I’ve been reading so much about copper boiler making and materials that I thought I would pull a page together on the subject of Brass Fittings and Boilers. The research was based around the miniature traction engine boiler and how I design the fixing for the cylinder on the top of the boiler. My first design used a brass block silver soldered to the boiler and allowed the cylinder to then be bolted to it.
However, brass fittings should not be used in boilers due to the fact that the zinc in brass leaches out over time, the brass becomes porous.
Copper is slightly more electronegative than zinc. Thus, if you put the two metals next to each other, some electrons will move from the zinc to the copper.
This is the selective leaching of zinc from the brass alloy [Ref 1]. Conditions that might increase dezincification [Ref 1,2] are:
- Slightly acidic or slightly alkaline water
- High levels of chlorine in the water
- More than 15% zinc in the brass alloy
- Low flow rates and low levels of aeration
- Relatively high tube wall temperatures
It appears as a red spongy region on the surface of the brass and a white “meringue” bulky white deposit. The porosity can result in leaks and brittleness.
De-Zincification Resistant brass was developed for use in water systems and is specified in BS2872 and BS2874. Brass CZ132 is made to a carefully controlled specification and is dezincification-resistant brass that is used in water systems. Although this is used in water systems, after some searching I’ve not as yet found a reference for it’s use in copper boilers.
The general guidance and experience from the model making community is that all bushes and all boiler fittings should be made from phosphor bronze. If you are concerned about the machinability of phosphor bronze then you should use Colphos.
- Yaofu Zhang, “Dezincification and Brass Lead Leaching in Premise Plumbing Systems: Effects of Alloy, Physical Conditions and Water Chemistry”, Masters Thesis, Blacksburg, Virginia, 2009
- NACE International – https://nace.org/home
- European Copper Institute – https://copperalliance.eu/
- “The Periodic Table, and Why Batteries Don’t Work the Way You Think”, Scientific American
- Electronegativity Values – Wikipedia