Analysis on the causes of corrosion on the outer surface of pressure parts of waste heat boiler

　　Why is the outer surface corrosion of pressure parts of HRSG? Let's discuss it together!

　　During the operation of the waste heat boiler, there is a leakage at the outer side of the elbow of a downcomer, which ejects steam. After the HRSG accident, the boiler was shut down to inspect the leakage part of the downcomer elbow. It was found that there was a 53mm perforation on the outside of the downcomer elbow. There was serious oxygen corrosion on the metal surface around the hole. The oxide skin thickness on the surface was 2-3mm. After the oxide scale was removed, the exposed metal wall was uneven The insulation layers of 6 downcomers and 10 risers were removed. It was found that all the risers and downcomers had serious corrosion, especially the lower part of the riser and downcomer. The thickness of oxide scale on the surface was 2-3mm.

　　After the oxide scale is removed, the exposed metal wall is also uneven, showing pitting and flaky corrosion pits. The thickness of oxide scale is 1-2 mm. After the insulation layer around the joint of the upper steam drum and the up pipe and down pipe is removed, it is found that the oxide skin thickness on the metal surface is 1-1.5mm, and the minimum wall thickness is 1714mm. The insulation layer of pressure parts of waste heat boiler adopts the structure of inner layer aluminum silicate fiber cotton and outer layer of refractory cement Inspection found that there are many cracks in the refractory cement layer. The waste heat boiler is not equipped with rain proof shed, but only depends on its refractory cement layer to prevent rainwater. In addition, according to the operation records of the HRSG, no overtemperature, overpressure and water shortage were found, and the water quality test results of the boiler met the standards. However, the inner wall of the tube was not covered with water scale (about 0.2 mm).

　　The cause of serious corrosion on the outer surface of pressure parts of boiler is mainly due to the rain water penetrating into the surface of pressure parts through the cracks of fire-resistant cement layer when it rains. At the same time, the infiltrated rainwater also brings in oxygen in the air and contacts with the metal surface of pressure parts, which causes oxidation and corrosion. Besides oxygen, acid rain also contains some acid substances, because of the infiltration of pressure elements The rainwater on the surface of the parts contains oxygen and acid substances at the same time. Under the action of oxygen and acid substances at the same time, the speed of metal surface being oxidized and corroded is greatly accelerated. Moreover, the infiltration of rainwater is not easy to evaporate due to the barrier of refractory cement layer, which prolongs the contact time between rain water and metal surface, and the oxidation corrosion time of metal surface is longer, which accelerates the corrosion of pressure parts And the speed of thinning. Attention should be paid to the anticorrosion of waste heat boiler.