I am wondering if AISI-302 stainless steel parts are suited for permantent contact with the nutrients? It is because I am thinking about using a non return valve which in the reservoir which has AISI-302 grade parts.
I don’t know how aggressive the nutrients are to metals and if this kind of stainless steel might bleed toxic stuff into the reservoir… Any idea?
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Not too familiar with the types but I would think that would work. The 302 is noted as having good corrosion resistance with the primary adjuncts in the metal being Carbon, Manganese, Phosphorus, Sulfur, Silicon, Chromium, Nickel, and Aluminum. 302 is resistant to solvents, acids, and chemicals but not as resistant as 304. 316 probably even better. Avoid mixing metals types (e.g. stainless to some other metal) to avoid galvanic corrosion. This type is sensitive to chlorides.
Types 302, 304, 304L and 305 austenitic stainless steels provide useful resistance to corrosion on a wide range of moderately oxidizing to moderately reducing environments. The alloys are used widely in equipment and utensils for processing and handling of food, beverages and dairy products. Heat exchangers, piping, tanks and other process equipment in contact with fresh water also utilize these alloys. Building facades and other architectural and structural applications exposed to non-marine atmospheres also heavily utilize the 18-8 alloys. In addition a large variety of applications involve household and industrial chemicals.
- Food and beverage industry
- Pressure Containing Applications
- Sanitary or Cryogenic Applications
This reference provides some additional insight: https://www.foodprotection.org/files/food-protection-trends/Oct-12-Schmidt.pdf
Pitting corrosion generally is the result of exposure to chlorides, bromides, and other halides and is accelerated by high temperature and lower pH level. Once formed, pitting corrosion has a tendency to continue to grow and is difficult to remove.
Also, you can be sure the surface of the metal is passivized to improve resistance to attack particularly if you plan to use any bleach for cleaning:
http://howtobrew.com/book/appendices/appendix-b/passivating-stainless-steel http://beersmith.com/blog/2017/01/09/passivating-stainless-steel-beer-brewing-equipment-to-prevent-corrosion/
Much better than brass or copper. A good quality plastic value would perhaps be even better for this application.
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Should be ok…‘should’, but plastic might be better if possible.
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Pls show me a good quality plastic non return valve because I couldnt find any 
What you mean by mixing metals? How could I mix the metals?
In the pdf you linked it shows a table and it says there that the 300 Series has low corrosion resistance and therefore its use is limited?
What is in the nutrients solution that is causing the oxidation?
You mean I can be sure or I have to make sure the stainless steel is passivized if I want to use bleach?
Actually it is a only a spring I have to deal with. Can I somehow do or order somewhere a spring in the right material myself? I guess it only seems easy … 
I don’t have anything off-hand. Will look. Do you have some specs?
Use of two different fittings of different types of metal. E.g. interconnecting a copper fitting to the stainless steel fitting and such.
Or, for that matter, the solution is rather conductive. Simply having different metals in the overall system may be sufficient to create what amounts to a weak electrolytic cell.
For stainless, the primary concern will be chlorides. So anything that is a chloride can cause some oxidation. Typically, chlorides are avoided in nutrient formulations but there will always be some (e.g. sodium chloride). How reactive the chlorides will be to passivized 304 stainless, I don’t really know. The chromium content (or the lack thereof) in the steel somewhat dictates this.
You can passivize the metals to help protect it from being attacked. Prepping the stainless by passivizing the surface before using it will help protect the underlying metal. Bleach (hypochlorite -> chlorides) may damage / remove the passivization, I believe. So, if you use bleach to clean the system, you may want to re-passivate the metal once done. And, avoid the use of bleach in a running system for long periods with the stainless fittings.
I would think, overall, the bigger concern would be mixing of the types of metal of fitting as that causes active corrosion. Electrolysis of the metals.
For instance,
Corrosion due to mixing of metals. But, even so, this is after several years.
I’d say use what you have but keep an eye on it and if there are any toxic symptoms look there and consider if it is affecting your system.
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I have a suction pump which has no parts which can corrode. But I have not yet found a non return valve. Nominal diameter is 1 inch.
Alternative would be to use a sump pump with that I have and which also has a non return valve integrated. But I have no idea if the parts can corrode or not. And I couldn’t find any information about the materials.
