News and Views from the
TYNESIDE SOCIETY of MODEL and EXPERIMENTAL ENGINEERS
Since the easing of restrictions earlier in May, there has been a welcome return of activity to the Club. The weather has been mostly kind – a bonus as things must still happen outside (at least for those who follow the rules !) It still seems possible that a full lifting of restrictions will happen after the 21st June.
Former TSMEE member Mark Croucher presented Chairman Peter Newby with a painting he had done some years ago depicting Jim Stephenson and others busy in the steaming bays. Note the exemplary social distancing !
A framed copy, printed on canvas, will be made and presented to Jim’s family and the original will be hung in the Clubhouse.
Members Musings …
Spot the difference
Jim Scott (continued from IM May 2021)
The voice from the kitchen…. “Jim, the kitchen tap is sweating” Sweating ?
Sweating? And so it was – tiny water droplets miraculously appearing through the chrome-plated surface of the swan-neck spout close to the bottom, slowly growing then running down into the sink. Gentle probing with a tooth-pick resulted in a greatly enhanced flow – could this be porosity in the casting, I wondered?
I soon realised that it wasn’t a casting but a thin walled tube, formed into a “D” section. But it was a brass tube, so not prone to corrosion. Nor was it exposed to hot water for very long periods so that should have been OK, shouldn’t it ? Anyway, a new tap was purchased and fitted and the leaking one put in the workshop pending destructive examination.
Eventually, and under pressure not to accumulate yet another piece of useless clutter, I made time to have a closer look at the problem. The outer surface of the spout showed that there were many small discoloured areas in the chrome finish, which I admit I had noticed before but attributed to over-enthusiastic cleaning by the kitchen supervisor.
Application of the toothpick to other suspect areas also produced holes so the next step was to see what it looked like from the inside. The spout was cut into several pieces using a junior hacksaw and the cut faces ground smooth then lightly etched in ferric chloride to show the extent of the attack. The “D” shape of the tube was also clear to see and the wall thickness measured as 1mm (0.040”).
The cause of the problem was now obvious – the internal surfaces had multiple areas of bright pink porous copper, each surrounded by a ring of white powdery material (zinc oxide), with the unaffected areas retaining their original drawn-brass finish. A section taken through the pink areas showed that many extended right through to the surface, effectively a hole capped by a thin layer of chromium plate.
I think most folk will know that the process at work here is dezincification, where zinc is leached from the brass alloy leaving behind a very weak and porous copper matrix. Eventually the porous copper will collapse with consequent escape of water or steam, potentially disastrous on a pressurised vessel such as a boiler. The process is well understood and can be avoided at the design stage by specifying resistant brasses containing less than 15% zinc, or those containing inhibiting additions such as arsenic. Typical composition of yellow brass is 70% Cu/30% Zn.
Summary of key points regarding dezincification
(As distilled from several “Big Boy’s Books of Corrosion/Metallurgy”).
Dezincification is an electrochemical reaction between the zinc content of the brass and chemical content of water in contact. The zinc is selectively removed from the brass alloy leaving behind a porous, copper-rich structure that has little mechanical strength. The zinc may be re-deposited locally as zinc oxide powder. Dezincification should not be confused with stress corrosion cracking or erosion/corrosion.
Brasses containing less than 15% zinc are usually immune from attack. However alloys with up to 35% Zn can also be made resistant by including inhibiters (eg arsenic) in the alloy during manufacture or by heat treatment (eg forging or annealing).
There are two distinct types of dezincification; “uniform area attack”, where the whole exposed surface is affected, and “plug”, which causes localised pitting.
Uniform-layer attack leaches zinc from a broad area of the surface. This is likely to occur under the following conditions:
– Water with high levels of oxygen and carbon dioxide.
– Stagnant or low flow rates.
– Slightly acidic water, low in salt content and at room temperature.
– Soft, low pH and low mineral water combined with oxygen, which forms zinc oxide.
– Waters with high chloride ion content.
Plug-type dezincification is localised in small areas with surrounding surfaces mostly unaffected. The pitting penetrates deeply into the wetted surface and may cause perforation of a valve or pipe fitting. This is likely to occur under the following conditions:
– Neutral or alkaline waters, high in salt content and at or above room temperature.
– Water with high levels of oxygen and carbon dioxide.
But what caused the taps to fail?
The direct cause of the failure was plug-type dezincification acting over a period of 15-20 years, with the spout probably manufactured from commercial 70:30 brass tube. This tap design does not provide separation of the hot and cold flows right to the outlet, ie the two flows are mixed right at the base, leaving a permanent “spout-full” of standing water above the valve. This amounts to perhaps 150ml and may have some bearing on the chemistry of the water in contact with the inner surface of the spout.
The reason plug -type dezincification occurred rather than uniform area type is probably due to the significant differences in the operating conditions noted in the summary above, specifically pH value, dissolved salt content and operating temperature.
With plug-type attack it is unlikely that even doubling the wall thickness of the spout to 2mm would have prevented the eventual failure, although it may have increased the service life by a few years.
The implications for Model Engineers
The use of brass in boiler fittings is the main area of concern, given the safety implications of a failure under steam. However it took many years of constant immersion before the taps failed and given that copper boilers are usually completely blown down after use the “wet” time is relatively short. The much higher boiler temperature may be a significant accelerator but even so it could be concluded that if the boiler is always stored dry the risk of dezincification affecting brass boiler fittings must be fairly small.
The design specification/drawings will indicate the materials to be used in boiler construction, including such items as bushes which, when silver-soldered in place, form a permanent part of a copper pressure vessel. Phosphor Bronze (PB102) is the usual choice for this purpose, being unaffected by dezincification or silver soldering and having superior mechanical properties to brass. Bronze or Gunmetal (SAE660) may be used for boiler fittings but I note that there are many commercial items available apparently manufactured from brass. Given that the fittings are removable, inspectable and replaceable it would seem that this has become an accepted practice which Boiler Inspectors will no doubt take into account when conducting hydraulic and steam tests.
For Model Engineers making their own boiler fittings it is considered good practice to use bronze or gunmetal, especially for those items located below the normal water level.
I thought the members may be interested to see my old Drummond. It was originally a treadle lathe but converted to electric. It is both surfacing and sliding via the feed shaft.
I believe it to be from a ship that was broken up at Ballcows at Blyth. It is over 100 years old and can still do a job and is very accurate.
When I got it it was in a wrecked state. I made new bearings, clasp nuts, and tail stock spindle. Stuffed inside the bed cavity was a page from a wartime Daily Express – the headlines were The “Sicily Landings”.
A reminder that the TSMEE Annual General Meeting is to be held on Wednesday 23rd June. It is anticipated that this will take place indoors at the Clubhouse.
Another month has gone by, and what a change in prospect we‟ve seen. Even the sun has come out to help us all throw off the lockdown-lassitude induced by months of winter gloom and state-sponsored inactivity. Remember, though, that we‟re not out of the woods yet, so care and prudence should remain our watch-words for some time yet.
Thanks as ever to this edition‟s contributors. I have a couple of pieces in reserve for July but can always use more.
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