Method of producing colored oxide films on valve metal products

Abstract

The essence of the invention is a method for obtaining colored oxide coatings on metal products by anodic oxidation in an aqueous electrolyte solution containing sodium or potassium hydroxide and a sodium or potassium salt of metasilicic or boric or metaphosphoric or orthophosphoric or hypophosphorous di-, tri- or poly-metaphosphoric acid or sodium aluminate or a mixture thereof with a total concentration not exceeding 10 wt% and at a temperature maintained between 5 and 40°C during the process carried out with polarization at direct current in the voltage range between the product and the counter electrode from +150 to +750 V or pulsed in the range from -200 to 0 V and from +200 to +750 V, with a frequency of 25 to 10,000 Hz, a duty factor of 10 to 100%, wherein the current density flowing through the product is limited to a value of 2.5 to 50 A / dm2, and the process duration is between 5 and 120 min, preferably rinsed in deionized water and dried, is characterized in that the product treated in this way is immersed in a dyeing solution at a temperature of 5 to 60°C for a time of 1 to 3600 s using ultrasound at a frequency of 20 to 120 kHz and a power in the range of 5 to 2000 W / dm2, preferably rinsed in deionized water and dried, and the thus colored and dried product is subjected to repeated anodic oxidation under the same conditions for a time of 5 to 1800 s.

Description

[0001] The present invention concerns a method for obtaining colored oxide films on metal products and as a result imparting color and protection against corrosion of metal products containing chemical elements that can be subjected to plasma electrolytic oxidation in alkaline solutions of silicates, phosphates, aluminates, or borates of sodium or potassium. The most prominent metals of technical importance that exhibit such properties are aluminium, magnesium, zinc, and titanium.

[0002] Oxide films belong to a wider family of technical coatings called conversion coatings. These coatings are produced due to the controlled corrosion process on the material's surface. Metal ions released from the metal matrix because of the electrode reaction, upon contact with the bath components, form nonsoluble deposits on the treated surface, which adhere tightly to the substrate. Such films are produced in electroless conditions and are characterized by relatively small thicknesses (up to a dozen or so micrometers) and quite low mechanical strength. Aluminum, magnesium, titanium, and, less frequently, zinc, as well as alloys containing these elements, can be successfully covered with oxide films in the process of anodic oxidation (or anodizing). During anodizing, processed parts are immersed into a suitable electrolyte and are connected to an electrical circuit as a positive electrode (anode), while an inert electrode material (e.g. stainless steel) is connected as a cathode. The flow of current forced by an external power supply drives electrode reactions, which cause thickening of the natural oxide (passive) film on the metal surface. Depending on the composition of the electrolytic bath and the electrical parameters, barrier layers (e.g. for the construction of capacitors) or porous coatings can be obtained. Pores are formed due to the local dissolution of the produced oxide coating, thanks to the appropriate composition of the electrolyte. Under constant voltage conditions, with the progress of the anodizing time, both the thickness of the coating and the depth of the pores increase, yet a thin barrier layer of constant thickness remains in direct contact with the metal.

[0003] Recently, a technique derived from conventional anodizing, called plasma electrolytic oxidation (PEO), also known as micro-arc oxidation (MAO) or micro-plasma oxidation (MPO), has been gaining attention. In contrast to classical anodizing methods, PEO utilizes environmentally friendly electrolytes with low concentrations of dissolved substances (generally less than 10 wt%). Thanks to the properties of the corresponding metal oxides, during anodic polarization it is possible to generate a high electric field, which, when the critical value is exceeded, can lead to dielectric breakdown and plasma generation. The plasma composition simultaneously includes elements originating from the gases generated in the process, the partially melted metal substrate, and the electrolytic bath components. For this reason, the oxide coatings produced by PEO are significantly enriched in elements contained in the bath, and the temporarily occurring high temperature of the treated surface leads to the crystallization of additional phases (e.g. corundum during PEO of aluminum products), unavailable during classical electrochemical treatment. However, the creation of plasma requires the use of a higher voltage than during conventional anodizing (from 250 to even 1000 V compared to 15 to 120 V, respectively).

[0004] Despite the benefits of increased hardness and thickness of PEO coatings compared to conventional anodizing, they often do not offer sufficiently good corrosion resistance due to the presence of micropores in the oxide structure. Only near the metallic substrate, there is a thin (generally between 0.5 and 1.0 µm thick), a continuous protective oxide layer, providing a barrier between the metal and the external environment. In the case of porous films produced in classical anodizing, pores have a columnar structure and a diameter of a few to several dozen nanometers. Such pores can be closed efficiently by hot water sealing. In addition, dyes can be absorbed on the walls of the pores before they are closed which allows a wide range of colors to be obtained on aluminium products.

[0005] Coloring of titanium products can be carried out using conventional anodizing in acidic or alkaline electrolytes, where the final color of the surface after treatment depends on the composition of the bath, the substrate, and the applied forming voltage. The surface color is created by the interference of light on the processed product. This is possible thanks to the optical properties of titanium dioxide in the oxide film, and the very good reproduction of the substrate surface morphology by the electrochemically formed oxide.

[0006] The disadvantage of traditional options for coloring the mentioned materials is that in the case of titanium products, interference color can only be obtained for relatively thin oxide layers (up to several hundred nanometers thick). Material colored in this way, if exposed to abrasive wear, completely loses its decorative value. Despite the greater thickness of oxide coatings (from a few to several dozen micrometers thick) typically produced on magnesium or aluminum products, their hardness and resistance to abrasive wear are not as good as for materials obtained in PEO processing. Although the oxide films produced in the PEO process are highly porous, traditional methods of surface coloring used for conventional anodizing do not provide satisfactory results. The capillary effect that allows for loading the pores with the coloring solution depends on the pore diameter, which is not uniform for layers produced in the PEO process. As a result, the coloring is generally uneven, and the dye is washed out over time. It is therefore necessary to use a method that allows for (i) uniform adsorption of the dye molecules in the entire volume of the oxide layer, (ii) enabling effective color fixation by an appropriate sealing operation.

[0007] There are known methods in technology according to which salts of transition metals, such as cobalt, chromium, copper, iron, manganese, molybdenum, nickel, titanium, vanadium or tungsten, are added to the electrolytic bath. From the scientific publication "Preparation and properties of blue ceramic coating by micro-arc oxidation on aluminum alloy" (C. Chen, Y. Zhang, W. Chen, H. Cheng, L. Wang, Mater. Rev., 31(5) (2017) 121-126) a method of forming blue oxide coatings on a substrate made of aluminum alloy AW5052 is known, where the color is given by Co(II) present in the electrolytic bath. It was noted that the higher the concentration of Co(OH) 2 (in the range of 1 to 3 g / dm 3< ) in the bath, the deeper the obtained surface color, although the corrosion resistance offered by the coating deteriorates with the increase of the concentration of cobalt(II) in the bath. The process is carried out under direct current conditions using the potentiostatic method.

[0008] From the scientific publications "Preparation process of green ceramic coating of magnesium alloy through micro-arc oxidation" (S. Wang, W.W. Ding, Adv. Mater. Res. 842 (2014) 228-232) and "Characterization of self-sealing MAO ceramic coatings with green or black color on an Al alloy" (W. Yang, D. Xu, J. Chen, J. Liu, B. Jiang, RSC Advances, 7(3) (2017) 1597-1605) methods of obtaining films in the PEO process with color from green to black depending on the concentration of potassium dichromate in the electrolytic bath are known. The electrolyte for the process is based on sodium hexametaphosphate (45 g / dm 3< ), sodium metasilicate (5 g / dm 3< ), potassium hydroxide (1,2 g / dm 3< ), potassium fluoride (3 g / dm 3< ) and the aforementioned Cr(VI) salt in the concentration range of up to 12 g / dm 3< . The process is carried out using a bipolar pulse rectifier at a frequency of 400 Hz and a pulse duty factor of 10% to a maximum voltage of 460 V for 15 minutes. Similarly to the case of cobalt(II) salt, an increase in the concentration of dichromate in the coating formation solution results in a deeper color (up to black at the highest concentration). The method is suitable for the anodizing of the magnesium alloy AZ91D and the aluminum alloy AW6061.

[0009] In a similar procedure, it is possible to impart color due to the presence of copper salt in the electrolyte for PEO treatment. The scientific publication "Effects of copper additive on micro-arc oxidation coating of LZ91 magnesium-lithium alloy" (S.-J. Lee, L.H.T. Do, Surf. Coat. Technol. 307 (2016) 781-789) provides a method by which brown oxide layers can be obtained by conducting the plasma electrolytic oxidation process in an electrolytic bath containing Na 3 PO 4 ·12H 2 O (5 g / dm 3< ), Na 2 SiO 3 (15 g / dm 3< ), NaOH (4 g / dm 3< ) with the addition of 3 to 5 g / dm 3< Cu 3 PO 4 . A copper(I) salt is introduced into the coating structure on a magnesium-lithium-zinc alloy LZ91 under bipolar pulse current conditions at 2 kHz, 30% duty cycle, and current density of 4 A / dm 2< for 5 min. The use of a higher concentration of copper results in a darker color of the film. From the scientific publication "Effect of copper additive on microstructure and anti-corrosion performance of black MAO films grown on AZ91 alloy and coloration mechanism" (R.Y. He, B.Y. Wang, J.H. Xiang, T.J. Pan, J. Alloy. Compd. 889 (2021) 161501) a method is known, where Cu 2 P 2 O 7 (0,5 to 3 g / dm 3< ) is used as an additive to the base bath for the PEO process with the composition of Na 2 SiO 3 (10 g / dm 3< ), KF (4 g / dm 3< ) and NaOH (4 g / dm 3< ). The coating formed on an AZ91 magnesium alloy substrate under unipolar pulse current conditions at a frequency of 700 Hz, a duty ratio of 20%, and a current density of 2 A / dm 2< for 2 minutes acquires a black color due to the presence of copper(II) oxide when there is at least 1.5 g / dm 3< of Cu 2 P 2 O 7 in the bath. From the scientific publication "New findings on properties of plasma electrolytic oxidation coatings from study of an Al-Cu-Li alloy" (Y.-L. Cheng, Z.-G. Xue, Q. Wang, X.-Q. Wu, E. Matykina, P. Skeldon, G.E. Thompson, Electrochim. Acta 107 (2013) 358-378) a method is also known in which brown oxide coatings are obtained on an aluminum-based alloy, where the color is determined by the presence of copper in the substrate alloy. The process is carried out under conditions of bipolar pulse current at 1 kHz, 20% duty cycle, and an average current density of 25 A / dm 2< for 10 min. In the case when the substrate material is an aluminum alloy with copper (3 wt%), the coating color is much lighter than in the case of the commercial alloy 2A97 with the chemical composition (w.t %): 3.8% Cu, 1.5% Li, 0.3% Mn, 0.4% Mg, 0.5% Zn, 0.12% Zr, and 0.07% Ti, Al - the rest. In this process, an electrolyte based on 8 g / dm 3< Na 2 SiO 3 ·9H 2 O and 1 g / dm 3< KOH is used.

[0010] From the scientific article "Thin films produced on 5052 aluminum alloy by plasma electrolytic oxide with red mud-containing electrolytes" (L. Sottovia, M.L.P. Antunes, C.A. Antonio, E.C. Rangel, N.C. Da Cruz, Mater. Res. 17(6) (2014) 1404-1409) a method is known for coloring of coatings in the PEO process on the substrate of aluminum alloy AW5052, where the electrolyte used in the treatment is an aqueous solution of KOH with a pH of 11, to which 5 g / dm 3< of the so-called "red mud" is added. The term "red mud" refers to waste from Bayer's technology for obtaining refined alumina from natural bauxite deposits. This waste is rich in ferric oxide, which is also the source of its color. The AW5052 alloy can be colored red in the PEO process in the aforementioned electrolytic bath under bipolar pulse conditions operated at a voltage of 600 V, a frequency of 200 Hz, and a duty cycle of 60% for 5 min. From the scientific publication "Application of thermally activated red mud in surface treatment of 5005 aluminum alloy" (S. Liu, J. Zeng, Prog. Org. Coat. 133 (2019) 276-288) there is an additional variant of the abovementioned process, where the "red mud" is previously subjected to heat treatment before being introduced into the base electrolyte (3 g / dm 3< KOH). The method is suitable for forming oxide coatings on the AW5005 aluminum alloy under bipolar pulse conditions, where a voltage of 480 V and a duty ratio of 40% are used during the anodic cycle, and in the cathodic cycle the parameters are the following: -200 V, duty ratio of 40%. The treatment is carried out at a frequency of 400 Hz for 60 min. In this way, depending on the heat treatment temperature, which is carried out for 4 hours, the final color of the coating can be obtained from dark brown (100°C), through orange (600°C) to yellow (1100°C).

