METHOD OF PRODUCING POROUS OXIDE LAYERS ON ALUMINUM CONTAINING POLYMERIC CORROSION INHIBITORS

Description

[0001] The subject of the invention is a method of producing porous oxide layers containing corrosion inhibitors formed by plasma electrolytic oxidation on aluminum and its alloys.

[0002] Aluminum and its alloys arc a group of metallic construction materials used in industries, where strength-to-weight ratio serves an important role. Parts made of aluminium and its alloys weight about 3 times less than those made of steel. For example, the benefit obtained from reducing the total weight of the vehicle allows for a reduction in specific fuel consumption, which makes these types of materials more and more popular in the production of automotive parts. Also the interest of automotive concerns in fully electric cars increases demand for lightweight constructions. Aluminum alloys are also eagerly used in aviation. Aluminum alloys with sufficiently high mechanical strength are obtained primarily by precipitation hardening. The disadvantage of this approach is a significant deterioration of the corrosion resistance of the aluminum alloy in relation to the material with a low alloying elements content. For this reason, appropriate corrosion protection measures have been adopted for construction materials based on aluminium. The most widespread method in the case of aluminum parts is anodizing conducted in acidic baths (often rich in chromium VI compounds). Recently, a relatively new technology of surface treatment of metals and alloys, called plasma electrochemical oxidation, has become more and more popular. The technique uses environmentally friendly dilute alkaline baths containing sodium and/or potassium metasilicate, orthophosphate, polyphosphates or aluminates and sodium hydroxide to form oxide ceramic coatings. This type of coatings is characterized by high hardness, wear resistance and improved corrosion resistance compared to the unprotected substrate. In addition, the methodology used in this process is not very complicated, as it does not require detailed surface preparation before the process and many stages of supplementary treatment to obtain results comparable or even better than those in the case of conventional anodizing. However, the oxide coatings obtained by plasma electrochemical oxidation have a relatively high porosity, which makes their corrosion resistance under operating conditions usually unsatisfactory. At the same time, the porous structure allows the properties of the top layer to be modified by impregnating it. This enables the production of self-repairing coatings which, in the face of the loss of layer continuity, release compounds from the inside of the layer that slow down the corrosion process or result in passivation of the substrate, e.g. corrosion inhibitors.

[0003] Patent No. EP1820882A1 describes a method of protecting the surface of non-ferrous metals by producing a self-healing conversion coating by conventional anodizing, hard anodizing or plasma electrochemical oxidation in baths containing polyoxometaiates (e.g. [SiMO₁₂O₄₀]^4-, [CeMo₁₂O₄₂]^8-, [SiW₁₂O₄₀]^4-, [PW₁₂O₄₀]^3-) and self-healing agent based on mixed oxide particles e.g. CaO·Al₂O₃ and CaO·2Al₂O₃ Sealing of the prepared surface is ensured by applying an external epoxy coating.

[0004] Patent no. CN101469425A discloses a method of modifying the oxide coating on a magnesium alloy obtained in the plasma electrochemical oxidation process, based on immersion in an acrylic acid solution for 0.5 to 2 hours at a temperature of 80 ± 3 ° C. Obtained coating is spin-coated with a layer of polyvinyl dimethyl siloxaue) and alumoxane which gives a superhydrophobic effect. Patent No. US20120031765A1 describes a method for protecting elements of valve metal alloys (e.g. magnesium, aluminum, titanium, tantalum, zirconium, hafnium, niobium, beryllium alloys) against corrosion by treatment with the plasma electrochemical oxidation method followed by immersion in an acid solution of fluorozirconate (50 g / dm³) for 2 minutes, drying for 1 h at 70 ° C and final powder coating based on a polyester. The samples prepared in this way can withstand up to 2000 h in salt spray chamber with acetic acid (procedure in accordance with ISO 9227).

