SWG chemical/corroding problems

[Rocket]

Put pool in this Summer. Pool guys filled it up and added to much salt (3900). So I had to drain down and add more water (3200 now). However after getting refilled water levels were way off. CA was 8. Calciaum hardness way low and FC way high. So they added proper chemicals, but I have since noticed skimmers screws and now automatic pool cover screws corroding like crazy. Not sure if happened before or after chemicals added. And it was the 2nd or 3rd time they tested the water and added chemicals. They run it thru a computer and it tells them what to add so I don't know why they can't get it right. Also I have a automatic SWG. The ORP is set to 650, but it never seems to be operating at 650. Before the last batch of chemicals were added it would read 615-620. Since the last batch was added now it says 690-710. It is supposed to make the right amount of chlorine to maintain 650, but as mentioned it has been all over the place. Also when I try to test the levels with strips. The strips are crazy off. I have two different types. One would say the FC was 15-20. The other would say 2-4. All the other levels tested were way off on the different strips from each other as well. Needless to say I am upset. I was wary of a SWG and corrosion after reading the blogspot pool guy, but my contractor said he has never had any problems. I am especially wary of it because unlike a few screws or a ladder, the pool cover cost about $10,000 and I am afraid the motor/rails/box etc will get eaten up. So far only a couple screws on it is corroding. But I am worried about the aluminum rails and the "coated steel box". Also concerned about the expensive stamped concrete surrounding the pool. Not sure what to do as obviously nothing is working right. Living in Midwest the season is drawing to a close.

[chem geek]

First of all, get yourself a good test kit, either the Taylor K-2006 you can get at a good online price here or the TF100 from tftestkits.net here with the latter kit having 36% more volume of reagents so is less expensive per test. Test strips are not accurate, especially for tests like CYA and they can't test Calcium Hardness (they only test Total Hardness).

I suspect that the corrosion is from the active chlorine being very high due to the Cyanuric Acid (CYA) getting lowered from the dilution and the SWG outputting too high so the Free Chlorine (FC) level is high. In an SWG pool, you need the FC to be a minimum of around 5% of the CYA level. If the chlorine is very high relative to CYA, then when combined with high salt levels this can be corrosive as you are seeing. It sounds like your ORP sensor or controller isn't very reliable as you should be able to dial it in and have a reasonably constant FC. You could just not use that part of it and use a fixed % on-time for the SWG which is what most people do. You will need to roughly figure out your chlorine usage if you go this route.

Read Water Balance for SWGs for more info on what you need to do for proper water chemistry in your SWG pool.

[Rocket]

Thanks. I am giving this info to my pool guy.

Two other questions.
#1 I have read somewhere on here that a sacrificial anode works well to stop corrosion. How do you do this? What would it be made out of? I want to stop the corrosion of the screws (hand rails are powder coated) and especially the concrete, but I also have aluminum rails on my automatic pool cover. I think I read something about zinc or magnesium?


#2 The pool guy put something in my skimmer basket (round metal thing) a few weeks ago that he said would help stop corrosion. Obviously it did not work. What is it and should I keep it in the basket?

[chem geek]

If there is aluminum touching the water, then the sacrificial anode must be made of magnesium. You only use a zinc anode if there is iron, stainless steel and copper -- not if there is aluminum. [EDIT] I have to qualify what I said. If there is aluminum, then it is better to use magnesium, but zinc would still likely have some protective effect. I originally looked at the electrochemcial series and not the galvanic series under real-world conditions where with the latter zinc is a little protective even for aluminum though magnesium provides more protection. [END-EDIT]

He may have put a zinc ball in the basket, but that really doesn't do much good. The zinc anode needs to be attached (connected electrically) to the bonding wire and then buried in moist soil.

[Rocket]

does anyone have a link to where you can buy a sacrificial zinc or magnesuim anode?