Issues regarding the corrosion of 304
Thread Content
Recently, while performing maintenance on a vacuum distillation unit, I encountered a problem. I’m not quite sure of the exact cause of this issue, so I’m reaching out to everyone here to learn from you. In our facility, the Changding oil and gas stream first passes through a heat exchanger; it is a counterflow shell heat exchanger made of 304 material, used for heat exchange with crude oil. After that, it passes through a low-fin air cooler also made of 304 material. The problem is as follows: During inspections, it was found that the 304 material in the heat exchanger had suffered severe corrosion; the manufacturer determined that this was chloride-induced stress corrosion, and the entire tube core needed to be replaced. However, during the inspection of the air cooler, no problems were found at all; not even the welds of the air cooler showed any signs of corrosion. I thought about it and wondered if the reason is as follows: since vapor is often present, and the temperature is relatively high, this creates favorable conditions for corrosion to occur. With air cooling, generally the inlet temperature is relatively lower, and in the case of air cooling the fluid is usually in a two-phase state of vapor and liquid, so corrosion is relatively less severe. I’m not a materials scientist, so I don’t know much about this area. Please help me out and analyze it.
The chloride ion content in the medium within stainless steel pipes must not exceed 0.25%; otherwise, corrosion can occur!
The concentration of chloride ions in the working medium and the operating temperature should be taken into consideration. Chloride ions have an impact on the corrosion resistance of stainless steel equipment, and both the concentration of chloride ions in the working medium and the operating temperature exert a significant effect on stress corrosion in stainless steel. For example, a water heater in a factory in Tianjin was severely corroded; after being replaced with all-stainless-steel material, leakage occurred just a few months after its installation. After careful analysis, it was found that hot water contains chloride ions and oxygen. Stainless steel cannot resist chloride ion corrosion at certain temperatures; especially in the presence of oxygen in the medium, as oxygen accelerates the corrosion process. In actual production, it has also been found that at certain concentrations and temperatures, the corrosion resistance of stainless steel is even lower than that of carbon steel; however, when the concentration of chloride ions is low or high but the temperature is not high, its corrosion resistance remains much better than that of carbon steel. In this regard, temperature has a greater impact on corrosion resistance than chloride ion concentration. Therefore, when selecting materials, in addition to considering the chloride ion concentration, special attention must be paid to the influence of temperature.