[0005] From the patent No. RU2528285C1 and the scientific publication "Modification of plasma electrolytic coatings on aluminum alloys with corrosion inhibitors" (S.V. Oleinik, V.S. Rudnev. A.Yu. Kuzenkovt T.P. Yarovaya, L.F. Trubetskaya. P.M. Nedozorov, Prot. Met Phys. Chem. Surf. 49 (7) (2013) 885-890) a method of obtaining oxide layers enriched with the addition of the corrosion inhibitors IFKhAN-39 and lFKhAN-25 (both based on oleate ions) on aluminum alloys is known. The treatment based on creating oxide layers on Russian alloys from the AMg-5, D16, V95 and AMtsM series by treatment with plasma electrochemical oxidation in a solution containing 90 g/dm³ Na₃PO₄·12H₂O, 26 g/dm³ Na₂B₄O_7·12H₂O and 4 g/dm³ Na₂WO₄·H₂O. The coating prepared in this way was further modified by immersion in aqueous solutions containing 1 g/dm³ of IFKhAN-39 or IFKhAN-25 at a temperature of 95-100°C for 50-60 minutes. Obtained oxide layers had a thickness between 8 and 15 µm. In the case of using the lFKhAN-25 inhibitor, the breakdown potential of the layer measured in a 0.01 mol/dm³ NaCl solution increased from -0.1 V vs. SHE for the unprotected alloy AMtsM up to 0.8 V vs. SHE.

[0006] Patent no. USS9765440B2 describes a method of preparing the surface of the AW-6082 alloy, from which the silicon etching chamber was made. Gaseous SiFf₆ is used in the etching process. As a result of treating the AW-6082 alloy with plasma electrochemical oxidation in a solution of 2 g/dm³ KOH and 1 g/dm³ Y(NO₃)₃, a surface enriched with anti-corrosive yttrium ions was obtained. As a result, the etching rate of the oxide layer under the operating conditions of the chamber was reduced from 85 nm/min obtained for the AW-6082 alloy when hard anodized, to 10 nm / min.

[0007] Patent No. CN III 5793A discloses a method of modification of aluminum surfaces by means of the plasma electrolytic oxidation process in baths containing borates, phosphates and silicates with the addition of rare earth metal salts and acrylic acid salts. The coatings obtained in this way take different colors, depending on the composition of the bath. After the electrochemical process, the obtained layers are subjected to an additional sealing process by covering them with a water-soluble acrylic resin and subsequent heating at a temperature of 150°C to 250°C for 5 to 30 minutes.

[0008] From the scientific publication "Elcctropolymerization of acrylic acid on carbon fibers for improved epoxy / fiber adhesion" (Bauer, A., Meinderink, D., Giner. I., Steger, H., Weitl, J., & Grundmeier, G. Surface & Coatings Technology 321 (2017) 128-135) a method for electropulymcrizing acrylic acid on the surface of carbon fiber materials is known. The coated part is cathodically polarized for approx. 10 minutes in a bath containing acrylic acid with the addition of a polymerization inhibitor, zinc salts and a polymerization initiator (e.g. N, N'-methylenebisacrylamide). In the first stage of the process, zinc-acrylic complexes are formed in the solution, which arc reduced on the surface of the workpieoe. Over time, more and more complexes are reduced at the cathode surface, resulting in a reduction in the concentration of the acrylic acid polymerization inhibitor at the electrolyte-cathode interface. Due to the absence of the polymerization inhibitor at the interface and the presence of the initiator in the solution, a layer of poly(acrylic acid) (PAA) is formed on the surface of the workpiece. The length of the PAA chain and the thickness of the layer increase with the increase of the process time.

[0009] The scientific publication "Graft co-polymerization of acrylic acid onto the linen surface induced by DBD in air" (C.S. Reo, D.Z. Wang, Y.N. Wang Surface & Coatings Technology 201 (2006) 2867-2870) describes the method of grafting acrylic acid polymer into flax fibers. The process can be divided into two main stages. In the first one, flax fiber undergoes the DBD (dielectric barrier discharge) process, in which active radicals are formed on the surface of the fiber. In the next stage of the process, the previously prepared fiber is immersed in an acrylic acid solution and then placed in boiling distilled water for 8 hours. In this way, the modified surface of the flax fibers is more dyeable due to the greater wettability achieved by the presence of the acrylic acid polymer.

[0010] Patent No. CN107190302B discloses a method for modifying the oxide layer obtained in the process of plasma electrochemical oxidation by placing it in a vacuum for 5-50 minutes, after which an inorganic inhibitor solution is added to the vacuum chamber, e.g. phosphate, fluorine silicate, vanadium salt and/or cerium or an organic inhibitor, e.g. 8-hydroxyquinoline or thiazole or imidazole derivatives. Finally, the layer is sealed with an epoxy coating.

