About the heat exchanger

                                                    2017-07-24 focus number:
                                                    1. Selection of material for heat exchanger: the determining 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 industrial cases where welded pipes cannot be used, welded pipes are used.

                                                    2. Metal corrosion of heat exchangers



                                                    1) The principle of metal corrosion: Metal corrosion refers to the destruction of the metal under the chemical or electrochemical action of the surrounding media, and is often caused by the joint action of physical, mechanical or biological factors, that is, the metal under the action of its environment.

                                                    2) Several common corrosion and damage types of heat exchangers:

                                                    A. Uniform corrosion: The macroscopic uniform corrosion damage produced on the entire surface exposed to the medium or over a large area is called uniform corrosion.

                                                    B. Contact corrosion: Two metals or alloys with different potentials are in contact with each other and immersed in electrolyte dissolved solution, and there is an electric current passing between them. The metal corrosion rate with positive potentials decreases, while the metal corrosion rate with negative potentials increases.

                                                    C. Selective corrosion: The preferential entry of an element in an alloy into the medium due to corrosion is called selective corrosion.

                                                    D. Hole erosion: the deep corrosion concentrated on some small points on the metal surface is called hole erosion, or hole corrosion and pitting corrosion.

                                                    E. Crevice corrosion: Severe crevice corrosion will occur in the crevices and covered parts of the metal surface.

                                                    F. Erosion corrosion: Erosion corrosion is corrosion accelerated by the relative motion between the medium and the metal surface.

                                                    G. Intercrystalline corrosion: Intercrystalline corrosion is a type of corrosion that preferentically corrodes the grain boundary and the area near the grain boundary of a metal or alloy, while the corrosion of the grain itself is relatively small.

                                                    H. Stress corrosion cracking (SCC) and corrosion fatigue SCC is the material fracture caused by the joint action of corrosion and tensile stress in a certain metal-medium system.

                                                    I. Hydrogen damage: Metal in electrolyte solution, due to corrosion, pickling, cathodic protection or electroplating, can produce damage caused by hydrogen seepage.

                                                    3. Influence of cooling medium on metal corrosion. The most commonly used cooling medium in industry is all kinds of natural water. There are many factors affecting metal corrosion. The main factors and their effects on several common metals are as follows:

                                                    1) Dissolved oxygen: Dissolved oxygen in water is an oxidant participating in the cathode process, so it generally promotes corrosion. When the concentration of oxygen in water is not uniform, oxygen concentration difference cells will be formed, causing local corrosion. For carbon steel, low alloy steel, copper alloy and some grades of stainless steel, melting oxygen is the most important factor affecting their corrosion behavior in water.

                                                    2) Other dissolved gases: IN the absence of oxygen 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 reduces the pH value of water and increases the corrosive action of water on metals.

                                                    3) Hardness: Generally speaking, increased hardness of fresh water reduces the corrosion of metals such as copper, zinc, lead and steel. Very soft water is highly corrosive. Copper, lead and zinc should not be used in this water. In contrast, lead is resistant to corrosion in soft water and produces pore erosion in hard water.

                                                    4) pH value: the corrosion of steel in water at pH>11 is small, and the corrosion increases when pH<7.

                                                    5) The influence of ions: chloride ions can damage the surface of passivated metals such as stainless steel and induce pore erosion or SCC.

                                                    6) Influence of scale: CaCO3 scale in fresh water. CaCO3 deposit is not good for heat transfer, but helps to prevent corrosion.

                                                    4. Influence of heat transfer process on corrosion: The corrosion behavior of metals is different with and without heat transfer. In general, heat transfer aggravates the corrosion of metals, especially under conditions of boiling, vaporization, or overheating. The effect of heat transfer is also different in different media or on different metals.

                                                    5. Anticorrosion methods: Knowing the causes of various corrosion of heat exchangers, reasonable selection of anticorrosion measures can achieve the purpose of efficient use of equipment.