Corrosion of stainless steel equipment
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This post was last edited by gedong456 on 2019-7-30 at 13:10. First, here are the pictures and short video, followed by an explanation: This pipeline is the backflushing pipeline for the bag filter; it is made of 304 stainless steel. After one year of operation, an increase in pressure difference was observed in this filter. Upon inspection and disassembly, it was found that both the internal cleaning pipeline and the frame of the filter bags were broken and scattered inside. They would break even with slight force applied by hand or by tapping. Attempts to replace the flange connections of the internal backflushing pipeline resulted in cracks appearing wherever TIG welding was carried out. After the change, the material becomes quite brittle; its color is similar to that of the modified part, only slightly lighter. Corrosion occurs mainly in the head area, while the lower part of the cylinder appears to be in its natural metal color. The main components of the system process are nitrogen (about 400 NM3/h), steam (about 30 Kg/h), and a small amount of polypropylene powder; the operating pressure is 2 KPa and the temperature is around 60°C. I would like to ask the experts: what are the causes of corrosion in this equipment? We are planning to replace it with 316 material – will this solve the corrosion problem? Thank you. Although the cause of the corrosion has not been specifically investigated, we plan to replace the equipment with one made of 316L material first. If the same issues occur with 316L as well, I will turn to you experts for help again.
This post was last edited by CHANGBAISHI on 2019-7-12 at 17:05. It is suggested to find more additional evidence, such as valves and fittings made of similar materials. For example, in the case of elbows, if it’s the same in such cases, it means it has nothing to do with the material. I suspect it's a material issue. Material. Check the origin of this “304 steel”. If it is corrosion, then it is a problem with the internal structure of the steel material. Generally, the material of pipe fittings such as elbows is quite reliable.
First, figure out what type of corrosion is causing it, and then take appropriate action
Nitrogen, steam, and a small amount of polypropylene powder are not sufficient to corrode 304 stainless steel; it is likely that there is an issue with the material itself.
We know that “stainless steel” gets its name because it tends to form a passive layer in oxidizing environments at room temperature, such as in the atmosphere, water, or strongly oxidizing acids. This protective oxide layer, primarily composed of chromium oxide (Cr2O3), results in an extremely low rate of corrosion; hence the name stainless steel. However, when the temperature increases or the oxidation capacity of the environment decreases, it shifts from a passive state to an active state, resulting in a significant increase in corrosion. In terms of operating conditions, it uses nitrogen, steam, a small amount of polypropylene powder, with an operating pressure of 2 KPa and a temperature of around 60°C. It can be seen here that the intergranular corrosion is caused by nitrogen (about 400 NM3/h). The reason is that this stainless steel is often used in an oxygen-free environment, so no oxide film primarily composed of chromium oxide (Cr2O3) forms on its surface. Additionally, it is known that alloy materials (Fe-Cr-C) are prone to stress corrosion cracking when exposed to a combination of water and steam. Therefore, intergranular corrosion or stress corrosion cracking of stainless steel occurs under such combined conditions. It is recommended that under such conditions, carbon steel material or carbon steel with anti-corrosion measures can be used, which would be a better option. This is my personal opinion.
Conduct a metallographic analysis to determine the main cause; it might be a problem with the material
If the material testing shows no issues, then phase analysis and sensitization analysis need to be carried out
The flanges in the backflush pipeline of this bag filter are all like this; the equipment cylinder is in relatively better condition, but it’s no longer possible to perform secondary welding. The part of the backflush pipe outside the equipment is fine
The equipment manufacturer’s response was that hydrogen corrosion was caused by steam condensation