Punctate corrosion on the outer coil; experts, please analyze the cause
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The diagram shows the external coil of the reactor’s head; the white areas indicate the locations of leaks. All leaks occur only in the innermost 3 layers of the coil. Leaks appeared 3 months ago, and the welds were repaired using argon welding; yet now similar punctual leaks have reappeared, again only in the innermost 3 layers. There are 2 units of this equipment in total, all using the same materials, welding materials, and welding procedures. One unit has no problems at all, while the other unit has leaks in its innermost 3 layers. I would like to know what could be the cause. The medium inside the internal coil is cooling condensate water
The chloride ion content is within the acceptable range; if chloride-induced corrosion were to occur over 3 months, the level would be extremely high. The problem lies in these 3 sections, and testing of the coil material taken from the site also revealed no issues
1. The coiled tubes differ in the cylinder section and the head section; the cylinder is generally formed by bending steel plates, while the head is made by splitting steel pipes, so the material to be sent for inspection should be pipes. 2. If air enters from above, it will condense and concentrate as it moves downward; chlorine ions are detected at the outlet at the lower level. 3. Steel plates are rolled, while small steel tubes are generally drawn; steel tubes are more prone to defects than steel plates.
It is possible to track back to the manufacturer to find out whether the problematic welds were done by the same person. It is estimated that these three sections had defects during welding, as the welding process was not followed strictly. For reference only
Are they all at the welds? Are they all close to the water inlet?
One idea is to consider whether stress concentration occurs due to the bending of the pipe, leading to pitting in the cooled state
There is a certain temperature difference between the inside and outside of the pipe, and the uneven expansion and contraction of the material result in slight stress deformations that cause minor tears in the welds. The greater the curvature of the pipe near the inner ring, the more likely this phenomenon occurs, as stress relief was not properly carried out during manufacturing.
In the stress-concentration zones of the outer half-pipe and the end cap, no appropriate treatment was carried out prior to welding; the alignment gap was too large, and alternating loads were generated as a result of thermal shocks, leading to cracks.
There is a patch in the lower right corner of Figure 2; it is a shield that was welded on after taking samples from the leaking outer casing using a cutting machine on site, so that the customer could continue using the equipment. That is the location where the samples were taken