Metal Surface Activation

Metal surface activation
Metal surface activation is an important pretreatment process in metal surface treatment. Its purpose is to remove the passivation film, oxide layer, and other inert substances on the metal surface, increase the chemical activity of the surface, create favorable conditions for subsequent electroplating, chemical plating, painting, and other processes, and enhance the bonding strength between the plating or coating and the metal substrate. When metal is exposed to air or after certain treatments, a passivation film will naturally form on the surface, such as the oxide film on aluminum and the passivation layer on stainless steel. Although these passivation films can provide protection to a certain extent, they will hinder the direct contact between subsequent treatment agents and the metal substrate, resulting in insufficient bonding strength and affecting product quality. Therefore, metal surface activation is a key step in ensuring the quality of subsequent processes.

Methods for metal surface activation mainly include chemical activation, electrochemical activation, and physical activation. Chemical activation is the most commonly used method, which uses acidic or alkaline solutions to react chemically with the passivation film on the metal surface, causing it to dissolve or convert into easily removable substances. For example, steel surfaces are often activated with dilute hydrochloric acid or sulfuric acid to remove surface oxide scale and passivation film; aluminum alloy surfaces are often activated with mixed solutions such as nitric acid and hydrofluoric acid to remove the natural oxide film and form a uniform active surface. Chemical activation is simple to operate and low in cost, and is applicable to a variety of metal materials, but the concentration, temperature, and treatment time of the solution must be strictly controlled to avoid excessive corrosion of the metal substrate.

Electrochemical activation is a method that uses electrode reactions to remove surface passivation films, usually in acidic or alkaline electrolytes. The metal workpiece is used as the anode or cathode, and by applying current, the passivation film on the surface is dissolved or peeled off under the action of the electrochemical reaction. During anodic activation, an oxidation reaction occurs on the surface of the workpiece, and the passivation film is oxidized and dissolved; cathodic activation uses the generation and reduction of hydrogen to remove the surface oxide layer. The advantages of electrochemical activation are fast activation speed, uniform effect, and the ability to accurately control the degree of activation. It is suitable for precision parts with high surface quality requirements, such as electronic components, aerospace parts, etc. However, this method requires dedicated electrolysis equipment and power supply, and the energy consumption is relatively high.

Physical activation is a method of removing the passivation film on the metal surface through physical means, including mechanical grinding activation, laser activation, and plasma activation. Mechanical grinding activation uses tools such as sandpaper and grinding wheels to polish the surface, removing the passivation film and oxide layer while increasing the surface roughness. Laser activation uses a high-energy laser beam to instantly irradiate the metal surface, causing the passivation film to decompose or evaporate due to heat. It has the advantages of being contactless, highly precise, and highly efficient. Plasma activation bombards the metal surface with high-energy particles in the plasma, breaking the chemical bonds of the surface atoms, removing the passivation film, and increasing surface activity. It is suitable for applications with extremely low surface damage requirements. Physical activation methods are environmentally friendly and do not produce chemical waste liquids, but the equipment investment is relatively large and is suitable for specific high-end applications.

The evaluation of metal surface activation effects is an important step in ensuring the quality of subsequent processes. Common evaluation methods include bonding strength testing, surface energy measurement, and electrochemical testing. The bonding strength test determines the bonding strength between the plating or coating and the metal substrate through experiments such as stretching and peeling; the surface energy measurement uses a contact angle meter to determine the surface energy of the activated surface. The higher the surface energy, the better the activation effect; the electrochemical test analyzes the electrochemical activity of the surface through methods such as polarization curves to evaluate the degree of activation. In practical applications, it is necessary to select appropriate activation methods and process parameters based on the type of metal material and the requirements of subsequent processes. With the development of materials science and surface engineering technology, new activation technologies such as nano-coating activation and bio-enzyme activation are being developed and applied. These technologies will further improve the activation effect and environmental performance of metal surfaces and promote the continuous advancement of metal surface treatment processes.