Sacrificial anode in central heating system

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Despite having excess Fernox in my C/H system and no black sludge visible when I drain off a little water from any radiator, I still get hydrogen* forming which I understand is indicative of corrosion. (*blue flame when lighting the gas during venting of the rad).

Would it be of any benefit to install a sacrificial anode in the pipework e.g. the Scalemaster corrosion master, or would that actually do the reverse and harm the system?

Any advice gratefully received.

Thanks.
 
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You'd be better off finding out why you're getting corrosion - it's almost certainly not electrolytic. If your rads are cold at the bottom they've got sludge in them. Take one off the wall and run a hosepipe through it - I think you'll see plenty of the black stuff coming out then. Is it an open vented or a sealed system?
 
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A few things to bare in mind that can cause excessive gas in the radiator system:
Boiler kettling.
Boiler over sized for the system.
Boiler not calibrated correctly for the system.
Boiler thermostat set too high.
Poor plumbing arrangement, not naturally venting.
Auto air vents if fitted not working.
Blockage/restriction in air separator (if fitted).
Excessive corrosion (which you seem to have dealt with)
 
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Despite having excess Fernox in my C/H system and no black sludge visible when I drain off a little water from any radiator, I still get hydrogen* forming which I understand is indicative of corrosion. (*blue flame when lighting the gas during venting of the rad).

Would it be of any benefit to install a sacrificial anode in the pipework e.g. the Scalemaster corrosion master, or would that actually do the reverse and harm the system?

You've got some already; you call them radiators. The steel is less noble than the copper on the electrolytic series/galvanic series, so the steel inner surface of the radiator corrodes, protecting the copper.

https://en.wikipedia.org/wiki/Galvanic_corrosion

It is galvanic/electrolytic/ bi-metallic corrosion, the hydrogen is released by electrolysis of the water, the oxygen forms corrosion sludge with the inside of your radiators.

A sacrificial anode is nonsense, concentrate your thinking on the electrolyte, the water.
Flush the system thoroughly to remove any acidic flux residues and (some of) the magnetite sludge corrosion products. Refill, add inhibitor.

If you're topping up the expansion vessel regularly (sealed system) it's leaking. If there's an F&E tank, check it's set up properly (1 or 2" of water above the Feed outlet when cold, the empty space is for Expansion), and check it doesn't pump over.
 
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Exactly what "Fernox" do you have in your system?

How do you know there is excess of it?

Tony
 
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It is fairly rare, but corrosion in a CH system can be cause by earth currents flowing in the pipework usually as a result of a incorrect or faulty house wiring.

But that would usually be a last resort to diagnose by measuring earth currents with a clamp meter.

A now retired heating engineer used to tell us of a new block of flats near Bristol that kept corroding all the rads which were replaced several times as no one could diagnose the problem.

Tony
 
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Thanks to all who responded to my post. I am determined to get to the bottom of this problem. Rather than reply to each individual post, I have answered you all in one shot!


Muggles: thanks for your response. All radiators are uniformly hot all over except the highest one, which is the towel rail in the upstairs bathroom. The top rung is usually cold. I have not taken a radiator off the wall to flush it but I have hammered one on the bottom edge with a rubber mallet when draining and there is no black sludge coming out.


Flying Fish: thanks for your response. Please see my responses to your points:

Boiler kettling. – I have never witnessed this.
Boiler over sized for the system – we have a 25kW wood pellet boiler in a 4 bedroomed detached dormer bungalow. The boiler has full modulation and typically ticks over at 5kW - 10kW.
Boiler not calibrated correctly for the system – I don’t know what you mean by this, sorry?
Boiler thermostat set too high – boiler thermostat is set at 65C.
Poor plumbing arrangement, not naturally venting – quite likely a "yes" to this one as the system is very prone to air locks when refilling after doing any work on the system. Hence auto air vents are fitted and do work. They are now only on the flow, not on the return, as we found that opening them on the latter caused air to be drawn into the system.
Auto air vents if fitted not working – see above.
Blockage/restriction in air separator (if fitted). I have recently bought a Spirovent deaerator to try and see if that helps. Not fitted yet.
Excessive corrosion (which you seem to have dealt with) – yes, excess Fernox in the system already (I know because I have the test kit).


OneTap - thanks for your response. Why is a sacrificial anode a nonsense? I thought by choosing a metal which has more negative electrochemical potential than the other metals then it will protect them all e.g. hence use of aluminium or magnesium as sacrificial anodes? I have a vented system and the water level in the F&E tank is indeed about 2 inches above the feed outlet when cold and there is no pumping over. Believe me, this has been checked many times!


Tony – thanks for your response. I use Fernox F1. I know I have about the double the required amount because I have the test kit to measure the concentration. How would I go about measuring earth currents? Do you mean putting a multi-meter between the radiator and an earth connection to see if there is any current flowing? With all the earth bonding I have, wouldn’t any electrical current be earthed anyway so there’d be nothing to measure?
 
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Why is a sacrificial anode a nonsense? I thought by choosing a metal which has more negative electrochemical potential than the other metals then it will protect them all e.g. hence use of aluminium or magnesium as sacrificial anodes?

They're meant to corrode away, hence sacrificial, you get the corrosion products in the water. The obvious example is galvanized (zinc coated) corrugated iron, or galvanized buckets.

