Corrosion of water heater elements - Polyphosphates

[Anonymous]

As I have reported here in the past, I, and many others, have been having terrible trouble with water heater elements on the Spanish coast, particularly round Almerimar. Isotherm have made up some stainless elements for me to try and they seem even worse than the copper clad ones.

I have today discussed this with the local Leroy Merlin who keep a huge range of domestic water conditioning equipment and I am told that everyone round here uses a 'filter' (actually a cartridge of chemical, not a filter but it is held in a filter housing) of polyphosphate. From the research I have done on the internet, this is safe to drink and coats the metal reducing rust and other corrosion. It is also said to 'condition' the water reducing scale and making it seem softer (though maybe not to soap).

I have bought a filter housing and cartridge and we will start tomorrow putting this in line with the hose.

I'd appreciate any comments - negative or positive - and will report back in due course. We plan to be here in Almerimar until next May so if we don't lose another element it will have been a resounding success. Give me a nudge if you haven't heard anything but it is safe to assume that no news is good news.

By the way, six months of the year we are cruising and on watermaker water which I assume is particularly safe for the elements. However, I am wondering what the pH is of deionised water and whether it has an affinity for CO2, forming carbonic acid gas, causing its own problem? Any chemists around?

 

[VicS]

[ QUOTE ]
Any chemists around?

[/ QUOTE ] Well ..... I was according to my profile.


The pH of totally deionised water will be 7 but virtually impossible to measure. In practice it will dissolve CO2 from the air and become slightly acidic.


Not sure about your "filter" We use a Britta filter at home to a limited extent. As far as I can determine that removes just the temporary hardness, ie the scale forming hardness, but not the permanent hardness, using an ion exchange resin. If tested I would expect to find the natural alkalinity due to the bicarbonates that are responsible for the temporary hardness to have been completely removed. It certainly eliminates scaling of a kettle element. The unit also filters out any particulates of course and includes what I think is a carbon filter to remove chlorine and certain other undesirable substances. We actually don't like the water from it as drinking water so as ours comes to us from wells in the chalk and has not already been drunk several times and laced with female hormones, antibiotics, pesticides and herbicides we drink it straight from the tap. (The Britta was bought on the advice of the Vet due to a urinary system problem that the cat suffers from!)

Your polyphosphate unit sounds very much like a Micromet (at least I think that is what it was called but I cannot find any reference to it now) dispensing unit that contains a very sparingly soluble (calcium salt ? ) variation on sodium hexametaphosphate, which at one time was sold as Calgon. (These days Cagon is not a phosphate, presumably due to environmental reasons. ) If I am right then its purpose would be to inhibit scale deposition from hard water. I don't think it would act to prevent corrosion at least not directly.

Not very helpful I'm afraid. There is a lot I have forgotten and it seems a lot I should know much more about! I can't even begin to suggest what is causing the problems you experience with heating elements.

 

[alant]

I was a 'water treatment' specialist with ICI many years ago.

Polyphosphates - ie Calgon, works by sequestering any scale potential salts (captures them when small preventing scaling).

If the correct stuff is used, shouldn't be any problem with potable supplies. Can also have a limited anti-corrosion effect. Its not quite as simple as just checking pH, since other factors will also determe whether scaling or corrosion will potentially exist.

"As far as I can determine that removes just the temporary hardness, ie the scale forming hardness, but not the permanent hardness, using an ion exchange resin."

These simply change any calcium/magnesium salts into soluble sodium salts. Usually on a larger scale recharges with good old sodium chloride. Drinking this 'softened water', can greatly increase your 'salt - sodium' intake & NOT recommended, because of the obvious relationship to increased heart attack potential. If these filters are simply an activated charcoal type, scrubbing out any other nasties, thats a different situation. Also, Silver can be used (cowboys in the old west, used to drop a silver dollar into drinking water) to render dodgy drinking water safe.
Don't know what is in Britta Filters, but be careful.

 

[Anonymous]

Thanks alan, Vic. That confirms my view that at worst the cartridge is completely harmless and at best might solve my problem. It might protect my water tanks and fitting, too.

