Negative CSI and metal corrosion

[PoolOwnerNumber9]

Split off of this topic. JasonLion

Using Jason’s poolcalculator.com shows that your CSI is about a -0.70. 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 would recommend that you try to get your CSI to a -0.35 which you could do by raising your alkalinity to 100 and your calcium to 150. For your pool, you would add 3 lbs of baking soda and 8 lbs of Calcium Hardness Increaser.

 

[AnnaK]

Re: The 1/3 Rule

From the Pool Calculator I see that the CSI is temperature dependent, increasing as the water warms up. Fluctuations in water temp are not something I can control easily. It's an above-ground in which the pump runs at night which means I have a fair amount of cooling overnight.

I will give some thought to increasing the CH. Other than the screws which hold the skimmer frame we don't have any metal objects in the pool.

 

[JasonLion]

Re: The 1/3 Rule

With a vinyl liner a very negative CSI is actually just fine. Low CSI is really only a risk to plaster, pebble, tile, and natural stone surfaces. Having a large margin before you get to scaling is good. Even if your PH accidentally gets very high you still won't have calcium scaling.

 

[Guest]

Re: The 1/3 Rule

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.

Blatently 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!

 

[PoolOwnerNumber9]

Re: The 1/3 Rule

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.

Second, there are two issues here, one is the water’s ability to dissolve and the other is the water’s ability to corrode. Corrosion is a very complex thing to understand. Corrosivity of water is most greatly influenced by chloride content, chlorine content, pH, conductivity, and carbonate concentration. Another factor in predicting the corrosive potential of a metal is whether or not the metal is able to form a passivation layer. A passivation layer protects the metal. Under normal conditions of pH and oxygen concentration, passivation is seen in such materials as aluminum, iron, zinc, magnesium, copper, stainless steel, titanium, and silicon. Ordinary steel can form a passivating layer in alkali environments (like calcium carbonate), as rebar does in concrete. Buffering chlorinated water with carbonate inhibits chlorine corrosion.

If the water is aggressive, it will tend to dissolve a metal’s passivation layer, or prevent it from forming, and expose the metal to more corrosive conditions.

Based on the OP’s specific conditions, I can agree with JasonLion’s assessment that no action needs to be taken, especially if the OP has reason to believe that they may need the negative CSI due to expected factors that would raise the CSI in the future. However, it is my personal opinion that my first recommendation is the correct thing to do.

I will give a second, optional recommendation: If your pH tends to rise, take no action and wait until your Total Alkalinity drops to 70 ppm before raising it. If your pH tends to drop, raise your Total Alkalinity to 120 ppm a little at a time as your pH drops.

 

[AnnaK]

Re: The 1/3 Rule

I will give a second, optional recommendation: If your pH tends to rise, take no action and wait until your Total Alkalinity drops to 70 ppm before raising it. If your pH tends to drop, raise your Total Alkalinity to 120 ppm a little at a time as your pH drops.

Is that advice in general or were you directing it at me, the OP, specifically?

My pH has been rock solid at 7.5 since about the end of May.

 

[Guest]

Re: The 1/3 Rule

Corrosivity of water is most greatly influenced by chloride content, chlorine content, pH, conductivity, and carbonate concentration. Another factor in predicting the corrosive potential of a metal is whether or not the metal is able to form a passivation layer.
Exactly, and this is NOT predicted by most of the saturation indicies in common use (Langelier, Ryznar, Puckorius, etc.) with the exception perhaps of the Larson-Skold index, and that only for steel.


