Adding Muriatic Acid directly to skimmer

[BobDenny2]

Perhaps this has already been hashed out. I read the previous threads, but they ended in 2007 so the fancy technology told me to post a new one.

The basic message I got from the prior thread was that MA *might* damage your equipment, with the copper pipe in your heater being the most vulnerable, followed by pump seals.

I've been adding MA to me skimmer for some time and noticed no problems, just assuming the dilution and short exposure would be OK. But the other thread alarmed me a bit, and it ended with essentially the message "we need some empirical testing"

So, I performed a really basic test. I took a short piece of standard 3/4" copper plumbing tubing, and put it in a shotglass of undiluted 31.45% MA.

Within less than a minute I could see that the MA had an effect on the tubing, essentially scouring off the oxidization. So I covered it with some saran wrap and left it in my garage while travelling for a week. Today I inspected it: the MA had discolored with some sort of blackish debris which floated to the top of the shot glass. I took out the copper, rinsed it off, and found the copper tube to be a bit pitted, but I found no observable change in it's thickness. In hindsight I probably should have used calipers to measure it, but oh well.

I then smacked it with a hammer to see if it had any structural damage. It of course bent, but did not lose its integrity.

If standard copper tubing can withstand a full week in full strength MA, I cannot see how diluted through the skimmer for 15 seconds per week can cause any damage. Maybe I'm missing something...

I've attached pictures, but not sure they tell the whole story.

Bob

 

[woodyp]

We're not real big on taking any chances with heaters worth 3 grand! Why not just pour it directly in front of a running return? That's what we recommend. That pitting you observed will eventually eat through and those black particles will end up as stains in your pool surface long before that.

 

[BobDenny2]

more pics

 

[BobDenny2]

We're not real big on taking any chances with heaters worth 3 grand! Why not just pour it directly in front of a running return? That's what we recommend. That pitting you observed will eventually eat through and those black particles will end up as stains in your pool surface long before that.

I understand the reluctance, and am not promoting or suggesting anyone do as I do. Only pointing out that the evidence does not support the notion that pouring MA into the skimmer is likely to damage the copper in your heater.

As I understand it, most heaters have about a 10 year life span. Pouring MA for 15 seconds, once per week, for 10 years adds up to about 2 hours of total exposure to Diluted MA. If copper pipe is not substantially compromised after a full week (168 hours) of Undiluted MA exposure, I don’t see how anyone could make the argument that this is a significant risk.

There was some discussion on the 2007 thread that folks found staining on the stainless steel shafts of pumps. But nowhere did anyone describe an actual failure of any equipment

The black particles could in theory be problematic for staining, I agree. But I have seen no such thing. Indeed I checked the glass of MA and copper pipe after 4 hours of soaking and there were no black particles at all. I was not able to observe how that progressed, but it took a full week to get to where you see it in the picture. Again, with only 2 hours of diluted exposure over 10 years, I don’t see how this can be a problem

 

[gwegan]

Interesting test.

What strength MA did you use? My HD sells MA in either 14.5% or 29% dilutions. They stack them right next to each other so its easy for anyone to make a mistake and grab the wrong one or not even notice the difference.

The MA and copper will only react until the MA has been used up by the reaction. Your test used a limited amount of MA over a limited time. In the real world its an unlimited amount of MA over a long time.

In a pool when you pour MA into the skimmer the dilution ratio is not as great as when you pour it into the pool, especially when you pour it in a couple feet out from a return.

We are not saying pouring it in the skimmer results in immediate damage to your pool. But the concentrated MA over time will often deteriorate pool surfaces faster than normal. Moreover, as suggested above, if we make a recommendation on this forum that pouring MA into a skimmer is ok we are essentially saying its ok to pour high concentration MA in the skimmer, because I assure you any caveats will get lost in the noise.

That's not what we want. So the general message of this forum is don't add MA or chlorine through the skimmer.

 

[grottoguy]

I'm not sure what the point of your experiment was. Do you find it easier to add the acid at the skimmer as opposed to pouring it by a return?

 

[kcindc]

Well, the problem is that it doesn't take too much copper to cause staining and to turn blonde hair green. Also, not all plumbing is copper and there are plenty of other parts in the system that you didn't test. Namely, all of the gaskets and internal parts of filters... I'm sure it wouldn't take too much acid to wear out my DE grids prematurely.

