I recently posted about my TA being too low. ~60-70. The general consensus was that since my PH kept rising pretty regularly I should keep it where it is. I went on vacation for 10 days and used pucks in my inline dispenser.. when i got back I noticed by Masterpools Valve wasn't spinning properly (It has like 5 holes in it with a gear that spins around to move the output between my floor cleaner nozzles). I took it apart and noticed the metal pieces of it were disintegrating (Its only about 3 months old, just replaced it).

Checked my TA and it now pretty solidly at 50 (Although the PH was fine about 7.4). Did my low TA destroy my metal components in my valve? Checking other stainless screws and stuff in my pool they all look fine. Including a stainless washer I used in the valve because the one included was damaged. However all the little metal washers and posts in my gear system all have very noticeable pitting on them.

Would a TA of 50 cause this? I added some baking soda to get it up to about 70 but I am wondering if I should raise it a bit more. That gear setup for the valve costs about $125 to replace, if I have to buy another one I would like to avoid destroying it as well. :\ I'm going to contact the vendor to see if they have had other people report the issue, but I suspect my actions caused it. Its probably not as high of a quality as the other stainless components in my pool so perhaps more susceptible to the lower TA?

I would more suspect out-of-balance pH than TA. A full set of test results is probably best.

Everything but the TA is within 'specs':

FC: 5
CC: 0
TA: Was 50, now 70.
CH: 280
CYA: 50
PH: 7.5

The pool does regularly drift up to 7.8 when I use liquid chlorine (Using pucks currently since I was just on vacation for 10 days, waiting for them to clear out and I will be back to liquid). I lower it using MA probably once or twice a week to drop it to 7.4 or so and let it drift back up.

With those numbers I doubt the chemistry is at fault. Hopefully one of the experts will have some idea what caused the problem, since I have none.

A TA at 50 ppm would not have destroyed metal fittings. Metal is most susceptible to corrosion from low pH and/or very high chlorine relative to the cyanuric acid level.

I would suspect corrosion from low pH and very high chlorine relative to the cyanuric acid level if the in-line chlorine feeder is before the valve, especially if the pump shuts off and the undiluted chlorine from the feeder gets into the valve.

The fact that some metal parts were OK and others were not, would lead me to believe it's a quality of the metal problem. The stainless washer you used in the valve was fine, but other metal in the valve was not = high quality stainless washer, and not so high quality other metal parts in the valve. That's my best guess anyway.

Unless I am mistaken, TA won't directly cause that sort of a problem. It only affects the ph. The ph, on the other hand, being a measure of acidity or alkalinity, will directly affect the water's propensity to attack metals or other substances. Since your ph is 7.4-7.5, it was not eating the valve. At least, not now.

I agree with JamesW and Ohm_Boy, low TA can't cause metal corrosion directly. There would only be a corrosion problem if the PH got significantly too low or the FC level was way way way too high. It is far more likely that the corrosion is the result of the use of inferior materials or pouring certain chemicals (notably muriatic acid) directly into the skimmer.

How do you add your muriatic acid?

Muriatic acid should be poured slowly in front of a return jet while the pump is running. The return jet will cause it to mix into the bulk pool water much more quickly than any other approach.

Thanks for the responses guys. My inline puckinator (LOL) is RIGHT before the valve. So I think (As JamesW suggested) when I am using pucks (Rarely, but a LOT in the last 45 days or so since I've been out of town) and my pump turns off the super-chlorinated and low PH water goes into the valve and just sits there for like 14 hours until my pump turns on again.

Any way to resolve there? There is only like 2 feet of PVC between the valve and the puck dispenser. Only thing I can think of is removing the gears when I am using pucks which will impact my circulation somewhat but at least stop me from destroying them.

You could allow the pump to run continuously, but that would waste a lot of electricity.

Removing the gears is the only option I can think of right now.