Rust on stainless steel caused by salt solution

Question

I am not a chemist and I could really use some help for a practical case.

I am fermenting vegetables (cabbage, onions, carrots, etc.). I recently switched from glass and ceramic vessels to stainless steel 304 ones (100L capacity). At the moment, I am only filling the vessels half-way, i.e. 50/60L and using a variable lid system. The vessel is airtight thanks to a plastic band around the lid, which is then placed right on top of the vegetables (it is a stainless steel lid and the plastic band around it is then pumped with air to compress it against the vessel). The lid also has an airlock system in the middle, allowing the co2 generated during the fermentation process to be pushed out. The brine created during the fermentation process also gets pushed out from that airlock (when the vegetables are put in the vessel, about 2% salt is added, so the brine contains salt). The brine ends up covering the lid and I removed it from time to time. It is not overflowing on the plastic band from the outside, but it seems that the brine is also pushed up against the plastic band and gets in between the plastic band and the vessel. After a few days, no more brine gets out.

At the end of the fermentation (after 4-5 weeks), when I removed the lid and emptied the fermentation vessel, I discovered traces of rust on the inside. It is right where the plastic sealed the vessel. It's not only rust, but the surface (or coating) of the stainless steel was even removed in some parts. There is no rust below or above that line (ie where the plastic sealed the vessel).

I am aware that stainless steel 304 is for sure not 100% rust-resistant but I was definitely not expecting such reaction after 1 usage! What a powerful brine...

My question is whether you believe that this reaction is due to the brine pushed up? Probably small amounts of brine end up getting stuck between the plastic and the vessel and thus potentially slightly exposed to air.

If that's the cause, I have two fixes in mind :

1. put the lid higher in the vessel so that the brine is not pushed up against the plastic/lid. I believe that the CO2 created should be sufficient to push out all the air below the lid.

2. continue to place the lid right above the vegetables. Once no more brine is pushed out, remove the lid to clean the hedges. Then put the lid back. I believe that the remaining fermentation process should push out the newly added air while not generating sufficient pressure to push new brine out.

I also came across another possible explanation while browsing full link

The same thing can happen at the water’s surface if the container is only half full. In this case, the steel above the waterline is exposed only to air, and the passive oxide layer is stable. Beneath the surface, the oxide layer is at a different potential and less stable because of the chloride ions. Now the crevice is represented by the waterline: stable area above, less stable but very large area below, and crevice corrosion occurs at the waterline. Usually this type of corrosion manifests as pitting or pinholes. The mechanism described is accelerated by localization, so a pit is most often the result.

Some pictures

1. the vessel
2. the rusted line / removed steel on the inside of the vessel
3. the lid on which the plastic band is then mounted

What do you think?

Answer 1

Chloride will pit/rust 304, not quite as much rust/pits on 316/317. Many things make it worse such as limited air access (under deposit or crevice corrosion). Oxygen can help rebuild the passive film. For your long term exposure ceramics and glass containers is the best fix. For metal containers, much higher alloy is needed such as Incoloy 825.

Answer 2

Here is a 2016 article that links the presence of ascorbic acid (A.A) to possible pit corrosion on stainless steel in the presence of tap water and chloride ions, to quote:

This study examined the effect of L-ascorbic acid (A.A) concentration on the pitting corrosion properties of 316L stainless steel (316L STS) of heat exchanger in synthetic tap water containing 400 ppm of Cl- ion...Above the 10^-4 M of A.A concentration, A.A generates soluble chelate rather than absorbs on the steel surface and it causes passive film deterioration and severe pitting corrosion.

I personally experienced pit corrosion on a stainless knife that was used to cut a mango (rich in Vitamin C), and which was carelessly left unwashed for days in a sink. On my part, an interesting, but unfortunate unintentional experiment.