The Knowledge - Velocette Owners Club

long period of time ?? This is most apparent in some OLD brass articles, & while the original hardness
caused in say sheet brass , while it was being spun or worked, remains forever I guess, - unless
softening is attempted, & I am pretty sure that this would be disastrous on articles getting on to 100
years old ? This was most apparent recently when Mark Gibson presented me with an acetylene gas
generator as a repair 'challenge' This article was at least 90 years old I should think, & the water tank
part of it was full of the typical 'splits' in the metal that is apparent in old worked brass articles, -
especially something that has been work hardened by 'spinning'. In this case though, one side of the
tank pretty much fell apart in little bits of brass 'honeycomb' the moment I started to smooth the dents,
& the reason was certainly that the brass was nothing more than frail 'zinc removed' sheet, typical of
electrolysis damage. This plus other experience with old brass, leads me to accept that the electrolytic
process can destroy brass over the years, simply through the action of changing temperatures, - the
local presence of suitable metal, (stored in a steel container ?) & the action of moisture in the air ? this
of course only being apparent in very old articles. Maybe Richard can comment on this ? As for the
repair challenge, it was carried out successfully, but only after light gauge sheet copper (easily
available) was patched on the inside of the destroyed part of the tank after smoothing & cleaning, &
another 'patch' cut to bring the lost section back to the original contour, & the whole thing sweat
soldered back together, with clamps & such holding everything into place. Pic available if you are
interested)
#4584 You've got me delving back into the dim past with this. We are now entering the realm of theory
with a bit of guesswork. Like you I'm familiar with electrolytic corrosion. The problem you outline with
old sheet brass items is somewhat different. The reference is 'Engineering Metallurgy Part 1' by R A
Higgins 1964 - this has been a useful book over the years. The subject is 'Cold -working' brasses. The
most ductile brass is high purity 70% copper, 30% zinc. (Even small levels of impurity lead to a marked
loss in ductility). It's commonly referred to as 'Cartridge Brass' or 'Deep drawing' brass and this would
be ideal for making items such as acetylene generators. For info only, such brasses are rather sensitive
to annealing temperatures and it's easy to 'burn' the alloy. (This trade term should not be confused with
oxidation. It is used to signify overheating, the result of which is rapid grain growth that on subsequent
working/pressing shows up as an 'orange peel' effect on the material surface). The problem that I
suspect you have experienced however is due to the fact that hard-drawn brasses are subject to
'season cracking'. Higgins says 'In such brasses, any corrosion which takes place is usual
intercrystalline, so that cracks will ultimately arise, as internal stresses are relieved, by fracture at the
relatively weak grain boundaries. This effect can often be seen in old brass electric-light switch-covers,
particularly when they have been subjected to corrosion in a damp atmosphere. Season cracking can
be avoided by giving the component a low temperature, stress relief anneal at about 250oC., after
fabrication'. In addition, somewhere in the back of my mind, I'm aware that acetylene gas has some
pretty corrosive effects on certain metals but hopefully someone can remind us.
#4585 The process you refer to in brass is known as 'de-zincification' and commonly wrecks water taps
& valves in hard water areas. The zinc is removed by electrolytic action leaving holed brass which is
also weaker. In water installations it was the case that a sacrificial anode was provided. Better alloys
are the choice today - unless you happen to have cheap taps!
#4586 Thanks chaps, it seems that I was basically correct, & the fact remains that any brass, especially
that of high zinc content is prone to this problem in the presence of certain factors. A lot of years is
probably a big factor ! The fact is that we as 'ooman beens' can only experience these happenings over
a short space of time in the big picture, & very few of us have the opportunity to actually note what
really happens to metals of all kinds eventually. One would expect that in the case of brass, in the end
after long periods of time, all that would remain is some form of copper residue, as we have been
taught that copper is one of the things that never really gives up ! I had one experience with marine
propellers where extremely expensive props were hooked to 2 Rolls Royce marine engines & fitted to a
70 foot steel boat, which was built privately in a yard close to me, & I had mentioned to the builder that
he would need to buy & fit decent sacrificial zinc anodes to the back of the hull. He was Dutch & had
previously built timber boats, & intimated that if you fitted 'decent' props, as he had done, the zinc was
unnecessary ! The boat was launched into the sea, & some Summer cruising enjoyed, the boat lay in
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