Enless Pool-Corrosion Issues PLEASE HELP

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Jeffm
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Enless Pool-Corrosion Issues PLEASE HELP

Postby Jeffm » Sun 19 Apr, 2009 20:35

I'm having some serious issues with the stainless in my pool that just won't go away. EP has been trying to help, but so far no luck

I'm getting corrosion spots near the hardware holes, they pop up in random locations.

The water chemistry is good: 7.6 pH, 200 Ca, 180 Alk. I bumped it up a little bit, just to be sure.

The stainless consists of sections of 316 stainless steel.

The frame of the tub is well bonded. I had no issues earlier, the problems began after I changed the liner and replaced the water about 2 months ago. Admittedly I started out with my chemistry too low. pH dropped to about 6.8 Alk was down to 80, and Ca was about 170. Bad test chemicals. SO, yes, I had corrosion issues then.

Now...chemistry is good, but the corrosion spots keep cropping up. For example Friday night I had spots on the "right side" in two sections, around the bolts. So I removed them, cleaned them up, but when I went to put them in, the other side had spots pop up....so I put the right side back in...removed the left side..... now the left side is out for cleaning...and the "front end" or drive housing is showing a spot around the bolt hole.

I tried putting a piece of zinc in the skimmer basket, but it had no effect.

The only thing I can think of is the connection point between the interior stailnness and the exterior steel frame is a bit loose...so maybe not a good ground connection??? (there are two self tapping screws that go threw the liner (fyi)

PLEASE HELP!!! I'm spending a ton of time on this and the wife is ready to fill it with concrete

We're not idiots...we both are engineers in the water industry so your suggestions won't be too technical


chem geek
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Enless Pool-Corrosion Issues PLEASE HELP

Postby chem geek » Sun 19 Apr, 2009 21:59

Since the corrosion is near the hardware holes and bolts, this could be galvanic corrosion where dissimilar metals meet. If you wanted to use a zinc anode, you don't just put that into the skimmer. What you want to do is electrically connect it (say, via a bonding wire) to the stainless steel and then bury the zinc anode in moist soil to ground it. That will have the zinc tend to put a positive charge on the stainless steel which should inhibit the corrosion.

It sounds like maybe some corrosion got started from the low pH and the passivity layer on the stainless steel didn't get fully reformed so corrosion is still occurring in those spots. Possibly brushing those areas with a very fine grit could expose more of the chormium in the stainless steel from which the passivity layer gets formed (i.e. just brush enough to remove the rusted surface). If you were able to use bolts that were of the same type of stainless steel as that which it touches, then that would help.

By any chance was your new water much saltier than the original water? You aren't using a saltwater chlorine generator (SWG) system for chlorine are you?

Richard
Jeffm
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Enless Pool-Corrosion Issues PLEASE HELP

Postby Jeffm » Sun 19 Apr, 2009 22:13

Richard, thanks for your help.

It's well water. probably 200 Alk, and 165 Ca from the tap. A little bit of iron, but I used some iron out initially, and we do have an iron filter. on the intake.

Standard cartridge filter.

The pool is in a sunroom, so burying a zinc anode isn't an option. I do know the outside tub is bonded to the wire mesh in the concrete and also to the pump and hydraulic unit and to ground wire on the circuit.
The interior (wet) stainless is connected to the outer tube by screws that go through the liner above the waterline. It is possible that, that is a poor connection. I plan to replace them with machine screws, nuts and washers.

I could attach the zinc to the exterior tub...but that's about it.

I'm using 316 hardware, and the interior metal is 316. It's possible that some of the hardware was corroded and put back into the pool.

Any time I've removed the stainless , I've buffed it with a scotch pad and applied passivating solution (77 plus). It's possible I didn't do a great job I guess...or maybe I need to get inside the bolt holes.
chem geek
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Enless Pool-Corrosion Issues PLEASE HELP

Postby chem geek » Mon 20 Apr, 2009 00:11

Well, it sounds like you are doing the right things. As for the zinc anode, at least having it electrically connected to the stainless steel will help, but without it being able to be in ground (an electrical "sink") it's effect will be lessened.

