Sciencemadness Discussion Board

Electrolysis of Salt Water

draythomp - 21-9-2011 at 22:09

I have a swimming pool with a salt water chlorine generator. When I first started with it there was a scale created between the plates of the generator and it was easily cleaned with HCl. Over time the scale became harder to remove. At first I thought it was just the concentration of acid, however...

Now, 29% HCl available in supply stores doesn't touch the scale. I can pour it straight in and it doesn't bubble at all and the scales just sets there. The acid seems to be fine, it will dissolve Calcium carbonate just fine.

The scale is a fine white powder and I'm currently removing it with a power washer.

Does anyone have any idea what this stuff is?

AndersHoveland - 21-9-2011 at 22:25

No idea.

But on a completely different topic related to pools, I have always wondered what the metal is that is used in handle bars found in pools. There are not many metals which can withstand constant contact with chlorine water, even gold is corroded.

Titanium apparently shows surprising corrosion resistance to chlorine, at least under alkaline conditions.
Although interesting titanium seems to be vulnerable to attack from alkaline solutions of hydrogen peroxide. It is thought that the HOO- anion forms a complex with the protective surface oxide layer, Ti(OH)2O2, with titanium still in the +4 oxidation state, the soluble complex later hydrolyzing into Ti(OH)4 and H2O2.

Another possibility is that the handle bars are anodized aluminum.

Silver would be corrosion resistant, but I doubt they would use silver to coat the handle bars.

Does anyone know for certain what the pool handle bars are made of?

draythomp - 21-9-2011 at 23:30

I'm not sure what you mean by handlebars. If you mean the handles on ladders and rungs on steps they are high grade stainless steel or pvc or polyethylene. The higher grades of stainless are pretty tolerant of the environment and do pretty well unless you let the chemistry get way out of balance. The various plastics work great, but have to be protected from UV; usually by a polymer paint.

AndersHoveland - 21-9-2011 at 23:52

I do not understand how stainless steel would be able to resist the corrosive effects of chlorine water.

draythomp - 22-9-2011 at 10:43

It's a matter of concentration. Sure, it's corrosive, but not that corrosive. Pools are kept at the lowest possible level of available chlorine possible and still kill bacteria and other organics. It can take days to clear an algae laden pool. The ph is kept just barely above neutral and concentrations of chemicals are measured in parts per million or billion. When you're dealing with 20000 gallons of water a couple of cups of 29% chlorine is not a very corrosive environment. This can be attested to by human eyeballs. The pool doesn't blind you when you jump in.

For example, in my salt generator cell, it causes a reaction that produces hypochloric acid and lye. This is in a relatively enclosed space, and even though there are many gallons of water flowing through there a minute, the environment is so caustic it will dissolve stainless in a matter of a few hours. However when the water shoots down the pipes and out into the pool, it is mixed with thousands of gallons of water and the concentration drops dramatically. Therefore, the cell plates are made out of titanium and cost several hundreds of dollars to replace when they finally succumb to the reaction.

In the pool itself, the stainless fittings come under a slow onslaught of various reagents and improper balancing of the various factors will, indeed, cause breakdown. If the ph goes too alkaline the various metal parts will start to deteriorate and problems happen like staining (rust) or green hair (copper in the water) bleaching of fabrics, scaling, etc.

My salt level must be monitored to not exceed 4000 ppm or it will cause metal to rust several feet away from the pool by splash or bad luck.... I have to keep the ph as close to 7.2 as possible at all times or things drop out of suspension and various crystals start forming. For this reason, I have no metal at all exposed in the pool. Everything is some kind of plastic or silica. This even includes the valve that lets fresh water in to compensate for evaporation. This valve is teflon and kynar with no metal at all in its construction.

So, take a close look at the mounting points of a metal ladder in a pool. You'll likely see a little staining somewhere. This means that at some time in the past, the operator didn't pay enough attention to the chemical balance and some damage happened.

Now, does someone out there have any idea what this new scaling is that resists high doses of HCl??

AndersHoveland - 22-9-2011 at 11:05

Perhaps the electrolysis cause some of the dissolved traces of hydrated silica to solidify out as insoluble polysilicates. Or it could be calcium sulfate, or a combination of the two. If it is a combination, it would be extremely difficult to dissolve with chemicals. The calcium acting as an inhibitor toward hydrofluoric acid, and the silica acting as an inhibitor towards strong acid.

