EP3294926B1

EP3294926B1 - Process for surface treating a magnesium alloy and process for electroless nickel plating a treated surface

Info

Publication number: EP3294926B1
Application number: EP15709985.4A
Authority: EP
Inventors: Emanuele Albanese
Original assignee: Dbs Group International Trust Co Srl
Current assignee: Dbs Group International Trust Co Srl
Priority date: 2015-01-27
Filing date: 2015-01-27
Publication date: 2021-07-14
Anticipated expiration: 2035-01-27
Also published as: EP3294926A1; WO2016120653A1

Description

TECHNICAL FIELD

The present invention relates to a method for superficial treatment of a magnesium alloy, in particular a electroless nickel plating of a magnesium alloy superficially pre-treated.

BACKGROUND ART

Magnesium is an alkaline hard metal with a density 1,76 g/cm3, of white silver colour and is one of the more widespread element on the planet.
Magnesium has a low thermal specific capacity and latent heat relatively low. Further, magnesium has a low affinity with the steel and is easily to recycle.
These properties make easy moulding and casting process with respect, for example, to the aluminium.
Magnesium has an excellent castability, high dimensional stability, high ratio resistance-specific weight, a low melting point, good electromagnetic shielding and very good mechanical properties.
The use of such material is impeded for two important limits. First, when magnesium is exposed to the air, is covered of magnesium oxide that, when is in form of dust, may develop an exothermic reaction in the presence of oxygen and water vapour. Second, the magnesium is less noble than other materials (lower potential electronegative) and tends to corrode easily.
To overcome the cited drawbacks, is known to apply coating on objects made of a magnesium alloy, to insulate the surface of the object from the corrosive environment, and/or add specific elements in the composition of the alloy to avoid trigger of inner corrosion processes of the crystalline structure.
Is known from the state of the art protecting magnesium alloys through proper superficial coatings, e.g. through painting.
As well known from the state are metallization method of a plastic or metallic material, as the ABS, polycarbonate, polyamide, steel, cast iron, brass, aluminium and copper.
Amongst the more spread metallization methods are nickel plating or electroless nickel plating, chromium plating, galvanizing, oxidizing, silver plating, gold plating, palladium plating.
To carry out these metallizations, the base material needs a specific preliminary superficial pre-treatment and the type of pre-treatment is specific for each kind of base material.
A common problem of metallization processes is the cleaning of the surface of the base material. This cleaning allows the coating material to best adhere to the base material.
Coating layers may also be manifold to sum effects of each layer. In the automotive sector, ABS or aluminium are coated with copper, nickel and chrome.
For magnesium alloys, are known anodizing processes to remove magnesium hydroxide, also known as magnesium oxide, from the surface of a component made of magnesium alloy. In these processes, magnesium hydroxide Mg(OH)₂ is removed from the component through degreasing. Following, the surface of the component is passivated with a chrome salt to realize a superficial conversion of the component. Once the passivation is concluded, the component is ready to be painted.
As an alternative, it's known to apply an hard anodizing process of a magnesium alloy component, to increase and compact magnesium hydroxide on the surface, and to form a protective layer of oxide (more than 10 µm of thickness) to protect the component from corrosion.
The hard anodizing allows only a black or white superficial coloration and thus is applied where no particular aesthetic finishing are required.
Furthermore, such process requires a large amount of energy to transfer, through the process gas, the current to the component. In the end, the protection offered by the cited treatments is weak, since it depends from the electrical current distribution during the process. When objects geometries are complex, the risk to expose the manufacture to burnt patches or regions not coated increases.
For example, the process known with the commercial name keronite is a process of superficial conversion of light alloys as aluminium, titanium and magnesium alloys, where, through a source of high energy within a gas, the natural oxide of the base material is expanded till a very compact and thick structure is realized, similar to ceramic carryovers. This compact structure makes the material very wear proof and gives a good corrosion protection degree.
An example of such process is described in the document GB 2469115 .
For this energy consumption and the used gases, the keronite is not applicable to all the typologies of products made of magnesium alloy. Furthermore, the high current density may bring defects in the objects with complex geometries and in particular in the undercuts and in some critical points where the deposit cannot arrive.
Solutions known from the state of the art does not resolve in an economic and effective way the problem of providing a surface treatment of a component made of magnesium alloy for a further metallization.
Further, method known from the state of the art does not improve adhesion of the metallization to the surface of a component made of magnesium alloy, in particular the metallization is a process of nickel plating.
Further, method known from the state of the art does not provide a method that allows to avoid use of carcinogen products, as chromic acid and its hexavalent salts, used in the traditional processes of metallization and passivation.
A further problem known from the state of the art is connected to the corrosion that often occurs during nickel plating process. During this process magnesium may corrode to protect nickel deposit.
TRAN T N ET AL: TRANSACTIONS OF THE INSTITUTE OF METAL FINISHING, vol. 90, no. 4, 1 July 2012 (2012-07-01), pages 209-214 discloses a process for preparing the surface of a magnesium alloy for electroless nickel plating. The process involves chemically degreasing and cleaning the magnesium surface using an alkaline cleaning bath, pickling the surface and then a two step activation. Magnesium fluoride is formed on the surface in the second activation step.

