Crumbling Monkey Metal

Fraid not. It is lead contamination that causes the zinc rot.

Wikipedia has some interesting stuff on Zamak/Mazak it seems it's a range of metals developed specifically to avoid zincpest by using high purity zinc.

Perhaps we do it a disservice it by classing it as standard 'monkey metal'?

Neil

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I am certain that 'we' do, Neil

Just like with 'Loctite' ... sloppy usage, and 'viral' opinion-forming have ridden roughshod over the manufacturers' good efforts.

MichaelG.

These lightweight cast “alloys” don’t need to be old to be useless. I’ve had to replace quite a number of parts on my Record Power belt and disc sander due to material failure after only a couple of years.

Neill, I have M10x1 taps here in Kings Bromley not far from you. If you want to PM me an address I can post to you.

Chris.

It's in the atmosphere trapped in H2O

Dixie Magnetos from America are notorious for failing due to hydrogen embrittlement. Because there is electricity passing through the body when the engine is running, any moisture gets gets split into oxygen and hydrogen. Most non critical aluminium castings on vintage cars contain large quantities of zinc. SU carbs are so bad you can only weld them with zinc based filler rods.

Always a problem laying hands on Mother Lode documentation because access is often via a University Library or has to be paid for!

However the Wikipedia Zinc Rot Article references a Dutch NLR paper 'Corrosion-induced cracking of model train zinc-aluminium die castings', which is available on the Wayback Machine as a PDF. (It's in English.)

Couple of quotes from the Report:

'The corrosion results from impurities in the metal, and requires the presence of moisture.'

'The problem needs thorough discussion because information available to the non-specialist, notably via the internet, often contains errors.'

'The origin of zinc die castings is a bit uncertain. Bierbaum (1923) mentions zinc-copper-aluminium die castings made as early as 1896, while Goodway (1985) and Gross (2003) state that the first commercial alloy was produced around 1907. By the early 1920s it was recognised that alloys based on the zinc-aluminium binary system were easy to cast and had good mechanical properties. At the same time, it became clear that the control of impurity elements was vital: small amounts of lead, cadmium and tin resulted in corrosion-induced cracking and swelling of the castings (Brauer and Pierce 1923).'

'Some alleviation of the corrosion problem was obtained by additions of copper to the impure binary alloys (Brauer and Pierce 1923), but its elimination was achieved only by the use of special high purity zinc (99.99%) and further additions of small amounts of magnesium (Goodway 1985; Gross 2003). These developments took place during 1926-1929, leading to introduction of the ZAMAK1 series of alloys, several of which are still in use today...'

The NLP report lists 11 references, the most recent being 'Gross, D.K., 2003, Zinc Die Castings – The Importance of Alloy Chemistry, Die Casting Engineer, Vol. 47, No. 2, pp. 30-31.'

The report goes into a lot more detail.

I'd add the observation that Zinc occurs naturally in ores containing Lead and Cadmium and refining it isn't easy. As Die Casting is often done for cheapness, it is rather likely that the alloy used for limited-life items might not be the best. It's also possible that a die caster might not consistently maintain absolute cleanliness throughout a series of melts, and there's always a risk of a numpty cutting corners or even chucking incompatible scrap into the mix...

Dave

If I can add to the 'embrittlement' idea:

This effect is known mainly on high tensile steel, and is caused when electroplating (such as zinc). When the steel is put under tension, hydrogen (from the effect of electrolysis) trapped under the plating percolates along the crystal boundaries of the steel, causing serious weakness. Just like a single drop of water can loosen the cohesion of a sugar lump.

If this effect happens in die-casting alloys in damp conditions, it may be because layers of different metals make tiny cells, generating small currents. This then creates the same sort of conditions as in steel. And the hydrogen comes from the water (even without electrolysis) as the metal - aluminium, zinc etc - reacts with moisture. This corrosion is called oxidation - the metal takes oxygen from water, and this leaves spare hydrogen ready to creep between the minute crystals.

The real problem is that neither the makers of model cars, nor those who made carburettors, or dashboard knobs, had any idea that their products would be treasured one hundred years later.

The same sorts of effects will destroy our favourite plastics, soon enough, don't you worry.

Cheers, Tim

So why are the envirofascists getting hot under the collar about plastics in the environment?

Andrew

Andrew - because when plastics fall apart they do not dissolve completely, but remain as tiny particles and shreds. The plastic is made by polymerisation - joining together long strings of molecules. These bonds are not always permanent, and the base molecules remain, cluttering the sea floor and the insides of critters, including you and me.

The main cause of failure for many plastics is ultra-violet radiation, but once the plastic gets into the sea, the radiation does not penetrate, so small un-expired bits are left, and we, eventually, eat them.

Enjoy your lunch ...

Cheers, Tim

Thanks Chris

Turns out it is M9x1!

I'm going to make a tap as it will be a nice 'short' for MEW.

Neil

To quote further from the reference Dave found, it seems moisture is essential, impurities make it worse, perhaps by causing evolution of hydrogen:

• Corrosion proceeds from the surface inwards, is intercrystalline, and attacks the β
phase.
• Moisture is essential to the occurrence of corrosion, which is also accelerated by
higher temperatures.
• Corrosion severity, as measured by swelling of the castings, depends on alloy
impurities, or other metals deliberately added, and also the alloy grain or crystal
size. In particular, the impurities lead, cadmium and tin result in very severe
corrosion. A finer crystal size, owing to more rapid solidification, causes a general
increase in corrosion. Brauer and Pierce suggested that the detrimental effect of a
finer crystal size is due to the greater number of intercrystalline boundaries where
corrosion can occur.
Evans (1923, 1925) provided an electrochemical explanation for the effect of impurities
on zinc corrosion. Evans observed that the cathodic reaction (evolution of hydrogen
gas) during zinc corrosion is facilitated by intercrystalline impurities and also by
impurities dissolved out of solid solution and redeposited as a sponge.
Evans' explanation fits firstly with the fact that lead and tin are virtually insoluble in
solid zinc and solid aluminium (Baker et al. 1992) and have melting points lower than
zinc and the eutectic. Lead and tin therefore solidify at the β/(α'+&beta boundaries, i.e. they
are intercrystalline impurities a priori. Secondly, small amounts of cadmium (up to
about 2.5 wt. %) will be in solid solution in the β phase (Mongeon and Barnhurst 1985;
Baker et al. 1992). Initial corrosion will attack the β phase via its crystal boundaries and
dissolve out the cadmium, which presumably redeposits as a sponge and then acts as an
intercrystalline impurity a posteriori.3

Thank you.