SWG and 'stainless' rusting

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M

mskovrinskie

0
May 31, 2008
40
PA
We installed an IG pool last year with a SWG and I love it! We've had a problem with chronic rusting on the screws on the returns, light, and skimmer boxes, and also very light rusting on the ladder and handrail. I've been careful to maintain the pH, FC, salt, Ca, and other levels within the guidelines in this forum. Iron levels in our water are not high at all. I had been running the filter 10-12 hours per day and we're about to open for the 2009 season.

The builder did put on an anode at construction time which they claim prevents this, but obviously something else is wrong. They have replaced the screws twice now and will probably do so several more times just this year unless it is fixed.

What could be causing this? I suspect a grounding/bonding issue, though everything checks out according to the builder. What else can be done?
 
mskovrinskie said:
What could be causing this? I suspect a grounding/bonding issue, though everything checks out according to the builder. What else can be done?
Click to expand...

That's certainly worth investigating. Current flowing through the water could cause rapid corrosion. The thing that makes me wonder is the skimmer. The screws have no electrical connection and aren't grounded.
 
You should post a full set of water test results, we might be able to spot something there.

Do you have a volt meter? There are some simple electrical tests you can do yourself with a volt meter and a long piece of wire.
 
My builder is literally opening the pool this afternoon, so it will be a few days before I can get stable water readings. I will certainly post once I can though!

Regarding the screws and their electrical connection: since it is a vinyl pool, there is a metail wall behind the vinyl and padding, which I believe is bonded. Is there any physical connection between the screws and that metal?

I always kept my Cl level around 2-3 last year and the SWG was running at the 20% level for 10-12 hours per day.
 
mskovrinskie said:
Regarding the screws and their electrical connection: since it is a vinyl pool, there is a metail wall behind the vinyl and padding, which I believe is bonded. Is there any physical connection between the screws and that metal?
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The screws in the ladder, railing and light certainly are. Thinking about it, I'm not sure the skimmer faceplate screws are isolated from the wall, but I believe they are. This would be a good thing to ask the builder.
 
Screws come in many different type of stainless. The most common type of SST screw is 18-8 which refers to the percentages of Cr and Ni. The best type of screw to use is made from 316L which has very low carbon content making it better at corrosion resistance. This is the material from which many marine parts are made.

Unfortunately, most people who sell things to the general public just say it's made from stainless steel without specifying the actual type of SST.

Good Luck
 
I wholeheartedly agree that the type of stainless steel is very important. We've seen several reports of above-ground pools, especially less expensive ones, with rusting screw problems after adding an SWG (and its associated salt) that were resolved when such screws were replaced with better stainless steel.

We've also seen where chlorine levels would attack even marine-grade stainless steel more rapidly, but this was when there was no Cyanuric Acid (CYA) in the water (it was an indoor pool) and the levels where high at 3-5 ppm FC. So chlorine can certainly have an effect, but usually if there is CYA in the water, the chlorine effect is greatly reduced and the effects of higher chloride (salt) levels become more apparent on weaker materials (see p. 28 in this report for example).

Richard
 
I finally got around to getting water level readings for you all before a SPECTACTULAR swimming weekend (90 degrees in April in Central PA is a rare thing)!

TC looks to be too high, though I had the SWG set higher than normal accidentally. It was at 5 but normally is around 2.
pH = 7.2 (will creep up with SWG)
T/A = 65
CYA = 45
Temp = 73 to start and got up to 77 yesterday!

My TA and CYA are low, so I added baking soda and CYA. This will bring the pH up too.
 

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The recommended TA level with a SWG is 60-80. It is 70-90 when you are using bleach.

Also, there isn't usually any need to adjust the TA level if it is reasonably close. Even if the range was 70-90, and you are at 65, then I wouldn't bother adjusting it just yet.
 
JasonLion said:
Do you have a volt meter? There are some simple electrical tests you can do yourself with a volt meter and a long piece of wire.
Click to expand...

I have the same problem with rust on my skimmer plate screws. I'd be curious to know what is the testing procedure you recommend with a volt meter and wire.
 
el_steak said:
JasonLion said:
Do you have a volt meter? There are some simple electrical tests you can do yourself with a volt meter and a long piece of wire.
Click to expand...
I have the same problem with rust on my skimmer plate screws. I'd be curious to know what is the testing procedure you recommend with a volt meter and wire.
Click to expand...

Keep in mind that by far the most likely possibility is that the screws are made of an inferior grade of metal.

However, someone mentioned bonding issues. Bonding problems are only indirectly related to corrosion problems. Still, it is always good to check for proper bonding, and it isn't all that difficult:

Connect one end of a long heavy gauge wire to the bonding connection on your pump or heater. Then measure the voltage and (if the voltage is very low) the resistance between the other end of the wire and each piece of metal within 3 feet of your pool. If the resistance is more than a couple of ohms or the voltage is more than one tenth of a volt then there is a bonding problem.
 
Here it is.

It's about an inch long and 3/16" in size. The head is 3/8" in diameter. The picture is a bit too dark, but you can see that there's a bit of rust in the 2 bottom thread.

screw.jpg
 
JasonLion said:
Connect one end of a long heavy gauge wire to the bonding connection on your pump or heater. Then measure the voltage and (if the voltage is very low) the resistance between the other end of the wire and each piece of metal within 3 feet of your pool. If the resistance is more than a couple of ohms or the voltage is more than one tenth of a volt then there is a bonding problem.
Click to expand...

Thanks for the info. I'll get my hands on a voltmeter and try it.
 

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