Corrosion inhibitor mechanism
At present, there is no unified understanding of the corrosion inhibition mechanism of water treatment agents and corrosion inhibitors. Below are several main theories introduced
adsorption theory
It is believed that corrosion inhibitors adsorb on the metal surface to form a continuous adsorption layer, isolating the corrosive medium from the metal and thus playing a protective role. Currently, it is widely believed that the corrosion inhibition effect of organic corrosion inhibitors is the result of adsorption. This is because the molecules of organic corrosion inhibitors are composed of two parts: one is hydrophilic polar groups that are easily adsorbed by metals, and the other is hydrophobic or lipophilic organic atomic groups (such as alkyl). One end of the polar group is adsorbed by the metal surface, while the other end of the water increasing group forms a directional arrangement upwards. As a result, the corrosive medium is squeezed out by the arrangement of corrosion inhibitor molecules, which separates the medium from the metal surface and plays a role in protecting the metal.
Phase forming film theory
The theory of phase forming film suggests that a layer of insoluble complex is formed on the surface of a metal, which is the product of the ion interaction between a metal corrosion inhibitor and a corrosion medium. For example, the inhibitor amino acid reacts with iron in hydrochloric acid to form [HORNH2] [FeCl4] or [HORNH2] [FeCl2] complex, covering the surface of the metal for protection. Quinoline reacts with Fe in concentrated hydrochloric acid to form a insoluble Fe complex on the surface of Fe, which stops metal contact with acids and slows down metal corrosion.
Electrochemical theory
From an electrochemical perspective, metal corrosion is an anodic and cathodic process that occurs in an electrolyte solution. The addition of corrosion inhibitors can block the progress of any process or both processes simultaneously, thereby achieving the effect of slowing down the corrosion rate. This effect can be represented by a polarization diagram, where increasing anode polarization or cathode polarization, or both, reduces the corrosion current I1 to I2. Of course, anodic polarization may also lead to enhanced cathodic depolarization, increasing the corrosion current to I2 ‘, thereby exacerbating corrosion. According to the above electrochemical principles, corrosion inhibitors can be divided into anode corrosion inhibitors, cathode corrosion inhibitors, and mixed type corrosion inhibitors.