The oxidation treatment of the spring is also called bluing, blackening, and boiling black. After oxidation, protective magnetic iron oxide is formed on the surface of the spring. This oxide film is generally blue or black, and sometimes dark brown. Its color depends on the surface state of the spring, the chemical composition of the spring material and the oxidation treatment process.
Spring blackening process
In the heated solution of the spring, iron ions (ferrous compounds) are produced due to the alkali corrosion on the surface of the spring. The addition of oxidizing agent-sodium nitrite in the alkali can change the oxidation process and form a dense oxide film on the surface of the spring (4 Ferric oxide). This oxide film is mainly composed of magnetic iron oxide, and the generation time of the oxide film is 30-60 minutes. If the time is extended, the thickness of the oxide film cannot be increased.
The oxidation treatment cost is low, the process formula is relatively simple, the production efficiency is high, the oxide film has a certain degree of elasticity, and basically does not affect the characteristic curve of the spring, so the oxidation treatment is widely used as the anti-corrosion and anti-corrosion treatment of formed coil springs, spring washers and leaf springs Decorative measures.
The quality inspection of the spring after oxidation treatment includes visual inspection and corrosion resistance inspection.
The oxidation treatment methods include: salty oxidation method, alkali-free oxidation method and electrolytic oxidation method. The alkaline oxidation method is mostly used.
The alkaline oxidation method is to put the spring in a sodium hydroxide solution containing an oxidant at a temperature of about 140°C for a certain period of time. The oxidant and sodium hydroxide react with iron to generate sodium ferrite and sodium ferrite, and then react with each other. This produces magnetic iron oxide.
The thickness of the oxide film layer is about 0.6~2μm. Although the oxide film can improve the corrosion resistance of the spring, because the film is thin and has pores, its protection ability is poor, and it can only be used for springs that work in less corrosive media. The level of its anti-corrosion performance depends on the compactness of the oxide film and its thickness, which is also determined by factors such as the concentration of sodium hydroxide, the concentration of the oxidant, and the temperature of the solution.
In order to improve the corrosion resistance and lubricity of the oxide film, the treatment before and after the oxidation treatment should be strengthened. Before oxidation treatment, the rust, oxide scale, oil stain, heat-treated salt residue and surface contact layer on the spring surface must be thoroughly removed. After oxidation treatment, the spring is usually filled in soap solution or dichromate, then washed with running warm water, blow-dried or dried, and finally replaced with a water film to replace the anti-rust oil or mechanical oil with a certain temperature Carry out oil immersion treatment.
Oxidation treatment has eroded the surface grain boundaries of some hot-rolled spring materials, which will reduce the fatigue strength to a certain extent. Therefore, care should be taken when using oxidation treatment.
The traditional oxidation treatment requires heating. In recent years, some domestic manufacturing units have used normal temperature blackening agent, which overcomes the shortcomings of the traditional blackening process and saves a lot of energy.
The blackening agent is blue-green concentrated liquefaction, no impurities, no peculiar smell, non-flammable, non-explosive, non-corrosive, and safe transportation. The blackening agent is diluted with water, and the commonly used dilution ratio is about 1:5 when the spring is blackened.
The operation process of the blackening agent is relatively simple, and the process route is: degreasing, rinsing, acid cleaning, rinsing, blackening (2~5min at room temperature), rinsing, and water film replacement anti-rust oil. It should be noted that the oil on the spring work piece must be removed, cleaned, and then put into the room temperature agent for oxidation treatment.