EP3649272B1

EP3649272B1 - A steel substrate bearing a cold sprayed coating

Info

Publication number: EP3649272B1
Application number: EP18737726.2A
Authority: EP
Inventors: Thomas URIOS; Anne FAGOT-VERDIER; Pierre-Emmanuel LEGER; Michel Jeandin; Maurice Ducos
Original assignee: ArcelorMittal SA
Current assignee: ArcelorMittal SA
Priority date: 2017-07-04
Filing date: 2018-07-02
Publication date: 2021-08-25
Anticipated expiration: 2038-07-02
Also published as: WO2019008405A1; WO2019008501A1; EP3649272A1

Description

The present invention relates to a steel substrate bearing a cold sprayed coating and a method for the manufacture of this cold sprayed coated steel substrate. The invention is particularly well suited for the industry, civil engineering and construction fields.
In industry, civil engineering and construction fields, metallic substrates are often use in order to produce metallic parts, in particular for marine metallic structures. For example, the metallic parts can be steel pilings, bearing piles, combined walls, moles, parts of offshore wind powers or plates and beams for offshore structures. The main problem of these parts is their resistance to corrosion. Indeed, the parts are used in corrosive areas, i.e. in sea area. Due to these corrosive areas, the lifetime of such parts can be divided by 5 when compared to the use of such parts on earth. Thus, there is a need to provide metallic substrates having a higher resistance to corrosion in order to improve their lifetime especially in corrosive atmospheres.
Some protections can be used to protect these parts. For example, it is known to produce these parts with a sacrificial thickness, the lifetime depending on the thickness of the part. However, even if the sacrificial thicknesses improve the lifetime of these parts, they do not really improve the corrosion resistance. It is also know to deposit paints or metallic coatings such as a zinc coating by hot-dip galvanizing. However, these deposition processes are very expensive and difficult to implement at industrial scale especially for long and heavy products.
Indeed, for example for the hot-dip galvanizing process, usually, a coil is firstly annealed and then hot-dipped in a zinc bath in the hot-dip galvanizing line. With the actual lines, it is difficult to cover parts such as marine metallic structures.
Indeed, for example, the steel pilings are produced by hot-rolling at about 1250°C. The semi-finished products called "beam blank" pass through walking beam furnace that are mobile. Then, oxides formed during the reheating are removed from the beam blank. The blanks pass through rolling mills. After, the length of the steel piling is about 100m. The steel piling is then cut in order to obtain a final length around 20m. The steel piling is finally cooled and can be implementing in the soil in the sea. Due to the length and the shape of the steel piling, it cannot be hot-dipped in continuous hot-dip galvanizing lines.
It is possible to cover these parts by a technique called "Cold spray", also called as "Cold Gas Dynamic Spray". Cold spray can be classified into two categories depending on different spraying pressures: low-pressure cold spray and high-pressure cold spray. Cold spray process mainly refers to a process in which tiny solid particles are sprayed onto the surface of a metallic or insulating substrate using a compressed gas flow with a supersonic speed, and deposited following strong plastic deformation, to form a coating. In the cold spray process, since the spraying is performed under a low temperature (below 600°C), phenomena such as oxidation, grain growth and phase transition are difficult to occur for spraying materials such that the properties of original materials can be well maintained. Meanwhile, the coating manufactured by a cold spray process has a denser microstructure, a lower heat-affected residual stress and a better long-acting protection for superstructures of the marine oil and gas equipments as compared to conventional thermally sprayed coatings, because during the cold spray, the process temperature is reduced, and solid particles is deposited mainly depending on the plastic deformation due to improvement in kinetic energy, and the particles are deformed more fully. The cold spray method can be used for covering metallic substrate in industry, civil engineering and construction field.
It is known to use some metallic coatings as a protection coating against corrosion using cold spray process. Metallic aluminum particles are generally recognized as relatively ideal cold spraying materials due to their low yield strength, good plastic deformation capacity and excellent corrosion-resistance property. A metallic zinc coating can be used as an anti-corrosive coating of metallic components, wherein the metallic zinc coating is used as a sacrificial anode material, providing a cathodic protection to protect metallic substrates from corrosion. However, the metallic zinc coating does not provide a sufficient barrier effect.
The patent application JP200730873A discloses an anti-corrosion coating of Al-Zn alloy with Al₂O₃ hard particles to be applied on steel plate substrate by cold spray. The patent application CN104946930 discloses a composite-structure aluminum-zinc protective coating and a preparation method thereof. The preparation method comprises the following steps: preparing a Zn-AI alloy composition, wherein the Zn-AI alloy composition comprises 5-35 wt.% of Al and the balance of Zn and inevitable impurities; preparing a Zn-AI alloy powder by atomization shock cooling to obtain dendritic structure powder with fine granule inside; and by using high-speed inert gas and/or reducing gas as carrier gas, carrying out high-speed injection on a steel structural member surface to deposit the Zn-AI alloy coating which is 100-500 mu m thick.
However, the anticorrosion properties of the aluminum-zinc metallic coating are not sufficient. Indeed, the high amount of zinc in the coating allows a sacrificial protection allowing a decrease or a prevention of red rust. Nevertheless, especially for metallic substrates used in very corrosive atmospheres, they have to be protected by a coating having a sacrificial protection and a barrier effect in order to significantly reduce the corrosion problems and therefore to greatly increase the life time of the metallic substrate.
Thus, the object of the invention is to provide a coated steel substrate having an improved resistance to corrosion, in particular having a barrier effect and a sacrificial protection. The object is also to provide a coated steel substrate having a longer life time especially in corrosive atmosphere. Finally, it also makes available an easy to implement method for the manufacture of such coated steel substrates.
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 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 steel substrate bearing a cold sprayed coating according to claim 1. The steel substrate bearing a cold sprayed coating can also comprise any characteristics of claims 2 to 8.
Another object is achieved by providing a method for the manufacture of a steel substrate bearing a cold sprayed coating according to claim 9. The method can also comprise any characteristics of claims 10 to 20. Finally, the object is achieved by providing the use of a steel substrate bearing a cold sprayed coating according to claim 21.
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 a schematic illustration of a cold spray device.
Figure 2 is a schematic illustration of a powder particle having a spherical shape.
Figure 3 is a schematic illustration of a powder particle having an angular shape.
Figure 4 is a schematic illustration of a powder particle having a spheroidal shape.