SUMMARY

The object of the present invention is to overcome drawbacks of the state of the art.
The cited drawbacks are overcome by a process to treat the surface of magnesium alloy comprising the following steps:

a) providing a component made of magnesium alloy and manufactured through a productive process;
b) removing impurities on the surface of the component following the productive process by chemical degreasing the surface;
c) removing magnesium hydroxide from the surface of the component by pickling the surface;
d) activating a first time the surface of the component generating magnesium fluoride on the surface;
e) uniforming the surface of the component by etching the surface itself;
f) activating a second time the surface of the component generating magnesium fluoride on the surface;

The process to treat the surface of magnesium alloy is a first object of the present invention. Advantageously , thanks to the present superficial treatment process, the electroless nickel plating adheres promptly to the surface made of magnesium alloy. Said superficial treatment prepares in an economic and effective way, the surface of the magnesium alloy object to receive a subsequent process of electroless nickel plating that represents the second object of the present invention.
According to a second aspect of the invention, the drawbacks mentioned are overcome by means of a electroless nickel plating process comprising the steps of:

treating a surface of a component made of a magnesium alloy according to the process to treat the surface of a component made of a magnesium alloy above mentioned;
electroless nickel plating the surface of the component immersing it in an alkaline bath containing nickel,

According to the electroless nickel plating process of the present invention, the magnesium alloy object is uniformly covered of a coating of nickel plate that allows to coat the object with further superficial finishings.
Furthermore, the deposit of nickel plate on the surface of the magnesium alloy object makes the surface electrically conductive, for applications requiring such feature.
Advantageously, the processes of the first and second objects of the present invention are not carried out in an acid environment, making them much safer than the ones known from the state of the art.
The electroless nickel plating not carried out in an acid environment, allow avoiding the corrosive phenomenon of the magnesium alloy that often occurs during electroless nickel plating of the alloy itself.
Further advantage of the processes objects of the present invention is that the nickel coating has an excellent adhesion degree to the substrate of magnesium alloy.
These and further advantages will be clear from the following description, of an exemplary not limiting embodiment with reference to annexed drawings.

BRIEF DESCRIPTION OF DRAWINGS

In the drawings:
Fig. 1 shows a block diagram according the present invention, that schematically shows the step of the superficial treatment method of a magnesium alloy and the electroless nickel plating of the pre-treated surface.