With the pool in a sunroom, are you using any Cyanuric Acid (CYA) in the water? If not, then your chlorine level is likely to be too high since the Free Chlorine (FC) level won't be moderated. For example, in most outdoor pools, the amount of active chlorine needed to prevent algae is an FC that is around 7.5% of the CYA level or an equivalent FC of around 0.06 ppm with no CYA. If your active chlorine level is high, then corrosion from oxidation will occur faster. Technical details about how CYA moderates chlorine is described in this 1974 paper . A derivation of why the FC/CYA ratio is a decent proxy for the effective FC with no CYA is here .

You don't want too much CYA, but having around 20 ppm and then 2-4 ppm FC would be 5-10 times less effective chlorine than if you used 2 ppm FC with no CYA.

Richard
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Enless Pool-Corrosion Issues PLEASE HELP

Postby Jeffm » Mon 20 Apr, 2009 08:24

Richard,
I typically keep my cholrine level around 1 ppm and avoid any algae. I also use the Zodiac Neptune CF sanitizer. With the pool being in the sunroom, with an opaque cover, 22 hours a day, algae really hasn't been an issue. I use a granual chlorine, (will check the name later) The slow release tablets won't work, because they contain an acid that will float on top and corrode the stainless.

EP actually reccomends using liquid bleach, but since it contains an alkalinty component, I prefer the granual.


I have a sump bucket outside the frame. I could drop the zinc into the bucket and run a wire to the hardware that bolts through the tub. Would using a copper or aluminum ground lug make a difference?
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Enless Pool-Corrosion Issues PLEASE HELP

Postby chem geek » Mon 20 Apr, 2009 21:56

Well, your active chlorine level is around 10 times higher than it needs to be if you were, say, at 2 ppm FC with 20 ppm CYA, but even so, such rapid corrosion is unusual at 1 ppm FC with no CYA.

Liquid bleach or chlorinating liquid is great, but the high pH will NOT raise your pool's pH over time (see this post for the technical reason). The initial addition raises the pH, but the consumption of chlorine is acidic so the net result is pH neutral except for a small amount of "excess lye" in the liquid chlorine. Clorox Regular unscented bleach (6%) is good because it has the least amount of "excess lye".

If your granular chlorine is Cal-Hypo, it has pretty much the same pH effect as bleach or chlorinating liquid, but for every 10 ppm FC you add, it also increases Calcium Hardness (CH) by 7 ppm.

If your granular chlorine is Dichlor, then this is net acidic with use, but for every 10 ppm FC you add, it also increases Cyanuric Acid (CYA) by 9 ppm.

A copper or aluminum ground lug would just make corrosion more likely to occur at the junction, but the beneficial effect would still be there as the positive charge would still continue to your stainless steel connected to that same lug.

Richard
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Enless Pool-Corrosion Issues PLEASE HELP

Postby Jeffm » Wed 22 Apr, 2009 12:31

A few updates (and I keep forgetting to check the chlorine type). You've lost me a little bit, but keep in mind, the pool is indoor, and covered 22 hours/day so I don't use much chlorine. I add 2-3 times week and raise it 2.0 ppm at the most.

Alkanity is actually 120, Ca Hardness I bumped up to 210/220 ish.
pH holding steady, 7.7
I had the copper checked and it's non existant.
TDS is 700.

I think I'd like to bump the Alk up just a bit.

I've had a piece of stainless isolated, just sitting in the middle with no contact, and it's in perfect condition, while the remaining pieces that are tied into the frame, and bonded/grounded (or I think they are), show corrosion pretty rapidly.

Three years ago when I first started up the pool I didn't watch my chemical at first..and had low pH, for quite a while, took some time for the corrosion to start. This is much much faster, almost seems like something is driving this.