If the water is aggressive, it will tend to dissolve a metal’s passivation layer, or prevent it from forming, and expose the metal to more corrosive conditions.
And the main factor determining this agressive nature in any of the indicies is the pH! pH is the main factor that will predict whether a passivation layer (scale) will dissolve.

ased on the OP’s specific conditions, I can agree with JasonLion’s assessment that no action needs to be taken, especially if the OP has reason to believe that they may need the negative CSI due to expected factors that would raise the CSI in the future. However, it is my personal opinion that my first recommendation is the correct thing to do.
Since the OP has a vinyl pool the CSI is NOT a major concern at all!
I will give a second, optional recommendation: If your pH tends to rise, take no action and wait until your Total Alkalinity drops to 70 ppm before raising it. If your pH tends to drop, raise your Total Alkalinity to 120 ppm a little at a time as your pH drops.
Where to put the TA really depends on whether the chlorination source is stabilized or unstabilzied. Stabilized chlorines (along with bromine tablets and MPS shock) in use are acidic and will cause both TA ahd pH to drop and benfit from a higher TA. Unstablized chlorine sources are pH neutral to mildly akalline in use and will exhibit a more stable pH at much lower TA levels because there will be much less outgassing of CO2.

 

[PoolOwnerNumber9]

Re: The 1/3 Rule

AnnaK, if your ph is staying stable, then I think that you don’t need to take any action. I see that your Total Alkalinity started at 100 and has dropped to 85, now. I am assuming that it will continue to drop. If it does, at some point, you will need to bring it back up. I think that you will be OK until you get as low as 70 ppm.

A question for WaterBear and JasonLion, how low, in your opinion, can the Total Alkalinity go before you recommend that it be raised? How about the Calcium? It seems, from your comments, that as long as the ph stays at 7.5 with no problems, then there is no need to worry about Total Alkalinity or Calcium?

WaterBear, I think that you are underestimating the carbonate concentration as a significant factor in the water’s aggressivity and corrosivity. Both the Alkalinity and Calcium readings give an indication of the carbonate concentration.

 

[Guest]

Re: The 1/3 Rule

A question for WaterBear and JasonLion, how low, in your opinion, can the Total Alkalinity go before you recommend that it be raised? How about the Calcium?
This depends in part on the pool surface. but carbonate alkalinity levels are really determined by whether the sanitation source is acidic or not.
Calcium levels once again depend on the pool surface. In vinyl pools the main purpose of calcium is to help prevent foaming since hard water does not foam a readily as soft water. With fiberglass there is empirical evidence that higher calcium levels help prevent staining and cobalt spotting but the mechanism for this does not seem to be completely understood. With plaster and aggregate finishes the the CSI does become a factor to consider.
In actual practice and assuming an unstabilized chlorine source a TA of 50 is probably the lowest to go in a vinyl pool while in other pools 70 is probably a good low end to shoot for. If there is a lot of aeration from such things as water features, ozonators, etc. then running the TA a bit lower might be beneficial to slow the outgassing of CO2 and the resultant pH rise.

It seems, from your comments, that as long as the ph stays at 7.5 with no problems, then there is no need to worry about Total Alkalinity or Calcium?
Not true at all and I really don't undestand where you get that idea. If you have read the pool school section of the forum and the further topics section at the end of pool school you would not make such comments as that.
WaterBear, I think that you are underestimating the carbonate concentration as a significant factor in the water’s aggressivity and corrosivity. Both the Alkalinity and Calcium readings give an indication of the carbonate concentration.

I thiink you are underestimating my understanding of water chemistry. Please explain how calcium readings will give an indication of carbonate concentration in the water (besides the fact that the usual titration test for calcium hardness is often expressed in ppm calcium carbonate).

BTW, in a different thread you stated that you work in the industry in 'service and repair'. Can I take that to mean you are a pool service tech (and possibly a CPO)?

 

[PoolOwnerNumber9]

Re: The 1/3 Rule

I did not mean to sound like I was questioning your understanding of water chemistry. Sometimes I have a hard time saying things clearly. I did not mean any offense. My question was due to the fact that you were not advising the OP in this thread about lower limits for Total Alkalinity and Calcium. In fact you said this:

“Since the OP has a vinyl pool the CSI is NOT a major concern at all!”

