation tests were carried out on the magnesium alloy members with surface treatment prepared in the working examples and comparative examples. The test results are reported in table 1 below. The results in the evaluation tests that are acceptable from a practical perspective are indicated with a rating of "more" or better. 1. Corrosion resistance after surface treatment Each magnesium alloy member with surface treatment (the sample) was tested for corrosion resistance directly without further processing. The sai spray method stipulated in JIS Z-2371 was used for the evaluation; the time of exposure to the salt spray was 72 hours. After finishing exposure to salt spray, the corrosion resistance after
of the surface treatment was evaluated by visual inspection of the state of development of oxidation in the sample. The results were rated using the following scale. ++: oxidized area less than 1% +: oxidized area of at least 1% but less than 3% •: oxidized area of at least 3% but less than 5% x: oxidized area greater than or equal to 5% Resistance Corrosion after paint application The sample for evaluation of corrosion resistance after paint application was prepared by applying 20 to 25 μm of a cationic electrodeposition paint (Electron 2000 from Kansai Paint Co., Ltd. ) on the surface of the magnesium alloy member with surface treatment and drying for 20 minutes at a temperature of 80 ° C. The salt spray method stipulated in JIS Z-2371 was used for the test. A cross was recorded in the paint film in the sample before the test. The exposure time to the salt spray was 720 hours. At the end of the exposure to the salt spray, the corrosion resistance after the application of paint was evaluated by measuring the blister width of only one side of the cross carried on the
sample. The results were qualified using the following scale. ++: the width of the blister on one side of the cross cover is less than 1 mm +: the width of the blister on one side of the cross cover is at least 1 mm but less than 3 mm •: the width blister on one side of the cross is at least 3 mm but less than 5 mm x: the blister width of only one side of the cross cover is equal to or greater than 5 mm. Adhesion of paint film The sample for evaluation of adhesion of paint film was prepared by means of the application of paint on the magnesium alloy member treated on its surface in accordance with that described above in 2. Resistance to corrosion after paint application. The adhesion of the paint film was evaluated based on the number of paint squares remaining in the test through the method of reticulation / detachment with tape to test the adhesion of a paint film (JIS K-5400, 1 mm x 1 mm, 100 squares). The evaluation was carried out both initially and after a water resistance test for 1,000 hours at 40 ° C. The results were
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qualified using the following scale. ++: absence of paint film detachment (number of remaining paint squares = 100/100) +: the number of remaining paint squares is less than 100/100 but is at least 98/100 •: the number of remaining paint squares is less than 98/100 but at least 95/100 x: the number of remaining squares of paint is less than 95/100 Table 1. Evaluation Results Material Deposit Treatment of Test P mg / m2 conversion Example 1 AZ91D 200 Manganese Example 2 AZ91D 130 Manganese Example 3 AZ91D 500 Manganese Example 4 AZ91D 1500 Manganese Example 5 AZ91D 12 Manganese Example 6 AZ91D 200 Zirconium Example 7 AM60B 180 Manganese Example 8 AZ31C 110 Manganese Example AZ91D 0 Manganese Comparative 1 Example AM60B 0 manganese
Comparative 2 Example AZ31C 0 Manganese Comparative 3 Example AZ91D 0 Zirconium Comparative 4 Example AZ91D Manganese Comparative 5 Example AZ91D 0 Manganese Comparative 6 Resistance to Film Adhesion Paint Corrosion PostPostInitial After Conversion Apply Water Resistance Testing Resistance
Example 1 ++ ++ ++ ++ Example 2 ++ ++ ++ ++ Example 3 ++ ++ ++ ++ Example 4 ++ ++ ++ ++ Example 5 + ++ ++ ++ Example 6 ++ ++ ++ ++ Example 7 ++ ------ ++ ++ Example 8 + + ++ ++ Example x ++ + Comparative 1
Example • x ++ + Comparative 2 Example xx ++ • Comparative 3 Example • • ++ + Comparative 4 Example • • ++ ++ Comparative 5 Example • • +++ Comparative 6 In Comparative Example 5, the deposit of the Magnesium phosphate coating was lower than the specified amount and in this case satisfactory properties were not obtained. Satisfactory properties were also not obtained in Comparative Example 6, which used sulfuric acid in the chemical attack process. As described above, the surface treatment of magnesium alloy members by the surface treatment method of the present invention produces a highly corrosion resistant surface having excellent paint adhesion. Since the surface treatment method of the present invention has the ability to induce the formation of a uniform, fine, dense conversion coating even on chemically heterogeneous surfaces, it can produce properties
stable that are resistant to the effects or influences of factors such as product form, region in the product, and emptying conditions. The chemical etch bath used in this invention does not contain hexavalent chromium which can be harmful to humans and the environment and, considering that what is known as a chromate agent is not used for the conversion treatment, it will be obtained as result a high commercial and industrial utility. While embodiments of the present invention have been illustrated and described, it is not contemplated that these embodiments illustrate and describe all possible forms of the invention. The words used in the specification are terms of description and not limitation, and it is understood that several changes may be made without departing from the spirit and scope of the invention.