Innovative low-toxicity method for stripping organic coatings from metal surfaces without damaging the substrate or the phosphate crystal structure

Abstract

The invention relates to a method to be used in various engineering fields such as automotive, aviation, and marine industries, for the effective and more environmentally friendly chemical removal of organic coatings (cataphoresis coating or electrostatic powder coating) from metal surfaces Moreover, the method does not damage the substrate or the phosphate crystals, thereby minimizing production downtime by eliminating the need for repeated pre-treatment steps and reducing scrap rates by enabling reuse of the substrate material. In addition, the stripping solution does not become contaminated during the process, allowing repeated use without generating waste. The organic coating swells in the developed solution and subsequently separates into a water bath. The dissolution of the organic coating in the water bath provides recycling potential and supports a zero-waste approach.

Description

[0001] DESCRIPTION

[0002] INNOVATIVE LOW-TOXICITY METHOD FOR STRIPPING ORGANIC COATINGS FROM METAL SURFACES WITHOUT DAMAGING THE SUBSTRATE OR THE PHOSPHATE CRYSTAL STRUCTURE

[0003] Technical Field

[0004] The invention relates to a method to be used in various engineering fields such as automotive, aviation, and marine industries, aiming to chemically remove organic coatings (cataphoresis coating or electrostatic powder coating) from metal surfaces in an effective and more environmentally friendly manner.

[0005] Background of the Invention

[0006] In order to maintain the effectiveness of organic coatings used across many engineering sectors such as automotive, aviation, and marine industries, it is important that these coatings are regularly inspected and renewed when necessary. In addition, to minimize issues arising from defective coatings and to reduce scrap rates, the coatings must be removed using appropriate operational methods.

[0007] Current applications include widely used methods for removing coatings from metal surfaces. These methods involve sandblasting, sanding, and certain aggressive acidic and alkaline chemical stripping processes. Each method has been experimentally tested and evaluated in terms of hardness, surface roughness, and corrosive impact.

[0008] Sandblasting propels abrasive particles onto the metal surface under high pressure and provides only partial removal of organic coatings. The process significantly increases surface roughness and may leave residual coating on hard surfaces. It also produces flaking and microcracks that weaken the substrate and reduce the adhesion performance of subsequent coatings. Due to its non-uniform action and inability to reach recessed or complex geometries, sandblasting is not an effective stripping method for many industrial components.

[0009] Sanding Method: Sanding is a mechanical technique used to remove coatings. However, it has been observed that coatings cannot be completely removed, particularly on surfaces with complex geometries. Sanding creates valleys and peaks on the surface, leading to increased roughness. The sanding method does not allowhomogeneous cleaning of the surface, which may result in weak adhesion and corrosive effects during the coating process.

[0010] Chemical Stripping Methods: Chemical stripping generally involves the use of acidic chemicals. This method aims to effectively remove residual coatings from the surface. However, due to the corrosive nature of the chemicals used, the solution penetrates deeper into the organic coating and forms microcracks on the substrate metal surface, weakening its mechanical properties. Since chemical stripping typically requires aggressive chemicals, it carries risks of corrosion and structural damage to the surface. This situation can create significant problems in subsequent coating processes. Particularly for parts with complex geometries, immersion-based chemical processes provide the most effective results; therefore, using chemical agents with low oxidative effect and strong dissolution capability yields successful outcomes.

[0011] N-Methyl-2-pyrrolidone (NMP): N-Methyl-2-pyrrolidone (NMP), commonly used in stripping processes, causes the coating to swell and subsequently dissolve in the solution. Although NMP provides an applicable process for coating stripping, it has several disadvantages. After stripping organic coatings, this solution causes minor damage to the metal surface and removes phosphate crystals from certain areas. Moreover, the toxicity of NMP poses significant health and environmental risks.

[0012] Phosphate Crystals: In the developed solution, preserving phosphate crystals without degradation and maintaining them in a homogeneous and desired size provides a significant advantage for subsequent organic coating processes. For this reason, compared to NMP-based methods, the developed solution offers a more effective and safer stripping method that avoids toxic materials.

[0013] Sulfuric Acid Application: Sulfuric acid is used as a fast and effective chemical solvent for coating removal. However, stripping with 98% sulfuric acid results in several disadvantages. High sulfuric acid concentration increases sulfate formation on metal surfaces and accelerates corrosion. During this process, microcracks and delamination occur. The iron sulfate (Fe2SO4) layer formed as a result of the reaction between sulfuric acid and metal weakens the structural integrity of the surface and accelerates corrosion. The aggressive nature of the chemical solvent used in the coating removal process increases the risk of pitting on the metal surface, negatively affecting material strength and surface quality. In addition, dissolved coating residues form a wastesolution within sulfuric acid. Experiments showed that even 10% sulfuric acid solutions caused significant surface damage. This indicates that alternative methods are needed.