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Perhaps something like?
https://www.pvcfittingsonline.com/valves/pvc-swing-check-valves/standard.html?size_in_inches=36
https://www.pvcfittingsonline.com/valves/pvc-swing-check-valves/clear-pvc-swing-check-valve.html?size_in_inches=36
check:
https://www.homedepot.com/p/Mueller-Streamline-1-in-PVC-Check-Valve-101-605HC/100200991
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Thank you very much!
Do you know if the type without a spring is also reliable?
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I didn’t look into the reliability of the different fittings or types.
The check type valve would typically have higher ratings against back flow (as compared to a non-return valve). A check valve is a non-return valve but not necessarily the other way around. Different ratings. Out of the listed examples, Hayward would likely be of the highest overall quality.
Check valve for industrial use would likely have the greatest reliability. But, you’ll need to search around to find some and they’ll likely be priced to reflect the performance. Search I performed was simply ‘PVC check valve’. There are also different plastic materials that may generally have better performance than PVC. Some plastics perform better than their metal counterparts under different circumstances.
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Thank you. Do you know if corrosion is always seen by the eye? Or maybe the metals can emit into the nutrients and you see nothing (maybe start of corrosion)?
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Sure, corrosion (generation of oxides, sulfides, or salts) or simply free metal ions that are dissolved into liquid can go uncaught. The rate at which this occurs would be the concern. A metal that is slow to corrode will take longer to detect and likewise would be contributing less metal ions into the solution. Stainless steel is generally slow to corrode in mild environments.
Think of this, most homes contain copper pipes for water. An exceedingly small number of copper ions are transferred into the water. The longer the water sits in the piping, the PH of the water, the temperature, and the mineral content determines the amount of copper in solution. You generally do not see the corrosion because you cannot view it and, under normal circumstances, the process is slow, slow, slow. The amount of copper in residential tap is likewise exceedingly small.
For a hydroponic system, folk have seen issues when using brass or copper in recirculating systems over extended period of time. Possibly, copper toxicity. Plants are sensitive to copper. @MicroDoser and @anon32470837 have experience and some testing with metallic toxicity in solution. Take a look into their threads and their experiences.
Stainless, though, is much more resistant than copper. The compound I’d be most concerned with would be nickel and chromium. Maybe aluminum. Though, I don’t think you’ll have a lot of problems here unless you are soaking the metal in chlorides or you never ever change the solution out.
But, again, if you can find a plastic fitting that is suitable that would eliminate the dissolved metals concern. Also, consider that nothing lasts forever. Building an increasingly reliable system also means that you are probably spending extra for higher quality components. I, personally, prefer reliability but sometimes it is equally wise to consider the trade-off of regularly replacing certain components instead of spending extra dough. Over time, you’ll be able to figure out where the extra money is worth the expense. Then upgrade.
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Yup, I can confirm that using copper leads to sulphur def, and iron leads to a P def eventually. I will say though that it took the iron casing to be very very rusted internally for any def to show up, a year or two at 5.8PH and a few runs with bleach. Copper piping causes a sulphur def almost immediately, no bleach or acid required. 12 brass fittings (15mm gate valve) used in a 100l recirculating system with 12 plants will gradually trend towards a sulphur def when they start to corrode. In my case all it took was 3 hours at 2PH and a dose of bleach to get them to start dumping much more copper into the water, although the valves were about 4 years old by that point.
I spent about a year trying to figure out deficiencies etc, looking at RH%, PH, and various things until I found out it was metal toxicity. Now though, I know nothing is wrong with my environment because I have addressed every aspect of it trying to chase down the defs…
I have shifted the way I view quality, product life, replacement schedules etc recently.
I used to buy a part, and when it failed I would replace it. Some parts I would buy a basic part, other parts I would buy higher quality. Most often it would be active parts, like pumps, that would get the most money thrown at them. Then I noted the cost of the failures.
Now, I think “What would it cost me if that part failed?” which sometimes leads me to have one of those parts just sat around to swap in, sometimes I shorted the replacement schedule, and other things get a very very good quality part and a spare sat around and a short replacement schedule. If my pump fails I don’t even want to wait the time it takes to go to the shop to replace it.
I also think “What is the way this part fails?” some parts just stop working, this is for me the ‘best’ failure. It is ‘failsafe’. Sometimes though they fail in a destructive way like corrosion or leaks. These failures can cause secondary problems or cascading failures. For the first type of failure, I just keep a spare and try to replace the part before failure. For the second I see when it starts to cause issues and replace that part with a much harsher schedule, ideally half the time between fitting and a problem.
