How to protect the corrosion of heat exchangers
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1. The choice of materials for the heat exchanger is determined by the economics. The pipe material is stainless steel, copper-nickel alloy, nickel-based alloy, titanium and zirconium, etc., except for the case where the welded pipe cannot be used industrially. Welded tubes are used, corrosion resistant materials are used only for tube lengths, and shell material is carbon steel.
2. Metal corrosion of heat exchangers 2.1 Principle of metal corrosion Metal corrosion refers to the destruction of metals caused by the chemical or electrochemical action of the surrounding medium, and often in combination with physical, mechanical or biological factors. That is, the metal is destroyed by the action of its environment.
2.2 Heat exchangers Several common types of corrosion damage 2.2.1 Uniform corrosion On the entire surface exposed to the medium, or on a large area, the macroscopically 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 solubilizing 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.
2.2.3 Selective Corrosion An element of an alloy that preferentially enters the medium due to corrosion is called selective corrosion.
2.2.4 Corrosion Concentration The corrosion of a large depth on individual small points on the metal surface is called pitting corrosion, or small hole corrosion and pitting.
2.2.5 Crevice Corrosion Violent crevice corrosion occurs in the gaps and covered parts of the metal surface.
2.2.6 Scour Corrosion Corrosion Corrosion is a type of corrosion that accelerates the corrosion process due to the relative motion between the medium and the metal surface.
2.2.7 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.
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-medium medium system.
2.2.9 Hydrogen Destruction Metals In the electrolyte solution, damage due to hydrogen permeation may occur due to corrosion, pickling, cathodic protection or electroplating.
3. The influence of the cooling medium on the metal corrosion The industrial cooling medium using zoi is a variety of natural water. There are many factors that affect metal corrosion, the main factors and their effects on several common metals:
3.1 Dissolved oxygen in dissolved oxygen water is an oxidant that participates 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 localized corrosion. For carbon steel, low alloy steel, copper alloys and certain grades of stainless steel, melting oxygen is an important factor affecting their corrosion behavior in water.
3.2 Other dissolved gases CO2 will cause corrosion of copper and steel in the absence of oxygen in water, but does 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 reduces the pH of the water and increases the corrosivity of the water to the metal.
3.3 Hardness Generally speaking, the hardness of fresh water is increased to reduce the corrosion of metals such as copper, zinc, lead and steel. Very soft water is very corrosive. In this water, it is not suitable to use copper, lead and zinc. In contrast, lead is resistant to corrosion in soft water and pitting corrosion in high hardness water.
3.4 pH steel corrosion is less in water with pH > 11 and corrosion increases at pH < 7 .
Effect of 3.5 ions Chloride ions can damage the surface of passivated metals such as stainless steel and induce pitting or SCC.
3.6 The effect of scale on CaCO3 scale in fresh water. The CaCO3 scale layer is detrimental to heat transfer, but is beneficial for preventing corrosion.
4. The effect of heat transfer on corrosion The corrosion behavior of metals is different under the conditions of heat transfer and no heat transfer. In general, heat transfer exacerbates the corrosion of metals, especially under conditions of boiling, vaporization or overheating. The effects of heat transfer are different in different media or on different metals.
5. Anti-corrosion methods Know the causes of various corrosion of heat exchangers, and rationally choose anti-corrosion measures to achieve the purpose of efficient use of equipment. In response to the corrosion conditions discussed above, the following preservative methods are proposed:
Here mainly introduces corrosion inhibitors and electrochemical protection.
1 Corrosion inhibitor The chromate-based corrosion inhibitor is commonly used in cooling water systems. Chromate ion is an anode (process) inhibitor. When combined with a suitable cathode inhibitor, it can be Satisfactory and economical anti-corrosion effect.
Chromate-zinc-polyphosphate: Polyphosphate is used because it has a clean metal surface and has corrosion inhibition ability. Polyphosphate can be partially converted into orthophosphate, which can also be formed with calcium. The colloidal cation inhibits the cathode process. Chromate-zinc-phosphonate: This method is similar to the previous method except that sodium phosphonate is used instead of polyphosphate. 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 uses 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.