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Action mechanism of water treatment agent corrosion inhibitor

2016-05-08 21:26:26

At present, there is no uniform understanding of the corrosion mechanism of water treatment agent corrosion inhibitor, and several main theories are introduced in the following.

(1) adsorption theory
It is considered that the corrosion inhibitor is adsorbed on the metal surface to form a continuous adsorption layer, which can protect the corrosion medium and metal. It is generally believed that the corrosion inhibition effect of organic corrosion inhibitor is the result of adsorption. This is because the organic corrosion inhibitor molecules are composed of two parts: part is easy to be metal adsorption of hydrophilic polar groups, and the other part is hydrophobic or lipophilic organic atomic group (such as alkyl). Very basic end is adsorbed on the metal surface, and the increasing water end up forming alignment, the corrosive medium is corrosion inhibition agent molecules arranged extrusion, such adsorption makes medium and the metal surface spaced to metal protective role.
(2) the theory of phase forming membrane
Phase film theory that metal surface to form a layer of insoluble complexes, the insoluble complex is the product of ionic interactions of metal corrosion inhibitor and corrosion medium, such as corrosion agent amino acid in the hydrochloric acid and iron is generated [HORNH2][FeCl4] or [HORNH2][FeCl2] complex covering plays a protective effect on the surface of the metal. In concentrated hydrochloric acid and Fe, the Fe is formed on the Fe surface, a difficult solution of the complex, so that the metal is no longer in contact with the acid, slow down the corrosion of metals.
(3) electrochemical theory
From the electrochemical point of view, the corrosion of metal is the anodic and cathodic processes in the electrolyte solution. Corrosion inhibitor Canadians can block any process or the arrest of two process, in order to achieve slow down the corrosion rate. This effect can be expressed by means of polarization, increase the anode polarization or cathodic polarization, or both at the same time increase, so that the corrosion current I1 reduced to I2. Of course, the anodic polarization may also cause cathodic depolarization strengthened, the corrosion current increased to I2 ', thus exacerbating the corrosion. According to the above electrochemical principles, corrosion inhibitor can be divided into anode corrosion inhibitor, cathodic inhibitor and mixed type corrosion inhibitor.
Oxide scale inhibitor
Corrosion inhibitor, directly or indirectly, and the formation of metal oxides or hydroxides, and to form a protective film on the metal surface, this kind of protective film, a thin and dense, strong adhesion with the base metal, closely combined with, obstructs the diffusion of the dissolved oxygen, metal corrosion reaction rate reduced. This kind of protective film in the formation process, the membrane would not have been thickening. When the oxide film increases to a certain thickness, a portion of the oxide will diffuse to the solution, when the oxide to the solution diffusion trend become barriers to thickening of the membrane, membrane thickness growth almost automatically stop. Therefore, the oxide film type corrosion inhibitor effect is good, and there is excess inhibitor will not produce scale.
Most of the film type inhibitor are heavy metal oxyacid salts, such as chromate, platinate and tungstate. Because of the heavy metal corrosion inhibitor, it is easy to cause environmental pollution, so it is generally used less.
Nitrite is formed by the aid of dissolved oxygen in the water to form an oxide film on the surface of the metal and the oxide film is formed by the oxidation of the oxide film. It is worth noting that the corrosion inhibitor in the use of water containing oxidizing agent, it will be oxidized into nitrate, the corrosion effect will be weakened, so it can not be used with the oxidation of bactericidal type (such as chlorine and so on) at the same time. In addition, nitrite in the long-term use, the system will be a large number of bacteria in the system, can be a large number of bacteria breeding, can be oxidized to nitrite nitrate, so that the effect of reducing the corrosion. Therefore, the use of nitrite is also subject to certain restrictions.
Ion precipitation film corrosion inhibitor in water
