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This guy also has four pure copper spas from Diamond Spas at the same residence. AFAIK these spas are similar to Bradford spas' units. There is some tile and some copper exposed. He uses inline chlorinators for these units and has not complained at all about corrosion.
Brad
That's interesting about the copper not showing signs of corrosion while the stainless steel did. I wonder if the chlorine levels are better maintained in his spa or if copper is more resistent to any potentially salt-amplified chlorine corrosion. Copper is more resistant than steel so perhaps even though chloride ion might interfere with any protective oxide layer that gets formed, that once removed the underlying steel in stainless steel corrodes faster than the underlying copper under any thin copper oxide protective layer (if any). This is all speculation on my part.
On a separate topic, I ran into this article about the damage to plaster from high levels of CYA. What is interesting is that the drop in CYA over time seems to indicate that even levels of CYA not much lower than 100 ppm can be detrimental to plaster. If you look at the line that starts with 200 ppm, it drops rather quickly down to around 130 ppm and then continues to drop more slowly to around 80-90 ppm. This may indicate that there is some plaster deterioration effect even at 80 ppm though it is clearly much slower. This would be another reason for SWG manufacturers to take a look at seeing if they can't figure out a way to operate more efficiently at lower CYA levels.
Richard
Richard,
It's not a matter of a salt system working efficiently at lower cya levels. Cya levels have nothing to do with efficiency. It has to do with retaining the chlorine in the water. Try the same with a bleach pool and low cya. The chlorine is quickly consumed by UV. Same thing with the chlorine produced by a salt system. UV.
We know, from trial and error more than anything, that the 60 -80 ppm range works. Since ORP controllers are affected by cya, 30 - 50 ppm is recommended.
But cya at lower than 25 ppm? You will drastically see an increase consumption of chlorine, salt generator or bleach, it doesn't matter the source.
Sean,
If the SWG manufacturer recommendation for CYA is solely based on retaining chlorine in the water (which is independent of whether you use an SWG or use another source of chlorine), then why did Evan (waterbear) and others see such a difference in efficiency going from 60 to 80 ppm? Yes, more CYA has the chlorine last longer, but it has diminishing returns and the difference between 60 and 80 isn't that high yet he (and others) saw a large difference -- that is, he was able to turn down his SWG a large amount.
The half-life of chlorine is determined by the separate half-lives of unbound chlorine (hypochlorous acid and hypochlorite ion) with a half-life of around 35 minutes AND the half-life of bound chlorine to CYA (i.e. the chlorinated isocyanurates) with a half-life of around 8.4 hours (some sources say 6 hours). The net result is shown in this chart which shows that the bulk of the benefit from CYA occurs at relatively low levels of CYA. This chart is similar to and consistent with the infamous Kent Williams "Cyanurics - Benefactor or Bomb?" article. The half-life of chlorine is about 6 hours at 30 ppm CYA while at 80 ppm this only increases to a little over 7 hours. That's not a huge difference. The reason for the small change is that the vast majority of chlorine, even at 30 ppm CYA, is stored in the form of chlorinated isocyanurates (that is, bound to CYA) so adding more CYA only cuts down the remaining free, unbound and disinfecting chlorine that is already at such a small level that it doesn't contribute much to the total loss. At 30 ppm CYA the disinfecting chlorine level is at 1/30th the level it would be if there was no CYA or put another way, almost 97% of the chlorine is bound to CYA and only 3% is unbound and getting cut in half every half-hour. At 80 ppm CYA, almost 99% is bound to CYA and only 1% is unbound. Though 1% is certainly much smaller than 3%, it is still a small number in an absolute sense -- it's what happens to the 97% or 99% that drives the total chlorine loss more than anything else.
It was interesting that the rate constants for CYA combining with hypochlorous acid turned out to be about the same amount of time that water flows through an SWG cell when the CYA level was somewhere in the neighborhood of 50-60 ppm or so. That's what had me think that this was why the CYA recommendation was higher along with what Evan (waterbear) and others were seeing in terms of efficiency. Also, it seemed quite strange that most non-SWG CYA recommendations are 30-50 ppm while only some of the SWG manufacturers were saying to have it at 70-80 ppm with "80 ppm being ideal". Don't you find that strange?
Richard
Last year I did an experiment with my pool and SWG and got different results than what others have experienced. I started out at 30 ppm of CYA and gradually increased it to 60 ppm and saw no increase in production of chlorine at all. My residual chlorine started out at 1 ppm and ended up at 1 ppm with the same pump run time and SWG setting. I know this is at odds with what others have experienced but I was fairly careful to do this during the spring and space out the CYA addition since it can take up to a week to fully dissolve.
Since you can only measure the chlorine residual which is a combination of SWG production and CYA retention, you cannot be sure that the increase in chlorine residual that some people experience is due to higher production. It could be that the UV exposure is high enough that they are simply increasing the retention of chlorine and thereby increasing the residual.
One thing I have noticed is a huge difference in production with water temp. This fall when the water temp dropped to 60 degrees I noticed that my production almost doubled. This is probably a combination of water temp and UV exposure but this seemed to have a much larger impact than CYA level did.
