Ok..I'll bite. What does the salt corrode?

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vejadu

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Mar 28, 2007
90
Collierville, TN (near Memphis)
TPG says that salt makes limestone and metals corrode. Is there any proof to this other than what he says? I have never actually seen this or heard of this from anyone I know who has pools.

I would like to hear from other builders/pool caretakers what problems they have noticed in salt pools that they have NOT noticed in non-salt pools.

I lived on the beach for years and I know that salt spray from the ocean does corrode metal quickly, so I can understand that salt would corrode metal parts such as pool poles for brushes and nets quicker than just leaving them out in the rain, etc. I assume that salt spray would have a much higher concentration of salt than the pool water. And doesn't all water have some salt in it?
 
Salt corrosion issues have been reported for at least limestone, sandstone, and slate in areas above the water line that get splashed. All porus stone is assumed to be affected. There have also been reports of lower quality stainless steel in and near the water rusting. Stainless steel comes in several grades, the lowest grade seems to be right on the edge at typical salt levels. Normally pools use a medium grade of stainless and don't have problems, but apparently some manufacturers skimp on some parts, particuarly screws. All of these have been observed to occur in less than a year in some situations.

There can also be serious problems with many materials if you run a SWG with a CYA level of zero, as indoor pools often do, and the FC level gets a bit too high for an extended period. In this case it is the chlorine that causes the corrosion, not the salt.

A wide variety of materials will corrode at very high salt levels. This is not an issue below the water line where salt levels are low. But above the water line splashout will evaporate leaving a salt reside. If salt accumulates in these areas for a while even more robust materials can corrode. In many areas rainfall is sufficent to wash the salt away. In arid climates you need to hose down the deck, and any other materials within splash range, occasionally.
 
The only modification I would make to what you said (which was summarized well) is that though it is the chlorine that actually corrodes (oxidizes) the stainless steel, the level of chlorides in the water DOES matter. Chlorides interfere with stainless steel's ability to reform a passivity (protective) layer (I have since talked to a couple of corrosion experts at two universities about this). The link to EPA documents describing this phenomenon are in my original investigative thread into the downsides of salt pools. Though salt does not cause the corrosion itself, it does accelerate it for stainless steel. The process is probably non-linear, however, which means that only getting near or reaching certain threshold values of salt and chlorine causes the run-away problem. Not having CYA in a pool has the disinfecting chlorine level be 20-30 times higher (or more) than in a typical outdoor pool with CYA so whereas 3000 ppm salt in an outdoor pool may not do much acceleration of corrosion for better stainless steel, but with no CYA it may be enough to push things over the edge with accelerated corrosion.

Since most indoor pools do not have CYA and don't seem to report rapid stainless steel corrosion but the ones that did on the Pool Forum used SWG -- that info plus the EPA info is where I came to the conclusions noted above.

Richard
 
So, for an outdoor pool, I would not need to worry about the interior of the pool corroding.

I have an LM3-40 that recommends I keep the salt around 4000 and no lower than 3000. From what I've read, that is higher than most SWCG recommend. I have noticed that there is a salt residue on the brick coping and on rafts and floats. This has never bothered me (It reminds me of the beach.)

Is it possible or probable for the salt to destroy the plastic or brick faster than normal elements?
 
Plastic is safe even at very high salt levels. There is some risk to the brick if the salt accumulates. If you have a visible salt crust that is probably enough to cause long term problems.

The difference between 3000 ppm and 4000 ppm probably only has a small effect. You don't want to ever go above 5000 ppm. Various materials have various thresholds below which corrosion is insignificant. There are a couple of things that start to react in the 5000 to 6000 ppm range.
 
If you notice that any material used in pool equipment is either Stainless Steel or plastic to avoid
corrosion. Chlorine is far more corrosive than salt. Salt seems to have little effect long term on concrete
as witnessed by its use a bridge supports in salt water. If there was structural problem with salt deteriorating
concrete the bridge builders certainly wouldn't want their bridges to collapse.

Some builders use this as an excuse for poor deck workmanship.

Cliff s
 
I agree with Cliff that solid concrete isn't as much of a problem with salt if it's formed correctly. This link talks about this in an easy-to-understand way (this link does as well). Nevertheless, there is a difference between a solid block of concrete that is well made to withstand high pressures (whether from salt crystallization or from water freeze/thaw cycles) and textured or stamped concrete with an uneven surface. Pilings are also not always just pure concrete (see this link or this link for example).

This is why my textured concrete installer recommended sealing the concrete that was textured and that this was optional (not necessary) with the solid blocks of coping concrete. Uneven surface texture is far more susceptible to flaking or damage from pressure than smooth solid blocks. So we have to be careful about making generalizations that all concrete is OK to use unsealed around pools even if it's well made. This may also be why flagstone, with its uneven surface, is more sensitive to problems even though it's not as soft a stone as some limestone.