[0014] In conclusion, although sulfuric acid provides a rapid solution for coating stripping, it must be evaluated carefully due to the structural damage and corrosion risks it introduces. It increases surface roughness, adversely affecting subsequent coating applications.

[0015] Nitric Acid (HNO3): Due to its strong oxidizing properties, nitric acid provides an effective stripping method. This process enables complete dissolution of the coating on the metal surface. For example, experiments showed that a cataphoresis coating applied on low-carbon steel was completely removed when exposed to nitric acid at 70 °C for 20 minutes. However, the interaction of nitric acid with the metal surface led to surface damage such as cracks and pitting. The reaction between nitric acid and metal generates corrosive byproducts such as nitrogen dioxide (NO2) and iron nitrate (Fe(NO3)2), increasing the risk of corrosion. The combination of nitric acid with air pollutants (sulfur dioxide and nitrogen oxides) also increases the risk of stress corrosion cracking. Therefore, while nitric acid provides a strong stripping capability, its potential damage and environmental impacts require careful application.

[0016] Piranha Solution Stripping: A solution consisting of 25% sulfuric acid and 75% hydrogen peroxide exhibits high corrosive effects. Although piranha solution rapidly strips organic coatings due to the strong oxidizing capability of hydrogen peroxide, it reacts aggressively with metal surfaces, causing deep damage. It accelerates oxidation reactions on the metal surface, leading to coating dissolution, but also causing cracks and other surface defects. Piranha solution forms microcracks in stainless steel structures, increasing surface roughness and making the metal more susceptible to corrosion. When exposed to high acid concentrations, stainless steel becomes more vulnerable to corrosive wear. Piranha solution exacerbates this vulnerability by weakening the structural integrity of the metal. Scanning Electron Microscope analyses revealed moderate cracking and significant surface damage on piranha-treated samples, demonstrating their detrimental impact. Therefore, while piranha solution is a strong chemical solvent for coating removal, it must be used cautiously. Alternative methods are recommended to preserve the structural integrity of metal surfaces.

[0017] 100% Hydrogen Peroxide (30% Concentration): A hydrogen peroxide solution with 30% concentration was used to remove organic coatings from surfaces. The phosphatecoating was largely degraded. However, surface morphology analysis revealed that while phosphate crystals remained intact in some areas, structural degradation occurred in others due to the corrosive effect of the high oxidant concentration. This caused some phosphate crystals to detach from the surface, disrupting its homogeneity. Cross-sectional images indicated that overall roughness was acceptable, but certain regions lacked phosphate crystals, demonstrating the non-uniformity of the stripping process. In summary, high-concentration H2O2partially destroyed phosphate crystals due to its strong oxidizing effect and caused undesired losses in the coating. This leaves the surface unsuitable for subsequent coating processes. In contrast, lower H2O2concentrations yielded effective results, successfully removing cataphoresis coatings while preserving the phosphate crystal structure and providing an ideal surface for recoating. Thus, using low-concentration H2O2offers a beneficial and efficient approach for controlled stripping while maintaining phosphate morphology.

[0018] One of the most significant technical problems arising from current applications is the increase in surface roughness. High roughness negatively affects adhesion strength of recoated layers after stripping and reduces overall performance, especially in corrosive environments. Additionally, mechanical methods such as sandblasting and sanding fail to provide homogeneous stripping on parts with complex geometries. In chemical methods, the corrosive effects of acidic solvents weaken the structural integrity of metal surfaces and cause weak adhesion during subsequent coating applications. Since chemical stripping is the most effective method for complex geometries, it is essential to develop less toxic and more effective solutions. Therefore, eliminating the deficiencies of existing methods and developing alternative techniques using fewer toxic materials is necessary.

[0019] As a result, due to the shortcomings described above and the inadequacy of current solutions in addressing the problems, improvements in this technical field have become essential.

[0020] Objective of the Invention

[0021] The invention has been developed by drawing inspiration from current practices and aims to overcome the disadvantages described above.

[0022] The primary objective of this invention is to preserve the integrity of the material by minimizing surface damage and roughness that occur during coating removalprocesses. In addition, the invention aims to perform the stripping operation without damaging the phosphate crystals applied prior to the organic coating. In this way, it becomes possible to obtain an efficient surface in a shorter time during subsequent coating processes, without the need for pre-treatment steps such as phosphating. The fact that the stripping solution does not become contaminated and does not generate waste, unlike other solutions, ensures a more environmentally friendly process and enables the stripping bath to be reused multiple times.

[0023] The mechanism developed for coating removal can be explained as follows: When the organic coating is immersed in the stripping solution, it undergoes deformation and swelling, after which it easily separates from the surface either in a hot water bath or through pressurized water spraying. Furthermore, the disintegration of the organic coating in water increases the potential for organic recycling, offering an effective approach for zero-waste processes.

[0024] One objective of the invention is to provide a stripping method using low-concentration and targeted chemicals. Instead of highly aggressive chemicals, the use of low-concentration solvents that remain effective while minimizing damage reduces microcrack formation and avoids deterioration of surface roughness.

