Method for producing an iron nitride coating on the surface of an iron or iron alloy substrate

Abstract

The present invention provides a method for producing an iron nitride coating on a surface of an iron or iron alloy substrate, comprising the steps of: The process comprises: (a) immersing an iron or iron alloy substrate acting as an anode in an electrolytic bath comprising an ionic liquid comprising nitrogen cations and / or nitrogen anions in the presence of a counter electrode acting as a cathode; applying an anodic current representative of a predetermined reference current density between the substrate and the counter electrode until a predetermined threshold voltage is reached, whereby an iron nitride coating having a predetermined thickness is produced on the substrate; and (b) applying an anodic current corresponding to a predetermined reference current density between the substrate and the counter electrode until a threshold anodic current representative of a predetermined threshold current density is reached, whereby an iron nitride coating having a predetermined thickness is produced on the substrate.

Description

[Technical field]

[0001] The present invention relates to a method for producing an iron nitride coating on the surface of an iron or iron alloy substrate, and to an article comprising an iron or iron alloy substrate and a surface coating thereon obtained by the method. [Background technology]

[0002] Nitriding is known as a surface treatment applied to ferrous substrates or ferrous alloys, such as steel or cast iron, to increase surface hardness and corrosion resistance.

[0003] Currently, nitriding of iron substrates or iron alloys is carried out using the following techniques: a) plasma assisted deposition, b) ion beam deposition, c) laser melting, d) vapor phase deposition, and e) cyanide-containing baths.

[0004] However, these nitridation techniques require high processing temperatures (typically 400-1000°C), long processing times (hundreds of hours), and complex systems (such as vacuum systems and high-temperature chambers). Furthermore, these nitridation techniques have significant environmental and safety issues, such as the use of highly toxic cyanide baths.

[0005] Therefore, conventional nitriding techniques are not suitable for most industrial applications, and processing large or thin parts is practically impossible due to their susceptibility to thermal deformation.

[0006] The processing temperature limitations of conventional nitriding techniques are particularly problematic: in fact, these techniques cannot be applied to parts or components that cannot be heated without damaging their geometrical properties (e.g. flatness and roughness).

[0007] The problem that the present invention aims to solve is therefore to provide a method for producing iron nitride coatings on the surface of iron or iron alloy substrates, such as steel or cast iron, which can be carried out at temperatures significantly lower than those required by the prior art, in particular at temperatures not exceeding 250° C., and which can be easily applied on an industrial scale. Summary of the Invention

[0008] The above problems are solved by a method for producing an iron nitride coating on the surface of an iron or iron alloy substrate, and by an article comprising the coating obtained by said method, as outlined in the appended claims, the definitions of which form an integral part of this specification.

[0009] A first object of the present invention is a method for producing an iron nitride coating on the surface of an iron or iron alloy substrate, the method comprising the following steps:

[0010] a) A process in which an iron or iron alloy substrate, acting as an anode, is immersed in an electrolytic bath containing an ionic liquid containing nitrogen cations and / or nitrogen anions in the presence of a counter electrode, acting as a cathode.

[0011] b) An electrochemical nitridation process of a substrate, which comprises at least one of the following steps:

[0012] A galvanostatic step of applying an anodic current representative of a predetermined reference current density between the substrate and a counter electrode until a predetermined threshold voltage is reached, whereby an iron nitride coating having a predetermined thickness is produced on the substrate.

[0013] A potentiostatic step in which an anodic current corresponding to a predetermined reference current density is applied between the substrate and the counter electrode until an anodic current representative of a predetermined threshold current density is reached, producing an iron nitride coating having a predetermined thickness on the substrate.

[0014] A second object of the invention is an article comprising an iron or iron alloy substrate and an iron nitride surface coating on said substrate, said surface coating being obtained by said method.

[0015] The method of the invention allows the production of iron nitride coatings at temperatures significantly lower than known processes, even at room temperature, and allows the production of homogeneous coatings without the need to adjust the morphology and size of the substrate, making the method particularly suitable for the production of surface coatings on non-heatable substrates, where geometrical features such as flatness and roughness would be lost.

[0016] Furthermore, the method of the present invention allows the production of an iron nitride coating in a short time, on the order of a few tens of minutes, making it particularly suitable for industrial-scale applications.

[0017] Due to the aforementioned advantages, the method of the present invention can be applied in all technical fields, from precision machinery to the aerospace industry, from the medical field to dental implantology and the automotive field.

