Chimney collar cast iron or aluminum

[Joha]

Hello, 

 

What's your opinion about the chimney collar made from cast iron or cast aluminum please? 

I need a new one and both options on the market... 

 

Many thanks already, 

 

Jo

[Jen-in-Wellies]

Hello Jo

Welcome to CWDF.

If this is a collar for a solid fuel stove, then the flue gases are rather corrosive. Cast iron ones seem to last decades. Only seen one that had to be replaced after ten years from corrosion. Having dissimilar metals in contact is a recipe for corrosion. You could see the aluminium being eaten away where the fixing bolts (steel), or roof (steel), or chimney (steel, or stainless steel) touch it. Not see an aluminium collar in use that I've known is aluminium, so don't know how long they last in the real world.

Aluminium is easier to cast than iron, due to the lower melting temperature. An aluminium collar is cheaper to deliver than cast iron, due to the lower weight. This may have something to do with their presence in the market.

As you may have guessed by now, I'd probably go for a cast iron collar!

 

Jen

Edited May 26, 2019 by Jen-in-Wellies

[Joha]

Many thanks Jen. It makes sense and I'll follow your advice. 

All the best! 

 

Jo

[blackrose]

Mine is mild steel. It's just short section of pipe (8mm wall thickness) welded into the roof. Been there for 14 years and shows no sign of corrosion.

 

I say mild steel, but that's just an assumption. Perhaps they make steel pipe with something else?

[philjw]

5 minutes ago, blackrose said:

I say mild steel, but that's just an assumption. Perhaps they make steel pipe with something else?

A lot is cast iron. Does go up to 8mm wall thickness.

[blackrose]

19 hours ago, Jen-in-Wellies said:

Hello Jo

Welcome to CWDF.

If this is a collar for a solid fuel stove, then the flue gases are rather corrosive. Cast iron ones seem to last decades. Only seen one that had to be replaced after ten years from corrosion. Having dissimilar metals in contact is a recipe for corrosion. You could see the aluminium being eaten away where the fixing bolts (steel), or roof (steel), or chimney (steel, or stainless steel) touch it. Not see an aluminium collar in use that I've known is aluminium, so don't know how long they last in the real world.

Aluminium is easier to cast than iron, due to the lower melting temperature. An aluminium collar is cheaper to deliver than cast iron, due to the lower weight. This may have something to do with their presence in the market.

As you may have guessed by now, I'd probably go for a cast iron collar!

 

Jen

 

You must have dissimilar metals all over your boat Jen?

 

I know I do - apart from the paint and sealant they're bedded in on. I've got 4 big aluminium cleats that have been there for over a decade without any ill effects; anodised aluminium windows, brass mushroom vents, etc, etc.

 

Dissimilar metals in contact are only subject to accelerated galvanic corrosion in the presence of an electrolyte. So if there's a puddle of standing water that sits there then the less noble metal may corrode, otherwise (on the topsides of a boat) it's really not a problem.

17 minutes ago, philjw said:

A lot is cast iron. Does go up to 8mm wall thickness.

Ok thanks, I didn't know that.

Edited May 27, 2019 by blackrose

[Jen-in-Wellies]

10 minutes ago, blackrose said:

You must have dissimilar metals all over your boat Jen?

 

I know I do - apart from the paint and sealant they're bedded in on. I've got 4 big aluminium cleats that have been there for over a decade without any ill effects; anodised aluminium windows, brass mushroom vents, etc, etc.

 

Dissimilar metals in contact are only subject to accelerated galvanic corrosion in the presence of an electrolyte. So if there's a puddle of standing water that sits there then the less noble metal may corrode, otherwise it's really not a problem.

True. However, a solid fuel stoves drip really nasty acidic condensate down over the collar at times, so there is your electrolyte. You see the stains on the collar roof and sides of many boats. That is why I am particularly un-keen on the idea of aluminium in contact with iron in a stove flue system. Steel flue pipes have a limited life and steel single skin chimneys can rot through in only a year or two. Even stainless chimneys and coolie hats get killed off eventually. Putting dissimilar metals in to that system just seems to be asking for trouble. You can minimise the flue gas condensate run off with how you operate your stove and with a properly fitted double skin chimney, but I'd prefer a cast iron collar in the system.

