I would like to know what causes the corrosion
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The last edit to this post was made by Dianxiaoer on 2019-12-6 at 11:33. The material of the heat exchanger tube sheet is Q345R, while the heat exchange tubes are made of 304. The fluid flowing through the tube side is water. I would like to ask those with experience in fusion welding what causes this corrosion
Judging from the current corrosion situation, the most likely cause is uneven welding quality.
The medium is water. . . Water isn’t pure either; it contains salt impurities, and this stuff is even more deadly due to its severe corrosive effect. The tube sheet can be made of 16Mn forgings, but when welding it to the heat exchange tubes, a layer of stainless steel must be welded on the surface of the tube sheet.
If there is no problem with the heat exchange tubes themselves, the issue lies in the tube joints. As mentioned above, there is no stainless steel surfacing welded on the tube sheet, and the welded joints between different types of steel are clearly not suitable to withstand the media present in the tube side.
The tube sheet is made of Q345R, while the heat exchange tubes are made of 304; there is a potential difference inherent in the direct welding between them. If the water contains corrosive substances such as disinfectants, potential difference corrosion can easily occur at the tube ends. Such a design is unreasonable. From the photos, it seems that the welding quality at the pipe end is also poor.
1. Is it an imported section? If so, it may be caused by long-term high-pressure erosion. 2. The quality of the welding rods is substandard, or inappropriate welding rods were used. 3. There are excessive corrosive substances in the water, or materials have entered the tube side. 4. The material used for the heat exchanger is of poor quality
From a corrosion perspective, there is some corrosion and resistance to intergranular corrosion, but it’s not sufficient; the welding quality is likely substandard
Was the welding wire used incorrectly? The corrosion isn’t uniform – in some areas it’s severe while in other areas there’s no corrosion at all. Could it be that the wrong welding wire was used during welding? Is it possible?
Selective corrosion may be caused by different welding processes. Is there any pattern? Given the significant weakness of the welded joint, it is necessary to weld a cladding layer onto the tube sheet before welding it to the heat exchange tubes, in order to avoid having joints between different steel types at the weakest point. . .
It seems that the welding wire wasn’t chosen properly, and corrosion was caused by the potential difference between different materials
At first glance, it seems that the welding material has fallen off; most likely, it’s a welding issue – either the welding material was not chosen appropriately or the welding technique was inadequate