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:

"Porosity" is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, as a percentage between 0 and 100% by volume,
the hardness can be defined by HV(0.1) or HV(0.01) according to the norm ISO 6507-1 and
"wt.%" means the percentage by weight.

The invention relates to a metallic substrate bearing a cold sprayed coating comprising:

from 1.0 to 10.0% by weight of zinc,
- between 10 and 20% by weight of Al₂O₃,
- the balance being aluminum,
- the coating not comprising magnesium,
- said coating having a porosity between 0.9 and 8.0% and
- wherein the microstructure of the coating does not comprise an intermetallic diffusion layer made of Fe and Al.

Without willing to be bound by any theory, it seems that the cold sprayed coated metallic substrate according to the present invention has a sacrificial protection and a barrier effect mainly due to the presence of zinc, Al₂O₃ and aluminum in the above specific amounts. Indeed, it is believed that Zn having a low electrochemical potential in addition to Al having a high barrier effect with specific proportions will provide a higher corrosion protection in corrosive areas Thus, the metallic substrate is well protected against corrosion, the appearance of the red rust is prevented or at least significantly delayed. Moreover, it seems that Al₂O₃ can improve the hardness of the coating so as to obtain a better adhesion of the coating on the metallic substrate. Finally, it is believed that the presence of magnesium will not provide a sufficient resistance corrosion.
The coating can comprise unavoidable impurities. For example, the optionally impurities are chosen from Fe, Si, Cu, the content by weight of each additional element being inferior to 0.3% by weight.
Preferably, the coating comprises from 2.0 to 8.0, more preferably from 3.0 to 6.0% by weight of zinc.
Preferably, the coating has a porosity between 3.0 and 7.0%.
On the contrary to conventional deposition processes, in particular the hot-dip deposition, the microstructure of the coating does not comprise an intermetallic diffusion layer made of Fe and Al. More preferably, the microstructure of the coating does not comprise an intermetallic diffusion layer made of FeAl₃ and Fe₂Al₅. More preferably, the coating microstructure does not include an intermetallic layer between the coating and the metallic substrate. Without willing bound by any theory, it seems that the specific microstructure of the present invention prevents the risks of coating brittleness and therefore improves the adherence of the coating on the metallic substrate.
Advantageously, the thickness of the coating is between 100 and 350µm, more preferably between 150 and 250µm.
Preferably, the coating hardness is between 40 and 80HV(0.1) and more preferably between 50 and 70HV(0.1). For example, the hardness of the coating is between of 60HV(0.1).
The metallic substrate is a steel substrate.
In a preferred embodiment, the metallic substrate is a part chosen from: a piling, a wire, a plate, a tube and a beam.
The method for the manufacture of a steel substrate bearing a cold sprayed coating according to the present invention comprises the following step:

A. The provision of a metallic substrate,
B. Optionally, a surface preparation step of the metallic substrate,
C. The cold spray deposition of a powder mixture comprising from 1.0 to 10.0% by weight of zinc, between 10 and 20% by weight of Al₂O₃, the powder not comprising magnesium, the balance being aluminum, on the metallic substrate, said deposition being performed in a cold spray device comprising a heating section, a powder feeder and a supersonic nozzle.

Preferably, when step B) is performed, the surface treatment is chosen from: shot blasting, pickling polishing, sand blasting and grinding. In this case, preferably, the surface preparation step is performed so as to obtain a surface roughness of the metallic substrate above 5µm, more preferably between 5 and 30µm and advantageously between 10 and 20µm. Indeed, without willing to be bound by any theory, it is believed that the surface preparation modified the surface texture of the metallic substrate allowing an increase of the coating adhesion. In this embodiment, the surface treatment is preferably shot basting.
Additionally, usually, the steel substrate can comprise an oxide layer on its surface originating from the elaboration route of the metallic substrate. In this case, it is believed that the surface preparation can remove oxides present on the metallic substrate to further improve the coating adhesion on the metallic substrate. For example, when the metallic substrate is a steel substrate, a layer of calamine is generally formed after the hot-rolling. In this case, the surface preparation can remove the millscale layer.
Advantageously, the step C) comprises the following sub-steps:

i. The injection of at least one gas in the cold spray device, said gas being under pressure,
ii. The heat treatment of a first part of the gas at a temperature between 200 and 600°C in a heating section,
iii. the transport of the second part of the gas through the powder feeder section so as to carrier the powder mixture,
iv. The mixing of the first and the second parts of gas before the ejector of the supersonic nozzle and
v. The acceleration of the mixing gases comprising the powder mixture at supersonic speed through the ejector of the supersonic nozzle so as to deposit the powder mixer on the metallic substrate.

Figure 1 illustrates a typical cold spray device wherein preferably, in step C), at least one gas under pressure is injected in the cold spray device.
Preferably in step C.i), the gas is chosen among: nitrogen, argon, helium, air or a mixture thereof.
Advantageously, the pressure of the gas is between 1 and 10 MPa. For example, the pressure of the gas is between 2 and 7 MPa.
Then, advantageously, in step C.ii), a first part of the gas is heated in the heating section 1 at a temperature 200 and 600°C, more preferably between 300 and 500°C and for example of 400°C.
Preferably, in step C.iii), the second part of the gas navigates through the powder the feeder section 2 in order to carrier the powder mixture.
In a preferred embodiment, steps C.ii) and C.iii) are performed simultaneously.
Preferably, the first part represents above 60% by volume of the gas and the second part represents below 40% by volume of the gas. More preferably, the first part represents above 70% by volume of the gas and the second part represents below 30% by volume of the gas. Advantageously, the first part represents above 80% by volume of the gas and the second part represents below 20% by volume of the gas.
Then, advantageously in step C.v), the first and the second parts of gas are mixed before the ejector of the supersonic nozzle 3.
After, preferably, the mixing gases comprising the powder mixture crosses the ejector of the supersonic nozzle 4 resulting in an acceleration of the gases at supersonic speed. Usually, the supersonic nozzle is a tube that is pinched in the middle, making a carefully balanced, asymmetric hourglass shape. Thus, the powder mixture particles impact the metallic substrate 4 to create a coating.
Advantageously, the supersonic speed is between 350 and 1200m.s^-1, preferably between 400 and 1000m.s^-1 and more preferably between 400 and 850m.s^-1.
In a preferred embodiment, the distance projection between the ejector supersonic and the metallic substrate is between 10 and 50mm, more preferably between 20 and 40mm and for example of 30mm.
Preferably, the projection angle between the supersonic nozzle and the metallic substrate is between 70 and 110°.
Preferably, the powder mixture comprises from 2.0 to 8.0, more preferably from 3.0 to 6.0% by weight of zinc.
The coating comprises between 10 and 20% by weight of Al₂O₃.
Preferably, the powder particles shape is spherical, spheroidal or angular. Figure 2 illustrates a spherical powder particle. Figure 3 illustrates an example of angular powder particle. For example, an angular particle can have a hexagonal shape, a cubic shape or a triangular shape Figure 4 illustrates a spheroidal powder particle.
Advantageously, the powder particles have a size above 10µm, more preferably between 10 and 150µm and more preferably between 10 and 100µm.
Advantageously, the hardness of the powder mixer is between 30 and 50HV(0.01).
The cold spray method according to the present invention can be used locally on the metallic substrate. In other words, it is possible to deposit the coating according to the present invention only on specific areas in order to obtain a metallic substrate having at least one coated area, the reminder being none coated. In this case, the productivity of the cold spray method increases. For example, it can be used on piling products in order to protect preferentially at least one specific area, the one subjected to the corrosion medium.
Moreover, it is also possible to deposit an additional coating on at least one specific area of a metallic substrate being already coated in order to highly improve the corrosion resistance in area(s) that will be subjected to very corrosive atmospheres.
Finally, sometimes, the coating of a metallic substrate can be damaged. It can be due to scratches for example during the transport or during the building of metallic structures. In this case, with the method according to the present invention, it is possible to repair the metallic coating by depositing the cold sprayed coating on the damaged area(s). The local deposition of the coating on the metallic substrate with the method according to the present invention is easy to implement and quick improving the productivity.
According to the present invention, the cold sprayed metallic substrate can be used for the manufacture offshore structure, offshore wind power, Marine current power, hull of a boat, coastal and port infrastructure, quay wall and, underground structure, rail and anchorage.
The disclosure will now be explained in trials carried out for information only.
The examples and trials are not according to the invention.