[0011] Patent No. CN102943297A describes a method for finishing the surface of titanium alloys by the PEO process in a bath containing 8-15 g / dm 3< of a phosphate salt, 0.3-1.6 g / dm 3< of an iron salt (not chloride or fluoride; preferably sulfate), 0.6-1.5 g / dm 3< of a manganese salt (not chloride or fluoride; preferably sulfate), 5-15 10 cm 3< / dm 3< of trolamine and 20-35 g / dm 3< of trisodium citrate. The PEO process leads to the formation of titanium oxide on the surface of the substrate, which visually resembles the blackened surface of a firearm produced on steel by the oxidation method.

[0012] From the scientific publication "Effects of KMnO4-added electrolytes on alumina coatings prepared by plasma electrolytic oxidation" (C.Y. Hong, H.P. Teng, F.H. Lu, J. Chinese Corros. Eng., 26(3) (2012) 141-146) a method for creating yellow oxide coatings on the surface of the AW6061-T6 aluminum alloy is known. The method consists of carrying out the PEO process under direct current conditions for 60 min, using a current density of 10 A / dm 2< . The process bath is an aqueous solution containing 0.1 M NaAlO 2 and 0.8-3.0 mM KMnO 4 . Whereas from the scientific article "Coloring and corrosion resistance of pure Mg modified by micro-arc oxidation method" (L.-H. Li, T.S.N.S. Narayanan, Y.K. Kim, Y.-M. Kong, G.-S. Shin, S.-K. Lyu, I.S. Park, M.H. Lee, Int. J. Prec. Eng. Manuf. 15(8) (2014) 1625-1630) a method for creating brown-red oxide layers on technically pure magnesium is known. For this purpose, an electrolyte based on 0.05 M Na 3 PO 4 , 0.2 M Na 2 SiO 3 , 1 M KOH and 15 mM KMnO 4 is used, while the oxidation process is carried out under direct current conditions for 3 min at a current density of 20-40 A / dm 2< .

[0013] From the scientific publication "Comparison of plasma electrolytic oxidation coatings on Mg-Li alloy formed in molybdate / silicate and aluminate / silicate composite electrolytes" (Z.J. Li, Y. Yuan, X.Y. Jing, Mater. Corros. 65(5) (2014) 493-501) a method for forming brown oxide coatings on Mg-Li alloy (5.6 wt% Li, 3.37 wt% Al, 1.68 wt% Zn, 1.14 wt% Ce, Mg - the rest) in the PEO process in an electrolytic bath with 15 g / dm 3< Na 2 SiO 3 , 0.6 g / dm 3< Na 2 MoO 4 , 3 g / dm 3< NaOH and 10 cm 3< / dm 3< triethanolamine is known. The process is carried out under unipolar pulsed conditions at a frequency of 2 kHz, a duty factor of 15% and a current density of 5 A / dm 2< for 10 min. Whereas, patent No. CN105463551A describes a method for processing aluminum alloys involving the PEO process in an electrolyte consisting of 0.5 M NaH 2 PO 4 , 0.5 M NaF, 0.1 M NH 4 F and 0.1 M Na 2 B 4 O 7 enriched with a manganese compound (in the form of MnO 4 -< ion with Mn concentration of not less than 0.5 g / dm 3< ) or molybdenum (in the form of MoO 4 2-< ion with Mo concentration of not less than 8 g / dm 3< ) with the possibility of using an additional source of cobalt (in the form of Co(OH) 2 dissolved in ammonia water, with Co concentration of not less than 0.1 g / dm 3< ). Oxidation is carried out at a voltage of 125-155 V and a current below 0.5 A. By selecting the appropriate concentration of bath components, different shades of the coating can be obtained, including blue, dark grey or orange.

[0014] From the scientific publication "Effects of nickel additive on micro-arc oxidation coating of AZ63B magnesium alloy" (W. Zhan, F. Tian, G. Ou-Yang, B.-Y. Gui, Int. J. Prec. Eng. Manuf. 19(7) (2018) 1081-1087) a method for creating brown coatings on the surface of AZ63B magnesium alloy from electrolytic baths based on 15 g / dm 3< Na 2 SiO 3 , 30 g / dm 3< anhydrous borax, 0.5 g / dm 3< nickel acetate, 30 g / dm 3< NaOH and 15 cm 3< / dm 3< triethanolamine is known. The PEO process is carried out under bipolar pulse conditions at a frequency of 400 Hz, a duty factor of 50%, and a current density of 3 A / dm 2< for 570 s. However, coatings obtained in this way are not uniformly colored and numerous white dots are visible on them.

[0015] From the scientific publication "Stable preparation and characterization of yellow micro-arc oxidation coating on magnesium alloy" (W. Yang, D. Xu, X. Yao, J. Wang, J. Chen, J. Alloys Compd. 745 (2018) 609-616) a method for creating colored oxide films as a result of PEO treatment on AZ80 magnesium alloy is known. The color of the coatings, from yellow to brown, can be obtained depending on the concentration of Na 2 SnO 3 (from 2 to 15 g / dm 3< ) in the base bath of 10 g / dm 3< KOH, 8 g / dm 3< KF, 5 g / dm 3< Na 2 SiO 3 , the pH of which is from 13.6 to 13.9. The PEO process is carried out in pulses at a voltage of 350 V, a frequency of 400 Hz, and a duty factor of 10% for 5 min. The corrosion resistance of the substrate is worse than that of the coating formed without the addition of a tin compound for Na 2 SnO 3 concentrations below 15 g / dm 3< .

[0016] From the scientific publication "Development of decorative and corrosion resistant plasma electrolytic oxidation coatings on AM50 magnesium alloy" (P.B. Srinivasan, J. Liang, C.Blawert, M. Störmer, W. Dietzel, Surf. Eng. 26(5) (2010) 367-370) a method for PEO treatment of AM50 magnesium alloy is known, where the resulting coating takes on a blue color. The method relies on conducting the process unipolarly with a pulse frequency of 45 Hz, a duty factor of 9%, and a current density of 3 A / dm 2< for 10 min in a water-based electrolytic bath consisting of Na 3 PO 4 , NH 4 OH, and a suspension (sol) of TiO 2 obtained from titanium alkoxide, ethanol, and triethanolamine. The intensity of the blue color of the coating increased with the increase of the TiO 2 sol content from 2 to 6%.

[0017] From the scientific publication "Effect of ammonium metavanadate on surface characteristics of oxide layer formed on Mg alloy via plasma electrolytic oxidation" (Y.G. Ko, K.M. Lee, D.H. Shin, Surf. Coat. Technol. 236 (2013) 70-74) a method is known of producing black oxide coatings on magnesium alloy Mg-8.29Al-0.83Zn-0.31Mn by PEO process in a bath containing 0.5 M KOH, 0.15 M K 4 P 2 O 7 and 0.08 M NH 4 VO 3 . pH and conductivity of the bath are 10.1 and 61.8 mS / cm, respectively. The process is carried out under direct current conditions at 10 A / dm 2< for 200 s. The scientific article "Comparison of effects of V2O5 and NaVOs on corrosion resistance of micro-arc oxidation coatings on magnesium alloys" (L.-Y. An, Y. Ma, Y.-P. Liu, Y.-S. Wang, S. Wang, Z.-Y. Wang, Chinese J. Nonferr. Met. 28 (8) (2018) 1542-1550) describes a method for obtaining brown-colored coatings from silicate baths enriched with the addition of V 2 O 5 solid particles and vanadium salt - sodium metavanadate (preferably at a concentration of 0.35 g / dm 3< NaVO 3 ). The layer owes its color to the formation of a spinel with the MgV 2 O 4 structure, regardless of which vanadium source is used. From the scientific publication "Development of vanadium impregnated flat absorber composite PEO coating on AA6061 alloy" (A.M. Pillai, A. Rajendra, A.K. Sharma, P. Bera, S. Poornima, S. Sampath, Surf. Coat. Technol. 410 (2021) 126891) a method for producing black coatings on the surface of the AW6061 alloy by the PEO process from a water-based bath containing 60 g / dm 3< Na 2 SiO 3 ·9H 2 O, 5 g / dm 3< NaOH and 10 g / dm 3< VOSO 4 ·xH 2 O is known. The PEO treatment is carried out unipolarly pulsed at a frequency of 50 Hz, a current density of 10A / dm 2< , a duty factor of 3.5%, and a time of up to 10 min. As the process time progresses in the vanadyl sulfate-based bath, the color of the layer becomes increasingly darker, with a uniform black color being obtained after at least 7 minutes.

[0018] The scientific publication "Preliminary study on preparation of black ceramic coating formed on magnesium alloy by micro-arc oxidation in carbon black pigment-contained electrolyte" (X. Liua, G. Liu, J. Xie, Procedia Engineer. 36 (2012) 261-269) and patent CN102367584B (the same authors) describe a method for creating black oxide coatings on AZ91D magnesium alloy and substrates made of other metals in PEO surface treatment. The process is carried out in an aqueous solution containing Na 2 SiO 3 (6-12 g / dm 3< ; preferably 6 g / dm 3< ), Na 2 WO 4 (0.5-2.0 g / dm 3< ; preferably 1.5 g / dm 3< ), KOH (2 g / dm 3< ), NaF (2 g / dm 3< ), sodium citrate (0.3-0.9 g / dm 3< ; preferably 0.3 g / dm 3< ) and a commercial carbon ink of the Chinese brand "Ostrich" (10-25 cm 3< / dm 3< ; preferably 25 cm 3< / dm 3< ) under bipolar pulse conditions at a frequency of 600 Hz, a duty factor of 20% and a current density of 1-4 A / dm 2< (preferably 3 A / dm 2< ) for 20 min.

[0019] Patent No. CN106702464A describes a method for producing black coatings on magnesium alloys, where the workpiece is subjected to the PEO process in a electrolytic bath with the chemical composition: (NaPO 3 ) 6 5-20 g / dm 3< (preferably 10 g / dm 3< ), KF 1-8 g / dm 3< (preferably 2 g / dm 3< ), KOH 1-8 g / dm 3< (preferably 3 g / dm 3< ), NH 4 VO 3 1-8 g / dm 3< (preferably 2 g / dm 3< ), Na 2 WO 4 2-10 g / dm 3< (preferably 4 g / dm 3< ). The treatment is carried out in bipolar pulse conditions using a current density in the range of 5-15 A / dm 2< , and the treatment time is between 8 and 15 minutes.

[0020] From the scientific publication "FexCo1-xWO4 films on titanium: plasma electrolytic synthesis, optical, electrochemical and photocatalytic properties" (Y.B. Budnikova, M.S. Vasilyeva, I.V. Lukiyanchuk, V.S. Egorkin, A.Y. Ustinov, V.G. Kuryavyi, D.H. Shlyk, J. Mater. Sci.-Mater. El. 34(28) (2023) 1973) there is a method known for producing colored coatings on the VT1-0 titanium alloy with the chemical composition (wt. %): 0.7 Al, 0.25 Fe, 0.10 Si, 0.07 C, 0.04 N, 0.2 O, 0.01 N; other impurities - 0.30; Ti - the rest. The coatings are produced in the PEO process using a electrolyte containing 0.0125-0.05 M FeC 2 O 4 , 0.0125-0.05 M Co(CH 3 COO) 2 , 0.15 M Na 2 WO 4 , and 0.05 M EDTA. The treatment is carried out under direct current conditions for 10 min at a current density of 10 A / dm 2< . Depending on the proportion of iron salt to cobalt salt, it was possible to produce coatings of various shades of gray and brown.