 

[JVTrain]

I'm not sure what the point of your experiment was. Do you find it easier to add the acid at the skimmer as opposed to pouring it by a return?

This was my initial thought. It's less convenient for me to add MA to my skimmer. When adding in front of a return, I can let the jug float in the pool water and it's easy to pour slowly with minimal arm strain and zero splashing.

 

[BobDenny2]

Interesting test.

What strength MA did you use? My HD sells MA in either 14.5% or 29% dilutions. They stack them right next to each other so its easy for anyone to make a mistake and grab the wrong one or not even notice the difference.

The MA and copper will only react until the MA has been used up by the reaction. Your test used a limited amount of MA over a limited time. In the real world its an unlimited amount of MA over a long time.

In a pool when you pour MA into the skimmer the dilution ratio is not as great as when you pour it into the pool, especially when you pour it in a couple feet out from a return.

We are not saying pouring it in the skimmer results in immediate damage to your pool. But the concentrated MA over time will often deteriorate pool surfaces faster than normal. Moreover, as suggested above, if we make a recommendation on this forum that pouring MA into a skimmer is ok we are essentially saying its ok to pour high concentration MA in the skimmer, because I assure you any caveats will get lost in the noise.

That's not what we want. So the general message of this forum is don't add MA or chlorine through the skimmer.

The MA is 31.45%. The big box near me sells the 31% and a 14%, but I’ve not been deceived.

I’m no chemist, but your point about the MA being “used up” is not lost on me—indeed the liquid in the shot glass, as you can see from the pictures, turned from yellow to clear, with the black floating residue. I assumed the MA had caused some kind of reaction that changed its structure. When I pulled the copper pipe out to rinse it off, the film still slightly burned my skin—still acidic?

But I checked the MA/copper after 4 hours and the MA was still the same yellow color, and there was no black debris. That still supports my position that 2 hours of exposure of 10 years should not be detrimental.

To the question of “why put it in the skimmer”: my pool uses the pool-floor pop-ups as the primary returns. There is one return that I can activate by disabling the pop-ups, but it is only in one corner of the pool. It is only my opinion, but it seems like the chemicals will be much more evenly distributed by the pop ups that are positioned all across the pool than by one return. And if there isn’t any real damage, why NOT use that distribution method?

 

[ping]

Muriatic acid by itself does not harm copper, what it does is cleans all the impurities off the copper. The black that you saw in your experiment are those impurities.

What does destroy copper and all the other fittings is th low pH of the water passing through the plumbing. Each time you add acid through the skimmer you are reducing the life span of those components due to the extremely low pH of the water.

It's your choice on how to add acid in your pool, we can only offer suggestions on how to best add it to protect all of the components. My suggestion would be to add it to a 5 gallon bucket and with the pool running slowly add it to the nearest pop-up and give the pool a good brushing.

 

[BobDenny2]

So, my point was, everyone here (in the deep end) seem to be scientists. Scientists usually value evidence. The arguments about adding MA to the skimmer seem to lack evidence.

I provided one portion of the evidence: significant MA exposure does not actually damage copper pipe.

Can we continue the scientific experiment? What would be next? It was suggested that simply a low pH would damage the copper…I don’t really see how this would be different than full-on Muriatic Acid exposure—wasn’t 4 hours of exposure enough?

 

[ping]

Like I said, MA does not harm copper, you could let it soak in it all you want and nothing will happen. What does happen is when the MA is added to water in the skimmer it lowers the pH of the water and it is this low pH level in the water that attacks the copper, seals, and pool surfaces.

 

[BobDenny2]

OK, so you are saying if I soaked in in 50% MA and 50% pool water that would actually get a LOWER pH than pure MA? That seems weird to me, but I can certainly re-run the experiment with those criteria. I doubt I’ll get any different results.

What other evidence would people like to see?

 

[JVTrain]

I'm not sure anyone is looking for more evidence. If it works for you, that's fine. In most situations, it's easier to add MA in front of a running return.

 

[JoyfulNoise]

Let me explain what you're seeing and why your experiment is wrong.

