Corrosion issues in concentration towers
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A concentration tower in a chemical plant, with media F21 and F22 as well as antimony pentachloride inside the tower; the temperature at the bottom of the tower is 50°C. Three towers under identical operating conditions, manufactured by the same manufacturer and in accordance with the same manufacturing requirements, using 16MnR material. One of the towers was in use for 10 days, and the weld connecting the discharge nozzle at the bottom of the tower to the bottom head suffered severe corrosion. Corrosion is manifested by pieces of the weld metal disappearing without a clear cause; there are perforations in some areas, and no cracks are visible around these holes. The other two towers are in better condition. What situations can cause this type of corrosion to occur?
In fact, among the three towers, only one tower has a weld that is severely corroded (is that correct?), which indicates that there are no issues with the material selection and manufacturing requirements. I think there is a defect in this weld; the absence of cracks does not mean that the weld is fine – it could be caused by other defects as well. As for this particular weld, it is possible that the wrong welding rod was used. It is likely that the manufacturing factory did not enforce strict quality controls during welding. If this weld is reworked using the same technical standards, the problem will be resolved.
Our factory’s quality assurance engineer now requires overall stress relief treatment to be carried out on the lower part of the newly constructed tower; (conventional repair work was only done on the welds of the original tower). What is the purpose of this?
For the heat treatment of the lower part of the new tower, in order to eliminate residual stresses (as the lower part of the tower has to bear its own weight), heat treatment of the entire tower is certainly subject to certain limitations. But routine treatments are sufficient only for the corroded weld areas; he believes that this corrosion is caused by residual stresses. I think that’s correct.
It seems that the welding rod used for welding this joint was not the right one.
Our company also has two reaction kettles with the same issue; further analysis revealed that the type of welding rod used was incorrect.
Based on my work experience, I agree with the view from the first floor. I’ve encountered similar situations too! Since the tower is a special type of equipment, high standards are required for welding. Proper treatment of the weld heat-affected zone, in order to eliminate residual stresses near the weld seams, is crucial for preventing localized corrosion!