[0011] From the scientific publication "PEO coatings with active protection based on in-situ formed LDH-nanoconiainers" (M. Serdechnova, M. Mohedano, B. Kuznetsov, C.L. Mendis, M. Starykevich, S. Karpushenkov, J. Tedim, M.G.S. Ferreira, C Blawert, M.L. Zheludkevich, J. Electrochem. Soc. 164 (2) (2017) C36-C45) there is a known method of modifying the surface of the AW-2024-T3 aluminum alloy by forming an oxide layer by plasma electrochemical oxidation and further securing it by creating layered double hydroxides (LDH) containing Al and Zn ions. LDHs can serve as nanometric reservoirs, which on the one hand absorb aggressive chloride ions from the corrosive environment, and on the other hand are able to desorb the inhibitor ions. In the described method, LDH was synthesized in-situ by contacting the oxide layer with a solution of 0.1 mol/dm³ Zn(NO₃)₂ and 0.6 mol/dm³ NH₄NO₃ with pH = 6.5 (adjusted by adding 1% NH₃ solution) for 30 minutes at 95°C. LDH-modified oxide layers were very effective when VO₃^- ions were used as an inhibitor, introduced into the coating by ion exchange with 0.1 mol/dm³ NaVO₃ solution at pH = 8.4 for 30 minutes at 50 ° C. The patent no. CN108330472A discloses a method for producing an LDH layer directly on magnesium alloys. Before protection, the surface is mechanically processed (grinding/polishing) and degreased. Then the surface is immersed in a solution containing a divalent ion (e.g. Mg²⁺), a trivalent ion (e.g. Al³⁺) and a passivator (e.g. K₂MoO₄) in a molar ratio of 6:2:1 with a pH of 12. The hydrothermal formation of the layer is carried out by at 110-160 ° C for 12-72 h. The surface is additionally immersed in a 2.5-5% aqueous solution of the corrosion inhibitor for 12-72 h.

[0012] The aim of the invention is to develop a method that allows to obtain porous oxide layers on a substrate of aluminum or its alloys enriched with a polymeric corrosion inhibitor formed during the plasma electrolytic oxidation process, to be incorporated into the pore structure, which will allow to provide the effect of an smart self-repairing layer.

[0013] The essence of the invention is a method of forming oxide layers in which an aluminum or aluminum alloy element is subjected to anodic polarization in a silicate bath preferably containing one of three salts: sodium or potassium silicate with a concentration of 1 to 120 g/dm³, sodium or potassium tetraborate with a concentration of 1 to 120 g/dm³ and potassium or sodium hexametaphosphate with a concentration from 2 to 240 g/dm³ or a mixture of these salts and sodium or potassium hydroxide with a concentration of 0.3 to 30 g/dm³, conducted under direct current or pulsed (monopolar or bipolar) current while maintaining the current density from 0.1 to 50 A/dm², frequency from 50 to 10 000 Hz, positive voltage from 250 to 800 V and negative voltage from 0 to -150 V, for 5 to 120 minutes, preferably rinsed in distilled water and dried, characterized in that the element so oxidized is immersed in the liquid monomer of the polymeric corrosion inhibitor for a period of 1 to 600 s and is allowed to dry after being drawn out. Preferably, the monomer of the polymeric corrosion inhibitor is acrylic acid or vinyl acetate. Preferably, after the drying process, the material is anodically re-polarized in a silicate, borate, phosphate bath or a mixture of these salts under direct current conditions to a voltage of 250 to 800 V for 5 to 600 s, preferably rinsed in distilled water and then dried.

