The present invention relates to a coated metallic substrate and a method for the manufacture of this coated metallic substrate. The invention is particularly well suited for automotive industry.

Zinc based coatings are generally used because they allows for a protection against corrosion thanks to barrier protection and cathodic protection. The barrier effect is obtained by the application of a metallic coating on steel surface. Thus, the metallic coating prevents the contact between steel and corrosive atmosphere. The barrier effect is independent from the nature of coating and substrate. On the contrary, sacrificial cathodic protection is based on the fact that zinc is a metal less noble that steel. Thus, if corrosion occurs, zinc is consumed preferentially to steel. Cathodic protection is essential in areas where steel is directly exposed to corrosive atmosphere, like cut edges where surrounding zinc will be consumed before steel.

However, when heating steps are performed on such zinc coated steel sheets, for example press hardening or welding, cracks are observed in steel which spread from the coating. Indeed, occasionally, there is a reduction of metal mechanical properties due to the presence of cracks in coated steel sheet after heating steps. These cracks appear with the following conditions: high temperature; contact with a liquid metal having a low melting point (such as zinc) in addition to stress; heterogeneous diffusion of molten metal with substrate grain bulk and boundary. The designation for such phenomenon is liquid metal embrittlement (LME), also called liquid metal assisted cracking (LMAC).

Other metallic coatings usually used for the production of automotive vehicle are aluminum and silicon based coatings. Usually, the steel sheets coated with such coatings are formed by press hardening process. When press hardening process is performed, there is no microcrack in steel due to the presence of an intermetallic layer Al-Si-Fe. Moreover, they have a good aptitude for painting. They allow for a protection by barrier effect and can be welded. However, they do not allow for a cathodic protection or they have a very low cathodic protection. The patent KR100775241 discloses a Zn-Mg alloy coated steel sheet and a method for manufacturing the Zn-Mg alloy coated steel sheet are provided, wherein the Zn-Mg alloy coated steel sheet has more improved adhesion and corrosion resistance by forming an Al-Si alloy film between a base steel sheet and a Zn-Mg alloy film. A Zn-Mg alloy coated steel sheet comprises an Al-Si alloy film which is formed on a base steel sheet and has a thickness of 0.1 to 1.0pm, and a Zn-Mg alloy film which is formed on the Al-Si alloy film and has a thickness of 0.5 to 10pm. The Zn-Mg alloy film comprises 10 to 15% by weight of Mg. The Al-Si alloy film is formed by an electron beam method or a sputtering method, and the Zn-Mg alloy film is formed by one method selected from the group consisting of a thermal evaporation method, an electron beam method, a sputtering method, an ion plating method, and an electromagnetic levitation method.

Despite, the Zn-Mg coated metallic substrate has a corrosion resistance, the high amount of Mg in the ZnMg coating can lead to surface defects and a decrease of corrosion due to the formation of Mg oxide on the coating surface. Moreover, the above combination of Al-Si with a thickness between 0.1 and 1.0μιη and Zn-Mg comprising 10 to 15% of Mg and having a thickness between 0.5 and ΙΟμιτι does not have a sufficient protection against corrosion, i.e. a barrier effect in addition to a sacrificial protection. Thus, the object of the invention is to provide a coated metallic substrate having a reinforced protection against corrosion, i.e. a sacrificial cathodic protection in addition to barrier protection, and which does not lead to LME issues. It aims to make available, in particular, an easy to implement method in order to obtain such coated metallic substrate.

In terms of sacrificial protective corrosion, electrochemical potential has to be at least 50mV more negative than the potential of the metallic substrate. For example, in case of a steel substrate, a maximum potential of -0.78V with respect to a saturated calomel electrode (SCE) is needed. It is preferable not to decrease the potential at a value of -1.4V/SCE, even -1.25V/SCE which would involve a fast consumption and would finally decrease the period of protection of steel.

This object is achieved by providing a coated metallic substrate according to claim . The coated metallic substrate can also comprise any characteristics of claims 2 to 16. Another object is achieved by providing a method for the manufacture of according to claim 17. The method can also comprise any characteristic of claims 18 to 20.