[0021] It should be noted that the addition of the above-mentioned metal salts (especially chromium or vanadium) to electrolytic baths makes the PEO technology less environmentally friendly, and the wastewater from the process should be treated as hazardous. In addition, the presence of additional components in the baths has an impact on other properties of the coatings, such as corrosion protection or resistance to abrasive wear. Furthermore, the range of colors available because of the addition of inorganic substances to the PEO process is limited to the colors of the appropriate oxides and metal salts that are components of the bath. The solution to the above problems may be the use of dyeing with organic substances.

[0022] Patent No. TW202227673A describes a method for producing colored oxide coatings on the surface of magnesium products using dyes. In the first step, an oxide coating is produced in the PEO process in an electrolyte solution consisting of alkali metal silicates, fluorides, hydroxides, and pore regulators (triethanolamine or diethanolamine at a concentration of 0.1-5 g / dm 3< ) at a voltage of 450-550 V for no less than 10 minutes. The coating produced in this way should be no less than 10 micrometers thick. Then the produced film is rinsed to remove the remaining solution from the pores. The coating is then colored in a dye solution, without disclosing what the dye is dissolved in or what its chemical structure or name is. Finally, the layer is sealed in a nickel acetate solution at a temperature of 25 to 35°C.

[0023] Patent No. CN101831683A describes a method for obtaining colored ceramic oxide coatings on the surface of magnesium alloys. The magnesium product is previously subjected to the PEO process in a phosphate bath (40-70 g / dm 3< (NaPO 3 ) 6 , 10-30 g / dm 3< NaOH, 5-20 g / dm 3< NH 4 F, 5-25 g / dm 3< hexamethylenetetramine, 5-15 g / dm 3< NH 4 H 2 PO 4 , 5-20 cm 3< / dm 3< glycerin, 60-100 cm 3< / dm 3< trolamine) at a current density of 1-5 A / dm 2< , frequency of 50-172.45 Hz for 1-15 min, which results in the formation of a coating with a thickness of 1-20 µm. The product is then transferred to a container with an aqueous solution of an organic dye (0.5-10 g / dm 3< ), of which the exact chemical composition has not been disclosed, with a pH between 4 and 6. The product is kept in the solution for 20 to 40 hours with mixing and then rinsed and dried.

[0024] Patent No. CN107217290A describes a method for dyeing oxide coatings produced in the PEO process on the surface of magnesium-based products. The detail previously subjected to the PEO process is immersed in an aqueous solution of a cationic dye (used in the textile industry) at a concentration of 1-10 g / dm 3< with the addition of a release agent (urea) at a concentration of 1-5 g / dm 3< and sodium sulfate at a concentration of 1-5 g / dm 3< . During dyeing, the solution should have a temperature of 20°C and a pH between 4,5 and 5. Dyeing is carried out at a temperature of 60-90°C, with stirring at a speed of 500-1000 rpm, and with the participation of ultrasound at a power of 100-200 W for 0.5 to 24 h. The longer the immersion time, the more intense the color of the product. In addition, another variant of the method for dyeing oxide coatings created in the PEO process on a magnesium substrate was identified from patent CN107227480A (by the same authors). What distinguishes the presented patent from the two previous ones is the type of dyeing substance, which in this case is a weakly acid dye, belonging to the class of monoazo dyes, used in a concentration of 1-10 g / dm 3< . Together with the dye, 1-5 g / dm 3< Na 2 SO 4 and 1-5 g / dm 3< of emulsifier are added to the dyeing bath based on deionized water. At a temperature of 20°C, the pH of the solution is between 4.5 and 5. Dyeing is carried out in a manner analogous to the description in the two patents mentioned above.

[0025] Patent No. US20230189465A1 describes a housing structure for electronic devices made of a metal substrate, which is first covered with an oxide coating characterized by a thickness of 2 to 15 µm in the PEO process. Then, a coloring coating consisting of water-based dyes in the amount of 3 to 10 wt%, surface-active substances in the amount of 0.3 to 2 wt% with a thickness of 1 to 5 µm is applied to the surface of the produced oxide. Finally, the surface is covered with a paint coating consisting of a primer coating, a base coat coating, and a topcoat coating. The patent does not disclose a method of coloring the oxide coating produced in the PEO process and only claims several substances from the group of water-based dyes (e.g. methylene blue, fuchsine or xanthene dyes).

[0026] From the scientific publication "Coloration of the aluminum alloy surface with dye emulsions while growing a plasma electrolytic oxide coating" (S.-C. Yeh, D.-S. Tsai, J.-M. Wang, C.-C. Chou, Surf. Coat. Technol. 287 (2016) 61-66) and patent no. TWI571536B a method for forming oxide coatings on the surface of products based on aluminum, magnesium or titanium in the PEO process with the participation of solvent dyes in the form of emulsion is known. At first, the coating is formed in an electrolyte with a pH in the range of 9 to 11 and conductivity of 8 to 10 mS / cm, which contains aluminate, citrate, fluorozirconate or hexametaphosphate ions, and sodium or potassium ions. Then, an aqueous emulsion is prepared in which the dispersed phase is a dye (with anthraquinone, methine, phthalocyanine, azo, or triarylmethane chromophore groups) dissolved in an organic solvent (e.g. toluene) or a colloid, where an organic or inorganic pigment is used (e.g. iron oxide red, cobalt blue or chrome green). The particle size in the dyeing medium is from 50 to 500 nm. The oxide is initially formed in an electrolyte devoid of dyes, and then, after reaching a sufficiently thick coating, the emulsion or colloid is added to the bath in portions and the forming voltage is increased each time, which results in further growth of oxidation products together with the dye.

[0027] From the scientific publication "Anodic dyeing of micro-arc oxidized aluminum with a cathodic pretreatment" (C.-M. Chen, H.-J. Chu, J.-L. He, Surf. Coat. Technol. 324 (2017) 92-98) and from patents No. CN107338466B and TWI623651B a method for dyeing metal products previously covered with an oxide coating in the PEO process using organic anionic dyes is known. After the PEO process, products are first subjected to cathodic pretreatment, which consists of immersing them in an aqueous solution of NaCl (from 0.01 to 0.1 mol / dm 3< ) and applying a negative voltage of -10 or -30 V between the processed detail and the steel counter electrode for 5-15 s. Then, an anodic dyeing operation is carried out in a solution containing from 0.01 to 0.1 mol / dm 3< ofNaCl and from 3 to 15 g / dm 3< of anionic dye at a positive voltage of 2 to 11 V for 60-300 s.

[0028] The purpose of the invention is to develop a method for producing colored, porous oxide coatings on a metal substrate subjected to the process of plasma electrolytic oxidation in alkaline solutions of sodium or potassium silicates, phosphates, aluminates, or borates. The color of the treated surface is given by introducing organic solvent dyes inside the pores of the oxide coating. At the same time, it is possible to enrich the composition with the presence of corrosion inhibitors, which additionally provide an anti-corrosion effect on the obtained surface.

[0029] The essence of the invention is a method for obtaining colored oxide coatings on metal products by anodic oxidation in an aqueous electrolyte solution containing sodium or potassium hydroxide and a sodium or potassium salt of metasilicic or boric or metaphosphoric or orthophosphoric or hypophosphorous di-, tri- or poly-metaphosphoric acid or sodium aluminate or a mixture thereof with a total concentration not exceeding 10 wt%. and at a temperature maintained between 5 and 40°C during the process carried out with polarization at direct current in the voltage range between the product and the counter electrode from +150 to +750 V or pulsed in the range from -200 to 0 V and from +200 to +750 V, with a frequency of 25 to 10,000 Hz, a duty factor of 10 to 100%, wherein the current density flowing through the product is limited to a value of 2.5 to 50 A / dm 2< , and the process duration is between 5 and 120 min, preferably rinsed in deionized water and dried, characterized in that the product treated in this way is immersed in a dyeing solution at a temperature of 5 to 60°C for a time of 1 to 3600 s using ultrasound at a frequency of 20 to 120 kHz and a power in the range of 5 to 2000 W / dm 2< , preferably rinsed in deionized water and dried, and the thus colored and dried product is subjected to repeated anodic oxidation under the same conditions for a time of 5 to 1800 s.