[0014] The method covered by the invention consists in the plasma electrochemical oxidation of aluminum or its alloy in alkaline solutions of sodium or potassium silicates, sodium or potassium tetraborates or sodium or potassium hexaphosphates or their mixture with additional anodic treatment after impregnating the previously formed oxide layer with an organic monomer polymeric substrate corrosion inhibitor. Additional anodic treatment, carried out in the same bath as the formation of the oxide layer by plasma electrochemical oxidation, aims to build up a corrosion inhibitor through accelerated passivation of the surface resulting from the flowing electric current, which also leads to the formation of plasma. Plasma micro-discharges taking place during additional treatment are characterized by a very low intensity, which allows for partial covering of the pores formed in the first stage of anodic treatment and avoiding excessive thermal degradation of organic compounds. The introduction of a monomeric form of a corrosion inhibitor between the first stage of treatment, aimed at giving the appropriate geometric and functional characteristics to the layer, and the second stage of sealing, allows to avoid contamination of the electrolyte bath with a large amount of degradation products of organic compounds and is beneficial from the point of view of the economic use of the compound and the reduction of the amount of environmentally harmful waste. In addition to building up the pores, micro-discharges generate local radicals that can initiate the polymerization process of monomers to the proper form of the inhibitor, which ultimately gives the formed layer the property of active anti-corrosion protection. If the monomer is acrylic acid, its polymerization allows the formation of poly (acrylic acid) chains. On the other hand, when the monomer is vinyl acetate, the polymerization first produces polyvinyl acetate), which then hydrolyzes in the presence of an alkaline bath environment to poly(vinyl alcohol). The surface of aluminum or its alloy prepared in such a way maintains corrosion resistance in an aggressive environment, even if it is damaged, which causes the inhibitor to be released from the inside of its pores. The thus described method of incorporating a corrosion inhibitor into the structure of the oxide layer formed on the surface of aluminum or its alloy translates into an increase in the corrosion resistance of the protected metal substrate, clearly different from that described in the patent RU2528285C1 dated 10/09/2014. In that patent, the sealing procedure consisted of successively creating an oxide layer using the PEO method, immersion in an aqueous solution of aliphatic (fatty) or aromatic carboxylic acids and sodium oleate at a temperature of 95 to 100°C for 50 to 60 minutes, finishing with the hydrophobization of the surface by the immersion in a solution of poly(tetrafluoroethylcne) in ethyl acetate. Contrarily to the described procedure, the method described in the present patent does not use elevated temperature. The consumption of organic compounds that may have a negative impact on human health and the environment is drastically reduced by the isolated application of the polymer (not in the same solution as the PEO coating bath), and the duration of the inhibitor application operation is also shortened. In addition, only dilute solutions of inorganic compounds are used to seal the layer. The patent PL238402 "Method for the production of porous oxide layers containing corrosion inhibitors" (M.Sowa, W. Simka, M. Wala) describes a similar method, where the corrosion inhibitors 8-hydroxyquinoline, 2-mercaptobenzouuaxole, po.ly(acrylic acid) or sodium lauryl sulfate dissolved in an amount of 1 to 10 g / dm³ in ethyl alcohol are applied to oxidized elements made of aluminum, magnesium, aluminum alloy or magnesium alloy by dripping or spraying, then the elements are dried and immersed in a silicate bath and polarized to high voltage in order to incorporate inhibitors in the structure of the formed oxide film. The present solution differs significantly from the aforementioned method, since pure substances are used in the process of introducing the corrosion inhibitors, not alcoholic solutions. Moreover, these inhibitors are formed by anodic polarization induced polymerization and/or post-reactions in the alkaline environment of an electrolyte bath, and are not limited to the partial closure of the pores in the oxide film.

[0015] As a result of the method, we obtain an oxide layer with a structure composed of two sublayers, i.e. an outer sublayer with high porosity (up to 20%) and an internal, dense layer adjacent to the metallic substrate, Its thickness is typically between 5 and 200 µm, with the thickness of the inner sublayer typically being about 50% of the total thickness. In the case of an aluminum-based substrate layer, the two phases of alumina, i.e. alpha (corundum) and gamma, can be detected therein. In addition, the formation of mullite (3Al₂O_2·2SiO₂) is possible when using a high concentration of silicate salt or a high current density. If sodium or potassium tetraborate is present in the bath, boron oxide (B₂O₃) or borate ions can also be identified. Baths containing sodium or potassium hexametaphosphate are enriched with orthophosphates. The outer part of the high porosity layer is used as a reservoir for the immersion of the polymeric monomers of the corrosion inhibitors. After evaporation of the solvent and electrochemical sealing, the layer does not change in terms of surface morphology and geometric dimensions, however, it increases the corrosion resistance offered by the oxide layer by additional passivation of the surface due to the presence of a corrosion inhibitor.