[0018] Further characteristics and advantages of the invention will become apparent from the following description of some embodiments given by way of non-limiting example. [Brief description of the drawings]

[0019] [Figure 1] FIG. 1 compares the Raman profile of a coating on a cast iron sample obtained by the method of the present invention (profile A) with the Raman profile of a coating on the same sample obtained by a conventional ferritic nitriding process (FNC) (profile B).

[0020] [Diagram 2] FIG. 2 shows the Glow Discharge Optical Emission Spectroscopy (GD-OES) profiles obtained on a cast iron sample coated with the method according to the invention and on the same cast iron sample that was not coated. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The present invention relates to a method for producing an iron nitride coating on the surface of an iron or iron alloy substrate, the method comprising the steps of a) immersing said substrate in an electrolytic bath containing an ionic liquid consisting of nitrogen ions in the presence of a counter electrode, followed by b) initiating an electrochemical nitriding process of said substrate, including a galvanostatic (constant current) and / or potentiostatic (constant voltage) step.

[0022] It has surprisingly been found that during the electrochemical nitriding process, nitrogen ions in the ionic liquid contained in the electrolytic bath decompose to generate nitrogen species which diffuse into the iron or iron alloy substrate, producing a surface coating of iron nitride. It has also surprisingly been found that the method according to the invention avoids adhesion problems between the substrate and the coating, since it involves surface modification of the substrate rather than deposition of a coating.

[0023] The term "ionic liquid" refers to chemical compounds consisting of ions (salts) that are in a liquid state under certain conditions of temperature and pressure. Ionic liquids are in all respects equivalent to molten salts, and by empirical definition, ionic liquids have a melting point below 100 °C, so they are in a liquid state even at low or room temperatures. In the latter case, they are called room temperature ionic liquids (or RTILs).

[0024] In one embodiment of the present invention, the ionic liquid contained in the electrolytic bath is a room temperature ionic liquid (RTIL).

[0025] The upper temperature limit at which the electrochemical process can be carried out is determined by the thermal stability of the ionic liquid, and therefore the electrochemical nitridation process is advantageously carried out at a temperature not exceeding the thermal decomposition temperature of the ionic liquid.

[0026] Preferably, the electrochemical nitridation process is carried out at a temperature below 250° C., more preferably below 200° C. Even more preferably, the electrochemical nitridation process is carried out at room temperature, the term “room temperature” meaning, for example, between about 20-25° C., between about 20-30° C.

[0027] Advantageously, both steps a) and b) of the method of the invention are carried out at said temperature.

[0028] Advantageously, during step a), the electrolytic bath in which the substrate is immersed is at a temperature below 250° C., preferably below 200° C., more preferably at room temperature.

[0029] Advantageously, the electrochemical nitridation process of step b) is carried out at a temperature below 250° C., preferably below 200° C., more preferably at room temperature.

[0030] This temperature not only simplifies the process operation and management, but also allows the production of iron nitride coatings (preferably Fe 2+ 1000 nm) with a lower crystallinity than iron nitride obtained by conventional nitriding techniques, for example using plasma technology, by using temperatures below 250 °C. 2-3 According to an embodiment, the method of the present invention has the advantage that it is possible to obtain Fe 2-3 A mixture of N (HCP structure, dense hexagonal) and Fe3N (rhombohedral structure) is obtained.

[0031] Surprisingly, it has been observed that the iron nitride coating obtained by the method of the present invention has higher corrosion resistance compared to iron nitride coatings obtained by conventional techniques. Without being bound by theory, it is believed that the low crystallinity of the iron nitride inhibits the expansion of grain boundaries, thereby reducing the number of reaction sites where oxidation processes can occur.

[0032] According to different embodiments, the electrochemical nitridation process includes a galvanostatic step or a potentiostatic step or both.

[0033] Galvanostatic Step

[0034] According to a first embodiment, the electrochemical nitridation process comprises at least one galvanostatic step.

[0035] During the galvanostatic step, an anodic current representative of a predetermined reference current density is applied between the substrate (acting as an anode) and a counter electrode (acting as a cathode) until a predetermined threshold electrical voltage is reached, resulting in an iron nitride coating having a predetermined thickness. The anodic current applied in the galvanostatic step can have a constant or pulsed tendency. If the anodic current has a pulsed tendency, the average value of the anodic current is representative of the predetermined reference current density.

[0036] In other words, during the galvanostatic step, an anodic current is applied having a current density that corresponds to a constant or average constant value until a predetermined threshold electrical voltage and thus a desired substrate surface coating thickness is reached. In other words, during the galvanostatic step, an anodic current is applied that is constant or has an average constant value (average constant value).