Thanks.

 

[VicS]

[ QUOTE ]
These simply change any calcium/magnesium salts into soluble sodium salts.

[/ QUOTE ] Having revisited the Brita website I can confirm that Brita filters contain a cation exchange resin in the H form that removes the temporary hardness due to Ca and Mg bicarbonates, but not permanent hardness due (mainly) to the sulphates. Presumably it is therefore a weak acid resin. It is a process that can be used in the deionisation of water with a high temporary hardness as a cost effective first stage. Other metals such as copper and lead are also removed and 67% removal of Al is reported.
Conventional salt regenerated water softeners remove all the hardness, replacing Ca and Mg with Na. The Brita does not increase the sodium concentration in this way.
The Brita filter also contains, as I said earlier, an activated cabon filter that removes chlorine, and pesticides etc.

Anyone using Brita filters might like to know that there is a freepost address on the website to return used cartridges to for recycling.

I also discovered that it is Brita not Britta (my mistake!)
[ QUOTE ]
Polyphosphates - ie Calgon, works by sequestering any scale potential salts

[/ QUOTE ] It is true that polyphosphates can be used in large enough concetrations to sequester the Ca and Mg and that effectively prevents scum formation with soap as well as limescale prevention but they can also be used at much lower concentrations, 0.5 to 5 ppm, to inhibit scale growth alone (known as "threshold" treatment). It is that which is exploited by the polyphosphate units described by Lemain but it is only effective at temperatures up to about 60C as above that the polyphoshate is hydrolysed.

Sorry still not helping with the heating element problem. Reduction of scale using the polyphospate unit might help if only by reducing overheating due to scale build up. You will have to keep some check on the water volume treated and recharge/ replace the units when necessary. Also if feasible turn the thermostat down to stay below 60C.

 

[Anonymous]

Thanks, Vic. Much of that ties up with what I had researched. Re temperature, it was mentioned on one website and they suggested that you put the cartridge in the cold water inlet to the heater (pretty obvious though, you would have thought /forums/images/graemlins/confused.gif). Most of the actual heating is presumably at a temperature well below 60 even though the maintained temperature might be 60+ Certainly this seems to be regarded as a 'must fit' item to all electric water heaters in this part of the world.

I know loads of people in the UK who get through domestic hot water cylinders regularly and I suspect that they need to investigate this sort of technology (polyphosphates might not be the answer, you need to analyse the water and check or ask a local specialist).

Thanks for looking into this for me David

 

[charles_reed]

Brita filters

are activated charcoal, no real effect on hardness.

 

[VicS]

Re: Brita filters

[ QUOTE ]
are activated charcoal, no real effect on hardness.

[/ QUOTE ] No, no effect on hardness. Removes chlorine which is good if your water supply is a bit over chlorinated as it tastes awful in tea and a number of organic compounds which many people find unattrctive in their drinking water.

You might find the FAQs sectin of the Brita website answers a few questions.

 

[VicS]

[ QUOTE ]
I know loads of people in the UK who get through domestic hot water cylinders regularly

[/ QUOTE ] Thats interesting. I have had to replace both in my house. One was probably 40 years old but the other was no more than 20 years old. Both failed adjacent to the inlet to the heating coil. I have theory based on temperature differentials but I have not thought through the electrochemistry yet. No doubt the water analysis is significant as well. Basically tap water does not corrode copper so something odd happens to corrode holes in a hot water cylinder.

 

[Anonymous]

[ QUOTE ]
Basically tap water does not corrode copper so something odd happens to corrode holes in a hot water cylinder.

[/ QUOTE ]I looked over dozens of websites last night and there were quite a few references to corrosion of copper by tapwater. One or two suggested the mechanism and best treatment. There are quite a few different types of treatment cartridge available though could be that a water softener (rechargeable ion exchange) would go a long way to reducing any problem in some cases. I lost a tank prematurely in Cornwall (7 years) and the plumbers said that I had done quite well for that area yet I remember homes back to my childhood where the copper tanks had apparently lasted many decades. I think it all depends on the water. Here in Almerimar, the water is known to be vicious though the local doctor says it is good for the health (I haven't spoken to her first hand about that so I don't know what the healthy properties are).

