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Thread: SWG and corrosion issues

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    SWG and corrosion issues

    Hi all. A couple of months ago I filled my new indoor SWG plaster pool after a couple years of planning and building. The pool was designed by a PE who specialized in designing and building pools for 20+ years. the pool was built by a company that has concentrated (mostly) on building and servicing (though not regular maintenance) commercial pools for about 30 years.

    I believe everything is properly bonded, except for maybe the solar heat exchanger.

    Here are my latest test results using TFT 100 kit:

    FC 2.5 (I am now generating to increase to 3.5+, FC is pretty stable, drops slowly)
    CC 0.0
    TC 2.5
    pH 7.6 (I have to add MA every day or two and I strive for now to keep it mostly in the range of 7.6 to 7.8)
    TA 70
    CH 270 (I recently added calcium chloride, and will add some more to push it above 300)
    Borate 0.0
    CYA 0.0
    Salt 3450 (recently added salt to raise from 2900, use white test strip)
    Temp. 82 (Usually increases to 84-85 for swimming)
    Liquid Solar Blanket (no comment because I really don't know what if anything it does for me)
    Sandstone coping and patio, coated with a sealant

    So anyway, a couple of days ago I came across a blog written by a guy who swears that SWG is worse thing to happen to pool consumers since people had to swim in unsanitized and unbalanced pools, back in the preindustrial days I guess. He has been working his blog for a number of years now and has posted what he claims to be un-contestable evidence that SWG systems are nothing but large profit centers for the pool companies and sooner or later will bring nothing but woe to the unsuspecting consumer. This guy is articulate and convincing.

    After I read some of his material I thought I should just rip out the SWG, let or help the water return to a non salt condition, and according to him, live happily ever after.

    I have since read many of the comments by contributors to this forum about this issue, and I have decided to take a more considered approach. In particular I will almost certainly add a sacrificial zinc into the water stream and properly bond it. If nothing else this should serve a an indicator of the existence of galvanic action: if the zinc is dissolving, that would indicate galvanic action and should provide some protection to many of the metal parts in the water. My concern here is adding zinc ions to the pool water.

    I will appreciate any comments about this issue that the participants would care to share.

    Thank you for your time.

  2. Back To Top    #2
    JasonLion's Avatar
    Join Date
    May 2007
    Silver Spring, MD

    Re: SWG and corrosion issues

    Welcome to TFP!

    People can easily be articulate and convincing and still be completely wrong. The corrosion risks with a SWG are not zero, but they are very very small. The vast majority of people with a SWG never experience any problems what so ever. And the vast majority of the people who do have problems have a couple of bolts made of inferior materials that need to be replaced, for a total expenditure of perhaps $20. That isn't to say that serious problems never happen, but they are very very rare. The largest risks are in indoor pools without any CYA and for people who have extensive natural stonework using one of the softer kinds of stone that is in direct contact with the water.

    We have perhaps ten thousand members with SWG systems and perhaps a dozen problems reported over the last several years, and only a few of those were anything I would consider remotely serious. SWG systems sell well because people love them. I have had one for a number of years and nothing could make me give it up. It isn't always true, but in many cases a SWG can even save you money over any of the alternatives.

    Adding a zinc anode is almost certainly a waste of time and money unless you have aluminum in contact with the water. The most common use of aluminum is in automatic pool covers with aluminum tracks or frames. If you have one of those, it can greatly benefit from a zinc anode. Otherwise there is unlikely to be any need for one. If you do add a zinc anode, there is no need to place it in contact with the water. It can be placed in damp soil outside the pool and be electrically connected to the bonding system.

    It would be helpful if you post your basic pool specs in your signature.

    Your CYA level is way too low, especially for use with a SWG. For an outdoor SWG pool CYA should be between 70 and 80. For an outdoor non-SWG pool CYA should be between 30 and 50. For an indoor pool CYA should be around 20. Raising the CYA level will significantly reduce the PH drift you are experiencing. You can also try letting TA go down to 60, which will also reduce PH drift.

    The liquid solar blanket helps prevent evaporation, reducing heat loss. However the liquid solar blanket products never work very well.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

  3. Back To Top    #3

    Re: SWG and corrosion issues

    Thanks for your prompt response JasonLion.

    "We have perhaps ten thousand members with SWG systems and perhaps a dozen problems reported over the last several years, and only a few of those were anything I would consider remotely serious."

    Yes, that is the message that I was able to tease out of the many other discussions about this in other threads on this website, although I would be more comfortable if many thousands of SWG systems are reported to be relatively trouble free after say ten or twenty years. I'm not sure just a few years is enough time for some problems due to galvanic action to manifest.

