rust spots in fermenters
some very small spots of rust have popped up in my fermenters. they're really small less than the size of a dime, but there are 6 or so.
Is there a tank polishing regimine? to take the steel back to the original 300 grit or whatever polish is on these bad boys. I am hesitant to take even really fine sand paper to it, but the rust can scrape off with my finger after the oxiclean step.
I'm hopping these are newly manufactured tanks. If they are then a small amount of ferrous material was on the tools during manufacturing and it has left some free iron that can allow your S.S. to rust. Unfortunately not that uncommon of an occurrence. The solution is to buy some "pickling compound" from a local welders supply shop and apply (follow all instructions...very nasty stuff). This is why some S.S. manufactures brag about there ferrous free manufacturing practices (i.e. no mild steel on the manufacturing floor).
The reason I am hoping new tanks is because otherwise you have managed to place free iron onto the tank from your own brewery! This free iron will have to be comming from somewhere in your process and will need to be identified (not easy). After removing the source then treat as above.
Note dime or even 1/2 a dime size is LARGE!
Hope this helps.
Last edited by Tbrew; 04-20-2005 at 06:50 PM.
Excellent advise above.
The rust MUST go. I suggest you have a few beers with an experianced Stainless welder about your situation.
(It's good to know a good one you can call in an emergency ...belive me.)
Just sanding is not an option as it may leave mineute scraches and the surface must be re-pascified.
You may want to re-look at what your doing that may have introduced the rust.
Do you ever climb inside your tanks?
You can often re-passivate by circulating a strong oxidising agent. We have used 5 % orthophosphoric acid, warm to hot (say 50 - 60 deg C) for an hour. If necessary repeat. The use of chlorine free sterilising agents is recommended, e.g peracetic acid. This will continually repassivate. If there is a major impurity in the steel, then this will probably not work.
The pickling compound used normally (always ?) contains hydrofluoric acid - as has already been said - spectacularly nasty stuff to get on skin / in eyes - so used with care.
As has already been said, grinding back with very fine abrasive may be necessary before pickling / passivating. Suggest you grind back, pickle, then passivate with H3PO4 etc
Along with the previous responses, localized heat will also start corrosion . If you ever noticed when you try and "arc" weld stainless, Corrosion will start soon after. ( it doesnt' take much heat either). Reasons for this are another subject.
We re-passivate using Nitric acid on all stainless before putting into service and then ever 6 to 12 months afterwards.
Again, Very nasty stuff! Caution is vital.
As for removing the "rust", try using wet paper, 600 to 1000 grit.
Same thing the Autobody shops use.
Flush very well after. Any grit going though the pump will raise caine with the pump seals.
You're right Matt.
The brain wasn't fully engaged. We actually use a mixture of nitric and phosphoric acid, but the greatest proportion is phosphoric (the proportion varies from supplier to supplier), and we tend to say we clean with phosphoric acid.
I'm going to wake this thread up... We have three fermenters that are going to need some rust spots treated. All the spots are in the inside on the weld seam where the walls transition to the dome top. I'm hesitant to use the pickling compound because of the difficulties of disposing of the materials and rinse water afterward. Is there a happy medium that people have had success with? My current thoughts are softly cleaning the spots with a stainless brush and some passivation blend?
Don't know if this helps..
Pickling should remove heat affected (oxidised) areas of surfaces following welding, rust, other impurities and metal parts.
Passivation, is the formation of a new protective oxide layer.
The only chemicals that should be used for pickling & passivating are based on nitric acid and hydrofluoric acid. Pickling agents should contain 10 - 15 % V/V nitric acid and 1 - 3 % V/V hydrofluoric acid.
Pickling agents with added hydrochloric acid, sulphuric acid, or other acids must not be used
Hydrofluoric acid is a reducing acid that in combination with the oxidising nitric acid dissolves the existing ‘fault’ oxide layer slowly and constantly.
Stainless Steel may be pickled with liquid chemicals, spray pickle liquids or compounds (pastes):
- liquids are used to pickle pipework by circulating them, or in a bath so smaller parts can be submerged;
- liquids may be sprayed on to pickle large areas in tanks;
- compounds can be used to pickle the outsides of the welds and other small areas.
Circulate and/or spray the pickling agent at a temperature between 15°C and 40°C for approximately 2 hours. Rinse thoroughly with halogen (chlorine, iodine, bromine) free water after pickling, ideally demineralised.
Wire brushing can be used as an alternative to pickling with paste, but the brushes must be made of stainless steel, nylon or other non-halogenated plastic abrasive such as Scotch Brite and must only be used on stainless steel for that purpose.
We didn't want to use scotch brite pads inside the fermenters... Is this ok then, or do we want to limit the use to the affected area's?
The chances are, unless you have highly polished stainless, that the scotchbrite will create finer scratches than the existing finish. Normally the finish of the main stainless sheet used is as rolled. The recommended grinding out after welding is coarser than that created by scotchbite. I can't remember what grit they reckon they should be finished to, but I have not doubt that scotchbrite will be considerably finer, though I accept not at mirror polish standard. But you would only need that for pharmaceutical, though I know some people prefer polished for yeast culture plants, but even they are not mirror polished like pharma.
The welders here regularly use scotchbrite pads, often with the paste or concentrated liquid to help get rid of the weld scorch marks quicker.
I would only use them on the affected parts, unless you think the whole of the inside would benefit from a precautionary treatment as well as the intensive treatment on the visible rust. suggest if you want to treat the whole vessel, assuming you can reach all parts with a brush, then tie wrap a couple of scotchbites round the broom head and use it that way. Less agressive physically, but probably enough if there are pin points of corrosion elsewhere.
Don't use scotch pads. This will strip your passivation layer. You need this layer. That is why there is rust. We use a phosphoric/nitric acid mix, loffler kms10, and run it at 5% for about 3 hours. Spray with a hose, gone. Did this today actually. I have a heating element in my HLT that rusts from time to time, I let it get a little bad the last time, went like 3 brews with it. Water is only in it for about 4 hours total. But do a strong acid CIP and it will come off. Good as new. Make sure your sprayball, if you are using one, isnt foaming the solution either. I never use scotch pads in my passivated tanks.
Scotchbrite pads are ok to use to get off thick rust, but MUST be followed up by careful passivation. Alternatively, use scotchbrite with the passivation liquid, but in this case you need to be very well protected, facemask, full waterproof suit if you use it for the whole vessel.
Awesome! Thanks guys! We will be going over them today. We'll let you know how it goes!
We passivate all new equipment, and anything that's been ground, tooled, sanded or welded, with a 20% solution of nitric acid. I won't play with hydrofluoric, period. After passivating for about an hour at an initial temp of 140 deg, we allow the equipment to air-dry without a rinse--this is an important part of passivation.
We periodically acid-wash all product-contact equipment with Wesmar BSR 10-40, 5%. BSR 10-40 contains phos and nitric acids (IIRC), and re-passivates while removing lime scale and beerstone. Again, allow to air-dry before rinsing.
We routinely clean with maroon Scotch-Brite pads. No problem.
We have no rust problems.
We are looking good so far... We have beer in two of the three tanks so we'll see how they look afterwards. Our head brewer climbed in and wire brushed with passivation solution and let air dry. Hopefully this will do the job.