 

[wizard]

I thought that most hot water cylinders had a sacrificial anode in them to prevent that problem due to stray currents from the mains.

 

[Anonymous]

Sacrificial anodes protect against electrolysis (dissimilar metals in the same electrolyte) and not stray currents. Could be that they are fitted, though, if dissimilar metals are used. I tried fitting one to my copper heating element to stop it corroding but it made no difference. That doesn't invalidate the technique for other systems, of course.

 

[Anonymous]

Vic - update. I am now seeing a completely different type of deposit in my electric kettle (modern flat plate element). I used to get an off-white deposit on the plate, the side walls and the strainer but now I am getting tiny flecks of brilliant white stuff that looks just like scratchings of white school chalk. There is no doubt that this is very different and (as I think it was you who observed) the concentration of polyphosphates must be very low given the method of dosing.

Given that the local plumbers merchant said that this is the cure for my problem and given that we can see a distinct and dramatic change in the precipitation, I am quietly confident (with fingers and toes crossed /forums/images/graemlins/smile.gif)

 

[VicS]

[ QUOTE ]
Vic - update. I am now seeing a completely different type of deposit in my electric kettle

[/ QUOTE ] That figures. Polyphosphate affects and inhibits the growth of (calcite) crystals when used at low concentrations. The effect is quite distinct from the softening when used in higher concentrations and measured by soap titration for example.

Lets hope the treatment solves your problems. You'll have to keep an eye on how long the units last before being recharged or replaced.

 

[Anonymous]

Two other points of interest....the kettle is over 60C and the water has been standing in the tank so conditioned water stays 'good' at least for a few days.

Those electrical conditioners that have been on the market for years need to be close to the point of use. Are they any use, do you know? I cannot see how they work.

 

[VicS]

[ QUOTE ]
Those electrical conditioners ....... Are they any use, do you know? I cannot see how they work.

[/ QUOTE ] I have no idea. I have often thought of getting one to try. I should have bought one while I was still working and fitted it to the supply to the laboratory still for 12 months. I would have had to monitor the water hardness though, but that would not have been difficult.

I think water that has been dosed with polyphosphate will stay "good" for some time even if not indefinitely.

 

[Anonymous]

Update... Everything had been working fine then I found that the dreaded scale was starting to form in the bottom of the kettle (this is a flat element type). I strongly suspect that the polyphosphates don't dissolve fast enough to give an adequate concentration when filling at anything like a normal rate.

Here in the Med it is customary to have your own designated water tap, and you use your own hose, so I am now filling at a very slow rate indeed - around 10 to 20 litres per hour - and I think that's an improvement. If that is not enough, I will try dropping the crystals direct into the tank but my concern at doing that is if they get through the primary pump filter and into the pump.

I'll keep this thread updated. Meanwhile, no significant scale has built up.

 

[VicS]

[ QUOTE ]
I will try dropping the crystals direct into the tank but my concern at doing that is if they get through the primary pump filter and into the pump.


[/ QUOTE ] I may already have mentioned it but the "system" for domestic water systems that was around 25 or 30 years ago was a little fine mesh plasic basket containing the stuff that you suspended in the coldwater storage tank. I had one at one time but regularly refilling it was such a chore that it fell into disuse fairly quickly. "Micromet" was the name at the time but there's no sign of it now. Maybe you could devise something along those lines. A winemaker's nylon straining bag would do maybe for an experiment.

 

[Anonymous]

Yes, I remember those well. They were not intended for food use whereas the polyphosphates I get over here are. As you say, could they be suspended in a bag...it is a mechanical problem. I don't want to have to open the tanks to fiddle around unless I must - certainly I must have a solution to the problem. The other way would be to dissolve the crystals in hot water in a controlled way and then add that each time I fill to approximate proportions. That's more difficult for me to do without a chemical balance or, more important, any idea of the concentration I'm aiming for!