Muriatic Acid vs. Sulfuric Acid

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Muriatic Acid vs. Sulfuric Acid

Postby Retep » Sun 22 May, 2011 18:25

In German public pools they use Sulfuric Acid instead of Muriatic Acid in order to lower the ph.
(Obviously only in pools where they use sodium hypochlorite or calcium hypochlorite as disinfectant. Most public pools use chlorine gas and -of course- then they need to raise the ph.
A gentleman from Germany ( who has 40 years experiance in pool water treatment )explained me the reason to use Sulfuric Acid is that there is no chloride being produced as well as having the advantage that Sulfuric Acid does not gas out in the mechanical room - therefore not creating any rust on metal surfaces, which is the case when Muriatic Acid is being used.

My question is why (to my knowledge) nobody uses Sulfuric Acid in North America ? :?:
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Muriatic Acid vs. Sulfuric Acid

Postby chem geek » Mon 23 May, 2011 02:26

Sulfuric acid will add sulfates to the water, so there is no free lunch here. Hydrochloric acid will increase chloride. Between the two, chloride is more innocuous as it takes fairly high levels before it becomes a problem. With sulfates, lower levels can increase stainless steel corrosion (when chlorides are also present). Besides, ALL sources of chlorine will result in increasing chloride levels in the pool so there is no avoiding it -- if this guy is that experienced, he should have known that. For every 10 ppm FC from ANY source of chlorine, it will result in 8.2 ppm salt when the chlorine is consumed/used. With chlorinating liquid, bleach, or lithium hypochlorite, there is an additional 8.2 ppm salt so a total of 16.5 ppm salt (sodium chloride though if using lithium hypochlorite it's really lithium chloride that is the result). See section VI in this EPA file that says the following:

Non-halide salts have little effect on stainless steels, but chlorides particularly tend to promote pitting, crevice corrosion, and stress-corrosion cracking. In some cases sulfates seem to aggravate the effects of chlorides. Chlorides present in amounts as little of 0.3% with sulfates present can produce severe corrosion. Even quite low concentrations of chlorides can cause corrosion when concentrated by occlusion in surface films. Oxidizing chlorides such as ferric or cupric chloride are specific for severe pitting, although halide salts can cause severe pitting and stress corrosion cracking. The austenitic stainless steels are, however, the most susceptible of all the stainless steels to “chloride” stress corrosion cracking.


As for fumes, one can just use half-strength Muriatic Acid (15-16% Hydrochloric Acid) which fumes a lot less.
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Muriatic Acid vs. Sulfuric Acid

Postby Retep » Tue 24 May, 2011 18:25

chem geek wrote: Besides, ALL sources of chlorine will result in increasing chloride levels in the pool so there is no avoiding it -- if this guy is that experienced, he should have known that.

[/quote]

Oh - he knows that very well. It's just that I haven't mentioned it in my posting (since this is well known anyways)
My point was that hydrochloric acid adds additional chlorides to the water compared to sulfuric acid.

According to the EPA file ( BTW - Thanks for posting this) it is my understanding that by having now chlorides ( from chlorine) and Sulfuric Acid creates now even more corrosion ?

Mmhh. . .I guess those Germans have no clue what they are doing. Yet it surprises me that there is
less corrosion in their pools compared to pools in North America. Then again - it might be linked to
lower chlorine levels as well.
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Muriatic Acid vs. Sulfuric Acid

Postby chem geek » Tue 24 May, 2011 22:00

You are correct with your reading of the EPA info. Unfortunately, they didn't list the chloride salt level where problems are seen or the corrosion rate vs. chloride level when sulfates are present. Note that this chloride/sulfate corrosion is talking specifically about stainless steel.

I believe you are correct with respect to indoor pools that have 1-2 ppm FC or higher and no CYA. The active chlorine level is much higher in such pools. In outdoor pools that often have CYA in them, you don't usually see the same corrosion problems -- assuming similar salt levels.

Also, the higher active chlorine level also results in more disinfection by-products and in particular more nitrogen trichloride which outgasses and can cause corrosion as well. Even more hypochlorous acid itself outgasses (as well as a little chlorine gas). All can cause corrosion in indoor environments. Finally, the Germans may have more stringent regs for air exchange.
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