    Frankly I had hoped that someone knows about the blogger that I referred to, since he seems to have been around for a long time, and would take the time to comment. He has been a prolific writer.

    Anyway, your comment about adding CYA to about 20 ppm to my indoor pool is something that I have seen elsewhere on this website. However I have also seen conflicting info about the need for an indoor pool, so I haven't done it yet. I guess it's possible that the upward pH drift is also related to the curing of the plaster, so I have waited. I will appreciate some more info about this.

    I had the TA at 60 until I recently raised it to 70 in order to experiment. It's too soon for me to comment about that.

    As for your comments about the usefulness of a zinc anode, well I'm not so convinced. For background I should inform you that in my younger days I designed, built, and operated boats at sea. The design, implementation, and operation of a proper system to mitigate corrosion due to galvanic action is crucial to the maintenance and safe operation of a sea going boat. Sacrificial zincs and proper bonding and ongoing maintenance of the system are the only practical way for anything except very large boats and ships. Now granted, a SWG pool is not a boat, but all the elements are there: salt water running through the systems, dissimilar metals exposed to that salt water, and the potential for stray currents in the salt water (or in the case of a SWG, deliberate presence of current in the salt water.)

    Now it has occurred to me that sea water (with much higher salt content than pools) and the difficulty in achieving a good ground when there is no earth to ground to, along with the proliferation of dissimilar metals on boats that are exposed to the sea water drive the necessity for protection from galvanic action on sea going boats. IMO there is a very high likelihood that the salt water in pools can become conductive for galvanic action due to the fact that salt water is known to be an electrical conductor and also due to the presence of dissimilar metals immersed in the salt water, and the action of the current passed through the water by the SWG, but I don't know that for sure. It is also reasonable to assume that a good earth ground on the electrical bonding system handles any stray currents, which is something that is impossible to achieve on a boat. Thus my idea of installing a zinc anode to test for the presence of galvanic action. Simply put, if galvanic action is present in the system, and if the earth ground is not working to mitigate it, the zinc will show that by (hopefully) slowly dissolving and thereby saving other metal parts that are exposed to the galvanic action in the salt water. It also occurred to me that even if there is galvanic action, it may be to small to worry about since the salt content is significantly lower than that in sea water. Regardless, IMO a properly installed zinc anode would protect against galvanic action damaging metal parts, if galvanic action is present.

    Now I have seen the various recommendations for protecting against galvanic action in pools: putting zinc balls in the skimmer basket, burying a zinc anode which is connected to the electrical bond in the soil (as you suggested), and some others. Well considering my experience with such issues on boats, my understanding about how the problem occurs, and the way I know to deal with it, none of these suggestions make any sense to me. AFAIK galvanic action is generated by electrical currents in the salt water, which has the potential to be an electrical conductor, and the best way to mitigate it, at least on boats, is to insert the zinc anode directly in the salt water and bond it. Thus any electrical currents present in the salt water is conducted to the zinc and to the ground through the grounded bond, instead of passing through the more noble and important metals in the system which are also grounded through the bond. Think path of least resistance. But AFAIK the sacrificial zinc must be directly exposed to the salt water which is where the galvanic action occurs and must be bonded to the electrical bonding system. If you know otherwise I will be happy to hear about it.

    Please accept these comments with a grain of salt (no pun intended) since I acknowledge that pools are not boats and my experience with salt water pools is next to nothing. But galvanic action is galvanic action, and if it's present in salt water pool systems, IMO the solution, if needed, is likely to be similar to the way we deal with it on boats.

    Thank you for your time.

    BTW I note that the certain blogger that I referred to used examples of corroded stone in which apparently all the examples were limestone. Now IMO it is almost common knowledge that limestone, unlike most rock, is very susceptible to corrosion (think limestone caverns). In geology class in school we learned to test for the presence of limestone by using a drop of HCL, since most other rock will not be affected by it. Prospective pool owners might consider that before deciding to use limestone around a pool.

  4. Back To Top    #4

    Join Date
    Apr 2007
    Ft Lauderdale, Florida

    Re: SWG and corrosion issues

    LS5555, it's not worth bringing up that blogger. He's a pool service maintenance guy in Texas, DFW area. I don't think he's been active for several years now. Like I said, not worth reviving his blogs.
    Sean Assam -
    National Accounts and Commercial Products Manager
    AquaCal Heat Pumps
    AutoPilot Salt Chlorine Generators

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