A system for developing a weakly reflective magnetite coating on the surface of stainless steel SS 420A

Abstract

A system (100) for developing a weakly reflective magnetite coating on a surface made of stainless steel SS 420A, comprising: a sample preparation arrangement (102) designed to prepare a sample of grade 420A stainless steel for surgical purposes from an ingot; a coating preparation arrangement (104) designed to apply the low-reflectivity magnetite coating to the stainless steel specimen; and a post-coating processing unit (106) comprising a rinsing unit (106a) for cleaning coated samples with deionized water and an air drying unit (106b) for drying the coated samples.

Description

FIELD OF THE INVENTION

[0001] The present disclosure relates to a system for developing a low-reflective magnetite coating on the surface of stainless steel SS 420A. More specifically, the present invention relates to a system for an optimal parametric low-reflective magnetite coating on the surface of stainless steel SS 420A with black color using a hot alkali chemical conversion coating technique. BACKGROUND OF THE INVENTION

[0002] Surgical instruments, especially those used in minimally invasive procedures, require materials with high strength, hardness, corrosion resistance, and biocompatibility. Martensitic stainless steel, particularly grade 420A, is widely used due to these properties. However, during keyhole surgery, the high-intensity light used for visualization is reflected from the instrument surface, compromising the surgeon's precision and posing a significant challenge. Existing surface modification techniques such as sandblasting and plasma treatments offer only limited improvements in minimizing light reflection due to the complex geometry of surgical instruments. Advanced coatings such as physical vapor deposition and diamond-like carbon coatings have shown potential but remain expensive and complex to implement.

[0003] To solve this problem, a low-reflective black oxide coating (magnetite, Fe3O4) must be developed on 420A stainless steel using a hot aqueous alkaline treatment. This cost-effective and efficient process reduces unwanted light reflections without altering the underlying properties of the material. To this end, the present invention provides a system for developing a low-reflective magnetite coating on the surface of 420A stainless steel. SUMMARY OF THE INVENTION

[0004] The present disclosure relates to a system for developing a low-reflective magnetite coating on the surface of SS420A stainless steel. The present invention provides a system for developing a low-reflective magnetite coating on surgical stainless steel SS420A using a hot alkali chemical conversion coating technique. The system comprises a comprehensive sample preparation unit equipped with blasting, degreasing, and pickling units for surface preparation, a coating preparation unit with precise salt solution mixing functions and a temperature-controlled bath for coating application, and a post-coating processing unit for final treatment. The system allows for controlled variation of sodium dichromate concentrations (250-350 g / L) and maintains precise bath temperatures up to 200°C, ensuring optimal coating formation.The integrated design of the invention enables efficient and reproducible production of homogeneous magnetite coatings on stainless steel surfaces.

[0005] The present disclosure is intended to provide a system for developing a low-reflective magnetite coating on the surface of SS420A stainless steel. The system includes: a sample preparation unit for preparing a 420A surgical stainless steel sample from an ingot; a coating preparation unit for applying the low-reflective magnetite coating to the stainless steel sample; and a post-coating processing unit including a rinsing unit for cleaning the coated sample with deionized water and an air drying unit for drying the coated samples.

[0006] An object of the present disclosure is to provide a system for developing a low-reflective magnetite coating on the surface of SS420A stainless steel.

[0007] Another object of the present disclosure is to provide a system for developing a uniform and low-reflective magnetite coating on SS 420A stainless steel surfaces by a controlled hot alkali chemical conversion coating.

[0008] Another objective of the present disclosure is to establish an automated surface preparation system that ensures optimal substrate conditions through precise blasting, degreasing and pickling processes.

[0009] Another objective of the present disclosure is to enable precise control of coating parameters including sodium dichromate concentration, bath temperature and immersion time for reproducible coating quality.

[0010] A further objective of the present disclosure is to enable efficient post-treatment of the coating through controlled rinsing and drying processes, thus ensuring the stability of the coating.

