Downsides to salt pools

[chem geek]

A related topic to the post above is "what is the conductivity of an SWG salt pool compared to a non-salt pool?" The conductivity of 3000 ppm salt water (at 25C or 77F) is 5.7 milliSiemens per centimeter. This conductivity is approximately linear with the amount of salt so an SWG pool is approximately 3-6 times as conductive as a non-salt pool (assuming typical 500-1000 ppm TDS for a non-salt pool).

In terms of what this conductivity means for resistance, that depends on the area of the metal that is in the water and the distance between them. The formula is C=G*(L/A) where C is specific conductivity in milliSiemens per centimeter, G is conductivity which is the inverse of resistance (R), L is the distance between the plates and A is the plate area. So, G=C*(A/L) and R/L = 1/(G*L) = 1/(C*A), but converting to Ohms (from the inverse of milliSiemens) gives R/L (ohms per centimeter) = 1000/(C*A) = 1000/(5.7*1) = 175 Ohms per centimeter.

For 1 meter square area plates, we have 1000/(5.7*100*100) = 0.0175 Ohms per centimter or 1.75 Ohms per meter. So clearly the area of the exposed metal is a critical factor.

The area of a light ring or a handrail (for the portion underwater) is probably around 200 square inches or 1300 square centimeters so that becomes 1000/(5.7*1300) = 0.135 Ohms per centimeter or 4.1 Ohms per foot. For comparison, a non-salt pool has a resistance (for the same amount of exposed metal) of around [EDIT] 12 to 24 [END-EDIT] Ohms per foot. In other words, an SWG salt-pool is more conductive (has lower resistance), but even a standard non-salt pool still conducts electricity quite a bit.

One factor to consider for which I have no detailed information (yet) is how water flow affects conductivity. I would think that there would be rather high resistance to net ion charge current flowing "upstream" of water flow and rather low resistance to that current going with the water flow. So actual resistance may be lower than just calculated when taking into account water flow. If this is true, then metal objects closer to the "return-to-skimmer" and "return-to-drain" flows would be more likely to corrode. However, there are also differences in chemistry in such water from the SWG through the returns since the water is higher in chlorine level and is higher in pH (is more alkaline) so is therefore more corrosive to metal (due to higher chlorine) and may precipitate hydroxides of metal ions that may be in the water, but this is really no different in principle, and in fact is much milder, than what occurs if one adds bleach or chlorinating liquid to the pool.

One thing that doesn't make a lot of sense is the graph shown in this post where it shows that the relative corrosion rate of steel in milligrams per square decimeter went up a very small amount (not even doubling) with even factors of 10 increases in salinity. Apparently, "normal" corrosion is not nearly dependent on the conductivity of the water and probably occurs directly as a chemical reaction rather than electrolytically so is probably most dependent on the concentration of the oxidizer (oxygen in air, dissolved oxygen in water, or disinfecting chlorine). Either that, or some localized corrosion has greater localized conductivity possibly due to some corrosion products. However, if there is a potential difference over a greater distance as with galvanic corrosion or electrolytic corrosion, then corrosion should be more directly and proportionately related (approximately linearly, in the range of TDS found in pools) to the salinity and specifically the conductivity of the water.

I received some confirmation regarding the SWG electronics and what I said was essentially correct in terms of a transformer (to lower AC voltage) followed by a rectifier bridge (to convert AC to DC) plus some other circuitry designed to ensure isolation of the AC voltages. The bonding wire is a secondary safety ground protection so does not connect to the plate wires. So if there were a voltage/current leak as I described in the previous post, then it would not be intentional (i.e. it's not a faulty design).

Another possibility again related to any voltage/current electrical leakage is if this leak went to "ground" which can be assumed to be a "sink" or "source" for electrons. The following shows what would happen in this case:

                                                                              < e-
                         < e-                               < e-      .........................
              ______________________ DC Voltage from _________________|__                     :
           e- |                      Transformer/Diodes                 | ^                   :
           v  |                                                         | e-                  :
              |                                                         |                     :
Negative Plate releasing electrons                        Positive Plate absorbing electrons  :
2H2O + 2e- --> H2(g) + 2OH-                               Cl- --> HOCl + H+ + 2e-             :
                                                                                              :
                                    LESS         LESS           LESS LESS                     : ^
             Cl-, OH- >              H+ + OH- --> H2O         < HOCl, H+                      : e-
                                                                                              :
                                                                                              :
                                       OH-                                                 Ground
                                        v

In this case, notice that though there isn't direct electrolytic corrosion, there is a buildup of charge (in this example it's negative charge, but it could instead be positive charge if the opposite plate were connected to ground instead). Essentially, the pool acts like a capacitor. A voltage would be measured from the pool to the ground and to any metal which may be bonded. However, if the bonding wire is not fully connected to ground (moist soil), then the charge in the pool can build up. If metal in the pool were bonded and such bond were connected to a good ground, then the charge in the pool would likely be discharged and would not build up (though this depends on the rate of the charge buildup relative to the discharge rate).

In the above example, the voltages are not usually sufficient to cause electrolytic corrosion and there is not an electron path to the pool metal, but if the bonding wire were connected to ground, then the Ground-to-Ground would be the electron path and corrosion would be possible just as with the example where a direct wire was connected (though the resistance through ground is clearly much higher than the direct wire example). Even with lower voltages, this still lowers the activation energy for corrosion so could still speed it up, but not as much as with electrolysis (remember that the oxidation potential of chlorine far exceeds what is needed to corrode iron so it is not a question of "if", but of "how quickly").

Richard