Downsides to salt pools

[nater]

Good point Carl.

Richard, here's an interesting article on CSCC (Chloride Stress Corrosion Cracking) that led to the failure of a stainless steel roofing structure over a community pool due to corrosion:
http://www.imoa.info/FileLib/swimming_pools.pdf

Here's a good link for general info on the different types of stainless:
http://en.wikipedia.org/wiki/Stainless_steal

It's hard to find Stainless Steel types listed on vendor sites for pool ladders, but most are advertised as 304 with a mirror polish.

[chem geek]

nater,

Thanks for the info. I did check the Wiki link a while ago, but I always try to find independent information since Wiki can sometimes be wrong (as it is modified by anyone), though usually it isn't wrong for too long. At any rate, you probably were still writing your post when I responded to Carl and gave this EPA PDF link which, out of the many many sources I've looked at, seems to distill the essence of stainless steel corrosion. It's focus is more on steel in dirt or atmospheric exposure near the sea (at least for some of its studies), but it also contains a wealth of information on the types of stainless steel and their relative corrosion resistance. Specifically, refer to the following sections:

II. Definitions of Alloys and Corrosion - talks about metal corrosion generally, not specific to stainless steel.

V. Seawater of Marine Environments - though this has higher salinity and other chemical and organic components compared to pool water, it still talks about various factors affecting corrosion rates.

VI. Types of Stainless Steels - the most useful section for understanding corrosion resistance of different types of stainless steel.

VII. General Corrosion of Stainless Steels - charts of specific corrosion measurements in multiple studies. This validates the general Class groupings described in section VI. It is this section that contains the following interesting paragraph:

Non-halide salts have little effect on stainless steels, but chlorides particularly tend to promote pitting, crevice corrosion, and stress-corrosion cracking. In some cases sulfates seem to aggravate the effects of chlorides. Chlorides present in amounts as little of 0.3% with sulfates present can produce severe corrosion. Even quite low concentrations of chlorides can cause corrosion when concentrated by occlusion in surface films. Oxidizing chlorides such as ferric or cupric chloride are specific for severe pitting, although halide salts can cause severe pitting and stress corrosion cracking. The austenitic stainless steels are, however, the most susceptible of all the stainless steels to “chloride” stress corrosion cracking.

I am not so concerned with stress corrosion cracking since that doesn't seem to be as applicable to the pool environment. It would be critical for a mountain climber, however (and see this link similar to the one nater gave above)! Note that the statement I put in bold above talks about 0.3% chloride which is 3000 ppm if the % chloride is measured as % salt (sodium chloride), but more likely this is literally % chloride which would be 3000 ppm chloride which is about 5000 ppm salt. Either way, it says that the chloride level close to what is found in salt pools can produce severe corrosion when sulfates are present (perhaps the 5000 ppm salt level is close to the 6000 ppm level reported in the SWG study, but I would be surprised if corrosion were truly "insignificant" in a 3000 ppm salt pool over more than one year, especially if there are sulfates in the water or if CYA is not used so that the chlorine level is too high). Unfortunately, it doesn't say what level of sulfates start to cause this problem, but be aware that dry acid (sodium bisulfate) and non-chlorine shock (potassium monopersulfate) both introduce sulfates into a pool so should probably be avoided in salt pools. It also means that fill water high in sulfates may make corrosion worse in salt pools.

IX. Copper and Copper Alloys - useful for understanding what might be found in a heat exchanger (in a gas-fired heater, for example).

XI. Specific Properties of Cast Copper Alloys - mentions how Copper combined with Nickel improves strength and corrosion resistance.

Richard

P.S.

I also found this study on the corrosion of Portland cement by salt (though at much higher levels of 5% which is 50,000 ppm -- higher than the sea, but with regular wetting and evaporation, could be achieved) and this study on Portland cement and blended concretes (at sea salt levels, probably around 35,000 ppm) in the presence of sulfate and though the sulfate did not make the initiation of corrosion start any faster, it did make the progression of corrosion (once initiated) faster. Also, magnesium sulfate was worse than sodium sulfate. Fortunately, dry acid has sodium while non-chlorine shock has potassium (which is chemically more similar to sodium than magnesium). However, fill water "hardness" typically has magnesium at about one-third to one-fourth the amount of calcium on a molar basis, but the bottom line is that the pool is mostly sodium and calcium, not magneisum (for posistive charged ions, aka cations). If the salt levels in these studies were closer to salt pool levels, then I'd probably fork over the money to get the full study to find out the sulfate levels, but it's not worth it when the salt level is so much higher. This link gives a decent overview of corrosion issues with concrete. I'm sure there's lots more, but what I am looking for is a valid scientific study that relates corrosion rates for specific materials to chloride and sulfate levels (if there's a study with actual pool water, that would be even better, of course, since calcium carbonate saturation *may* reduce corrosion rates for certain materials). With that kind of information, we can set some guidelines for the kinds of materials to be used, estimate their expected life, and make recommendations with regard to chlorine level (including CYA) and identify other risk factors (e.g. sulfates) and their impact. It would be nice if manufacturers would take up the slack in this area, but as we have seen from the "lack of full information" on the chlorine/CYA relationship (independent of salt pools), this is something we may have to do ourselves first.

