From the point of view of the damage of the heat exchanger, corrosion is a very important reason, and the corrosion of the heat exchanger is ubiquitous. To solve the corrosion problem is to solve the fundamental problem of heat exchanger damage. In order to prevent the corrosion of the heat exchanger, it is necessary to clarify the root cause of the corrosion. The reasons for the corrosion of the heat exchanger are discussed in the following aspects.
First, the choice of materials for heat exchangers:
The determinant of the material used is its economy. The pipe material is stainless steel, copper-nickel alloy, nickel-based alloy, titanium and zirconium. In addition to the industrial use of welded pipes, welded pipes are used. For tube process, the shell material is carbon steel.
Second, the metal corrosion of the heat exchanger:
The principle of metal corrosion: Metal corrosion refers to the chemical or electrochemical action of the surrounding medium, and often the destruction of metal caused by the interaction of physical, mechanical or biological factors, that is, the metal in its environment. The damage caused by the action.
Several common types of corrosion damage in heat exchangers:
1. Uniform corrosion, which is generated on the entire surface exposed to the medium, or on a large area, and the macroscopically uniform corrosion damage is called uniform corrosion.
2. Contact corrosion. Two metals or alloys with different potentials are in contact with each other and immersed in the electrolyte solution. There is a current passing between them. The metal corrosion rate of the positive potential is reduced, and the metal corrosion rate of the negative potential is increased.
3. Selective corrosion. The phenomenon in which an element in an alloy preferentially enters the medium due to corrosion is called selective corrosion.
4. Pitting corrosion is concentrated on individual small points on the metal surface. Corrosion is called pitting corrosion, or small hole corrosion and pitting.
5. Crevice corrosion, severe crevice corrosion occurs in the gaps and covered parts of the metal surface.
6. Scour corrosion Corrosion is a corrosion that accelerates the corrosion process due to the relative motion between the medium and the metal surface.
7. Intergranular corrosion, intergranular corrosion is a kind of corrosion in which the grain boundary of the metal or alloy and the vicinity of the grain boundary are preferentially etched, and the grain itself is less corroded.
8. Stress Corrosion Cracking (SCC) and Corrosion Fatigue SCC is a material fracture caused by the combined action of corrosion and tensile stress in a certain metal-medium medium system.
9. Hydrogen destruction, metal in the electrolyte solution, due to corrosion, pickling, cathodic protection or electroplating, can cause damage caused by hydrogen permeation.
Third, anti-corrosion method: corrosion inhibitor, electrochemical protection
1. Corrosion inhibitor, corrosion inhibitor with chromate as the main component is commonly used in cooling water system. Acid ion is an anode (process) inhibitor. When combined with a suitable cathode inhibitor, it can be obtained. Satisfactory and economical anti-corrosion effect.
Chromate-Zinc-Polyphosphate: Polyphosphate is used because it has a clean metal surface and has corrosion inhibition properties. Polyphosphate can be partially converted to orthophosphate, which can also be formed with calcium. The colloidal cations inhibit the cathode process.
Chromate-zinc-phosphonate: This method is similar to the previous method except that sodium phosphonate is used instead of polyacid salt. Carbamyl phosphate can also be used for the pH specified by polyphosphate. High occasions. The carbamate phosphonate prevents scale formation and controls the precipitation of calcium salts even at a pH of 9.
Chromate-zinc-hydrolyzed polyacrylamide: Due to the dispersion of the polyacrylamide hydrolyzed by the cationic copolymer, it is possible to prevent or inhibit scale formation.
2, electrochemical protection, using cathodic protection and anode protection. The cathodic protection uses an external DC power source to make the metal surface become a cathode to achieve protection. This method consumes a large amount of electricity and is expensive. The anode protection is to protect the heat exchanger from the anode of the external power supply to form a passivation film on the metal surface to protect it.