SS passivation a myth?

[bracconiere]

🗳 and i'm putting in a request for a brew kettle icon under smileys.......

 

[jtratcliff]

hmmm....I'm not a chemist or metallurgist, but I find the ASTM argument compelling... I've also used BKF for surface rust spots assuming that it also passivates... but usually get spots again in a similar spot..

Makes sense that you might be abrasively removing *,most* surface iron then forming an imperfect chromium oxide layer from air but not reacting w/ acid to remove *all* surface iron...

I dunno... I'd say when in doubt passivate w/ Citric acid...

 

[Rickdude]

An abbreviated version of our passivation process (based on the NASA studies):

-Make sure everything is completely, completely clean. Any soil or film will prevent effective passivation.

-Decide if you are going to recirculate your passivation solution or completely fill your vessels with the solution, but the key is to ensure that there are no dry spots.

-Use 4% citric acid on a weight basis (so 10 gallons of water = 83.3# water; you add 3.3# of citric acid).

-Heat the solution to 140 F and either soak or circulate. Do this for 30 - 60 minutes. (Don't go longer because there's actually a drop off in effectiveness with time.)

-Drain the solution completely and leave everything open to air dry. You are not done yet, though...

-Let the air-dried surfaces remain exposed to the air. 8 hours should be sufficient, but there's no time limit.

I'm attaching the slightly longer version, too. Hope that's helpful!

Rick

 

[Brewbuzzard]

How many folks has he sent to the moon again?

I didn't know the moon was SS, I thought it was made of cheese.

 

[Zinge]

An abbreviated version of our passivation process (based on the NASA studies):

-Make sure everything is completely, completely clean. Any soil or film will prevent effective passivation.

-Decide if you are going to recirculate your passivation solution or completely fill your vessels with the solution, but the key is to ensure that there are no dry spots.

-Use 4% citric acid on a weight basis (so 10 gallons of water = 83.3# water; you add 3.3# of citric acid).

-Heat the solution to 140 F and either soak or circulate. Do this for 30 - 60 minutes. (Don't go longer because there's actually a drop off in effectiveness with time.)

-Drain the solution completely and leave everything open to air dry. You are not done yet, though...

-Let the air-dried surfaces remain exposed to the air. 8 hours should be sufficient, but there's no time limit.

I'm attaching the slightly longer version, too. Hope that's helpful!

Rick

So, after 8 hours do you need to rinse, or scrub/clean?

Are folks in general passivating all stainless vessels, including hlt, mash? I have some rust spots inside a stainless pump head so I imagine recirculating acid through that is a good idea?

Any concern with citric acid on heating elements?

 

[passedpawn]

So, after 8 hours do you need to rinse, or scrub/clean?

Are folks in general passivating all stainless vessels, including hlt, mash? I have some rust spots inside a stainless pump head so I imagine recirculating acid through that is a good idea?

Any concern with citric acid on heating elements?

As I mentioned before, I clean my coffee pot with it. It gets up to 200+F. So I don't see any problem cleaning your elements, even with them on.

 

[RPh_Guy]

So, after 8 hours do you need to rinse, or scrub/clean?

Rinse with water after draining the acid.
The surface needs to be thoroughly cleaned before passivation, therefore cleaning and/or scrubbing afterwards is not needed.

Are folks in general passivating all stainless vessels, including hlt, mash? I have some rust spots inside a stainless pump head so I imagine recirculating acid through that is a good idea?

I just recently passivated everything (all stainless equipment that contacts the wort/beer/water) since I got a new kettle, new chiller, new pump, and new false bottom.... I love my new system.
Passivation took care of some rust spots on the pump head and false bottom.

Any concern with citric acid on heating elements?

Nope!

Cheers

 

[Gruel]

1. Passivation does not occur at room temperature.
2. Air does not selectively remove iron from the surface of metal (i.e. passivation does not occur spontaneously).
3. Lemon juice contains a lot of other substances that may interfere with the process.

The acid (nitric or citric) removes the free iron from the surface. The chromium oxide then forms all by itself - this is the passivation - , as long as there is oxygen around. At room temperature.

 

[RPh_Guy]

The acid (nitric or citric) removes the free iron from the surface. The chromium oxide then forms all by itself - this is the passivation - , as long as there is oxygen around. At room temperature.

The acid does not remove iron at room temperature.

 

[eric19312]

The acid does not remove iron at room temperature.

If you look up the definition of passivation this is spot on. Passivating is removing the free iron so that the chromium oxide layer can spontaneously form. Removing the iron with acid is normally done at specific concentration, temperature and time. And these variable depend to the type of stainless steel (eg 304).

 

[Gruel]

The acid does not remove iron at room temperature.

Mhmm. ASTM A-967 7.1.1.3 seems to say otherwise.

