Production process of a zinc conversion film for corrosion protection

Description

PRODUCTION PROCESS OF A ZINC CONVERSION FILM FOR CORROSION PROTECTION Field of Invention

[001] The present invention discloses a green process for applying an inorganic coating to a zinc surface, providing this surface with greater protection against corrosion. The present invention falls within the fields of Chemistry, Materials Engineering, Chemical Engineering and Metallurgical Engineering, more specifically in surface treatment and corrosion protection. Background of the Invention

[002] Galvanizing and zinc electroplating have long been widely applied methods of corrosion protection, primarily for the protection of steel. Zinc metal has superior atmospheric corrosion resistance to steel due to the formation of hydroxides, carbonates, and basic carbonates that are more protective than the respective corrosion products of steel. Increased corrosion resistance can be achieved through so-called conversion coatings. These coatings form without external polarization of the metal surface, solely through contact between the metal and the chemical bath. The resulting chemical and / or electrochemical reactions convert the zinc metallic surface into a film with properties different from the oxide film formed naturally in air.

[003] The most efficient and widely used treatment is based on baths containing chromate ions. This ion acts as an oxidant during the conversion process, resulting in the reduction of Cr(VI) to Cr(III) in the formation of Cr(OH)3 films, which provide good corrosion resistance.

[004] The basic reactions of the process are then: (a) Zn ^ Zn2++ 2e- (b) CrO42-+ 3e-+ 5H+^ Cr(OH)3 + H2O Petition 870250059100, dated 11 / 07 / 2025, page 12 / 27 2 / 12

[005] The oxidation of the substrate by chromate also promotes the incorporation of zinc ions into the coating. Even with reactions (a) and (b), there is still a surplus of residual Cr(VI) ions that are incorporated into the interior of the conversion layer, enabling the curing of defects that may have been caused in the film.

[006] In this case, reduction of Cr(VI) exposed by the damage can form Cr(OH)3 according to reactions (a) and (b) and recover the conversion layer. As chromate-based conversion films often show cracks as a morphological characteristic (cracked earth appearance), this self-healing mechanism is essential for long-lasting corrosion resistance. In addition, chromate also plays an important role as an inhibitor of oxygen reduction on the metal.

[007] However, due to environmental problems and the recognized toxicity and mutagenic and carcinogenic properties of chromate, Cr(VI)-based processes have been largely replaced, in most cases, by processes based on baths containing Cr(III) ions. The problems manifest themselves in practice through the handling of chromate baths and the subsequent exposure of workers, through the disposal of baths containing chromate, and through wash waters with diluted chromates.

[008] Unlike chromate, Cr(III) ions are not oxidants. Chromate is replaced in its substrate oxidant function generally by the addition of nitrate or H2O2 to the Cr(III) bath. In this way, these processes also allow the formation of solid compounds containing Cr and Zn by precipitation, but not from the electrochemical reaction (b). With the elimination of chromate, the conversion layer also loses its self-healing capacity. Cr(III) ions also do not act as inhibitors of oxygen reduction. For these reasons, it is essential for Cr(III)-based films to have a more homogeneous morphology, with as few defects as possible. Petition 870250059100, dated 11 / 07 / 2025, page 13 / 27 3 / 12

[009] In some military and aeronautical applications, Cr(VI) continues to be used in first world countries, even though it is highly toxic, due to the corrosion protection power of chromate treatment.

[0010] Patent documents such as US2006 / 0266438A1, JP2009041092(A), US4578122, US4359345, which teach about conversion layers on the Cr(III) base, mention a range of additives, such as Co, Ce, Mn, Fe, Ni, Mo, La ions, among others. And, generally, the function of these additives is described as “activation” of the surface.