[0011] Another objective of the present disclosure is to develop a comprehensive coating system capable of producing homogeneous black magnetite coatings suitable for biomedical applications.

[0012] To further clarify the advantages and features of the present disclosure, a more detailed description of the invention will be given with reference to specific embodiments thereof, which are illustrated in the accompanying drawings. It should be noted that these drawings represent only typical embodiments of the invention and are therefore not to be considered limiting its scope. The invention will be described and explained in additional detail and in greater detail with reference to the accompanying drawings. SHORT DESCRIPTION OF THE FIGURE

[0013] These and other features, aspects, and advantages of the present disclosure will be better understood when the following detailed description is read with reference to the accompanying drawings, in which like characters represent like parts throughout. Where: Fig.1 shows a block diagram of a system for developing a low-reflective magnetite coating on an SS420A stainless steel surface according to an embodiment of the present disclosure.

[0014] Furthermore, those skilled in the art will appreciate that elements in the drawings are shown for convenience and may not necessarily be drawn to scale. For example, the flowcharts illustrate the method by key steps to enhance understanding of aspects of the present disclosure. Furthermore, with respect to device construction, one or more components of the device may have been represented in the drawings by conventional symbols, and the drawing may show only the specific details relevant to understanding embodiments of the present disclosure in order not to clutter the drawings with details that would be readily apparent to those skilled in the art who would benefit from the description herein. DETAILED DESCRIPTION:

[0015] To facilitate an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and described in specific language. It is to be understood, however, that no limitation on the scope of the invention is intended thereby, since such changes and further modifications of the illustrated system, and such further applications of the principles of the invention as illustrated therein, are contemplated as would normally occur to one skilled in the art to which the invention pertains.

[0016] It will be understood by one skilled in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the invention and are not intended to be limiting thereof.

[0017] References in this specification to "one aspect," "another aspect," or similar language mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. Therefore, occurrences of the phrase "in one embodiment," "in another embodiment," and similar language throughout this specification may or may not all refer to the same embodiment.

[0018] The terms "comprises," "comprising," or other variations thereof are intended to cover non-exclusive inclusion, such that a process or method comprising a list of steps not only includes those steps, but may also include other steps not expressly listed or inherent in such process or method. Likewise, one or more devices or subsystems or elements or structures or components preceded by "comprises...a" does not preclude, without further limitation, the existence of other devices or other subsystems or other elements or other structures or other components or additional devices or additional subsystems or additional elements or additional structures or additional components.

[0019] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The system, methods, and examples provided herein are for illustrative purposes only and should not be considered limiting.

[0020] Embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings.

[0021] Fig. 1 shows a block diagram of a system (100) for developing a low-reflective magnetite coating on an SS420A stainless steel surface according to an embodiment of the present disclosure.

[0022] With reference to Fig.1, the system (100) comprises a sample preparation unit (102) configured to prepare a sample of 420A surgical stainless steel from a bar; a coating preparation unit (104) configured to apply the low-reflective magnetite coating to the stainless steel sample; and a post-coating processing unit (106) comprising a rinsing unit (106a) for cleaning the coated sample with deionized water and an air drying unit (106b) for drying the coated samples.

[0023] In one embodiment, the sample preparation unit (102) comprises: a blasting unit (102a) that uses a plurality of glass beads to remove surface contaminants from the surface of the cut sample; a degreasing mechanism (102b) for cleaning the sample using an alkaline solution; a pickling mechanism (102c) configured to treat the sample with 20% diluted hydrochloric acid; and a sample holding mechanism (102d) configured to hold the processed and cleaned sample made of 420A surgical stainless steel.