7.1.1.4 and 7.1.1.5 then allow other recipes as longs as the parts afterwards pass the test requirements, like dunking in water for an hour, drying for an hour, repeating twelve times, and showing no surface rust. Or keeping a wet cloth on the surface for at least an hour, and then letting the surface air dry, before inspecting for rust.

 

[Gruel]

So you have like a 1 quart boil kettle?

I have 3 ten gallon kettles in my system.. how many lemons is that anyway?

Very funny. I actually do have a few quart-size pots that I also clean and treat using this method. In any case, I'm not filling my stainless pots with acid, I just keep the surface wet. But as I said, there's nothing wrong with making your own acid if you need a lot.


I didn't follow the oxalic acid (barkeepers friend) discussion in all detail, but I wanted to add that
a) oxalic acid works very well to electro-etch austenitic steels, to reveal the grain structure after polishing, and
b) oxalic acid is not (to the best of my knowledge) mentioned in the above standard on chemical passivation treatments for stainless steels. On the other hand, section 8.2 allows - if I read it correctly - any other treatment, as long as the parts meet the test requirements.
So, to slightly paraphrase Red Green, if it works, it's an accepted solution...

 

[Bilsch]

Very funny. I actually do have a few quart-size pots that I also clean and treat using this method. In any case, I'm not filling my stainless pots with acid, I just keep the surface wet. But as I said, there's nothing wrong with making your own acid if you need a lot.


I didn't follow the oxalic acid (barkeepers friend) discussion in all detail, but I wanted to add that
a) oxalic acid works very well to electro-etch austenitic steels, to reveal the grain structure after polishing, and
b) oxalic acid is not (to the best of my knowledge) mentioned in the above standard on chemical passivation treatments for stainless steels. On the other hand, section 8.2 allows - if I read it correctly - any other treatment, as long as the parts meet the test requirements.
So, to slightly paraphrase Red Green, if it works, it's an accepted solution...

Ok thats cool.. but what is the resistance to using citric acid? Obviously its not the 'it's natural' argument if you have no troubles with the not natural oxalic acid in BKF. Secondly, electro etching would work with many different acids or even salt water as the electrolyte because that process is an electrochemical reaction which is a whole different deal then we are talking about here. And lastly.. one would think that if Red did specific testing on oxalic acid's ability to enrich the chromium on the surface of stainless that the ASTM would have heard about it by now. But hey you do whatever you think is best for the maintenance of your equipment.

 

[RPh_Guy]

Mhmm. ASTM A-967 7.1.1.3 seems to say otherwise.
View attachment 683202
7.1.1.4 and 7.1.1.5 then allow other recipes as longs as the parts afterwards pass the test requirements, like dunking in water for an hour, drying for an hour, repeating twelve times, and showing no surface rust. Or keeping a wet cloth on the surface for at least an hour, and then letting the surface air dry, before inspecting for rust.

I base my suggestions on data. Take it or leave it.

See page 22 (Phase 1 conclusions):
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110001362.pdf
"several citric acid treatments especially those at room temperature, performed worse than no passivation treatment."
See page 63 for data.

 

[passedpawn]

Ok thats cool.. but what is the resistance to using citric acid? Obviously its not the 'it's natural' argument if you have no troubles with the not natural oxalic acid in BKF. Secondly, electro etching would work with many different acids or even salt water as the electrolyte because that process is an electrochemical reaction which is a whole different deal then we are talking about here. And lastly.. one would think that if Red did specific testing on oxalic acid's ability to enrich the chromium on the surface of stainless that the ASTM would have heard about it by now. But hey you do whatever you think is best for the maintenance of your equipment.

Great conversation guys. I'm really enjoying it. Wish I understood chemistry better as these subjects usually leave me with more questions than answers. My thoughts:

• I think oxalic acid is very natural, at least as much as citric. Isolated directly from plants, same as citric.
• I would think that repassivation of chromium is a lot faster if a strong oxidizing acid is used. This conversation could benefit from discussion of redox equations and/or ORP values
• I wonder if something simple, like hydrogen peroxide, would accelerate the repassiviation of the chromium after an acid was used to remove free iron.

 

[Vale71]

AFaIK oxalic acid removes Cr and Ni as well, so it cannot be equivalent to citric which has been shown to remove Fe only.

 

[shoengine]

Great conversation guys. I'm really enjoying it. Wish I understood chemistry better as these subjects usually leave me with more questions than answers. My thoughts:

• I think oxalic acid is very natural, at least as much as citric. Isolated directly from plants, same as citric.
• I would think that repassivation of chromium is a lot faster if a strong oxidizing acid is used. This conversation could benefit from discussion of redox equations and/or ORP values
• I wonder if something simple, like hydrogen peroxide, would accelerate the repassiviation of the chromium after an acid was used to remove free iron.

