I admit I should have checked this a year ago. Gas HWS in motor home.

The anode was  nearly all sacrificed, and caused a weird odor in the hot water.

Most anodes are cheaper on Ebay. Got to it time before tank corrosion sets in.

how old was your hot water heater?

We have had our van 2years I check the hot water Anode regular ,it is like new, we had 4weeks away last year and 3months away the year before a trip around the block, i think the reason for the anode being so good all our water has been put through a twin water filter then into the water tanks of the van, I may be wrong but that is the way I see it.

Lance C

Phil C wrote:

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Gday folks

Being ex RAAF I have a bad/good habit of preventative maintenance (some call it OCD)

Every 6 months I check the anode irrespective of the water quality. Most times that check will be OK and the next check will be 3 months after that. Our anodes are from ebay as well. Found the aluminium anodes last longer than the magnesium, I believe thats something to do with water hardness.

Be safe and dont do dumb stuff..

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 Hi Phil 

Sometimes lasting longer is  not a good thing

Magnesium which are normally fitted by the makers give best protection for normal town quality water

Aluminium are an option with highly mineralised water & give longer life than Magnesium in that situation, but even then Magnesium will give the best protection

in other words Aluminium will not give good, let alone best  protection with low mineral content water

The makers do know best

PeterQ

Olley46 wrote:

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We have had our van 2years I check the hot water Anode regular ,it is like new, we had 4weeks away last year and 3months away the year before a trip around the block, i think the reason for the anode being so good all our water has been put through a twin water filter then into the water tanks of the van, I may be wrong but that is the way I see it.

Lance C

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Hi Olley

It is dissolved minerals in the water that affect anode life, not so much organic matter unless it produces acids

If your filter only remove organics [not a chemical filter] it is more likely that you are just using good quality low mineral content water

PeterQ 

My chemistry is a little rusty (I forgot to change the anode), but here are the electrode potentials for iron, magnesium, and aluminium:

http://en.wikipedia.org/wiki/Table_of_standard_electrode_potentials


Mg²⁺ + 2e^-  Mg(s)     -2.372V

Al³⁺ + 3e^-  Al(s)     -1.662V

Fe²⁺ + 2e^-  Fe(s)     -0.44V

Fe³⁺ + 3e^-  Fe(s)     -0.04V

Ta Dorian, not a huge difference between magnesium and aluminium but still lower than iron/steel

Is that the triboelectric series? Im so rusty with all that stuff now.

Cheers

Phil C wrote:

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Is that the triboelectric series?

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Triboelectric charging occurs when you rub one material with another, ie friction.

Electric cell potentials are a galvanic or electrochemical effect, as in a battery.

I've been trying to understand the specs for HWS anodes, but I can't make sense of them.

Here are two anodes:

http://www.anodesonline.com.au/products-page/rv-ho****er-anode

Aluminium RV Anode CDAR-A5-17-240A
Primarily used in hard water, where total dissolved solids (TDS) are between 600-2500mg/L
Exposed Length: 210mm
Overall Length: 240mm
Anode diameter: 17mm
Electrochemical Potential: -1.00 to -1.05V (with respect to Cu/CuSo4 Reference Electrode)
Theoretical anode consumption rate in hot drinking water: 6kg/A.year

Magnesium RV Anode CDMR-M2-21-0240-A
Primarily used in soft water, where total dissolved solids (TDS) are between 40-600mg/L. 
Exposed Length: 210mm
Overall length: 240mm
Diameter: 21mm
Electrochemical Potential: -1.45V (with respect to Cu/CuSo4 Reference Electrode)
Theoretical anode consumption rate in hot drinking water: 6kg/A.year

The first thing that bothers me is that the difference in the electrochemical potentials between the two anodes is about 0.45V. The Wikipedia page suggests that the difference should be 0.71V (= 2.372V -1.662V). I realise that Wikipedia is using a "standard hydrogen electrode" as its reference electrode whereas the other site is using a Cu/CuSo4 Reference Electrode, but I would have thought that the difference between Al and Mg would have remained the same.

The other thing I don't understand is the identical figure for anode consumption rate. IIUC, the spec is saying that a continuous current of 1A over a period of a full year results in the loss of 6kg of anode material.

Here are my calculations:

If a current of 1 ampere flows for one year, then the total amount of charge transferred is 1 ampere.year.

To determine how many electrons have been transferred, we need to divide the above result by 1 "elementary charge", ie the charge carried by 1 electron (= 1.6022 x 10-19 coulombs).

Magnesium and aluminium atoms have valencies of 2 and 3, respectively. This means that they have 2 and 3 electrons in their outermost shells, and that they release 2 and 3 electrons when they are ionised. Therefore, to determine the number of atoms that have been ionised, we divide the above result by 2 or 3, as appropriate.