Though Taylor does not appear to offer a sulfate test kit (they have a sulfite test, but that's not the same thing), there do appear to be test kits from Hach, Hanna Instruments, and LaMotte and probably others as well. Any data gathering that is done on pools to try and figure out causes of corrosion should probably test for sulfates in addition to all of the other standard water chemistry parameters (pH, TA, FC, CC, CYA, Salt, Borates, Temp).

[EDIT]
P.P.S.

I want to remind everyone that most people (and servicers/installers) on this forum are NOT reporting corrosion problems with salt (SWG) pools. I do not want people scared off of SWG just because I'm trying to investigate what is going on with a few reports of corrosion and some servicers/installers who believe they see more. I am trying to be as unbiased as possible and just want facts that can be disseminated as information so that people can make intelligent choices. That's all.

waste, Ben, and others who have experience servicing multiple pools (some with SWG, some without), please, please give us your feedback.
[END-EDIT]

[waste]

Hi all! Nater and Richard have requested my 'take' on this, here it comes:

Davenj (post #2) is wondering about the 'white sludge' in his anchor cups - DavidD and I talked about it in http://www.poolforum.com/pf2/showthread.php?t=6237 (#5+)

I have also been in 2 threads discussing SS rusting (http://www.poolforum.com/pf2/showthread.php?t=5114 / http://www.poolforum.com/pf2/showthread.php?t=3310)

The 'bolded' section in the last post may well have something to do with PatL34's admonition against using dry acid (sodium bisulfate) in SWCG pools.

Aside from this, I don't really have much to say, if there's been any degridation of deck or equipment - I haven't noticed it, but we've only been using these units for a few years (the co. I worked for in Va used the Lectronator and did mostly shotcrete pools w/ precast coping- but that was 12 years ago and I wasn't looking for premature failing due to salt and my memory isn't so great that I can remember if any of those pools had coping or deck problems at the ladders or stairs)

The only other thing I've noticed with salt pools is the accumulation of salt 'crust' at the exit areas, and rust on NON STAINLESS bolts on deck chairs, etc where people sit with salinated dripping wet bathing suits.

One thing is for sure; next season I'm going to take a very close look at the deck areas which are routinely exposed to the water from a SWCG pool! (ok, so this is the 'long term approach', but good studies are done over long periods - BTW, who's funding this study )


{I've been 'subscribed' to this since it went to page 2 and will follow it through - it's an excellent discussion on a possible pitfall to having a salt pool} - Waste

[chem geek]

Thanks waste. What I think is important is to look out for corrosion in both salt and non-salt pools and to take careful measurements of water chemistry parameters when such corrosion is found. We don't want to bias ourselves by only "looking" for corrosion in salt pools. I knew about your response to one of the two corrosion links I had, but didn't know about the other. Thanks for those references -- every piece of information helps to put the puzzle together, including the reference to avoiding sulfates (dry acid) in some situations.

I reread this statement from the EPA document that I quoted earlier, and this time am including a different section in bold.

Non-halide salts have little effect on stainless steels, but chlorides particularly tend to promote pitting, crevice corrosion, and stress-corrosion cracking. In some cases sulfates seem to aggravate the effects of chlorides. Chlorides present in amounts as little of 0.3% with sulfates present can produce severe corrosion. Even quite low concentrations of chlorides can cause corrosion when concentrated by occlusion in surface films. Oxidizing chlorides such as ferric or cupric chloride are specific for severe pitting, although halide salts can cause severe pitting and stress corrosion cracking. The austenitic stainless steels are, however, the most susceptible of all the stainless steels to “chloride” stress corrosion cracking.

