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In the water treatment industry, sewage treatment equipment is very important for prevention of corrosion, the sewage in the microorganisms on the damage of sewage treatment facilities is particularly serious, therefore, corrosion resistant materials, corrosion protection measures are very common in the sewage treatment industry.

Corrosion prevention methods can be divided into two broad categories: one is the correct selection of anti-corrosion materials and other anti-corrosion measures; two, the choice of reasonable process operation and equipment structure. Strict adherence to the technical procedures of chemical production can eliminate the corrosion that should not occur, and even if a good corrosion resistant material is used, the corrosion procedures in the operation process will also cause severe corrosion. At present, the corrosion prevention methods commonly used in chemical production are as follows.

1, the correct selection and design

It is the most effective method to understand the corrosion resistance of different materials and choose the corrosion resistant material correctly and reasonably. As everyone knows, a lot of variety of materials, the corrosion rate of different materials in different environments have different material, personnel shall satisfy a specific environment corrosion rate is low, the price is cheaper, physical and mechanical properties of the design requirements of the material, in order to obtain economic and reasonable equipment service life.

2, adjust the environment

If you can eliminate various factors caused by corrosion environment, corrosion will stop or slow down, but most of the environment can not be controlled, such as air and water in the soil and in the water of oxygen can not be removed, and the chemical production process can not be changed. But some local environment can be adjusted, such as boiler water to oxygen (adding deoxidier sodium sulfite and hydrazine), can protect the boiler from corrosion; and as the first water out of air into the sealed warehouse, also can avoid the storage of metal parts to prevent rust; cooling water heat exchangers and other equipment cause scaling and perforation, in addition of alkali water or acid to adjust the pH value to the optimal range (neutral); often with alkali or ammonia refining process in the production of fluid remain neutral or alkaline. When the temperature is too high, it can be cooled or cooled on the wall of the machine, or lined with refractory bricks on the inner wall of the equipment, and so on. All these are the methods adopted under the premise of changing the environment without affecting the products and processes. Under the premise of allowing, it is recommended that the medium should be used instead of the strong corrosive medium.

3. Add corrosion inhibitors

Usually, a small amount of inhibitor is added to the corrosive environment to greatly reduce the corrosion of metals. We usually divide them into three classes: inorganic, organic and gas phase inhibitors, and their corrosion inhibition mechanisms are also different.

4. Cathodic protection

Cathodic protection is a method of reducing or eliminating metal corrosion by relying on an applied DC current or sacrificial anode to protect the metal from being a cathode. Because before the application of cathodic protection, corrosion of the metal structure most are cathode region and anode region, if the anode region all into cathode region, the metal component into the cathode, also can eliminate the corrosion. For a specific project, many problems should be considered before choosing the cathodic protection system:

5, anodic protection

When the device is used as an anode, the current passing through the outside will accelerate the corrosion, and the corrosion current will increase with the polarization of the anode. But there will be another case of passivation of the metal, when the potential with the current rise to passive potential, the corrosion current decreased rapidly, even tens of thousands of times can be reduced, with potential rise, current is constant, until far blunt. Using this principle, the equipment needs to be protected as an anode, and the current is introduced to keep the potential in the middle of the passivation zone. The corrosion rate can be kept low, and the current through it will indicate the corrosion rate of the equipment.

6, alloying

The alloy component which can promote passivation is added into the base metal, and when the amount is added to a certain proportion, the material with good corrosion resistance is obtained. Such as iron in adding more than 12% chromium, called stainless steel; nickel chromium steel added, can expand the scope of the passivation, but also can improve the mechanical properties; and as adding 14% silicon iron, have high silicon acid resistance excellent, etc.. In addition, the addition of small amounts of ultra low voltage cathodic noble metals in certain reactive metals can promote passivation. Such as stainless steel and titanium are activated in certain concentration and temperature in sulfuric acid, such as 0.1~0.15% in the base metal palladium or platinum, will be distributed on the surface of alloy become numerous micro cathode, promote local corrosion battery operation, the cathode current increases quickly, quickly reach the passivation region, the metal corrosion resistance.

7, surface treatment

When the metal is contacted with the environment, it is treated with a passivating agent or a film forming agent to form a stable and dense passive film on the surface, and the corrosion resistance is greatly increased. It is different with the inhibitor method, it is in use in the environment do not need to add inhibitor, aluminum by anodic treatment, can be generated on the surface is more dense than film generated in the atmosphere, this kind of film has excellent corrosion resistance in mild corrosive environment, the surface of steel bluing this is the principle of iron parts.

8, metal coating and coating

A thin layer of more corrosion-resistant metal protects the base of the steel. Commonly used method is electroplating, generally plated 2~3 layer, only a few microns thick, and inevitably exist microporous, solution can penetrate the pores, will constitute a coating - corrosion battery. If the coating is precious metal, its potential is higher than iron, will become a cathode, will accelerate the underlying iron corrosion. Therefore, this kind of coating is not suitable for strong corrosive environment, but it can be used in atmosphere, water and other environments. The corrosion products produced by slow forming can plug the micropores, increase the resistance and obtain a certain service life. If the polarity of a corrosive cell is made of cheap metals, the reverse is made to protect the steel from cathodic protection and to maintain a longer life. In addition to electroplating, hot dip coating (molten dipping), flame spraying, steam plating and integral metal sheet plating are also used. The latter because of no microporous, corrosion resistance, longer life, but the price is slightly higher.

9, coating

Using organic coatings to protect the metal structure in the atmosphere is the most widely used means of corrosion protection. The coating covers the metal surface and forms a porous film after drying. Although it can not completely separate the metal from the medium, it increases the value and reduces the corrosion current through the diffusion resistance of the micropores and the resistance of the solution. In the relaxed environment, such as air, water, micropore corrosion of metal corrosion products can be slow, clogging pores, long service life, but it is not suitable for highly corrosive solution, because the metal corrosion speed, and accompanied by the production of hydrogen, can make the surface rupture.

10, lining

The lining is usually made of a whole piece of material suitable for contact with strongly corrosive media. Tanks, such as hydrochloric acid and dilute sulphuric acid, are lined with rubber or plastic, and steel tanks for storing nitric acid are lined with stainless steel sheets. Enamel is actually a kind of glass lining. It is called enamel glass in industry. It has strong acid resistance. It is widely used in food, medicine and other industries. It can guarantee the quality of products, but it can not burn too much equipment.

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