To determine the mass of anode material that has been consumed, we need to multiply the above result by the atomic mass of each atom. This is 24.305 amu for magnesium and 26.98 amu for aluminium.

http://www.google.com/search?q=1+ampere+x+1+year+%2F+%281+elementary+charge%29+%2F+2+x+24.305+atomic+mass+units+in+kilograms

http://www.google.com/search?q=1+ampere+x+1+year+%2F+%281+elementary+charge%29+%2F+3+x+26.98+atomic+mass+units+in+kilograms

The results are 2.94kg for aluminium and 3.97kg for magnesium.

Could it be that the valance electrons are in a higher energy level? but it makes sense because magnesium gets eaten quicker than aluminium as PeterQ suggested. Then again I may be very wrong, I havnt even thought about quantum mechanics for donkies ages.

BTW the two anodes you have compared are different diameters Mg 17mm Al 21mm could be the cause of the quandary.

Love your research Dorian, my mind is having fun picking this over.

Cheers

Hi AK

Actually they are each, very basic primary cells [Electric battery]
The anode is the POS terminal,
The steel shell is the Neg
The pos & neg output is a closed circuit via the anode thread
The water is the electrolyte
With pure water very little current will flow .

As  Dorian has said Galvanic action between dissimilar metals

Remember that is also what most corrosion is
PeterQ 

-- Edited by oldtrack123 on Wednesday 5th of November 2014 08:30:30 PM

It was made in 1996 Blue.I remember checking the anode and replaced it 4 years ago when I had

the gas igniter electrode replaced at the same time when the heater did not ignite.The heater works ok,

no leaks or rusty water, flushed and refilled the smell is gone (wonder what caused that).

It's now in the maintenance book, no note and I forget these days.

"Q"...Wonder if there is a difference between gas heaters and electric heaters with elements regarding

the effect on the anode. Does the element create electric current or is it not relevant at all ? The service expert told me there

is also an electronic charged anode available, and needs to be wired in. I stick with the original, less problems I think.

 Cheers and many thanks for the advice. 

(It's not old age, it's absent minded)

-- Edited by Vanderee on Wednesday 5th of November 2014 09:04:28 PM

-- Edited by Vanderee on Wednesday 5th of November 2014 09:18:43 PM

Thanks Dorian. I am reading all that again, amazing research.

Brain box is a bit slow. Something will stick .

Thanks.

(It's not old age, it's absent minded)

Vanderee wrote:

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Thanks Dorian. I am reading all that again, amazing research.

Brain box is a bit slow. Something will stick .

Thanks.

(It's not old age, it's absent minded)

 

---

basically Vanderee..

if I can put it quite simply.. 

water heaters are a ticking time bomb.. (so to speak)

depending on variables..

the arse is going to rust out on it when the anodes have been eaten away and most folk will just blindly go out and buy another one.. whether it be for the van water heater or the one at home..

dorian may find the Australian Science forum more challenging... not that we don't enjoy his conversation on here...

Vanderee wrote:

---

It was made in 1996 Blue.I remember checking the anode and replaced it 4 years ago when I had

the gas igniter electrode replaced at the same time when the heater did not ignite.The heater works ok,

no leaks or rusty water, flushed and refilled the smell is gone (wonder what caused that).

It's now in the maintenance book, no note and I forget these days.

[1]"Q"...Wonder if there is a difference between gas heaters and electric heaters with elements regarding

the effect on the anode.

[2]Does the element create electric current or is it not relevant at all ? The service expert told me there

[3]is also an electronic charged anode available, and needs to be wired in. I stick with the original, less problems I think.

 Cheers and many thanks for the advice. 

(It's not old age, it's absent minded)

 

-- Edited by Vanderee on Wednesday 5th of November 2014 09:04:28 PM

-- Edited by Vanderee on Wednesday 5th of November 2014 09:18:43 PM

-- Edited by Vanderee on Wednesday 5th of November 2014 09:26:14 PM

---

HI

[1]Gas Or electric makes no difference, it is how/ what the tank is made o,f that counts

Some tanks are Stainless steel , & do not need anodes copper tanks do not need anodes

Steel tanks with an internal coating to protect the steel[ often referred to as glass or ceramic lined] do need anodes

The lining may have or develop small pinholes . these are a source of corrosion if not protected by an anode

[2] the electric element is sheathed & the sheath is connected to the tank,  no potential difference can exist

[3]Electrically supplied low voltage[impressed voltage] is another form of protection, it replaces consumable anodes but requires careful control

These methods are generally referred to as "Cathodic Protection"

PeterQ

Phil C wrote:

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I enjoy the way Dorian thinks, My mind has been challenged and some old facts (or fiction) have been recalled. Thanks Dorian.

