Learning to do maintenance on my own gear, Apeks

[Tanks A Lot]

He is right that halogenated polymers will emit harmful substances when they burn. Specifically, fluorinated ones, which FKM is, can emit carbonyl fluoride COF2, which is incredibly toxic. It's LC50 is 360ppm, which makes it ten times more deadly than carbon monoxide at 3700ppm. There is another fallacy often coming into the mix here in SCUBA which is: "Well, if my regulator burns, the toxicity of the gas is my least problem!" This statement misses the mark of what is happening and possible. It assumes that combustion is always catastrophic, which isn't true at all. Anyone that has worked on enough oxygen equipment will eventually see charred high pressure seats, where partial combustion took place. The user likely never noticed a thing. In such a case, trace amounts of toxic gases can be produced.

There have been high profile cases in the medical industry, where people died in hospitals. One such case was in France in 1993, where two people died after a PCTFE seat burned in a manifold without anyone noticing. PCTFE is even worse than only fluorinated polymers, as it contains chlorine. It's combustion product can contain phosgene, which was responsible for well over 85,000 deaths during World War I. France banned PCTFE in oxygen equipment afterwards.

If you made it through the above you might be wondering: "Well, I either use something that doesn't burn easily, but if it does emits deadly gases, or I take something that ignites rather quickly?!". And that is in essence the conundrum that oxygen gas engineers are faced with and there is no easy way out. The current consensus seems to be that FKM is a better choice, even with the added risk of toxic gases. The risk of EPDM or NBR catching fire outweighs it.
But the designer takes this into account, as designing for oxygen service is so much more than just chucking in a few FKM O-rings and call it a day. How does the gas flow? Which are the critical areas, where do I see high flow rates, where will adiabatic heating take place? What if the high pressure seat ignites, have I designed my system in a way that puts O-rings, springs and diaphragms out of harms way?

I write all of the above, not to put you off your stated goal, quite the contrary. But I would like you to approach the subject with the right mindset. The SCUBA industry is often a bad example for many things. It's a small industry, that is virtually unregulated. Look at other industries, such as the medical one and try to extrapolate from there.

A very basic approach to cleaning your SCUBA gear for oxygen would look as follows:
1. Wash the parts with dishwaser. Any will do really, this is just our pre-clean. Maybe seek non-scented ones, but thats just my personal preference.
2. Rinse the parts generously.
3. Inspect the parts carefully, as this will bear heavily on the order below. Are the parts coated with silicone grease? Well, that needs to go before the acid bath, as otherwise the acid will just sit on top of the silicone grease.
4. Alkaline bath. Specific formulas exist, but in a pinch one can mix something with washing soda. Surfactants play a key role, dishwashing concentrate often contains anionic ones, which are excellent for alkaline cleaning. A little surfactants go a long way.
5. Rinse again.
6. Acid bath, hopefully not with vinegar... Again surfactants are key. Look for something containing amphoteric or nonionic surfactants. Some dishwashing concentrates may have these. Don't worry about shifting the PH of the solution with these added, as you'd have to add ludacrious amounts to make a difference.
7. Rinse yet again!
8. Neutralization bath with baking soda.
9. Clean the parts with one the Simple Green alternatives.
10. Rinse one last time, preferably not with tap water, but don't sweat it too much if you can't find any distilled water, although any hardware store carries it.

Things to keep in mind:
In SCUBA, the worst offenders for hydrocarbons are silicone grease and what it traps and compressor oil. Despite what many, many people will claim, you can't get rid of silicone grease with dishwashing liquid, nor simple green, nor the acid bath, nor most alkaline cleaners. Silicone grease has a PDMS oil, which has a siloxane backbone. This siloxane backbone is very unimpressed by any of the above. You need to grab something like toluene or xylene to remove it. This is a major consideration when converting something for oxygen service that has seen.
Modern compressors run on synthetic oils, which alkaline cleaning is well equipped to remove. However, if you used gas that had mineral oil in, this isn't the case. Mineral oils do not have ester links, so again they are largely unimpressed by our alkaline cleaning to remove oils, as it just doesn't saponify.

In SCUBA, oxygen cleaning has often been conflated with "Just use Simple Green!", but that is non-sense. You must approach the topic systematically and if you suspect a certain impurity present, use an appropiate removal tool. Simple Green is great, but it isn't the one-stop answer many make it out to be.

Do not forget to test your parts for cleanliness either. While non of these will pick up trace amounts, water shake tests, beading tests and visual inspection can give you some peace of mind.

Again, I by no means want to put you off your stated goal however means for you that you must approach the subject carefully and diligently. Oxygen cleaning isn't something magical that only certain people can do, but it takes patience and attention to detail.

 

[Mobulai]

Whenever I see a notification for a post by @Tanks A Lot I know I’m in for a treat of a learning experience; Tank you Tank you for that awesomely educational „rant“ (and fixing a bunch of misconceptions on my side)
(More than I bargained for)

 

[DANDM]

This was very interesting and informative, thank you. I do have a couple of questions.

4. Alkaline bath. Specific formulas exist, but in a pinch one can mix something with washing soda. Surfactants play a key role, dishwashing concentrate often contains anionic ones, which are excellent for alkaline cleaning. A little surfactants go a long way.

