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Thread: SWG and 'stainless' rusting

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    SWG and 'stainless' rusting

    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?
    21,000K IG Vinyl Free Form, 1.5 HP Pump, Sta-Rite filter, 5 jets, 2 drains, SWG, 200K BTU gas heater

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    Mod Squad JohnT's Avatar
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    Re: SWG and 'stainless' rusting

    Quote Originally Posted by mskovrinskie
    What could be causing this? I suspect a grounding/bonding issue, though everything checks out according to the builder. What else can be done?
    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.
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    Re: SWG and 'stainless' rusting

    What is your chlorine level? High chlorine levels are much more corrosive to metals than the salinity level...especially if the metal is constantly submerged underwater.
    Sean Assam - Sean@teamhorner.com
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    Re: SWG and 'stainless' rusting

    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.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
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    Re: SWG and 'stainless' rusting

    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.
    21,000K IG Vinyl Free Form, 1.5 HP Pump, Sta-Rite filter, 5 jets, 2 drains, SWG, 200K BTU gas heater

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    Re: SWG and 'stainless' rusting

    Quote Originally Posted by mskovrinskie
    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?
    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.
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    Re: SWG and 'stainless' rusting

    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
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    Re: SWG and 'stainless' rusting

    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
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
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    Re: SWG and 'stainless' rusting

    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.
    21,000K IG Vinyl Free Form, 1.5 HP Pump, Sta-Rite filter, 5 jets, 2 drains, SWG, 200K BTU gas heater

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    Re: SWG and 'stainless' rusting

    Leave the TA where it is! It will help with the pH stability with a SWG but get your CYA up! (this will also help with the pH stability).

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    Re: SWG and 'stainless' rusting

    Quote Originally Posted by waterbear
    Leave the TA where it is! It will help with the pH stability with a SWG but get your CYA up! (this will also help with the pH stability).
    I thought the desired TA range for a SWG/Vinyl/gas heater pool was 70-90. Is that wrong?
    21,000K IG Vinyl Free Form, 1.5 HP Pump, Sta-Rite filter, 5 jets, 2 drains, SWG, 200K BTU gas heater

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    Re: SWG and 'stainless' rusting

    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.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
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    Re: SWG and 'stainless' rusting

    Quote Originally Posted by JasonLion
    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.
    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.
    14X24 ig vinyl pool 3ft-4 1/2ft deep - 7000 gal
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    Re: SWG and 'stainless' rusting

    Quote Originally Posted by cobra46
    The best type of screw to use is made from 316L which has very low carbon content making it better at corrosion resistance.
    Any idea where I can get 316T skimmer/return screws online?
    14X24 ig vinyl pool 3ft-4 1/2ft deep - 7000 gal
    Aqua Genie skimmer and return - no main drain, 1 hp hayward pump, 27" Hayward filter with Zeobrite,
    Aqua Logic P4 controller + SWG, 2 lights
    192 square feet of solar panels controlled by P4, 5KW electric heater (just in case...)

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    Re: SWG and 'stainless' rusting

    If you posted a pic of your screw I may be able to help.
    11000 Gals, Intelliflo, Sta-Rite Cartridge, Polaris 360

    Pool I built in my old house: my-new-pool-build-t4534.html

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    Re: SWG and 'stainless' rusting

    Quote Originally Posted by el_steak
    Quote Originally Posted by JasonLion
    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.
    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.
    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.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
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    Re: SWG and 'stainless' rusting

    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.

    14X24 ig vinyl pool 3ft-4 1/2ft deep - 7000 gal
    Aqua Genie skimmer and return - no main drain, 1 hp hayward pump, 27" Hayward filter with Zeobrite,
    Aqua Logic P4 controller + SWG, 2 lights
    192 square feet of solar panels controlled by P4, 5KW electric heater (just in case...)

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    Re: SWG and 'stainless' rusting

    Quote Originally Posted by JasonLion
    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.
    Thanks for the info. I'll get my hands on a voltmeter and try it.
    14X24 ig vinyl pool 3ft-4 1/2ft deep - 7000 gal
    Aqua Genie skimmer and return - no main drain, 1 hp hayward pump, 27" Hayward filter with Zeobrite,
    Aqua Logic P4 controller + SWG, 2 lights
    192 square feet of solar panels controlled by P4, 5KW electric heater (just in case...)

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    Re: SWG and 'stainless' rusting

    I can't tell exactly what the size is but my best guess can be found here. You can compare the drawing to your screw.

    http://www.mcmaster.com/#90198a573/=1oodw0. This is a 316 screw. I will have to look a bit harder to find 316L.

    McMaster does requires you to buy a whole bag of the smaller screws.
    11000 Gals, Intelliflo, Sta-Rite Cartridge, Polaris 360

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    Re: SWG and 'stainless' rusting

    Thanks, they provide a diagram with exact dimensions so I'll verify with my caliper tonight.
    14X24 ig vinyl pool 3ft-4 1/2ft deep - 7000 gal
    Aqua Genie skimmer and return - no main drain, 1 hp hayward pump, 27" Hayward filter with Zeobrite,
    Aqua Logic P4 controller + SWG, 2 lights
    192 square feet of solar panels controlled by P4, 5KW electric heater (just in case...)

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