Moreover, the object is achieved by providing the use of a coated metallic substrate according to claim 21.

Finally, the object is achieved by providing an installation for continuous vacuum deposition of coatings on a running metallic substrate according to claim 22. The installation can also comprise any characteristic of claims 23 to 25.

Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention.

To illustrate the invention, various embodiments and trials of non-limiting examples will be described, particularly with reference to the following Figures:

Figure 1 is an illustration of the specimen used for the LME test.

Figure 2 is a corrosion cycle according to the norm VDA 233-102.

In all the figures, the thickness of the layers represented is exclusively for purposes of illustration and cannot be considered to be a representation of the different layers to scale.

The following terms will be defined:

- "wt.%" means the percentage by weight.

The inventions relates to a coated metallic substrate comprising at least a first coating based on aluminum comprising optionally less than 50% of zinc, such first coating having a thickness below 5pm and being directly topped by a second coating comprising from 0.5 to 5.9% by weight of magnesium, the balance being zinc.

Without willing bound by any theory, it is believed that the above coated metallic substrate allows for a high protection against corrosion. Indeed, it seems that the first layer based on aluminum comprising optionally less than 50% of zinc provides a high barrier effect mainly due to the presence of aluminum. It is believed that the thickness of the first coating has to be below 5pm since the barrier effect is highly obtained and above 5pm there is a risk to significantly reduce the productivity and over complicate the deposition method. Moreover, it is believed that the second coating allows for a high sacrificial protection mainly due to the combination of magnesium and zinc, the magnesium being in a specific amount. Additionally, when the amount of Mg is above 5.9%, it seems that powdering problems occurs in the second coating. Finally, the coated metallic substrate having the first and second layers has a high resistance to LME. Consequently, with the specific coated metallic substrate according to the present invention, it is possible to obtain a coated metallic substrate having a high barrier effect, a high sacrificial protection in addition to a resistance to LME.

Preferably, the first coating comprises less than 30% of zinc and optionally, less than 5% of zinc.

Advantageously, the first coating comprises at least one element chosen among: Si and Mg.

In another preferred embodiment, the first coating does not comprise at least one of the following elements chosen from: zinc and silicon. Indeed, without willing to be bound by any theory, it seems that the presence of zinc in the first coating can decrease the LME resistance and the presence of silicon can reduce the sacrificial protection.

Preferably, the first coating consists of aluminum, i.e. the amount of aluminum in the first coating is above 99.0% by weight.

In a preferred embodiment, the first coating has a thickness between 2 and

4pm.

Preferably, the second coating has a thickness between 1 and 10pm.

Advantageously, the second coating comprises from 0.5 to 4.5%, more preferably from 0.5 to 2.0%, by weight of magnesium. In another preferred embodiment, the second coating comprises from 2.0 to 5.0% by weight of magnesium.

In a preferred embodiment, the second coating does not comprise at least one of the following elements chosen among: aluminum, silicon and copper.

Preferably, the second coating consists of zinc and magnesium.

Advantageously, the microstructure of the second coating comprises Mg ₂ Znn phases in a Zn matrix. Indeed, without willing to be bound by any theory, if the second coating comprises at least 95% by weight of Mg ₂ Znn in Zn matrix instead of less than 95% by weight of Mg ₂ Zn-n phases in a Zn matrix, there is a risk to obtain a brittle coating allowing powdering problems. For example, the second coating comprises less than 85%, preferably less than 75% by weight of Mg ₂ Znn in Zn matrix.

Preferably, an intermediate layer is present between the metallic substrate and the first coating, such intermediate layer comprising iron, nickel, chromium and optionally titanium. Without willing to be bound by any theory, it seems that the intermediate coating layer improves the adhesion of the first coating on a metallic substrate.

Thus in a preferred embodiment, the intermediate layer comprises at least 8% by weight nickel and at least 10% by weight chromium, the rest being iron. For example, the layer of metal coating is 316 stainless steel including 16-18% by weight Cr and 10-14% by weight Ni, the balance being Fe.