[0024] In one embodiment, the coating preparation assembly (104) comprises: a salt solution preparation unit (104a) configured to prepare an alkaline salt solution by dissolving sodium hydroxide (NaOH), sodium nitrate (NaNO3), and sodium dichromate (Na2Cr2O7) in distilled water; a heating assembly (104b) comprising a stainless steel bath with a temperature control circuit for temperature control and a heating coil capable of heating the alkaline salt solution to 200°C; and an immersion mechanism (104c) connected to the sample holding mechanism (102d) and configured to immerse samples for coating into the alkaline water bath operated at a specific temperature.

[0025] In one embodiment, the coating preparation assembly (104) further comprises: a concentration control unit (104d) configured to vary the weight percentage of sodium dichromate; a temperature monitoring system (104e) configured to maintain a precise bath temperature; and a timing mechanism (104f) configured to regulate the coating duration.

[0026] In one embodiment, coating formation is achieved by chemical reactions between SS 420A and an alkaline salt solution, resulting in magnetite formation.

[0027] In one embodiment, the development of a homogeneous coating on the surface of the stainless steel sample depends on various coating variables, such as coating time, bath temperature, and the chemical composition of the salt solution.

[0028] In one embodiment, the preparation of an alkaline salt solution carried out by a salt solution preparation unit comprises preparing a solution with different weight percentages (wt%) of sodium dichromate in the composition, taking into account three different weight percentages of sodium dichromate, including weight percentages of 250, 300 and 350 g / l.

[0029] The present invention relates to a system for developing a low-reflective magnetite coating on the surface of stainless steel SS420A. In this invention, an optimal parametric low-reflective magnetite coating with a black color is applied to the surface of SS420A using a hot alkali chemical conversion coating technique. The coated surface of the stainless steel is subjected to bioactivity studies.

[0030] In one embodiment, a 25 mm x 25 mm x 3 mm 420A surgical stainless steel sample is prepared from the ingot. The prepared sample is subjected to a blasting process, using glass beads with a mesh size of 60-80 and a grain size of 0.250-0.180 mm for efficient blasting. The stainless steel sample is then degreased using an alkaline solution. The degreased sample is then pickled using 20% ​​diluted HCl and cleaned under running water. The alkaline salt solution is prepared by dissolving laboratory-grade sodium hydroxide (NaOH), sodium nitrate (NaNO3), and sodium dichromate (Na2Cr2O7) in distilled water. A solution with a different weight percentage of sodium dichromate is prepared for each test.The prepared alkaline mixture is poured into a stainless steel bath, where the temperature of the stainless steel bath is controlled using a control loop together with a heating coil. The control loop can supply heat up to a temperature of 200 °C via the heating coil. The prepared stainless steel samples are immersed in the alkaline bath for the coating process. The equations below describe the chemical reaction between 420A stainless steel and the prepared alkaline salt solution, which leads to the formation of magnetite on the surface of the 420A stainless steel sample. 4Fe + NaNO3 + 5NaOH → 3Na2FeO2 + FeO + H2O + NH3 (1) 7Fe + 5Na2FeO2 + Na2Cr2O7 + 2NaNO3 + 5NaOH + 13H2O → 3Fe3O4 + Fe2O3 + FeO + 2Cr(OH)3 + 19NaOH + 2NH3 (2) (2) 7Fe + 5Na2FeO2 + Na2 Cr 2O7 + 2NaNO 3 + 13H 2O → 3Fe3O4 + Fe2O3 + FeO + 2Cr(OH3 + 14NaOH + 2NH3 (3) (3)

[0031] The coated stainless steel sample is then cleaned in deionized water and dried in air.