[0025] Another objective of the invention is to provide a stripping method through a hybrid solution formulation. Hybrid solutions formed by combining chemical components, for example hydrogen peroxide and organic solvents, enable both effective and rapid coating removal while maintaining the integrity of the surface and minimizing phosphate damage.

[0026] Another objective of the invention is to provide a stripping method through controlled application durations. Carefully adjusting the contact time of the chemical solvents with the surface minimizes the risk of acidic attack and oxidation.

[0027] A further objective of the invention is to provide a stripping method that preserves the phosphate crystal structure. The chemicals used possess properties that enable selective removal of the organic coating while allowing the existing phosphate crystals to remain intact on the surface, thereby enabling subsequent coating processes to be performed successfully without the need for re-phosphating.To achieve the objectives described above, the invention provides a method for removing organic coatings from metal surfaces, the method comprising treating the coated part with a solution containing toluene, acetone, and hydrogen peroxide.

[0028] The structural and characteristic features of the invention, along with all of its advantages, will be understood more clearly from the detailed description provided below, and therefore the evaluation should be carried out with consideration of this detailed explanation.

[0029] Description of the Figures

[0030] Figure-1 presents the flow diagram of the process comprising the application, removal, and reapplication of the organic coating.

[0031] Detailed Description of the Invention

[0032] In this detailed description, the method and preferred embodiments of the invention are explained solely for the purpose of improving the understanding of the subject matter.

[0033] The invention relates to a method for removing organic coatings from metal surfaces, the method comprising treating the coated part with a solution containing toluene, acetone, and hydrogen peroxide.

[0034] According to one embodiment of the invention, the coated part is treated with a solution containing 40-60% toluene, 20-40% acetone, and 10-30% oxidant.

[0035] According to another embodiment of the invention, the coated part is treated with a solution containing 50% toluene, 30% acetone, and 20% oxidant.

[0036] According to another embodiment of the invention, during the stripping process the coated part is treated with the solution at 25-80 °C for 5-45 minutes.

[0037] The coated parts referred to herein include parts coated with organic layers such as cataphoresis coating or electrostatic powder coating.

[0038] Figure 1 illustrates the flow diagram of the process comprising the application, removal, and reapplication of the organic coating. A detailed examination of the process shown in the figure demonstrates that, following stripping, the phosphate crystals remain undamaged, allowing the coating to be directly applied to the surface without the needfor pre-treatment steps previously required before the organic coating. Since the process causes minimal damage to the surface, the substrate material can be reused without being scrapped, or, if necessary, alternative coating techniques other than organic coating can be applied.

[0039] The developed solution, comprising a combination of an oxidant, acetone, and toluene, provides a significant innovation from an environmental protection perspective by offering an alternative to existing toxic solvents. This method allows preservation of the surface properties and enables recoating while causing minimal damage to the substrate. Furthermore, the phosphate crystals formed to enhance adhesion and provide additional corrosion resistance prior to the organic coating are not damaged by the developed stripping solution, thereby enabling direct coating after stripping. In this way, the phosphating procedure can be bypassed, leading to savings in time and cost. Figure 1 presents all steps of the organic coating process. After the stripping process, seven steps required for recoating (hot degreasing"!, rinsing2, activations, phosphating4, rinsing5, passivations, rinsing?) were skipped, and the organic coating was applied directly to the surface. The sample recoated after stripping exhibited similar salt spray test results to a normally coated sample. For both cataphoresis coatings applied via the normal process and the recoating after stripping, no red rust was observed for 55 cycles 11320 hours.

[0040] Within the scope of the invention, eight different chemical stripping processes were carried out as shown in Table 1 below. Among these, the most effective method for coating removal was the sixth formulation, containing 50% toluene, 20% hydrogen peroxide, and 30% acetone, which provided an efficient stripping outcome.

[0041] Table 1. Chemical methods used for stripping processes

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[0043]

[0044] Time (dk.)

[0045]

[0046] After the stripping process, the obtained part may be recoated with an organic coating and subjected to rinsing and curing procedures. Consequently, the invention offers a wide range of applicability for industrial operations seeking sustainable and efficient solutions for removing coatings from metal surfaces.

Claims

CLAIMS1. A method for removing organic coatings from metal surfaces, characterized in that the coated part is treated with a solution comprising toluene, acetone, and hydrogen peroxide.

2. The method according to claim 1, characterized in that the coated part is treated with a solution comprising 40-60% toluene, 20-40% acetone, and 10-30% hydrogen peroxide.

3. The method, according to claim 1, characterized in that the coated part is treated with a solution comprising 50% toluene, 30% acetone, and 20% hydrogen peroxide.

4. The method according to claim 1, characterized in that the coated part is treated with the solution at 25-80 °C.

5. The method according to claim 1, characterized in that the coated part is treated with the solution for 5-45 minutes.

6. The method according to claim 1, characterized in that the coated part comprises an organic coating such as cataphoresis coating or electrostatic powder coating.

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