In another preferred embodiment, the intermediate layer comprises Fe, Ni, Cr and Ti wherein the amount of Ti is above or equal to 5 wt.% and wherein the following equation is satisfied: 8 wt.% < Cr +Ti < 40 wt.%, the balance being Fe and Ni, such intermediate coating layer being directly topped by a coating layer being an anticorrosion metallic coating.

The metallic substrate can be chosen among: aluminum substrate, steel substrate, stainless steel substrate, copper substrate, iron substrate, copper alloys substrate, titanium substrate, cobalt substrate or nickel substrate.

The present invention also relates to a method for the manufacture of a coated metallic substrate comprising the following step:

A. The provision of a metallic substrate, B. Optionally, a surface preparation step of the metallic substrate,

C. the deposition of the first coating based on aluminum and comprising optionally less than 50% of zinc, such first coating having a thickness below 5pm and

D. The deposition of the second coating comprising from 0.5 to 5.9% by weight of magnesium, the balance being zinc.

Preferably, in when step B) is performed, the surface treatment is chosen from: shot blasting, pickling, etching, polishing, sand blasting, grinding and the deposition of an intermediate layer comprising iron, nickel, chromium and optionally titanium. Preferably, the surface treatment consists in depositing an intermediate layer on the metallic substrate. Moreover, the intermediate layer permits to avoid the removal of the oxide layer naturally present on the metallic substrate.

Preferably, In steps C) and D), the deposition of the first and second coatings independently of one another are performed by a hot-dip coating, by electro-deposition process or by vacuum deposition. For example, the first coating is deposited by hot-dip and the second coating by vacuum deposition. In another example, the first and seconds coatings are deposited by vacuum deposition.

In steps C) and D), when vacuum deposition is performed, the first and second coatings independently of one another are deposited by magnetron cathode pulverization process, jet vapor deposition process, electromagnetic levitation evaporation process or electron beam physical vapor deposition. For example, the first coating is deposited by magnetron cathode pulverization process and the second coating is deposited by jet vapor deposition process.

The present invention also relates to the use of the coated metallic substrate according to the present invention for the manufacture of a part of an automotive vehicle.

Finally, the present invention relates to an installation for continuous vacuum deposition of coatings on a running metallic substrate comprising:

A. Optionally, an intermediate section comprising a magnetron cathode pulverization device, B. A first section comprising an electron beam evaporation device to deposit a first coating based on aluminum comprising optionally less than 50% of zinc and

C. A second section comprising a jet vapor evaporation device.

Preferably, when the intermediate section is present, the magnetron cathode pulverization device comprises a vacuum deposition chamber comprising one target made of iron, chromium, nickel and optionally titanium and a plasma source to deposit an intermediate layer comprising iron, nickel, chromium and optionally titanium on the metallic substrate.

In this case, inert gas is injected into the vacuum deposition chamber being at a pressure between 10 ^"4 and 10 ^~7 bar. The plasma source creates a plasma which electrically charges the gas. Surface atoms of the target are physically ejected by the bombardment of energetic particles (ions) created in a plasma. Ejected atoms deposit on the metallic substrate.

In the first section, the electron beam evaporation device comprises a vacuum deposition chamber comprising an evaporation crucible comprising metal based on aluminum comprising optionally less than 50% of zinc, a heating device and an electron gun.

In the second section, the jet vapor evaporation device comprises a vacuum deposition chamber comprising at least one evaporation crucible, a vapor jet coater and at least one evaporation crucible suited to feed the vapor jet coater with a vapor comprising from 0.5 to 5.9% by weight of magnesium, the balance being zinc.

Preferably, the vapor jet coater is a sonic vapor jet coater and the evaporation crucible comprises an induction heater.

Advantageously, a recharging furnace is placed beneath the evaporation crucible and is adapted to be maintained at atmospheric pressure. More preferably, the recharging furnace is connected to a metal ingot feed means.

