What does Calcium Hardness really do?

[TreeFiter]

There is a poster on the wall where I work made by AquaCheck. On this poster it describes three points of pool water chemistry in everyday terms. It talks about the difference between pH and Alkalinity, Free Chlorine and Chloramines, and Hardness.

The first two seem fine, but the part about Hardness doesn't make any sense to me, and I can't find anything to support their claims. It says; "Check the water hardness to prevent damage to your pool or hot tub. When the hardness level is too high, ugly scale deposits can develop. If the hardness is too low, pipes and fixtures will become damaged and corroded."

So the claim is that soft water will corrode metal parts, like the copper heat exchanger in your pool heater.

Is there any science to back this up? To me it seems that if soft water will corrode copper pipes, it wouldn't be a good idea to install a water softener into residential plumbing that is usually copper pipes. I don't hear about too many catastrophic pipe failures due to soft water, so what is really going on.

For that matter, what is happening when hard water corrodes metal fixtures in residential plumbing? Is it an ion replacement, kind of like how salt eats up concrete?

I'd like to dispel the myth if there is no truth to it, and hopefully save customers a lot of money wasted on Calcium Chloride for pools that really don't need it. (I'm talking about pools with vinyl liners here).

 

[Casey]

We never recommend vinyl lined pool owners to add calcium. NEVER. Now whether your place of employment does is out of our hands, we at TFP don't.

Definitions and Abbreviations

Scroll down to Calcium Hardness. Pool School has your answers. You just have to get in there and start opening links and reading.

 

[JasonLion]

So the claim is that soft water will corrode metal parts, like the copper heat exchanger in your pool heater.

Is there any science to back this up? To me it seems that if soft water will corrode copper pipes, it wouldn't be a good idea to install a water softener into residential plumbing that is usually copper pipes. I don't hear about too many catastrophic pipe failures due to soft water, so what is really going on.

There is never any plumbing corrosion due to low calcium levels. It is all a misunderstanding from a long long time ago when someone was trying to figure out the ideal chemical levels for closed circuit boiler water at a time when the chemistry involved was only partially understood.

Low calcium levels can cause corrosion of plaster/grout/pebble/quartz/stone surfaces, and high calcium levels are a risk for any kind of pool.

 

[TreeFiter]

Taken from the Definitions page:

"This is the amount of calcium in your water and is most important if you have a plaster pool (marcite, exposed aggregate, quartz, etc.). It is not as important in vinyl pools. In fiberglass pools it helps prevent staining and cobalt spotting. Too high a calcium level can lead to scaling and cloudy water, too low can damage plaster surfaces. The recommended range is between 200-400 ppm for both plaster and fiberglass. It really is not an issue for vinyl pools unless it is very high. Anything above about 130 ppm is probably ok for vinyl and above 200 ppm for fiberglass. If there is a warranty on your pool or liner that specifies a calcium level then follow that to keep your warranty! It is tested with a titration (drop count) test that changes color from pink to blue."

This makes it sound like below 130ppm would not be OK for vinyl.

Beyond what TFP reccomends, I would like to hear more about the chemistry involved.

 

[JasonLion]

There isn't any chemistry involved, there never was. Neither metal or plastic plumbing, or a vinyl liner, cares if there is calcium in the water or not (unless the CH level is high enough to cause scaling). There simply aren't any chemical reactions happening that make any difference, so no chemistry to explain.

The recommended CH level for vinyl pools has been going down as we gain more experience with pools that have very low CH levels. The 130 CH level recommendation is from quite a while back when we were less confident and hedging our bets. We have a lot of written material, we miss updating old material sometimes.

 

[TreeFiter]

There isn't any chemistry involved, there never was. Neither metal or plastic plumbing, or a vinyl liner, cares if there is calcium in the water or not (unless the CH level is high enough to cause scaling). There simply aren't any chemical reactions happening that make any difference, so no chemistry to explain.

The recommended CH level for vinyl pools has been going down as we gain more experience with pools that have very low CH levels. The 130 CH level recommendation is from quite a while back when we were less confident and hedging our bets. We have a lot of written material, we miss updating old material sometimes.

You are more or less confirming what I thought. I just want to be sure before I go shaking things up at work. From what I've been able to find online, and my understanding of the chemistry involved, there is a higher risk of corrosion from a pool with higher Calcium Hardness than lower (due to ion replacement, like a faucet exposed to hard water for a long time).

 

[JasonLion]

The big risk for metal plumbing/fittings is from low PH. Anything below about 7.0 can be an issue for metals, especially copper heat exchangers. The lower the PH and the longer the contact time the more of a problem it will be.