[0037] The predetermined reference current density is such as to ensure at least partial decomposition of the nitrogen cations and / or anions of the ionic liquid to produce nitrogen species available for forming a nitride coating on a substrate.

[0038] Preferably, the predetermined reference current density is at least 0.025 mA / cm 2 , at least 0.050mA / cm 2 , at least 0.075mA / cm 2 , at least 0.1mA / cm 2 , at least 0.2 mA / cm 2 , at least 0.3 mA / cm 2 , or at least 0.4 mA / cm 2 , or at least 0.5 mA / cm 2 , or at least 0.6 mA / cm 2 , or at least 0.7mA / cm 2 , or at least 0.8mA / cm 2 , or at least 0.9mA / cm 2 , or at least 1mA / cm 2 It is.

[0039] Preferably, the predetermined reference current density is 50 mA / cm 2 Not to exceed 45mA / cm 2 Not to exceed 40mA / cm 2 Not to exceed 35mA / cm 2 Not to exceed 30mA / cm 2 , or 25mA / cm 2 , or 20mA / cm 2 , or 15mA / cm 2 , or 10mA / cm 2 , or 5mA / cm 2 not exceed.

[0040] Preferably, the predetermined reference current density is 0.5-50 mA / cm 2 , e.g. 0.5-20mA / cm 2 , or 0.5-10mA / cm 2 It has a value between .

[0041] The value of said reference current density varies depending on the electrical resistance of the substrate to be coated, in particular the amount of native oxide present on the substrate surface, which means that the thicker the native oxide layer covering the substrate (up to a thickness of a few nanometers), i.e. the more resistive the substrate, the higher the value of the reference current density.

[0042] During the galvanostatic step, the applied anode current can be either DC or AC.

[0043] Preferably, said predetermined threshold electrical voltage is at least 1V, at least 2V, at least 3V, at least 4V, at least 5V, at least 6V, at least 7V, at least 8V, at least 9V, or at least 10V.

[0044] Preferably, said predetermined threshold electrical voltage is 20V or less, 19V or less, 18V or less, 17V or less, 16V or less, 15V or less, 14V or less, 13V or less, 12V or less, 11V or less.

[0045] Preferably, the predetermined threshold electrical voltage is between 1-20V, more preferably between 1-12V, even more preferably between 1-10V, for example 5V.

[0046] Preferably, when said threshold electrical voltage is reached, an average coating thickness of at least 0.040 μm, preferably at least 0.1 μm, more preferably at least 1 μm, even more preferably 1-5 μm is obtained.

[0047] The higher the threshold voltage, the faster the growth rate of iron nitride on the substrate and the formation of the nitride film. However, the higher the voltage, the more gaseous products are generated from the electrolytic bath. A threshold voltage in the range of 1-20 V was found to provide a good balance between film growth rate and process efficiency.

[0048] The duration of the galvanostatic step depends on the applied anode current and the time it takes to reach a certain threshold voltage as mentioned above.

[0049] As already mentioned, the thickness of the iron nitride film obtained in the galvanostatic step depends on the threshold voltage reached and therefore also on the time required to achieve it, in other words, on the duration of the galvanostatic step.

[0050] Preferably, the galvanostatic step has a duration of at least 5 minutes, at least 10 minutes, at least 15 minutes, at least 20 minutes, or at least 25 minutes.

[0051] Preferably, the galvanostatic step has a duration of not more than 180 minutes, not more than 120 minutes, not more than 60 minutes, not more than 55 minutes, not more than 50 minutes, not more than 45 minutes, not more than 40 minutes, not more than 35 minutes, not more than 30 minutes.

[0052] Preferably, the galvanostatic step has a duration of from 10 minutes to 180 minutes, or from 15 minutes to 60 minutes, for example about 60 minutes.

[0053] Potentiostatic Step

[0054] According to a second embodiment, the electrochemical nitridation process comprises at least one potentiostatic step.

[0055] During the potentiostatic step, a voltage having a value corresponding to a predefined reference current density is applied between the substrate (acting as an anode) and the counter electrode (acting as a cathode) until a threshold anode current representative of the predefined reference current density is reached, resulting in an iron nitride coating having a predefined thickness. The voltage applied during the potentiostatic step can have a constant or pulsatile tendency. If the voltage has a pulsatile tendency, its average value is equal to said predefined reference voltage.

[0056] In other words, during the potentiostatic step, a constant or average constant voltage is applied until the threshold current density and thus the desired thickness of the coating on the substrate surface is obtained. Thus, the voltage applied during the galvanostatic step has a constant or average constant value (constant average value).

