Common methods for blackening stainless steel, treatment effects, and process steps

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This post was last edited by cy1125915 on 2015-12-17 16:01. In industrial production, there are two types of surface treatment: physical treatment processes and chemical treatment processes. Blackening the surface of stainless steel is a common technique in chemical treatment. Principle: Through chemical treatment, an oxide film is formed on the metal surface, and this oxide film is used to achieve the purpose of surface treatment. The principle behind this surface treatment process is to use appropriate equipment to create an oxide layer on the metal surface, which acts as a barrier preventing direct contact between the metal and the external environment. In simple terms, the surface blackening process refers to a chemical surface treatment method that utilizes an oxide film formed on the metal surface to achieve the desired surface treatment effects. 1 Common methods Category 1: Acidic coloring methods (1) Molten dichromate method. The stainless steel parts are immersed in a molten sodium dichromate salt solution and stirred thoroughly for 20–30 minutes to form a black oxide film. Remove it, cool it, and then wash it with water. The temperature of the molten salt is between 450 and 550°C; below this temperature, sufficient oxidation cannot occur, and the black color cannot be achieved. (2) Chromate black chemical oxidation method. The color change of this film layer occurs from light to dark; when it changes from light blue to dark blue (or pure black), the time interval is only 0.5–1 minute. If this optimal point is missed, the color will revert to light brown, requiring the film to be removed and repainted. Curing treatment can improve wear resistance. With a color locator, a uniform black oxide film can be obtained. Strict operational requirements are necessary. This method is also known as the Inko method; it has been around for several decades and was invented by the British. After the process was improved, it has been widely applied, yielding significant benefits. At present, compared with advanced **, the main gap in our country lies in colored electronic monitoring devices. This type of equipment is already being used in industrial production abroad to achieve consistent colors, but there are no reports of its use in China; therefore, developing electronic monitoring equipment for coloring is an urgent task. 2. The sulfidation method can produce an attractive black film; acid washing with aqua regia is required before oxidation. 3. Alkaline oxidation: This method involves using a solution prepared from hydrogen peroxide and sodium hydroxide, with an oxidation time of 10–15 minutes. The black oxide film has excellent wear resistance and does not require any curing process. The salt spray test time is generally 600–800 hours. It can maintain the excellent rust-resistant quality of stainless steel. Category 2: Electrolytic oxidation method. Solution preparation: 20–40 g/L of methyl dichromate, 10–40 g/L of manganese sulfate, 10–20 g/L of boric acid, with a pH of 3–4. The colored film is immersed in a 10% HCl solution at 25°C for 5 minutes; if there is no change in color or no peeling of the inner layer, it indicates that the film has good corrosion resistance. 1Crl7 ferritic stainless steel was blackened through electrolytic treatment, with a fast blackening rate; after further hardening, a black oxide film was obtained that had a uniform color, elasticity, and a certain degree of hardness. It is characterized by a simple processing method, fast blackening speed, good coloring effect, and excellent corrosion resistance. It is suitable for the blackening treatment of various stainless steels, making it highly useful. Third category: QPQ heat treatment method. Performed in specialized equipment; the coating is strong and highly wear-resistant ； However, due to stainless steel, especially austenitic stainless steel, its rust resistance after QPQ treatment is not as good as that of the untreated material. The reason is that the chromium content on the surface of austenitic stainless steel has been compromised. This is because during the pre-processing step of QPQ, namely the nitridation process, carbon and nitrogen elements penetrate in, causing damage to the surface structure. It rusts easily; even in a salt spray environment, it starts to rust after just a few hours. Due to this weakness, its practicality is limited. 2. Treatment effect: Color: The black-treated stainless steel comes in a shiny black or matte black color. There are no scratches on the surface, and the color is uniform. Film thickness: 1.5–2.5 microns; this value does not affect the precision of the product. Values above or below 1.5–2.5 have a significant impact on the product’s dimensions, resulting in defective products. Corrosion resistance. Select 2 pieces for the neutral salt spray test; it is desirable for them to withstand 600–800 hours of this test. If they cannot hold up to 600 hours, the performance is poor. The surface should be clean and tidy, free of any blackish-gray deposits, with a secure coating layer. 3 Process Steps: Cleaning ； Keep the product surface clean and tidy. Degreasing: The workpiece must be completely immersed in the degreasing solution ； The pH value of the degreasing solution should be between 12 and 14; the treatment time is 10–30 minutes. The solution should be shaken up and down several times every 3–5 minutes. If the pH value drops below 12, more degreasing powder should be added ； wash with water ； Pickling: The pH value of the pickling solution is 2–4, and the treatment time is 5–10 minutes ； wash with water ； Blackening: Pool solution concentration pH value of 2.5–3.5, treatment time of 10–12 minutes ； wash with water ； Dry and apply oil.