Pitting corrosion of S30403
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The heat exchanger we designed uses S30403 as the material for its heat exchange tubes, and pitting corrosion occurs at the points where the gas and liquid phases meet within these tubes. The likely reason for this is the accumulation of chloride ions in the area near the gas-liquid interface. I would like to ask the experts: are chloride ions the only factor that can cause pitting corrosion? Also, in such cases, how should the material for the heat exchanger be chosen, and how can corrosion be prevented?
Material impact can also cause pitting. Are there any photos of corrosion?
Is the heat exchanger you designed vertical? What medium flows in the shell side and the tube side? In vertical heat exchangers, since water flows in the shell side, the cooling water outlet is located between the upper tube sheets, creating a space where no water can reach. In this area, the tube bundle is exposed to an environment of alternating wet and dry conditions or a gas-liquid interface, which facilitates the concentration of chloride ions. Additionally, the cooling water evaporates at the gas-liquid interface, resulting in the release of dissolved oxygen. Oxygen acts as a cathodic depolarizer, thereby contributing to and promoting SCC failures. Take a look at this case: it involves the top condenser of the acetonitrile dehydrogenation unit in an acrylonitrile plant. The tube bundle is made of S304L material, and the heat exchange tubes are joined to the tube sheet by expansion fitting. The shell side is fed with treated circulating cooling water at 32°C/40°C, while the tube side contains hydrocyanic acid at 58°C/38°C. After several years of use, more than 10 heat exchange tubes broke; the fractures occurred at a distance of 14–54 mm from the lower end of the upper tube sheet. Metallographic analysis confirmed that these cracks were caused by stress corrosion cracking
The chloride ion concentration is low; there is only pitting corrosion at the gas-liquid interface, with no corrosion in other areas
The heat exchanger is vertical; I forgot what the medium is. I’ll check it again when I start working on Monday
The accumulation of chloride ion concentration at the gas-liquid interface is equivalent to dew point corrosion; isn’t it the same thing?
How can this situation be resolved? Is there S2205 available for heat exchange tubes?
I have encountered such situations before; switching directly to titanium material is safer
Consideration can be given to the position and structure of the outlet nozzle to make the outlet level with or above the lower surface of the upper tube sheet. From a process perspective, it is also possible to add an inlet pipe space to disperse gas beneath the upper tube sheet.
This can also happen with circulating water systems; everything is made of titanium, which makes the cost way too high