[0057] Preferably, said threshold current density corresponds to a current density value below which the efficiency of the electrochemical process decreases, and the thickness of the coating remains substantially constant, taking into account the gradual increase in electricity consumption.

[0058] The predetermined reference voltage is such that it produces an anodic current that at least partially decomposes the nitrogen cations and / or anions of the ionic liquid to produce nitrogen species available for forming a nitride film on a substrate, and the predetermined reference voltage is such that it overcomes the electrical resistance of the nitride film being formed.

[0059] Preferably, the predetermined reference voltage is at least 1V, at least 2V, at least 3V, at least 4V, at least 5V, at least 6V, at least 7V, at least 8V, at least 9V, or at least 10V.

[0060] Preferably, said reference electrical voltage is 50V or less, 45V or less, 40V or less, 35V or less, 30V or less, 25V or less, 20V or less, 15V or less.

[0061] Preferably, said predetermined reference electrical voltage is between 1V and 20V, or between 1V and 10V, for example about 5V.

[0062] The value of said reference electrical voltage varies depending on the electrical resistance of the substrate to be coated, i.e. the more resistive the substrate, the higher the value of the reference electrical voltage will be in order to overcome the electrical resistance of the nitride coating to be formed.

[0063] During said voltage constant step, the applied voltage may be either continuous or alternating.

[0064] Preferably, the predetermined threshold current density is at least 20 μA / cm 2 , or at least 25 μA / cm 2 , or at least 30 μA / cm 2 , or at least 40 μA / cm 2 , or at least 45 μA / cm 2 , or at least 50 μA / cm 2 has a value of

[0065] Preferably, the predetermined threshold current density is 80 μA / cm 2 Not to exceed 75 μA / cm 2 Not exceeding 70μA / cm 2 Not exceeding 65μA / cm 2 Not to exceed 60 μA / cm 2 or 55 μA / cm 2 has a value not exceeding

[0066] Preferably, the predetermined threshold current density is 20-80 μA / cm 2 Between 30 and 70 μA / cm 2 and more preferably between about 50 μA / cm 2 It is.

[0067] Preferably, when said threshold current density is reached, an average coating thickness of at least 0.040 μm, preferably at least 0.1 μm, more preferably at least 1 μm, even more preferably 1-5 μm is obtained.

[0068] The duration of the potentiostatic step depends on the applied electrical voltage and the time required to reach a threshold anode current.

[0069] As already mentioned, the thickness of the iron nitride film obtained during the potentiostatic step depends on the threshold current density and therefore on the time required to achieve it, in other words, on the duration of the potentiostatic step.

[0070] Preferably, the potentiostatic step has a duration of at least 5 minutes, at least 10 minutes, at least 15 minutes, at least 20 minutes, or at least 25 minutes.

[0071] Preferably, the potentiostatic step has a duration of not more than 180 minutes, not more than 120 minutes, not more than 60 minutes, not more than 55 minutes, not more than 50 minutes, not more than 45 minutes, not more than 40 minutes, not more than 35 minutes, not more than 30 minutes.

[0072] Preferably, the potentiostatic step has a duration of 15 to 180 minutes, or 15 to 60 minutes, for example about 60 minutes.

[0073] Galvanostatic and potentiostatic steps

[0074] According to a preferred embodiment, the electrochemical nitridation process comprises, in sequence, a galvanostatic step and a potentiostatic step, where the potentiostatic step follows the galvanostatic step, the two steps being as described above.

[0075] Preferably, the electrical voltage applied during the potentiostatic step corresponds to a threshold electrical voltage reached in the galvanostatic step.

[0076] It has been found experimentally that obtaining a coating having a desired average thickness, preferably 0.040-5 μm, more preferably 0.5-5 μm, and even more preferably 1-5 μm, maximizes the efficiency of the electrolytic nitriding process.

[0077] Without being bound by theory, during the galvanostatic step, nitrogen species resulting from the decomposition of nitrogen cations and / or anions in the ionic liquid diffuse in a controlled manner to the substrate and form iron nitrides. With the formation of said nitrides, the electrical resistance of the substrate increases. During potentiostatic, the electrical voltage is kept equal on average to a predefined value to ensure efficient diffusion of the nitrogen species into the substrate.

[0078] Preferably, the electrochemical process comprising a galvanostatic step followed by a potentiostatic step has a duration of 5 to 180 minutes, more preferably 10 to 120 minutes, even more preferably 30 to 60 minutes, for example about 60 minutes, the duration depending on the temperature used for the electrochemical process.

