Understanding "Corrosive to Plaster" a bit more....

[eqbob]

I'd like to understand the CSI and corrosive to plaster aspect a bit more in depth.

The impetus for this question is that this is of course winter and pool temps have plummeted. Since temp can't be controlled, only pH can be. CH and TA also play a role obviously, but even a 0.2 difference in pH can be the difference between "ok" and "corrosive" at low water temps. My water temp is currently 46.

Additionally, playing with pool math show essentially to keep pH at 7.8 or 8.0, but since the testing materials only go to 8.0, you don't really know if you're at 8.0 or above that. Getting much above that isn't any good either.

My TA is currently at 60. CH 320.

It's truly "amazing" how little muriatic it takes to drop the pH. 4 ounces into 12,000 gallons of water can dump pH 0.4. My pump/water feature run 12x7 so it does go back up if it drops too low.

So that brings up the real set of questions.

• How long does "corrosive" take to cause damage?
• What visible changes would I see for damage? Or is this accumulated damage over years and it really only results in a reduced life span of the plaster 10 years from now?
• What are the instant vs. cumulative effects of "corrosive"? For example, if I got too much MA in there and my pH went down to 7.6 until it aerated back up I would be "corrosive" If that process of going back up took .... say a day... would my plaster be totally screwed? Or would it be the accumulation of 200 of those days over years that would cause the damage?

I know there are some experts out here who say to ignore CSI, there's no use for it, they've never calculated it, etc. I am not trying to knock that approach. For *me* however, it's a valid indicator of keeping several distinct parameters in range as they interact. I also find the science behind it interesting and would like to understand the short and long-term impacts of occasional "dips" into the corrosive phase of things.

 

[JasonLion]

If CSI is between -0.6 and +0.6 then you don't really have anything to worry about in that instant. But you do need to take into account how the CSI might change over time. For example, if CSI is 0.6, odds are the PH will go up and thus CSI will go up and you will have a problem at some point.

CSI does not really tell you anything about how quickly an actual problem will develop, rather it is about if there could be a problem or not. As a general rule, CSI being a little out of range for a short period isn't likely to cause any issues at all, while if CSI is further out of range or out of range for longer or both then the odds of problems increase and the possible severity increases. But the actual rates at which problems develop are not predictable.

Problems might cover quite a range of things. Calcium scaling can happen in days and cause fairly major issues or take months to become visible. Plaster erosion can develop very slowly over years, causing the surface to get rough only several years later. Plaster erosion can also happen quickly, ruining the plaster in a few weeks.

The standard PH test goes up to 8.2. Your color chart may or may not show the 8.2 color, depending on which one you have. But regardless, the 8.2 color is different than the 8.0 color.

This line of questioning isn't going to be especially productive. Most of what you are asking about is simply not known. Unless you want to embark on a multi-year research project it is unlikely you will ever find out the answers to several of your questions.

 

[eqbob]

This line of questioning isn't going to be especially productive. Most of what you are asking about is simply not known. Unless you want to embark on a multi-year research project it is unlikely you will ever find out the answers to several of your questions.

Ok. That alone is good information. I didn't know if there were solid answers to these musings or not. An answer of not known is still an answer. You've provided some additional information that's helpful as well. Can't learn unless I ask, so I ask, albeit at the risk of appearing stupid or angering the membership / admins.

Can't really embark on the research side without having pools for testing and I only have one and don't want to ruin it <G>

My base testing kit is a Taylor 2006. BTW, I find PoolMath incredibly useful and helpful so thank you for that implementation. I also use PoolPal and I keep on my phone since that's with me when I test.

 

[chem geek]

I discuss various factors that can contribute to the rate and amount of scaling or dissolving/pitting of plaster (we usually reserve the term "corrosion" to refer to metal corrosion that is most affected by pH and not by CSI) in this post and in this post.

The short answer is that we don't know for sure how the rate is affected definitively but we do know that low pH accelerates dissolving/pitting because having plaster exposed to water with 0 ppm CH does not result in fast pitting but having the surface exposed to very low pH does result in more rapid pitting. This implies that hydrogen ions may play a role as a catalyst in accelerating the dissolving of calcium carbonate from plaster. Basically, the dissolving of plaster isn't just from water molecules hitting the surface and dislodging caclium and carbonate as ions and then those same ions in saturated water reforming calcium carbonate again. Hydrogen ions may dislodge carbonate to form bicarbonate ions so may accelerate the degradation process. This could occur if the acid catalyzed mechanism involved bicarbonate ion as shown below:

CaCO₃(s) + H⁺ <---> HCO₃^- + Ca²⁺ <---> CO₃^2- + H⁺ + Ca²⁺

However in the above the rate of the right hand side reaction is very fast so the rate limiting scale formation may not involve hydrogen ion so not be directly affected by pH though it is indirectly affected in that the rate may be dependent on bicarbonate ion concentration.

The amount of dissolving or scaling may also be affected by what is known as the Calcium Carbonate Precipitation Potential (CCPP) which uses the CSI to calculate how much scale or dissolving can occur. Basically, the formation of scale or the dissolving of plaster is self-extinguishing mostly because forming scale lowers the pH and dissolving plaster raises the pH. As a general rule, the CCPP is higher (at the same CSI) when the pH buffering is higher so when the TA is higher or when borates are used (borates don't show up much as TA but still buffers the pH).

If I use your numbers of 60 ppm for TA and 320 ppm for CH and assume 30 ppm CYA and 50ºF temperature and a pH of 7.8, then I get a CSI of -0.18 which is not very low at all. The CCPP is only -2.4 which is also very small so not a problem. If you were to have 50 ppm Borates and a TA of 66 ppm for the same -0.18 CSI, then the CCPP is -6.6 so there can be a greater quantity of dissolving plater before it self-extinguishes from rising pH. The CCPP magnitude is the ppm of calcium carbonate for dissolving or scaling that brings the CSI to 0. The 6.6 quantity in 10,000 gallons is 0.55 pounds of calcium carbonate.

Also keep in mind that all chemical reactions slow down at lower temperature so the lower water temperature is at lower risk of dissolving very much even when the CSI is negative because any such dissolving is significantly slower. We don't know the activation energy of the reaction but if it's similar to chlorine reactions temperature dependence, then the rate is cut in half for every 13ºF drop in temperature.