[0030] The method according to the invention is also presented in Figure 1 and consists of forming an oxide coating on a metal substrate in the process of plasma electrolytic oxidation, wherein in the structure of the formed oxide one can distinguish a multitude of pores with dimensions ranging from the submicron scale to the size of a few micrometers, where additional operations are used to fill the free spaces in in the coating with dyeing substances, giving decorative effect to the finished product. After creating the oxide coating and cleaning of the pores from residual electrolyte after the PEO process, the metal product is immersed in a solution of a dye or dyes belonging to anthraquinone, azo, azine, quinoline, phthalocyanine, pyrene, or triarylmethane compounds and their metal complexes at a concentration of 1 to 100 g / dm 3< . The composition may be enriched with the presence of organic corrosion inhibitors, such as heterocyclic compounds (e.g. imidazoline or quinoline derivatives), fatty acids, quaternary amines, or naphthalene acids, also in the concentration range of 1 to 100 g / dm 3< . The solvent for the dyeing bath is a suitable organic solvent, e.g. ethanol, isopropanol, benzene, or toluene. After immersing the product previously treated in the PEO process in the dyeing bath, the temperature of which is maintained in the range of 5 to 60°C, an ultrasound generator is started with a frequency of 20 to 120 kHz and power in the range of 5 to 2000 W / dm 2< . Dyeing lasts from 30 to 3600 s, after which the ultrasound generator is switched off, and the dyed product is rinsed and dried. Finally, to prevent the washing out of organic substances introduced into the coating, the product is reintroduced into the plasma electrolytic oxidation system and the anodic treatment is continued for another 5 to 1800 s, after which the product is rinsed and dried. The oxide that grows in the next PEO process seals the previously dyed oxide coating, and the organic substances present in the system are partially destroyed because of the action of the plasma and the high voltage causing anodic oxidation. As described in the scientific publication "Coloration of the aluminum alloy surface with dye emulsions while growing a plasma electrolytic oxide coating" (S.-C. Yeh, D.-S. Tsai, J.-M. Wang, C.-C. Chou, Surf. Coat. Technol. 287 (2016) 61-66) and patent no. TWI571536B, it is possible to conduct the PEO process in such a way that at the appropriate moment of processing, emulsions containing solvent organic dyes are introduced into the system, which leads to the formation of uniformly colored oxide coatings. The drawback of the method presented by the inventors from Taiwan is the contamination of the PEO process bath with coloring substances each time, which causes the need to replace the solution before the next production cycle, because, as the authors themselves admitted, conducting the plasma electrolytic oxidation treatment in a solution containing dyes gives an unsatisfactory effect from the very beginning. The coatings produced in this way are unevenly colored, and the bath significantly changes its composition during subsequent processing cycles. This effect happens because during PEO the highest intensity of plasma microdischarges is usually observed at the very beginning of the process. Therefore, in this patent, the dye is introduced in a separate tank, which minimizes contamination of the bath. In addition, the dyes used in the solution are insoluble in water, so when the colored product is immersed back into the PEO electrolyte, it is not washed out into the bath. This part of the dye that can be destroyed in the additional PEO process is subjected to the action of plasma, and therefore mainly oxidized to simple products of oxidation of organic substances, i.e. water, carbon dioxide, nitrogen. Thanks to this, the electrolyte remains free from organic contamination. In other cited methods (in patents CN107217290A, CN107227479A, or CN107227480A), to achieve satisfactorily intense color, very long dyeing times measured in tens of hours are used. The method described in this patent allows one to obtain an intense color even after a few minutes of dyeing. Moreover, in comparison with the method from the previously described publications "Anodic dyeing of micro-arc oxidized aluminum with a cathodic pretreatment" (C.-M. Chen, H.-J. Chu, J.-L. He, Surf. Coat. Technol. 324 (2017) 92-98) and patents no. CN107338466B and TWI623651B, the solution described in this patent does not require the use of NaCl solution to activate the oxide coatings produced in the PEO process to color them. Although the Chinese authors did not present the results regarding the anti-corrosion properties of the coatings they prepared, it can be expected that anodic treatment in a solution containing aggressive chloride ions will lead to serious defects in the produced protective coatings. Example I. A workpiece made of magnesium alloy AZ91D was subjected to preliminary degreasing with isopropyl alcohol in an ultrasonic cleaner for 10 minutes and after drying it was immersed in an aqueous solution containing 5 g / dm 3< of sodium hexametaphosphate, 0.6 g / dm 3< of sodium metasilicate, 0.8 g / dm 3< of sodium aluminate and 2.8 g / dm 3< of potassium hydroxide at a temperature of 15°C. A stainless steel counter electrode was also immersed in the bath with the solution and the wires supplying current to the electrolytic system were connected. Then a unipolar pulse current was applied to the system with a maximum voltage of +440V, a limiting current density of 18 A / dm 2< , a frequency of 50 Hz, and a duty factor of 33%. The plasma electrolytic oxidation process was carried out for 40 minutes, after which the detail was removed from the solution and rinsed. The dried part was immersed in a solution of 50 g / dm 3< of nigrosin spirit soluble (Solvent Black 5) in ethanol at a temperature of 20°C. Dyeing was carried out using ultrasound at a frequency of 45 kHz and a power of 25 W / dm 2< for 5 minutes. After removal, the part was rinsed in deionized water, and then an additional process of plasma electrolytic oxidation was carried out under the same conditions for 4 minutes. As a result of the method, a dark purple coating (Fig. 2) with closed pores was obtained, which was visible under an electron microscope both in the planar view (Fig. 3) and in the cross-section (Fig. 4). Its thickness was 30.0±5.1 µm. The corrosion resistance of the protected substrate surface increased by 2 orders of magnitude as demonstrated by electrochemical impedance spectroscopy experiments in a solution containing 0.1 mol / dm 3< Na 2 SO 4 and 0.5 g / dm 3< NaCl, where the polarization resistance after 2 h of immersion for the unprotected surface was 4.5±0.3 kΩ·cm 2< , and for the surface with a colored oxide coating 219±27 kΩ·cm 2< . Example II. A workpiece made of aluminum alloy AW6061 was subjected to preliminary degreasing with acetone in an ultrasonic cleaner for 5 minutes and after drying it was immersed in an aqueous solution containing 12 g / dm 3< of sodium metasilicate, 3 g / dm 3< of sodium hexametaphosphate and 4 g / dm 3< of potassium hydroxide at a temperature of 20°C. A stainless steel counter electrode was also immersed in the bath with the solution and the wires supplying current to the electrolytic system were connected. Then a bipolar pulse current was applied to the system at a maximum positive voltage of +425 V and negative of -150 V, a limiting current density of +24 A / dm 2< and negative of -30 A / dm 2< , a frequency of 100 Hz and a duty factor of 100%. The plasma electrolytic oxidation process was carried out for 35 minutes, after which the part was removed from the solution and rinsed. The dried part was immersed in a solution of 25 g / dm 3< of nigrosin spirit soluble (Solvent Black 5) and 10 g / dm 3< of 8-hydroxyquinoline (quinoline derivative) as a corrosion inhibitor in ethanol at a temperature of 20°C. Dyeing was carried out using ultrasound at a frequency of 24 kHz and a power of 45 W / dm 2< for 10 minutes. After removal, the part was rinsed in deionized water, and then an additional plasma electrolytic oxidation process was carried out under the same conditions for 2 minutes. As a result of the method, a dark purple coating was obtained (Fig. 5). Its thickness was 36.0±2.3 µm. Corrosion resistance measurements of the alloy surface determined by the potentiodynamic polarization method in 3.5 wt% NaCl solution showed that the breakdown potential of the passive coating on the aluminum alloy before and after protection increased from -0.530±0.027 to 1.044±0.169 V (Fig. 6). Example III. A workpiece made of ZRE1 magnesium alloy was subjected to preliminary degreasing with isopropyl alcohol in an ultrasonic cleaner for 5 minutes and after drying it was immersed in an aqueous solution containing 8 g / dm 3< of sodium hexametaphosphate and 2 g / dm 3< of potassium hydroxide at a temperature of 15°C. A stainless steel counter electrode was also immersed in the bath with the solution and the wires supplying current to the electrolytic system were connected. Then a bipolar pulse current was applied to the system at a maximum positive voltage of +430 V and negative of -10 V, a limiting current density of +15 A / dm 2< and negative of -15 A / dm 2< , a frequency of 500 Hz and a duty factor of 25%. The plasma electrolytic oxidation process was carried out for 25 minutes, after which the part was removed from the solution and rinsed. The dried sample was immersed in a solution of 25 g / dm 3< of the commercial solvent dye Unisol Red BE (Solvent Red 129) and 10 g / dm 3< of palmitic acid in isopropyl alcohol at a temperature of 20°C. Dyeing was carried out using ultrasound at a frequency of 45 kHz and a power of 35 W / dm 2< for 3 minutes. After removal, the part was rinsed in deionized water, and then an additional plasma electrolytic oxidation process was carried out under the same conditions for 2 minutes. As a result of the method, a red coating was obtained ( Fig. 7). Its thickness was 38.7±1.7 µm. The contact angle for the surface treated only with PEO was 41.6±3.3°, while for the surface colored with the addition of palmitic acid, the value of this parameter increased to 120.9±2.3° ( Fig. 8). Example IV. A workpiece made of Ti6Al4V titanium alloy was subjected to preliminary degreasing with isopropyl alcohol in an ultrasonic cleaner for 5 minutes and after drying it was immersed in an aqueous solution containing 35 g / dm 3< of potassium hydrogen orthophosphate and 5 g / dm 3< of sodium hydroxide at a temperature of 15°C. A stainless steel counter electrode was also immersed in the bath with the solution and the wires supplying current to the electrolytic system were connected. Then a direct current was applied to the system at a maximum voltage of 350 V and a limiting current density of 10 A / dm 2< . The plasma electrolytic oxidation process was carried out for 5 minutes, after which the part was removed from the solution and rinsed. The dried part was immersed in a solution of 35 g / dm 3< Unisol Blue KFL (Solvent Blue 70) in toluene at a temperature of 18°C. Dyeing was carried out using ultrasound at a frequency of 45 kHz and a power of 75 W / dm 2< for 10 minutes. After removal, the part was rinsed in deionized water, and then an additional plasma electrolytic oxidation process was carried out under the same conditions for 1 minute. As a result of the method, a blue coating of 5.6±0.7 µm thickness was obtained, with roughness expressed as Ra = 2.55±0.22 µm and Rz = 17.65±1.07 µm (10% smoother surface than the initial undyed surface). The corrosion resistance of the coating is improved due to the presence of the dye, which was confirmed by measurements of linear polarization resistance in 3.5 wt% NaCl solution (an increase from the initial value of 14.4±1.9 to 115.4±2.8 MΩ·cm 2< ). Example V. A workpiece made of zinc alloy ZL5 was subjected to preliminary degreasing with isopropyl alcohol in an ultrasonic cleaner for 10 minutes and after drying it was immersed in an aqueous solution containing 18 g / dm 3< of sodium hexametaphosphate, 32 g / dm 3< of potassium dihydrogen orthophosphate, 30 g / dm 3< of sodium aluminate, 28.3 g / dm 3< of sodium tetraborate and 3.1 g / dm 3< of sodium hydroxide at a temperature of 5°C. A stainless steel counter electrode was also immersed in the bath with the solution and the wires supplying current to the electrolytic system were connected. Then a pulse current was applied to the system at a maximum positive voltage of +150 V, a limiting positive current density of +2.5 A / dm 2< , a frequency of 25 Hz, and a duty factor of 10%. The plasma electrolytic oxidation process was carried out for 5 minutes, after which the part was removed from the solution and rinsed. The dried part was immersed in a dyeing solution at a temperature of 5°C for 1 s using ultrasound at a frequency of 20 kHz and a power of 5 W / dm 2< , and dried. Prepared in this way detail was subjected to repeated anodic oxidation under the same conditions for 5 s. The dyeing solution contains: one solvent dye at a concentration of 0.1 g / dm 3< , belonging to the group of anthraquinone compounds, in the form of 1-hydroxy-4-(4-methylanitino)anthracene-9,10-dione (Solvent Violet 13) one corrosion inhibitor of the substrate material at a concentration of 0.1 g / dm 3< , from the group of fatty acids in the form of lauric acid the rest is a solvent, which is isopropanol. As a result of the method, an oxide coating with a thickness of 2.52±0.15 µm and a light purple color was obtained. The presence of both the dye and the fatty acid caused the surface wettability angle relative to demineralized water to increase from 15.2±6.1 ° for the undyed surface to 84.6±2.3 °. Example VI. A workpiece made of the Zircaloy 2 zirconium alloy was subjected to preliminary degreasing with isopropyl alcohol in an ultrasonic cleaner for 2 minutes, and after drying it was immersed in an aqueous solution containing 22.1 g / dm 3< of sodium metasilicate, 11.2 g / dm 3< of potassium hydrogen orthophosphate, 7.5 g / dm 3< of sodium diphosphate, 9.4 g / dm 3< of sodium aluminate and 2.5 g / dm 3< of potassium hydroxide at a temperature of 20°C. A stainless steel counter electrode was also immersed in the bath with the solution and the wires supplying current to the electrolytic system were connected. Then a bipolar pulse current was applied to the circuit with a maximum positive voltage of +500 V and negative of -75 V, a limit current density of +25 A / dm 2< and negative of -25 A / dm 2< , a frequency of 5000 Hz and a duty factor of 50%. The plasma electrolytic oxidation process was carried out for 70 minutes, after which the detail was removed from the solution and rinsed. The product treated this way was immersed in a dyeing solution at a temperature of 35°C for 1800 s using ultrasound at a frequency of 78 kHz and power in the range of 1000 W / dm 2< , rinsed in deionized water and dried, and the detail so colored and dried was subjected to repeated anodic oxidation under the same conditions for 1000 s. The dyeing solution contains: two solvent dyes at a concentration of 50 g / dm 3< each, where the solvent dye is Unisol Orange 2RV (Solvent Orange 62) and Unisol Black RE (Solvent Black 27) two corrosion inhibitors at a concentration of 50 g / dm 3< each, where the corrosion inhibitor is acrylic acid and stearic acid the rest is the solvent, which is ethanol. As a result of the method, an oxide coating thick at 72.3±4.5 µm with a dark brown color was obtained. The presence of corrosion inhibitors, as well as dyes, allowed one to obtain a hydrophobic surface with a wetting angle of 115.3±2,0° for demineralized water. The angle of droplet slippage from this surface was 37.2±4.9°.

Examples

Embodiment Construction

. A workpiece made of magnesium alloy AZ91D was subjected to preliminary degreasing with isopropyl alcohol in an ultrasonic cleaner for 10 minutes and after drying it was immersed in an aqueous solution containing 5 g / dm 3Fig. 2) with closed pores was obtained, which was visible under an electron microscope both in the planar view (Fig. 3) and in the cross-section (Fig. 4). Its thickness was 30.0±5.1 µm. The corrosion resistance of the protected substrate surface increased by 2 orders of magnitude as demonstrated by electrochemical impedance spectroscopy experiments in a solution containing 0.1 mol / dm 3Fig. 5). Its thickness was 36.0±2.3 µm. Corrosion resistance measurements of the alloy surface determined by the potentiodynamic polarization method in 3.5 wt% NaCl solution showed that the breakdown potential of the passive coating on the aluminum alloy before and after protection increased from -0.530±0.027 to 1.044±0.169 V (Fig. 6). Example III. A workpiece made of ZRE1 magnesium a...

[0001] The present invention concerns a method for obtaining colored oxide films on metal products and as a result imparting color and protection against corrosion of metal products containing chemical elements that can be subjected to plasma electrolytic oxidation in alkaline solutions of silicates, phosphates, aluminates, or borates of sodium or potassium. The most prominent metals of technical importance that exhibit such properties are aluminium, magnesium, zinc, and titanium.