[0079] The following description relates to an embodiment in which the electrochemical nitridation process consists of only one galvanostatic step, an embodiment in which the electrochemical nitridation process consists of only one potentiostatic step, and an embodiment in which the electrochemical process includes one galvanostatic step and one potentiostatic step in sequence.

[0080] As mentioned above, the galvanostatic and / or potentiostatic steps are performed with steady or pulsed trends of the anode current and electrical voltage, respectively.

[0081] In a preferred embodiment, the galvanostatic and / or potentiostatic steps are performed with pulse trends of the anode current and electric voltage, respectively. According to this embodiment, preferably the amplitude of the current and / or voltage pulses is at least ±10% relative to the average value. A smaller amplitude would result in a pulse trend having an effect substantially equivalent to a stationary trend. Preferably, each current and / or voltage pulse has a duration of at least 100 ms (milliseconds).

[0082] Advantageously, by carrying out a galvanostatic and / or potentiostatic step with a pulse tendency, it is possible to limit the formation of by-products at the surface of the substrate, reduce the generation of gaseous by-products and improve the local mixing of the ionic liquid, all of which contribute to increasing the efficiency of the process.

[0083] According to a preferred embodiment, the electrochemical nitridation process according to step b) of the method of the present invention is carried out under an inert atmosphere, for example under a nitrogen flow, which can increase the efficiency of the nitridation process and reduce the occurrence of the side reaction of iron oxide (e.g. rust) formation before nitridation.

[0084] According to a preferred embodiment, during said step a), the substrate is immersed in the electrolytic bath by a support structure (so-called rack) which has the function of keeping it immersed in the electrolytic bath and at the same time applies the current and / or voltage supplied by a current and voltage generator, respectively. In other words, said support structure advantageously ensures the electrical contact between the voltage / current generator and the substrate to be nitrided. Furthermore, said support structure advantageously makes it possible to measure the voltage and current flow applied to the substrate. Preferably, said support structure is made of metal, for example titanium or stainless steel.

[0085] According to an embodiment of the present invention, the non-aqueous electrolyte bath consists of said ionic liquid. According to this embodiment, no solvent is added to the electrolyte bath, the ionic liquid acting as both reagent and solvent ("pure reaction").

[0086] According to an alternative embodiment, the non-aqueous electrolytic bath consists of said ionic liquid and a polar solvent, preferably an aprotic polar solvent. Preferably, said polar solvent is selected from the group consisting of acetonitrile, dimethylformamide, dimethylsulfoxide, and acetone. The addition of a polar solvent to the electrolytic bath can adjust the viscosity of the ionic liquid, increase the conductivity of the ionic liquid, and increase the Coulombic efficiency of the electrochemical process. This allows thicker nitride films to be obtained.

[0087] Desirably, during the electrochemical process the electrolyte bath is forcefully mixed, for example by a mixer and / or stirrer, to improve the efficiency of the process.

[0088] As already mentioned, the ionic liquid consists of nitrogen cations and / or nitrogen anions.

[0089] Preferably, the nitrogen cation is selected from the group consisting of pyrrolidinium, imidazolium, morpholinium, piperidinium and ammonium cations. Preferably, the nitrogen cation is functionalized with an alkyl group, preferably selected from methyl, ethyl, propyl and butyl.

[0090] Preferably, the nitrogen anion is selected from the group consisting of dicyanamide, tricyanomethane, bis(trifluoromethylsulfonyl)imide, bis(fluorosulfonyl)imide, phosphate, hexafluorophosphate, and nitrate anions.

[0091] According to a preferred embodiment, the ionic liquid is composed of nitrogen cations and nitrogen anions.Preferably, the ionic liquid is selected from the group consisting of 1-propyl-1-methylpyrrolidinium dicyanamide, 1-ethyl-1-methylpyrrolidinium dicyanamide, 1-propyl-1-methylimidazolium dicyanamide, 1-ethyl-1-methylimidazolium dicyanamide, 1-ethyl-3-methylmorpholinium dicyanamide, tributylmethyl tributylmethylammonium bis(trifluoromethylsulfonyl)imide, butyltrimethylammonium bis(trifluoromethylsulfonyl)imide, choline bis(trifluoromethylsulfonyl)imide, 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide, 1-ethyl-3-methylimidazolium The compound is selected from the group consisting of bis(trifluoromethylsulfonyl)imide; 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide; 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide; 1-methyl-1-propylpiperidinium bis(trifluoromethylsulfonyl)imide; 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide; and 1-ethyl-3-methylimidazolium nitrate.