Separate replaceable sacrificial anodes were used on sectional steel water storage tanks, steel water heaters, steel pipelines, steel ships, etc., where the corrosion products wouldn't cause a problem.

I can recall looking into a big sectional steel Braithwaite tank and seeing the magnesium sacrificial anode, the size of a football, sitting amongst a small pile of corrosion debris. You don't want loose solid corrosion products in a closed circulating system, they will cause problems with the pump, and possibly the boiler when they move around the system. I think (not entirely certain) that sacrificial anodes are now prohibited in potable water systems by the Water Regulations; certainly all the large enamelled steel water heaters have changed over to electronic corrosion protection devices.

The most common cause of galvanic corrosion is acidic soldering flux residues. Your radiators are already acting as sacrificial anodes; that is not a good thing.

I don't think any of the big water treatment companies (Fernox, Sentinel, etc.,) offer a sacrificial anode device for heating systems. Why do you think that might be?
 
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speak to kamco, they have said in the past they can make up an inhibitor for gasey systems on request, maybe helpful to you.
 
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speak to kamco, they have said in the past they can make up an inhibitor for gasey systems on request, maybe helpful to you.

BigSnoopy01 - thanks for your response. This is from the Kamco website:

"Occasionally we hear of a system which continues to gas after every thing else has been tried, and for no plausible reason. Chemists term this ‘runaway’ corrosion and fortunately it is rare. In such cases, we can formulate for the engineer a special version of Systemsafe DM inhibitor, with a much higher than normal level of "yellow metal inhibitor", and this often stops the unusual corrosion by filming out on copper, brass and bronze, and effectively suppressing any galvanic corrosion."

I'll give them a ring in the morning. Thanks for the tip.
 
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They're meant to corrode away, hence sacrificial, you get the corrosion products in the water. The obvious example is galvanized (zinc coated) corrugated iron, or galvanized buckets.

Separate replaceable sacrificial anodes were used on sectional steel water storage tanks, steel water heaters, steel pipelines, steel ships, etc., where the corrosion products wouldn't cause a problem.

I can recall looking into a big sectional steel Braithwaite tank and seeing the magnesium sacrificial anode, the size of a football, sitting amongst a small pile of corrosion debris. You don't want loose solid corrosion products in a closed circulating system, they will cause problems with the pump, and possibly the boiler when they move around the system. I think (not entirely certain) that sacrificial anodes are now prohibited in potable water systems by the Water Regulations; certainly all the large enamelled steel water heaters have changed over to electronic corrosion protection devices.

The most common cause of galvanic corrosion is acidic soldering flux residues. Your radiators are already acting as sacrificial anodes; that is not a good thing.

I don't think any of the big water treatment companies (Fernox, Sentinel, etc.,) offer a sacrificial anode device for heating systems. Why do you think that might be?

I hadn't considered solid particles of the anode floating about - fair point. I have tested the pH of the fluid in the C/H system and it is 7 i.e. neutral as opposed to acidic. Is it possible that there are localised "clumps" of flux in the rads so that although the overall pH is neutral, there could be "hotspots" of lower pH? I should add that there hasn't been any alteration to the system in the last 2 years - wouldn't any flux residues have been fully dispersed in that time though?
 
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I should add that there hasn't been any alteration to the system in the last 2 years - wouldn't any flux residues have been fully dispersed in that time though?

Why? If it hasn't been flushed, where would they go?

Most domestic corrosion inhibitors contain sodium molybdate and are slightly acidic. SFAIK, the advantage of the molybdate is that you can't overdose the system (within reason) and you don't need a test kit. I don't know the theory about how it works as a corrosion inhibitor.

The commonest commercial treatment regime used to be to keep the water alkaline at about 8 or 9 pH and use an oxygen scavenger, such as sodium sulphite at about 50 ppm, to absorb any dissolved oxygen before it corrodes the ferrous components The scavenger concentration needs regular checking with a suitable test kit (£!). The rate of the oxygen scavenger depletion indicates the rate at which oxygen is getting into the system.

Keeping the water alkaline stops galvanic corrosion dead. You could demonstrate that to your self with a bucket of water with a bit of copper tube and a bit of steel pipe immersed and connected through a multi-meter set to measure mA. With tap water there will be little or no potential difference/current.

Making the water acidic (few drops of lemon juice, vinegar, HCl, whatever) will cause an increase in the current. The current is stripping ions from the steel, corroding it. The more acidic the electrolyte, the bigger the current. If you had wanted to generate a current, you'd use the strongest acid you could get (as in car batteries; Breaking Bad reference here).

Active flux contains ammonium hydroxide (I think) that changes to hydrochloric acid on heating; soluble, instant conductive water/electrolyte, instant recipe for bimetallic corrosion.

Making the water slightly alkaline (caustic soda, baking soda, whatever) stops the current.

... wouldn't any flux residues have been fully dispersed in that time though?

PS I just realised what you'd meant by that. The corrosion is not caused by the acidic compounds dissolving the steel, which would deplete the acid. It's caused by the acidic compounds making the water conductive, completing a bi-metallic corrosion cell and allowing galvanic/bi-metallic corrosion to start. That will continue indefinitely.
 
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