Since I have very high CH fill water, I am planning a refill soon while the CH is at it's lowest. This gives me another opportunity to repeat the same experiment and see if I get different results this time.
Getting back to the original topic of this thread, which is what are the downsides to salt pools, I am curious if Australia, which has been using salt pools for about 30 years now, has modified their construction techniques or has had problems with corrosion, expecially in some of the older pools with SWG's. Also, I believe that many of the pools there use a much higher salt concentration than we do in the U.S.
I think that finding out this info would be most useful.
Mark,Quote:
Originally Posted by mas985
I remember when you posted these results before and waterbear commented how the largest gain was seen approaching 80 ppm CYA so if you do your experiment be sure to get the CYA up to that high a level. That would be a good test, especially if your SWG manufacturer is one of the ones that recommends the 70-80 ppm CYA level. We can then put this issue to rest one way or the other (and I'll apologize to gunslinger who's NY skepticism may have been correct; and to PoolSean) -- it's an important issue since it would be easy for us to recommend lowering the CYA to the more "normal" levels closer to 30 ppm for most pools which would give more disinfecting chlorine at the 3 ppm FC level (probably enough to keep away mustard/yellow algae which one user on this forum couldn't keep away without 5-6 ppm FC in his high CYA SWG pool). I just didn't want to recommend that if it had a negative effect on efficiency.
Thanks,
Richard
I agree with you. In the P.S. to this post I talk about a pool installer in Australia with whom I've had E-mail correspondence. This is what I wrote in that post:Quote:
Originally Posted by waterbear
I communicated with a pool installer from Australia who indicated that they see very little corrosion in their pools (SWG or non-SWG), but nearly all of their installations use the marine-grade 316 stainless steel, the paving is limestone or clay that is fired and that the supplier warrants for use in salt pools or it is coated to make it salt/sulfate resistant. They normally install heat pumps with titanium coils, rather than gas heaters (and such copper and cupro-nickel heaters do show corrosion). They also typically see higher TDS due to higher salt and sulfates in their tap water so even non-SWG pools have high salt levels, especially after seasons of chlorinating liquid usage.
Now since this is just one guy I talked with, this isn't a representative sample and it's based on his experience and opinion. It does seem that they take more care in selection of materials, but it also seems that they did this even before SWG pools were introduced due to their already very high salt (I'm guessing this is partly hardness, not just sodium chloride) and sulfates water.
If anyone else out there knows pool installers in Australia, it would be helpful to get some more information to confirm, counter, or expand upon what was said above. I did find this link to South Australian water quality and it appears that in the cities (metropolitan areas) the water hardness is around 100 ppm and the TDS (presumed to be mostly salt after accounting for hardness) is around 330 ppm. However, in the country, the water hardness is much higher with most in the 200-500 ppm range while TDS was also higher with most in the 350-900 range but with some over 2000 ppm. This link describes how "Australia's ancient weathered landscapes and some surface waters are naturally salty." There are other links as well, but generally they indicate a saltier environment (though I should point out, not at SWG levels).
[EDIT] It is also true that older SWG systems recommended salt levels of around 5000-6000 ppm, but most newer SWG systems are around 3000 ppm. So it would be reasonable to assume that since Australia had SWG pools earlier, that more of them (using the older SWG cells) would be at higher salt levels. That would make any potential corrosion issues worse so is another reason they would be more inclined to use more chlorine/sulfate/chloride resistant materials. [END-EDIT]
With everything said up to this point, I'm not too concerned with metal corrosion in SWG pools that use CYA (i.e. most outdoor pools). I think what we have learned that may not have been known before is that indoor pools using an SWG with no CYA risk corrosion of their stainless steel, even at "normal" FC levels of 3-5 ppm. The study Sean gave us and some of Brad's customer experiences also show that any pool where the chlorine is not monitored and goes sky high is also at risk and that this tends to happen more with indoor pools (probably because there is no sunlight to limit how high the chlorine level can get). As for hardscape materials, that still seems to be an open question but I'm leaning towards recommending more diligence in care of such materials in an SWG environment (either stronger materials or regular rinsing/diluting and/or sealing) though stone materials should really be cared for in any environment. The other open issue is sulfates in SWG pools and at this point I would recommend minimizing the use of non-chlorine shock and dry acid in such pools, but this is just being cautious and not yet born out by a lot of experience (except PatL34's caution referred to by waste in this thread).
Thanks,
Richard
Another thought that just occurred to me on this topic can best be summed up by "Location, Location, Location". Where you live might be (and most likely is) a factor in how your pool is constructed. I happen to live in Florida on a a 2 block wide barrier island between the Atlantic and the Intercoastal Waterway and salt spray is everywhere. We just accept it as a fact of life--one of the prices you pay for living by the ocean. Therefore, materials used in all construction tend to be possibly a bit more salt resistant than materials that might be used elsewhere. (This thought occurred to me as I was rinsing the salt off my car!:eek::rolleyes:) This goes along with what Chemgeek said about water quality in Australia and the material they use there as a result.
Perhaps a better title for this thread might be "Is your pool constructed of corrosion resistant material?" intstead of "Downsides to salt pools";).
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:
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.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+
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:
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).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-
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