[0092] According to another embodiment, the ionic liquid is composed of nitrogen cations or nitrogen anions.Preferably, the ionic liquid is selected from the group consisting of tributylmethylphosphonium bis(trifluoromethylsulfonyl)imide, diethylmethylsulfonium bis(trifluoromethylsulfonyl)imide, 1-methyl-1-propylpiperidinium tetrafluoroborate.

[0093] As already mentioned above, a substrate of iron or an iron alloy, for example cast iron or steel, is immersed as an electrode in an ionic liquid in the presence of a counter electrode, the substrate acting as the anode and the counter electrode acting as the cathode.

[0094] Preferably, the counter electrode is made of a body of graphite, stainless steel, titanium or aluminium. Advantageously, said materials have high electrical conductivity and at the same time do not degrade in the ionic liquid. More preferably, the counter electrode is graphite.

[0095] According to different embodiments, said counter electrode consists of a body immersed in the electrolytic bath or of the same container of the electrolytic bath in which the substrate is immersed.

[0096] According to a preferred embodiment, the method of the invention comprises a step of pre-treating the substrate, said pre-treatment step being carried out upstream of step a) of immersing the substrate in the electrolytic bath.

[0097] Preferably, said pretreatment step comprises a "degreasing" step, during which traces of oils, greases and / or lubricating oil coolant fluids are removed from the surface of the substrate. Preferably, during said "degreasing" step, the substrate is immersed in a polar solvent for a predefined time, preferably at least 30 seconds, optionally with the application of ultrasound. The polar solvent is for example water, ethanol or a mixture thereof, optionally with the water containing a surfactant. The substrate is then washed with water and air dried.

[0098] Preferably, said pretreatment step includes a "surface treatment" step to remove traces of oxides from the substrate surface. Preferably, during said "surface treatment" step, the substrate is immersed in an acidic solution for a predetermined time, preferably at least 1 minute, and then washed with water. The acidic solution is, for example, a 5% wt aqueous solution of hydrochloric acid.

[0099] Preferably, the pretreatment step includes both the "degreasing" step and the "surface treatment" step.

[0100] The substrate, which following said pretreatment step is subjected to steps a) and b) of immersion in an electrolytic bath and electrochemical nitriding, respectively, advantageously exhibits an electrically conductive surface. Said pretreatment step is particularly preferred. In fact, if the surface of the substrate is contaminated with processing residues (e.g. lubricant and coolant liquids), islands of poor electrical conductivity may form on which the nitride coating grows inhomogeneously.

[0101] According to a preferred embodiment, the method of the invention comprises a step of post-treating the substrate, which is carried out downstream of step b), i.e. downstream of the electrochemical nitridation process.

[0102] Preferably, the post-treatment step includes a "rinsing" step to remove any ionic liquid residues from the nitrided surface of the substrate. Preferably, during the "rinsing" step, the coated substrate is immersed in a polar solvent for a predetermined time, preferably at least 30 seconds. The polar solvent is, for example, water, ethanol, or a mixture thereof. The substrate can then be washed with distilled water. Preferably, the substrate is then air-dried.

[0103] Preferably, the post-treatment step further comprises a subsequent "heat treatment" step to improve the corrosion resistance of the substrate. During the "heat treatment" step, the substrate is heated to a temperature of, for example, between 50-150°C, such as 50-200°C, for a predetermined time, preferably at least 30 seconds.

[0104] As already mentioned above, an object of the present invention is also an article consisting of a substrate of iron or an iron alloy (for example cast iron or steel) and an iron nitride surface coating on said substrate, said surface coating being obtained by the method described above.

[0105] Preferably, the surface coating is iron nitride Fe 2-3 It contains a mixture of N and iron nitride Fe3N.

[0106] The articles of the present invention are not limited to any particular shape or form, as the methods of the present invention are capable of processing any complex shape that is capable of being immersed in an electrolytic bath.

[0107] Preferably, said article is a part of an automobile or motorcycle, or a part of said part.

[0108] In a particular embodiment, the article is a brake caliper of a braking system of an automobile or motorcycle, or a part of said brake caliper.

[0109] In another embodiment, the item is a brake disc of a braking system of an automobile or motorcycle, or a part of such a brake disc, such as a brake band.

[0110] Experimental Section

[0111] First, the cast iron samples were immersed in an ultrasonic ethanol bath for more than 30 s, then immersed in an acidic solution for more than 1 min, and then washed with water.

