Corrosion damage of bellows expansion joints mostly occurs on the expansion joints of flue gas pipes, which can cause perforation and cracking of expansion joints. Perforation is mainly caused by Cl- and dew point corrosion, and cracking is caused by stress corrosion under acidic medium.
(1) Corrosion and perforation of bellows: Cl- in flue gas mainly comes from the decomposition of raw materials and steam. When the expansion joint forms Cl-containing corrosion condensate from the surface peak, if the concentration exceeds the critical concentration, pitting corrosion will occur at the peak. For more impurities on the inner surface or where the weld is not smooth, the pitting effect will be strengthened and eventually lead to perforation. When the surface temperature of the expansion joint is low, the steam in the flue gas is lower than the dew point temperature. SO2 and SO3 in the flue gas condense on the inner surface of the expansion joint to form a mixed acid corrosion solution, and chemical corrosion and electrochemical corrosion occur.
In most cases, according to the sulfur content in the raw material and the steam content in the flue gas, it can be known that the flue gas dew point temperature is below 150 ° C, so the possibility of dew point corrosion exists. In fact, the pitting corrosion of the bellows is often the result of several corrosion effects.
(2) Corrosion cracking of bellows: The stresses on bellows expansion joints in service mainly include additional stress caused by manufacturing and installation, working stress, thermal stress and residual stress caused by welding deformation. When the expansion joint is partially in an acidic corrosion electrolyte solution, stress corrosion cracking (SCC) of the bellows expansion joint occurs.
The SCC of the bellows expansion joint mainly occurs in the transitional section (between the peak and the valley) and the ring joint of the barrel section. In the discontinuous zone of the transition section, the radial stress was found to be very large, up to 60 MPa, and the residual stress in this part was also very large. According to relevant literature reports, a plant found that the highest value can reach 180MPa after dissecting the expansion joint sample. The stress superposition of the two is much greater than the yield limit of the material σs and the critical value of SCC σscc.
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