1. Impurity of impurities dissolved in water in an ionic or molecular state
(1) The main constituents of calcium salts in water are Ca(HCO3)2, CaCl2, CaSO4, and CaSiO3. Calcium salts are the main component of the heat exchanger fouling.CaSO4 is a hard, fine-grained scale with loose structure and low adhesion.It is a relatively soft sludge that is separated from the water and is easy to remove even if it is attached to the heated surface.
(2) The main components of the magnesium salt in water are Mg (HCO3), MgCl2, and MgSO4.
After the magnesium is dissolved in water, it forms a Mg(OH)2 precipitate after being thermally decomposed, and Mg(OH)2 is also a sludge-type scale.MgCl2, MgSO4 dissolved in water, when the water pH is <7, acid corrosion of the metal wall due to hydrolysis.
(3)The sodium salt is mainly composed of NaCl, Na2SO4, and NaHCO3. NaCl does not form scale, but the presence of free oxygen in the water accelerates the corrosion of the metal wall.If the content of Na2SO4 is too high, salt will form on the accessories behind the evaporator, which will affect safe operation.NaHCO3 in water decomposes NaCO3, NaOH, and CO2 under the action of temperature and pressure, which will damage the metal grains.
2. Hazard of dissolved oxygen gas
There are many reasons for corrosion in heat exchangers, but the most severe and the fastest is oxygen.
On the atomic order table, the potential of iron is above hydrogen. In neutral water containing no oxygen, the iron atoms on the surface of the system metal lose electrons into divalent ions (Fe-2e→Fe2+).
Fe2+ ions and OH- ions in water combine [Fe2++2OH-→Fe(OH)2] under electrostatic attraction and establish the following balance in water: Fe2++2OH-=Fe(OH)2.When oxygen is present in the water, Fe(OH)2 is further oxidized to insoluble iron hydroxide precipitated:4Fe(OH)2+O2+2H2O→4Fe(OH)3↓. Due to the precipitation of Fe(OH)3, the iron ions around the anode are transferred into the aqueous solution, which accelerates the progress of corrosion.
It can be seen from the above reaction that water and oxygen are necessary conditions for corrosion, the anode portion is a corroded portion, and the cathode portion is a portion where corrosion products are piled up.When corrosion is carried out uniformly over the entire metal surface, the rate of corrosion is not so fast, so the hazard is not large, and this corrosion is called general corrosion.When corrosion is concentrated on certain parts of the metal surface, it is called localized corrosion. The local corrosion rate is very fast and it is easy to wear. Pit erosion is a common local corrosion in heat exchangers, so it is very harmful.