The term "oxidizing chlorides" may not be a specific issue with having only compounds of chloride. It may also occur with the combination of a strong oxidizer in the presence of chloride, but this is just my speculation. Interestingly, the "ferric" form of iron is indeed an oxidizer with a rather high (standard) reduction potential of +0.771V while the "cupric" form of copper is a much weaker oxidizer with a low reduction potential of +0.3419V. By comparison, oxygen has a very high reduction potential of +1.229V while hypochlorous acid has an even higher reduction potential of +1.482V. One can only compare these potentials from a molar equivalent basis and in actual pool water the actual potentials (based on actual concentrations or activities) are completely dominated by hypochlorous acid (hypochlorite ion is tied to this as well, with a lower +0.81V molar reduction potential) followed by dissolved oxygen. In other words, there may be a very strong effect between chlorides and both sulfates and oxidizers (e.g. chlorine) in terms of severe corrosion (pitting) of stainless steel. The reference to "halide salts" would include sodium chloride (and other metals with chloride, including iron and copper already mentioned), but this paragraph from the EPA is wholly qualitative and not quantitative and therefore unsatisfactory. It can guide us for what questions to ask, but does not give us specific answers.

Richard

[waste]

Richard, (as often happens when I try to keep my 'word count' down), I misrepresented what I was trying to say (go figure ). Though I specificly said I'd look at the SWCG pool areas, it was my own 'shorthand' for 'looking' at them compared to pools of the same age and ~ usage which diddn't have the SWCGs. This also raises the issue of my not being 'blind' (ie 'double-blind experiment') - undoubtably, I'll report some more or less corrosion to the pools in question. However, since I work in a relatively small area, many other factors will be removed from my observations. (in my 'miss-spent' youth, I studied psychology, and as it's a 'soft science', they made sure to 'pound-in' the necessity of using the 'hard science' methodology)

[Waterworks]

I just found another potential problem, or at least a very good source for testing. A residential customer of ours with an indoor salt pool called and said his ladders and light were corroding, so I went to check it out with my service man. The ladders were so corroded that they were falling apart. His pool is full-tile and his original white grout lines are now black, his original blue tiles are now greenish. The light ring is black also. We went and checked his cartridge filters and they were black, caked with rust. The pump basket was also black. The ladder rails that are above water are rusting, as you'll see in the pictures, but nowhere near as bad as the underwater parts.
Outside the pool there is a slide, metal deck drains, metal exhaust vents, tile decking, tile safety grip coping and a flagstone deck section with grout. The slide posts showed no corrosion, but I think they are aluminum. One of the metal deck drains is showing some rust, one is just fine. the metal exhaust vents show no corrosion. The flagstone and tile deck sections show no corrosion.

We installed the SWG in May 2005. The pool was installed around 2000. Before the SWG came along there was no corrosion. We went on a service call in March of last year to clear up the pool, as it was cloudy. At that point, the generator had been turned all the way down. We had not had any complaint of corrosion, and did not actively look for it at this point. My service man turned it up to 30% and told the customer to keep checking the chlorine levels and to keep them high until the pool was clear again, then turn the SWG down. The customer has not brought in a water sample since, and while I was speaking with him today he did not know where his test kit was. When I looked at the SWG today, it was still set on 30%. The pool room had absolutely no chlorine smell and was not hard on my eyes. His kids use the pool almost every day and have never complained about anything.

Here are the results of my water test:
Chlorine - I used my FAS-DPD test kit. I filled the sample to 10ml, then added two scoops of powder. As the powder hit the water it turned bright pink, but then went back to clear. As I shook the sample, it would periodically turn to pink, but then back to clear. I'd seen this before so I added my R0871 and after 25 drops the sample had turned pink and stayed that way. I kept adding drops until I had done 110. Then I mixed 1 part pool water with 3 parts distilled water and it still took 100 drops in a 10 mL test, meaning a chlorine reading of somewhere between 55-100 ppm. I did an OTO test and the sample turned bright red.
pH - phenol red test was off the charts so I used my Hannah meter and got 8.4
TDS (Our city water is about 650ppm) - 2850
Salt - 3200 on the Aquarite machine and 3000 on my Taylor Kit
Alkalinity - 80
Calcium - 140
CYA - 0

It seems to me that this info coincides with what I already thought. Most SWG's create WAY too much chlorine for indoor pools. This excess chlorine causes corrosion. Since this corrosion ties in with the installation of the SWG, the SWG takes the blame.