Pardon my ignorance, but what is the goal of an alkaline bath? It doesn't seem like a common step for non-oxygen cleaning servicing. What pollutants is it aiming to clean? You mentioned compressor oils, but why wouldn't a thorough (extreme) simple green/detergent wash not achieve this?

With citric or phosphoric acid, what concentrations and time would you aim for?

 

[Tanks A Lot]

Alkaline cleaning is squarely aimed at removing many oils and organic residues. When cleaning something for oxygen, there are three main areas you should think about. Admittedly, these are rather vaguely defined, but broadly fall into opposite ends of the pH scale and the square middle.

1. Alkaline cleaning​

Alkaline cleaning works on many organic residues. The OH⁻ ions from the alkaline, which has a very high pH (many OH⁻ ions), are apt at removing a lot of organic residue. This works because the OH⁻ ions break down the triglycerides which are present in many oils, fats and greases. The products are glycerol and what we commonly call soap. These are water-soluble, which oils and greases are not, and it allows us to remove the converted oils and greases with the water.

People often conflate dishwashing liquid and other household soaps with alkaline cleaning, but that is incorrect. The early soaps we produced (already thousands of years ago) were truly alkaline. But today’s soaps are often virtually pH neutral. Imagine that wasn't the case. Every time you’d be doing the dishes or go into the shower, you would douse yourself in a strong alkali. Clearly, that isn't the case.

While some soaps and shampoos may be very, very slightly alkaline, the bulk of them aren’t at all. They often work with surfactants. These don’t just lower the liquid’s surface tension, which is essential for proper cleaning, but they also encapsulate dirt particles in something called micelles. These are tiny structures that have a hydrophobic head and a hydrophilic tail. In essence, the hydrophobic head buries into the grease particle, while the hydrophilic tail sticks into the water. Once enough of these have buried into a dirt particle to form a sphere around it, it’s called a micelle. This micelle can easily be suspended in water, as the outer layer is made up of the hydrophilic tails. Thus, grease and oils can be suspended within water, which they normally can’t. This is often referred to as emulsification.

The way surfactants clean is very distinct from how alkalines work. One chemically breaks down the oils and fats, while the other suspends them in solution.

If you think about alkaline cleaners, think about sodium hydroxide (lye), oven cleaner, and similar substances. These things truly cut through grease, unlike most household soaps. Something I have used when I wanted to mix something myself is sodium carbonate, often called washing soda or soda ash. You won’t reach extreme pH values, as it maxes out around 12-ish or so, but it is still a much better true degreaser than many soaps. True alkaline cleaning must be approached with a similar mindset to acid cleaning. This stuff is very harsh on your skin and causes similar burns to acids. Especially industrial oven cleaners can go up to close to 14 on the pH scale, which eats through skin.

So no, soaps generally do not count as alkaline cleaning at all, very few truly are. In a nutshell, alkaline cleaning aims to remove many greases, oils and fats. These often contain hydrocarbons, which easily ignite.

2. Acid cleaning​

Acid cleaning works on mineral residues. When you go diving in saltwater, or rarely very mineral-rich freshwater, mineral deposits will be left behind on your equipment. That white stuff you see on your regulator? That is calcium carbonate (CaCO₃). This stuff forms very differently from, say, common table salt (NaCl). While the latter more or less simply precipitates out of evaporating saltwater, CaCO₃ is formed by some easy-to-follow chemical formulas, which involve carbon dioxide, carbonic acid, carbonate and calcium. The thing about CaCO₃ is that, unlike many other salts that form from saltwater evaporating, this one isn’t water-soluble, not even a bit. Acid cleaning is almost exclusively used to remove the CaCO₃ salts from equipment. It has very little other use. It does so by introducing H⁺ ions, which essentially reverse the forming formulas (simplified a bit).

Now this also makes for the most satisfying part of any cleaning, because you can see it, it bubbles and makes the gear shiny. That is why it often has special status for technicians, it is something that they can see. But the truth is that, at least as far as oxygen cleanliness is concerned, this type of cleaning pales in importance compared to alkaline cleaning, which removes hydrocarbons.

For a soaking solution, I would dilute phosphoric acid to around 10%. For an ultrasonic cleaner, aim at 5% or below. Citric acid does well in similar concentrations, although I might slightly increase them by a couple of percentage points. Citric acid is inherently weaker as an organic acid compared to the mineral phosphoric acid. Temperature is a key ingredient in any thoughtful cleaning process, think about the Sinner’s Circle, which I won’t expand on here. Aim for 50°C to 60°C, but avoid temperatures that would cause too much vapour. Acid vapour is nasty, even when it comes from "safe" household acids such as phosphoric or acetic acid.

You could use vinegar, it works as "well". But while phosphoric acid and citric form insoluble chromium compounds, acetic acid produces highly soluble ones. So no, phosphoric acid and citric acid will not leave the chrome untouched, they also attack it. The difference is that they generally attack the surface layer and then form a sort of protective barrier with the insoluble products. This isn’t a perfect protection layer, as it’s not a hard patina, but it gives at least some very crude protection.