[0002] Oxide films belong to a wider family of technical coatings called conversion coatings. These coatings are produced due to the controlled corrosion process on the material's surface. Metal ions released from the metal matrix because of the electrode reaction, upon contact with the bath components, form nonsoluble deposits on the treated surface, which adhere tightly to the substrate. Such films are produced in electroless conditions and are characterized by relatively small thicknesses (up to a dozen or so micrometers) and quite low mechanical strength. Aluminum, magnesium, titanium, and, less frequently, zinc, as well as alloys containing these elements, can be successfully covered with oxide films in the process of anodic oxidation (or anodizing). During anodizing, processed parts are immersed into a suitable electrolyte and are connected to an electrical circuit as a positive electrode (anode), while an inert electrode material (e.g. stainless steel) is connected as a cathode. The flow of current forced by an external power supply drives electrode reactions, which cause thickening of the natural oxide (passive) film on the metal surface. Depending on the composition of the electrolytic bath and the electrical parameters, barrier layers (e.g. for the construction of capacitors) or porous coatings can be obtained. Pores are formed due to the local dissolution of the produced oxide coating, thanks to the appropriate composition of the electrolyte. Under constant voltage conditions, with the progress of the anodizing time, both the thickness of the coating and the depth of the pores increase, yet a thin barrier layer of constant thickness remains in direct contact with the metal.

[0003] Recently, a technique derived from conventional anodizing, called plasma electrolytic oxidation (PEO), also known as micro-arc oxidation (MAO) or micro-plasma oxidation (MPO), has been gaining attention. In contrast to classical anodizing methods, PEO utilizes environmentally friendly electrolytes with low concentrations of dissolved substances (generally less than 10 wt%). Thanks to the properties of the corresponding metal oxides, during anodic polarization it is possible to generate a high electric field, which, when the critical value is exceeded, can lead to dielectric breakdown and plasma generation. The plasma composition simultaneously includes elements originating from the gases generated in the process, the partially melted metal substrate, and the electrolytic bath components. For this reason, the oxide coatings produced by PEO are significantly enriched in elements contained in the bath, and the temporarily occurring high temperature of the treated surface leads to the crystallization of additional phases (e.g. corundum during PEO of aluminum products), unavailable during classical electrochemical treatment. However, the creation of plasma requires the use of a higher voltage than during conventional anodizing (from 250 to even 1000 V compared to 15 to 120 V, respectively).

[0004] Despite the benefits of increased hardness and thickness of PEO coatings compared to conventional anodizing, they often do not offer sufficiently good corrosion resistance due to the presence of micropores in the oxide structure. Only near the metallic substrate, there is a thin (generally between 0.5 and 1.0 µm thick), a continuous protective oxide layer, providing a barrier between the metal and the external environment. In the case of porous films produced in classical anodizing, pores have a columnar structure and a diameter of a few to several dozen nanometers. Such pores can be closed efficiently by hot water sealing. In addition, dyes can be absorbed on the walls of the pores before they are closed which allows a wide range of colors to be obtained on aluminium products.

[0005] Coloring of titanium products can be carried out using conventional anodizing in acidic or alkaline electrolytes, where the final color of the surface after treatment depends on the composition of the bath, the substrate, and the applied forming voltage. The surface color is created by the interference of light on the processed product. This is possible thanks to the optical properties of titanium dioxide in the oxide film, and the very good reproduction of the substrate surface morphology by the electrochemically formed oxide.

[0006] The disadvantage of traditional options for coloring the mentioned materials is that in the case of titanium products, interference color can only be obtained for relatively thin oxide layers (up to several hundred nanometers thick). Material colored in this way, if exposed to abrasive wear, completely loses its decorative value. Despite the greater thickness of oxide coatings (from a few to several dozen micrometers thick) typically produced on magnesium or aluminum products, their hardness and resistance to abrasive wear are not as good as for materials obtained in PEO processing. Although the oxide films produced in the PEO process are highly porous, traditional methods of surface coloring used for conventional anodizing do not provide satisfactory results. The capillary effect that allows for loading the pores with the coloring solution depends on the pore diameter, which is not uniform for layers produced in the PEO process. As a result, the coloring is generally uneven, and the dye is washed out over time. It is therefore necessary to use a method that allows for (i) uniform adsorption of the dye molecules in the entire volume of the oxide layer, (ii) enabling effective color fixation by an appropriate sealing operation.

[0007] There are known methods in technology according to which salts of transition metals, such as cobalt, chromium, copper, iron, manganese, molybdenum, nickel, titanium, vanadium or tungsten, are added to the electrolytic bath. From the scientific publication "Preparation and properties of blue ceramic coating by micro-arc oxidation on aluminum alloy" (C. Chen, Y. Zhang, W. Chen, H. Cheng, L. Wang, Mater. Rev., 31(5) (2017) 121-126) a method of forming blue oxide coatings on a substrate made of aluminum alloy AW5052 is known, where the color is given by Co(II) present in the electrolytic bath. It was noted that the higher the concentration of Co(OH) 2 (in the range of 1 to 3 g / dm 3< ) in the bath, the deeper the obtained surface color, although the corrosion resistance offered by the coating deteriorates with the increase of the concentration of cobalt(II) in the bath. The process is carried out under direct current conditions using the potentiostatic method.

[0008] From the scientific publications "Preparation process of green ceramic coating of magnesium alloy through micro-arc oxidation" (S. Wang, W.W. Ding, Adv. Mater. Res. 842 (2014) 228-232) and "Characterization of self-sealing MAO ceramic coatings with green or black color on an Al alloy" (W. Yang, D. Xu, J. Chen, J. Liu, B. Jiang, RSC Advances, 7(3) (2017) 1597-1605) methods of obtaining films in the PEO process with color from green to black depending on the concentration of potassium dichromate in the electrolytic bath are known. The electrolyte for the process is based on sodium hexametaphosphate (45 g / dm 3< ), sodium metasilicate (5 g / dm 3< ), potassium hydroxide (1,2 g / dm 3< ), potassium fluoride (3 g / dm 3< ) and the aforementioned Cr(VI) salt in the concentration range of up to 12 g / dm 3< . The process is carried out using a bipolar pulse rectifier at a frequency of 400 Hz and a pulse duty factor of 10% to a maximum voltage of 460 V for 15 minutes. Similarly to the case of cobalt(II) salt, an increase in the concentration of dichromate in the coating formation solution results in a deeper color (up to black at the highest concentration). The method is suitable for the anodizing of the magnesium alloy AZ91D and the aluminum alloy AW6061.

[0009] In a similar procedure, it is possible to impart color due to the presence of copper salt in the electrolyte for PEO treatment. The scientific publication "Effects of copper additive on micro-arc oxidation coating of LZ91 magnesium-lithium alloy" (S.-J. Lee, L.H.T. Do, Surf. Coat. Technol. 307 (2016) 781-789) provides a method by which brown oxide layers can be obtained by conducting the plasma electrolytic oxidation process in an electrolytic bath containing Na 3 PO 4 ·12H 2 O (5 g / dm 3< ), Na 2 SiO 3 (15 g / dm 3< ), NaOH (4 g / dm 3< ) with the addition of 3 to 5 g / dm 3< Cu 3 PO 4 . A copper(I) salt is introduced into the coating structure on a magnesium-lithium-zinc alloy LZ91 under bipolar pulse current conditions at 2 kHz, 30% duty cycle, and current density of 4 A / dm 2< for 5 min. The use of a higher concentration of copper results in a darker color of the film. From the scientific publication "Effect of copper additive on microstructure and anti-corrosion performance of black MAO films grown on AZ91 alloy and coloration mechanism" (R.Y. He, B.Y. Wang, J.H. Xiang, T.J. Pan, J. Alloy. Compd. 889 (2021) 161501) a method is known, where Cu 2 P 2 O 7 (0,5 to 3 g / dm 3< ) is used as an additive to the base bath for the PEO process with the composition of Na 2 SiO 3 (10 g / dm 3< ), KF (4 g / dm 3< ) and NaOH (4 g / dm 3< ). The coating formed on an AZ91 magnesium alloy substrate under unipolar pulse current conditions at a frequency of 700 Hz, a duty ratio of 20%, and a current density of 2 A / dm 2< for 2 minutes acquires a black color due to the presence of copper(II) oxide when there is at least 1.5 g / dm 3< of Cu 2 P 2 O 7 in the bath. From the scientific publication "New findings on properties of plasma electrolytic oxidation coatings from study of an Al-Cu-Li alloy" (Y.-L. Cheng, Z.-G. Xue, Q. Wang, X.-Q. Wu, E. Matykina, P. Skeldon, G.E. Thompson, Electrochim. Acta 107 (2013) 358-378) a method is also known in which brown oxide coatings are obtained on an aluminum-based alloy, where the color is determined by the presence of copper in the substrate alloy. The process is carried out under conditions of bipolar pulse current at 1 kHz, 20% duty cycle, and an average current density of 25 A / dm 2< for 10 min. In the case when the substrate material is an aluminum alloy with copper (3 wt%), the coating color is much lighter than in the case of the commercial alloy 2A97 with the chemical composition (w.t %): 3.8% Cu, 1.5% Li, 0.3% Mn, 0.4% Mg, 0.5% Zn, 0.12% Zr, and 0.07% Ti, Al - the rest. In this process, an electrolyte based on 8 g / dm 3< Na 2 SiO 3 ·9H 2 O and 1 g / dm 3< KOH is used.

[0010] From the scientific article "Thin films produced on 5052 aluminum alloy by plasma electrolytic oxide with red mud-containing electrolytes" (L. Sottovia, M.L.P. Antunes, C.A. Antonio, E.C. Rangel, N.C. Da Cruz, Mater. Res. 17(6) (2014) 1404-1409) a method is known for coloring of coatings in the PEO process on the substrate of aluminum alloy AW5052, where the electrolyte used in the treatment is an aqueous solution of KOH with a pH of 11, to which 5 g / dm 3< of the so-called "red mud" is added. The term "red mud" refers to waste from Bayer's technology for obtaining refined alumina from natural bauxite deposits. This waste is rich in ferric oxide, which is also the source of its color. The AW5052 alloy can be colored red in the PEO process in the aforementioned electrolytic bath under bipolar pulse conditions operated at a voltage of 600 V, a frequency of 200 Hz, and a duty cycle of 60% for 5 min. From the scientific publication "Application of thermally activated red mud in surface treatment of 5005 aluminum alloy" (S. Liu, J. Zeng, Prog. Org. Coat. 133 (2019) 276-288) there is an additional variant of the abovementioned process, where the "red mud" is previously subjected to heat treatment before being introduced into the base electrolyte (3 g / dm 3< KOH). The method is suitable for forming oxide coatings on the AW5005 aluminum alloy under bipolar pulse conditions, where a voltage of 480 V and a duty ratio of 40% are used during the anodic cycle, and in the cathodic cycle the parameters are the following: -200 V, duty ratio of 40%. The treatment is carried out at a frequency of 400 Hz for 60 min. In this way, depending on the heat treatment temperature, which is carried out for 4 hours, the final color of the coating can be obtained from dark brown (100°C), through orange (600°C) to yellow (1100°C).

[0011] Patent No. CN102943297A describes a method for finishing the surface of titanium alloys by the PEO process in a bath containing 8-15 g / dm 3< of a phosphate salt, 0.3-1.6 g / dm 3< of an iron salt (not chloride or fluoride; preferably sulfate), 0.6-1.5 g / dm 3< of a manganese salt (not chloride or fluoride; preferably sulfate), 5-15 10 cm 3< / dm 3< of trolamine and 20-35 g / dm 3< of trisodium citrate. The PEO process leads to the formation of titanium oxide on the surface of the substrate, which visually resembles the blackened surface of a firearm produced on steel by the oxidation method.