It also seems odd that and anode provides cathodic protection... just a play on words lads...

 Cheers all

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 Hi Phil

It does seem a little confusing ,but it is the correct term ,as used in the industry[Another line that I was involved with]

It is the cathode [ heater shell]that is being protected.

Anodic protection is actually the reverse

The part to be protected is made positive  & only used in special circumstances using applied voltage /current

One use is for the protection of tanks containing highly caustic materials

Any applied voltage system [can be used for both Cathodic & Anodic protection] needs carefull current control &monitoring

Stringent rules apply, where it can interfere with Mains systems[earthing]

PeterQ

-- Edited by oldtrack123 on Thursday 6th of November 2014 01:24:29 PM

Aus-Kiwi wrote:

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So electrolysis has nothing to do with it ?

---

 Hi

Electrolysis is a different process

Simple expanation ,used for breaking down material into it's seperate chemical

components or elements

e.g

Electrolysis of water [H2O ]gives 2 hydrogen atoms  & one  oxygen atom,per molecule of water

Actually chargeing a lead acid battery is a sort of electrolysis process [A chemical reaction by using electric current] 

PeterQ

AFAICT, AS2239-2003 is the primary Australian standard for sacrificial anodes. Unfortunately I haven't been able to locate a free copy (I refuse to pay $106 for 34 pages).

Here is a document summary - "Galvanic (sacrificial) anodes for cathodic protection":
http://www.saiglobal.com/pdftemp/previews/osh/as/as2000/2200/2239.pdf 

The following paper has some relevant information, especially as it relates to Australia:
http://www.cathodicanodes.com.au/copy/pdf/When%20Anodes%20Ain't%20Anodes.pdf

A Magnesium High Potential anode (AS2239-M1) is specified for those locations where the Total Dissolved Solid (TDS) Content is 0 - 40 mg/L. 

A Magnesium Low Potential anode (AS2239-M2) is specified when the TDS content is 40 - 600 mg/L. 

An Aluminium AS2239-A5 anode (AS2239-A5) is specified when the TDS content is 600 - 2500 mg/L. 

In the case of Rheem, M1 anodes have green caps, M2 anode caps are orange or black, and A5 caps are blue.

The anode material is an alloy, not a pure metal. AS2239 stipulates that M1 magnesium anodes must contain around 1% manganese. M2 anodes must contain approximately 3% aluminium, around 1% zinc and manganese, and 0.04% calcium. A5 anodes must contain approximately 4.5% zinc, and about 0.15% tin.

Magnesium anodes have an efficiency of only 50% whereas aluminium anodes are 90% efficient.

The theoretical consumption rate of a magnesium anode is 2200 Ah/kg. This equates to 3.98 kg/Amp-year, which is close to my calculated result.

The density of aluminium is 2.7 gm/cc and for magnesium it is 1.74 gm/cc.

The volume of an anode (including the steel rod in the centre) is given by ...

volume = pi x radius squared x length

The mass of the anode material is given by ...

mass = density x volume

So the mass of aluminium and magnesium in the RV anodes is 129 grams and 127 grams, respectively (assuming the steel rod is not significantly massive). [Sorry, my initial calculations were incorrect and have been edited.]

http://www.google.com/search?q=pi+x+%2821+%2F+2+mm%29+%5E2++x+210+mm+x+1.74+gram+per+cc+in+grams
http://www.google.com/search?q=pi+x+%2817+%2F+2+mm%29+%5E2++x+210+mm+x+2.7+gram+per+cc+in+grams

Aus-Kiwi wrote:

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Similar as used on boats, radiators in cars.. Any two metals being heated gives off electricity..
Alloy 

 

 

Thermocouples operate on a different principal to  cathodic or anodic protection which does not require heat.

 

Thermocoples work on what is referred to as the "Seebeck effect"

You require a hot junction of metals sufficiently different on the scale & a cold junction for a very small voltage with a very low current.

A range of dissimilar metals are used

Depending on temp range from a iron< copper [low temps] hot junction, to a platinum/ irridium junction for high temps, up to around 1400C[ from memory]

Care must be taken with connecting cables that they do not, in thermselves ,form additional HOT junctions which could add or subtract from the true reading of the measured point.

Personally, have made up many special  thermocouples for industries where accurate measurement, control  & recording was required.

Installed & serviced the  temp control, & recording equipment.

 

 

PeterQ

 

Phil C wrote:

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Cheers PeterQ, I was just playing with the words Anode and Cathode....

So cathodic protection with an anode.. Oh well it made me giggle...

Cheers mate.

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HI Phil

Did you note this from Dorian's post

"Here is a document summary - "GALVANIC[SACRiFICIAL]ANODES FOR CATHODIC PROTECTION":

Regards

PeterQ