View attachment 683209

"didn't your mole used to be on the other side?"

 

[Gruel]

Ok thats cool.. but what is the resistance to using citric acid? Obviously its not the 'it's natural' argument if you have no troubles with the not natural oxalic acid in BKF. Secondly, electro etching would work with many different acids or even salt water as the electrolyte because that process is an electrochemical reaction which is a whole different deal then we are talking about here. And lastly.. one would think that if Red did specific testing on oxalic acid's ability to enrich the chromium on the surface of stainless that the ASTM would have heard about it by now. But hey you do whatever you think is best for the maintenance of your equipment.

Bilsch, I have no resistance to using citric acid. I'm not harvesting clover to make my own cleaner...
I was simply adding that lemon juice is easy to come buy, and seems to work.

Regarding the oxalic acid, I was just adding that it does something to the surface of stainless steels like 304 and 316. You are of course correct, electro-etching is very different from just soaking. I didn't mean to imply otherwise. Also, as Vale71 points out, it makes sense that to reveal the grain structure oxalic acid has to remove everything (all elements in the alloy), I assume with preference along some crystal/grain axis.

 

[Gruel]

I base my suggestions on data. Take it or leave it.

See page 22 (Phase 1 conclusions):
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110001362.pdf
"several citric acid treatments especially those at room temperature, performed worse than no passivation treatment."
See page 63 for data.

RPH_Guy, I did not know that. Maybe newer versions of the ASTM standard take that into account; my quote is from the 2007 version.

Added: A very interesting read. So they (NASA) did not accept the ASTM standard until doing their own tests. But then their conclusion is that citric acid (at 140F) indeed works better than nitric acid.

Regarding their test method: Blasting your samples with iron grit seems a very aggressive test; you are embedding iron contamination in your surface. It sounds like it might be worse than using the wrong tools to machine stainless. Hopefully our tools and vessels are not treated that way during production.

Coming back to our application: if there is no reason nor indication to assume a brewing vessel or other implement has iron contamination on the surface, then why go through the whole passivation rigamarole? I'd argue to leave good enough alone, unless you have reason to believe you have an iron problem somewhere. And then citric acid (at lemon juice strength, 4%, and 140F), according to NASA, is the way to go....

 

[Gruel]

Great conversation guys. I'm really enjoying it. Wish I understood chemistry better as these subjects usually leave me with more questions than answers. My thoughts:

• I think oxalic acid is very natural, at least as much as citric. Isolated directly from plants, same as citric.
• I would think that repassivation of chromium is a lot faster if a strong oxidizing acid is used. This conversation could benefit from discussion of redox equations and/or ORP values
• I wonder if something simple, like hydrogen peroxide, would accelerate the repassiviation of the chromium after an acid was used to remove free iron.

View attachment 683209

We don't seem to have any chemists here, at least none that chose to reveal themselves, and metallurgists (like Palmer) are apparently not good enough... ;-) Brewers are a tough crowd.

 

[day_trippr]

What revision is "How To Brew" up to these days? 6? 7?

Cheers!

 

[Bilsch]

We don't seem to have any chemists here, at least none that chose to reveal themselves, and metallurgists (like Palmer) are apparently not good enough... ;-) Brewers are a tough crowd.

Chemists?? We don't need no stinking chemists, we got RPh-Guy!

 

[Agtronic]

So, very interesting read. I’m a certified welder and have forgotten a lot of the specifics over the years, reading this thread has brought back a lot of the theory.

I build parts for race cars, and use mostly 304, 316L and 321. Some observations:

Same-grade examples vary by quality depending on where they are sourced. In my experience, 304 from a quality source, intended for the food and beverage industry, out performs similar looking 304 sourced from overseas in many ways. I once stretched a piece of the good stuff from 3.0” diameter to over 4.0” using a tube expander (used to create slip fits) and I couldn’t believe it didn’t break. It just kept stretching. I tried the same with a piece of imported 304 (markings clearly visible) and it cracked with the slightest bit of pressure applied. You would think that if they are calling it 304, it w have the same composition right? Well it means little if the source is not trustworthy. I’ve seen similar results with rusting of exhaust systems. The good stuff stays clean while the cheap stuff starts to surface rust. 304 is not even intended for heat applications, but the good stuff works well under the stresses of exhaust heat, all the way up to the turbo.

So why do I mention this? I imagine that the same varieties of material exist in the brewing world. Just because it’s 304 or 309 or 316, it doesn’t mean much unless they used good quality stuff from a reputable source.