[0011] Patent documents US2006 / 0266438A1, JP2009041092(A), US4578122, US4359345, as well as others, use baths containing the Cr(III) compound and activators, some of which supposedly also act as Cr(III) oxidants. In this way, if Cr(VI) (chromate) is formed, it will occur within the bath. Even assuming that the metallic surface favors the electrochemical reaction and some of the Cr(VI) remains in the conversion film, some of the Cr(VI) will remain in the bath, which, at the end of its useful life, becomes contaminated with appreciable amounts of Cr(VI). It is also important to point out that several ions used in these patents, such as those mentioned above, are electrochemically active and react with each other and with the oxidizing agent. As a consequence, less stable oxidants, such as H2O2, then decompose through parallel reactions and not by oxidizing the substrate. This therefore requires a more rapid replacement of the bath contaminated with Cr(VI).Generally, it becomes difficult to maintain the properties of the bath within tolerable limits for extended periods.

[0012] In contrast, the present invention overcomes these problems of the prior art by separating the steps of formation of the Cr(III) ion-based conversion layer and the subsequent oxidation of Cr(III) within the conversion layer by contact in additional treatments with a Cr(III) oxidizing agent. It is then possible to oxidize Cr(III) to Cr(VI) within the conversion layer and thus preserve the self-healing capacity without the use of baths. Petition 870250059100, dated 11 / 07 / 2025, p. 14 / 27 4 / 12 of Cr(VI), respectively without contamination of Cr(III) baths with Cr(VI) ions.

[0013] US patent 4349392 discloses an aqueous acidic solution and a process for treating metallic surfaces, particularly zinc and zinc alloy surfaces, to deposit a passivating film with greater clarity and hardness and to impart greater corrosion resistance. The solution contains effective amounts of chromium ions, substantially all in the trivalent state, hydrogen ions to provide a pH of about 1.2 to about 2.5, an oxidizing agent, a bath-soluble and compatible organic carboxylic acid or its metallic salts present in an amount to impart greater initial hardness and clarity to the passivating film, and at least one additional metallic ion selected from the group consisting of iron, cobalt, nickel, molybdenum, manganese, lanthanum, cerium, lanthanide mixtures, as well as mixtures thereof. The treatment solution may optionally also contain halide ions and a wetting agent.Thus, the US patent uses a Cr(III) bath, along with oxidants, including H2O2. Cr(III) ions as well as the oxidant H2O2 remain in the same bath. If oxidation of Cr(III) to Cr(VI) occurs, it will happen within the bath. However, there is no separation between the coating formation stage and the oxidation stage, therefore unlike the invention now proposed.

[0014] Patent GB2097024 discloses metallic surfaces, particularly zinc surfaces and zinc alloys, treated with an acidic aqueous solution containing effective amounts of A) hydrogen ions to provide a pH of about 1.5 to about 2.2, B) an oxidizing agent, C) at least one of iron, cobalt, nickel, molybdenum, manganese, aluminum, lanthanum, lanthanide mixtures or cerium ions or mixtures thereof, or instead of C) iron and cobalt ions. Other treatment solutions also incorporate D) chromium ions, substantially all of which are in the trivalent state, and iron ions in combination with an additional metal of C) or cerium ions, or A), B), C) and D) and F), a compatible and bath-soluble silicate compound or Petition 870250059100, dated 11 / 07 / 2025, page 15 / 27 5 / 12 A), B), C) and D) and G), a mixture of 1-hydroxyethylidene-1,1-diphosphonic acid and citric acid or mixtures of A), B), C) and D) with two or more of E), F) and G). The treatment solution may optionally also contain halide ions and a wetting agent.). Therefore, the British patent teaches the decomposition of H2O2 by reactions with metal ions contained in the same bath and suggests the use of nitrate instead of H2O2. The decomposition of H2O2 occurs because the oxidant is mixed with the other agents in the same bath, and it is at this stage that patent GB2097024 differs from the present invention, since H2O2 is applied in a later step.

