Accidental K-Meta experiment

[Jblyth]

K-Meta + Stainless steel = Rust!

For all of those chemistry majors out there, i'd like to find out what happened recently while sanitizing some wine equipment. I was getting ready to press some grapes skins recently from my wine, and had prepped a k-meta sanitizing solution in a bucket filled with my stainless steel collander and other equipment. Long story short is that I got interrupted, and let it sit there much longer than anticipated (a few hours). When I got back and opened the bucket, my collander had started to severely rust on top! Oddly, the items which were nearer to the solution at the bottom of the bucket were fine, but the collander had a thick, mucus-like rust on top that required quite a bit of cleaning.

What happened?

 

[TonyP]

Chemistry is not my area but I've done research on stainless steel (SS) and SO2. I can tell you damp or wet SO2 is fairly corrosive, but SS graded at 20% chromium or higher should be fine. Lower grade SS has less resistance and SO2 can react aggressively with low-grade SS.

The problem, of course, is that you seldom know the chromium content of your SS or other metals. Thus, it pays by-and-large to avoid k-meta solution around metals, especially expensive ones like pumps. FYI, dry k-meta is only mildly corrosive and shouldn't be as big an issue.

Tony P.

 

[BeerAlchemist]

Tony suggested I post this here as well from a conversation we are having on another thread.

"304 and 316 SS both have 18% chromium; however, professional wineries use the more expensive 316 stainless because of its higher resistance to pitting. Particularly they use 316 in areas exposed to the gas as that is much more corrosive, so things like lids will be 316. I believe the difference is the added Molybdenum.

"A 20% concentration of sulphur dioxide dissolved in water at 20 degC can be expected to give a corrosion rate between 0.1 and 1.0 mm/year on 304 types, but a rate below 0.1mm/year on 316 types and hence is 'mildly' corrosive."
http://www.bssa.org.uk/topics.php?article=37

So, I'd guess you were dealing with a low grade of SS in which the passivated layer that protects it from corrosion was readily ripped apart by the SO2 as Tony said.

 

[Dend78]

same thing happens in water treatment plants, stainless steel boxes and such last a short period of time around the tanks, and we drink the water a few steps later

 

[Jblyth]

same thing happens in water treatment plants, stainless steel boxes and such last a short period of time around the tanks, and we drink the water a few steps later

That's rather scarry to hear that So2 can be that corrosive, yet we put it in our wine (obviously not at those concentrations). Thanks to all for the lesson in chemistry.