PVC is more or less impervious to anything you might encounter in a pool.

 

[chem geek]

You can read a discussion among corrosion experts in this link. Basically, there were studies on municipal pipe that showed that saturating or slightly over-saturating the water with calcium carbonate would reduce corrosion rates because a hard layer of calcium carbonate would form on the pipe surfaces protecting them from corrosion. This is mostly limited to some municipal water distribution systems, but most instead have softer water and use phosphates or other chemicals for corrosion control and they maintain the pH to be higher.

http://www.gewater.com/handbook/cooling_water_systems/ch_24_corrosion.jsp

Theoretically, controlled deposition of calcium carbonate scale can provide a film thick enough to protect, yet thin enough to allow adequate heat transfer. However, low-temperature areas do not permit the development of sufficient scale for corrosion protection, and excessive scale forms in high-temperature areas and interferes with heat transfer. Therefore, this approach is not used for industrial cooling systems. Controlled calcium carbonate deposition has been used successfully in some waterworks distribution systems where substantial temperature increases are not encountered.

The situation described above is precisely the situation in swimming pools with gas heaters. The temperature at the surface of a heat exchanger in a gas heater is around 30ºF hotter which makes the saturation index roughly +0.2 higher.

http://www.wqa.org/sitelogic.cfm?ID=366

Because the Langlier (calcium saturation) Index is lowered in water that has had calcium removed, skeptics sometimes consider softened water to be more corrosive. But softening of water via cation exchange does not make water more corrosive. In fact, the U.S. Environmental Protection Agency (USEPA) and the American Water Works Association have both recently corrected their enclosed brochures as to the misconception that ion exchange softening has an effect on the corrosivity of water.

Municipal water systems often use calcium carbonate saturation indices to help control precipitation in city water mains. This information is useful where utilities try to lay down a protective film in hopes of retarding the rate of corrosion in municipal distribution systems. The Langlier Index (LI) is such a calcium carbonate saturation index that measures the potential of a water to deposit calcium carbonate scale. Water with an LI greater than zero tends to be of higher hardness and alkalinity and therefore to be scale forming. An LI less than zero represents water that tends to dissolve CaCO3.

However, these calcium carbonate saturation indices do not rate the corrosive tendency of the water itself, nor the effect of scale in household plumbing. While some scales are capable of such protection, scales in a household water system are often porus or soft and thus non-protective. It is rare that hardness scale formation is uniform in household plumbing, for the heaviest scale usually forms at points of greatest heat transfer and at low points in a system. In a water heater, for example, most scale forms at the bottom where heat is applied, while the top of the heater tank may show little or no scale. Thus, even in hard scale-forming water, thousands of water heaters can show that corrosion has occurred under or through the scale, or in locations where protective scale has not formed. Thus, it is clear that corrosion protection in household plumbing is not assured simply because a water heater will precipitate calcium carbonate, as indicated by various scale indices. Further, none of these indexing methods take into account the effects of dissolved oxygen, ammonia, chlorides, hydrogen sulfide and other sulfur compounds, water flow velocities, the presence or absence of solid particles or the volume of water through the system which markedly affect water corrosivity.

Here again this is similar to pools that have a primary body of water and a hotter area in gas heaters that are only hotter when cycled on. The pool environment is not consistent.

http://www.euro-inox.org/pdf/map/paper/Ledion_water_EN.pdf

This concept of aggressiveness (with respect to calcium carbonate) is not directly related to the notion of corrosiveness of the water (with respect to a particular metal). An aggressive water may be non corrosive towards a stainless steel, for example. Nevertheless, the situation of the water with respect to the calcium-carbonate equilibrium can have an indirect influence on the corrosion of a metal. Some types of corrosion in cold water are practically never observed unless the water is aggressive (e.g. type I pitting in copper, corrosion of galvanised steel). Moreover, metals, which owe their corrosion resistance to the presence of a layer of corrosion products at the surface can show highly variable behaviour depending on whether the conditions are on one side or the other of the calcium-carbonate equilibrium.

In practice, the two primary metals found in pools are stainless steel in railings and light rings and copper in heat exchangers. Bare steel is rare though some galvanized steel may be used for some screws (not a great idea since the steel is galvanized with zinc which corrodes from chlorine). Having zero calcium in the water is really no different than being just somewhat under-saturated with calcium carbonate. In both situations you won't get any sort of protective layer and as noted above forming a consistent hard scale layer is very difficult to achieve and for metals forming passivity layers (including stainless steel and to some extent copper) calcium carbonate scale may not even be beneficial.