[0015] US patent 4578122 discloses an aqueous solution free of acid peroxide and a process for treating receptive metallic surfaces to impart a chromium passivating film containing chromium ions, substantially all present in the trivalent state, hydrogen ions to provide a pH of about 1.2 to about 2.5, at least one additional metal ion selected from the group consisting of iron, cobalt, nickel, molybdenum, manganese, aluminum, lanthanum, cerium, lanthanide mixtures, as well as mixtures thereof, present in an amount effective to activate the formation of the chromate passivating film, and nitrate ions as the essential oxidizing agent, present in an amount to provide a molar ratio of nitrate ions to chromium ions and activating metal ions of at least about 4:1 and sufficient to activate the hydrated trivalent chromium to form a chromate film on the substrate.The aqueous acidic solution may optionally, and preferably, also contain a controlled amount of sulfate ions, halide ions, organic carboxylic acids, a soluble and bath-compatible silicate compound, and one or a combination of wetting agents to further improve the characteristics of the passivating film produced. Therefore, the US patent uses nitrate as an oxidizing agent, in addition to additives to increase the hardness of the conversion layer, and the Cr(III) is mixed with the oxidizing agent in the same bath, which is different from the present invention. Petition 870250059100, dated 11 / 07 / 2025, page 16 / 27 6 / 12

[0016] US patent 7691498 refers to a coating composition that includes a binder, a trivalent chromium solid, and a stabilized oxidizing agent. The coating composition is used in a method to protect the surface of a metallic article, such as an aluminum article. The trivalent chromium solid forms a soluble hexavalent chromium compound as a result of reaction with the oxidizing agent when activated in the presence of a corrosive environment. The released water-soluble hexavalent chromium migrates to the coating defects to protect the metallic article. Thus, the US patent teaches a solution to the same problem as the invention now proposed, but the process steps are significantly different. Cr(III) and oxidant are within the conversion layer, but the oxidant is stored intercalated in a lamellar structure compound (LDH, graphite). The oxidant leaves this structure only when chloride enters the film.Ion exchange then occurs, and the oxidant comes into contact with Cr(III). It should also be mentioned that the product of the invention now proposed is not a conversion film, but rather a polymeric film containing the inorganic components in powder form. In comparison, the current invention differs in the simplicity of the process, which provides considerable economic advantages.

[0017] As noted, the state of the art would benefit from a solution that presents a process for producing a conversion layer, formed in a Cr(III) bath, which recovers its self-healing capacity through subsequent treatment with an oxidizing agent, which oxidizes part of the Cr(III) ions in the conversion layer to Cr(VI). Furthermore, the invention now proposed comprises a process without the application of Cr(VI) salts in the bath, without contaminating the Cr(III) bath with Cr(VI) ions and without producing other effluents containing Cr(VI), such as wash water or Cr(VI) baths to be disposed of after being exhausted. Petition 870250059100, dated 11 / 07 / 2025, p. 17 / 27 7 / 12 Summary of the Invention

[0018] In a first object, the present invention relates to a process for forming a coating for protection against corrosion of zinc surfaces, which comprises a step (a) of forming the protective film from a Cr(III) bath, followed by a step (b) which aims to oxidize part of the Cr(III) ions incorporated in the coating to Cr(VI), by means of a bath with an oxidizing agent.

[0019] In a second object, the invention now proposed discloses the use of the conversion film for protection against corrosion, in which it is applied to surfaces of zinc, zinc alloys, galvanized steel and electrodeposited zinc.

[0020] In a third object, the present invention relates to a process that produces a coating with self-healing properties by the residual presence of Cr(VI) ions, without handling baths containing Cr(VI) ions, without Cr(VI) residues in the process effluents and without the presence of leachable Cr(VI) ions from the coating. This coating obtained by a green process is produced in two steps defined in the first object.

