Don't ever risk your hardware based off a 1 1/2 page thread from 2 years ago on OCN. There's alot of false info to be had on OCN in tiny little threads like that. Not saying whether that info's reliable or not, but there's tons of unreliable info floating around here. There's only a few people that i'd take their research as absolute facts. Mayhem with dyes and coolants; Martin, IT DIVA, Bundy, Stren, BNEG in WC components; IT DIVA in anything electronics related.
Also remember, no matter what you choose for coolant or water it WILL become conductive in a loop after time. There's nothing you can do about this except flush your system periodically.
Copper/Brass/Nickel doesn't matter "Aside from bad plating" is absolutely correct. And bad plating on one block will not only give noticeable corrosion, it will corrode every single block and radiator in your loop. That's the problem
At this point, everybody's plating METHOD's are correct. But there isn't a company in the world that has 100% success with every last piece they manufacture. Take my 290x for instance. If that 290x was actually a bad plating job, my entire loop could be destroyed by the time i notice. And loops become expensive quickly.
It's not something that you should "expect" to happen. Just like you shouldn't "expect" to receive a bad CPU/Mobo/GPU, but it happens