[0012] From the scientific publication "Effects of KMnO4-added electrolytes on alumina coatings prepared by plasma electrolytic oxidation" (C.Y. Hong, H.P. Teng, F.H. Lu, J. Chinese Corros. Eng., 26(3) (2012) 141-146) a method for creating yellow oxide coatings on the surface of the AW6061-T6 aluminum alloy is known. The method consists of carrying out the PEO process under direct current conditions for 60 min, using a current density of 10 A / dm 2< . The process bath is an aqueous solution containing 0.1 M NaAlO 2 and 0.8-3.0 mM KMnO 4 . Whereas from the scientific article "Coloring and corrosion resistance of pure Mg modified by micro-arc oxidation method" (L.-H. Li, T.S.N.S. Narayanan, Y.K. Kim, Y.-M. Kong, G.-S. Shin, S.-K. Lyu, I.S. Park, M.H. Lee, Int. J. Prec. Eng. Manuf. 15(8) (2014) 1625-1630) a method for creating brown-red oxide layers on technically pure magnesium is known. For this purpose, an electrolyte based on 0.05 M Na 3 PO 4 , 0.2 M Na 2 SiO 3 , 1 M KOH and 15 mM KMnO 4 is used, while the oxidation process is carried out under direct current conditions for 3 min at a current density of 20-40 A / dm 2< .

[0013] From the scientific publication "Comparison of plasma electrolytic oxidation coatings on Mg-Li alloy formed in molybdate / silicate and aluminate / silicate composite electrolytes" (Z.J. Li, Y. Yuan, X.Y. Jing, Mater. Corros. 65(5) (2014) 493-501) a method for forming brown oxide coatings on Mg-Li alloy (5.6 wt% Li, 3.37 wt% Al, 1.68 wt% Zn, 1.14 wt% Ce, Mg - the rest) in the PEO process in an electrolytic bath with 15 g / dm 3< Na 2 SiO 3 , 0.6 g / dm 3< Na 2 MoO 4 , 3 g / dm 3< NaOH and 10 cm 3< / dm 3< triethanolamine is known. The process is carried out under unipolar pulsed conditions at a frequency of 2 kHz, a duty factor of 15% and a current density of 5 A / dm 2< for 10 min. Whereas, patent No. CN105463551A describes a method for processing aluminum alloys involving the PEO process in an electrolyte consisting of 0.5 M NaH 2 PO 4 , 0.5 M NaF, 0.1 M NH 4 F and 0.1 M Na 2 B 4 O 7 enriched with a manganese compound (in the form of MnO 4 -< ion with Mn concentration of not less than 0.5 g / dm 3< ) or molybdenum (in the form of MoO 4 2-< ion with Mo concentration of not less than 8 g / dm 3< ) with the possibility of using an additional source of cobalt (in the form of Co(OH) 2 dissolved in ammonia water, with Co concentration of not less than 0.1 g / dm 3< ). Oxidation is carried out at a voltage of 125-155 V and a current below 0.5 A. By selecting the appropriate concentration of bath components, different shades of the coating can be obtained, including blue, dark grey or orange.

[0014] From the scientific publication "Effects of nickel additive on micro-arc oxidation coating of AZ63B magnesium alloy" (W. Zhan, F. Tian, G. Ou-Yang, B.-Y. Gui, Int. J. Prec. Eng. Manuf. 19(7) (2018) 1081-1087) a method for creating brown coatings on the surface of AZ63B magnesium alloy from electrolytic baths based on 15 g / dm 3< Na 2 SiO 3 , 30 g / dm 3< anhydrous borax, 0.5 g / dm 3< nickel acetate, 30 g / dm 3< NaOH and 15 cm 3< / dm 3< triethanolamine is known. The PEO process is carried out under bipolar pulse conditions at a frequency of 400 Hz, a duty factor of 50%, and a current density of 3 A / dm 2< for 570 s. However, coatings obtained in this way are not uniformly colored and numerous white dots are visible on them.

[0015] From the scientific publication "Stable preparation and characterization of yellow micro-arc oxidation coating on magnesium alloy" (W. Yang, D. Xu, X. Yao, J. Wang, J. Chen, J. Alloys Compd. 745 (2018) 609-616) a method for creating colored oxide films as a result of PEO treatment on AZ80 magnesium alloy is known. The color of the coatings, from yellow to brown, can be obtained depending on the concentration of Na 2 SnO 3 (from 2 to 15 g / dm 3< ) in the base bath of 10 g / dm 3< KOH, 8 g / dm 3< KF, 5 g / dm 3< Na 2 SiO 3 , the pH of which is from 13.6 to 13.9. The PEO process is carried out in pulses at a voltage of 350 V, a frequency of 400 Hz, and a duty factor of 10% for 5 min. The corrosion resistance of the substrate is worse than that of the coating formed without the addition of a tin compound for Na 2 SnO 3 concentrations below 15 g / dm 3< .

[0016] From the scientific publication "Development of decorative and corrosion resistant plasma electrolytic oxidation coatings on AM50 magnesium alloy" (P.B. Srinivasan, J. Liang, C.Blawert, M. Störmer, W. Dietzel, Surf. Eng. 26(5) (2010) 367-370) a method for PEO treatment of AM50 magnesium alloy is known, where the resulting coating takes on a blue color. The method relies on conducting the process unipolarly with a pulse frequency of 45 Hz, a duty factor of 9%, and a current density of 3 A / dm 2< for 10 min in a water-based electrolytic bath consisting of Na 3 PO 4 , NH 4 OH, and a suspension (sol) of TiO 2 obtained from titanium alkoxide, ethanol, and triethanolamine. The intensity of the blue color of the coating increased with the increase of the TiO 2 sol content from 2 to 6%.

[0017] From the scientific publication "Effect of ammonium metavanadate on surface characteristics of oxide layer formed on Mg alloy via plasma electrolytic oxidation" (Y.G. Ko, K.M. Lee, D.H. Shin, Surf. Coat. Technol. 236 (2013) 70-74) a method is known of producing black oxide coatings on magnesium alloy Mg-8.29Al-0.83Zn-0.31Mn by PEO process in a bath containing 0.5 M KOH, 0.15 M K 4 P 2 O 7 and 0.08 M NH 4 VO 3 . pH and conductivity of the bath are 10.1 and 61.8 mS / cm, respectively. The process is carried out under direct current conditions at 10 A / dm 2< for 200 s. The scientific article "Comparison of effects of V2O5 and NaVOs on corrosion resistance of micro-arc oxidation coatings on magnesium alloys" (L.-Y. An, Y. Ma, Y.-P. Liu, Y.-S. Wang, S. Wang, Z.-Y. Wang, Chinese J. Nonferr. Met. 28 (8) (2018) 1542-1550) describes a method for obtaining brown-colored coatings from silicate baths enriched with the addition of V 2 O 5 solid particles and vanadium salt - sodium metavanadate (preferably at a concentration of 0.35 g / dm 3< NaVO 3 ). The layer owes its color to the formation of a spinel with the MgV 2 O 4 structure, regardless of which vanadium source is used. From the scientific publication "Development of vanadium impregnated flat absorber composite PEO coating on AA6061 alloy" (A.M. Pillai, A. Rajendra, A.K. Sharma, P. Bera, S. Poornima, S. Sampath, Surf. Coat. Technol. 410 (2021) 126891) a method for producing black coatings on the surface of the AW6061 alloy by the PEO process from a water-based bath containing 60 g / dm 3< Na 2 SiO 3 ·9H 2 O, 5 g / dm 3< NaOH and 10 g / dm 3< VOSO 4 ·xH 2 O is known. The PEO treatment is carried out unipolarly pulsed at a frequency of 50 Hz, a current density of 10A / dm 2< , a duty factor of 3.5%, and a time of up to 10 min. As the process time progresses in the vanadyl sulfate-based bath, the color of the layer becomes increasingly darker, with a uniform black color being obtained after at least 7 minutes.

[0018] The scientific publication "Preliminary study on preparation of black ceramic coating formed on magnesium alloy by micro-arc oxidation in carbon black pigment-contained electrolyte" (X. Liua, G. Liu, J. Xie, Procedia Engineer. 36 (2012) 261-269) and patent CN102367584B (the same authors) describe a method for creating black oxide coatings on AZ91D magnesium alloy and substrates made of other metals in PEO surface treatment. The process is carried out in an aqueous solution containing Na 2 SiO 3 (6-12 g / dm 3< ; preferably 6 g / dm 3< ), Na 2 WO 4 (0.5-2.0 g / dm 3< ; preferably 1.5 g / dm 3< ), KOH (2 g / dm 3< ), NaF (2 g / dm 3< ), sodium citrate (0.3-0.9 g / dm 3< ; preferably 0.3 g / dm 3< ) and a commercial carbon ink of the Chinese brand "Ostrich" (10-25 cm 3< / dm 3< ; preferably 25 cm 3< / dm 3< ) under bipolar pulse conditions at a frequency of 600 Hz, a duty factor of 20% and a current density of 1-4 A / dm 2< (preferably 3 A / dm 2< ) for 20 min.

[0019] Patent No. CN106702464A describes a method for producing black coatings on magnesium alloys, where the workpiece is subjected to the PEO process in a electrolytic bath with the chemical composition: (NaPO 3 ) 6 5-20 g / dm 3< (preferably 10 g / dm 3< ), KF 1-8 g / dm 3< (preferably 2 g / dm 3< ), KOH 1-8 g / dm 3< (preferably 3 g / dm 3< ), NH 4 VO 3 1-8 g / dm 3< (preferably 2 g / dm 3< ), Na 2 WO 4 2-10 g / dm 3< (preferably 4 g / dm 3< ). The treatment is carried out in bipolar pulse conditions using a current density in the range of 5-15 A / dm 2< , and the treatment time is between 8 and 15 minutes.

[0020] From the scientific publication "FexCo1-xWO4 films on titanium: plasma electrolytic synthesis, optical, electrochemical and photocatalytic properties" (Y.B. Budnikova, M.S. Vasilyeva, I.V. Lukiyanchuk, V.S. Egorkin, A.Y. Ustinov, V.G. Kuryavyi, D.H. Shlyk, J. Mater. Sci.-Mater. El. 34(28) (2023) 1973) there is a method known for producing colored coatings on the VT1-0 titanium alloy with the chemical composition (wt. %): 0.7 Al, 0.25 Fe, 0.10 Si, 0.07 C, 0.04 N, 0.2 O, 0.01 N; other impurities - 0.30; Ti - the rest. The coatings are produced in the PEO process using a electrolyte containing 0.0125-0.05 M FeC 2 O 4 , 0.0125-0.05 M Co(CH 3 COO) 2 , 0.15 M Na 2 WO 4 , and 0.05 M EDTA. The treatment is carried out under direct current conditions for 10 min at a current density of 10 A / dm 2< . Depending on the proportion of iron salt to cobalt salt, it was possible to produce coatings of various shades of gray and brown.

[0021] It should be noted that the addition of the above-mentioned metal salts (especially chromium or vanadium) to electrolytic baths makes the PEO technology less environmentally friendly, and the wastewater from the process should be treated as hazardous. In addition, the presence of additional components in the baths has an impact on other properties of the coatings, such as corrosion protection or resistance to abrasive wear. Furthermore, the range of colors available because of the addition of inorganic substances to the PEO process is limited to the colors of the appropriate oxides and metal salts that are components of the bath. The solution to the above problems may be the use of dyeing with organic substances.

[0022] Patent No. TW202227673A describes a method for producing colored oxide coatings on the surface of magnesium products using dyes. In the first step, an oxide coating is produced in the PEO process in an electrolyte solution consisting of alkali metal silicates, fluorides, hydroxides, and pore regulators (triethanolamine or diethanolamine at a concentration of 0.1-5 g / dm 3< ) at a voltage of 450-550 V for no less than 10 minutes. The coating produced in this way should be no less than 10 micrometers thick. Then the produced film is rinsed to remove the remaining solution from the pores. The coating is then colored in a dye solution, without disclosing what the dye is dissolved in or what its chemical structure or name is. Finally, the layer is sealed in a nickel acetate solution at a temperature of 25 to 35°C.

