304 Stainless Steel Corrosion problem at distillery
A discussion started in 2007 but continuing through 20182007
Q. I own a small vodka distillery and we are having problems with 304 stainless steel filtration pipes corroding. The pipes contain coal carbon and are used to filter both raw grain alcohol (195 proof) and distilled grain alcohol (95 - 100 proof). The corrosion starts as tiny pinholes. Any suggestions on cleaners or food-grade sealants/coatings that might prevent this corrosion? Any explanations as to why this might happen (scientifically/technically)?Susan Ashton
hobbyist/distiller - Portland, Oregon
A. Chlorides are the most probable cause. My guess is it is the carbon that you are using. Once it pits, it is a down hill battle from there. Plan on replacing it in the somewhat near future. Neither pickling or passivation nor electropolish gets to the bottom of the pit, so it will continue to grow. New pits will be retarded, but not prevented.James Watts
- Navarre, Florida
A. Hi Susan,
Not really a mystery.
Stainless steel is corrosion-resistant only in an oxidising environment, because an oxidising environment creates a sub-microscopic film of tightly-adherent, non-porous, chrome+nickel+iron oxide which protects the underlying metal from contact with the service environment.
Unfortunately, your service environment is precisely the opposite of oxidising. Both the activated carbon, and more certainly the alcohol, are powerful chemical deoxidants or reducers, and ensure that there is no possibility of oxidising conditions at the surface of the pipe and therefore no way that the protective surface layer can form.
The pipe will therefore be corroded by the liquids in the pipe, including the water component of the spirits, almost as fast as if it were ordinary mild steel pipe.
There will actually be a general all-over attack of the inner surface of the pipe, but of course that attack will not be precisely even to the closest thousandth of a millimetre, so in one or a few places it will become evident as a pinhole first. At that stage, you take it out of service, so you don't see that the wall of the pipe has become thinner overall.
All of the stainless steels rely on an oxidising environment, not just 304, so all will suffer from the same problem, although the rates of attack might be very different.
You really need to go to a nickel-base material for the pipes, but that would be so expensive that I'd be inclined to try 2205 stainless steel first, as it should have a service life much longer than 304 and might be cost effective.
Rather than continue to use an inadequate material and try to add a protective coating to the inside to make up for the inadequate corrosion resistance (and the coating itself would require monitoring and maintenance), I strongly suggest you use more suitable material in the first place and not have to worry about a coating.
Have the pipes passivated internally before you put them into service, to be sure that the naturally occurring oxide film starts out as thick as possible, to provide as much protection as possible. If and when that layer is reduced by the oxygen demand of the liquid, the 2205 base metal itself will be more resistant than 304 base metal.
Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.
A. We have had a lot of experience with maintenance of distillery tanks and pipes through the years. With proper cleaning and repassivation maintenance the tanks should last many years with little problem. Starting with 316L or 2205 grades of stainless steel will give you much better results, of course.
Once the pits start forming it is VERY difficult to get rid of them, and the corrosion sites will continue to grow. If you do not want to replace the unit, you can do constant maintenance on the system, or you can grind off the surface down to base metal and passivate it well before using it. Then regular maintenance can keep the pits from reforming usually.
adv.: Let us know if we can help.
Stellar Solutions, Inc.
We deal with a few laser manufactures and a distillery that use 304 SS and the previous answers are absolutely correct. We have helped solve the corrosion problem by plating the SS surfaces with Electroless nickel. The process we use holds up well against chlorides and temperature extremes. The distillery had us plate kettles, mash tuns, piping and fittings for the filters and bottler. He said when he buys more he'll do the same process because it eliminated the rust problem. Lots of distilleries use copper or copper lined vessels which traditionally seem to hold up well. Hope this helps.
- Denver, Colorado
A. Materials aging are easier oxided. My faster and easier recommendation is to grind (at least 0.001") the metal surface in order to eliminate the layer weaker resistant to oxide.
Keep me informed please.
- Minneapolis, Minnesota, USA
304 and 201 stainless shot glasses - liquor challenge?August 25, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread
Q. Hi -
I've got a situation that is left me baffled and I figured this community probably is the best resource to ask - so here goes.
I've got two shot glass samples from china for measuring and drinking liquor.
1 sample is "201" stainless intended for kitchenware and less $$
1 sample is "304" stainless intended for drinkware and more $$
They come from different suppliers.