I brought one of his ladders back to my shop so I could try to do some kind of test on it. I was thinking of setting up 2 buckets, one with high chlorine and one with high salt and setting each leg of the ladder (because they are evenly corroded at this point) in it's own bucket. I would take pictures before and after and compare the effects. I would also test the water every day or two and keep it balanced. I wish I had a way to test a third part so I could do high chlorine with CYA. Does anyone think this test would be worthwhile, and does anyone have any tips to make the test better?

If anyone can tell me how to post a picture, I will post a couple pics of the ladder.

Brad

[chem geek]

I brought one of his ladders back to my shop so I could try to do some kind of test on it. I was thinking of setting up 2 buckets, one with high chlorine and one with high salt and setting each leg of the ladder (because they are evenly corroded at this point) in it's own bucket. I would take pictures before and after and compare the effects. I would also test the water every day or two and keep it balanced. I wish I had a way to test a third part so I could do high chlorine with CYA. Does anyone think this test would be worthwhile, and does anyone have any tips to make the test better?

If anyone can tell me how to post a picture, I will post a couple pics of the ladder.

Brad, I think that this forum is now limiting uploading of pictures, but if you send them to me (via E-mail -- click on my name on any post and there will be a link to E-mail to me) then I can host them on the website I use for the other pool stuff I have and then your post can link to that.

Sorry to hear about this customer, but man, that much chlorine with no CYA is horribly corrosive. As for what test to do vis-a-vis salt, part of what you want to compare is moderate chlorine levels (say, 5 ppm FC with no CYA) with salt at 3000 ppm vs. the same chlorine levels with salt at around 300 ppm. We already know and understand the effects of CYA on chlorine so that isn't as useful. What we really want to know is whether the salt in the presence of chlorine accelerates the corrosion so you want the same chlorine level in both buckets with the only difference between the two being the salt level. To make this a reasonably fair test, go ahead and add baking soda to raise the TA and maybe even some calcium chloride for CH. That would make it very much like pool water (so try and keep the pH around 7.5 as well). Whenever you test or add chemicals, be sure and do the same sort of mixing (circulation) in the two buckets as that also affects corrosion (if there is no circulation, then the buildup of "rust" products probably slows down additional corrosion).

This story really emphasizes the need for customer education about SWG and especially about testing your water regularly. I can hear Carl's words now...

Stay ahead of your water!
Take 2 to 5 minutes every day for pool maintenance!

Richard

[chem geek]

I thought of another possible reason why SWG manufacturers recommend high levels of CYA (typically 70-80 with 80 "best"). In this thread (starting at around post #30) I said that the reason for high CYA in SWG pools is that this is more efficient at producing chlorine. Though this appears to be true (based on pool owner's experiences with runtime), I didn't say what happens when there is a buildup of chlorine at the plate. In addition to production of chlorine, there is also a competing reaction of producing oxygen. If the reaction producing chlorine slows down, then the reaction rate of producing oxygen can speed up in its place, though the production of oxygen is normally less favored than that of chlorine (technically, the production of oxygen is more likely to occur in an equilibrium sense, but it has a much higher activation energy than the production of chlorine so results in an overvoltage such that chlorine is more favored than oxygen).

It is interesting to note that different SWG systems use a different voltage to drive their cells. Some operate in the range of 6-9 volts, but others are in the range of 22-30 volts. Though the voltage level required to only produce chlorine and not oxygen is quite low (perhaps between 1.16V and 1.45V at normal pool concentrations of pH and chlorine), higher voltages will probably produce both chlorine and oxygen with chlorine being produced more. I do not know if higher voltages change this relationship, but suspect that it might, with the oxygen proportion increasing somewhat at higher voltages (though still produced in lower quantity than chlorine). At any rate, having lower CYA levels may very well increase the oxygen proportion and this can be dangerous since the other plate is producing hydrogen. Though hydrogen by itself is not explosive and in water does essentially nothing, the combination of oxygen and hydrogen can be dangerous if this combination of gasses were to accumulate (if the pump shuts off but the SWG cell keeps running, for example).

The blog link at the first post in this thread has been reporting some explosions of SWG systems and I wonder if those pools were using lower CYA levels than recommended and produced more oxygen that could have been at explosive levels with hydrogen. It is also possible that the explosions weren't actual ignitions of hydrogen and oxygen, but high pressure bursts from the buildup of gas pressure. It should be understood that under normal operating conditions when the SWG only operates when the pump is running, that the risk of explosion is virtually nil. I'm just trying to sort out some unusual circumstances (where the salt cell didn't shut off when the water flow stopped) to better understand what is going on.

