Negative CSI and metal corrosion

[PoolOwnerNumber9]

I can agree that the CSI is not necessarily tightly correlated to metal corrosion. And, I do agree that a somewhat negative CSI is OK in a vinyl liner pool. Although, I do think that you are underestimating the importance of the carbonates and bicarbonates in reducing corrosion, especially corrosion due to chlorine.

I would also like to comment of this quote from you:

“Continued use of Trichlor tablets especially in a skimmer resulted in corrosion of copper heat exchangers in gas heaters, but this was due to low pH more than anything else.”

I agree that the low pH contributed to the copper corrosion, but I think that the chlorine was probably a larger factor due to it being an oxidizer.

 

[Guest]

A negative CSI means that the water is aggressive or tends to be corrosive. It can damage any mineral or metal based things like screws or ladders. A positive CSI means that the water tends to want to scale or be cloudy.

I can agree that the CSI is not necessarily tightly correlated to metal corrosion.

?????????????????????

When you posted the first comment I answered "Blatently false" ( and received repercussions for my statement, btw.)

You answered :

WaterBear said “Blatantly false. The CSI ONLY predicts scaling or aggressive water in terms of calcium (plaster). It has NO bearing whatsoever in predicting if the water is going to be aggressive toward metals!”

I have to disagree with you, WaterBear. First, I do understand that the CSI is specifically designed to measure the water’s ability to dissolve or hold Calcium. However, I believe that it is also a good way to measure the water’s ability to dissolve or hold other minerals or metals such as magnesium, copper and iron alloys, such as steel.

So now I am confused as to what you believe...does SI predict metal corrosion or does it not, in your opinion?

 

[chem geek]

“Continued use of Trichlor tablets especially in a skimmer resulted in corrosion of copper heat exchangers in gas heaters, but this was due to low pH more than anything else.”

I agree that the low pH contributed to the copper corrosion, but I think that the chlorine was probably a larger factor due to it being an oxidizer.

The fastest way to strip the copper from a heat exchanger is to regularly pour Muriatic Acid into the skimmer, which of course has no added chlorine in it. Of course, that's not a fair comparison since the pH of the Muriatic Acid is far lower than that of Trichlor tabs, even if one pours very slowly.

I do agree that Trichlor adds both concentrated chlorine and acidity, but we generally haven't heard of people ruining their gas heaters by pouring chlorinating liquid or bleach into the skimmer (which produces a high pH as well as high chlorine levels). I generally recommend against that, but I'm just being conservative. In terms of actual damage seen, it seems that it's most likely from Muriatic Acid, then from Trichlor and the jury is out on chlorine alone (well, really, only the jury in my own mind -- most everyone else believes it's OK).

Richard

 

[PoolOwnerNumber9]

WaterBear, it's mostly semantics. My statements are not contradictory. As I said before, there is the ability of water to dissolve and the ability to corrode. I said that "A negative CSI means that the water is aggressive or tends to be corrosive" and "I can agree that the CSI is not necessarily tightly correlated to metal corrosion." Just because a negative CSI "tends to be" corrosive, does not necessarily mean that it is "tightly correlated". I think that my previous posts are quite clear on the subject. Corrosion is a very complicated matter, at best, with many relevant factors.

Richard, chlorine is strong oxidizer. It will definitely cause corrosion, especially on copper heat exchangers. Chlorine corrosion of copper is accelerated by a low pH due to the greater oxidizing strength of hypochlorous acid at low pH. At higher pH levels the corrosivity of the chlorine will be reduced. Therefore, I believe that my statement, that "the chlorine (from Trichlor) was probably a larger factor" than the low pH, is accurate. I did agree that the low pH contributed. I think that you are underestimating the chlorine's role in the corrosion. How low do you think that the pH goes when there are (5) 3 inch tabs in the skimmers with a total flow rate of about 75 gpm?

 

[chem geek]

I think that you are underestimating the chlorine's role in the corrosion. How low do you think that the pH goes when there are (5) 3 inch tabs in the skimmers with a total flow rate of about 75 gpm?

The lower pH and the higher chlorine level from Trichlor tabs in a skimmer when the pump is running is negligible. They simply don't deteriorate that quickly. Even if the skimmer had a low 20 GPM flow rate and even if one tablet were to dissolve in two days at this rate, then that is one 8-ounce tablet of Trichlor in 57,600 gallons equivalent so with recirculation of the same water it's really no different then just dissolving in the bulk pool water. In a 10,000 gallon pool, that would be an increase in FC of 5 ppm and a pH drop from 7.5 to 7.4 assuming a TA of 100.

