Does nitrogen sealing provide protection against alkaline corrosion of stainless steel?
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Hello, teachers. I have encountered a problem here and would like to ask for your advice. In the processing unit under construction, the alkali washing tanks require that the alkali solution used for washing be reused. This reused alkali solution is at a temperature of around 80–90°C, contains approximately 8% NaOH, and also about 3% NaC3 resulting from the washing process. After researching relevant information, it was decided to use 321 stainless steel. In a post I read, there was a chart showing that using hydrogen to isolate air can effectively slow down the rate of corrosion. Since nitrogen sealing is widely used in the Class A factory where this equipment is located, would using nitrogen sealing to isolate air from this equipment also provide some degree of corrosion protection?
It is called “nitrogen sealing” mainly because of its inert properties, which are used to prevent the leakage of the medium being protected. As for anti-corrosion, it seems to not have this function.
Hydrogen protection as described in the literature takes advantage of its reducing properties while also isolating air; the use of nitrogen sealing is also aimed at preventing air from entering. I mainly want to ask whether, under such conditions that are not too demanding, preventing O2 from entering can also help to reduce alkali corrosion to some extent Corrosion caused by chloride ions should be relatively weaker under alkaline conditions, and the use of 321 has already minimized its impact as much as possible.
Oh, if I’m not qualified enough for design work, I can only share my opinions on your question. That’s how I understand it.
What you call nitrogen sealing should refer to the area outside the pipeline; it serves an anti-corrosion purpose. However, inside the pipeline, since sodium hydroxide (8%) is used at a temperature of 80–90 degrees, there is a risk of alkali corrosion, so careful consideration needs to be given to the choice of material
Chloride pitting has little to do with acids and bases; it is mainly the chloride ions that damage the passivation layer of stainless steel.
Chloride ions do not corrode stainless steel because of the reaction between acid and iron that produces hydrogen