This seemed to me to indicate that the Total Alkalinity and Calcium could be zero without any problems as long as the pH stayed OK. If the CSI is totally irrelevant then why would you need to have any calcium at all? If the pH was staying stable, then why would you need Total Alkalinity if the CSI was irrelevant?

I am a service tech and a CPO. Are you saying that calcium carbonate is not a factor in determining the carbonate concentration?

 

[SeanB]

Re: The 1/3 Rule

This is a reminder to ALL posters. Please keep a civil tone when having discussions and especially disagreements. None of us knows it all and we all benefit from productive discussions.

SeanB

 

[Guest]

Re: The 1/3 Rule

I did not mean to sound like I was questioning your understanding of water chemistry. Sometimes I have a hard time saying things clearly. I did not mean any offense. My question was due to the fact that you were not advising the OP in this thread about lower limits for Total Alkalinity and Calcium. In fact you said this:

“Since the OP has a vinyl pool the CSI is NOT a major concern at all!”
Saturation index, per se, is not a concern in a vinyl pool since we are not dealing with a reactive pool surface. Water still should be within recommended guidlines.
If you go back to our pool school section and read the "Recommended Levels" section you will see what our stance is on this (and actually it it pretty much in line with industry recommendations)
category/pool-school/recommended_levels


This seemed to me to indicate that the Total Alkalinity and Calcium could be zero without any problems as long as the pH stayed OK
It would be next to impossible for TA to be 0 when pH was in recommended range. Bicarbonates would form in the water simply because CO2 would dissolve from the atmosphere and move toward equalibrium (formation of bicarbonate ions). If the pool was filled with softened water it conceivably would have 0 ppm calcium hardness, but it could not have 0 ppm carbonate alkalinity at normal pool pH range.

If the CSI is totally irrelevant then why would you need to have any calcium at all? If the pH was staying stable, then why would you need Total Alkalinity if the CSI was irrelevant?
Understand that TA (carbonate alkalinity) will work the same in hard or soft water as far as the pH buffering effect (actually the pH raising effect towards a pH of 8..2) is concerned. The saturation index for calcium only becomes important in a system that is comprised of calcium (a plaster pool). As far as scaling tendencies are concerned pH is the primary predictor. If it was not it would be impossible to maintain pools with high calcium levels. As you know it might be difficult at times but by keeping tabs on pH it is not impossible. There really is no correlation between corrosive tendencies towards metals and the CSI. Too many other factors come into play and other ionic species such as sufates and hypochlorites and chlorides play an important part here.


I am a service tech and a CPO. Are you saying that calcium carbonate is not a factor in determining the carbonate concentration?

Well, calcium carbonate doesn't actually exist in pool water. It does exist as scale deposits or as a precipitate that can cloud a pool. What exists in the pool at normal pH are bicarbonate ions along with other anionic species and calcium ions along with sodium and other cationic species. Carbonate ions, per se, do not exist at normal pool pH in any appreciable concentration. The usual factor that can cause calcium carbonate to precipitate out is high pH, which will convert bicarbonate to carbonate.
If you are looking at what determines carbonate concentration in pool water it would have to be pH. Like I said, at normal pool pH the majority will be in the form of bicarbonate ions and very little as carbonate. If we raise the pH above about 9 then we will start having cabonate ions and above 10 there is practically no bicarboante left.
Likewise, at low pH there is very little bicarbonte (measurable TA) since most will have converted into carbonic acid (essentially, but not exactly the same as, carbon dioxide dissolved in water.)
Perhaps this thread might explain it better:
viewtopic.php?f=67&t=4979
The calcium concentration of the water is basically immaterial since this will occur no matter what the calcium hardness is. The only difference would be at what point the water becomes 'saturated' and calcium carbonate precipitates out.