The problem with Trichlor pucks in a skimmer has to do with what happens when the pump is off. In that case, the tablets dissolve in a much smaller volume of water and without circulation that can become very acidic and high in FC. If even 1/10th of a tablet were to dissolve in the equivalent of 10 gallons of water, that would lower the pH to 4.3 and have the chlorine get to 550 ppm. However, the CYA would also be at 332 ppm and would bind to a lot of the chlorine so the hypochlorous acid concentration would get to 167 ppm -- still high, but not as high as if CYA were not also released.

The hypochlorous acid concentration of bleach or chlorinating liquid will be far lower. 6% bleach has a hypochlorous acid concentration of around 5.2 ppm due to the 11.5 pH (for Clorox Regular), but has a very high hypochlorite concentration of over 60,000 ppm and this is pure and undiluted (i.e not poured into a skimmer). The net oxidizing potential is lower than the Trichlor example. However, oxidizing potential doesn't directly translate to metal corrosion rates.

If given a choice between a pool with 200 ppm FC vs. a pool with a pH of 4 or less, it's a tough call but I'd probably take the former over the latter. Several pools that have been reported on forums with low pH (< 4.5 since the TA test turned red immediately) but not abnormally high chlorine levels (say, from continued use of Trichlor in feeders, not in the skimmer, but with no pH adjustments over time) often have metals in the water from corroded parts of the pool. So for me, the low pH is serious by itself.

Richard

 

[PoolOwnerNumber9]

A pH of 4.5 is 1,000 times more acidic than a pH of 7.5. I have seen many instances of heaters being destroyed by people who used Trichlor in their skimmer, and who ran their pump continuously. I can assure you that the chlorine does oxidize the copper. As I noted, the low pH does cause some of the corrosion, partly due to its effect of making the chlorine more reactive. Which factor is actually larger would depend on just how low the pH went. I seriously doubt that it goes as low as 4.5. It probably doesn't drop below 6.5, which would make the pH 10 times more acidic. I will try to find out.

 

[Guest]

A pH of 4.5 WITHOUT any chlorine is able to leach copper. I think you underestimate the effects of pH on corrosion of copper.

 

[Guest]

WaterBear, it's mostly semantics.

\
I have to disagree with you here. It's not semantics, it's chemistry we are talking about. Your statements were not ambiguous but your position on the subject is.

 

[JasonLion]

Low pH is highly correlated with reported heater problems. Using trichlor in the skimmer is only somewhat correlated with reported heater problems. In practice, low pH is a significantly more accurate predictor of problems than trichlor in the skimmer is. The actual corrosion mechanism may well involve chlorine, but at normal FC levels, problems are not reported with any meaningful frequency unless the pH goes low.

Of course, all of these factors are cross correlated. Using trichlor in the skimmer tends to cause periods of low pH when the pump turns on. Using trichlor in general tends to run the risk of very low pH.

 

[chem geek]

I have seen many instances of heaters being destroyed by people who used Trichlor in their skimmer, and who ran their pump continuously.

Though I can certainly understand how there could be a problem from having the pump cycle on and off, I don't see how from the chemistry that Trichlor in the skimmer with continuous 24/7 flow would lead to a problem. Of course, we don't recommend putting Trichlor in the skimmer (unless it's the kind that do not dissolve when there is no flow such as BioGuard Smart Sticks). The pH simply does not drop enough and the chlorine levels do not rise enough to be an issue if the flow is continuous so something else must be going on. Perhaps the tablet doesn't dissolve continuously and once in a while "falls apart" with a piece that can then potentially get lodged somewhere downstream and cause a problem with local physical contact -- but that's just speculation on my part trying to jibe with your experience.

When researching corrosion in SWG pools, I was sent this PDF file about studies that showed that high chlorine of 20 ppm FC with no CYA in the water was highly corrosive, and 30 ppm extremely corrosive, at least to stainless steel. This study was comparing chlorine levels vs. salt levels and didn't look at pH as a factor (presumably the pH was maintained near 7.5, but the study doesn't explicitly state that), but it does indicate, as you say, that high levels of hypochlorous acid (so high levels of chlorine with no CYA or very high levels of chlorine with some CYA as found with Trichlor) can be highly corrosive. Consider the source when looking at studies, however (Eltech provided technology for SWG systems). Even so, we have seen stainless steel corrosion in salt pools at higher chlorine levels with no CYA (around 5 ppm FC with no CYA) after about a year, but not consistently. Stainless steel corrosion is a "runaway" type of corrosion since it is competing between rapid corrosion of steel vs. reforming of a passivity layer of chromium oxide (which is interfered by chloride ion). Copper corrosion, on the other hand, is just direct corrosion and slower than seen in plain (not stainless) steel and does not have the same "runaway" effect.