[0032] The ranges of coating parameters in the hot alkaline conversion treatment process were determined through extensive testing. The formation of a uniform black oxide coating on the surface of SS 420A depends on various coating factors, including coating time, bath temperature, and the chemical composition of the salt solution. The alkaline salt solution was prepared by adjusting the weight percentage (wt%) of sodium dichromate in its composition. After conducting numerous experimental trials, three specific wt% values ​​for sodium dichromate were selected: 250, 300, and 350 g / L. It was observed that at concentrations below 250 g / L, a uniform black coating did not form, while concentrations above 350 g / L resulted in an undesirable golden-yellow coating. The coating time was optimized to 50 to 60 minutes, and the bath temperature was maintained between 115 and 125 °C.Experimental results showed that lower bath temperatures and shorter durations resulted in uneven black coatings on the substrate surface. Conversely, durations exceeding 60 minutes and temperatures exceeding 125°C resulted in undesirable color changes in the coating. The concentrations of sodium hydroxide and sodium nitrate in the bath were kept constant at 400 g / L and 320 g / L, respectively, at a fixed pH of 8. The average percent reflectance (R) represents the proportion of visible light reflected from the substrate surface. A Varian Cary 5000 UV-VIS-NIR spectrophotometer (with a spectral range of 175–3300 nm) was used to measure the visible light reflectance from the coated surfaces.

[0033] To determine the optimal parameter combination to minimize the average percentage reflectance (R), a Taguchi Orthogonal Array (OA) was used. The coating time (D), bath temperature (T), and weight fraction of sodium dichromate (C) were selected as control factors, the values ​​of which are listed in Table 1. The signal-to-noise ratio (S / N) analysis was performed using Minitab-17 software. The S / N ratio is categorized according to the desired output characteristics, e.g., the lower the better, the higher, or the more nominal. The average percentage reflectance (R) corresponded to the "lower the better" criterion, since a lower R value is essential for achieving an effective, low-reflective coating on the SS 420A substrate surface.To determine the optimal combination of control factors, the "higher the better" condition was applied to analyze the main effects plot. A high S / N ratio indicated the optimal coating parameters and ensured minimal experimental noise. Furthermore, analysis of variance (ANOVA) and regression analysis were performed to validate the results. Contour plots were used to investigate the relationship between the output features and two different control variables by analyzing their discrete contours. A conventional L9 OA designed for three parameters at three levels is shown in Table 2. Table 1. Selected parameters and experimental levels Input parameters symbol Levels A B C Coating time (min) D 50 55 60 Bath temperature (°C) T 115 ± 2 120 ± 2 125 ± 2 Wt% sodium dichromate (g / L) C 250 300 350 Table 2. L9 OA, experimental results for average percent reflectivity. Sample Code OA Average percentage reflection (R%) Signal-to-noise ratio (R dB) D T C I 1 1 1 9.75 -19.7801 II 1 1 2 9.64 -19.6815 Chapter Three 1 1 3 7.93 -17.9855 IV 2 2 1 9.01 -19.0945 V 2 2 2 8.21 -18.2869 VI 2 2 3 8.27 -18.3501 VII 3 3 1 7.9 -17.9525 VIII 3 3 2 9.04 -19.1234 IX 3 3 3 7.93 -17.9855

[0034] X-ray photoelectron spectroscopy (XPS) was used to analyze the composition and chemical state of the coating applied to the SS 420A substrate. The spectra were generated using a PHI 5000 Versa Probe II under specific test parameters: a 200 µm beam spot and a monochromatic A1-Kα X-ray stimulation source (hv = 1486.6 eV) with a voltage of 15 kV. Survey scans were recorded using a 50 W X-ray source with a pass energy of 187.85 eV, while high-resolution spectra of key elements were acquired at a pass energy of 46.95 eV. Morphological studies and elemental composition analyses of the coating were also performed using a scanning electron microscope in conjunction with energy-dispersive X-ray analysis.