To prevent the scale from building up, typically powdered borax or sodium carbonate (sold as soda ash at pool stores) is used to "soften" the water. Alternatively, the pool water can be run through a "water softener" system. The water can be changed out, with the calcium removed by this system before the chlorine is added. Here is how such a system works, if you are curious:
http://www.popularmechanics.com/home/improvement/interior/12...

Be sure that the chlorine chemicals you are adding to you pool do not contain calcium. Solid calcium hypochlorite is a common additive, you might switch to liquid sodium hypochlorite or TCCA.

I have always found it rather unbelievable that chlorine is still used in pools. Chlorine tends to react with organics fairly easily, especially in sunlight, to create all sorts of carcinogenic compounds, and pool swimmers often have a higher incidence of skin cancer.

I would also wonder whether, or how much, the chlorine in pools contributes to the degredation of the ozone layer. Why is Freon banned, but chlorine in pools is not?

Ag+3 complexes, for example, have shown very strong inhibiting properties against the growth of algae.

http://www.havuz.org/pool_pool/pool_wizard/pool_wizard.htm

Alternative methods utilize periodic "shock treatments" when people are not in the water, using an ozone generator in combination with H2O2, also known as the "Peroxone" method, which generates hydroxyl radicals to sterilize the water.

Another method that could be used is to neutralise away most of the chlorine an hour prior to swimming in it. This can, ironically, be accomplished by adding H2O2 into the pool. Although H2O2 is also an oxidizer, like chlorine, it acts as a reducing agent in this case, reducing the chlorine to chloride.

[Edited on 22-9-2011 by AndersHoveland]

draythomp - 22-9-2011 at 11:40

I think you got it!! Thank you. The dirt here is high in both calcium carbonate and calcium sulphate. My guess is that over time I have precipitated the carbonate out of solution and removed most of it from the pool water. That leaves the sulphate running around which is precipitated in the hot electrolosis reaction and grabs onto the titanium plates. This stuff won't dissolve in HCl and therefore, my problem.

At least so far, this makes the most sense. Thank you. Now, if someone has a suggestion on how to get this stuff off the plates of the chlorinator without scraping them. A power washer does a reasonable job, but it doesn't get all of it.

Thanks again.

Endimion17 - 22-9-2011 at 13:18

Quote: Originally posted by draythomp  
I'm not sure what you mean by handlebars. If you mean the handles on ladders and rungs on steps they are high grade stainless steel or pvc or polyethylene. The higher grades of stainless are pretty tolerant of the environment and do pretty well unless you let the chemistry get way out of balance. The various plastics work great, but have to be protected from UV; usually by a polymer paint.


I believe it's chrome steel. One of the types, actually. Almost every piece of metal used in such places, as well as boats, yachts, sailboats, is chrome steel. Even the metal in designer kitchens. :D
It resists salt and weak aqueous solutions of chlorine quite well. I'd say it's one of the best metals to use for making laboratory stands and wall grids. It's not cheap, though...

draythomp - 22-9-2011 at 15:12

Well, it's easy to find out. Just google for "swimming pool ladder" and watch how many stainless steel choices you get to choose from vs. more exotic metal combinations.

AndersHoveland - 22-9-2011 at 15:28

That would make sense, I suppose. Chromium is attacked under more alkaline conditions by chlorine, whereas iron tends to be attacked under more acidic conditions. So an alloy of both metals would likely be resistant to chlorine under a fairly wide pH range.

After doing some quick research, it seems that common stainless steel is quickly corroded by chlorine, but an alloy know as Hastelloy C is very resistant under these types of conditions. It is an nickel alloy with Mo, Cr, and often lesser quantities of Fe, Ti, Co, or Mn.

Have you considered ultrasonic cleaning to break up the scale?

[Edited on 22-9-2011 by AndersHoveland]

draythomp - 22-9-2011 at 15:49

Ultrasonic cleaning would probably do it, but the generator is about 14 inches long and 6 inches wide. That would take a pretty good sized cleaner and would be way over what I'd be willing to spend.

I looked into Hastelloy for a project I had once and that stuff isn't cheap. Most people would be perfectly willing to by three or four lesser priced items and replace them as needed. Remember, a pool isn't too caustic; it can't be, we let our children play in it for hours.