My curiosity lies in this: If you are not using heat, ie: using a vessel for fermenting, what would cause iron to make it back to the surface? I would expect that passivation would only be required once, not on a regular basis. I always figured that applying heat was the real issue for SS. I’ve used passivation acids to clean welded areas on manifolds and other critical parts and it always lasted the life of the part. So what’s different with brewing equipment?

By the way, I LOVE stainless steel. It’s been my bread and butter for a long time.



edit: many autocorrected words, typing this on my phone was painful.

 

[Bobby_M]

It probably has to do with paranoia more than anything. It would be easy enough to test I suppose. If you run a CIP cycle with citric acid and then send a sample of the solution to a lab to test for iron content, that would tell you if it did anything positive or not. If you don't find any iron, it was a waste of time.

 

[Vale71]

My curiosity lies in this: If you are not using heat, ie: using a vessel for fermenting, what would cause iron to make it back to the surface?

Surface wear due to cleaning and disassembling/reassembling of various parts would be my first guess. Although if you only CIP (either with a spray ball or a full soak) and never disassemble anything (not a good idea unless you're 100% sure that dirt cannot collect anywhere no matter what) I suppose you could go decades without further treatment.

 

[Bobby_M]

Surface wear due to cleaning and disassembling/reassembling of various parts would be my first guess. Although if you only CIP (either with a spray ball or a full soak) and never disassemble anything (not a good idea unless you're 100% sure that dirt cannot collect anywhere no matter what) I suppose you could go decades without further treatment.

Yeah, it's much more likely that all the accessories that get handled and banged together after every use is what really needs it as opposed to the interior of a fermenter that doesn't see any abrasive contact.

 

[day_trippr]

Damn that's some fancy grade TIG work right there
Turbo displacement looks bigger than that 4 banger

Cheers!

 

[RPh_Guy]

You guys (and gals) referencing Palmer,
Are you talking about what he says in the first edition of How to Brew as some kind of evidence against needing to passivate?

If so... That's incorrect and outdated. But, who better to tell you that than John Palmer?

Here's a more recent (2015?) article by Palmer:
Preventing corrosion
Here he talked about using nitric acid and none of that nonsense about just-clean-it-and-air-will-passivate. However this article is obviously also outdated since he does not mention citric acid and he references an outdated standard.
Here's a 2019 MBAA podcast where John Palmer and Ashton Lewis discuss passivation and tout the benefits and rationale for using citric or nitric acid for removing surface iron.

I hope this helps kill the Palmer myth that chromium-rich passivation occurs without following a protocol using citric or nitric acid.
It's not just the oxide layer that's important for keeping the surface passive. The composition of the metallic surface is also important -- iron oxide is exactly what we do not want, so iron should be minimized.

Maybe some day Palmer will learn about Fenton reactions just as he eventually learned about passivation.

• I would think that repassivation of chromium is a lot faster if a strong oxidizing acid is used. This conversation could benefit from discussion of redox equations and/or ORP values
• I wonder if something simple, like hydrogen peroxide, would accelerate the repassiviation of the chromium after an acid was used to remove free iron.

I don't know of any published scientific evidence to answer these questions. I've heard "expert" opinions range from the oxide layer forming instantaneously upon air exposure to several weeks.
I've heard of H2O2 being used to help passive some aluminum alloys but not steel.

If you are not using heat, ie: using a vessel for fermenting, what would cause iron to make it back to the surface?

Theoretically, chloride (present in wort/beer) or chlorine compounds (often present in tap water) can damage the passivation layer, as can some cleaning compounds. Furthermore any mechanical scrubbing, scrapes, or scratches can possibly expose surface iron. I know off hand that there are in fact published studies that show that the presence of stainless steel can affect H2S production during fermentation, which proves that the oxide layer can be penetrated under normal circumstances and therefore passivation can have an positive impact.

In my opinion, how often you should passivate or whether you passivate at all depends on why you want to passivate. If you're just worried about corrosion, passivation is optional, just insurance. If you're worried about the effects of iron on your beer, then more frequent passivation is probably warranted. If you're not worried about either of those potential issues, then just go about your business.

Passivation is not a difficult or expensive procedure. I imagine it's a lot less expensive to passivate numerous times than to get a single iron test with ppb sensitivity.

 

[passedpawn]

Theoretically, chloride (present in wort/beer) or chlorine compounds (often present in tap water) can damage the passivation layer.

I put some bleach in a cornie keg and left it for too long (days). Etched a hole right through the steel. I had to recycle the keg. True story. Learned a valuable lesson that day. So yea, strong enough chlorine will remove the passivation layer

 

[Agtronic]

That makes sense. If someone would tell me that scrubbing or scratching SS would lead it to rust, I’d say I disagree. You can grind SS down as much as you want and it will not rust when exposed to water/air. But as I expected, in brewing the worry is not rust but rather the effect trace amounts of iron may have on the beer.