There is also a difference between surface discoloration or pocking and severe structural damage or erosion. A pocked or discolored surface doesn't matter much for a bridge piling, but it does matter for a pool deck (if one is concerned with aesthetics rather than structural integrity). As for comparing chlorine vs. salt, it depends on concentration and the substance being attacked (i.e. whether the substance is susceptible to oxidizers or to physical "pressure" processes or to chlorides).

As for stainless steel, there are many varieties with different resistance to corrosion. I talk about that starting with this post in the Pool Forum and refer to this EPA PDF file that has lots of information about stainless steel corrosion. Though chlorine is an oxidizer and is corrosive, chlorides (i.e. salt) is particularly destructive to stainless steel by preventing reforming of its passivity layer. The question is, of course, at what level do serious problems start to occur. There are hints in that document, but not definitive answers.

Richard
 
Hmmm..... wonder if this is a "blip on the screen" or a more troubling trend for SWGs. Seems like there's a lot of "if's".... if the right metal/grade, if in the wrong place, ....then rusting might happen. Putting my SS DE filter and new heater at risk is a little concerning. OTOH, I read that the aussies have been using SWGs for years.

Kevin
 
Apparently in Australia the water tends to be fairly corrosive without adding any salt, so they have been using sturdy materials all along and didn't need to do anything special for SWGs. The average salt level used in Austraila is generally higher, into the range where some metals commonly used here would corrode, but those metals were already corroding there before they added salt so they never noticed.
 

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I'm not sure where to post my comments, or questions on this subject, but this looks as good as any...

We're in the midst of a pool build... ACTUALLY had progress today, which hasn't been seen from my PB in over a month. Waterline tile was installed today. Coping scheduled to be installed Monday. (scheduled being the key word there....)

Coping material is the comment... or at least the subject of my post.

My coping will be used clay brick. From my brief travels around this lovely country of ours, I know that brick aren't necessarily a common building material everywhere... but in the south, we like to veneer our houses and lots of other stuff with brick.

In my little corner of the world, it's fashionable to use old/used brick from demolished buildings to veneer our new houses to make them look old(ish). And being the (somewhat) fashionable folks that we are... we have old/used brick. And since I wanted our pool to "match" our house... we will have old/used brick as our coping and hot tub veneer, too.

You don't have to look at too many bricks to realize that they're a relatively soft material. Maybe "very" soft. Maybe softer than that.

Anyways.... my plan is to use old brick, seal it up, and turn on the salt (along with it's chlorine generator).

As of now... the brick is not really an option... as it's being installed Monday (so they say). If anything is optional, its the SWCG.

Am I crazy for mixing the two?



Steve
 
stevenbrla said:
Am I crazy for mixing the two?

Steve
Click to expand...


Guess it depends on your willingness for risk. Maybe you can have the bricks sealed? I would think bricks would hold up much better than natural stone because their fired (in a kiln) and not as soft, even the older ones you mentioned. But thats my opinion.... Also, are you going to have kids jumping in and out of the pool splashing water on them? Something to consider.

:-D
 
Based on my past... I'm not averse to risk.

I definately will seal the bricks & mortar.

I have a 6 y.o. lil girl who I hope will be splashing around with her friends. If she's not, I'll be most disappointed.


Truly there's risk in everything, right? Just a matter of management.... (at least in your head... for a while)

I wonder about the "hardness" of brick vs. the natural "stones" that are commonly used. Might have to do a google on that one.

Hardness and brittleness of metals are well documented, even hardnesses of wood are recorded. Of course just like those, the variation in clay, stone, are probably even greater than those in metal and wood.

Thanks for the response... anyone heard/seen poor results of brick and swcg's?

Steve
 
stevenbrla said:
Hardness and brittleness of metals are well documented, even hardnesses of wood are recorded. Of course just like those, the variation in clay, stone, are probably even greater than those in metal and wood.
Click to expand...

I would think that its not the hardness, but the porous nature of brick that is the potential problem. Sealed up I'll bet they would work very well.
 
kevreh said:
Hmmm..... wonder if this is a "blip on the screen" or a more troubling trend for SWGs. Seems like there's a lot of "if's".... if the right metal/grade, if in the wrong place, ....then rusting might happen. Putting my SS DE filter and new heater at risk is a little concerning. OTOH, I read that the aussies have been using SWGs for years.

Kevin
Click to expand...

Yeah, down here corrosion doesn't seem to be a problem. Most of the pools have salinity of 5000-6000ppm, with some operating on seawater and channel water, with the salinity of 17000-22000ppm. The only rare occasions of corrosion of stainless steel are due to electrolytic corrosion. Addition of SWG is usually not the cause of the problem, but merely the trigger.
 

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