[0023] Patent No. CN101831683A describes a method for obtaining colored ceramic oxide coatings on the surface of magnesium alloys. The magnesium product is previously subjected to the PEO process in a phosphate bath (40-70 g / dm 3< (NaPO 3 ) 6 , 10-30 g / dm 3< NaOH, 5-20 g / dm 3< NH 4 F, 5-25 g / dm 3< hexamethylenetetramine, 5-15 g / dm 3< NH 4 H 2 PO 4 , 5-20 cm 3< / dm 3< glycerin, 60-100 cm 3< / dm 3< trolamine) at a current density of 1-5 A / dm 2< , frequency of 50-172.45 Hz for 1-15 min, which results in the formation of a coating with a thickness of 1-20 µm. The product is then transferred to a container with an aqueous solution of an organic dye (0.5-10 g / dm 3< ), of which the exact chemical composition has not been disclosed, with a pH between 4 and 6. The product is kept in the solution for 20 to 40 hours with mixing and then rinsed and dried.

[0024] Patent No. CN107217290A describes a method for dyeing oxide coatings produced in the PEO process on the surface of magnesium-based products. The detail previously subjected to the PEO process is immersed in an aqueous solution of a cationic dye (used in the textile industry) at a concentration of 1-10 g / dm 3< with the addition of a release agent (urea) at a concentration of 1-5 g / dm 3< and sodium sulfate at a concentration of 1-5 g / dm 3< . During dyeing, the solution should have a temperature of 20°C and a pH between 4,5 and 5. Dyeing is carried out at a temperature of 60-90°C, with stirring at a speed of 500-1000 rpm, and with the participation of ultrasound at a power of 100-200 W for 0.5 to 24 h. The longer the immersion time, the more intense the color of the product. In addition, another variant of the method for dyeing oxide coatings created in the PEO process on a magnesium substrate was identified from patent CN107227480A (by the same authors). What distinguishes the presented patent from the two previous ones is the type of dyeing substance, which in this case is a weakly acid dye, belonging to the class of monoazo dyes, used in a concentration of 1-10 g / dm 3< . Together with the dye, 1-5 g / dm 3< Na 2 SO 4 and 1-5 g / dm 3< of emulsifier are added to the dyeing bath based on deionized water. At a temperature of 20°C, the pH of the solution is between 4.5 and 5. Dyeing is carried out in a manner analogous to the description in the two patents mentioned above.

[0025] Patent No. US20230189465A1 describes a housing structure for electronic devices made of a metal substrate, which is first covered with an oxide coating characterized by a thickness of 2 to 15 µm in the PEO process. Then, a coloring coating consisting of water-based dyes in the amount of 3 to 10 wt%, surface-active substances in the amount of 0.3 to 2 wt% with a thickness of 1 to 5 µm is applied to the surface of the produced oxide. Finally, the surface is covered with a paint coating consisting of a primer coating, a base coat coating, and a topcoat coating. The patent does not disclose a method of coloring the oxide coating produced in the PEO process and only claims several substances from the group of water-based dyes (e.g. methylene blue, fuchsine or xanthene dyes).

[0026] From the scientific publication "Coloration of the aluminum alloy surface with dye emulsions while growing a plasma electrolytic oxide coating" (S.-C. Yeh, D.-S. Tsai, J.-M. Wang, C.-C. Chou, Surf. Coat. Technol. 287 (2016) 61-66) and patent no. TWI571536B a method for forming oxide coatings on the surface of products based on aluminum, magnesium or titanium in the PEO process with the participation of solvent dyes in the form of emulsion is known. At first, the coating is formed in an electrolyte with a pH in the range of 9 to 11 and conductivity of 8 to 10 mS / cm, which contains aluminate, citrate, fluorozirconate or hexametaphosphate ions, and sodium or potassium ions. Then, an aqueous emulsion is prepared in which the dispersed phase is a dye (with anthraquinone, methine, phthalocyanine, azo, or triarylmethane chromophore groups) dissolved in an organic solvent (e.g. toluene) or a colloid, where an organic or inorganic pigment is used (e.g. iron oxide red, cobalt blue or chrome green). The particle size in the dyeing medium is from 50 to 500 nm. The oxide is initially formed in an electrolyte devoid of dyes, and then, after reaching a sufficiently thick coating, the emulsion or colloid is added to the bath in portions and the forming voltage is increased each time, which results in further growth of oxidation products together with the dye.

[0027] From the scientific publication "Anodic dyeing of micro-arc oxidized aluminum with a cathodic pretreatment" (C.-M. Chen, H.-J. Chu, J.-L. He, Surf. Coat. Technol. 324 (2017) 92-98) and from patents No. CN107338466B and TWI623651B a method for dyeing metal products previously covered with an oxide coating in the PEO process using organic anionic dyes is known. After the PEO process, products are first subjected to cathodic pretreatment, which consists of immersing them in an aqueous solution of NaCl (from 0.01 to 0.1 mol / dm 3< ) and applying a negative voltage of -10 or -30 V between the processed detail and the steel counter electrode for 5-15 s. Then, an anodic dyeing operation is carried out in a solution containing from 0.01 to 0.1 mol / dm 3< ofNaCl and from 3 to 15 g / dm 3< of anionic dye at a positive voltage of 2 to 11 V for 60-300 s.

[0028] The purpose of the invention is to develop a method for producing colored, porous oxide coatings on a metal substrate subjected to the process of plasma electrolytic oxidation in alkaline solutions of sodium or potassium silicates, phosphates, aluminates, or borates. The color of the treated surface is given by introducing organic solvent dyes inside the pores of the oxide coating. At the same time, it is possible to enrich the composition with the presence of corrosion inhibitors, which additionally provide an anti-corrosion effect on the obtained surface.

[0029] The essence of the invention is a method for obtaining colored oxide coatings on metal products by anodic oxidation in an aqueous electrolyte solution containing sodium or potassium hydroxide and a sodium or potassium salt of metasilicic or boric or metaphosphoric or orthophosphoric or hypophosphorous di-, tri- or poly-metaphosphoric acid or sodium aluminate or a mixture thereof with a total concentration not exceeding 10 wt%. and at a temperature maintained between 5 and 40°C during the process carried out with polarization at direct current in the voltage range between the product and the counter electrode from +150 to +750 V or pulsed in the range from -200 to 0 V and from +200 to +750 V, with a frequency of 25 to 10,000 Hz, a duty factor of 10 to 100%, wherein the current density flowing through the product is limited to a value of 2.5 to 50 A / dm 2< , and the process duration is between 5 and 120 min, preferably rinsed in deionized water and dried, characterized in that the product treated in this way is immersed in a dyeing solution at a temperature of 5 to 60°C for a time of 1 to 3600 s using ultrasound at a frequency of 20 to 120 kHz and a power in the range of 5 to 2000 W / dm 2< , preferably rinsed in deionized water and dried, and the thus colored and dried product is subjected to repeated anodic oxidation under the same conditions for a time of 5 to 1800 s.

