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