Patent Application
20190003056
The present application claims priority on European Patent Application No. 17461560.9 filed Jun. 28, 2017, the entire content of which is incorporated herein by reference.
The present relates generally to corrosion resistant coatings for metal substrates, and more particularly to methods for forming ceramic coatings having high corrosion protection and wear resistance.
Spark discharge oxidation is a plasma electrolytic oxidation process used to form oxide coating on metallic substrate. Plasma electrolytic oxidation allows hard ceramic coatings to be applied on diverse metallic alloys. Micro-discharges occurring on the electrode surface during the process promotes the creation of ceramic oxides phases, which may improve mechanical properties of the coating.
However, if the metallic substrate to be coated has a complicated spatial shape, a complicated surface configuration, through holes, and/or blind holes, obtaining a uniform coating over the surface of the substrate can be challenging.
There is accordingly provided, in accordance with an aspect of the present invention, a method of forming a corrosion-resistant ceramic coating on a metallic substrate, the method comprising: providing a passivation layer on a surface of the metallic substrate by electrochemical passivation of the metallic substrate under a first electrical current and using a first electrically conducting solution; and forming the corrosion-resistant ceramic coating on an outermost surface of the metallic substrate by plasma electrolytic oxidation of the metallic substrate with the passivation layer, in a second electrically conducting solution and under a second electrical current having a discharge voltage, the outermost surface in use adapted to be exposed to a corrosive environment; wherein the first and the second electrically conducting solutions comprise a tetrafluoroborate compound.
The method as described above may further include selecting the tetrafluoroborate compound from the group comprising potassium tetrafluoroborate, sodium tetrafluoroborate, lithium tetrafluoroborate and ammonium tetrafluoroborate
In the method as described above, the electrochemical passivation and the plasma electrolytic oxidation may be performed separately, and the first and the second electrically conducting solutions are different solutions.
In one particular embodiment, the first electrically conducting solution has a tetrafluoroborate concentration of 0.1-50 g/L. In another more particular embodiment, the second electrically conducting solution has a tetrafluoroborate concentration of 0.1-20 g/L.
In the method as described above, at least the second electrically conducting solution may be circulated to flow around the substrate.
In the method as described above, the substrate may be moved during at least plasma electrolytic oxidation in the second electrically conducting solution, and the substrate may further be continuously moved.
In the method as described above, the substrate may be moved by continuous flow of the first electrically conducting solution and/or the second electrically conducting solution around the substrate.
In the method as described above, the plasma electrolytic oxidation may be an electrolytic spark discharge oxidation and the discharge current is injected through the second solution by impulse, each impulse having an impulse time divided into a flow-on time, during which a current passes through the second solution, and a flow-off time, during which no current passes through the second electrically conducting solution, wherein the ratio between the flow-on time and the impulse time is of from 10% to 100%. The ratio between the flow-on time and the impulse time may also be from 30% to 40%.
The method as described above may further include adapting a distance between the outermost surface of the substrate to be coated, and at least one electrode delivering at least one of the first electrical current and the second electrical current.
The method as described above may also, alternately, including providing the passivation layer and the corrosion-resistant ceramic coating in a single step. The electrochemical passivation is a first stage of the plasma electrolytic oxidation, the first and the second electrically conducting solutions are a single and same solution; and the passivation layer is provided during onset of the electrical current up to the discharge voltage.
In accordance with another aspect of the present invention, there is also provided an apparatus for forming a corrosion-resistant ceramic-oxide coating on a metallic substrate, the apparatus comprising: an electrolytic cell including a housing containing an electrically conducting solution and adapted to receive the metallic substrate therein; and an anode and a cathode that are spaced apart and operable to supply current though the electrically conducting solution between the anode and the cathode; and wherein the electrically conducting solution comprises at least one tetrafluoroborate compound.
In the apparatus as described above, the tetrafluoroborate compound may be selected from the group comprising potassium tetrafluoroborate, sodium tetrafluoroborate, lithium tetrafluoroborate and ammonium tetrafluoroborate.
In the apparatus as described above, the electrically conducting solution may have a tetrafluoroborate concentration of 0.1-50 g/L.
The apparatus as described above may further include a mounting assembly comprising: an upper arm connected in electrical contact to the anode; and a lower arm having a first end connected in electrical contact to the upper arm and a second end adapted to be connected in electrical contact to the substrate to be coated, the lower arm being movable relative to the upper arm to allow movement of the substrate within the electrically conducting solution.