[EDIT] A further note: Even the most dangerous levels of hydrogen gas and oxygen gas in a 2:1 ratio will not spontaneously explode unless the pressures are very high (much higher than what pipes would handle, so bursting would occur before explosion) or the temperatures are very high (at least 400C) so generally some sort of spark or flame is required to trigger a literal thermal explosion. In other words, the risk of oxygen production and explosion is low; the risk of a pressure burst is high if the salt cell operates in a closed valve environment (which would only occur if some component failed to operate properly). [END-EDIT]

Richard

[giroup01]

At any rate, having lower CYA levels may very well increase the oxygen proportion and this can be dangerous since the other plate is producing hydrogen.

Richard,

Can you expand a bit on the chemistry behind low CYA levels favoring O2 production ?

Thanks,

[chem geek]

Can you expand a bit on the chemistry behind low CYA levels favoring O2 production ?

Sure.

First let's take a look at the only reaction that appears to be reasonably possible at the cathode (the negatively charged plate). The other reactions that can occur at this plate include the plating of metals (iron, copper) that are in the water. Also, the [EDIT] high (sorry about saying "low" at first -- I was "thinking" the right thing but "wrote" the wrong word) [END-EDIT] pH in the region of this plate make scaling of calcium carbonate much more likely.

2H+ + 2e- --> H2(g) ...................... Eo = 0V

In other words, this is the generation of hydrogen gas. It can also be written as coming from water to produce hydroxyl ion:

2H2O + 2e- --> H2(g) + 2OH- .......... Eo = -0.8277V

The different Eo readings reflect the fact that Eo is defined to be at standard conditions which means that each of the species dissolved in water have a concentration of 1 mole/liter and each gaseous component has a partial pressure of 1 atmosphere.

Now let's look at the two different competing reactions that appear to be reasonably possible at the anode (the positively charged plate):

2H2O --> O2(g) + 4H+ + 4e- ........... Eo = -1.229V

2Cl- --> Cl2(g) + 2e- ....................... Eo = -1.35827V
Cl2(g) + H2O --> HOCl + H+ + Cl-
----------------------------------------
Cl- + H2O --> HOCl + H+ + 2e- ......... Eo = -1.482V

The first reaction is where oxygen is produced from water. The other reactions are the production of chlorine where I show the additional reaction of chlorine dissolving in water to produce hypochlorous acid. You can see that the Eo value is more negative after chlorine gas dissolves in water to make hypochlorous acid and that would seem to imply that the reaction is not favored, but again this is only because the Eo is defined for standard conditions (which are theoretical and may not always be actually achievable) of 1 atmosphere pressure of chlorine gas with 1 mole/liter concentration of hypochlorous acid AND 1 mole/liter concentration of hydrogen ion (i.e. a pH of 0), AND 1 mole/liter concentration of chloride ion.

For the cathode, I listed the reduction potentials as oxidation potentials (i.e. with negative numbers) to indicate that a voltage needs to be applied to make the reaction occur. Note that it takes a lower (absolute) voltage to produce oxygen than chlorine. This represents a difference of about 25 kilojoule per mole in favor of oxygen formation over chlorine formation. HOWEVER, chemical reactions have what is known as an activation energy which is an energy barrier that must be overcome before the reaction can happen. In electrochemistry, this barrier is called overvoltage which represents the additional electrical potential needed to get an electrochemical reaction started. Oxygen has a very high overvoltage of 700 mV while chlorine has an overvoltage of only 50 mV.

So if we add the overvoltages to the standard potentials, we see that to produce oxygen it takes 1.229+0.7 = 1.929V while to produce chlorine it takes 1.35827+0.05 = 1.40827V. Or we can look at producing hypochlorous acid directly as 1.482+0.05 = 1.532V. Now these voltages do not account for the actual concentrations of the various species in pool water as I noted above.

If we want to really know about the true voltages required and which reactions are favored, we need to convert from standard conditions to actual concentrations. There is a formula for doing that called the Nernst equation as follows:

E = Eo - RT/nF * ln(K)

where "K" is the ratio of product concentrations (raised to powers corresponding to their stoichiometric quantity in the formula) to reactant concentrations. "R" is the gas constant, "T" is the temperature in Kelvin, "n" is the number of electrons in the equation and "F" is the Faraday constant. Sometimes this Nernst equation is written with a "+" and the "K" value is inverted to have reactants divided by products.