I know from personal experience that a Trichlor feeder parked near stainless steel poles in my pool rusted the mounts and also rusted a stainless steel thermometer in the skimmer when I briefly had Trichlor in there but not run continuously (these were the early days when I first owned a pool and didn't learn pool water chemistry). So there is no question that Trichlor can be a serious problem in the region where it dissolves if there is not good circulatory flow at all times.

Anyway, the debate isn't that important since we know that both acidic conditions and high hypochlorous acid levels are a problem and should be avoided. If someone wants to do some experiments with some copper pennies (actually copper coated zinc unless the penny is older than 1982) comparing acidic conditions with low chlorine vs. high chlorine levels at normal pH, that would be interesting.

Richard

 

[PoolOwnerNumber9]

WaterBear, for a pool where the only concern was for high quality stainless steel, I would not be overly concerned about a negative SI. As a rule I would advise someone to not allow it to go much below -0.35, although it could go lower without too much concern as long as they maintained proper levels for chlorine, pH, and Total Alkalinity. If I was taking care of the pool, I would not let it go below -0.35. For a pool with a copper heat exchanger I would avoid a negative SI. Here are the readings of a pool, with a heater, that I was at today:
CL: 4.0
pH: 7.5
Total Alkalinity: 175
Calcium: 280
Cyanuric: 40
I do understand what a pH of 4.5 will do. That's why I noted that it was 1,000 times more acidic than a pH of 7.5. When I'm talking about low pH, I mean 6.8 to 7.2, not 4.5. When I said that the high chlorine was a larger factor than the lowered pH while using Trichlor in a skimmer, I assumed a starting pH of 7.5, an Alkalinity of 150 ppm and a lowered pH of 7.0. I do agree that, if the pH started at 7.0, or lower, and that the starting Alkalinity was lower than 150, then the lowered pH would be at least as large of a factor as the chlorine. I also believe that the chlorine and pH, working together, are more destructive than the sum of both working alone. Any time I have ever seen someone use Trichlor in their skimmer, it always destroyed the heat exchanger.

 

[Guest]

Any time I have ever seen someone use Trichlor in their skimmer, it always destroyed the heat exchanger.

Have you ever bothered to check the pH of the water in the skimmer? You might find it enlightening.

 

[PoolOwnerNumber9]

Have you ever bothered to check the pH of the water in the skimmer? You might find it enlightening.

And, just exactly what do you think it would be? I will do a test as soon as I can and let you know the results. What is your position on using trichlor in the skimmers for a pool that has a heater?

 

[chem geek]

I'm sure PoolOwnerNumber9 knows that the water near a dissolving Trichlor tablet is very, very acidic (<< 4.5). I believe his comment with regard to somewhat low acidity water just below 7.0 was with regard to the pool as a whole. What may not be as clear is how many Trichlor pools actually get down to a pH of 4.5 or below for the entire pool water. [EDIT] waterbear does not promote having a Trichlor puck in the skimmer; his point is that the area near a dissolving puck is VERY acidic unless there is continuous water flow. [END-EDIT]

Here's an example for what can happen with a pool as a whole. Start with a pool that has a pH of 7.5, TA of 120, CYA of 30. Now use Trichlor tablets at a rate of 2 ppm FC per day (about one 3" 8-ounce tablet every 3 days in 10,000 gallons). After just one month, the pH will be down to 6.4 and the TA down to 78. After the second month, it's down to a pH of 5.8 and a TA of 35. After the third month, the pH is down to 3.9 and the TA is -6 (yes, TA can be negative, but will measure as 0 since the indicator will immediately turn red in the TA test). Even if you assume only a 1 ppm FC per day usage, you get the same result after 6 months. If someone does not monitor their pH and add a base periodically, then they can run into serious trouble in a matter of months. We've seen this reported a surprising number of times on several pool forums. When the TA test turns immediately red, your pH is at or below 4.5 (assuming a normal carbonate level to start with).

So now I see the misunderstanding since I was referring to really low pH levels (near dissolving Trichlor with no circulation) compared to high chlorine levels. At only somewhat low pH levels, like 6.8 or even 6.0, I would agree that high chlorine way above the CYA level (i.e. a high hypochlorous acid level) can be more damaging more quickly. It's just that it's not usual to get such high chlorine levels while also having the pH be near normal since hypochlorite sources will raise the pH when added.

PoolOwnerNumber9, the CSI for the numbers you posted (assuming a temperature of 85F) is +0.28, mostly due to the higher TA. Are you saying that the copper heat exchanger was shot in such a pool and that this was due to Trichlor pucks in a skimmer? I wasn't clear on the details of the situation you were describing.

Richard

 

[Guest]

Have you ever bothered to check the pH of the water in the skimmer? You might find it enlightening.