In a particular embodiment, the lower arm is pivotably connected to the upper arm as to be movable in an upright direction between at a first position and at least one second position, the first and second positions being angularly separated by an angle of from 90 to 135°.
The apparatus as described above may further include a cathode frame having a plurality cathode frame portions, each cathode frame portion being disposed along a wall of the housing and being removably connected to at least another of the cathodes frame portions, the cathodes frame portions having reversible connecting features as to be movable relative to each other, the cathode comprising a plurality of cathode plates connected to the cathode frame portions and disposed around the housing.
The apparatus as described above may further include: a filter connected to the electrolytic cell and adapted to filter a stream of the electrically conducting solution received from the electrolytic cell; a heat exchanger connected downstream the filter and upstream the electrolytic cell, the heat exchanger being adapted to remove heat from the stream of the electrically conducting solution exiting the filter before the stream is circulated back to the electrolytic cell; and a flow generating device operably connected to the electrolytic cell as to circulate the electrically conducting solution around the substrate, from the electrolytic cell to the filter and the heat exchanger, and from the filter and heat exchanger back to the electrolytic cell. The flow generating device may be a pump, the pump including a compressor operable to supply a gas and circulate the electrically conducting solution.
Reference is now made to the accompanying figures in which:
A method of treating a substrate to form a corrosion-resistant coating, such as a corrosion-resistant ceramic oxide coating, is described herein. Ceramic coatings, and more particularly ceramic oxide coatings, are sometimes used in applications when the coated component may be exposed to corrosive and/or wear conditions. For example, the corrosion-resistant coating can be formed on a component of a gas turbine engine.
It is known that the components of a gas turbine engine operate under harsh conditions: high temperatures, erosion of fly ash and sand particles, wear out of adjacent moving surfaces, oxidation and corrosion produced by the passage of hot combustion gas, etc. Therefore, certain components of a gas turbine engine may be protected by a corrosion-resistant coating.
Referring to
The coated substrate as described herein can therefore be a surface of a component of such a gas turbine engine 10, such as casing or housing thereof. For example, the component having a coated surface may include a gearbox housing, a gas generator casing, or a turbine casing, and the like, wherein wear resistance and corrosion protection is desired. Referring to
As can be seen in
In a particular embodiment, the substrate is a metallic based substrate. For example, magnesium substrates, titanium substrates or aluminum substrates are generally used in aviation industry. The substrate can thus comprise magnesium, titanium, aluminum or any alloys thereof. In other industries the metallic substrate can comprise valve metal, such as vanadium, tungsten, zirconium, niobium, hafnium, tantalum or stainless steel.
The method comprises the step of providing a first pre-treatment layer (hereafter “the first layer”), also called a passivation layer, on a surface of the substrate. The substrate can be immersed in a first electrically conducting solution (hereafter “the first solution”) while applying electrical current through the first solution. Such process is generally performed in an electrolytic cell comprising electrodes, and wherein a current is applied between the electrodes (i.e. the anode and the cathode).
The first solution, as used in the present method, comprises tetrafluoroborate ionic compound. The tetrafluoroborate ionic compound can be one of potassium tetrafluoroborate, sodium tetrafluoroborate, lithium tetrafluoroborate and ammonium tetrafluoroborate.
It is understood that the tetrafluoroborate ionic compound is an additive introduced in a pre-existing electrically conducting solution. For example, the tetrafluoroborate ionic compound can be added in a solution of fluoride ionic compound. Therefore, the first solution can comprise both tetrafluoroborate ionic compound and fluoride ionic compound. The cation of the fluoride ionic compound generally correspond to the cation of the tetrafluoroborate compound. For example, depending on the tetrafluoroborate ionic compound to be used, the first solution can be one of a solution of potassium fluoride, sodium fluoride, lithium fluoride, and ammonium fluoride.
Referring to the particular embodiment of
The first fluoride solution with tetrafluoroborate additives allows the formation of a fully passivated surface on the metallic substrate. In the first solution, the concentration of fluoride compounds can be between 0.5 and 200 g/L. In a more specific embodiment, the concentration of fluoride compounds is between 20 and 60 g/L. In a further more specific embodiment, the concentration of tetrafluoroborate compounds is be between 0.1 and 20 g/L, and may be for example between 0.5 and 5 g/L.
