Application of surfactants in metal processing

Metal processing includes metal cleaning, surface coating and metal turning, milling, planing, cobalt, rolling, casting and other processing processes. In order to reduce labor intensity, improve product quality, save energy and reduce cost, some auxiliaries are often used in various processing processes, among which surfactants play a very important role.

1 Surfactants and their properties

Surfactants are substances that can significantly reduce the surface tension of liquids or the interfacial tension of liquids. Their molecules are composed of polar hydrophilic groups and non-polar hydrophobic groups. The common hydrophilic groups include carboxyl group, sulfate group, sulfonic group, amine group, quaternary amine group, hydroxyl group, etc. Hydrophobic groups are represented by long chain alkanes and aromatics. Because they have both hydrophilic and oil-philic "amphiphilic" properties, the surface (or interface) properties of the cleaning solution can be changed during chemical cleaning, so that it has a lower surface tension. Surfactants can be divided into anionic, cationic, nonionic and amphoteric types according to the charge of the surface active part. Surfactants have the properties of wetting, foaming, solubilizing and defoaming.

2 Application of surfactants in metal processing

2.1 Application in metal cleaning

Metal cleaning actually runs through the entire metal processing process. The cleaning agent mainly consists of three components: cleaning agent, corrosion inhibitor and surfactant. Surfactant in chemical cleaning plays the role of adsorption, penetration, emulsification, dissolution, washing, etc., in chemical cleaning not only as an auxiliary agent, but also as the main component and has been widely paid attention to, especially in pickling, alkali washing, corrosion inhibition, oil removal, biocide and other cleaning process has played an increasing role. At present, water-based cleaning with surfactants as the main body has overcome the shortcomings of chemical pickling, chemical alkali washing and other cleaning agents, and has become the main development direction of metal cleaning.

Advantages of water-based metal cleaning agent: high cleaning efficiency, strong fouling force; Good safety performance, does not pollute the environment; Save energy, low washing cost; The washing process has no damage to the parts to be washed, and the parts will not be corroded after washing.

The water-based cleaning agent is mainly composed of a mixture of non-ionic surfactant and anionic surfactant, plus a variety of auxiliary agents. The former has a large fouling capacity and good rust prevention and corrosion inhibition ability, while the latter can improve and improve the comprehensive performance of cleaning agents. The cleaning mechanism is to use the wetting, penetration, emulsification, solubilization, dispersion of surfactants to weaken the adhesion of oil on the metal surface, and then apply mechanical agitation, washing, heating, ultrasonic and other physical methods to make the oil out of the metal surface, and into the lotion is emulsified and dispersed.

The surfactants commonly used in metal cleaning are fatty alcohol polyethylene oxide ether, alkyl phenol polyethylene oxide ether, sodium dodecyl benzene sulfonate, dodecyl glycol amide, sodium methoxyfatty amide benzene sulfonate, etc.

Wang Yu et al. studied the cleaning performance of metal cleaning agent based on two kinds of Gemini surfactants, dioctyl polyoxyethylene ether and dialkyl benzene disulfonate. The results show that the metal cleaning solution containing negative/non-gemini surfactants has high surface activity, and there is a good synergistic effect between negative/non-gemini surfactants.

2.2 Application in metal phosphating

Phosphating is the treatment of metals with a dilute solution containing phosphoric acid, phosphates and other chemicals. The metal surface reacts chemically with phosphoric acid and phosphate medium in the above solution and transforms into a complete insoluble metal phosphate layer with medium corrosion protection. This phosphate conversion film on the metal has a variety of uses, it can improve the paint and metal bonding and corrosion resistance; It can also be used as a carrier for anti-rust oil, a carrier for lubricants in metal cold working and a lubricant for friction tables.

In order to improve the structure of the phosphating film and improve the phosphating effect, the phosphating solution also contains other additives, among which the surfactant has the effect of emulsifying and solubilizing the oil adhering to the metal surface, and can prevent the reprecipitation of dirt; Make the solution wet and permeate, which is conducive to the contact between the phosphating solution and the metal surface; Reducing the surface tension of the liquid is conducive to the escape of hydrogen in the phosphating reaction, strengthening the polarization of the cathode, and thus accelerating the phosphating film formation process.

The surfactants commonly used in phosphating are formed by the condensation of ethylene oxide and various hydrophobic hydrocarbon long chains, that is, non-ionic surfactants.

2.3 Application in metal plating

In the plating process, in order to improve the performance of the electrolyte and improve the quality of the coating, it is often necessary to add some additives, among which the surfactant is one of the main varieties, it has the effect of leveling, bright, diffusion, changing the physical properties of the coating, inhibiting the escape of acid fog.

Surfactants commonly used in electroplating include Pinga, OP emulsifier, sodium dodecyl sulfate, sodium methylene binaphthalene sulfonate, fluorinated surfactants, fatty acid polyethylene oxide ester, polyethylene oxide castor oil, etc.