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Thread: Calcium Hardness and Corrosivity?

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    In the Industry

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    Jul 2012
    Saugerties, NY

    Calcium Hardness and Corrosivity?

    I've posted in the past with questions about how necessary it is to maintain Calcium Hardness in Vinyl Liner pools. The responses I received essentially said that it wasn't necessary to worry about Calcium Hardness in Vinyl Liner pools. Recently I learned a little bit more about corrosivity, and the Langelier Saturation Index. I spent some time creating what if scenarios with an online LSI Calculator and it seems that Calcium Hardness plays a pretty significant role in preventing corrosivity. So I'm wondering if I am misunderstanding what I was told in the past, or if my understanding of the LSI and how it applies is wrong. To be clear, the issue is not how it will effect the liner, but how it will effect metal parts, such as copper heat exchangers in heaters.

    Pool Technician
    Saugerties, NY

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    Mod Squad woodyp's Avatar
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    Apr 2010
    East Texas

    Re: Calcium Hardness and Corrosivity?

    I'm sure chem geek will be along shortly on that one!
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    mas985's Avatar
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    May 2007
    Pleasanton, CA

    Re: Calcium Hardness and Corrosivity?

    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
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    Patrick_B's Avatar
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    Jun 2011
    Midland TX

    Re: Calcium Hardness and Corrosivity?

    Generally speaking, we really don't give much thought to LSI for pools, as CSI is really more suitable.
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    Join Date
    May 2015

    Re: Calcium Hardness and Corrosivity?

    Quote Originally Posted by mas985 View Post

    Thanks for this, i recently got my Taylor 2006 and was a little worried about my CH. It was at 100 and I thought it was low, good thing I checked here before increasing it.
    20k gallon IG vinyl pool.
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    Join Date
    Mar 2007
    San Rafael, CA USA

    Re: Calcium Hardness and Corrosivity?

    Not having the water saturated with calcium carbonate, which is all that the LSI and CSI indexes do (they are nearly the same, by the way, I just updated the index to be more accurate than some of the over-simplifications in some versions of the LSI), is necessary to protect pool surfaces that have calcium carbonate in them, such as plaster/gunite and grout. It is not needed for vinyl pools.

    As for metal corrosion, I suggest you read this discussion to understand that metal corrosion is mostly caused by low pH and other factors such as dissolved oxygen and is NOT at all caused by a lack of calcium in the water. The idea that saturating the water with calcium carbonate will prevent metal corrosion is dicey at best since you'd have to very carefully maintain a level that created a thin film without scale, but in something like a copper heat exchanger the temperature varies by at least 30F when on vs. off so has the index swing back and forth by about 0.3 units and with the pump also going on and off with huge changes in water flow through that copper pipe it's virtually impossible to maintain a calcium carbonate film layer to protect the metal.

    The pool industry did a disservice when it used the term "corrosive" for a low saturation index number. It is NOT corrosive, but rather "plaster dissolving" more specifically "calcium carbonate dissolving". The water would be corrosive to metal if the pH were low and most often a low pH is associated with a low saturation index because pH directly affects that index, BUT one could have VERY high CH water and still the low pH would corrode the metal even if the saturation index were 0 or positive. In practice, a low pH will create a negative saturation index because even with 5000 ppm CH and 200 ppm TA the saturation index turns negative below a pH of 6.4

    My tap water has a CH of 55 ppm and a saturation index of -0.7 but it does not corrode the copper pipes in our house (or anyone else's) because the pH is at 7.7. The water district does add 300-500 ppb orthophosphate in order to further reduce the possibility of corrosion but the low CH itself does NOT cause corrosion. The use of such corrosion inhibitors is primarily to minimize the amount of lead and to some degree copper that gets into the water system. The amounts they are trying to control are very, very small in terms of pipe thickness. It is NOT to prevent pipes from bursting/leaking, but rather to minimize the number of ions getting into the water supply for drinking water. This is why it's often zinc orthophosphate that is used since this tends to inhibit lead and copper leaching the best. Also, the phosphates form a thin film with the metal to protect it, but do so without the delicate balancing act of calcium carbonate saturation that is often difficult to achieve without causing significant scaling somewhere in the system (say at a different water temperature or where other water parameters have changed).
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