In the vacuum deposition chamber, beside the face of the metallic substrate which has to be coated, there is a vapor jet coater. This coater is suited to spray a metal alloy vapor comprising from 0.5 to 5.9% by weight of magnesium, the balance being zinc onto the running substrate. The vapor jet coater is mounted on an evaporation crucible suited to feed the vapor jet coater with a vapor comprising from 0.5 to 5.9% by weight of magnesium, the balance being zinc. In one preferred embodiment, one evaporation crucible is suited to contain a metal alloy bath comprising magnesium and zinc generating the vapor to be deposited on the substrate. In another preferred embodiment, two evaporation crucibles are used, one evaporator crucible containing magnesium and one evaporator crucible containing zinc, the mixture of zinc and magnesium vapors is deposited on the substrate. Preferably, the at least evaporation crucible is preferably located in the deposition chamber.

The evaporation crucible can be provided with heating means enabling the metal alloy vapor to form and to feed the vapor jet coater. The evaporation crucible is advantageously provided with an induction heater which has the advantage of making the stirring and the composition homogenization of the metal alloy bath easier.

The pressure in the evaporation crucible depends on the bath temperature and on the metal bath composition. It generally varies between 10 ^"3 and 10 ^"1 bar. Accordingly, the pressure in the deposition chamber is kept below the one in the evaporation crucible (typically around 10 ^"4 bar).

The at least evaporation crucible can be connected to a continuous feeding system of zinc and/or magnesium. The continuous feeding comprises preferably a recharging furnace suited to feed the evaporation crucible with zinc in molten state and capable of maintaining a constant level of liquid in the evaporation crucible . the recharging furnace is preferably located outside of the vacuum deposition chamber. The recharging furnace is preferably placed beneath the evaporation crucible and adapted to be maintained at atmospheric pressure. Due to the height between the evaporation crucible and the recharging furnace and to the pressure difference created between them, the molten main element goes up in the evaporation crucible by barometric effect as the metal bath evaporates. This ensures a continuous feeding of the evaporation crucible and contributes to maintaining a constant level of liquid in the evaporation crucible, whatever the line speed. Preferably, the continuous feeding further comprises a feeding unit suited to be fed with magnesium in solid state and suited to feed the evaporation crucible with the at least one additional element indifferently in molten state, in solid state or partially in solid state.

The invention will now be explained in trials carried out for information only. They are not limiting. Examples

For all samples, the chemical composition of the steel sheet used is as follows: 0.2% by weight of C, 1.5% by weight of Si, 2% by weight of Mn, 0.04% by weight of Al, the balance being iron.

For Trial 1 , a coating of Zn was deposited by electrodeposition on a steel sheet.

For Trials 2 and 3, a coating comprising 4% by weight of magnesium, the balance being zinc was deposited by magnetron cathode pulverization process.

For Trials 4 and 5, an intermediate layer being 316 stainless steel including 16-18% by weight Cr and 10-14% by weight Ni, the balance being Fe was deposited by magnetron cathode pulverization, a first coating of aluminum was deposited by electron beam deposition and a second coating comprising 4% by weight of magnesium, the balance being zinc was deposited by JVD. For Trial 6, no coating was deposited on the steel sheet. Example 1 : LME test

In order to measure LME sensitivity, high temperature tensile test was performed in conditions as close as possible to spot welding conditions.

Tensile specimens are illustrated in Figure 1. Two holes in the heads are machined to inserts pins and thus ensuring no sliding during the test.

Then, a preload force of 1 kN is applied on each specimen. The specimens are heated at a heating rate about 1000°C/s to reach a temperature between 750 and 950°C. When the temperature is reached, displacement is applied to specimens until a full failure. A strain rate of 3mm/s was used during the test.

Stress-strain curves are determined and analyzed. The derived of this stress-strain curve is calculated and drawn. Steel elongation when the strain derivative is minimum, corresponds to the fracture of the specimen. If this fracture results from an LME phenomenon, i.e. if liquid zinc is present in cracks, this elongation is defined as the "critical LME elongation value". If this fracture is a ductile fracture, the elongation is defined as "critical elongation value". This value then plotted as a function of the temperature of the tensile test.