DETAILED DESCRIPTION

The following description of one or more exemplary embodiments refer to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The scope of the invention is defined by the appended claims.
The technical details, the structure or the characteristics described as follows can be combined between each other in any suitable way.
To properly comprising the idea underlying the present invention, reference to figure 1 is made.
The superficial treatment process 100 of an object made of magnesium alloy comprises various steps described in detail below.
Initially, (step 110) an object or component realized at least in its surface with a magnesium alloy is provided.
Said object may be realized according to different moulding or manufacturing techniques, for example extrusion, lamination, die, casting or moulding. This last may be the moulding process commercially known as Thixomolding.
The component may optionally rinsed with water after the moulding.
Subsequently, the impurities on the surface, for example oil, are removed. With the impurities on the surface of the component may be as well present magnesium hydroxide spontaneously formed on the magnesium alloy for protective reasons.
When the component to pre treat has a low superficial finishing, e.g. because has scraps or chips deriving from the manufacturing process, it is possible superficially finishing the component with a mechanical machining with the aid of an alkaline substance, for example the substance known with the trade name Quakercool 3890 of the company Quaker.
The use of acid substances has the drawback to trigger corrosion reactions in the magnesium alloy, for this reason acid soaps or substances with pH lower than 5 are to avoid.
A tape or brush polishing or a vibro-finishing with dedicated machines can be used as mechanical machining.
Once the optional phase of superficial finishing is over, the surface of the component is chemically degreased (step 120) through an agitated water solution, preferably with insufflated air, and containing salts of borax, like borax decahydrate, tetra borate sodium, decahydrate or borate sodium in a weight percentage comprised between 25% and 50 %, pyrophosphate salts, as pyrophosphate sodium, in a weight percentage comprised between 2,5% and 10 %, sulfonated sodium salts, as dodecylbenzene sulfonated sodium, in a weight percentage comprised between 2,5% and 10 %, and a surfactant, e.g. ethoxylate alcol, in a weight percentage comprised between 2,5% and 5 %.
The solution, or bath, is in a tank made of steel 316L, internally coated in plastisole to avoid degreasing agents act also on the tank. The tank is heated to ensure a steady temperature of 70°C-85°C. This temperature bring in solution the salt and let evaporating the alcohol in the bath (if any), augmenting the degreasing effect.
The time span of the step 120 of chemical degreasing, the immersion time of the component in the bath, depends from the type of alloy. The time span of this step 120 is inversely proportional to the presence of aluminium in the alloy. For example, the time of immersion for alloys like AZ91, AZ31 and AZ21 is between 5 and 10 minutes, whilst for alloys AM 50, AM 60, ZK30 and ZK60 the time of immersion is between 10 and 30 minutes.
Further, the time span of the step 120 depends, for the same alloy, and the type of casting used. For an AZ91 alloy made by casting and/or moulding is preferable a time of immersion between 10 and 15 minutes, whilst if the component is made by injection the time of immersion is preferably of 5 - 8 minutes, because the surface is less porous and more compact. If the component is made by extrusion or lamination, the time of immersion is preferably of 20 - 30 minutes, because the surface is more porous and manufacturing residues may be present. This step 120 may thus last from 5 to 30 minutes according the manufacturing process by which the component is realized.
Subsequently to the chemical degreasing the surface of the component made of a magnesium alloy may be rinsed to remove degreasing substances of the previous chemical degreasing step (step 120).
This step occurs to avoid degreasing substances of the previous steps are present in the phases subsequent the rinsing.
To carry out an optimal rinsing, is preferable using two distinct tanks containing mains water and not demineralized water, wherein two distinct rinsings occur. Mains water is more hard, due to the salt minerals contained, and makes easy the removal of degreasing residues that otherwise could interfere with the quality of the subsequent step, other than pollute chemical substances contained. Even more preferably, a rinsing is carried out in three distinct tanks with cascade rinsing.
The use of several tanks allows the tanks downstream the first to be free from soaps or residues released with the rinsing in the first tank.
Alternatively, is possible to use a unique tank with continue change of water.
It is preferable the water flows on the surface of the component during the present phase of rinsing. The temperature of the water may ambient temperature.
The rinsing phase may be ended when the operator looking on the surface of the component sees a uniform film of water on it. The present rinsing phase ends in 15 minutes.
Subsequently the first rinsing of the surface of the component the method comprises a further step of removal of magnesium hydroxide from the component surface by pickling the surface (step 130).
In this step 130 other than magnesium hydroxide, that is spontaneously formed on the magnesium alloy, are removed superficial imperfections, for examples chippings or scraps of the metal, that could in turn oxidized from the production process.
The step 130 comprises the immersion of the component in a bath containing fluoride salts in a concentration of 80-100 g/l, sulphuric acid (H₂SO₄) or phosphoric acid (H₃PO₄) in a concentration of 60-110 ml/l.
Sulfuric acid, or alternatively phosphoric acid with the same concentration, generates an almost neutral etching on the surface of the component. It is preferable using fluoride salts as hydrofluoric acid (HF) or ammonium bifluoride ((NH₄)HF₂), for the behaviour of the magnesium to bond with the fluoride to form magnesium fluoride (MgF₂).
It is further possible to add sodium fluoride or ammonium chloride, in a concentration of 1/3 of the total ammonium bifluoride.
Preferably said bath is at ambient temperature and is in a tank realized in steel 316L, internally coated in plastisole or Moplen (Polypropylene-H) to avoid degreasing agents act also on the steel instead of the component. Furthermore the ammonium bifluoride is corrosive for the steel.
The tank is mechanically agitated or through air to mix the solution and thus makes the reaction more effective.
Times of immersion of the component in the bath (step 130) depend on the type of alloy as a function of the oxide in the component. Different alloys have different affinities to develop magnesium hydroxide on their surface. The more magnesium hydroxide is present on the surface of the component and the longer is the length of the step.
For example, for the alloy AZ91 an immersion time of 2 - 5 minutes is preferable, for alloys AZ31, AZ21, ZK30 and ZK60 an immersion time of 20 - 60 seconds is preferable, whilst for alloys AM50 and AM60 an immersion time of 1 - 3 minutes is preferable.
Further the time span of the superficial oxide removal step (step 130) depends, for the same alloy, from the type of casting used. Manufacturing techniques influences production on the formation of oxide on the surface. If the component is made by extrusion or lamination time of immersion is of 2 - 3 minutes is preferable, by injection a time of immersion of 30 - 90 seconds is preferable, while components made by casting and/or moulding a time of immersion between 1 and 5 minutes is preferable.
This step 130 may thus last from 20 seconds to 5 minutes according the manufacturing process by which the component is realized and the type of magnesium alloy in which the component is made.
Subsequently to the removal phase of oxide from the surface of the component through pickling (step 130), the surface of the component made of magnesium alloy may be rinsed in demineralized water at ambient temperature. Preferably the water is demineralized to avoid the salts contained influence the reactions of the following steps.
It is further preferable the tank containing the rinsing water be agitated by air insufflation to augment the efficacy of the step. The rinsing step ends in 5 minutes.
After the preceding step of rinsing of the surface, the component has for a first time a superficial activation (step 140) to enucleate on the component surface magnesium fluoride (MgF₂).
Following the previous removal of the magnesium hydroxide from the surface (step 130), this step 140 generates magnesium fluoride (MgF₂) between the α primary, α eutectic and β phases of the crystalline structure of the magnesium alloy.
Magnesium has three crystalline structures: α primary, α eutectic e la intermetallic β, where the first is more regular than the second, whilst the third is more nodular. Between the bounds of these crystalline structures α and β exist zones of the magnesium alloy where the ammonium bi-fluoride, or any acid fluoride, reacts in optimal way with the intermetallic component of the phase β to grow the magnesium fluoride (MgF₂) on the surface. The composition of the phase β depends on the typology of alloy, for example the alloy AZ91 the step β is Mg₁₇Al₁₂ and easily reacts with ammonium bifluoride.
