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Coloring and Characterization Techniques for Stainless Steel Screws

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Stainless steel screws have excellent corrosion resistance, wear resistance, and excellent decorative effects, and are widely used in the interior and exterior decoration of automotive vehicles. Black and blue stainless steel screws are used for connecting passenger cars due to their excellent heat absorption, corrosion resistance, processability, and welding performance. In recent years, extensive research has been conducted on the coloring technology of automotive stainless steel screws, and certain results have been achieved. This article discusses the coloring and characterization techniques for stainless steel screws.

1.Staining method for stainless steel screws

1.1 Physical vapor deposition

By using physical vapor deposition technology, a certain color of oxide film or alloy film can be obtained on the surface of stainless steel screws. Physical vapor deposition includes sputtering coating, vacuum coating, ion plating, ion implantation, etc. Commonly used targets include TiO2, Al2O3, SnO2, Cr, Ti, Ni, and alloys. Vacuum evaporation plating, resistance heating evaporation plating, induction heating evaporation plating, etc. Sputtering coating includes ion beam sputtering coating, magnetron sputtering deposition, etc. These technologies do not use harmful chemical reagents and drugs, do not emit any toxic and harmful gases or waste liquids, and do not pollute the natural environment. The film has a magnificent luster, good corrosion and oxidation resistance, does not fade, is not easy to scratch, and is not easily broken, peeled, or scratched when bent over 90 °. It is also not affected by ultraviolet radiation and can be used as a unique automotive decorative material.

1.2 Chemical treatment coloring

After chemical treatment on the surface of stainless steel screws, they exhibit various colors under the action of interference light, and their color mainly depends on factors such as the chemical composition, organizational structure, surface smoothness, and film thickness of the surface oxide film. The advantage of chemical coloring method is that coloring stainless steel screws can be complex and obtain uniform colors, but the operating temperature is high and the reproducibility of colors is difficult to ensure. The main methods include:

① Sulfurization method.

The activated stainless steel screws were immersed in a coloring solution mainly composed of sodium hydroxide and inorganic sulfides for sulfurization reaction, resulting in a black sulfide film layer with good decorative effect on the surface of the stainless steel screws. But this kind of coating has poor corrosion resistance and needs to be coated with a gloss coating.

② Alkaline chemical blackening method.

Coloring stainless steel screws in a strong alkaline aqueous solution containing oxidants and reducing agents has the advantage of eliminating the pre-treatment process of removing the oxide film on the surface of stainless steel screws before coloring, and chromium salts are not used during coloring; The disadvantage is that the coloring temperature is too high.

③ Salt dissolution method.

Immerse the stainless steel screw in a mixture of molten sodium dichromate and potassium dichromate, and the surface of the stainless steel screw is oxidized into a black, matte, but firm film by newly formed oxide atoms. The temperature is 450-500 ℃, and the time is 10-20 minutes. Due to the high viscosity and slow diffusion of molten salt, it is difficult to obtain uniform color for stainless steel screws manufactured using this method.

④ Dichromate staining method.

Stainless steel screws are subjected to immersion coloring treatment in Na2Cr2O7H2SO4 aqueous solution. The color of the product surface can be controlled based on the immersion time. The process operation is simple, the process is stable, the product qualification rate is high, and the production cost is low. The main components of this oxide film are Cr2O3, Cr, and Fe, which are thicker than the INCO method oxide film and have better conductivity, so the weldability is good.

⑤ INCO method.

Immerse the polished and activated stainless steel screws in a mixed solution of chromic anhydride sulfuric acid. Over time, the surface of the stainless steel screws is oxidized to form oxide films of different thicknesses, which produce different colors due to light interference. At present, this method has adopted microcomputer control, multiple varieties, and automated production, making it one of the earliest and most widely used methods.

⑥ One step method for coloring hard film.

An improvement has been made on the INCO method by applying a small cathodic current to the stainless steel screw electrode while chemical coloring. The potential difference method is used to control the color of the stainless steel screw, thus combining chemical coloring with electrochemical hardening into one step. The traditional coloring method will change the original color of stainless steel screws more or less due to the increase in oxide film thickness during the later stage of hard film treatment. Adopting a one-step method not only simplifies the coloring process, improves color controllability and reproducibility, but also enhances the wear resistance and corrosion resistance of stainless steel screw colors.

⑦ Pulse laser irradiation method.

A new technology for manufacturing stainless steel screw colors has been developed using pulse laser and computer-aided design. The stainless steel screw is first immersed in a 5% nitric acid solution, and then irradiated with a pulse laser with an energy density of 0.8J/m2. The irradiation time is controlled, and the stainless steel screw will appear in seven colors, including blue, green, and golden yellow. This method requires less cost than other coloring processes.

⑧ Deposited metal – chemical coloring method.

After depositing copper, zinc, titanium, nickel, or other alloy layers on the surface of stainless steel screws, chemical coloring is carried out on the deposited metal by utilizing the easy coloring characteristics of the deposited layer. There are many types of coloring, with bright and beautiful color quality, easy to control, and easy to operate. It can also create complex patterns or color schemes.

1.3 Electrochemical coloring

The advantages of electrochemical coloring are good color controllability and reproducibility, low coloring treatment temperature, and some processes can be carried out at room temperature, resulting in less environmental pollution. However, special fixtures are required for electrolytic coloring, and the processed stainless steel screws are not easily complicated, otherwise uneven distribution of power lines will lead to uneven color.

① Current method.