Muriatic acid, like all strong acids, attacks and removes the passivation layer on metals, that is the metal oxide film. So on your copper pipe, the minute you placed it into muriatic acid, the copper oxide is removed and, at extremely low pH (the pH of 20 Baume MA is -1) pure metal surfaces are stabilized. The metal itself will only very slowly dissolve in pure acid, the rate is typically to low to measure over short time spans. You can see this PDF document on etch rates of various materials and etchant types used in the manufacturing of MEMS devices (MEMS are Micro-Electrical-Mechanical Systems and the air bag accelerometer sensor in your car is an example of a MEMS device). In the acid solution, the metal will not re-oxidize. The process you are performing is called "pickling" in the metal finishing industry and it is used all the time to clean and remove unwanted oxide films and chemical impurities from finished metal surface. Once the surface is exposed to air (or neutral pH water) and new oxide passivation layer will form. If you want to do a real experiment, you would have two containers, one filled with acid and the other filled with water. Then you would cycle them back and forth and you would, after many cycles, see and measure a fair amount of copper loss.

[EDIT] A better experiment (to avoid acid drag-out contamination of your rinse water) would be to use a container of MA and a garden hose. Dip the copper tube into the acid for 1 min and then rinse it completely with the garden hose. Repeat that process. After some time, you will see the surface become roughened and the Cu loss will be more apparent. [END-EDIT]

This is exactly what happens in your pool. You spike your pH with acid, it removes the copper oxide passivation layer, and then as the pH is restored, that layer reforms. This cycle is what destroys the heat exchanger.

If you want to do and experiment, make two beakers - one dilute acid (5:1 with water) and the other one with the same dilution and an small amount of hydrogen peroxide added. Add two identical pieces of Cu tubing. You can witness and record what happens to the copper when an oxidizer is present to reform the passive oxide layer.


Sent from my iPhone using Tapatalk

 

[bdavis466]

I would also think that to validate the test further, there would need to be water under flowing across the surface in a similar fashion to water flowing through a heater. The low pH eroding the copper would be exacerbated by the force of the water as well.

There are several threads from pool owners showing pictures of heat exchangers that were destroyed by poor water chemistry, namely long-term trichlor use (very acidic).

I don't really see how adding acid to the skimmer saves any time or is any better than pouring in front of a return.

 

[ping]

In the OP's defence, his pool does not have any wall returns, just the pop-up floor cleaning system, so adding chems is not as easy as pouring them over a return.

 

[bdavis466]

I'd argue is even better so long as the acid was poured over the zone that was currently active. This would seemingly prevent isolated areas of low pH on the plaster surface as opposed to pouring in front of a return and being a little heavy handed... The acid can form a cloud that stinks to the bottom and would sit there unless brushed shortly after.

 

[JoyfulNoise]

One other fact to keep in mind is that erosion and thinning of the metal tube in the heat exchanger is NOT the most likely failure mechanism. When acids attack a metal surface, they remove the metal oxide passive layer and then, unless an oxidizer is present (like the peroxide experiment I suggested), the removal rate drops to close to zero. However, this is really only true for a single crystalline metal surface. Copper tubing is a polycrystalline metal (lots of tiny crystals grains of metal all oriented at different angles to one another). So in a real metal surface the grain boundaries (areas where different crystal orientations come into contact) are typically attacked much more quickly than the metal surface of the grain. This has many technical reasons but it is the case. So, when you constantly lower the pH of the water flowing through the heat exchanger, you remove the passive layer in general and you expose the grain boundaries and erode those boundaries more quickly. This eventually leads to weakened metal structure and crevice/pitting corrosion. Once the crevice or pits get large enough, they become points of mechanical weakness and it is at these points that the tube will likely crack or burst.

So low pH acid dosing of the heat exchanger leads to increased metal contamination into the water as well as structural weakness of the tube itself. This all works to lower the operational life of the heat exchanger.

And I agree with Brian that the in-floor system makes chemical distribution better, not worse. If the OP wants to add acid, it can be diluted in a small bucket of water first (1 gallon would be sufficient) and then broadcast slowly around the pool perimeter with the IFS cleaner running. The in-floor system will quickly mix the acid up and keep it from "pooling" in any one area of the pool. A quick brushing of the pool walls will further mix it all up and keep the pH from lowering too far.

 

[alben]

I know with certainty that pouring acid into the skimmer will take out the pump impeller shaft seal. Each year I usually have to lower pH at least one time. Being a bit lazy I poured the muratic acid into the skimmer instead of pulling the safety cover back. I did this on 3 separate occasions with the heater bypassed. On each occasion the pump impeller shaft seal failed within a day. After this happened the third time I put 2 and 2 together that adding MA to the skimmer was making the pump seal fail. When inspecting the seal, the rubber was actually deteriated.