I'm going to test a few things:
I'll have two small sections of stainless in the water, not in contact with each other, one will be connected to the frame with a copper wire, the other will not.
I intend to watch them for 2 days, only running the pump, then two days running the heater, and see if there is any difference. I'm wondering if I have a short in heater, or possibly the pump. I guess this is a good way to check.

I'm also planning to run a seperate wire from my bonding tie ins to the frame, with a multimeter in between, I should have a complete circuit, if not then I need to look at the bonding. How it would change in 6 months, I have no idea. I didn't do anything to it.

After the tests are done, I'll add my extra grounding and the zinc.
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Enless Pool-Corrosion Issues PLEASE HELP

Postby chem geek » Wed 22 Apr, 2009 12:55

If your pH is stable, I wouldn't raise the TA. A higher TA will just tend to increase the rate of pH rise, though this is minimized with the pool being covered. You can calculate the saturation index using The Pool Calculator . Yours is slightly positive, which is fine. No need to increase the TA.

So your test showing that the isolated stainless steel didn't corrode does point to something on the bonding wire essentially putting a more negative potential voltage on it which would tend to have metal corrode much faster. Usually corrosion is fastest where dissimilar metals are touching (galvanic corrosion), but in your case it does sound like there's an extra driver forcing this to be much faster. You could try measuring the voltage difference between the bonding wire and general ground or the pool itself -- similar to what you have said you will do.

Good luck and keep us posted.
Jeffm
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Enless Pool-Corrosion Issues PLEASE HELP

Postby Jeffm » Wed 22 Apr, 2009 14:23

Okay, I'll keep the TA as is, thought I'd just try to be safe. My pH is very stable, relatively speaking. A swim in place pool tends to have a higher pH rise due to the aeration caused by swimming and using the propulsion system.

As for my Test...it was mostly by accident, and and observation. An actual test is going to start soon.

I'll give you a mental picture of my grounding/bonding.

I have a bonding wire into my concrete, and tied into my mesh in 2 points (that I can no longer access due to a tile deck around the pool now. I have bonding wires coming off the pool frame (galvanized steel) in two locations. I can still access one of them. I have another wire coming off my aluminum framed sunroom, those all tie into together underneath the tile deck. No access, but no reason to think anything happened to them. They go from the sunroom to my adjoining basement, (now I can see them) and tie into the hydraulic power unit, the heater, and the pump, then to a ground lug on a metal conduit box, that's grounded to the GFI breaker on the breaker box.

The stainless inside the pool is connected to itself, forming a complete circle. But not to the tub/frame itself except for two places. In theory, if you disconnected those to spots, it could be lifted out of the pool as one unit. Now, the points it does connect are above the water line (duh) by means of two self tapping screws. I intend to replace them with some machine screws, nuts and washes, put grounding lugs on and wire to the frame, and then probably to a zinc anode in my sump bucket.

I can't believe that that bonding wire was broken between decemeber and now. But, I think it should be easy enough to check by running a loose wire from a visible point on the mechanicals, to the pool tub, and putting a multimeter in between. I should get a complete circuit.

Here's the odd part, if it's true that the corrosion is only occuring on the parts that are connected to the frame, it seems a bit counter-intuative, and that would lead me to believe that I have a short in my heater, generating a current conducting through the water, then the stainless benches, and into the frame, completing the circuit through the grounding wire.
Jeffm
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Enless Pool-Corrosion Issues PLEASE HELP

Postby Jeffm » Wed 22 Apr, 2009 22:21

Bonding/Grounding verified, more or less. I checked for continuity:
From the stainless in the pool to the frame outside, and back. Entire loop was about 0.3 ohm
Then I checked the loop from the frame, through the bonding wire to the point where all the grounding/bonding from the heater, pump and power unit tie into the rigid conduit, which I assume is grounded back to the circuit panel. Guess I check that.

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