I generally find that the cost of a failed or corroded part is around 1% of the cost to my plants from it failing. I keep finding higher quality parts, at higher prices, and so far the cost is easily justified. While I got it cheaper the R.R.P. of my main pump is around £800. Plastic head, magnetic drive, no metal in contact with the solution. I got it precisely for the reasons this thread was made, to limit metals leaching into my nutrient solution causing lockouts/deficiencies. The recent costs of not having that pump are heading closer to ten times that amount.
Of course, all this depends on how important it is for your plants to reliably grow in a healthy manner. Some people are fine with a failure every now and then. Losing 1/4 of your yield, or losing half a cycle refitting things is not such a big problem to some, but if getting a certain yield, or keeping to a particular timetable are important things then you should at least consider making a chart or calendar with all your parts and when they need to be replaced marked on it to remind you. It is good practice anyway.
I feel your pain trying to find plastic parts, I am hunting for 15mm gate or ball valves so I can stop using brass ones. All the plastic valves seem to be 4 times the size of metal ones, understandably, and none I have found fit 15mm plastic piping. I may have to change my whole system to be 22mm throughout.
I know I have a year or two to do that before my brass fittings start to become a problem, and when they do I know the signs now. This, for example, is too much copper. Too much iron shows later in flower and manifests as a P def where older leaves suffer necrosis starting at the leaf edge.
After swapping out the brass valves for new ones, improvement happened almost immediately, within the week.
EDIT :
I would say the corrosion would be visible to the eye, although it may be internal to the valve/pipe/pump. When my brass fittings needed replacing they looked like this. Externally my pumps looked fine but when I looked inside investigating my problems they were very very rusted indeed.
I would say if the internal surface still looks new, the chances of toxic levels of metals leaching into your solution is low.
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Very helpful answers. Thank you very much! 
I would like to contradict myself as I had problems even with the new brass parts. Bright yellow plants after 3 weeks…
I have replaced them with plastic ones and the problem has now gone for good.
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I am having trouble with some of my plants. I am not sure if there might be leeching something into the nutes. Also I can rarely change the nutes…
“Older leaves show necrosis starting at the leaf edges…”
That is something that I am observing.
How can I find out what is the source of these nutrient deficiencies?
Plants are in 3 weeks bloom.
Now even of some newer leaves of some plants there is already yellowing. Why is that? But I am not sure, maybe it is some bleaching with the newer leaves.
Btw, do you know if the hunter solenoids which are for lawn irrigation are good for use with fertilizer?
In order of operation, I always double-check PH. Then if PH is good I refresh the nutrient solution.
Your problem there looks to me like a P def, which is rare and suggests either incorrect PH or contamination. It could also be magnesium which is also likely if you have an RO system. Both would be fixed by fresh nutes and correct PH. Check your PH, if it is over 6 it could be the cause of a P def, if it is under 5.8 that could be the cause of a Mg def.
If that does not fix the problem then your problem is not a lack of nutrients, nor a lack of availability from incorrect PH. If that is the case, iron may be leaching into your nute solution.
Once you have eliminated the most common causes, check for any metal parts or other possible sources of contamination. After my experiences in the last year, I would no longer have any metal parts in a recirculating system in contact with the nutrient solution.
I am unfamiliar with Hunter solenoids, sorry. Maybe find some technical specs or a datasheet.
IME parts designed for run to waste flow are often unsuitable for recirculating systems.
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Why this graphic shows a completely different range for ph of potassium availability in hydro???
Hmmm, I get these mixed up all the time and sometimes think “P” when it is actually “K”, etc. Perhaps, it’s just me. But, as a reminder:
Potassium uses the symbol “K”.
Phosphorus uses the symbol “P”.
Also, these charts are based on nutrient availability before they start to get “locked out”. The transition from available to unavailable is not necessarily a hard cutoff as visualized in such graphics, but rather it decreases outside of the optimal range. It becomes more difficult for the plant to uptake the mineral as the chemistry of a mineral species changes due to the PH change (also somewhat dependent on the overall chemistry in the system).
I believe that there is some interpretation when generating such charts. They are more intended as a rough guideline.
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I do too. Would be easier to remember if it was Po and Ph.
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ok I changed the nutients completely. After I put in the nutrients in the RO water, it has a ph of 5.3. What you suggest where I should start with my ph considering the actual state of the plants? Low or high?