[0035] The cytotoxicity of the coating was assessed using the direct extraction method according to ISO 10993-5. The L-929 cell line served as the basis for this study. Before testing, both coated and uncoated substrates were sterilized by autoclaving. Cultures were incubated for 24–26 hours at 37°C in minimal medium (MEM) supplemented with fetal bovine serum (FBS). Extracts were prepared by immersing disinfected samples in culture medium for 72 ± 2 hours at 50 ± 2°C. Positive and negative controls were created by diluting phenol solutions and incubating ultra-high molecular weight polyethylene (UHMWPE) in culture medium under identical conditions. The extracts were then adjusted to concentrations of 100%, 50%, and 25% by diluting with the culture medium.To assess the cytotoxic effects, the prepared extracts, as well as positive and negative controls, were applied to the L-929 cell line and incubated for 24–26 hours at 37 ± 1 °C. Cellular responses were analyzed by observing the cultured cells under a microscope after incubation with the test, positive, and negative control samples.

[0036] The corrosion resistance of the samples was evaluated using potentiodynamic polarization (PDP) tests conducted according to ASTM G5-94 standards. A three-electrode cell with a capacity of 250 ml was used. The setup included a saturated calomel electrode (SCE) as the reference electrode, platinum as the auxiliary electrode, and coated or uncoated SS 420A samples as the working electrode. Simulated body fluid (SBF) with a pH of 7.4, which mimics natural tissue conditions, served as the electrolyte. The SBF was prepared using the Kokubo method and maintained at 37 ± 1 °C.

[0037] A 1 cm 2A large area of ​​the working electrode was exposed to the electrolyte, and the samples were immersed for 3600 seconds to establish a stable open circuit potential (OCP). The OCP was swept from -1000 mV to +1000 mV at a rate of 0.0005 mV / s. Potentiodynamic tests were performed on both coated and uncoated SS 420A samples, and using Tafel extrapolation, the corrosion current density (Icorr) and other parameters were determined, confirming the corrosion resistance properties of the coated samples.

[0038] Additionally, a salt spray test was conducted according to ASTM B117 standards. A solution of 5 ± 1 parts by mass of NaCl dissolved in 95 parts of water was prepared, ensuring a specific gravity between 1.0268 and 1.0413 and a pH of 6.5 and 7.2 at 35 °C. The samples were exposed to a salt spray chamber at 35 ± 2 °C for 24 hours. The reliability of the test depended on the type of samples, the evaluation criteria, and the operating conditions.

[0039] To evaluate the adhesion properties of the black oxide coating under sterilization conditions, repeated sterilization tests were conducted according to ISO 17665-1. Conventional sterilization involved steam temperatures of 121–134 °C for 15–21 minutes. A rapid sterilization cycle was performed at 134 °C for 3 minutes in an autoclave, followed by a cooling phase. The complete sterilization process took 45 minutes to 1 hour. Surface morphological investigations of the black oxide (Fe3O4)-coated SS 420A samples, optimized with the ideal parameter combinations, were performed using a scanning electron microscope.

[0040] The results showed that the optimal combination of coating parameters for a minimum average percentage reflectance (R) was achieved as D3-T3-C3 (D = 60 min, T = 125 °C, and C = 350 g / l). The reflectance (R) of uniformly black coated SS 420A was reduced from the original 56.14% for uncoated samples to 7.546%. It was found that the chemical composition of the bath and the bath temperature had the greatest influence on the formation of magnetite (Fe3O4) on SS 420A. This coating effectively minimizes the problems surgeons face due to scattering and reflection of visible light from the surfaces of SS 420A surgical instruments. The coating was characterized by XPS analysis, which confirmed the formation of magnetite (Fe3O4). An in vitro cytotoxicity test using the L-929 cell line showed that the coating is non-toxic.Corrosion resistance studies revealed that magnetite-coated SS 420A exhibited superior corrosion resistance compared to the uncoated substrate. Additionally, a sterilization resistance test was conducted, including repeated autoclaving cycles. After 100 cycles, the average reflectance value (R) increased slightly to 8.32%, which is still within the desirable range. These results demonstrate that the developed coating is suitable for the production of biocompatible, low-reflective black coatings for surgical instruments made of medical-grade 420A stainless steel.