All acids will attack chrome, but some do so more than others. And contrary to what many would like you to believe, it is not just down to the concentration of the acid, but often what products it forms with chromium oxide.

The time in the acid should be as little as possible, because like I mentioned above, they all attack chromium oxide to some extent, it’s just the nature of the H⁺ ions. With 10% phosphoric acid, it will not take more than a couple of minutes. Watch your gear and the white deposits of CaCO₃. If these are gone and the gear has stopped bubbling, you are done with acid cleaning. It does not clean anything else other than these mineral deposits.

3. Removal of anything that alkalines and acids don't touch​

Alkalines remove many greases and oils, acids remove minerals. But stuff like silicone grease needs another approach, such as toluene or xylene. Equally, old PFPE grease is impossible to remove without a specialized cleaner. The thing with PFPE grease is that it is fluorinated, and the chemical saying “like dissolves like” rings true here. Short of industrial substances like Tribolube® KO², Christo-Kleen 1, or Krytox MS-250 Re-Move™, I have not found something that reliably removes them. The trouble is that to remove such a halogenated grease, one likely needs a halogenated solvent and these are not exactly common in the household. It’s something I have not found an easy DIY solution for.

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You rightfully ask why alkaline cleaning might be in order when Simple Green or similar is used. There are two trains of thought here. The first would be: it isn’t necessary.

This might be true under most circumstances, especially if we just look at SCUBA. If you look at the MSDS of Simple Green Extreme, you’ll find that it has some very basic alkaline properties, often due to triethanolamine and potassium silicate. A lot of their cleaning comes from surfactants or propylene glycol butyl ether. These are very apt at removing oils and greases, but are not alkaline. When heavily diluted, it won’t leave you with much alkalinity. So yes, these cleaners are great at cutting through most greases and dirt, but more so due to the surfactants and propylene glycol butyl ether, and they often suffice. But there is the slight off-chance you’re working with something that is truly heavily contaminated, such as an unknowledgeable person using marine grease or some such thing. Admittedly, these are edge cases, but something to keep in mind. Simple Green is not a magic bullet.

Secondly, and far more important for me, the stuff isn’t cheap, at least not for me. Reusing it can have real value. If you remove pretty much all oils and greases and other contaminants before your gear touches the Simple Green, you can keep using it for many cycles. I like to use it as my last step, well, second-last if you count the final rinse. This has the added virtue of cleaning any residue that my earlier cleaning may have left. Silicone grease is terrible, but so are the residues of toluene or xylene I may have not fully removed. Think about the Simple Green (or whatever other compound in its place) as the final step to truly remove any leftover that you have missed earlier. A last step to give you peace of mind, not a magic bullet.

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None of what I have suggested above is what was used back in the old days. There are things that clean greases and oils far better than what I described, and there are acids which outshine phosphoric acid by a mile. Carbon tetrachloride and trichloroethylene were extensively used for degreasing. Nitric acid and chromic acid are apt at removing calcium carbonate and more importanly, they do not attack the chrome layer, or better said somewhat reform it.
But these are not used anymore, as they are either very toxic, super harmful to the environment, carcinogenic, or all three. They are truly nasty substances which I would steer well clear of.

 

[Tanks A Lot]

With regards to acids, I find the following pictures illuminating. These were chrome layered brass rings I cut into pieces and half immersed in acid. I have done this a few times, always with the same results.

Acetic acid

Citric acid

Phosphoric acid

It should be crystal clear to anyone that acetic acid should be the last choice for what we are doing. Unfortunately you will hear the vinegar suggestion ad nauseum, even explicitly stated in many service manuals. It has become somewhat religious for many people, god knows for what reason....
It is the worst choice that you have at your disposal. If you ever bothered to look at what the commercial cleaners, such as Cressi’s ALR 5000 or GSM’s Regulator Cleaner Solvent contain, you won't be surprised to find that they are always based on phosphoric acid.

Vinegar is not the right choice and anyone who ever bothered with citric acid or phosphoric will never go back, ever.

 

[DANDM]

@Tanks A Lot brilliant response, thank you for taking the time to write such a detailed response. It's been a few years since I studied chemistry as part of my degree, and I was by no means talented at it, but your post truly juggled my memory and made a lot of sense. I even made soap during chemistry, so your explanation really made sense to me!

I'm adding citric acid to my shopping list. Will have to check if phosphoric acid is accessible and affordable down under in my corner of Australia.

It's great to read how oxygen cleaning works. I've been trying to explore whether that's something I want to do. I've learnt to service my gear from a Hog service clinic course and the RSlingers workshop, but oxygen cleaning is one step that I am a bit concerned about. I'd love to do it, but every manual just goes on about getting proper training. Unfortunately I can't find anyone offering an oxygen cleaning course in Australia. The only course is the TDI Oxygen cleaning course, but it requires the gas blending course as a prerequisite (paying for a course I don't intend on using?!) and having to work for a dive shop. That's just not something I'm interested in investing at the moment. Maybe with a bit more reading and preparation I'll take the challenge to O2 clean my regs some day, for now I'll just inform myself about the process.