Hi (hope we're in right place - sorry if not - we're new to this) =96 we are having serious corrosion/galvanic erosion with our Ariston Unvented Stainless Steel Hot Water System. We are on our 3rd copper thermostat tube (=A380!) and 3rd immersion element (magnesium sacrificial anode included - =A340) in 18 months. Ariston have no answers nor do our plumbers (=A3250!) =96 just arrived home to water again leaking from the thermostat tube! Can you please put us in touch (via email/telephone...) with anyone who might be able to shed some sort of light on a solution/cause of our problem?. The house/system is 6 years old =96 original plumber doesn=92t know what the problem is. Have noticed a small (200mV) on the earthed pipes joining steel tank - but can't prove electrolysis is fault. Thanks in case you can help. Regards Tony
Defective earth connection? How can you get a voltage from an earthed pipe, when earth is 0V? It isn't earthed. What are you using as 0V for the measurement of the PD in the pipe?
Whereabouts are you?
Electolysis is the cause. The currect (!) Arsiton water heaters have an electronic anti-corrosion device. Maybe you could get one of those.
Thanks for the reply. Have checked whole electric system now - it would app= ear there is 200mV showing across the Service box earth (PME from road) and= a "ground rod" which we've buried (banged 2' into) into the ground outside= , if you see what I mean. This 200mV shows up throughout the house on earth= connections in sockets etc and the new "Earth".
There is however only 20mV showing across new Earth and "Neutral" - ALL p= ower switched off at Service box, of course.Think the 200mV might be coming= courtesy EDF? Is it enough for corrosive electrolysis?
The immersion element includes a sacrificial anode on our model - Ariston j= ust said they don't use them anymore, didn't say why! Thanks! Have taken ou= t immersion (sic, anode)and put "blank" in pro-temp - hope the won't make c= orrosion worse without the anode.
Electronic anti-corrosion device? Will invstigate - but not sure Ariston Te= chnical will be able to help - have you any knowledge/experience? Would be = cheaper than the Thermastore we've been looking at online this afternoon!!
Sorry about the long reply - just hoping it makes some sense.
ppear there is 200mV showing across the Service box earth (PME from road) a= nd a "ground rod" which we've buried (banged 2' into) into the ground outsi= de, if you see what I mean. This 200mV shows up throughout the house on ear= th connections in sockets etc and the new "Earth".
ALL power switched off at Service box, of course.Think the 200mV might be = coming courtesy EDF? Is it enough for corrosive electrolysis?
just said they don't use them anymore, didn't say why! Thanks! Have taken = out immersion (sic, anode)and put "blank" in pro-temp - hope the won't make= corrosion worse without the anode.
Technical will be able to help - have you any knowledge/experience? Would b= e cheaper than the Thermastore we've been looking at online this afternoon!= !
I believe that sacrificial anodes were stopped being used because the Water Regulations prohibited their use, or the metal corrosion products they drop into potable water systems (I've haven't come across anything that confirmed this). All the unvented water heater manufacturers started using electronic devices some years (8+??) ago. I don't know the details, they just make apply a positive charge to the heater, making it the cathode and protecting it. If you look at the installation instructions for the recent Ariston water heaters, they have them fitted.
You need the sacrificial anode, once it has gone the heater casing starts to corrode.
I'm not an electrician, but the stray earth current sounds wrong and I'd suspect that would be the cause of your problems. It could cause corrosion problems.
Such voltages are perfectly normal and are generally of no consequence whatever. I very very much doubt that they're anything to do with your corrosion problem.
In the PME (aka TN-C-S) system the earth and neutral conductors in the street mains and your service cable are one and the same, so voltage drop along the neutral due to normal load current causes a potential difference between the supply 'earth' and the local 'ground'. This can reach several volts - 200 mV is 'nothing'.
Your wiring earth and neutral are bonded at the PME earth terminal in the supply cut-out (the unit containing EDF's main fuse). On no load the PD should be zero, but stray earth currents flowing quite normally in your main bonding conductors could cause such a tiny PD - where exactly did you put the meter probes?
That's highly unlikely, IMO. You're measuring with the meter set to AC volts, I presume? Try putting the meter on DC - any DC offsets in the earthing and bonding would be far more likely to drive electrolytic corrosion, but even than you've have to consider what current path is relevant to the problem. Do you live near anything that could put significant DC current into the ground, e.g. LV electric railway, tramway, telephone exchange?
Cost reduction, increased spares sales...?
The cylinder is a closed surface, electrically. Faraday sayeth that there can be no electric field in such a closed vessel, unless it's introduced somehow. The voltage on the surface relative to earth is quite irrelevant. To get internal currents there would have to be internal electrochemical PDs (as there will be with those sacrificial anodes), or else a PD or current introduced through one of the penetrations.
HTH but I've no more to suggest ATM. I'd continue to badger Ariston. Your water company might help too - is the pH of your water unusually high or low?
I think you're confusing this with something else.
It is bimettalic (aka Galvanic) corrosion. There are two different metals (water pipes and cylinder shell) connected and with an electrolyte. The least-noble metal (usually the steel/SS cylinder) corrodes. It's to do with a current passing through the water by hitching on the metal ions. Putting a sacrificial anode in protects the cylinder shell, so long as the anode is replaced regularly.
Hi Onetap - interested to hear your views on the sacrificial anode - having= been told on Friday by Ariston "Tech" that "...they don't use them anymore= " - was talking to another "Tech" this morning about another poss solution = and he said that they certainly do still have the anode - on the immersion =
- to protect the immersion! So, I said, if it's only there for that, withou= t the immersion (which is currently on the floor in the airing cupboard - t= he anode won't be missed either? (Have blanked off hole with BRASS plug!!) = What about the copper thermostat tube - that corroded and started to leak i= n the first place, when the last anode disintegrated? I asked. No answer to= that!
What do you think is going to happen next without the anode in,Onetap? The= rmostat tube bursts again? - SS cylinder develops leak ? Copper pipework in= rest of house starts to corrode and leak? =20
Devil and the deep blue, here. Put immersion + anode in - lasts maybe 2 mon= ths, needs new immersion+anode.(=A340) Don't replace it - themostat tube co= rrodes maybe 6months - replace (=A380). Aaahhhh!
It was an attempt to clarify the issue. You seem to...
...have missed the point though. The 200 mV (AC) to an arbitrary earth point is 'A' and won't cause the corrosion. Electrolytic effects that do, 'B,' are independent of 'A'. There are probably a few million cylinders that are a few volts AC above the local earth and that don't have corrosion problems.
A more general question for the panel might be "how common are corrosion problems in unvented cylinders?"
Good point, Andy - talking to a company of "Corrosion Experts" today and th= e guy there was asking me questions - why? It seems his mother had just for= ked out =A3800 on 2 plumbers to come and sort out corrosion problems with t= he immersion element anode on her ARISTON Unvented system! Coincidence? I t= hink not!=20
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