brown versus black: The Big Fight...Live!

[bster]

Power flushed a system today and when dumping it was a very thick mid brown (jet brown as i've heard it described before ) colour and not your 'average' black death colour.

so what makes the CH water very, very brown opposed to very, very black.

My theory is that initial stages of corrosion reslut in the black magnatite sludge and that the brown colour is a result of further corrosion of the magnatiite itself. This is based on my experiment - I placed a magnet under the outlet of the powerflush dump hose and it did not attract as much metal compared to a 'black' system the week before.

 

[pannierstan]

get two glass bottles and throw a nail in each. Then fill one with water devoid of oxygen and the other with tap water

You should find that one will go black and the other brown.

Black sludge is just corrsion devoid of oxygen.

Brown sludge is usually from a system that is getting air. Usually poor configuration(pumping over or dragging in air)
Hope you were reconfiguring if it was an open system.

 

[bster]

the system was previously open vented but the system was updated (this is a guess) 10 years ago with a sealed system (combi) so, in theory, there should have been no air within the system yet it was as brown as jet brown gets?

 

[Agile]

Current oxidisation is reddish, recent is brownish and it goes black in 3-9 months.

Its useful to know that when diagnosing faults!

Tony

 

[bster]

Agile - Pannisar is still correct about the oxygen thing as well?

 

[chrishutt]

bster, have a look at this if you really want to know about iron oxides.

Broadly speaking, oxides with lots of Oxygen (ferric) are the reddish colour and oxides with little Oxygen (ferrous) are black, as has been explained. If you pour some black sludge from a heating system onto the ground and leave it, it will react with the Oxygen in the air and turn to the familiar rust colour.

Many people on this forum refer to the black oxides from heating systems as Magnetite (ferrous ferric oxide) and that is characteristic of underwater corrosion, but is only one of many oxide forms that appear black. I would expect Oxides in heating systems to include more complex forms due to the abundance of water, perhaps with hydroxides and hydrates present.

 

[Agile]

Its even more complicated because much of the very thin black deposits in boilers and the inside of coper pipe are a copper oxide.

Dissolve it in sulphuric acid and you will get the familiar blue/green copper sulphate colour, add salt solution and you will get a whitish green precipitate of copper chloride.

The black on Stan's nail is not due to water totally devoid of oxygen but due to water with less oxygen.

Its less relevant knowing the exact chemical aspects then being able to determine if the problem is current, recent or old. Then of course how best to deal with it !

Tony

 

[ChrisR]

There's calcium on there from the tap water, chlorides from fluxes, and whatever if any inhibiting chemicals too. Inhibitors eg are designed to passivate surfaces with molybdates. That will include the "rust" to varying degrees (using up the inhibitor). Some cleaners "flocculate" the particles by attaching to them and getting them into suspension.
I don't know what they all look like, but varying shades of mud colour would be a good bet I expect!

Fresh oxidation is quite a "bright brown" colour.
One inhibitir, I think it's Fernox MB-1, gives a definite brown colour to the sludge.

 

[Fluffster]

Anyone tried putting a flame to an open bleed screw in the dark??
Does not bode well if you have net curtains next to it lol.
Pete

 

[Fluffster]

Anyone tried putting a flame to an open bleed screw in the dark??
Does not bode well if you have net curtains next to it lol.
Pete