 

[JasonLion]

Re: The 1/3 Rule

The lowest I ever take TA is around 50, and that only when there are significant CO2 outgassing issues. Very low TA levels allow the PH to fluctuate too easily, which generally leads to problems fairly quickly.

There is extensive experience with CH levels between 50 and 100 in vinyl pools without any reports of problems with metal corrosion. CH levels below 50 are extremely rare, so empirical data is not available in that range.

There are a large number of factors that influence the rate of metal corrosion. In swimming pools, PH seems to be by far the dominant effect. The vast majority of metal corrosion incidents reported involve PH levels below 6.8. Nearly all of the remaining metal corrosion incidents reported involve extremely high FC levels, well above the levels we recommend, typically combined with low or zero CYA. In practice, CH levels do not show any correlation with reported metal corrosion problems.

 

[chem geek]

Re: The 1/3 Rule

I just want to reiterate what waterbear has said about a low saturation index not being a predictor for corrosion of metal. The saturation index is solely and only a measure of the level of saturation of calcium carbonate in the water. The only relationship it has with regards to metal corrosion is that a low saturation index *may* be due to a low pH and a low pH IS a very significant factor with metal corrosion. The other factors in metal corrosion that are significant are the level of dissolved oxygen in the water, the level of other oxidizers such as hypochlorous acid ("active" chlorine), the conducitivity of the water and a looser relationship to the amount of buffering in the water with lower buffering (TA) being somewhat more aggressive IF the other factors are already significant.

In addition to the above, there are other substances in the water that can inhibit corrosion by slowing down its rate, such as the presence of phosphates in the water. In fact, I recently tested my tap water for phosphates (for the heck of it) and found it to be high and E-mailed my water company who E-mailed back that they intentionally add zinc phosphate to the water at a rate of 300-500 ppb and that this is done specifically for corrosion control. They use monochloamine in tap water in the pipes at a level of roughly 1 ppm. My phosphate level in my pool is currently somewhere in the neighborhood of 2000-3000 ppb, yet it doesn't get algae because of my keeping the chlorine level properly maintained. I do notice a slightly higher chlorine demand this year so perhaps this winter I'll use winter rains to do a roughly half dilution of the pool water and see if that makes a difference.

Even for scaling vs. dissolving of plaster, the saturation index does not predict whether this will occur or how fast this will occur. It only predicts if it CAN occur. If the index is negative, then plaster CAN dissolve and scaling will NOT happen. If the index is positive, then plaster will NOT dissolve and scaling CAN happen.

There is some speculation that a thin layer of calcium carbonate from having slightly saturated water can protect metal from corrosion, but this is hotly debated and for the most part seen as a minor and inconsistent form of protection -- the layers of scale are usually not uniform and corrosion can occur from cracks in the scale and be even worse due to a local environment that is not well circulated.

There are specific potential problems with corrosion of stainless steel that is made worse at higher chloride levels so SWG pools do have to be a bit more careful, but in practice we have only seen this be a problem when no CYA is used in the pool as chlorine levels are then very high. Stainless steel bars that get splashed with salty water can corrode more quickly and we've seen that problem on this forum.

Richard

 

[Guest]

Re: The 1/3 Rule

Richard, Thank you!

 

[PoolOwnerNumber9]

Re: The 1/3 Rule

Richard, The presence of calcium carbonate in the water causes a film to form which slows the flow of dissolved oxygen and prevents corrosion. Calcium carbonate also functions to lower and possibly eliminate the current flowing through the water. Reference: http://www.cee.vt.edu/ewr/environmental ... osion.html

Corrosivity of water is most greatly influenced by chloride content, chlorine content, pH, conductivity, dissolved oxygen and carbonate concentration. Salt Water Pools are affected by the high chloride content and higher conductivity. Also, aeration increases the levels of dissolved oxygen in the water.

Although you do mention “…buffering in the water with lower buffering (TA) being somewhat more aggressive IF the other factors are already significant”, I think that you are underestimating the importance of the carbonates as a factor in controlling corrosion. Buffering chlorinated water with carbonates specifically inhibits chlorine corrosion.