[0112] The samples thus treated were mounted on a support structure (also called a rack, i.e. a metal bar to hang the samples on) and immersed in an electrolytic bath consisting of tributylmethylammonium bis(trifluoromethylsulfonyl)imide, with a graphite counter electrode also immersed in the bath.

[0113] The samples were then subjected to an electrochemical treatment at room temperature, which included a first galvanostatic step and a second potentiostatic step.

[0114] Specifically, the average current density is 10 mA / cm 2 An anodic current of 0.01 μA / cm was applied between the sample and the counter electrode until a voltage of 5 V was reached. The first galvanostatic step lasted for about 10 min. Then, an average voltage of 5 V was applied between the sample and the counter electrode, resulting in a current density of 50 μA / cm. 2 The second galvanostatic step lasted for approximately 50 min.

[0115] After the electrochemical nitridation process was completed, the samples were immersed in an ethanol bath for more than 30 seconds, after which they were washed with distilled water and air-dried.

[0116] The resulting samples were coated with a homogeneous iron nitride layer approximately 0.5-1.0 μm thick, which had low crystallinity and a corrosion potential 78 mV higher than that of the uncoated sample, indicating that the samples coated in this way were more corrosion resistant than the uncoated samples.

[0117] Figure 1 compares the Raman profile of a coating obtained by the method described above (profile A) with that of a coating obtained by conventional ferritic nitriding (FNC) (profile B). Comparing the two profiles, it can be seen that the coating obtained by the method of the present invention has a lower degree of crystallinity and a different concentration of oxidation by-products (e.g. Fe2O3, FeOOH). In a rough estimation, the degree of crystallinity can be evaluated by observing the width at mid-height of the peaks; that is, the peaks of the coating obtained by the method according to the present invention have a larger width at mid-height compared to the coating obtained by the FNC technique.

[0118] Figure 2 shows the Glow Discharge Optical Emission Spectroscopy (GD-OES) profiles obtained for a cast iron sample coated with the method according to the invention and the same cast iron sample without a coating. In comparison to the uncoated sample, the nitrided sample has 1) a nitrogen concentration peak just below the sample surface and 2) a non-negligible nitrogen concentration away from the sample surface. This confirms that the coating obtained by the method of the invention is integrated into the sample matrix.

[0119] It is clear that only one particular embodiment of the invention has been described herein, and a person skilled in the art can make all the necessary modifications to adapt the method for producing an iron nitride coating on the surface of an iron or iron alloy substrate to his particular conditions, without, however, departing from the scope of protection defined in the appended claims.

Claims

1. A method for forming an iron nitride coating on the surface of an iron or iron alloy substrate, (a) a process of immersing the iron or iron alloy substrate acting as the anode in an electrolytic bath containing an ionic liquid containing nitrogen cations and / or nitrogen anions in the presence of a counter electrode acting as the cathode; A galvanostatic step in which an iron nitride film having a predetermined thickness is formed on the substrate, comprising: a step of applying an anode current representing a predetermined reference current density between the substrate and the counter electrode until a predetermined threshold voltage is reached; and A method comprising an electrochemical nitriding process (b) of a substrate, comprising a potentiostatic step of applying a voltage having a value equal to a predetermined reference voltage between the substrate and the counter electrode until a threshold anode current representing a predetermined threshold current density is reached, thereby generating an ionized iron nitride film having a predetermined thickness on the substrate.

2. The method according to claim 1, wherein the ionic liquid is at 250°C or lower, preferably 200°C or lower, and more preferably at room temperature.

3. The method according to any one of claims 1 or 2, wherein the electrochemical nitriding process (b) is carried out at a temperature of 250°C or lower, preferably 200°C or lower, and more preferably at room temperature.

4. The electrochemical nitriding process (b) includes, in order, the galvanostatic step and the potentiostatic step, The method according to claim 1, wherein the potentostatic step follows the galvanostatic step.

5. The method according to claim 1, wherein the anode current tends to remain constant during the galvanostatic step.

6. During the galvanostatic step, the anode current is pulsed. The method according to claim 1, wherein the average value of the anode current represents the predetermined reference current density.

7. The method according to claim 1, wherein the voltage remains constant during the potentostatic step.

8. During the potentostatic step, the voltage is pulsed. The method according to claim 1, wherein the average value of the voltage is equal to the predetermined reference voltage.

9. The predetermined reference current density is 0.5–50 mA / cm². ２ Preferably 0.5–20 mA / cm ２ , or 0.5–10 mA / cm ２ The method according to claim 1, having a value.

10. The method according to claim 1, wherein the predetermined threshold voltage is 1-20V, preferably 1-12V, and more preferably 1-10V.

