Development And Application of Surfactants in Corrosion Inhibition And Rust Prevention Industry

Surfactants contain oleophilic non-polar groups and hydrophilic polar groups. Since most metal surfaces are charged, hydrophilic groups are adsorbed on the surface of the metal by physical or chemical action. During adsorption, the hydrophilic group is oriented towards the metal, and the oilophilic group is oriented away from the metal. When the hydrophilicity of the surfactant is strong, the surfactant has a strong adsorption force on the metal, but the dispersion is poor at this time. If the surfactant has a strong lipophilicity, it has a good dispersion, but the adsorption is poor. Therefore, in different cases, surfactants are required to have different hydrophilic and lipophilic values (HLB values). Due to the adsorption of surfactants on the metal surface, on the one hand, the properties of the interface and the charge distribution state of the metal surface are changed, so that the energy state of the metal surface tends to be stable, the activation energy of the corrosion reaction increases (the energy barrier increases), and the corrosion rate slows down. On the other hand, the oleophilic group of the adsorbed surfactant forms a hydrophobic protective film on the metal surface, preventing the movement of charges and substances associated with the corrosion reaction and reducing the corrosion reaction rate.

The charge state of the metal surface is particularly important for the adsorption of surfactants. The adsorption of surfactant on metal surface mainly depends on electrostatic attraction and van der Waals force, of which electrostatic attraction is the most important. If the potential (φ0=0) when there is no charge distribution on the metal surface is the basis, when the metal is in the corrosive medium, if the corrosion potential is greater than zero (φ1>φ0), the metal surface is positively charged, and it is easy to adsorb anionic surfactants. If the corrosion potential is less than zero (鯸-1<鯸-0), the metal surface is negatively charged, so it is easy to adsorb cationic surfactants. If the corrosion potential is equal to (φ1=φ0), the metal surface is not charged, and the chemical adsorption of surfactant molecules often occurs.

No matter what kind of adsorption occurs, as long as an effective protective film can be formed on the metal surface, it can slow down the corrosion of the medium to the metal. In actual industrial reactors, most metals are negatively charged (cathode suppressed), so most of the cationic surfactants containing sulfur, nitrogen and phosphorus are used as metal preservatives, and the lone pair electrons contained in such surfactants can form ions with hydrogen ions or other cations. These ions are attached to the surface of the metal in a single molecular layer.

When this cation is adsorbed on the negatively charged metal surface, it is difficult for the hydrogen ions in the acidic medium to approach the metal surface again (the overpotential of H+ increases) and the reduction reaction with the metal occurs, so that the corrosion is inhibited. Due to the different corrosion potentials of metals in different media, it is necessary to pay attention to the selectivity of surfactants to metals. For example, the strongly alkaline dibutylammonium ion (C4H9N) + does not have a good anti-corrosion effect on 1N sulfuric acid. Because the corrosion potential of iron in 1N and sulfuric acid is (φ1=-0.28V), and the zero potential of iron is φ1=-0.37V, that is: φ1>(φ0, so the energy surface is positively charged, it is difficult to adsorb cations. If a small amount of anions can be strongly adsorbed by metal iron, such as: chloride ions, iodine ions, thiocyanogen ions, etc. These ions are first adsorbed on the surface of metal iron and the surface of iron is negatively charged, so that the anti-corrosion effect of quaternary ammonium salt will be greatly strengthened.

In addition, when the amount of amines in the acidic medium is insufficient, it will not only not play the anti-corrosion role, but also accelerate the corrosion reaction. This is because ions tend to discharge in the cathode region and accelerate the cathode reaction. The reaction is as follows:?

Not only do polar groups have such an important effect on the corrosion rate, but non-polar groups also have a very important effect on the corrosion rate. With the increase of the carbon chain in the non-polar group, the anti-corrosion effect is gradually enhanced. Due to the growth of the carbon chain, the attraction between the carbon chains is enhanced, and the shielding effect on the metal surface is enhanced, so the corrosion reaction is more difficult to occur.

In addition to the physical adsorption described above, there is chemisorption. This adsorption belongs to monolayer adsorption and is irreversible. In addition, the double chain, triple chain and π electrons on the benzene ring in the surfactant can form chemical bonds with the metal and produce adsorption, which enhances the corrosion resistance of the metal.

In fact, the basic mechanism of action of organic corrosion inhibitors and anti-rust additives is the same. Inhibitor molecules form an adsorption film on the surface of the metal, shielding water or other corrosive media to prevent corrosion or rust.