[0021] These and other objects of the invention will be immediately valued by those skilled in the art and by companies with interests in the segment and are described in sufficient detail for their reproduction in the following text. Brief Description of the Figures:

[0022] For a better understanding of the present invention, figures are presented. The figures are described individually as follows: Figure 1 illustrates the schematic block diagram of the post-oxidation Cr(III) conversion procedures. Petition 870250059100, dated 11 / 07 / 2025, page 18 / 27 8 / 12 Figure 2 illustrates comparative potentiodynamic voltammetry with zinc samples with a Cr(III) conversion layer, zinc with a Cr(VI) conversion layer, and untreated zinc, showing similar pitting and repassivation potentials for the Cr(III) and Cr(VI) treatments, respectively. Detailed Description of the Invention

[0023] The present invention describes a process for producing a conversion film for corrosion protection that can be applied to surfaces galvanized with pure zinc (Zn), or with Zn-Al and Zn-Al-Si alloys such as in the commercial coatings Galfan®, Galvalume® and Magnélis®, as well as surfaces coated with Zn by electrodeposition.

[0024] The process comprises the following steps: surface preparation (cleaning); formation of a conversion layer from a bath containing Cr(III) ions; post-treatment with an oxidizing spray bath, for partial oxidation of Cr(III) contained in the film to Cr(VI). Cleaning

[0025] The surface to be treated by the process described in the present invention must be free of oily products, which must be removed by immersion in a bath with dimensions sufficient to submerge the structure to be treated, without exceeding 10% of this volume, containing commercial degreasers, alkaline or not, with a concentration of 2 g / L to 4 g / L, preferably 3 g / L. After degreasing, the surface must be cleaned with pure deionized water in order to remove the cleaning products applied.

[0026] In the presence of other impurities on the surface that were not eliminated by the previous procedure, such as oxides and residual oily materials not removed by degreasing; cleaning with acidic products may be considered, for example with sulfuric or hydrochloric acid at a concentration of 0.001 to 0.1 mol / L, preferably 0.01 mol / L. The use of nitric acid (HNO3), also at a concentration of 0.001 mol / L to 0.1 mol / L, preferably 0.01 mol / L for cleaning purposes, described in other processes often as Petition 870250059100, dated 11 / 07 / 2025, p. 19 / 27 9 / 12 “activation” can also be considered. The volume to be used depends on the size of the structure; therefore, a PVC tank or other similar polymer, with dimensions that allow the structure to be submerged, without exceeding this volume by more than 10%, is used. Formation of a conversion layer from a bath containing Cr(III) ions.

[0027] The process of forming a coating for corrosion protection of zinc surfaces begins with the surface being properly prepared and cleaned. Subsequently, the surface is immersed in a bath with Cr(III), in a PVC tank or other similar polymer, with dimensions that allow the structure to be treated to be submerged, without exceeding this volume by more than 10%. The process comprises the following composition: Cr(III) sulfate Cr2(SO4)3 or alkaline Cr(III) sulfate, Cr(SO4)5(OH)2, in which the concentration of these salts can vary between 0.005 mol / L and 0.4 mol / L, preferably 0.1 mol / L. Additives may be added provided they do not affect the stability of the Cr(III) ions. The bath temperature can vary between 20°C and 55°C, preferably 25°C, and the immersion time between 1 minute and 3 minutes, preferably 2 minutes, depending on the zinc content of the galvanized surface.After the immersion time, the surface should air dry naturally at a temperature of 20°C to 25°C, preferably 25°C, for 1 to 10 minutes, ideally 2 minutes. Depending on site conditions (temperature below 20°C, relative humidity above 70%), the drying time may be extended to 10 minutes. This step will lead to the formation of the Cr(III)-based conversion layer. Production process for a coating with self-healing properties by in situ oxidation of some of the Cr(III) ions in the film to Cr(VI)