[0016] Example 1. An aluminum workpiece of the AW 7020 grade is pre-ground and degreased in isopropyl alcohol using ultrasound, and after evaporating the solvent, it is immersed in a solution containing 10 g/dm³ Na₂SiO₂, 12 g/dm³ Na₂B₄O₇ and 2.8 g/dm³ KOH. After the electrical connections are brought to the workpiece and the stainless steel counter electrode, its polarization begins under the conditions of 50 Hz alternating current, square wave, with peak anodic current equal to 33 A / dm² and cathodic current 33 A / dm². This treatment is carried out for 1 h. After the initial layer formation, the workpiece is rinsed with distilled water and air dried. The surface is then saturated with pure acrylic acid and the surface is allowed to dry completely. Next, the workpiece is immersed in the same alkaline solution of sodium silicate and sodium tetraborate and subjected to anodic polarization to a cell voltage of 400 V increasing linearly in 60 seconds. After reaching the maximum voltage, the workpiece is kept under these conditions for another 120 seconds, the element is taken out of the bath. rinsed in distilled water and air dried.

[0017] As a result of the process, a layer with a thickness of 65.3 µm was formed, the chemical composition of which, determined on the basis of energy-dispersive X-ray spectroscopy, confirmed the presence of such elements as Al, B, O, Na and K. Corundum was also detected (by XRD) in the sample, indicating significant surface wear resistance. The corrosion resistance tests carried out in a 0.1 M NuCl solution showed a shift of the corrosion potential towards the positive potential (in relation to the surface without the inhibitor) by over 100 mV and a reduction of the passivation current from 1 120 nA / cm² to 215 nA / cm².

[0018] Example II. A workpiece made of AW 6061 aluminum alloy is initially ground and degreased in isopropyl alcohol using ultrasound, and after evaporation of the solvent, it is immersed in a solution containing 20.21 g / dm³ Na₂B₄O₇ and 5.6 g / dm³ KOH. After the wires are connected to the workpiece and the counter electrode made of stainless steel, its polarization begins in the conditions of bipolar impulse current with a frequency of 5 000 Hz, with peak values of anode current equal to 22 A/dm³ and cathode current of 27.5 A/dm³. This treatment is carried out for 45 minutes. After the formation of the layer in the PEO process, the workpiece is rinsed in distilled water and dried in the air. The element is then immersed in 50 ml of acrylic acid for 1 minute. After the workpiece has surfaced from the solution, it is air-dried for 20 minutes. Next, the workpiece is immersed in the same alkaline sodium tetraborate solution and subjected to anodic polarization to a cell voltage of 450 V increasing linearly over a period of 120 s. After reaching the maximum voltage, the part is kept under these conditions for another 200 s. After the process is completed, the workpiece is removed from the bath, rinsed in distilled water and air dried.

[0019] As a result of the process, a layer was formed, the thickness of which was determined to be 20.2 µm, its composition contained Mg, Al, Si, B, K, Na and O. The high porosity of the resulting layer favored wetting its surface with an alcoholic solution of the inhibitor. After applying and sealing the layer with an inhibitor, in the corrosion resistance tests in a 3.5% NaCl solution, an increase in the polarization resistance of the samples was observed by one order of magnitude after the addition of the inhibitor to the layers and its sealing by electrochemical means.

[0020] Example III. A workpiece made of AW 1050A aluminum (99.5% pure aluminum) is initially ground and degreased in isopropyl alcohol with the help of ultrasound, and after evaporation of the solvent, it is immersed in a solution containing 16.5 g/dm³ Na₂SiO₃ and 4.8 g/dm³ of KOH. After the wires are led to the workpiece and the stainless steel counter electrode, its polarization begins in the conditions of bipolar pulsed current with a frequency of 500 Hz, with peak values of anode current equal to 27.5 A/dm² and cathode current of 36.5 A/dm². This treatment is carried out for 50 minutes. After the initial layer formation, the workpiece is rinsed with distilled water and air dried. The surface is then saturated with vinyl acetate and allowed to dry. Next, the element is immersed in the same alkaline sodium silicate solution and subjected to anodic polarization to a cell voltage of 425 V increasing linearly over 60 s. After reaching the maximum voltage, the element is held under these conditions for another 240 s. After the process is completed, the workpiece is removed from the bath, rinsed in distilled water and air dried.