In one embodiment, the first solution comprises potassium fluoride at 0.5-200 g/L and potassium tetrafluoroborate at 0.1-20 g/L. In another embodiment, the solution comprises sodium fluoride at 0.5-200 g/L and sodium tetrafluoroborate at 0.1-20 g/L. In a further embodiment, the solution comprises lithium fluoride at 0.5-200 g/L and lithium tetrafluoroborate at 0.1-20 g/L. In yet a further embodiment, the solution comprises ammonium fluoride at 0.5-200 g/L and ammonium tetrafluoroborate at 0.1-20 g/L.
The immersion of the substrate in the first solution can be performed under stable voltage. It is contemplated that the electrical parameters of the passivation process can be determined as to optimize the formation of the first layer. In a particular embodiment, the current used for the electrochemical passivation has a voltage of between 1 and 120 V. In one particular embodiment, the voltage is about 50 V. Constant voltage, direct current, or impulse current can be applied. The anodic current density can be self-setting and successively decreasing as electrochemical passivation runs. In a particular embodiment, the anodic current density starts from 10 A/dm2 and achieves, at the end phase, about 0.01 A/dm2. In another embodiment, the anodic current density achieves about 0.001 A/dm2 at the end phase. The temperature of the first solution in the electrochemical passivation is kept between 0 and 100° C. In a particular embodiment, the first solution is kept at about 20° C.
The method then comprises the step of providing the corrosion-resistant ceramic coating by treating the substrate with the first layer. The ceramic coating is formed on an outermost surface of the substrate, and provides protection against corrosion and wear. The term «outermost surface» to the surface of the metallic substrate being, in use, likely to be exposed to a corrosive environment.
The corrosion-resistant ceramic coating may be formed by plasma electrolytic oxidation of the substrate covered with the first layer. Plasma electrolytic oxidation is an electrochemical treatment allowing the formation of oxide coating on surfaces. This process generally uses voltages higher than those used in the electrochemical pre-treatment passivation.
It is understood that the first layer applied during the first passivation step provides higher stability during the plasma electrolytic oxidation operating and better properties of the ceramic coatings. For example, a fluoride layer allows obtaining better properties for the outermost ceramic coating such as lower surface roughness, lower scattering of the ceramic coating so that the thickness of the ceramic coating stays within tolerance band. The presence of the fluoride layer also decrease the onset time for the electrolytic spark discharge oxidation, thereby increasing of the yield of the process.
Still referring to
The second solution also comprises tetrafluoroborate ionic compounds additive, wherein the tetrafluoroborate compounds can be at least one of potassium tetrafluoroborate, sodium tetrafluoroborate, lithium tetrafluoroborate and ammonium tetrafluoroborate. The concentration of tetrafluoroborate compounds in the second solution can be between 0.1 and 50 g/L. In a particular embodiment, the concentration of tetrafluoroborate compounds is between about 0.5 and about 5 g/L.
It is understood that the tetrafluoroborate compound is an additive introduced in a pre-existing electrically conducting solution. For example, as shown in
In one embodiment, the second solution comprises potassium hydroxide at 1-20 g/L, potassium silicate at 1-100 g/L and potassium tetrafluoroborate at 0.1-50 g/L. In another embodiment, the second solution comprises sodium hydroxide at 1.0-20 g/L, sodium silicate at 1-100 g/L and sodium tetrafluoroborate at 0.1-50 g/L. In a further embodiment, the second solution comprises lithium hydroxide at 1-20 g/L, lithium silicate at 1-100 g/L and lithium tetrafluoroborate at 0.1-50 g/L. In yet a further embodiment, the second solution comprises one of potassium, sodium and lithium hydroxide at 1-100 g/L, one of potassium, sodium and lithium silicate at 1-50 g/L and ammonium tetrafluoroborate at 0.1-50 g/L.
Therefore, in the particular embodiment of
In another particular embodiment, the first layer is formed in a first stage of a plasma electrolytic oxidation process, while the voltage of the electrical current is progressively increased. In such case, the first layer can be an oxide layer, such as a magnesium oxide layer, a titanium oxide layer, an aluminum oxide layer, etc., depending on the type of metallic substrate to be coated.
In the process of electrolytic spark discharge oxidation, constant or impulse current of monopolar or bipolar nature can be applied. The current may be monopolar impulse nature. The impulse current may have a frequency of from 0.01 kHz to 10 kHz. In a particular embodiment, the impulse current is between about 0.5 kHz and about 1 kHz. The voltage of the current applied may be from 100 V to 600 V. In a particular embodiment, the voltage is between about 200 V and about 450 V. In addition, the anodic current density is between 0.1 A/dm2 and 20 A/dm2. In a particular embodiment, the anodic current density is between about 2 A/dm2 and about 10 A/dm2. The temperature of the second solution during plasma electrolytic oxidation may be kept between 0 and 60° C. In a particular embodiment, the temperature of the second solution is kept between about 3 to about 7° C.