[0028] The Cr(III) conversion layer, formed as in the first object, is exposed to a solution containing an oxidant. The bath for the oxidation step of the Cr(III) base layer has the following composition: Petition 870250059100, dated 11 / 07 / 2025, p. 20 / 27 10 / 12 - Hydrogen peroxide (H2O2) with a concentration of at least 20% (20 volumes) or up to 50%, preferably 30%, or: - Hydrogen peroxide with a concentration of 30% to 50% and HNO3 with a concentration of 0.01 mol / L, with or without the addition of sodium nitrate (NaNO3) with a concentration of 0.1 mol / L, preferably, for lower costs, H2O2 with a concentration of 30%. The application of the oxidizing bath occurs at room temperature. It is important that this step be applied to the newly formed conversion layer, that is, immediately after complete drying for 1 to 10 minutes at room temperature, preferably for 2 minutes, as the layer undergoes chemical changes, such as dehydration, over time.

[0029] The process of the present invention is preferably applied to smooth surfaces, for example, galvanized sheets, which must be kept in a horizontal position for application of the bath by spraying with a suitable paint gun, without the applied solution running, since the aim is not to remove the previously applied Cr(III) bath, which is soluble. It is then allowed to dry naturally for a minimum of 1 minute to 10 minutes, preferably 2 minutes.

[0030] One application should be sufficient in this case, but the process can be repeated once or twice after drying.

[0031] A final rinse with pure water is not necessary, but is recommended only when the oxidizing bath contains HNO3 in higher concentrations.

[0032] The application of the oxidizing bath aims at the partial oxidation of Cr(III) to Cr(VI), allowing the production of films from Cr(III), but with self-healing capacity due to the minimally formed and retained Cr(VI) within the oxidized conversion layer. In the case of using H2O2 as an oxidant, the following electrochemical reactions can occur: (c) H2O2 + 2H++ 2e-^ 2H2O (d) Cr3++ 4H2O ^ CrO42-+ 3e-+ 8H+ Petition 870250059100, dated 11 / 07 / 2025, p. 21 / 27 11 / 12

[0033] Cr(VI) forms on the metal surface, which catalyzes the reduction of H2O2 and therefore the oxidation of Cr(III) to Cr(IV), which will be retained in the conversion layer, without producing leachable residues contaminated by chromate on the film surface. It was observed that chromate formation is facilitated when the process is carried out by spraying, rather than immersion, since in the latter case, loss would occur due to solubilization of Cr(III). During this spray process, applied for example by spraying with a paint gun, a thin layer of liquid forms on the surface, which allows rapid access of oxygen from the atmospheric air. The reduction of oxygen increases the pH on the surface and possibly accelerates reaction (d) by eliminating H+.

[0034] Application to inclined or vertical surfaces must be done carefully, with the minimum spray volume per sheet area, so that the Cr(III) solution does not run. Reapplication of oxidant by spray gun one or two more times is necessary in this case.

[0035] Other application techniques that produce a thin film using roller or brush type tools would theoretically also be possible, but economically unviable for application in larger areas. The use of paint guns, which can be regulated according to the viscosity of the liquid, represents the best alternative; the viscosity of the oxidizing bath varies, for example, according to the concentration of H2O2 in the bath.

[0036] The following example shows the results of the potentiodynamic voltammetry test, after treatment of the zinc described herein in 0.1 mol / L basic Cr(III) sulfate followed by spraying with 30% H2O2 compared with treatment in 0.1 mol / L chromic acid and zinc without any treatment. The example shown here is intended only to illustrate one of the numerous ways of carrying out the invention, however without limiting its scope.