I believe that a negative SI almost certainly will dissolve plaster, while a positive SI only indicates the possibility of scaling.

Also, at least in concrete/plaster pools, I have noticed significant suppression of algae corresponding to higher carbonate alkalinity and calcium levels.

 

[Guest]

Re: The 1/3 Rule

Also, at least in concrete/plaster pools, I have noticed significant suppression of algae corresponding to higher carbonate alkalinity and calcium levels.

Interesting, this goes agains what is commonly known about algae growth. CO2 is necesssary for photosyntysis of algae. In fact, aquarists will inject CO2 into a planted aquarium to promote algae growth. Higher TA levels will prouduce greater carbonation of the water and will acutally foster conditions beneficial to algae growth.

 

[AnnaK]

Re: The 1/3 Rule

This thread has moved far, far away from my initial question about the 1/3 rule. I'm unsubscribing from it now. Thanks nonetheless.

 

[meatloaf]

Re: The 1/3 Rule

I agree, it's starting to resemble a political debate.

 

[chem geek]

Re: The 1/3 Rule

PoolOwnerNumber9,

The use of calcium carbonate saturation as a way of preventing corrosion is not absolute as implied by the link you gave. One discussion of this among corrosion experts is here though I got pretty much the same information when I called up some professors at universities who had published information on corrosion when I was researching it as part of this thread at The Pool Forum. There are papers, such as this one that show that corrosion of gray cast iron (at high temperatures) is accelerated by higher chlorides and decelerated by bicarbonate/carbonate. This report implies that anions such as carbonate can inhibit crevice corrosion, but the primary mechanisms are not known and the pH buffering of the bicarbonate/carbonate system could be the primary factor since it reduces local acidity. This has nothing to do with calcium carbonate saturation.

As for practical experience from pool users, PBs, etc. on multiple pool forums, we've only seen metal corrosion come up in a small number of situations that had explanations other than just a saturation index. 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 also experienced this myself with a Trichlor feeder that corroded stainless steel near where it "parked"). Addition of salt (for a saltwater chlorine generator, SWG) to inexpensive above-ground pools using inferior bolt materials was another situation that was remedied by replacing the bolts with stainless steel (and this corrosion was independent of saturation index since some of the pools had high hardness water with a >0 saturation index). Addition of salt for an SWG in indoor pools where no CYA was used resulted in fairly rapid (< 1 year) corrosion of stainless steel in some cases. So the pH and conductivity of the water (and presence of chlorides, especially for stainless steel corrosion) along with the amount of oxidizer (i.e. hypochlorous acid) appear to be the primary factors in pools.

The only situation where we don't recommend saturation (or near saturation) of calcium carbonate is for pools without plaster/grout surfaces, specifically vinyl pools. Fiberglass pools may have some protection of the gelcoat by more calcium so we recommend having some even if not fully saturated. We don't recommend having no pH buffering though for pools -- it's mostly the Calcium Hardness (CH) level that has a different recommendation. Anyone can certainly saturate the water in their vinyl pools if they want to -- we just say it is not necessary and have not seen any metal corrosion as a result of that (PoolSolutions and The Pool Forum were started in 1997 by Ben Powell; Trouble Free Pool was started in 2007 after The Pool Forum no longer accepted new registrations and Ben went out of communication).

We'll have to look out for higher calcium and carbonate levels inhibiting algae. That doesn't seem to be correlated based on algae blooms when people let their pools go. There seem to be as many vinyl pools with low CH that have a problem as plaster pools saturated with calcium carbonate. If FC gets too low relative to CYA, algae develops in many pools. The ones where they don't are more likely to be low in nutrients (e.g. phosphates or nitrates) or have some other algae inhibition (e.g. borates, PolyQuat, copper).

Richard