Causes of pitting corrosion of plate heat exchanger plates
Thread Content
The methanol heater of our formaldehyde device uses methanol and formaldehyde for heat exchange. The thermal measurement is 30% formaldehyde (the rest is water), the temperature is about 75 degrees, and the chloride ion is about 2.8 mg/l.； The cold side is about 95% methanol (the rest is water). After heating, the temperature is about 45 degrees, and the chloride ion is about 37mg/l. After using this plate replacement for 4 months, the plates were perforated and corroded (the plate material is 304,) and more than 100 of the more than 200 plates had perforations in different situations. see photos: Relevant parties have been consulted about this situation. There should be no problem with the material selection and there are performance records. The reasons are tentatively analyzed as follows.: 1. Corrosive substances in the medium remain at the angles of the plate exchangers, which will form chemical pitting corrosion over time. ； There is stress corrosion (it can be understood that the clamping size is too large or too much) ； 2. When the temperature reaches about 75 degrees, chloride ion corrosion will occur. ； 3. The accumulation and accumulation of chloride ions or other corrosive media in the gap area, leading to corrosion ； But the specific reason is still unclear. How to deal with it? I still need to change it to 316L material. Please enlighten me, thank you!
The pitting corrosion of the plate heat exchanger plates you described may be caused by many factors, such as chloride ion corrosion in the medium, corrosion accumulation in the crevice area, etc. Since it involves chemical equipment, it is recommended to contact a professional chemical equipment maintenance company or heat exchanger manufacturer for inspection and repair. At the same time, considering changing to 316L material is also a solution. Hope it helps. .
There is another reason. This device has been stopped for a period of time. There is material in the plate replacement. Will it cause corrosion?
If the shutdown time is long and there is material inside the plate replacement, corrosion will easily occur. During shutdown, chemical substances in the medium may undergo certain chemical reactions, such as oxidation, decomposition, etc., to generate corrosive substances, which will accumulate and accumulate during plate replacement, causing corrosion. Therefore, in order to prevent this from happening, you can consider emptying the plate to replace the internal medium during shutdown, or taking anti-corrosion measures, such as opening the feed opening, water drain, etc., to ensure internal ventilation and dryness and avoid the accumulation and accumulation of chemical substances. .
It has been confirmed that the material in the system was emptied with a pump the day after the device was shut down at the site. However, the system was cleaned and it is estimated that there is still a small amount of residual material.
Now our analysis must be scientifically verified in the end. We need to take samples and find an authoritative metal research unit to conduct failure analysis. After getting the reasons, we can choose suitable materials. Another method is to follow the analysis of the main reasons and find suitable material changes to try.
Summary of the introduction to corrosion of plate heat exchangers.
I found an article about the corrosion of formaldehyde circulating water plate heat exchangers. The situation is similar to the problem you encountered, I hope it can be helpful to you.
Take a look at this post, it may help you https://bbs.hcbbs.com/thread-3152341-1-1.html