This step 140 of the process has the double object of protecting the surface, since the magnesium fluoride (MgF₂) is substituted by magnesium hydroxide (Mg(OH)₂) that normally would form on the surface, and makes the surface catalyzable at electroless nickel plating. Surfaces area where magnesium fluoride (MgF₂) is formed are area in which the nickel reacts.
Normally, the surface of the magnesium alloy not treated would not initiate a oxidation-reduction reaction of the chemical nickel. Now after the step of first superficial activation (step 140), a plurality of catalyzable zones are realized on the surface of the material to coat and the oxidation-reduction reaction of the chemical nickel may start.
To realize the first superficial activation, step 140, the component is immerse in a bath containing fluoride salts with a concentration of 280-345 g/l and phosphoric acid (H₃PO₄) with a concentration of 20-70 ml/l.
Preferably fluoride salts may be hydrofluoric acid (HF) or ammonium bifluoride ((NH₄)HF₂), for the behaviour of the magnesium to bond with the fluoride to form magnesium fluoride (MgF₂). Phosphoric or sulphuric acid augments the solubility of the fluoride salts in the bath.
The bath is at a temperature comprised between 35°C and 50°C, preferably 40°C, and occurs in a tank, preferably realized in steel 316L, internally coated in plastisole or Moplen (Polypropylene-H) to avoid degreasing agents of the bath act also on the steel of the tank instead of the component. Furthermore the ammonium bifluoride is corrosive for the steel.
The tank is mechanically agitated and comprises a filter pump with a porous septum, preferably of
5-10 µm, to avoid that solids detailed from the component surface is again in contact with the surface of the component immersed in the bath. The filter pump continually cleans the solution in the tank. It is not advisable the use of tank agitate by air insufflation, since the oxygen may modify the activation reaction. The presence of too much oxygen may shift the reaction toward the formation of magnesium hydroxide, Mg(OH)₂ instead of magnesium fluoride (MgF₂).
Since the crystalline structure of the magnesium alloy depends on the composition of the alloy and on the manufacturing process used for obtaining the component, immersion times in the bath are function of these two aspects.
For alloys AZ91A, AZ91B is preferable immerging the component in the bath for 2 - 4 minutes, for alloys AZ91C, AZ91D, AZ91F, AZ31 and AZ21 is preferable immerging the component in the bath for 30-90 seconds, for alloys ZK30 and ZK60 is preferable immerging the component in the bath for 20 - 60 seconds, whilst for alloys AM50 and AM60 is preferable immerging the component in the bath for 1-3 minutes.
For the same alloy used, e.g. AZ 91 alloy, immersion times depend of the manufacturing technique used. For moulding or cast, is preferable an immersion time of 2-5 minutes. For injections, is preferable an immersion time of 30-90 seconds. For extrusion and lamination, is preferable an immersion time of 2-3 minutes.
The time span of the present step 140 is proportional to the superficial porosity degree.
The time span of this step 140 may thus last from 20 seconds to 5 minutes according to the manufacturing process used and the magnesium alloy in which the component is made. Subsequently to the first activation phase (140) of the component surface, the surface may be rinsed in demineralized water at ambient temperature. Preferably, the water is demineralized to avoid that salts disturb the reactions of the subsequent steps of the process.
It is further preferable the tank containing rinsing water be agitated by air insufflation or alternatively by ultrasounds, to augment the efficacy of this step. The present rinsing step lasts preferably less than 90 seconds, to avoid the surface of the component, that is extremely active from the previous phase, reacts with hydrogen and oxygen to form again magnesium hydroxide (Mg(OH)₂).
This rinsing step allow to eliminate traces of the previous steps that could react with the pyrophosphate salt of the next step.
Following the rinsing step after the first superficial activation (step 140), the surface of the component is uniformed by etching (step 150).
During the step 150 the surface of the component is uniformed where the crystalline structure is more discontinuous. With respect to the pickling, the porosities on the surface are levelled.
The step 150 comprises the immersion of the component in a bath at a temperature comprised between 75°C and 85°C, preferably 80°C. The bath contains, dissolved in water, pyrophosphate salts as tetrapyrophosphate potassium (K₄P₂O₇), in a concentration of 30-50 g/l, and nitrate salt, as potassium nitrate (KNO₃), in a concentration of 30-50 g/l.
The tank is agitated, preferably mechanically agitated, and comprises a filter pump. The filter of the pump has a porous septum of 5-10 µm, to avoid that solids detached from the component is again in contact with the surface of the component immersed in the bath. The filter pump continually cleans the solution in the tank.
The tank is preferably realized in steel 316L and internally coated in plastisole or Moplen (Polypropylene-H) to avoid degreasing agents react with the steel instead of the component, and to avoid that nitrates damage the steel of the tank.
The above mentioned temperature allows nitrates to work the surface in optimal way. Nitrate salts attack the surface of the component and better solubilize the pyrophosphate salts. Pyrophosphate salts in turn bond well enough to fluorides easing the formation of magnesium fluoride (MgF₂), in the next second step of superficial activation, and further are good promoters of the resistance to corrosion of the magnesium.
The time span of this step 150 depends on the porosity degree of the surface at the beginning of the step.
Important factors for the etching step 150 are the type of manufacturing process and the type of alloy.
Some elements in the magnesium alloys are more sensible to pyrophosphate salts e.g. alloy without aluminium as ZK30 and the time span of the etching is brief, while for alloys with more aluminium, as ZK30, the time span of the etching lasts more.
In general, in the case of alloys AZ91 and AZ21 is preferable immerging the component in the bath for 2-4 minutes, for the alloy AZ31 is preferable immerging the component in the bath for 40-90 seconds, for alloys ZK30, ZK60, AM50 and AM60 is preferable immerging the component in the bath for 20-60 seconds
For the same alloy used, e.g. AZ91 alloy, immerging times depend from the manufacturing process.
For casting or moulding is preferable a time of immersion between 2 and 4 minutes, whilst if the component is made by injection the time of immersion is preferably of 1-3 minutes, if the component is made by extrusion or lamination the time of immersion is preferably of 3 - 8 minutes. The time span of this step 150 is directly proportional to superficial porosity degree. This step may vary from 20 seconds to 8 minutes according to manufacturing process used for realizing the component and to the type of magnesium alloy of the component.
Next to the etching step of the component surface for uniforming the surface (step 150) there is the rinsing of the component surface in demineralized water at ambient temperature. Preferably the water is demineralized to avoid the salts contained influence the reactions of the following steps.
It is further preferable the tank containing rinsing water be agitated by air insufflation or alternatively by ultrasounds, to augment the efficacy of this step. The present rinsing step lasts preferably less than 90 seconds, to avoid that the surface of the component, that is extremely active from the previous phase, reacts with hydrogen and oxygen to form again magnesium hydroxide (Mg(OH)₂). This further rinsing step allows eliminating traces of the previous steps that should pollute the subsequent one.
After the preceding step of rinsing of the surface, the component has for a second time a superficial activation (step 160) to enucleate on the surface further magnesium fluoride (MgF₂). The second superficial activation (step 160) repeats the previous first superficial activation step (step 140), therefore the features and details of the two steps (steps 140 and 160) substantially coincide.
If the component is realized through injection the following steps are not necessary, therefore it is possible to extend the time span of the second activation until 10 minutes. The components realized by injection have a poor porosity and that allows to obtain a quick degree of levelling of the surface and a compact and uniform growth of magnesium fluoride.
After the second superficial activation step 160, the component made of magnesium alloy may be rinsed in demineralized water at ambient temperature. Preferably the water is demineralized to avoid the salts contained influence the reactions of the following steps.
It is further preferable the tank containing the rinsing water be agitated by air insufflation to augment the efficacy of this step. The present rinsing step lasts preferably less than 60 seconds, to avoid that surface of the component, that is extremely active from the previous phase, reacts with hydrogen and oxygen to form again magnesium hydroxide (Mg(OH)₂).
If the component is realized with a manufacturing process, or moulding process, giving a surface particularly porous and irregular it is preferable keep going the superficial treatment of the component with the following steps. It is therefore preferable keep going the process for components manufactured with techniques different to injection.
Following the rinsing step after the secondo step of superficial activation, the surface of the component is newly uniformed by etching, to level the residual imperfections of the component surface.
