There's been talk about various sources of electrochemical corrosion and stray voltages and currents so I want to address some of this so that we can try and figure out what is really going on. First, I want to distinguish between different types of corrosion sources for metal in conductive solutions or exposed to moist soil or moist air:
1) "Normal" corrosion. This is where there is a single metal exposed to some oxidizer (usually oxygen in air and dissolved oxygen in water, though in pools chlorine is by far the more powerful oxidizer). Such corrosion is usually slow unless some deep pits form at which point the corrosion begins to look more like an electrolytic cell with a lower concentration of oxidizer deep in the pit and with electron flow in the metal and ion flow in the solution between the surface and the bottom of the pit.
2) "Galvanic" corrosion. I use this term to refer to when dissimilar metals are touching or connected electrically (by a wire, or a common connection to ground, etc.). In this case, the more anodic metal will corrode and will generally do so much more quickly than with corrosion of a single metal. This technique is used intentionally to protect steel pipes from corroding by connecting them electrically to zinc "sacrificial anodes". Zinc will corrode much faster than steel and when connected together, it essentially imparts a net negative potential on the steel which slows down its corrosion.
3) "Electrolytic" corrosion. I use this term to refer to a forced potential difference that is applied to two metals in a conductive solution (or moist ground, etc.). The potential difference must be DC in nature in order for electrolysis to occur and it may come from stray voltages or may be intentional (as in an electrolytic cell). The corrosion rate is high for this type of corrosion and is related to the potential difference, but it is non-linear and the rapid corrosion rate will only start to occur after the appropriate potential difference is achieved, though this is typically only a volt or two.
4) "Passivity" corrosion. I use this term to refer to corrosion that occurs when there is interference with the formation of a passivity or protective layer that normally prevents or slows down corrosion. Stainless steel is the most common example and I've discussed elsewhere in this thread how chlorides (from salt) can interfere with the formation of the passivity layer and how sulfates may accelerate this process.
A valid question is whether the SWG cell itself can be a source of DC voltages, so to discuss this let me first show what normally goes on if there is no leakage of voltage/current:
Code:
< 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-
Cl-, OH- > H+ + OH- --> H2O < HOCl, H+
Notice that what goes on is that there is a current flow of electrons in the wire between the plates and that this current is driven from a voltage source, typically stepped down AC voltage from a transformer that is then converted to DC voltage by a diode bridge or equivalent circuit. There is also an ion charge flow of negative ions moving from the left to the right and positive ions moving from the right to the left -- both of which represent a NET negative charge flow from left to right. I also show how chloride ions will diffuse from left to right since they are used up (converted to chlorine) on the right and how chlorine (HOCl) diffuses from the right to the left. What mostly goes on, however, is that the hydroxyl ion produced on the left combines with the hydrogen ion produced on the right to form water.
In the above cell, there is water flow (from the pump) and this will tend to push both the positive and negative ions (and neutral species as well) downstream possibly out of the cell and into the pipe. However, both positive and negative charges are in the flow so there is no NET charge flow and therefore no ion current (except between the plates as described above). If there were some sort of net charge flow down the pipe, then there would have to be some sort of net charge creation in the cell. This is possible, but only if there is a leak of electrons so that the circuit is complete. This is shown in the following diagram:
Code:
< 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- ^ :
v Fe(2+) :
e- > :
Metal in Pool .........................................:
Fe(s) --> Fe(2+) + 2e-
Notice that now there is an electron current leak (it's a positively charged leak in the sense that it wants to "receive" electrons) and this causes less positively charged hydrogen ions to be produced at the plate so when all ions are pushed down the pipe with the water flow, there is a net negative charge due to an excess of hydroxyl ions (OH-) over hydrogen ions (H+). In addition, at the metal in the pool, the metal (iron, in this example) corrodes so forms ferrous ions in the water and these are excess positive charges that migrate back toward the cell (through the floor drain and skimmer since the ions won't generally diffuse "upstream" through the returns). So there is a net negative ion current flow from the salt cell to the metal in the pool, thus completing the circuit. In essence, the electrical connection between the wire connected to the positive plate in the salt cell and the metal in the pool effectively "extends" this plate to be partly in the cell and partly in the pool. The problem is that the metal in the pool isn't made out of platinum, titanium, graphite, or another material that does not corrode, so instead of making chlorine, the metal corrodes since that reaction is much more favored. The resistance of this long flow path is obviously much higher than between the plates so the current will be small, but even a current of 2.5 milliamps would result in 1.6 ounces of metal corrosion in a year of "on" time (25 milliamps would corrode 1 pound of metal in a year).
The question becomes, "can the plate or wire on one side of the salt cell (especially the positively charged side) become connected to metal in the pool?" There is a bonding wire and it seems as if it is intentionally connected to the SWG, but I do not understand why. I can see that for most metal that is in devices that carry electricity, such as the pump, that you want to bond it to other metal in the pool so that no potential difference exists (essentially shorting out an electrolytic circuit), but you obviously can't connect the two plates in the salt cell together or else they will short out and no electrolysis will occur. You really shouldn't connect one side either, for the reasons shown above in the diagram. Perhaps the salt cell is bonded in case the electrical housing gets wet, but that seems strange to me. Does anyone know how the bonding to the SWG is done? Has anyone looked inside their SWG power box to see what the bonding wire is connected to and if there is any way for it to be electrically connected to a wire going to a plate?
If DC voltages are measured at the pool, say between the pool water and some metal or the ground, then it should be trivial to turn off the SWG (perhaps even unplug it) and see if the DC voltages remain or go away (or measurably decline). If the SWG is not the source of DC voltages, then these could be traced back to the source. It is not uncommon to find AC voltage differences around a pool due to the electrical distribution system which shunts the neutral to ground in the power system. The neutral is not completely neutral so ground voltages (and currents) are possible. However, AC voltages do not cause electrolysis and therefore will not cause corrosion. Bonding all pool metal together (as well as a grounding grid in coping) removes this potential difference by shunting the current to the bonding wire instead of inside you!
Richard
Re: Downsides to salt pools
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Originally Posted by mas985
) This goes along with what Chemgeek said about water quality in Australia and the material they use there as a result. 
.
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