[0030] The method according to the invention is also presented in Figure 1 and consists of forming an oxide coating on a metal substrate in the process of plasma electrolytic oxidation, wherein in the structure of the formed oxide one can distinguish a multitude of pores with dimensions ranging from the submicron scale to the size of a few micrometers, where additional operations are used to fill the free spaces in in the coating with dyeing substances, giving decorative effect to the finished product. After creating the oxide coating and cleaning of the pores from residual electrolyte after the PEO process, the metal product is immersed in a solution of a dye or dyes belonging to anthraquinone, azo, azine, quinoline, phthalocyanine, pyrene, or triarylmethane compounds and their metal complexes at a concentration of 1 to 100 g / dm 3< . The composition may be enriched with the presence of organic corrosion inhibitors, such as heterocyclic compounds (e.g. imidazoline or quinoline derivatives), fatty acids, quaternary amines, or naphthalene acids, also in the concentration range of 1 to 100 g / dm 3< . The solvent for the dyeing bath is a suitable organic solvent, e.g. ethanol, isopropanol, benzene, or toluene. After immersing the product previously treated in the PEO process in the dyeing bath, the temperature of which is maintained in the range of 5 to 60°C, an ultrasound generator is started with a frequency of 20 to 120 kHz and power in the range of 5 to 2000 W / dm 2< . Dyeing lasts from 30 to 3600 s, after which the ultrasound generator is switched off, and the dyed product is rinsed and dried. Finally, to prevent the washing out of organic substances introduced into the coating, the product is reintroduced into the plasma electrolytic oxidation system and the anodic treatment is continued for another 5 to 1800 s, after which the product is rinsed and dried. The oxide that grows in the next PEO process seals the previously dyed oxide coating, and the organic substances present in the system are partially destroyed because of the action of the plasma and the high voltage causing anodic oxidation. As described in the scientific publication "Coloration of the aluminum alloy surface with dye emulsions while growing a plasma electrolytic oxide coating" (S.-C. Yeh, D.-S. Tsai, J.-M. Wang, C.-C. Chou, Surf. Coat. Technol. 287 (2016) 61-66) and patent no. TWI571536B, it is possible to conduct the PEO process in such a way that at the appropriate moment of processing, emulsions containing solvent organic dyes are introduced into the system, which leads to the formation of uniformly colored oxide coatings. The drawback of the method presented by the inventors from Taiwan is the contamination of the PEO process bath with coloring substances each time, which causes the need to replace the solution before the next production cycle, because, as the authors themselves admitted, conducting the plasma electrolytic oxidation treatment in a solution containing dyes gives an unsatisfactory effect from the very beginning. The coatings produced in this way are unevenly colored, and the bath significantly changes its composition during subsequent processing cycles. This effect happens because during PEO the highest intensity of plasma microdischarges is usually observed at the very beginning of the process. Therefore, in this patent, the dye is introduced in a separate tank, which minimizes contamination of the bath. In addition, the dyes used in the solution are insoluble in water, so when the colored product is immersed back into the PEO electrolyte, it is not washed out into the bath. This part of the dye that can be destroyed in the additional PEO process is subjected to the action of plasma, and therefore mainly oxidized to simple products of oxidation of organic substances, i.e. water, carbon dioxide, nitrogen. Thanks to this, the electrolyte remains free from organic contamination. In other cited methods (in patents CN107217290A, CN107227479A, or CN107227480A), to achieve satisfactorily intense color, very long dyeing times measured in tens of hours are used. The method described in this patent allows one to obtain an intense color even after a few minutes of dyeing. Moreover, in comparison with the method from the previously described publications "Anodic dyeing of micro-arc oxidized aluminum with a cathodic pretreatment" (C.-M. Chen, H.-J. Chu, J.-L. He, Surf. Coat. Technol. 324 (2017) 92-98) and patents no. CN107338466B and TWI623651B, the solution described in this patent does not require the use of NaCl solution to activate the oxide coatings produced in the PEO process to color them. Although the Chinese authors did not present the results regarding the anti-corrosion properties of the coatings they prepared, it can be expected that anodic treatment in a solution containing aggressive chloride ions will lead to serious defects in the produced protective coatings. Example I. A workpiece made of magnesium alloy AZ91D was subjected to preliminary degreasing with isopropyl alcohol in an ultrasonic cleaner for 10 minutes and after drying it was immersed in an aqueous solution containing 5 g / dm 3< of sodium hexametaphosphate, 0.6 g / dm 3< of sodium metasilicate, 0.8 g / dm 3< of sodium aluminate and 2.8 g / dm 3< of potassium hydroxide at a temperature of 15°C. A stainless steel counter electrode was also immersed in the bath with the solution and the wires supplying current to the electrolytic system were connected. Then a unipolar pulse current was applied to the system with a maximum voltage of +440V, a limiting current density of 18 A / dm 2< , a frequency of 50 Hz, and a duty factor of 33%. The plasma electrolytic oxidation process was carried out for 40 minutes, after which the detail was removed from the solution and rinsed. The dried part was immersed in a solution of 50 g / dm 3< of nigrosin spirit soluble (Solvent Black 5) in ethanol at a temperature of 20°C. Dyeing was carried out using ultrasound at a frequency of 45 kHz and a power of 25 W / dm 2< for 5 minutes. After removal, the part was rinsed in deionized water, and then an additional process of plasma electrolytic oxidation was carried out under the same conditions for 4 minutes. As a result of the method, a dark purple coating (Fig. 2) with closed pores was obtained, which was visible under an electron microscope both in the planar view (Fig. 3) and in the cross-section (Fig. 4). Its thickness was 30.0±5.1 µm. The corrosion resistance of the protected substrate surface increased by 2 orders of magnitude as demonstrated by electrochemical impedance spectroscopy experiments in a solution containing 0.1 mol / dm 3< Na 2 SO 4 and 0.5 g / dm 3< NaCl, where the polarization resistance after 2 h of immersion for the unprotected surface was 4.5±0.3 kΩ·cm 2< , and for the surface with a colored oxide coating 219±27 kΩ·cm 2< . Example II. A workpiece made of aluminum alloy AW6061 was subjected to preliminary degreasing with acetone in an ultrasonic cleaner for 5 minutes and after drying it was immersed in an aqueous solution containing 12 g / dm 3< of sodium metasilicate, 3 g / dm 3< of sodium hexametaphosphate and 4 g / dm 3< of potassium hydroxide at a temperature of 20°C. A stainless steel counter electrode was also immersed in the bath with the solution and the wires supplying current to the electrolytic system were connected. Then a bipolar pulse current was applied to the system at a maximum positive voltage of +425 V and negative of -150 V, a limiting current density of +24 A / dm 2< and negative of -30 A / dm 2< , a frequency of 100 Hz and a duty factor of 100%. The plasma electrolytic oxidation process was carried out for 35 minutes, after which the part was removed from the solution and rinsed. The dried part was immersed in a solution of 25 g / dm 3< of nigrosin spirit soluble (Solvent Black 5) and 10 g / dm 3< of 8-hydroxyquinoline (quinoline derivative) as a corrosion inhibitor in ethanol at a temperature of 20°C. Dyeing was carried out using ultrasound at a frequency of 24 kHz and a power of 45 W / dm 2< for 10 minutes. After removal, the part was rinsed in deionized water, and then an additional plasma electrolytic oxidation process was carried out under the same conditions for 2 minutes. As a result of the method, a dark purple coating was obtained (Fig. 5). Its thickness was 36.0±2.3 µm. Corrosion resistance measurements of the alloy surface determined by the potentiodynamic polarization method in 3.5 wt% NaCl solution showed that the breakdown potential of the passive coating on the aluminum alloy before and after protection increased from -0.530±0.027 to 1.044±0.169 V (Fig. 6). Example III. A workpiece made of ZRE1 magnesium alloy was subjected to preliminary degreasing with isopropyl alcohol in an ultrasonic cleaner for 5 minutes and after drying it was immersed in an aqueous solution containing 8 g / dm 3< of sodium hexametaphosphate and 2 g / dm 3< of potassium hydroxide at a temperature of 15°C. A stainless steel counter electrode was also immersed in the bath with the solution and the wires supplying current to the electrolytic system were connected. Then a bipolar pulse current was applied to the system at a maximum positive voltage of +430 V and negative of -10 V, a limiting current density of +15 A / dm 2< and negative of -15 A / dm 2< , a frequency of 500 Hz and a duty factor of 25%. The plasma electrolytic oxidation process was carried out for 25 minutes, after which the part was removed from the solution and rinsed. The dried sample was immersed in a solution of 25 g / dm 3< of the commercial solvent dye Unisol Red BE (Solvent Red 129) and 10 g / dm 3< of palmitic acid in isopropyl alcohol at a temperature of 20°C. Dyeing was carried out using ultrasound at a frequency of 45 kHz and a power of 35 W / dm 2< for 3 minutes. After removal, the part was rinsed in deionized water, and then an additional plasma electrolytic oxidation process was carried out under the same conditions for 2 minutes. As a result of the method, a red coating was obtained ( Fig. 7). Its thickness was 38.7±1.7 µm. The contact angle for the surface treated only with PEO was 41.6±3.3°, while for the surface colored with the addition of palmitic acid, the value of this parameter increased to 120.9±2.3° ( Fig. 8). Example IV. A workpiece made of Ti6Al4V titanium alloy was subjected to preliminary degreasing with isopropyl alcohol in an ultrasonic cleaner for 5 minutes and after drying it was immersed in an aqueous solution containing 35 g / dm 3< of potassium hydrogen orthophosphate and 5 g / dm 3< of sodium hydroxide at a temperature of 15°C. A stainless steel counter electrode was also immersed in the bath with the solution and the wires supplying current to the electrolytic system were connected. Then a direct current was applied to the system at a maximum voltage of 350 V and a limiting current density of 10 A / dm 2< . The plasma electrolytic oxidation process was carried out for 5 minutes, after which the part was removed from the solution and rinsed. The dried part was immersed in a solution of 35 g / dm 3< Unisol Blue KFL (Solvent Blue 70) in toluene at a temperature of 18°C. Dyeing was carried out using ultrasound at a frequency of 45 kHz and a power of 75 W / dm 2< for 10 minutes. After removal, the part was rinsed in deionized water, and then an additional plasma electrolytic oxidation process was carried out under the same conditions for 1 minute. As a result of the method, a blue coating of 5.6±0.7 µm thickness was obtained, with roughness expressed as Ra = 2.55±0.22 µm and Rz = 17.65±1.07 µm (10% smoother surface than the initial undyed surface). The corrosion resistance of the coating is improved due to the presence of the dye, which was confirmed by measurements of linear polarization resistance in 3.5 wt% NaCl solution (an increase from the initial value of 14.4±1.9 to 115.4±2.8 MΩ·cm 2< ). Example V. A workpiece made of zinc alloy ZL5 was subjected to preliminary degreasing with isopropyl alcohol in an ultrasonic cleaner for 10 minutes and after drying it was immersed in an aqueous solution containing 18 g / dm 3< of sodium hexametaphosphate, 32 g / dm 3< of potassium dihydrogen orthophosphate, 30 g / dm 3< of sodium aluminate, 28.3 g / dm 3< of sodium tetraborate and 3.1 g / dm 3< of sodium hydroxide at a temperature of 5°C. A stainless steel counter electrode was also immersed in the bath with the solution and the wires supplying current to the electrolytic system were connected. Then a pulse current was applied to the system at a maximum positive voltage of +150 V, a limiting positive current density of +2.5 A / dm 2< , a frequency of 25 Hz, and a duty factor of 10%. The plasma electrolytic oxidation process was carried out for 5 minutes, after which the part was removed from the solution and rinsed. The dried part was immersed in a dyeing solution at a temperature of 5°C for 1 s using ultrasound at a frequency of 20 kHz and a power of 5 W / dm 2< , and dried. Prepared in this way detail was subjected to repeated anodic oxidation under the same conditions for 5 s. The dyeing solution contains: one solvent dye at a concentration of 0.1 g / dm 3< , belonging to the group of anthraquinone compounds, in the form of 1-hydroxy-4-(4-methylanitino)anthracene-9,10-dione (Solvent Violet 13) one corrosion inhibitor of the substrate material at a concentration of 0.1 g / dm 3< , from the group of fatty acids in the form of lauric acid the rest is a solvent, which is isopropanol. As a result of the method, an oxide coating with a thickness of 2.52±0.15 µm and a light purple color was obtained. The presence of both the dye and the fatty acid caused the surface wettability angle relative to demineralized water to increase from 15.2±6.1 ° for the undyed surface to 84.6±2.3 °. Example VI. A workpiece made of the Zircaloy 2 zirconium alloy was subjected to preliminary degreasing with isopropyl alcohol in an ultrasonic cleaner for 2 minutes, and after drying it was immersed in an aqueous solution containing 22.1 g / dm 3< of sodium metasilicate, 11.2 g / dm 3< of potassium hydrogen orthophosphate, 7.5 g / dm 3< of sodium diphosphate, 9.4 g / dm 3< of sodium aluminate and 2.5 g / dm 3< of potassium hydroxide at a temperature of 20°C. A stainless steel counter electrode was also immersed in the bath with the solution and the wires supplying current to the electrolytic system were connected. Then a bipolar pulse current was applied to the circuit with a maximum positive voltage of +500 V and negative of -75 V, a limit current density of +25 A / dm 2< and negative of -25 A / dm 2< , a frequency of 5000 Hz and a duty factor of 50%. The plasma electrolytic oxidation process was carried out for 70 minutes, after which the detail was removed from the solution and rinsed. The product treated this way was immersed in a dyeing solution at a temperature of 35°C for 1800 s using ultrasound at a frequency of 78 kHz and power in the range of 1000 W / dm 2< , rinsed in deionized water and dried, and the detail so colored and dried was subjected to repeated anodic oxidation under the same conditions for 1000 s. The dyeing solution contains: two solvent dyes at a concentration of 50 g / dm 3< each, where the solvent dye is Unisol Orange 2RV (Solvent Orange 62) and Unisol Black RE (Solvent Black 27) two corrosion inhibitors at a concentration of 50 g / dm 3< each, where the corrosion inhibitor is acrylic acid and stearic acid the rest is the solvent, which is ethanol. As a result of the method, an oxide coating thick at 72.3±4.5 µm with a dark brown color was obtained. The presence of corrosion inhibitors, as well as dyes, allowed one to obtain a hydrophobic surface with a wetting angle of 115.3±2,0° for demineralized water. The angle of droplet slippage from this surface was 37.2±4.9°.

Examples

Embodiment Construction

. A workpiece made of magnesium alloy AZ91D was subjected to preliminary degreasing with isopropyl alcohol in an ultrasonic cleaner for 10 minutes and after drying it was immersed in an aqueous solution containing 5 g / dm 3Fig. 2) with closed pores was obtained, which was visible under an electron microscope both in the planar view (Fig. 3) and in the cross-section (Fig. 4). Its thickness was 30.0±5.1 µm. The corrosion resistance of the protected substrate surface increased by 2 orders of magnitude as demonstrated by electrochemical impedance spectroscopy experiments in a solution containing 0.1 mol / dm 3Fig. 5). Its thickness was 36.0±2.3 µm. Corrosion resistance measurements of the alloy surface determined by the potentiodynamic polarization method in 3.5 wt% NaCl solution showed that the breakdown potential of the passive coating on the aluminum alloy before and after protection increased from -0.530±0.027 to 1.044±0.169 V (Fig. 6). Example III. A workpiece made of ZRE1 magnesium a...

Claims

1. Method of obtaining colored oxide coatings on metal products by anodic oxidation in an aqueous electrolyte solution containing sodium or potassium hydroxide and a sodium or potassium salt of metasilicic or boric or metaphosphoric or orthophosphoric or hypophosphorous di-, tri- or poly-metaphosphoric acid or sodium aluminate or a mixture thereof in a total concentration not exceeding 10 wt% and at a temperature maintained between 5 and 40°C during the process carried out with polarization at direct current in the voltage range between the product and the counter electrode from +150 to +750 V or in a pulsed range from -200 to 0 V and from +200 to +750 V, with a frequency from 25 to 10,000 Hz, a duty factor from 10 to 100%, where the current density flowing through the product is limited to a value of 2.5 to 50 A / dm2 and the duration of the process is between 5 and 120 min, preferably rinsed in deionized water and dried, is characterized in that the product treated in this way is immersed in a dyeing solution at a temperature of 5 to 60°C for a time of 1 to 3600 s using ultrasound at a frequency of 20 to 120 kHz and a power in the range of 5 to 2000 W / dm2, preferably rinsed in deionized water and dried, and the product thus colored and dried is subjected to repeated anodic oxidation under the same conditions for a time of 5 to 1800 s, wherein the dyeing solution contains: - one or two solvent dyes in a concentration of 0.1 to 100 g / dm3, where the solvent dye is a substance belonging to the group of anthraquinone, azo, azine, quinoline, phthalocyanine, pyrene, or triarylmethane compounds or their metal complexes, - one or two corrosion inhibitors of the substrate material in a concentration of 0.1 to 100 g / dm3, where the corrosion inhibitor is a compound from the group of fatty acids or quinoline or triazole derivatives or acrylic acid, poly(acrylic acid), poly(ethylene glycol) or poly(vinyl alcohol), - a solvent that is ethanol, isopropanol, acetone, dichloromethane, chloroform, hexane, benzene, or toluene.

2. The method according to claim 1, is characterized in that the metal product contains Be, Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W or any mixture thereof, preferably being a magnesium alloy or an aluminum alloy or a titanium alloy.

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