[0041] The drawings and the foregoing description provide examples of embodiments. Those skilled in the art will recognize that one or more of the described elements may well be combined into a single functional element. Alternatively, certain elements may be separated into multiple functional elements. Elements from one embodiment may be added to another embodiment. For example, orders of the processes described herein may be changed and are not limited to the manner described herein. Furthermore, the actions of any flowchart need not be implemented in the order shown; nor do all actions necessarily need to be performed. Also, those actions that are not dependent on other actions may be performed in parallel with the other actions. The scope of the embodiments is in no way limited by these specific examples.Numerous variations, whether explicitly stated in the specification or not, such as differences in structure, dimensions, and material use, are possible. The scope of the embodiments is at least as broad as indicated in the following claims.

[0042] Advantages, other benefits, and solutions to problems have been described above with reference to specific embodiments. However, the advantages, benefits, solutions to problems, and any components that may cause an advantage or solution to occur or become more apparent are not to be construed as a critical, required, or essential feature or component of any or all of the claims. REFERENCES 100 A system for developing a low-reflective magnetite coating on the surface of stainless steel SS420A. 102 Sample preparation unit 102a blasting machine 102b Degreasing mechanism 102c Pickling mechanism 102d Sample holding mechanism 104 Coating preparation plant 104a Saline solution preparation unit 104c Immersion mechanism 104d Concentration control unit 104e Temperature Monitoring System 104f Time control mechanism 106 Aftertreatment plant 106a Flushing unit 106b Air drying unit

Claims

[1] A system (100) for developing a low-reflective magnetite coating on a stainless steel SS 420A surface, comprising: a sample preparation assembly (102) configured to prepare a 420A surgical grade stainless steel sample from a billet; a coating preparation arrangement (104) configured to apply the low-reflective magnetite coating to the stainless steel sample; and a post-coating processing unit (106) comprising a rinsing unit (106a) for cleaning coated samples with deionized water and an air drying unit (106b) for drying the coated samples. [2] System (100) according to claim 1, wherein the sample preparation unit (102) comprises: a blasting unit (102a) using a plurality of glass beads to remove surface contaminants from the surface of the cut sample; a degreasing mechanism (102b) for cleaning the sample using an alkaline solution; a pickling mechanism (102c) configured to treat the sample with 20% diluted hydrochloric acid; and a sample holding mechanism (102d) configured to hold the processed and cleaned sample made of 420A surgical stainless steel. [3] The system (100) of claim 1, wherein the coating preparation assembly (104) comprises: a salt solution preparation unit (104a) configured to prepare an alkaline salt solution by dissolving sodium hydroxide (NaOH), dissolving sodium nitrate (NaNO 3) and sodium dichromate (Na2 Cr2 O7) in distilled water; a heating arrangement (104b) comprising a stainless steel bath with a temperature control circuit for temperature control and a heating coil capable of heating the alkaline salt solution to 200 °C; and an immersion mechanism (104c) connected to the sample holding mechanism (102d) and configured to immerse samples for coating into the alkaline water bath operated at a specific temperature. [4] The system (100) of claim 1, wherein the coating preparation assembly (104) further comprises: a concentration control unit (104d) configured to vary the weight fraction of sodium dichromate; a temperature monitoring system (104e) configured to maintain a precise bath temperature; and a timing mechanism (104f) configured to regulate the coating duration. [5] System (100) according to claim 1, wherein the coating formation is achieved by chemical reactions between SS 420A and an alkaline salt solution leading to magnetite formation. [6] The system of claim 1, wherein the development of a homogeneous coating on the surface of the stainless steel sample depends on various coating variables, such as coating time, bath temperature and chemical composition of the salt solution. [7] The system according to claim 3, wherein the preparation of the alkaline salt solution by the salt solution preparation unit comprises preparing a solution having a different weight percentage (wt%) of sodium dichromate in the composition, taking into account three different weight percentages of sodium dichromate, including weight percentages of 250, 300 and 350 g / l.

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