During this step the surface of the component is uniformed where the crystalline structure is more discontinue. With respect to the pickling the porosity (if any) are in practice levelled.
The second step of uniformation comprises the immersion of the component in a bath at a temperature of 75°C and 85°C, preferably 80°C.
The bath contains, dissolved in water, pyrophosphate salts, as tetrapyrophosphate potassium (K₄P₂O₇), in a concentration of 30-50 g/l, and nitrate salts, as potassium nitrate (KNO₃), in a concentration of 30-50 g/l.
The tank is agitated, preferably mechanically agitated, and comprises a filter pump. The filter pump has a porous septum, preferably of 5-10 µm, to avoid that solids detached from the component (if any) is again in contact with the surface of the component immersed in the bath. The filter pump continually cleans the solution in the tank. The tank is preferably realized in steel 316L and internally coated in plastisole or Moplen (Polypropylene-H) to avoid the substances of the bath react also on the steel instead of the component. This to avoid that the nitrates damage the steel of the tank.
The above mentioned temperature allows nitrates to work the surface in optimal way. Nitrate salts attack the surface of the component and further better solubilize the pyrophosphate salts. Pyrophosphate salts in turn bond well enough to fluorides easing the formation of magnesium fluoride (MgF₂) in the next third step of superficial activation and further are good promoters of the resistance to corrosion of the magnesium.
The time span of this step depends from the porosity degree of the surface at the beginning of the step. Determinant factors for time span of the etching step are typology of the manufacturing process and the type of alloy.
Some elements in the magnesium alloys are more sensible to pyrophosphate salts e.g. alloy without aluminium as ZK30 alloy and the time span of the etching is brief, while for alloys with more aluminium, as AZ91, the time span of the etching lasts more.
In general, in the case of alloys AZ91 and AZ21 is preferable immerging the component in the bath for 2-4 minutes, for the alloy AZ31 is preferable immerging the component in the bath for 40 - 90 seconds, for alloys ZK30, ZK60, AM50 and AM60 is preferable immerging the component in the bath for 20 - 60 seconds.
For the same alloy used, e.g. AZ91 alloy, immerging times depend from the manufacturing process.
For casting and/or moulding is preferable a time of immersion between 2 and 4 minutes, whilst if the component is made by injection the time of immersion is preferably of 1-3 minutes. If the component is made by extrusion or lamination the time of immersion is preferably of 3 - 8 minutes. The time span of this step is directly proportional to superficial porosity degree.
The duration of this step may vary from 20 seconds to 8 minutes according on function of the manufacturing process and of the type of magnesium alloy of the component.
After the second step of uniforming the surface, the component made of magnesium alloy may be rinsed in demineralized water at ambient temperature. Preferably the water is demineralized to avoid the salts contained influence the reactions of the following steps.
It is further preferable the tank containing the rinsing water be agitated by air insufflation to augment the efficacy of the step. The rinsing step ends preferably in less than 60 seconds, to avoid that surface of the component, that is extremely active from the previous phase reacts with hydrogen and oxygen to form again magnesium hydroxide (Mg(OH)₂).
After the preceding step of rinsing of the surface, the component has for the third time a superficial activation to enucleate on the surface further magnesium fluoride (MgF2). Following the previous removal of the magnesium hydroxide from the surface (step 130), this step 140 generates magnesium fluoride (MgF2) between the α primary, α eutectic and β phases of the crystalline structure of the magnesium alloy.
This step of the process has the double scope of protecting the surface, since the magnesium fluoride (MgF₂) is substituted by magnesium hydroxide (Mg(OH)₂) that normally would form on the surface, and making the surface catalyzable at electroless nickel plating.
To realize the third superficial activation, the component is immersed in a bath containing fluoride salts with a concentration of 280-345 g/l and phosphoric acid (H₃PO₄) with a concentration of 20-70 ml/l.
Preferably fluoride salts may be hydrofluoric acid (HF) or ammonium bifluoride ((NH₄)HF₂), for the behaviour of the magnesium to bond with the fluoride to form magnesium fluoride (MgF₂). Phosphoric or sulphuric acid augments the solubility of the fluoride salts in the bath.
The bath is at temperature comprised between 35°C and 50°C, preferably 40°C, and occurs in a tank preferably realized in steel 316L, internally coated in plastisole or Moplen (Polypropylene-H) to avoid degreasing agents act also on the steel of the tank instead of the component. Furthermore the ammonium bifluoride is corrosive for the steel.
The tank is mechanically agitated and comprises a filter pump with a porous septum, preferably of 5-10 µm, to avoid that solids detached from the component is again in contact with the surface of the component immersed in the bath. The filter pump continually cleans the solution in the tank. It is not advisable the use of tanks agitate by air insufflation, since the oxygen may modify the activation reaction.
Since the crystalline structure of the magnesium alloy depends on the composition of the alloy and on the manufacturing process used for obtaining the component, immersion times in the bath are function of these two aspects.
Despite of this, the present step can end later with respect of the previous second superficial activation step, since this is the last phase of the superficial treatment of the component made of magnesium alloy.
In particular, in the case of AZ31 alloy is preferably to immerge the component for 2-3 minutes, while for the other kind of magnesium alloys the duration should be 8-12 minutes.
For the same alloy used, e.g. AZ 31 alloy, immersion times depend of the manufacturing technique used. For moulding or cast, is preferable an immersion time of 7-12 minutes. For extrusion and lamination, is preferable an immersion time of 2-4 minutes.
The time span of the present step is proportional to the superficial porosity degree.
The time span of this step may thus last from 2 minutes to 12 minutes according to the manufacturing process used and the magnesium alloy in which the component is made.
In the second and third activation phase, leaving the component in immersion for longer times, is not derivable a significant growth of magnesium fluoride (MgF₂) and, vice versa, a unpleasant phenomena of chalking of the surface of the component occurs.
Further, when the activation generates too much areas of catalysation on the surface of the component, during the next electroless nickel plating step the nickel cannot catalyse all areas, creating porous zones on the nickeled surface.
After the third step of generation of the magnesium fluoride on the surface component, the component made of magnesium alloy may be rinsed in demineralized water at ambient temperature. It is further preferable the tank containing the rinsing water be agitated by air insufflation to augment the efficacy of the step. The rinsing step ends in less than 60 seconds, to avoid the the surface of the component, that is extremely active from the previous phase, reacts with hydrogen and oxygen to form again magnesium hydroxide (Mg(OH)₂).
The surface of the component is ready for the electroless nickel plating. The surface has only few traces of magnesium hydroxide but vice versa many areas where magnesium fluoride (MgF₂) is present.
The pre-treated component is then immersed in an alkaline bath containing nickel with a low quantity of phosphor (step 170), namely from 1 to 3 in weight percentage.
The bath may have a temperature comprised between 74°C and 85°C and containing nickel sulphate (NiSO4), sodium hypophosphite (NaPO₂H₂) and regulators of the chemical oxidation-reduction reaction.
The concentration of nickel sulphate (NiSO4) is preferably comprised between 5 g/l and 6,5 g/l and that of sodium hypophosphite (NaPO₂H₂) is preferably comprised between 15 g/l and 25 g/l. The sodium hypophosphite (NaPO₂H₂) is the regulating element of the concentration of phosphor in the alloy and augmenting it augments the concentration of phosphor in the alloy.
The nickel sulphate reacts by oxidation-reduction with sodium hypophosphite and metal nickel falls on the surface of the component. Compounds fluoride are promoter of the oxidation-reduction reaction between nickel sulphate and sodium hypophosphite, the magnesium alloy becomes catalytic and begins to cover with nickel metal. Consequently, the nickel metal deposits on the surface of the component. In this process sodium hypophosphite becomes orthophosphite sodium.
In particular, regulators of the chemical reaction of oxidation-reduction may comprise: carbocyclic acids with a complex concentration between 20 and 90 g/l, e.g. succinic acid (C₄H₆O₄), malic acid (C₄H₆O₅), lactic acid (CH₃H₆O₃) e citric acid (CH₆H₈O₇); ammonium fluoride (NH₄F) with a content between 7,2 and 12,3 g/l; ammonium sulphate ((NH₄)₂SO₄) with a content between 1,2 and 2,8 mg/l; and trisodium citrate (Na₃C₆H₅O₇) with a content between 3,8 and 11,7 g/l.
The ammonium fluoride and ammonium sulphate are accelerator of the reaction, whilst the carbocyclic acids and trisodium citrate are complexants and stabilizers of the reaction.
The bath is contained in a tank comprising:

means for maintaining the content of nickel sulphate (NiSO₄) and sodium hypophosphite ((NaPO₂H₂) within predetermined values;
means for maintaining an alkaline pH in the bath;
means for agitating the bath; and
means for heating the bath.

The aforementioned means for maintaining the content of nickel sulphate and sodium hypophosphite may comprise two containers containing the solution of nickel sulphate and sodium hypophosphite connected with the tank where the component is immersed and a controller configured to regulate the dispensing of the two compounds in the bath to maintain steady contents.
Means for maintaining an alkaline pH in the bath may comprise a pHmeter to control if the pH lowers than 6 and a pump taking sodium hydroxide, preferably with a content equals to or inferior to the 10%, from another container. Sodium hydroxide is not directly inserter in the tank with the electroless nickel plating, otherwise the nickel hydroxide flocculates forming by reaction between the sodium hydroxide and nickel phosphate. It is thus preferable dosing the sodium hydroxide inside the pump installed.
Means for agitating the bath may be mechanical, as mobile frame moved in the bath, wherein the components to treat are attached.
The tank may comprise a filter pump to delicately agitate the bath without air insufflation. It is preferable a filtration with 6-7 exchanges per hour. In the case of a tank of 100 litres, the pump has preferably a capacity of 600-700 l/h.
These means are useful also to ensure a uniform circulation of the bath on the component and obtaining a uniform coating of nickel plating in all the points of the surface. Further, it allows avoiding wrinkles on the surface that may generate aesthetic and functional defects.
The tank may be in stainless steel 316L with anodic protection, preferably at 650 mVolt, and steam heating. It is preferable to avoid heating system with candles in pirex glass, these would be damaged from substances in the nickel solution, e.g. from fluorides.
With this nickel process a yield of the reaction of 12-20 µm/hour is achievable.
The deposit generated on the surface of the component has a thickness and an aspect substantially uniform and furthermore a low content of phosphor.
Further, maintaining an alkaline pH:

the reaction is fastened and on the surface of the component more nickel and less phosphor are deposed;
the corrosion of the magnesium during electroless nickel plating is minimized;
the tank management and the relative bath are safer for working personnel.

The deposit of the nickel so obtained is sufficiently hard, few passive and adapted to receive further layers of coating.
Further the component so nickel plated is particularly resistant to the corrosion and the abrasion, because the layer of nickel obtained on the component makes a uniform barrier between air and the surface of the material made of magnesium alloy.
The superficial nickel plating obtained has a good electrical conductivity.
Further features of the nickel plating are summarized in the following:

hardness comprised between 800 and 900 HV according to the type of coating of nickel;
resistance to corrosion lower than 48 h NSS (Neutral Salt Spray), according to ISO 4527, for a thickness of nickel with low content of phosphor of about 15 µm;
conductibility comprised between 0,01 mohm e 0,05 mohm;
resistance to UV rays;
resistance to a thermic shock higher than 5 cycles of 30 minutes at a temperature of 125 °C alternated by 5 cycles of 30 minutes at a temperature of -55 °C;
minimum thickness of 5 µm.