And, just exactly what do you think it would be? I will do a test as soon as I can and let you know the results. What is your position on using trichlor in the skimmers for a pool that has a heater?

I don't believe in using trichlor in the skimmer, period! It will not only damage heaters but also pump seals and cartridges because of the continued exposure to low pH. It is akin to pouring acid into the skimmer on a continuous basis.

 

[PoolOwnerNumber9]

PoolOwnerNumber9, the CSI for the numbers you posted (assuming a temperature of 85F) is +0.28, mostly due to the higher TA. Are you saying that the copper heat exchanger was shot in such a pool and that this was due to Trichlor pucks in a skimmer? I wasn't clear on the details of the situation you were describing.

Those numbers were being given as an example of the levels that I keep a pool with a heater. It is also a concrete/plaster pool so the positive SI is also due to concern for the plaster. The pool looks very good. As I noted before, I also find reduced algae problems with the higher alkalinity and calcium.

It is a pool that I had taken over after the previous service person used Trichlor in the skimmer. Not only was the heater ruined, the plaster was a horrible stained mess due to the copper and iron from the heater. The heater was replaced about 10 years ago and the pool has been replastered. Everything works and looks great.

I think that we can all agree that using Trichlor in the skimmer is not the thing to do. I think that the lowered pH and raised chlorine work synergistically, creating much more damage than either one alone.

 

[Guest]

With the numbers you gave I wonder how the pool is being chlorinated. IF you are using an unstabilized chlorine source then you will have better pH stability by raising the CH and lowering the TA since there would be much less outgasing of CO2. If you are using a stabilized chlorine source then the higher TA makes sense but I wonder how you maintain the CYA given the level the FC is kept.

Also, how are you maintaining the pH in this pool. How often do you need to add acid and what is the pH before you do? How fast does the pH rise?

A pool is not a stationary object that can be described by one set of test results. It is dynamic and the trends that occur in a pool need to be tracked and if there are any negative ones (such as constant pH rise that can lead to scaling or a crashing pH and TA due to continued trichlor usage with no adjustments to TA to counter it) they should be corrected by water balance adjustments.

 

[PoolOwnerNumber9]

Approximately:
55% Trichlor tabs, 15% Bromine tabs (Bromine + Chlorine) and 30% Sodium Hypochlorite. The only other chemicals I need to add are Sodium Bicarbonate and Calcium as needed. Note, at a highly positive SI and high Calcium, Calcium Hypochlorite should not be used unless it can be filtered. The water does not accept it well and there tends to be a lot of undissolved precipitate. Also, the pool remains covered most of the time with an automatic cover

 

[Guest]

Approximately:
55% Trichlor tabs, 15% Bromine tabs (Bromine + Chlorine) and 30% Sodium Hypochlorite. The only other chemicals I need to add are Sodium Bicarbonate and Calcium as needed. Note, at a highly positive SI and high Calcium, Calcium Hypochlorite should not be used unless it can be filtered. The water does not accept it well and there tends to be a lot of undissolved precipitate. Also, the pool remains covered most of the time with an automatic cover

So this is a bromine pool! (Organic bromine wll build up in the water and any chlorine added will reactive the bromine so unless the pool is drained the old adage "Once a bromine pool, always a bromine pool" holds true in this case. The dimethyhydantoin in the bromine tablets have a stabilizing effect on bromine is a similar way that CYA stabilizes chlorine and it also makes the bromine persistent. Why in the world are you using trichlor with bromine? The chlorine will reactivate the bromide in the water but the combination of the acidic trichlor and the acidic organic bromine certainly is why this pool requires such a high TA to keep the pH from crashing and the water becoming corrosive. It's simply a pH issue in this case and not a SI issue. (Occam's Razor in effect here!) You are right that cal hypo would cause this pool to cloud but that is a function of the high TA. (Once again Occam's Razor.)
The fact that the pool remains covered is a benefit since it's bromine. If it was exposed to sun it would be difficult to maintain the bromine levels. Even though dimethylhydantion does somewhat stabilize bromine against UV light it is NO WHERE near as effective as CYA is with chlorine. It tends more to make the bromine persistent in the water (as opposed to plain sodium bromide).

 

[PoolOwnerNumber9]

WaterBear, it actually works quite well. Anyone using bromine should keep it to an absolute minimum, perhaps 10% to 15%. I adjust the amount of each chemical based on the chemical readings. The pH stays very stable. I personally like the high alkalinity.

Now, back to the original topic; Calcium carbonate provides multiple protections for metal including insulating it from chlorine, dissolved oxygen and electrical currents. It also neutralizes acids that come into contact. In fact, Tums, the antacid, is made out of calcium carbonate.