Corrosion Prevention Methods Commonly Used in Chemical Production

Anti-corrosion methods in general can be divided into two categories: first, the correct selection of anti-corrosion materials and other anti-corrosion measures; The second is to choose a reasonable process operation and equipment structure. Strict compliance with the process procedures of chemical production can eliminate the corrosion phenomenon that should not occur, and even if the use of good corrosion resistant materials, no corrosion procedures in the operation process will cause serious corrosion.

At present, the commonly used corrosion prevention methods in chemical production are as follows:

1. Correct material selection and design

It is the most effective way to understand the corrosion resistance of different materials and select the anti-corrosion materials correctly and reasonably. As we all know, there are many varieties of materials, different materials in different environments in the corrosion rate is also different, material selection personnel should be for a specific environment to choose low corrosion rate, cheaper, physical and mechanical properties to meet the design requirements of the material, so that the equipment to obtain economic and reasonable service life.

2. Adjust your environment

If the various factors causing corrosion in the environment can be eliminated, the corrosion will be stopped or slowed down, but most of the environment is uncontrollable, such as the water in the atmosphere and soil, the oxygen in seawater, etc., can not be removed, and the chemical production process can not be changed at will. But some local environment can be adjusted, such as boiler water first remove oxygen (adding deoxidizer sodium sulfite and hydrazine, etc.), can protect the boiler from corrosion; If the water is removed before the air enters the closed warehouse, the stored metal parts can also be prevented from rusting.

In order to prevent the cooling water from causing scaling and perforation of heat exchangers and other equipment, alkali or acid can be added to the water to adjust the PH value to the best range (near neutral); In refining process, alkali or ammonia is often added to keep the production fluid neutral or alkaline. When the temperature is too high, it can be cooled in the wall, or lined with firebrick heat insulation in the inner wall of the equipment, etc. These are the methods used under the premise of changing the environment and not affecting the product and process, under the premise of allowing, it is recommended to use a moderate medium instead of a strong corrosive medium in the process.

3. Add corrosion inhibitor

Usually, adding a small amount of corrosion inhibitor in the corrosive environment can greatly slow down the corrosion of metals, we generally divide it into inorganic, organic and gas phase corrosion inhibitors three categories, the corrosion inhibition mechanism is also different.

3.1 Inorganic corrosion inhibitor

Some corrosion inhibitors will slow down the anode process, called anodic corrosion inhibitors, which include oxidants that promote anode passivation (chromate, nitrite, iron ions, etc.) or anodic film forming agents (alkali, phosphate, silicate, benzoate, etc.), which mainly react in the anode region to promote anode polarization. General anode corrosion inhibitor will generate a protective film on the anode surface, in this case the corrosion inhibition effect is better, but there is also a certain risk, because if the dose is insufficient, it will cause the protective film is incomplete, the bare metal area exposed to the film defect is small, the anode current density is large, and it is more likely to perforate.

Another type of corrosion inhibitor is the reaction at the cathode, such as calcium ions, zinc ions, magnesium ions, copper ions, manganese ions and the cathode produce hydroxide ions, forming insoluble hydroxide, which covers the surface of the cathode in a thick film, thus blocking oxygen diffusion to the cathode and increasing concentration polarization. In addition, there are also mixed corrosion inhibitors that block the anode and cathode at the same time, but the amount of addition generally needs to be determined by testing.

3.2 Organic corrosion inhibitor

Organic corrosion inhibitors are adsorptive, adsorbed on the metal surface to form an invisible film several molecules thick, which can block the anode and cathode reactions at the same time, but the influence on the two is slightly different. Organic compounds containing nitrogen, sulfur, oxygen and phosphorus are commonly used as inorganic corrosion inhibitors, and their adsorption types can be divided into electrostatic adsorption, chemical adsorption and π bond (unlocated electron) adsorption depending on the molecular configuration of organic matter. The development of organic corrosion inhibitors is very fast, and the use of it is very wide, but the use of it will also produce some shortcomings, such as contaminated products, especially food, corrosion inhibitors may be beneficial to this part of the production process, but into another part of the harmful substances, it may also inhibit the required reaction, such as pickling so that the film removal speed is too slow, etc.

3.3 Vapor phase corrosion inhibitor

This type of corrosion inhibitor is a highly volatile substance, containing corrosion inhibition groups, generally used to protect metal parts in storage and transportation, and is mostly used in solid form. Its steam is decomposed by atmospheric water into effective corrosion inhibition groups, which are adsorbed on the metal surface to achieve the purpose of slowing down corrosion. In addition, it is also an adsorbent corrosion inhibitor, and the protected metal surface does not require rust removal treatment.

4. Cathodic protection

Cathodic protection is a method of reducing or eliminating metal corrosion by relying on external direct current or sacrificial anode to make the protected metal become a cathode. Because before the application of cathodic protection, there are cathode and anode regions on most corroded metal structures, if all the anode regions can be turned into cathode regions, and the entire metal component becomes cathode, the purpose of eliminating corrosion can be achieved.