You beat me to this topic by just a couple days. I actually had it all written up and have been waiting to make sure the sulfur smell didn't come back before posting. I only came across this thread when I was making some edits in preparation for posting and trying to find a reference from another thread. I don't want to re-edit what I've already done so here it is, there is some duplicate information from earlier in this thread :
Recently a friend of mine purchased some brand new Perlick 650SS flow control taps. Unfortunately, it seems that some 650SS taps are prone to causing a sulfur odour when used with low pH beer or cider. Numerous complaints can be found on this forum and others, AIH even posts a warning on
their 650SS page. In our experience, the sulfur odour happens after the beverage has been sitting in the line for at least an hour; after a pour or two it will go away until it is left stagnant for a while. After some research and experimentation, I'm confident that we've determined the source of the sulfur smell and how to fix it. There is a chance that the theories presented below are not entirely correct, but the procedure did fix the sulfur issue for us.
Source of the sulfur
The 650SS is very similar to the design of the 630SS, with the addition of flow control. However, these flow control parts are (probably) fabricated from 303 stainless instead of 304 stainless. 303 was likely chosen since it is easier to machine – a useful property for small, intricate flow control parts. Why is it easier to machine? Because of
sulfur additions in the steel (also phosphorus, but that isn't important here), which allow the tool to slide more easily over the surface and promote clean shearing of the steel. The sulfur/rotten egg smell comes from residual elemental sulfur that remains on/in the surface. The sulfur should have been cleaned off before the steel was passivated.
Passivation is a process which builds up a protective oxide layer on top of the just-cleaned stainless.
303 and other 'free-machining' stainless steels need a slightly different procedure from the standard passivation in order to remove residual surface sulfur – essentially just a pre- and post-soak in alkaline solution, preferably sodium hydroxide. ASTM A967 mentions this the in appendix (paragraph X1.3.3), but does not specify a procedure. Basically, the acid used for passivating the stainless dissolves the MnS inclusions, leaving behind elemental sulfur in the resulting pits which is not removed by typical post-passivation cleaning. Check out
this paper which provides a more thorough discussion of what is happening.
Why does Perlick let this happen?
Since this problem does not occur consistently with all 650SS taps, I have a couple guesses as to the cause:
- Perlick has multiple vendors/factories doing their cleaning/passivation, some who know how to treat 303SS and some who do not.
- Their passivation treatment method is borderline ok, taking away most of the residual sulfur, but certain batches of 303 have higher sulfur content resulting in leftover sulfur on the final product .
These are simply my guesses and I do not have any concrete knowledge of Perlick's actual operating practices or materials used, just my own experiments which have a very small sample size. I emailed Perlick with the information in this post, but have not received a response from them. I will update this post if I do.
Experimentation
Since we had two 650SS taps, we wanted to test whether the NaOH soak was sufficient to correct the problem, or if a full cleaning and passivation treatment was required. One of the taps only underwent a NaOH soak (5 wt% @ 70°C for 30min). For the other tap we performed the NaOH soak, followed by passivation in citric acid (as per ASTM A967 Citric I, 10 wt% citric acid @ 60-70°C for 1 hr), followed by another NaOH soak same as before. Citric acid was used since most homebrewers would not (and probably should not…
have access to the nitric acid and sodium dichromate bath traditionally specified by ASTM A967.
We've been using the taps on a cider (pH unknown – it’s all gone now) and berliner weisse (pH 3.3) for the past month or so and have not had the sulfur smell return on either tap. There may have been a couple times where the taps were not used for several days and they was a hint of sulfur, but I'm unsure whether this was just me being paranoid. It was subtle enough that I doubt I would have noticed it if I wasn't actively looking for it. Therefore, I believe a single alkaline soak treatment is enough to solve the problem.
How to remove the sulfur
The following procedure details the process to desulfurize your Perlick 650SS flow control taps. You should only need to do this once and the sulfur will be permanently removed from your taps.
CAUTION: Sodium hydroxide, and to a lesser extent, citric acid, are hazardous chemicals which should be treated with greater care than your standard brewery chemicals such as PBW or starsan. This website provides a reasonable overview of the hazards. Always read the MSDS and take appropriate safety precautions before using highly caustic or acidic chemicals.
Follow this procedure at your own risk, I am not responsible for your safety.
You may be able to find both sodium hydroxide and citric acid at your local homebrew store. However, I found it was 1/10 the price at a local DIY soapmaking store, and I could buy it dry instead of a premixed liquid form. Hardware stores may also stock it.
- Completely disassemble the Perlick 650SS tap. Don’t worry about removing the o-rings; they are Buna-N/Nitrile, which is compatible with all of the chemicals being used.
- Separate the 303SS parts that need to be cleaned, rinse them in hot water, and set aside. The 303SS parts are handle lever (item 5A), flow compensator (item 9) and/or the flow control lever shaft (item 11) in the diagram on page 2 of this link. I’m not entirely sure if all these parts are 303 but this procedure isn’t going to damage anything if they aren't. You could probably throw the whole tap in the bath, but I was worried about etching the polished finish on the outside of the tap. After further research this probably isn’t a concern, but proceed at your own risk.
- Prepare a 5 wt% NaOH (sodium hydroxide) solution in a stainless steel or borosilicate glass container large enough to fully submerge the parts. My starter flask worked well for this. This can be done by mixing 5g of dry NaOH per 100 mL of water. ALWAYS add the NaOH slowly to cold water, not the other way around! Dissolving NaOH is an exothermic reaction (gives off heat) and may spatter if water is added to the NaOH. Distilled water is preferred, especially if your tap water contains a lot of minerals.
- Heat the solution to 70°C. Make sure you have adequate ventilation in the area.
- Place the parts in the hot NaOH solution and soak for 30 minutes, maintaining temperature.
- Rinse parts thoroughly in water.
- Rinse parts in isopropyl alcohol. I'm unsure whether this step is necessary, but Shen mentions it in their paper as reducing the surface residue so I don't think it hurts.
- Rinse parts thoroughly in water and allow to dry.
- Smell or taste the parts. If you can still detect sulfur, something may have gone wrong. You can try installing the taps and using them, or try the full alkaline-acid-alkaline passivation treatment with citric acid. To perform full alkaline-acid-alkaline passivation treatment, proceed the same as above, completing step 6. Then prepare a 10 wt% citric acid solution in the same manner as the NaOH solution, using a ratio of 10g/100mL. Heat up the citric acid solution to ~60-70°F and soak for 1 hour. After the citric acid soak, rinse the parts in water, and continue again at step 3, repeating the NaOH wash (steps 3-9).
Let me know if you have any questions!
References:
ASTM A967 - Standard Specification for the Chemical Passivation of Stainless Steels
The Cleaning of 303 Stainless Steels (T.H. Shen, 2004))
Carpenter Technology - Passivating and Electropolishing Stainless Steel Parts
Onto this thread:
Based on my write-up above, I think you should try the NaOH soak again, possibly for longer. Also, it sounds like you just soaked the levers last time? You should soak all three parts you show in
pic08
I hadn't come across the information stpug raised regarding the reaction of buna-N/nitrile with starsan. I wouldn't worry about it unless nitrile gives off sulfur in all acidic solutions (more specifically beer/cider/wine/etc.). However, I don't have much background with polymers so I could be missing something. To back up your point that the acidic starsan is releasing sulfur from the stainless parts, I did notice a strong sulfur smell when performing the citric acid soak and I was not using nitrile gloves (though I probably should have used some for safety).