Examples

For all samples, the metallic substrate is a steel substrate having the following composition according to the EN 10248-1 norm:

C (wt.%) Si(wt.%) Mn(wt.%) S(wt.%) P(wt.%)

0.18 0.21 1.32 0.003 0.013
Trial 1 is a cold sprayed coated steel sheet wherein the coating comprising 3.9% by weight of Zn, the balance being Al.
Trial 2 is a cold sprayed coated steel sheet wherein the coating comprising Al.
Trial 3 is a cold sprayed coated steel sheet wherein the coating comprising 15% by weight of Al₂O and Al.
For all Trials, the steel sheet was first shot blasted to obtain a surface roughness of 14.5µm. Then, the powder mixture was deposited by cold spray process. The parameter of the cold spray process was as follows:

gas: nitrogen at a pressure of 2.5MPa,
heat treatment at a temperature of 400°C,
second part of the carrier gas: 7% by volume,
projection distance: 30mm,
projection angle: 90°,
supersonic speed: between 450 and 800m.s^-1.

Example 1: Electrochemical behavior test:

Trials 1 to 3 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 1 :

Trials	Composition			Powder mixture			Coating
Trials	Al	Zn	Al2O3	size (µm)	shape	Hardness (HV)	Thickness (µm)	Porosity (%)	Hardness (HV)	Coupling potential (V/SCE)
1	95	3.9	-	Between 15-55	spherical	40	200	1.9	60	-1.01
2	>99	-	-	Between 15-55	spherical	28	200	5.2	60	-0.73
3	85	-	15	Between 15-55	spherical	ND	200	2.7	63	-0.76

ND: Not Done.

Trial 1 according to the present invention is highly sacrificial compared to Trials 1 to 3. Coupling potential of Trial 1 is under -0.78V/SCE as required.

Example 2: Salt spray test

Trials 1 to 3 were prepared and subjected to a corrosion test to evaluate the protection of the coated steel sheets.
Firstly, all trials were scratched through the coating till the metallic substrate on a width of 2mm. Then, a test, consisting in submitting coated steel sheet to corrosion cycles according to the norm ATSM-B-117, was realized. All Trials were incorporated in an enclosure wherein a salt fog was sprayed. The salt fog comprises 5% by weight of NaCl. The salt fog had the following characteristics:

pH = 7,
temperature = 35°C and
debit flow 1,5ml/h.

The presence of corrosion on coated steel sheet around scratches was observed by naked eyes: 0 means excellent, in other words, there is little or no corrosion around scratch and 5 means very bad, in other words, there is a lot of corrosion around scratch. Results are shown in the following Table 2:

Number of days

Trials

1 3 8

1 0 0 0

2 0 1 2

3 0 1 2
Trial 1 shows excellent protection against corrosion when compared to Trials 2 and 3. Moreover, Trial 2 shows red rust after 15 days of salt spray test.

