[Heat Exchanger Corrosion] Causes of Heat Exchanger Corrosion How to Repair Heat Exchanger Corrosion
Causes of Heat Exchanger Corrosion
Under the chemical or electrochemical action of the surrounding medium, and often under the combined action of physical, mechanical or biological factors, the metal is destroyed, that is, the metal in its damage caused by the environment.
When the tube-and-tube heat exchanger is made, the welding of tube sheet and tube is generally performed by manual arc welding, and the shape of the weld has various degrees of defects, such as depressions, pores, slag inclusions, etc. The stress distribution is also not uniform. When in use, the tube sheet part is generally in contact with industrial cooling water, and impurities, salts, gases and microorganisms in the industrial cooling water will cause corrosion to the tube sheet and welds. Studies have shown that industrial water, whether it is fresh water or sea water, will have various ions and dissolved oxygen, among which the concentration of chloride ions and oxygen changes, which plays an important role in the corrosion shape of metals.
In addition, the complexity of the metal structure also affects the corrosion morphology. Corrosion with a large depth concentrated on individual small points on the metal surface is called pitting corrosion, or pinhole corrosion and pitting corrosion. Severe crevice corrosion occurs in crevices and covered areas of metal surfaces. Therefore, the corrosion of the welds between the tube sheet and the tube is mainly pitting corrosion and crevice corrosion. From the appearance, there will be many corrosion products and deposits on the surface of the tube sheet, with bubbles of different sizes distributed. When seawater is used as the medium, galvanic corrosion will also occur. “Bimetallic corrosion” is also a common phenomenon of tube sheet corrosion.
Corrosion Hazards of Heat Exchangers
Heat exchangers are one of the key equipment in production plants. Statistics show that about 60% of a large number of daily failures and emergency repairs are due to corrosion and damage to heat exchanger pipes. It seriously affects the safe, stable and full-load operation of the production equipment. Due to the different factors such as the structure, material, operating conditions and cooling water quality of the heat exchanger, corrosion and leakage problems at the tube bundles, nozzles and tube sheets are often caused, resulting in condensate pollution, which will seriously deteriorate the quality of the water supply and aggravate the “four-tube boiler”. “Fouling and corrosion, causing a “four-pipe” blasting accident in severe cases.
How to repair heat exchanger corrosion
1, the choice of heat exchanger materials
The deciding factor of which material to use is its economy. The pipe materials include stainless steel, copper-nickel alloy, nickel-based alloy, titanium and zirconium, etc., except for the cases where welded pipes cannot be used in industry For welded pipe, corrosion resistant material is only used for the pipe side, and the shell side material is carbon steel.
2. Metal corrosion of heat exchangers
2.1 The principle of metal corrosion Under the action of electrochemistry, and often under the combined action of physical, mechanical or biological factors, the metal is destroyed, that is, the metal is destroyed under the action of its environment.
2.2 Several common types of corrosion damage in heat exchangers
2.2.1 Uniform corrosion occurs on the entire surface exposed to the medium, or in a larger area, macroscopic The uniform corrosion damage is called uniform corrosion. 2.2.2 Contact corrosion Two metals or alloys with different potentials are in contact with each other and immersed in the electrolyte solute solution, and current flows between them. The corrosion rate of metals with positive potential decreases, and the corrosion rate of metals with negative potential increases.
2.2.3 Selective Corrosion The phenomenon that an element in the alloy preferentially enters the medium due to corrosion is called selective corrosion.
2.2.4 The pitting corrosion is concentrated on individual small points on the metal surface and the corrosion with a greater depth is called pitting corrosion, or pinhole corrosion and pitting corrosion.
2.2.5 Crevice corrosion Severe crevice corrosion occurs in the crevices and covered parts of the metal surface.
2.2.6 Erosion Corrosion Erosion corrosion is a kind of corrosion in which the corrosion process is accelerated due to the relative motion between the medium and the metal surface.
2.2.7 Intergranular corrosion Intergranular corrosion is a kind of corrosion that preferentially corrodes the grain boundaries of metals or alloys and the regions near the grain boundaries, while the grain itself corrodes less. 2.2.8 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-media system.
2.2.9 Destruction of metals by hydrogen In the electrolyte solution, due to corrosion, pickling, cathodic protection or electroplating, damage due to hydrogen permeation can occur.
3. Influence of cooling medium on metal corrosion
The most widely used cooling medium in industry is various natural water. There are many factors affecting metal corrosion, the main factors and their effects on several commonly used metals:
3.1 Dissolved oxygen dissolved oxygen in water is an oxidant participating in the cathodic process, so it generally promotes corrosion. When the concentration of oxygen in the water is not uniform, a concentration cell of oxygen will be formed, causing local corrosion. For carbon steels, low alloy steels, copper alloys and certain grades of stainless steel, molten oxygen is the most important factor affecting their corrosion behavior in water.
3.2 When other dissolved gases are oxygen-free in water, CO2 will cause corrosion of copper and steel, but will not promote corrosion of aluminum. Trace amounts of ammonia corrode copper alloys, but have no effect on aluminum and steel. H2S promotes corrosion of copper and steel, but has no effect on aluminum. SO2 lowers the pH of water and increases the corrosiveness of water to metals.
3.3 Hardness Generally speaking, the increase of hardness of fresh water reduces the corrosion of metals such as copper, zinc, lead and steel. Very soft water is highly corrosive, and copper, lead, and zinc should not be used in such water. Conversely, lead is resistant to corrosion in soft water and pitting in hard water.
The corrosion of 3.4 pH steel is less in pH>11 water, and the corrosion increases when pH<7.
3.5 Effect of ions Chloride ions can damage the surface of passive metals such as stainless steel and induce pitting corrosion or SCC.
3.6 Influence of scale CaCO3 scale in fresh water. The CaCO3 scale layer is not good for heat transfer, but it is good for preventing corrosion.
4. The effect of heat transfer process on corrosion
The metal under the condition of heat transfer and no heat transfer, Corrosion behavior is not the same. Generally speaking, heat transfer increases the corrosion of metals, especially under conditions of boiling, vaporization or superheating. In different media, or for different metals, the effect of heat transfer is also different.
5. Anti-corrosion method
Knowing the causes of various corrosion of heat exchangers, and choosing reasonable anti-corrosion measures, can To achieve the purpose of efficient use of equipment.