 

[TreeFiter]

Thank you chem geek. What I'm taking away from all of this is that the idea of Calcium Hardness preventing corrosion is rooted in the formation of a coating of CaCO3 that prevents contact between the water and the metal. It sounds like the water is somewhat corrosive to metals, which is why this idea of using CaCO3 scale to protect it came about. Is the idea more that the water may at times have a lower pH and become corrosive, or is it that metals will dissolve slightly in most conditions (equilibrium is reached with a very small amount of dissolution).

 

[chem geek]

There is always some degree of metal corrosion if there are oxidizers in the water (not just chlorine, but also dissolved oxygen). How fast depends on many factors, but in pools the primary factor is the pH of the water. Low pH is more corrosive to metals. My tap water has around 55 ppm CH and the saturation index is around -0.7, but the pH is at 7.7 and around 300-500 ppb orthophosphate is added to the water as a corrosion inhibitor. In swimming pools using CYA so that the active chlorine level is low, you don't typically have metal corrosion issues if the pH is maintained in a normal range, especially 7.5 or higher. Of course, one can get metal corrosion from using inferior metals such as zinc or low quality stainless steel, but high quality stainless steel and copper are usually OK.

In saltwater chlorine generator pools with the higher 3000 ppm salt levels the increase in conductivity of the water increases the rate of metal corrosion. Also, the higher chloride level attacks stainless steel by inhibiting reformation of the passivity layer. So in these pools it's even more important to use high quality stainless steel. For gas heater heat exchangers, one can use cupro-nickel or titanium for greater corrosion resistance. Usually the metal seen to corrode in such pools is aluminum in header bars for vanishing automatic covers and that can be prevented by protecting the aluminum (say, via the bonding wire) by burying a zinc sacrificial anode in moist soil.

 

[TreeFiter]

Thanks again chem geek. I just sent an email to AquaChek, who put out the poster that started this whole conversation. It will be interesting to hear what they have to say (if anything).

But if I understand you correctly, there will always be a small risk of corrosion if you have metals in water. All we can do is try to slow it down to where it won't cause major problems in the life of a pool. If there are problems with corrosion it is almost certainly due to pH being too low.

 

[TreeFiter]

I got a response from AquaChek today. The conversation is as follows:

I saw an Aquachek poster that says low Calcium Hardness levels result in corrosion of metals. I just checked the FAQ on the website, and it says the same thing.

How does soft water cause corrosion? It seems to me that if soft water were corrosive to metals, every house with a water softener would have major problems due to corroded copper pipes.

So what is this claim based on? Chemically, what about soft water makes it corrosive? How does soft water in a swimming pool put metal parts at risk, but not in residential plumbing?

Regarding your email below on water hardness. It is correct to say that soft water increases the corrosiveness of water. Soft water causes corrosion by increasing its affinity for dissolved metals. In the case of a swimming pool, if you lower the hardness of the water chemically, or if your water is naturally soft, it can cause corrosion of the plaster walls a which are made up largely of calcium and it can cause corrosion of the pipes in your pump. However, in the case of soft water we use in our home the story is a little different. Most water softeners are of the ion exchange type, what they do is remove the calcium and magnesium ions from the water and replace it with a sodium ion, thus making the water softer. By replacing one metal ion with another the propensity for the water to become corrosive is lowered, therefore softened water from an ion exchange water softener is not corrosive as naturally soft water can be corrosive or water that has been treated to remove calcium and/or magnesium from your pool/spa.

It seems to me that if a pool has sodium present, hardness wouldn't be an issue based on this argument. Since most pH increasers and Total Alkalinity have Sodium in them, and Salt Water Pools have sodium in them, I can't see too many situations where the lack of calcium would matter.

 

[JasonLion]

Their reasoning doesn't make any sense (except when talking about plaster). What about naturally soft tap water? That doesn't cause any significant amount of metal corrosion, nor does it have any significant amount of salt.

 

[TreeFiter]

I agree. I wrote back again, and I get the impression that this guy is kind of frustrated by my question. I certainly don't feel like I've been given an answer that holds water. I'll post more of his responses as I get them. I keep hoping that he might shine some light on something I wasn't aware of, but I'm doubtful.

 

[chem geek]

They don't know what they are talking about. They are basically saying that if you don't have calcium, then just add sodium chloride salt and everything will be fine. That's baloney. Increasing conductivity of the water increases corrosion rates. Exchanging calcium for sodium is pointless at preventing corrosion. Why even waste your time with these people unless you are trying to educate them?