Claims

A metallic substrate bearing a cold sprayed coating comprising:
- from 1.0 to 10.0% by weight of zinc,

- between 10 and 20% by weight of Al₂O₃,

- the balance being aluminum,

- the coating not comprising magnesium,

- said coating having a porosity between 0.9 and 8.0%,

- wherein the microstructure of the coating does not comprise an intermetallic diffusion layer made of Fe and Al and

- wherein the metallic substrate is a steel substrate.
A metallic substrate bearing a cold sprayed coating according to claim 1, wherein the coating comprises from 2.0 to 8.0% by weight of zinc.
A metallic substrate bearing a cold sprayed coating according to claim 2, wherein the coating comprises from 3.0 to 6.0% by weight of zinc.
A metallic substrate bearing a cold sprayed coating according to anyone of claims 1 to 3, wherein the coating has a porosity between 3.0 and 7.0%.
A metallic substrate bearing a cold sprayed coating according to anyone of claims 1 to 4, wherein the microstructure of the coating does not comprise an intermetallic diffusion layer.
A metallic substrate bearing a cold sprayed coating according to anyone of claims 1 to 5, wherein the thickness of the coating is between 100 and 350µm.
A metallic substrate bearing a cold sprayed coating according to anyone of claims 1 to 6, wherein the coating hardness is between 40 and 80 HV(0.1).
A metallic substrate bearing a cold sprayed coating according to claims 1 to 7, wherein the metallic substrate is a part chosen from: a piling, a wire, a plate, a tube and a beam.
A method for the manufacture of a steel substrate bearing a cold sprayed coating comprising the following step:
a. The provision of a steel substrate,

b. Optionally, a surface preparation step of the steel substrate,

c. The cold spray deposition of a powder mixture, comprising from 1.0 to 10.0% by weight of zinc, between 10 and 20% by weight of Al₂O₃, the coating not comprising magnesium, the balance being aluminum, on a steel substrate, said deposition being performed in a cold spray device comprising a heating section, a powder feeder and a supersonic nozzle.
A method according to claim 9, wherein step C) comprises the following sub-steps:
i. The injection of at least one gas in the cold spray device, said gas being under pressure,

ii. The heat treatment of a first part of the gas at a temperature between 200 and 600°C in a heating section,

iii. the transport of the second part of the gas through the powder feeder section so as to carrier the powder mixture,

iv. The mixing of the first and the second parts of gas before the ejector of the supersonic nozzle and

v. The acceleration of the mixing gases comprising the powder mixture at supersonic speed through the ejector of the supersonic nozzle so as to deposit the powder mixer on the steel substrate.
Method according to claim 9 or 10, wherein when step B) is performed, the surface treatment is chosen from: shot blasting, pickling polishing, sand blasting and grinding.
Method according to claim 10 or 11, wherein in step C.i), the gas is chosen among: nitrogen, argon, helium, air or a mixture thereof.
Method according to anyone of claims 10 to 12, wherein in step C.i), the pressure of the gas is between 1 and 10 MPa.
Method according to anyone of claims 10 to 13, wherein in step C.ii), the first part represents above 60% by volume of the gas and the second part represents below 40% by volume of the gas.
Method according to anyone of claims 10 to 14, wherein in step C.v), the supersonic speed is between 350 and 1200m.s^-1.
Method according to anyone of claims 10 to 15, wherein in step C.v), the distance projection between the ejector supersonic and the metallic substrate is between 10 and 50mm.
Method according to anyone of claims 10 to 16, wherein in step C.v), the projection angle between the ejector supersonic and the metallic substrate is between 70 and 110°.
Method according to anyone of claims 9 to 17, wherein the powder mixture shape is spherical, spheroidal or angular.
Method according to anyone of claims 9 to 18, wherein the powder mixture has a size above 10µm.
Method according to anyone of claims 9 to 19, wherein the hardness of the powder mixer is between 30 and 50HV (0.01).
Use of a metallic substrate bearing a cold sprayed coating according to anyone of claims 1 to 8 or obtainable according to anyone of claims 9 to 20, for the manufacture of offshore structure, Offshore wind power, Marine current power, hull of a boat, coastal and port infrastructure, quay wall and, underground structure, rail and anchorage.