Results are shown in the following Table 1 : 0 means excellent, in other words, there is no liquid zinc in cracks, i.e. the fracture is ductile; 1 means bad, in other words, liquid zinc is present in cracks corresponding to LME cracks.

Coating Temperature Critical LME LME

(°C) elongation or cracks

Trials Thickness Thickness Critical

1 ^st coating 2 ^nd coating

(μ ^η ) (μηι) elongation value

(mm)

750 06 I

800 07

Zn

1 7.5 - - 850 12

(EG) 1

900 OJS

950 06 1

750 4 0

800 3.8 0

ZnMg(wt.4%

4 ^* Al 3 2.5 850 3.6 0

)

900 3 0

950 3.2 0

750 3.8 0

800 3.2 0

Zn g(wt.4%

5 ^* Al 3 5 850 2.8 0

)

900 2.9 0

950 3.7 0

750 3.2 0

800 3.8 0

6 - - - - 850 3.2 0

900 2.9 0

950 3.1 0

'according to the present invention.

Trials 4 and 5 according to the present invention show excellent resistance to LME at the same level as the bare steel, i.e. Trial 6. Example 2: Electrochemical behavior test:

Trials 1 to 5 were prepared and subjected to an electrochemical potential test.

A test consisting in measuring the electrochemical potential of the coated steel surface sheet was realized. Steel sheets and coatings were separated and dipped in a solution comprising 5% by weight of sodium chloride at pH 7. A saturated calomel electrode (SCE) was also immersed into the solution. The coupling potential of the surface was measured over time. Results are shown in the following Table 2: Coating Electrochemical

Trials 1 ^st Thickness Thickness potential

coating (Mm) coating (Mm) (V/SCE)

Zn -1.05

1 7.5 - -

(EG)

ZnMg -1.05

2 2.5 - - (wt.4%)

ZnMg -1.05

3 5 - - (wt.4%)

ZnMg(wt. -1.06

4* Al 3 2.5

4%)

ZnMg(wt. -1.06

5 ^* Al 3 5

4%)

^* according to the present invention.

All tested coatings have a similar electrochemical potential and are sacrificial for steel. Coupling potential of all Trial is under -0.78V/SCE as required, i.e. the minimum electrochemical potential for having sacrificial protection against corrosion.

Example 3: Corrosion test:

Firstly, Trials 1 to 5 were scratched through the coating until the metallic substrate on a width of 1 mm.

A test, consisting in submitting these Trials 1 to 5 to corrosion cycles according to the norm VDA 233-102, was then realized. Trials were put in a chamber wherein an aqueous solution of sodium chloride of 1 % by weight was vaporized on trials with a rate of flow of 3ml_.h ^"1 . The temperature varied from 50 to -15°C and the humidity rate varied from 50 to 100%. Figure 2 illustrates one cycle corresponding to 68 hours, i.e. one week.

The amount of red rust was observed around the scratch, i.e. the front face, and in the scratch with a software and by naked eyes: 0% means excellent, in other words, there is no red rust and 100% means very bad, in other words, 100% of the Trial is covered by red rust. Results are shown in the following Table 3: Coating Front face - Red rust (wt.%) scratch -Red rust (wt.%)

Trial 1 3 6 7 1 3 6 7

1 ^s ' Thicknes Thicknes

s cycl cycle cycle cycle cycl cycle cycle cycle coating s (Mm) coating s (Mm)

e s s s e s s s

Zn

1 7.5 - - 1 8 87. 100 0 0 100 100 (EG)

Zn g

2 (wt.4% 2.5 - - 0 5 23 31 0 0 100 100 )

ZnMg

3 (wt.4% 5 - - 0 1 4 6 0 0 100 100 )

ZnMg

4* Al 3 (wt.4% 2.5 0 1 4 7 0 0 16 20

)

ZnMg

5 ^* Al 3 (wt.4% 5 0 0 1 1 0 0 12 13

)

*according to the present invention.

Trials 4 and 5 according to the present invention have a lower amount of red rust compared to Trials 1 to 3.