In industrial production, there are two types of surface treatment: physical treatment process and chemical treatment process. Blackening of stainless steel surface is a commonly used process in chemical treatment.
Principle: By chemical treatment, a layer of oxide film is generated on the metal surface, and the surface treatment is achieved through the oxide film. The principle used in this surface treatment process is to create an oxide film on the metal surface under the action of corresponding equipment, which can isolate the metal from direct contact with the external environment.
The common methods for blackening stainless steel are as follows:
Category 1: Acid coloring method
(1) Molten dichromate method. Immerse stainless steel parts in molten sodium dichromate solution and stir thoroughly for 20-30 minutes to form a black oxide film. Remove and cool, then rinse with water.
(2) Chromate black chemical oxidation method. The process of color change of this film layer is from light to dark. When it changes from light blue to deep blue (or pure black), the time interval is only 0.5-1 minute. If this optimal point is missed, it will return to light brown and can only be removed and re colored.
2. The vulcanization method can obtain a beautiful black film, which needs to be pickled with aqua regia before oxidation
3. Alkaline oxidation method. Alkaline oxidation is a solution prepared with sodium hydroxide, with an oxidation time of 10-15 minutes. The black oxide film has good wear resistance and does not require curing treatment. The salt spray time is generally between 600-800 hours. Can maintain the excellent quality of stainless steel without rust.
Category 2: Electrolytic oxidation method
Solution preparation: (20-40g/L dichromate, 10-40g/L manganese sulfate, 10-20g/L boric acid, 10-20g/L/PH3-4). The colored film was soaked in a 10% HCl solution at 25C for 5 minutes, and there was no color change or peeling of the inner film layer, indicating good corrosion resistance of the film layer. After electrolysis, 1Cr17 ferritic stainless steel is blackened rapidly, and then hardened to obtain a black oxide film with uniform color, elasticity, and a certain degree of hardness. The characteristics are simple process, fast blackening speed, good coloring effect, and good corrosion resistance. It is suitable for surface blackening treatment of various stainless steels and therefore has considerable practical value.
Category 3: QPQ Heat Treatment Method
Conducted in specialized equipment, the film layer is firm and has good wear resistance; However, due to the fact that stainless steel, especially austenitic stainless steel, does not have the same rust prevention ability as before after QPQ treatment. The reason is that the chromium content on the surface of austenitic stainless steel has been damaged. Because in the previous process of QPQ, which is the nitriding process, carbon and nitrogen content will infiltrate, causing damage to the surface structure. Easy to rust, salt spray poor will only rust within a few hours. Due to this weakness, its practicality is limited.
Post time: Aug-29-2024