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.