Degreasing of metal surfaces
Metal surface degreasing is one of the key processes in the metal surface treatment process. Its main function is to remove grease, oil stains and other organic pollutants on the metal surface, providing a clean surface for subsequent processes such as rust removal, phosphating, painting, and electroplating, ensuring that subsequent treatments can adhere evenly and firmly to the metal surface, thereby improving the quality and performance of the product. The grease on the metal surface comes from a wide range of sources, including cutting fluids and lubricants used in the processing process, oil stains contaminated during storage and transportation, and mixtures of dust and grease in the environment. If these greases are not thoroughly removed, they will form an isolation film on the metal surface, hindering the contact between subsequent treatment agents and the metal substrate, leading to problems such as coating shedding and plating blistering, seriously affecting the service life and appearance quality of the product. Therefore, metal surface degreasing is the premise and basis for ensuring the effectiveness of subsequent treatments.
There are many methods for degreasing metal surfaces. According to the different degreasing principles, they can be divided into physical degreasing, chemical degreasing, electrochemical degreasing, etc. Physical degreasing mainly removes surface grease through mechanical action, heating, adsorption, etc. Common methods include wiping, high-pressure water washing, ultrasonic degreasing, etc. The wiping method is suitable for removing small areas and localized oil stains. It is simple to operate but has low efficiency. The high-pressure water washing method uses the impact force of high-pressure water flow to wash away surface grease. It is suitable for large areas and workpieces with relatively flat surfaces. The ultrasonic degreasing method uses the cavitation effect generated by ultrasonic waves in the liquid to separate the grease from the metal surface and disperse it into the degreasing liquid. It has the advantages of high degreasing efficiency and the ability to process workpieces with complex shapes. It is widely used in the degreasing of precision parts.
Chemical degreasing uses chemical reagents to react with oils and fats, converting them into water-soluble substances. Common chemical degreasing agents include alkaline degreasing agents, acidic degreasing agents, and solvent-based degreasing agents. Alkaline degreasing agents are among the most widely used degreasing agents in industry. They are primarily composed of alkaline substances such as sodium hydroxide, sodium carbonate, and sodium silicate. They remove animal and plant oils and fats through saponification and mineral oils through emulsification. They offer advantages such as low cost and excellent degreasing effectiveness, making them suitable for degreasing metal materials such as steel and aluminum alloys. Acidic degreasing agents are suitable for removing certain greases that are difficult to remove with alkaline degreasing, and can also remove surface scale and rust. However, they are somewhat corrosive to metal substrates and require the addition of corrosion inhibitors. Solvent-based degreasing agents, such as gasoline, alcohol, and trichloroethylene, achieve degreasing by dissolving oils and fats, resulting in rapid degreasing. However, they are flammable, explosive, and environmentally polluting, and are currently being replaced by more environmentally friendly degreasing agents.
Electrochemical degreasing, based on chemical degreasing, utilizes the mechanical stirring and emulsification effects of gases (hydrogen or oxygen) generated by electrode reactions through the application of direct current to accelerate grease removal, improving degreasing efficiency and quality. Electrochemical degreasing can be divided into two methods: anodic and cathodic. Anodic degreasing uses the workpiece as the anode, generating oxygen bubbles on the surface, resulting in effective degreasing but susceptible to surface corrosion. Cathodic degreasing uses the workpiece as the cathode, generating hydrogen bubbles on the surface. This results in faster degreasing and less corrosion, but can easily lead to impurity adsorption. In practical applications, alternating anode and cathode degreasing is often used to balance degreasing effectiveness and workpiece quality. Electrochemical degreasing is suitable for applications requiring high degreasing standards, such as precision instruments and automotive parts.
The effectiveness of metal surface degreasing directly impacts the quality of subsequent processes, necessitating rigorous testing and control of degreasing results. Common testing methods include the wiping method, the water film method, and contact angle measurement. The wiping method involves wiping the degreased surface with a clean white cloth and observing it for traces of oil. The water film method involves spraying water onto the degreased surface. A uniform, continuous film that persists for a period of time indicates good degreasing. The contact angle method assesses degreasing effectiveness by measuring the contact angle of water on the metal surface; a smaller contact angle indicates a more thorough degreasing. During the degreasing process, the degreasing agent and process parameters, such as temperature, time, and concentration, must be carefully selected based on the type of metal material and the nature and amount of oil contamination to ensure effective degreasing. With growing environmental awareness, the development of efficient, environmentally friendly, and low-toxic degreasing agents and processes is becoming a trend. For example, biodegradable degreasing agents and low-temperature degreasing processes are being used to reduce environmental pollution and achieve green production in metal surface treatment.