Surprising results are obtained with an alloy AZ91D , wherein the morphology of the deposit obtained on magnesium alloy is spherical, compact and free of porosity.
Alloy AZ91D is the magnesium alloy more widespread and comprises about 9% aluminium and 1% zinc.
After the electroless nickel plating the process comprises a rinsing phase with demineralized water in an agitated tank.
The nickel plated component according the process of the present invention may subsequently have further superficial treatments, with functional or aesthetic features, according one of the following processes.
The first process of further superficial treatment comprises a neutralization step, an alkaline copper plating a further electroless nickel plating with a content medium/high of phosphor a passivation step to increment the resistance to corrosion and a drying step and a thermic treatment.
An nickel plate alloy with a medium content of phosphor comprises a weight percentage of phosphor comprised between 4 and 9, whilst an alloy with a high content of phosphor comprises a weight percentage of phosphor comprised between 10 and 12.
This first process allows improving resistance to corrosion and wear of the component.
A second process of further superficial treatment comprises a neutralization step, a step of alkaline copper plating, a further electroless nickel plating with a content medium/high of phosphor, a superficial activation step, a step of lamellar zinc and a final superficial coating with painting.
This second process allows further improving resistance to corrosion and represents an alternative to the classical painting.
A third process of further superficial treatment comprises an activation step of superficial activation and a DLC step (Diamond Like Carbon).
This third process allows improving resistance to wear of the component reaching superficial hardness up to 5200 HV.
A fourth process of further superficial treatment comprises a neutralization step and a step of alkaline zinc plating.
This fourth process allows improving resistance to wear of the surface of the component.
A fifth process of further superficial treatment comprises a step of neutralization, a step of alkaline copper plating, a step of neutralization a further step of acid copper plating, a step of neutralization, a step of electroless nickel polished plating, a step of electrolytic nickel opaque, a step of neutralization, a step of activation, a step of micro porous chrome plating, and in the end a step of polished chrome plating, satin finish chrome plating, opaque or black.
The polished nickel plating provides polished finishing to the next chrome plating, whilst the opaque nickel, as known as nickel columnar, provide a higher corrosion resistance.
This fifth process allows improving the aesthetic aspect of the component and augmenting the resistance to corrosion.
A sixth process of further superficial treatment comprises a step of superficial activation and a step PVD (Physical Vapor Deposition).
This sixth process has many uses for components for the medical sector, electronics, furniture and automotive.
A seventh process of superficial treatment comprises a step of neutralization, a step of alkaline copper plating, a step of neutralization, a next step of acid copper plating, a step of neutralization, a polished electrolytic nickel plating, a step of neutralization, a step of activation, and in the end a step of polished chrome plating, satin finish chrome plating, opaque or black.
This seventh process allows component to be used for every common application, for example for bath fitting components or door knobs.
A eight process of further superficial treatment comprises a neutralization step, an alkaline copper plating, a further step of neutralization, a further step of alkaline copper plating, a step of neutralization, an acid copper plating, a step of neutralization, a step of tin-copper alloy plating, a new step of neutralisation, a golden, palladium or silver plating step, a step of gun metal or white bronze. Gun metal is a deposit dark and polished, while white bronze is a silver looking deposit.
This eight process allows component to have particular aesthetic features useful for the fashion and jewellery sector.
A ninth process of further superficial treatment comprises a step of neutralization, a passivation step, a step of cubicatura. The cubicatura, also known as water transfer printing, is a coating that exploits the superficial tension of the water and the interaction of this with an alkaline substance sprayed on a hydro soluble film.
This ninth process allow the component having a particularly pleasant aesthetic superficial finishing.
Finally, it's clear that the process for treating the surface and the nickeling so obtained can be varied and adapted in several ways, all comprised in the present invention, further, all the features are substitutable with technically equivalent elements. Practically, the materials, the substances and the quantities can be varied according to the technical requirements.
Finally are described hereinafter some advantageous embodiments:

I) Process according to claim 4, said at least a salt comprises salts of borax between about 25% and 50 % in weight, and pyrophosphate salts between 2,5% and 10 % in weight.
II) Process according to previous clause, wherein the agitated bath has a temperature comprised between about 70°C and 85°C and the component is immersed for a time comprised between 5 and 30 minutes, said time being function of the manufacturing process used for realizing the component and the type of magnesium alloy.
III) Process according to claim 6, wherein the component is immersed for a time comprised between 20 seconds and 5 minutes, said time being function of the manufacturing process used for realizing the component and the type of magnesium alloy.
IV) Process according to claim 7, wherein the component is immersed in an agitated bath having a temperature between 35 °C and 50 °C and containing 280-345 g/l of fluoride salts and 20-70 ml/l of phosphoric acid (H₃PO₄) or 20-70 ml/l of sulphuric acid (H₂SO₄).
V) Process according to the claim 7 or previous clause, wherein the component is immersed for a time comprised between 20 seconds and 5 minutes, said time being function of the manufacturing process used for realizing the component and the type of magnesium alloy.
VI) Process according to claims 9 or 10, wherein the component is immersed for a time comprised between 20 seconds and 8 minutes, said time being function of the manufacturing process used for realizing the component and the type of magnesium alloy,
VII) Process according to the claim 12, wherein the surface of the component is uniformed for a second time immerging the component in a bath having a temperature comprised between 75 °C and 85 °C, containing 30-50 g/l of pyrophosphate salts and 30-50 g/l of nitrate salts.
VIII) Process according to the preceding clause, wherein the bath is agitated and constantly cleaned by means of a filtering system, wherein the component is immersed for a time comprised between 20 seconds and 8 minutes, said time being function used for realizing the manufacturing process of the component and the type of magnesium alloy.
IX) Process according to claim 12 or clauses VII) or VIII), comprising the further step of rinsing the component for less than 60 seconds with demineralized water in an agitated bath to remove residues on the surface of the component left by the previous step of the process.
X) Process according to claim 12 or clauses VII) or VIII) or IX), comprising the step of generating for a third time magnesium fluoride between the α primary, α eutectic and β phases of the crystalline structures of the magnesium alloy.
XI) Process according to clause X), wherein the component is immersed in an agitated bath having a temperature between 35 °C and 50 °C and containing 280-345 g/l of fluoride salts and 20-70 ml/l of phosphoric acid (H₃PO₄) or 20-70 ml/l of sulphuric acid (H₂SO₄).
XII) Process according to clause X) or XI) wherein the component is immersed for a time comprised between 2 minutes and 12 minutes, said time being function of the manufacturing process used for realizing the component and the type of magnesium alloy.
XIII) Process according any of the clause from X) to XII), further comprising the step of rinsing the component surface for less than 60 seconds with demineralized water in an agitated bath to remove residues on the surface of the component left by the previous step of the process.
XIV) Process of electroless nickel plating according to claim 13, wherein the component is immersed in an alkaline bath at a temperature comprised between about 74°C and 85°C and containing nickel sulphate (NiSO4), sodium hypophosphite (NaPO₂H₂) and regulators of the chemical oxidation-reduction reaction.
XV) Process of electroless nickel plating according to previous clause, wherein the content of nickel sulphate (NiSO4) is preferably comprised between 5 g/l and 6,5 g/l and the content of sodium hypophosphite (NaPO₂H₂) is comprised between 15 g/l and 25 g/l.
XVI) Process of electroless nickel plating according to clause XIV) or XV), wherein said regulators of the chemical oxidation-reduction reaction comprise carbocyclic acids with a content between 20 and 90 g/l, ammonium fluoride (NH₄F) with a content between 7,2 and 12,3 g/l; ammonium sulphate ((NH₄)₂SO₄) with a content between 1,2 and 2,8 mg/l; and trisodium citrate (Na₃C₆H₅O₇) with a content between 3,8 and 11,7 g/l.
XVI) Process of electroless nickel plating according to any of the clause from XIV) to XVI), wherein the bath is contained in a tank comprising: means for maintaining nickel sulphate (NiSO₄) and ammonium sulphate ((NH₄)₂SO₄) concentrations about predetermined values; means for maintaining an alkaline pH in the bath; means for agitating the bath and means for heating the bath.

Claims

Process for treating the surface of a magnesium alloy comprising the steps of:
I - providing a component made of magnesium alloy and manufactured by a productive process;

II- removing impurities on the surface of the component following the productive process by chemical degreasing the surface;

III- removing magnesium hydroxide from the surface of the component by pickling the surface;

IV- activating a first time the surface of the component generating magnesium fluoride on the surface;

V- uniforming the surface of the component by etching the surface;

VI- activating a second time the surface of the component generating magnesium fluoride on the surface;
wherein the process comprises at least a step of rinsing of the surface of the component with water between two consecutive steps of the process, to remove residues left on the component surface from the previous step.
Process according to claims 1, wherein water of the rinsing step between the steps II and III contains mineral salts, whilst the water between the other couples of consecutive steps is demineralized.
Process according to claims 1 or 2, wherein a superficial finishing of the component with an alkaline substance is provided between steps I and II.
Process according to any of the preceding claims, wherein the chemical degreasing comprises the step of immerging the component in an agitated bath containing water, at least a salt and a surfactant.
Process according to any of the preceding claims, comprising, between the step II and step III, letting run water with mineral salts on the surface of the component in at least two separate tanks or in a single tank having a continuous water turnover, for a time lower than 15 minutes.
Process according to any of the preceding claims, wherein the step III comprises immerging the component in an agitated bath at ambient temperature containing 80-100 g/l of fluoride salts and 60-110 ml/l of sulphuric acid (H₂SO₄) or 60-110 ml/I of phosphoric acid (H₃PO₄).
Process according to any of the preceding claims, wherein in step IV and step VI, the magnesium fluoride (MgF₂) is generated between α primary, α eutectic and β phases of the crystalline structures of the magnesium alloy.
Process according to any of preceding claims, wherein the rinsing step of the component between the steps IV and V or between the steps V and VI occurs in an agitated bath with air or ultrasounds wherein the component is immersed for less than 90 seconds.
Process according to any of the preceding claims, wherein at the step V the component is immersed in a bath at a temperature comprised between 75°C and 85°C, and containing 30-50 g/l of pyrophosphate salts, 30-50 g/l of nitrate salts.
Process according to the preceding claim, wherein the bath is mechanically agitated and constantly cleaned by a filtering system.
Process according to any of the preceding claims, wherein after the step VI the process comprises the further step of rinsing the surface of the component for less than 90 seconds with demineralized water in an agitated bath to remove residues on the surface of the component left by the previous step of the process.
Process according to claim 11, comprising the step of uniforming for a second time the surface of the component by etching the surface.
Process of electroless nickel plating comprising the step of:
- treating a surface of a component made of magnesium alloy according a process according to any of the preceding claims;

- electroless nickel plating the surface of the component immerging it in an alkaline bath containing nickel.
Process according any of the preceding claims, wherein the magnesium alloy is selected among the following types of alloy: AZ91, AZ31, AZ21, AM50, AM60, ZK30 or ZK60.
Process according any to any of preceding claims, wherein the manufacturing process is selected among the following ones: extrusion, lamination, die, moulding or injection.