Apply a controllable current signal to the stainless steel screw sample to cause an oxidation-reduction reaction of the stainless steel screw in the coloring solution, thereby obtaining the desired color. There are two commonly used current methods: pulse current method and constant current method. By using the former to color SUS304, a specific thickness of coloring film can be obtained by controlling the total amount of input electricity, thereby achieving the goal of color control. The color reproducibility is better than that of chemical coloring methods. Using the constant current method to anodize stainless steel screws in golden yellow, the resulting color film is bright and beautiful, showing a golden yellow color, with excellent corrosion resistance and good adhesion.

② Voltage method.

On the basis of the natural potential of stainless steel screws, a positive or negative potential is applied to cause the sample to undergo an oxidation-reduction reaction in the coloring solution and coloring. The methods are also divided into pulse potential method and constant current method. Austenitic stainless steel screws have a fast coloring speed under room temperature and square wave voltage pulse conditions. Due to the fact that the natural potential of different stainless steel screws cannot be completely consistent, operation is relatively difficult. When using constant potential anodic polarization coloring, the applied polarization potential varies, resulting in different colors.

1.4 Organic coating and coloring

The organic coating coloring method involves degreasing, phosphating, and chromating the surface of stainless steel screws, applying organic paint, and then baking. The commonly used coatings for coating include alkyd resin, polyester resin, acrylic resin, polyvinyl chloride, polyamine resin, polysiloxane resin, epoxy resin, etc. The color coating prepared for stainless steel screws not only has good coloring, formability, and weather resistance of organic matter, but also has the mechanical properties and easy processing properties of stainless steel screws. It can be used for processing such as punching, bending, deep drawing, riveting, welding, etc. It is widely used in various industries such as vehicles, furniture, and electrical appliances.

2. Characterization methods

2.1 Color film composition and surface morphology

XPS was used to analyze the elemental composition of the passivation film formed by the electrochemical deposition of polyaniline on SUS304 stainless steel screws in a 0.1mol/L H2SO4 solution. The results indicate that the content of Cr in the generated passivation film is approximately twice that of the passivation film formed by anodic polarization, and the ratio of Cr2O3/Cr (OH) 3 is much higher than that of the latter. Coloring stainless steel screws using square wave current pulse method and using AES deep analysis to study the content of elements in the color film is lower than that in the stainless steel screw matrix, and decreases with the prolongation of coloring time. However, the change in Cr and Ni element content is small and can be ignored. During the chemical coloring process of stainless steel screws, a small cathode current is introduced simultaneously, combining chemical coloring and electrolytic hardening into one step. The surface morphology of the color film was observed using SEM; Before applying cathodic current, there are many large and deep micropores on the surface of the color film, with a pore size of approximately 10-20nm. The film is loose, soft, and has poor wear resistance, and is prone to contamination. When the cathodic current gradually increases to 6.3mA/dm2, the surface of the color film becomes smooth due to the disappearance of micropores, and the wear resistance is also enhanced. Atomic force microscope (AFM) can directly observe the microstructure of the facial mask deposited on the surface of stainless steel screws, and characterize the surface morphology and defects of the film.

2.2 Thickness and refractive index

When the refractive index of the oxide film on the surface of stainless steel screws is constant, the interference color depends on the thickness of the oxide film and the angle of natural light incidence. When the angle of incident light is constant, the color of the color film depends on its thickness. Auger electron spectroscopy depth analysis and UV visible spectrophotometry are commonly used methods for measuring film thickness. Elliptical polarization method is an advanced technology for measuring optical parameters such as nanoscale thickness and refractive index of thin films. It has the advantages of high measurement accuracy (d<10nm), non-destructive, and fast. According to the changes in polarization state of polarized light before and after reflection, the thickness and refractive index of the surface film of stainless steel screw samples can be measured. The migration mechanism and film formation region of oxygen and metal ions during the formation of color films on SUS304 stainless steel screws in H2SO4-CrO3 refractive index solution were studied using O18 tracing and nuclear reaction techniques. The thickness of oxide films with different colors was measured using elliptical polarization method. The results showed that the thickness of the brown, blue, golden, red, and green films were approximately 130, 156, 229, 325, and 780nm, respectively, which is consistent with the measurement results of AES in the experiment.

2.3 Wear resistance of color film

After coloring treatment, the surface of stainless steel screws is covered with a layer of porous and soft oxide film, which contains multiple 10~20nm curved and connected micropores, accounting for about 20% of the total area. The structure of this colored film is not compact, and the use of a hard film treatment can fill the micropores on the surface of the colored film, greatly improving the wear resistance and corrosion resistance of the stainless steel screw colored film. When automotive stainless steel screws are used for decoration, they require brightness and smoothness. Therefore, in production, the glossiness and surface roughness of the color film also need to be considered. The glossiness can be measured using a glossmeter, and atomic force microscopy (AFM) is commonly used to measure the roughness of the film.

2.4 Corrosion resistance of color film

Stainless steel screws have excellent performance and, after coloring treatment, have good decorative effects, making their application fields more extensive. Electrochemical impedance technology is an important method for analyzing electrode process dynamics, double layer and diffusion, and studying the mechanisms of electrode materials, solid electrolytes, conductive polymers, and corrosion protection. SUS304 stainless steel screws were colored using traditional chemical methods, INCO methods, and square wave current pulse methods, and their pitting resistance in HCI solution was tested using electrochemical impedance spectroscopy technology. The results showed that the color film prepared by the INCO method had the best corrosion resistance, followed by the square wave current pulse method, and the color film prepared by the traditional chemical method had the worst corrosion resistance.

3. Outlook

In recent years, stainless steel screw coloring technology has achieved many research results and practical applications, but the reproducibility, surface hardness, and corrosion resistance of stainless steel screw colors need to be further improved. The coloring of stainless steel screws in automobiles not only has a bright and gorgeous appearance but also excellent corrosion resistance. With the continuous improvement of coloring technology, stainless steel screw coloring will have a wider application prospect.
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