[0037] EXAMPLE 1 - Potentiodynamic Voltammetry Test with 0.2 mV / s For this example, pure zinc samples (99.99% Zn) were used. The surface of the samples was sanded with SiC sandpaper and polished with 1 µm diamond paste and neutral soap. Subsequently, the samples were cleaned in a bath. Petition 870250059100, dated 11 / 07 / 2025, page 22 / 27 The samples were treated with 12 / 12 ultrasound solution using deionized water and finally with isopropyl alcohol. Some samples received no additional treatment. For the remaining samples, the Cr(VI) conversion film was applied by immersion for two minutes in a 0.1 mol / L CrO3 solution, and the Cr(III) film by immersion for two minutes in a 0.1 mol / L Cr4(SO4)5(OH)2 solution. Both films were dried for two minutes. In-situ oxidation of the Cr(III) film was performed by spraying with 35% H2O2 and finished with a two-minute drying period. The temperature for both treatments was 25°C. After drying, the surfaces of the samples with conversion films were finally cleaned with deionized water. Comparative voltammetry tests with untreated zinc samples, zinc with a Cr(VI) conversion film, and zinc with a Cr(III) conversion film were performed in a 0.1 mol / L NaCl solution, saturated with air at 25°C.The test cell consisted of the sample, a platinum counter electrode, and an Ag / AgCl reference electrode. A potential scan rate of 0.2 mV / s was applied using an Autolab PGSTAT302N potentiostat. The test results (Fig. 2) clearly show the protection of zinc against corrosion by the conversion films: the two samples with conversion films have pitting (Epite) and repassivation (Erep) potentials around 200 mV higher than untreated zinc: Epite(Zn) = -700 mV; Epite(Cr(VI)) = -520 mV; Epite(CrIII) = -500 mV. Erep(Zn)=-800mV; Erep(CrVI)=-590mV; Erep(CrIII)=-580mV. The comparison between Cr(VI) film and Cr(III) film shows that the degree of corrosion protection provided by in-situ oxidation treatment of Cr(III) is comparable to the protection provided by Cr(IV) film, with slightly better values ​​for in-situ oxidation treatment.

[0038] It should be understood that the present description does not limit the application to the details described herein and that the invention is capable of other embodiments and of being practiced or performed in a variety of ways, within the scope of the claims. Although specific terms have been used, such terms should be interpreted in a generic and descriptive sense, and not for the purpose of limiting the scope of the invention.

Claims

1. Production process for a conversion film for corrosion protection characterized by comprising the following steps: (a) formation of a conversion layer from a Cr(III) bath; (b) in situ oxidation of the product obtained in step (a).

2. Process according to claim 1, characterized in that step (a) takes place in a Cr2(SO4)3 bath of 0.005 mol / L and 0.4 mol / L, preferably 0.1 mol / L, or Cr^SO^OH^ of 0.005 mol / L and 0.4 mol / L, preferably 0.1 mol / L, wherein step (a) optionally comprises added additives.

3. Process according to claim 2, characterized in that step (a) the bath temperature is between 25°C and 55°C, preferably 25°C, and the immersion time is between 1 and 3 minutes, preferably 2 minutes.

4. Process according to claim 3, characterized in that the conversion film is dried at a temperature of 20°C to 25°C, preferably 25°C, for 1 minute to 10 minutes, preferably for 2 minutes.

5. Process according to claim 1, characterized in that step (b) occurs at a temperature of 20°C to 25°C, preferably 25°C, in H2O2 with a concentration of 20% to 50%, preferably 30%; or in which in a solution of H2O2 with a concentration of 30% to 50% and HNO3 with concentrations of 0.01 M, with or without the addition of NaNO3 with a concentration of 0.1 mol / L, preferably with H2O2 with a concentration of 30%.

6. Process according to claim 5, characterized in that step (b) occurs with the conversion layer in Cr(III).

7. Process, according to claim 5 or 6, characterized in that step (b) occurs by means of spraying with a paint gun on smooth surfaces. Petition 870260065381, dated 02 / 07 / 2026, page 6 / 10 2 / 2 8. Process according to claim 7, characterized in that step (b) is repeated for inclined or vertical surfaces.

9. Process according to claim 8, characterized by step (b) on inclined or vertical surfaces and natural drying for 1 minute to 10 minutes, preferably for 2 minutes.

10. Use of the conversion film for corrosion protection, as defined in any one of claims 1 to 9, characterized by being applied to surfaces of zinc, zinc alloys, galvanized steel and electrodeposited zinc.

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