Calcium for metal

[chem geek]

Calcium, along with carbonate (alkalinity) and appropriate pH, is needed when you have a plaster pool or other grout in order to prevent them from corroding (dissolving) into the pool water. A vinyl pool does not need calcium for this purpose.

The belief that calcium is needed for heaters or for other metal surfaces is that in certain environments that can be normally corrosive to metal, having a "protective" layer of calcium carbonate scale can help to protect the metal from corroding. However, corrosion of metal is not at all the same thing as corrosion (dissolving) of plaster. The fact that you do not have calcium does not make your metal corrode any faster -- only lower pH, higher temperature, increased dissolved oxygen or oxidizing chlorine (HOCl) levels, and higher conductivity (higher salt or TDS) can cause that. In practice, if the pH stays above 7.0, then corrosion effectively does not occur (in pool water; sea water is another matter, but that's due to higher levels of dissolved oxygen and salt and even then it is iron that normally corrodes well before copper).

It is not clear whether you can realistically maintain pool water in a slightly scaling state (for calcium carbonate) to have this deposit on metal pipe surfaces (typically copper) in your pool heater and to do so as a thin film that is enough to prevent corrosion and yet does not continually build up to the point of blocking water flow. In heat exchangers this balance between corrosion vs. scaling is a serious issue, but in a pool it is much less clear that calcium carbonate water balance is important for anything except plaster/grout pools and even then it appears that you have to stray very far away from ideal balance before you run into problems.

So, bottom line, for your vinyl pool, don't worry about calcium. If you are worried about corrosion of metals, then keep your pool's pH in the normal range (well above 7.0) and don't use Tri-Chlor tablets for your regular chlorine source as these are highly acidic.

Richard

[mbar]

Thanks Richard for clearing this up. I am still hearing different takes on if you should use calcium in a fiberglass pool. Now I can see that it really isn't necessary.

[chem geek]

Thanks Richard for clearing this up. I am still hearing different takes on if you should use calcium in a fiberglass pool. Now I can see that it really isn't necessary.

Marie,

If anyone has either real-world experience with metal corrosion due to low or no calcium (as opposed to low pH) or if they are aware of scientific reasons why calcium is needed, I'd like to hear about it. Please don't take my word for it just because the science seems to indicate that it isn't a problem. I have been wrong before and will no doubt be wrong again so if real-world experience contradicts my beliefs, I certainly want to know about it. I'm just relating my understanding as it is at this time.

Thanks,
Richard

P.S. My own personal experience with metal corrosion in my own pool (we have bars in the pool just under the water line) is that the only place corrosion occurred was near where the floater with Tri-Chlor tablets had been and that I had the pool pump running 8-10 hours/day (i.e. not 24/7). I also had a thermometer corrode when I put Tri-Chlor in my skimmer. So localized low pH conditions can certainly cause corrosion. Needless to say, I don't use Tri-Chlor anymore and all of this occurred long before I got involved with this forum.

[mbar]

The experience I have had is that when I first had my fiberglass pool, the pool company put in lots of calcium. When I switched from baquacil to chlorine, they put in mega doses of calcium, then shocked it with calhypo. I ended up with lime green water, then milky water, then a pool that was completely stained yellow. That's when I took control of my own pool, I found this forum, and have been happy ever since. Last year I ran my pool with calcium of 25, I had no problems. This year I bumped it up to 125, because of all I read about fiberglass leaching out if there is no calcium - it didn't make sense to me. I know that fiberglass has trouble with plating - but in my own experience, and my opinion, it is high ph along with high chlorine that brings on that situation. So far this year by running my pool with a good sequesterer, ph no higher than 7.4, and alkalinity on the low side - about 80, I have not had any staining problems. I have read on some sites that fiberglass pools will not be warranteed if there is not calcium of 300 in the water. My pool didn't come with that provision. I can understand that calcium is needed in a plaster pool, but I don't know why fiberglass would dissolve unless the water was acidic.

[RavenNS]

ChemGEEK (lol, I love your name),

you mentioned that temperature would have "play" in corrosion...
I heat my pool to 90F... would this indicate that i should have higher calcium levels to prevent corosion of the Heat Pump ( & filter pump)?
( I'm sure most people don"t regularly leave their pools that hot...lol)

[chem geek]

I heat my pool to 90F... would this indicate that i should have higher calcium levels to prevent corosion of the Heat Pump ( & filter pump)?
( I'm sure most people don"t regularly leave their pools that hot...lol)

Actually, we run our pool, which is solar heated most of the time (for 5 months and gas heated about two months a year), at 88F since my wife uses the pool for swim therapy and I've always liked the warmer water temperature (I'm still pretty skinny).

The best research on corrosion that I can find shows a rough doubling in corrosion rates for each 10C (18F) rise in temperature. The temperature inside a gas heater could be, near the metal surface, up to 30F higher so that is the most likely place for a problem (if any) but typically it isn't corrosion that people are worried about (since they keep their pH well above 7.0) but rather scaling since the higher temperature (ironically) makes it more likely to precipitate or scale calcium carbonate. So in plaster/grout pools, some people (Ben, for example) keep their pools on the slightly corrosive side so that the higher gas heater temperatures don't cause scaling. However, I've been educated on this forum by how extreme the water balance must get before either corrosion or scaling of plaster starts to occur (the LSI or equivalent index seems to tolerate even 1.0 without problems rather than the "official" 0.3 or 0.5 limit) so I really don't have a good answer except that we (including myself) probably worry about this too much.

So if I were you, I wouldn't worry about corrosion or scaling and just keep you pH well into the normal range above 7.0. As far as temperature goes, spas are typically at 104F and I don't know if anyone adds calcium to spas nor have I heard of corrosion problems in spas that are not due to low pH. Of course, the relatively low volume of water in a spa compared to the relatively large number of people who can use a spa makes maintaining pH balance a real headache! Again, if anyone has any contradictory experience, I'd like to hear about it.

Richard

[RavenNS]

ChemGeek, thanks for the leason... most facinating!
( how come i can totally get all this stuff that you post & I can't figure the darn test kits?...lol)

As for Spa / Hot Tubs
yes, you bet calcium is added... usually the companys bottle it up under a "different name" ( & charge at least 4 times as much for it )
Beachcomber ( one of the tub brands that we have) have been selling us " Protect"... there is no content label on this stuff. I only found out that it was calcium this year, which means that I have been paying $13 a tiny bottle for quite some time for calcium

The BBB method works just fine for hot tubs, just substitude bromine for Bleach...lol

Oh & BTW, I find maintaining hot tubs is actually easier than pools... ( I dunno why)...lol

[chem geek]

As for Spa / Hot Tubs
yes, you bet calcium is added...

The BBB method works just fine for hot tubs, just substitude bromine for Bleach...lol

Oh & BTW, I find maintaining hot tubs is actually easier than pools... ( I dunno why)...lol

Actually, I want to go off-topic here, so let me create a new thread for this. Please meet me at this linked thread.

Richard

[chem geek]

I just noticed in this post that Steve (medvampire) found that his pool with CH 350 and TA 200 gets cloudy when the pH gets above 7.8 so that means roughly a saturation index (LSI or CSI) of around 0.7 - 0.75 (depending on temperature which I don't know). So my statement about a 1.0 saturation index was true for one person, but not for another.