Anyway, if I assume reasonable initial conditions of a pH of 7.5 to get [H+] = 3.7x10^(-8), normal dissolved oxygen levels (based on 21% oxygen in the air and using Henry's Law) to get [O2] = 2.6x10^(-4) moles/liter and converting to 0.0065 atmospheres (partial pressure), using normal chlorine levels of 3 ppm with 30 ppm CYA to have [HOCl] = 5.8x10^(-7) moles/liter and chloride concentration of 3000 ppm to have [Cl-] = 0.052 then this adjusts the oxygen equation by (0.02585/4)*ln(0.0065*(3.7x10^(-8))^4) = -0.47V so E = -0.75V for oxygen. The chlorine equation is adjusted by (0.02585/2)*ln(5.8x10^(-7) * 3.7x10^(-8) / 0.052) = -0.37V so E = -1.11V for chlorine. I'm not certain on the handling of the oxygen quantity in terms of molarity vs. partial pressure, (I believe I did it correctly) but it turns out it doesn't matter much in this case so let's keep going.

Applying the overvoltages again gives is E = -1.45V for oxygen while we get E = -1.16V for chlorine. So at least initially, the production of chlorine is favored over the production of oxygen and if the voltage applied to the salt cell is set to be BETWEEN 1.16V and 1.45V, then only chlorine can be generated and oxygen cannot. [EDIT] I neglected to do the concentration adjustment calculation for the cathode for the production of hydrogen, but that would modify the overall voltage by the same amount regardless of whether oxygen or chlorine were produced at the anode so it's not that relevant to this discussion. [END-EDIT]

If higher voltages are used, then both chlorine and oxygen will be produced with chlorine favored over oxygen. If you have ever done the "home" experiment of taking two carbon cores from D-cell batteries and connecting them to a 6V transformer (as I have), then putting this into salt water produces both chlorine and oxygen (you can smell the chlorine and you can "pop" the gasses with a flame where the oxygen and hydrogen combine) and can somewhat vary the ratio of the two depending on the salt concentration. If you use sodium bicarbonate as the electrolyte, then you get only hydrogen and oxygen and not chlorine.

The salt cells appear to use voltages that are much higher than these minimum amounts that would ensure that only chlorine gets generated. I'm sure that this is partly due to the rather low chloride concentration in pool water (especially today, since early salt generators wanted 6000 ppm instead of 3000 ppm). Otherwise, you would need very large generating plates to get decent generation rates. So with voltages of 6-9 volts for some manufacturers and 22-30 volts for others, it would seem that both chlorine and oxygen could get produced. My guess is that the higher voltage units have a greater tendency to produce more oxygen proportionately than the lower voltage units, but that's just a guess.

Now what I do not know is whether the materials (coatings) used for the generating plates in a salt cell are designed to somehow inhibit the oxygen generation, but I suspect that is not the case. (If anyone knows about this, please let us know).

As the electrolysis proceeds, the product concentrations build up at the generating plates (though there is water flow to help keep them lower) and this slows down the generating reactions. Note that the reaction with oxygen produces hydrogen ion so is acidic and that the production of chlorine AND dissolving of that chlorine in water also produces hydrogen ion so is also acidic. However, the oxygen reaction produces one hydrogen for each electron while the chlorine reaction produces half a hydrogen for each electron. This means that as the pH gets lowered near the plate, the chlorine reaction will become more favored, assuming that the chlorine gas is able to dissolve in water rapidly. Having CYA in the water helps make this happen because it combines with the hypochlorous acid that is produced so that the reaction of chlorine dissolving in water can continue at a rapid pace. So the lower buildup of products in the chlorine reaction, due to CYA binding with the generated hypochlorous acid, means that this reaction becomes even more favored compared to the oxygen generation.

This can be netted out as follows:

2Cl- --> Cl2(g) + 2e-
Cl2(g) + H2O --> HOCl + H+ + Cl-
HOCl + CYA --> Cl-CYA + H2O
----------------------------------------
Cl- + CYA --> Cl-CYA + H+ + 2e-

Each succeeding reaction above moves the products into another form and that takes them away from being able to go "backwards" to create reactants again (or to inhibit or slow down the creation of more products). It is almost as if one turned up the water flow with respect to sweeping away chlorine gas, but that this is done chemically.

Richard