[0021] As a result of the process, a layer was formed, the thickness of which was 36.0 µm. in thickness. The analysis of the chemical composition of the layers showed the presence of elements present in the electrolyte bath (Si, O, Na, K) as well as a substrate made of technically pure aluminum (Al). Apart from aluminum oxide (in gamma and alpha variants), the mullite phase (3Al₂O₃-2SiO₂) was found in the layer. The poly(vinyl alcohol) present in the layer contributed to the increase in the corrosion resistance of the substrate, which was found on the basis of the increase in polarization resistance measured in a 0.1 M sodium sulphate solution with the addition of 500 ppm NaCl from 2.64 to 34.7 MΩ·cm² compared to the layer not modified with vinyl acetate.

[0022] Example IV. The AW 2024-T6 aluminum alloy, after prior grinding and degreasing, is subjected to anodic polarization in a silicate bath containing sodium silicate at a concentration of 80 g/dm³, sodium hexametaphosphate at a concentration of 120 g/dm³ and sodium hydroxide 12 g/dm³ conducted under direct current conditions while maintaining current density 20 A/dm³, positive voltage 500 V, for 45 min. Then it is rinsed in distilled water, dried and the material prepared in this way is immersed in acrylic acid and then dried. The material is then anodically re-polarized in a silicate bath under direct current conditions to 485 V for 300 s and then dried. The monomer of the polymeric inhibitor is acrylic acid.

[0023] As a result of the process, a layer with a thickness of 102.3 µm was created, characterized by high roughness. It consisted of a highly porous top part (capable of absorbing the monomer) and a barrier oxide layer in the immediate vicinity of the metal substrate. The layers were brown before applying the inhibitor (coming from the copper present in the alloy). The layer formed after being subjected to corrosion tests in 0.1 M NaCl showed a shift of the corrosion potential from the value of --900 mV vs. SCE up to -400 mV vs. SCE and lowering the passivation current from 453.3 nA1cm² to 3.2 nA/cm².

[0024] Example V. An aluminum part of the AW 1050A grade (99.5% pure aluminum) is subjected to anodic polarization in a silicate bath containing potassium hexametaphosphate with a concentration of 123 g/dm³ and potassium hydroxide with a concentration of 5.6 g/dm³, in the pulsed regime with maintaining the current density 0.5 A/dm², frequency 9000 Hz, positive voltage 700 V and negative voltage -150 V, for 60 minutes, rinsed in distilled water and dried at ambient temperature. Vinyl acetate is applied to this polarized material by immersion and then dried at ambient temperature. After the drying process, the material is subjected to another anodic polarization in a hexametaphosphate bath under direct current conditions to a voltage of 600 V for 600 s, rinsed in distilled water, and then dried in the ambient temperature for 120 minutes.

[0025] As a result of the process, the layer was 185.3 µm thick (including 72.2 µm constituted a barrier layer with a highly porous part of the coating above it). The barrier sublayer consisted mainly of alumina. Corundum was detected in the layer. The corrosion tests showed that in comparison with the non-oxidized surface (pure alloy;, the layer to be sealed was much more resistant to corrosion after 2 hours of immersion in 3.5% NaCl solution. The polarization resistance determined for the surface without the layer was 511.3 kQ · cm², and for the fully protected surface was 118.2 MΩ · cm².

Claims

1. A method of forming oxide layers, where an element made of aluminum or its alloy is subjected to anodic polarization in a silicate, borate, hexametaphosphate bath or a mixture thereof, preferably containing sodium or potassium silicate with a concentration from 1 to 120 g/dm³, sodium or potassium tetraborate with a concentration from I to 120 g/dm³, sodium or potassium hexametaphosphate with a concentration of 1 to 240 g/dm³ and sodium or potassium hydroxide with a concentration of 0.3 to 30 g/dm³, where polarization is carried out in the direct current or pulsed current regimes while maintaining the current density from 0.1 to 50 A/dm², frequency from 50 to 10,000 Hz, positive voltage from 250 to 800 V and negative voltage from 0 to -150 V, for 5 to 120 minutes, preferably rinsed in distilled water and dried characterized in that the element so oxidized is immersed in the liquid monomer of the polymeric aluminum corrosion inhibitor, then extracted and dried.

2. The method according to the process of claim 1, wherein the monomer is acrylic acid.

3. The method according to the process of claim 1, wherein the monomer is vinyl acetate.

4. The method according to claims from 1 to 3, characterized in that after the drying process, the material is re-anodized in a silicate, borate, hexametaphosphate bath or their mixture under direct current conditions to a voltage from 250 to 800 V for 5 to 600 s, preferably rinsed in water distilled and then dried.