As shown in
The method can further comprise circulating at least one of the first and the second solution around the substrate during the formation of the first layer and/or during the treatment of the substrate having the first layer to provide the ceramic coating. However, while providing the first layer it may be sufficient to solely ensure full contact between the substrate and the first solution. The circulation of the first and/or the second solution can be continuous or intermittent. For example, pumps or compressors can be used to put the first solution and/or the second solution into motion. Moving the first and/or the second solution around the substrate maximizes the contact between the electrolytes in the respective one of the first and/or the second solution and the surface of the substrate
In the particular embodiment of
The method can further comprise moving the substrate in at least one of the first solution and the second solution during the formation of the first layer and/or during the treatment of the substrate having the first layer to provide the ceramic coating. As mentioned above, while providing the first layer it can be sufficient to solely ensure full contact between the substrate and the first solution.
In the embodiment of
It is understood that the substrate can be moved using any suitable techniques. For example, the first and/or the second solution can be circulated around the substrate using pumps or compressors and the flow generated can put the substrate into motion. The substrate could also be moved by an mechanical device generating an automatic movement of the substrate in the first and/or the second solution. Moving the substrate within the first and/or the second solution allows the formation of a coating having a uniform thickness and reproducible properties regardless of the geometry of the substrate.
The method can also comprise adapting the distance between the electrode delivering the electrical current during at least one of the electrochemical passivation and plasma electrolytic oxidation. For example the cathode may be moved closer to the substrate to be coated depending on the size and shape of the metallic substrate. It is contemplated that moving the electrode, and more particularly the cathode, allow to further improve the uniformity of the ceramic coating formed on the outermost surface of the substrate.
The thickness of the corrosion-resistant coating formed by the present method depends on the parameters used: voltage, frequency, concentration of the electrolytes and additives in the first and second solutions, duration of the electrolytic arc-discharge oxidation process, duty cycle, circulation of the first and/or second solution, motion of the substrate, etc. For example the thickness of the corrosion resistant coating can be between 5 and 20 μm. In a particular embodiment, the corrosion resistant ceramic coating obtained has a thickness of about 10±2 μm after 10 to 20 minutes of the electrolytic spark discharge oxidation with average value of current density equal to 2-5 A/dm2, current frequency 0.5-2 kHz, and duty cycle of 20 to 40%. The substrate coated with said 10 μm corrosion-resistant ceramic coating can resist from 220 hours to 240 hours when tested in standard salt chamber (5% water solution of sodium chloride, at 35° C.).
The corrosion-resistant coating resulting from the present method can comprise magnesium fluoride, magnesium oxide, magnesium fluoro-oxide and silicon dioxide. Such materials are resistant to corrosion and abrasion thereby providing a wear resistant and corrosion resistant coating.
An apparatus for forming a corrosion-resistant ceramic-oxide coating on a metallic substrate is also described.
Referring to
The electrically conducting solution 116 comprises at least one tetrafluoroborate compound. The electrically conducting solution 116 can be used in electrolytic passivation (first solution) and can have a concentration of tetrafluoroborate compound of between 0.1 and 20 g/L. The electrically conducting solution 116 can be used in plasma electrolytic oxidation (second solution) and can have a concentration of tetrafluoroborate compound of between 0.1 and 50 g/L. The tetrafluoroborate compound and the electrically conducting solution 116 are as described herein.
In the embodiment of
As shown in
Still referring to
Movement of the lower arm 128 relative to the upper arm 124 leads to movement of the substrate within the electrically conducting solution. In the embodiment of
As shown in
The mounting assembly 122 can moves between the first and the second positions in a period of time of 0.5 to 1.5 minutes. The mounting assembly 122 allows rotation of the substrate to be coated in relation to the anode by a total angle of at least 180°. When the substrate has complicated spatial shape and complicated surface configuration, through holes and/or blind holes and/or deep cavities, movement of the substrate within the electrically conducting solution provides a corrosion-resistant coating having uniform thickness and reproducible properties regardless of the geometry of the substrate.
Referring to
Referring to
As shown in
The flow generating device 244 can be a pump or a compressor. As illustrated in
Referring to
Referring to
Referring to
In
The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. For example, any suitable material having the properties described with respect to the aluminide layer, bond layer or ceramic layer may be used. Any suitable method of applying the different layers may be used. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 17461560.9 | Jun 2017 | EP | regional |