When scotch is left in both of these glasses overnight the color of the liquid turns dark?
The questions is why?
- Is the passivation non-existent or just poorly executed by both suppliers?
- Is this common or to be expected when leaving 80 proof booze overnight in stainless?
The second question is: can this be fixed?
Would another round of passivation stop this issue? I'm not sure its truly corrosion as it happens pretty quickly (24 hours)
Thanks for your consideration or suggestions!
buyer - New York
August 25, 2014
The problem of liquor finishing off an insufficient passivation job and thereby ending up with unacceptable levels of iron in the beverage is in fact the very origin of using citric acid for passivation, pioneered by Coors in Germany when they first started using stainless steel kegs in the late 1970s.
I would suggest you passivate unused examples of both stainless samples with a citric acid method and redo your overnight scotch test.
adv.: Let us know if we can help with this.
Stellar Solutions, Inc.
August 25, 2014
A. My guess is that the problem is atmospheric oxidation of the scotch rather than reaction with the metal.
However - it should be made a criminal offence to leave good scotch overnight and no gentleman would dream of drinking it from a stainless 'glass' except in emergency.
August 27, 2014
PVD Consultant - San Diego, California
August 26, 2014
Ray - thanks for the input. I am working on getting more samples to perform the test you suggest. Passivation followed by 24 hour soak.
I did get some response that passivation can be done with a type of powder? In my novice mind passivation is done in a bath of acid - like any other bath with liquid.
I'm concerned i've got a lost in translation issue as I'm unsure how a factory working in stainless could have no experience with passivation.
And ---- Geoff, you sir are correct. No gentleman would sip scotch from a stainless glass - that is for sure. We are sure the discoloration isn't from the air as we have a control sample in a glass and its unaffected after 24 hours.
Any other thoughts?
- New York
September 19, 2014
The only "powder" that is useful for passivation is dry citric acid, right before you mix it with water so you can put your parts in. If somebody is telling you differently, they are misinformed.
You would be surprised how many companies working with stainless steel do not know what passivation is.
Stellar Solutions, Inc.
Coating stainless barware for corrosion resistanceAugust 24, 2018 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread
Q. Hi! I have some issues with an SS product that's in contact with alcohol. I understand that nickel level is key, but it also affects the welding capabilities and forming.
products: flask, jiggers, and other bar utensils.
They corrode and change the taste and color of the alcohol.
The manufacturer in China uses SS from 202 - 304 and some others 4xx.
I was wondering if I could finish them with Melonite (QPQ)
Besides the nice finish, it will help with corrosion and will also harden them which will be great.
So, I have 2 questions:
- Is QPQ something I can use on food-related items?
- Is there another plating or process that will do better, considering price? I don't need them to be 'immortal' but I really want them to outperform in quality and safety.
I read about the electroless Nickel plating, but I'm concerned about toxicity and also that it will fail with time affecting the appeal of my product.
Thanks in advance for your answer, I really appreciate it.
- Michigan, USA
A. Hi Catalina. Usually we pick materials of construction and coatings to best suit a number of requirements rather than a single one. For this reason, while I'm not sure whether QPQ processed stainless is food-safe, I think that it would be rarely employed in applications close to yours anyway. Electroless nickel might be suitable, as noted above by Gary Wolf.
There are other issues though. Adding a coating to such items would probably cost more than the item; and getting the inside of items like flasks properly coated could be problematic.
I think the right answer for high quality barware which will "outperform" is to have it made out of 316 stainless steel and then properly passivated; I would suspect that (with the exception of flasks because they hold alcohol for extended periods) 304 is good enough. Good luck.
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"
August 26, 2018
Thanks Ted Mooney. Your answer is really appreciated.Catalina Diaz [returning]
- Michigan , USA
October 3, 2018
I would try electro-polishing the stainless steel barware. It provides superior passivation (eliminating iron and bringing the corrosive resistant metals like chrome and nickel to the surface) its also very cosmetically appealing. Keep in mind not all stainless steels are created equal. Alloys with better corrosive resistance will ultimately turn out better. I think your best bet as Ted stated above to go with a 316 or 304 alloy.
Advanced Finishing Systems - Gloucester, Virginia USA
October 7, 2018
Thank you Aaron G. I'll have this in mind.Catalina Diaz [returning]
- Sterling Heights, Michigan, USA
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