The deposited film corrosion inhibition agent can and dissolved in the generation of ions in water insoluble salts or complexes, precipitation in the metal precipitated on the surface, resulting in the formation of corrosion film, this film is porous, thick, loose, mostly with the base metal adhesive poor. Therefore, it prevents oxygen to diffuse to the surface of metal incomplete, anti-corrosion effect is not very ideal, if the amount of corrosion inhibitor was too much, generated by the film thickness will continue to increase, due to the fouling layer thickened affect heat transfer, if temperature is higher, and the water bicarbonate, will generate carbonate scale and dissolve more carbon dioxide, so the system is unfavorable. The deposition film type corrosion inhibitor is a poly phosphate, silicate and zinc salt, etc..
Usually think of polyphosphate is an anodic type inhibitor, because it is mostly dominated by Fe2O3 and FePO4 protective film, it can inhibit the anodic reaction, but oilfield water Ca2, Mg2 concentrations were higher, poly phosphate easily and they generate complex deposition of the protective film is mainly calcium polyphosphate and poly phosphoric acid magnesium etc., they are deposited on the cathode surface, inhibits the cathodic reaction. Therefore, in the use of poly phosphate as corrosion inhibitor, the water should have a certain concentration of Mg2, Ca2 ions, so the corrosion effect is more significant. If using a poly phosphate as corrosion inhibitor, measures must be taken to control microbial growth, because the polyphosphates are microbial growth nutrients. It will promote microbial reproduction, resulting in a lot of conjunctival bacterial, the deterioration of water quality, so the effective sterilization measures is necessary.
Zinc salt is a cathodic inhibitor, which is the role of the zinc ion, which produces Zn (OH) 2 in the cathode part of precipitation, the role of protective film. Zinc salt anions generally does not affect its corrosion inhibition ability, zinc chloride, zinc sulfate and zinc nitrate and can be selected, but if used zinc chloride, chloride ion content in water will increase, it can destroy the protective film, resulting in a number of adverse consequences, should be noted that when in use, the zinc salt is generally not used alone, if it and other corrosion inhibition agent used in combination, will have obvious synergistic effect. The use of zinc salt has some limitations, it is very serious pollution of the environment, if you can find other non polluting corrosion inhibitor, then should avoid the use of heavy metals as corrosion inhibitor.
The metal ion precipitation membrane inhibitor
This inhibitor is due to the activation of the metal dissolved, and the metal ions in the site and the corrosion inhibitor to form precipitation, resulting in a dense film, the corrosion inhibition effect is good. After corrosion film formation, even under the corrosion inhibitor in excess. The film also stop growing, because of the formation of corrosion film once, it will metal wrapped up, but not with corrosion inhibitor continues, will stop precipitate, anti corrosion film is no longer thickening. The protective effect because it is formed on the surface of the copper body of the chelate, thereby inhibiting corrosion. This kind of corrosion inhibitor and heterocyclic thiol, etc.. Mercaptobenzothiazol phosphate and polyphosphate used together, has a good effect on preventing metal corrosion.
The adsorption of membrane type inhibitor
Adsorption of membrane type corrosion inhibition agent is organic compounds, the molecule is hydrophilic and hydrophobic groups and hydrophilic groups of the molecules can effectively adsorb on clean metal surface, and hydrophobic gene toward the aqueous side, which prevents to metal surface diffusion of dissolved oxygen and water, so as to inhibit the corrosion reaction. This kind of corrosion inhibitor anti inhibition effect with the metal surface cleanliness has the very big relations, if a lot of dirt on the metal surface, the adsorption film formation not tight, not to isolate corrosion medium, the corrosion in local will be very serious. Adsorption of membrane type inhibitor with amine compounds and other surfactant compounds.
To sum up, the oxide film type corrosion agent such as chromate although the effect is good, and the economy is reasonable and feasible, but in consideration of environmental protection, is expected in the future there will be great development, metal ion precipitation membrane type corrosion inhibition agent development is not large. So far, mainly used in the water and ion precipitation of the film type inhibitor, that is, poly phosphate and zinc salt. In recent years, many researches have been made on the adsorption film type corrosion inhibitor, and the experiment is also carried out in China.

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