 

Jen

Who picked up a degree in materials science a long time ago and still remembers a bit of the corrosion and electrochemistry lectures.

[dmr]

We have two aluminium chimney collars and I don't see any major corrosion issues but they do need some maintenance.

 

The paint (powder coating?) started to fail after about 10 years and the gunge from the stove started to give some corrosion. I fixed this after a couple of years and it was only superficial and removed easily with a wire brush. My first repair/repaint used etch primer and failed after a year or two. My second attempt used surface tolerant epoxy primer and is doing fine.

 

These are rather nice chimney collars with "Colecraft Engineering" in raised lettering and both have a cast aluminium wedge under them so that the chimneys are truly vertical.

 

.................Dave

[Jen-in-Wellies]

11 minutes ago, dmr said:

We have two aluminium chimney collars and I don't see any major corrosion issues but they do need some maintenance.

 

The paint (powder coating?) started to fail after about 10 years and the gunge from the stove started to give some corrosion. I fixed this after a couple of years and it was only superficial and removed easily with a wire brush. My first repair/repaint used etch primer and failed after a year or two. My second attempt used surface tolerant epoxy primer and is doing fine.

 

These are rather nice chimney collars with "Colecraft Engineering" in raised lettering and both have a cast aluminium wedge under them so that the chimneys are truly vertical.

 

.................Dave

Good to hear some actual real life and long term experience with these collars, rather than our speculation.

Jen

[dmr]

44 minutes ago, Jen-in-Wellies said:

Good to hear some actual real life and long term experience with these collars, rather than our speculation.

Jen

The only evidence of almost significant corrosion was between the head of the steel mounting bolt and the aluminium collar, but even this was noting too worrying. When I did the refurb I painted a biggish washer in epoxy to hopefully prevent this in future.

 

I general I think we worry too much about dissimilar metal corrosion, I did extensive stainless to mild steel repairs on my old VW camper and never saw a problem. Stick dissimilar metals in sea water and that's a different kettle of fish altogether.

 

Currently in Salthouse dock and note there are a number of long term narrowboats here now, I wonder how they get on?.

 

...........Dave

[Jen-in-Wellies]

7 minutes ago, dmr said:

The only evidence of almost significant corrosion was between the head of the steel mounting bolt and the aluminium collar, but even this was noting too worrying. When I did the refurb I painted a biggish washer in epoxy to hopefully prevent this in future.

The epoxy coated washer is a good idea to break the corrosion circuit. At least with any contact between roof and collar the collar will corrode in preference to the roof, but there is usually a nice thick layer of paint there as well as sealant. Starting to think these aluminium collars are not as bad after  my initial instinctive dislike of the idea.

 

Jen

[dmr]

40 minutes ago, Jen-in-Wellies said:

The epoxy coated washer is a good idea to break the corrosion circuit. At least with any contact between roof and collar the collar will corrode in preference to the roof, but there is usually a nice thick layer of paint there as well as sealant. Starting to think these aluminium collars are not as bad after  my initial instinctive dislike of the idea.

 

Jen

Maybe an ali collar makes a sacrificial anode for the boat roof .

 

The roof under the collar is a high rust risk area due to trapped water and heat, so getting a good seal is really important here. I have currently got a homemade neoprene seal under the back cabin collar as a trial, though we don't light the back stove very often. The main cabin stove collar is done with plumbers mate and this has lasted well. This collar is due for a refurb this summer so it will be interesting to see what the roof looks like under it. The back cabin roof had some significant pitting when I did that about three years ago.

 

................Dave

[Joha]

Many thanks for All for such an interesting discussion. It looks like both collars may be right choice if fixed properly. 

 

All the best. 

 

Jo

[system 4-50]

I got fed up with the iron flue on my stove because it stank when it got properly hot, something to do with the paint it came with, and it got stuck in the collar with rust, so I replaced it with a stainless steel one. This does not stink, but the stove cement between it and the iron stove no longer remains tidy but expands furiously like crystals growing in a tank.