304 Stainless Steel Corrosion problem at distillery

A. Hi Robert. A quick answer: yes, stainless steel should always be passivated.

But there are surely finishes than 304SS, including using 316SS as suggested by Lee Kremer, electroless nickel plating it as suggested by Gary Wolf, and replacing it with glass as implied by Geoff Smith.

Luck & Regards,

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.

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.

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.

A. Howdy,

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.

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.

A. Chris,
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.

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.

Geoff,

Great post!

Jim Treglio - scwineryreview.com
PVD Consultant & Wine Lover
San Diego, California
August 27, 2014

A. Chris,
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.

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.

Regards,

 Thanks Ted Mooney. Your answer is really appreciated.

A. Catalina,
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.

  Thank you Aaron G. I'll have this in mind.