11. The method according to claim 1, wherein the predetermined reference voltage is 1-50V, preferably 1-20V, and more preferably 1-10V.

12. The predetermined threshold current density is 20-80 μA / cm². ２ Preferably 30-70 μA / cm ２ More preferably, about 50 μA / cm ２ The method according to claim 1.

13. The method according to claim 1, wherein the duration of the electrochemical nitriding treatment by the electrochemical nitriding process (b) is 5 minutes to 180 minutes, preferably 10 minutes to 120 minutes, and more preferably 30 minutes to 60 minutes.

14. The method according to claim 1, which produces an iron nitride coating having an average thickness of 0.040–5 μm, preferably 0.5–5 μm, and more preferably 1–5 μm.

15. The method according to claim 1, wherein the ionic liquid is a room-temperature ionic liquid (RTIL).

16. The aforementioned ionic liquid A nitrogen cation selected from the group consisting of pyrrolidinium, imidazolium, morpholinium, piperidinium, and ammonium cations, preferably wherein the ammonium cation is functionalized with an alkyl group selected from methyl, ethyl, propyl, and butyl. and / or A nitrogen anion selected from the group consisting of dicyanamide, tricyanomethane, bis(trifluoromethylsulfonyl)imide, bis(fluorosulfonyl)imide, phosphate ion, hexafluorophosphate ion, and nitrate ion. The method according to claim 1, including the method described in claim 1.

17. The ionic liquid comprises a nitrogen cation and a nitrogen anion. Preferably, the ionic liquid is 1-propyl-1-methylpyrrolidinium dicyanamide; 1-ethyl-1-methylpyrrolidinium dicyanamide; 1-propyl-1-methylimidazolium dicyanamide; 1-ethyl-1-methylimidazolium dicyanamide; 1-ethyl-3-methylmorpholinium dicyanamide; tributylmethyl tributylmethylammonium bis(trifluoromethylsulfonyl)imide, butyltrimethylammonium bis(trifluoromethylsulfonyl)imide, choline bis(trifluoromethylsulfonyl)imide, 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide, 1-ethyl-3-methylimidazolium The method according to claim 1, selected from the group consisting of trifluoromethylsulfonylimide bis(trifluoromethylsulfonylimide)bis(trifluoromethylsulfonylimide)bis(trifluoromethylsulfonylimide)bis(trifluoromethylsulfonylimide)bis(trifluoromethylsulfonylimide)bis(trifluoromethylsulfonylimide)bis(trifluoromethylsulfonylimide)bis(trifluoromethylsulfonylimide)bis(trifluoromethylsulfonylimide).

18. The method according to claim 1, wherein the ionic liquid is tributylmethylphosphonium bis(trifluoromethylsulfonyl)imide, or diethylmethylsulfonium bis(trifluoromethylsulfonyl)imide, or 1-methyl-1-propylpiperidinium tetrafluoroborate.

19. The method according to claim 1, wherein during process (a), the substrate is immersed in an electrolytic bath by a support structure (so-called rack).

20. The method according to claim 1, wherein the counter electrode is composed of a body made of graphite, stainless steel, titanium, or aluminum.

21. The method according to claim 1, wherein the counter electrode consists of a main body immersed in the electrolytic bath or a container of the electrolytic bath.

22. Upstream of the process (a), a pretreatment step is included for pretreatment of the substrate. The aforementioned preprocessing step is: A degreasing step comprising immersing the substrate in a polar solvent for a predetermined time, preferably at least 30 seconds, while optionally applying ultrasound, and / or The method according to claim 1, comprising a surface treatment step of immersing the substrate in an acid solution for a predetermined time, preferably at least 1 minute, and then washing it with water.

23. Downstream of the electrochemical nitriding process (b), a post-treatment step is included for post-treating the substrate. The aforementioned post-processing step is: The method according to claim 1, comprising the steps of immersing the substrate in a polar solvent for a predetermined time, preferably at least 30 seconds, and optionally heating it to a temperature of 50-200°C for a predetermined time, preferably at least 30 seconds.

24. An article comprising an iron or iron alloy substrate and an iron nitride surface coating on the substrate, An article wherein the iron nitride surface coating is obtained by the method described in claim 1.

25. The iron nitride surface coating is made of iron nitride Fe ２-３ N and iron nitride Fe ３ The article according to claim 24, comprising a mixture with N.

26. The article according to claim 24 or 25, which is a brake caliper or brake disc, or a part of a brake caliper or brake disc, for a brake system of an automobile or motorcycle.

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