I made a shocking discovery: stainless steel seems to corrode with bleach X___X

[DiTBho]

I brought home a damaged industrial device used to clean glass bottles. It is a kind of mini autoclave that allows water washing at 120°C.

I fixed it, and I did an interesting experiment with continuous cleaning cycles:

1. autoclave-open, pre-wash with water + 20% industrial bleach at 24C
2. autoclave-open, steam water at 100°C
3. autoclave-close, controlled pressure, washing at 120°C
4. autoclave-open, air cooling
5. 50 minute pause
6. visual inspection, every 10 cycles
7. repetition

All automated and controlled by an industrial PLC.

Two guinea pigs

• a stainless steel bar
• a titanium bar 3-2.5

After ~60 cycles {~ 6 inspections} the steel bar starts to corrode, not much but it's starting to be noticed, while the titanium bar shows absolutely no defects

This is very interesting - prolonged use of bleach corrodes stainless steel? *very* interesting - for the cleaning procedure I use, adding bleach to the pre-wash and a steam of water, both save 5 minutes and make the process more efficient, then I autoclave the water at 120 °C, for another 5 minutes, 10 minutes in total to automatically clean the water bottle.

It's crazy as hell, but I love this thing where I come home, put my bicycle in a hot air tent to dry the water off, put my water bottle in that contraption, and while those two PLC-controlled machines are working, I can take a shower and relax, and when I come back it's all work that I don't have to do.

[themadhippy]

prolonged use of bleach corrodes stainless steel

Its the chlorine that done it,and no need to use bleach in an autoclave,the whole point of them is to get stuff sterile without any chemicals.

[DiTBho]

no need to use bleach in an autoclave, the whole point of them is to get stuff sterile without any chemicals.

yup, bleach was the solution I used previously to clean. Out of laziness I did not disassemble the straw and the pump that injects the bleach into the glass container where I put the two guinea pigs, and it is the same container on which the autoclave cups are then closed.

It makes no sense to use bleach, washing at 120 °C already eliminates 100% of any organic thing that can nest in a bottle of water!

However, I was surprised by the fact that, due to my laziness, I saw the stainless steel corrode: I really did not expect that!
I used supermarket bleach, not industrial bleach, so it's not just diluted bleach, it also contains other things... d'oh 

[DiTBho]

you always learn something, I should have read something like differences(chlorine, bleach) 

[floobydust]

Are the pieces on a metal rack, touching other metals? To avoid any electrolysis or galvanic corrosion.
I find dishwasher soap pucks will intermittently attack the finish on stainless steel cutlery and it apparently has to due with poor QC on the soap.

[ejeffrey]

It depends on the alloy but chlorine compounds attack many types of stainless steel   304 is one of the most common alloys and it's known to be vulnerable to saltwater (sodium chloride) corrosion.  316 is significantly more resistant to salt but I think high temperature bleach can still corrode it.  You can use dilute bleach at room temperature to disinfect some stainless steels but need to keep the contact time short and wash it away thoroughly.

[jonpaul]

We use ultrasonic cleaner with hot water ordish soap.

Also 99% iso alchol

j

[T3sl4co1l]

The best part is you're probably making hexavalent chromium in the process!

Not very much, just whatever comes out of the surface layer.  But, that's something to think about.

Specifically, when alkaline bleach (hypochlorite solution) is used, the high redox potential and pH oxidize the protective chrome oxide layer to sodium chromate in solution.  The bright yellow color is probably too weak to see over the greenish color of the solution as-is, but doing it repeatedly and comparing to fresh bleach may show some difference.

Also, if you're autoclaving the bleach itself, that decomposes the hypochlorite into chloride and chlorate, still a strong oxidizer, but relatively innocuous in neutral pH at least.  It does decompose in acidic conditions, releasing chlorine gas, or if concentrated enough (both chlorate and acid), the explosive ClO2 can even be produced.

Lesson learned, it's stainless, not stainproof

Tim