Pot metal conservation

[jboger]

Probably most of us have at some point encountered deteriorated pot metal parts.

The first question I have is, Will these parts undergo further deterioration? They are now more than 100 years old (or so) and perhaps, after so many years, they are now stable and won't degrade any further. I would then suggest to leave them well enough alone.

The second question is this, and it follows on the first, If pot metal parts are subject to further degradation, can they be stabilized? For example, I've seen pieces with open cracks. Could epoxy be inserted into the crack to stabilize them? My fear here is that whatever one inserts will have a different coefficient of expansion than that of the pot metal and this could make matters worse.

Any thoughts?

[jboger]

I found a website for a company known as Muggy Weld:

https://www.muggyweld.com/videos/pot-me ... se%20metal.

[edisonclassm]

The Pot metal problem was explained to me years ago by a friend who was in the die casting business in Cleveland Ohio. This business had been in his family for several generations and they started the business at the time the technology was being introduced into American manufacturing. The metal used in die casting was zinc. They would lubricate the dies with a lead oxide so that the parts would break free from the die easily. The technology was entering into American manufacturing around 1900 and about somewhere around 1930 it was brought to the industry's attention that the parts manufactured years earlier began to swell and deteriorate in a peculiar way. Research found the culprit was the contamination from the lead oxide lubricant used to dislodge the parts from the dies. The deterioration caused by the lead oxide was known as inter-granular corrosion. They would not apply the lubricant to the dies every time they poured so the parts that came out of the die when it was first lubricated tended to deteriorate more rapidly than the parts that were produced just before the dies were re-lubricated. There might have been 10 -20 pours between lubrications. That is why you see parts severely deteriorated and others that seemingly are unaffected by deterioration. My friend told me the industry went into a real panic and found ways to remedy the issue. That is why, die castings manufactured after 1930ish do not suffer the same deterioration that earlier castings suffer.

[edisonclassm]

BTW forget trying to weld die cast parts that have suffered from inter-granular corrosion. The parts just crumble when you apply heat to them. These die cast weld products are for parts made after 1930 and have broken due to misuse.

[JerryVan]

If pot metal parts are subject to further degradation, can they be stabilized? For example, I've seen pieces with open cracks. Could epoxy be inserted into the crack to stabilize them? My fear here is that whatever one inserts will have a different coefficient of expansion than that of the pot metal and this could make matters worse.

Any thoughts?

My thinking is that if further degradation is occurring and you apply whatever means to successfully stabilize a crack, the continued part growth will only cause further cracking elsewhere. However, if the degradation has ceased, introducing a stabilizing agent may give back some part strength and utility. I have used an epoxy with water-like consistency that wicks into the voids. Who knows if it helped or not.

[MarkELynch]

Paul, thanks for explaining your expert friend’s theory about the mold release agent being the culprit. From available documentation it seems likely that any addition of lead would lead to the pot metal part swelling and eventual failure.
However, I can find no documentation about a lead-containing release agent being used in zinc die casting. In an effort to educate myself can you substantiate this theory and provide a reference?

While a lead release agent theory is possible it is more likely that the actual lead-containing zinc alloy is the culprit. In 1929 the New Jersey Zinc Company developed Zamak (zinc, aluminum, magnesium and copper alloy) which is 99.9% lead free. This may be why the 1930 and later potmetal Orthophonic soundboxes with the RCA marking are usually fine.

Here is an interesting reference about “Zinc Pest”. While Wikipedia is not the most reliable source this seems fairly well researched. https://en.wikipedia.org/wiki/Zinc_pest

Thanks,
Mark

[edisonclassm]

Mark,
I am relaying what was told to me by a man who is now deceased but was a die cast manufacturer and whose family was in the die casting business for several generations. Basically, the information I received came from the horse's mouth.

[dennis]

Could the cracks be treated with light use of SuperGlue to stabilize them?

[MarkELynch]

Paul, thanks for your reply. The release agent is a very critical part of the die casting process, without it the casting would solder itself to the mold making removal difficult and possibly damaging the mold.

I’ve contacted a modern zinc diecasting company to see if they have any knowledge of lead-containing release agents that might have been used in the 1920’s. This is a fascinating area of study. I’ve rebuilt 25 different Victor first-style changers as used in the 10-50 and the other three Electrola variants and am very familiar with bad pot metal. As some of you know, there are five key elements that were cast from a zinc alloy by the Alumac company. The vast majority of these components are terribly swollen and cracked which prevents these machines from working. Some of the original pot metal parts look perfect or show less cracking but I’ve found that they are still horribly out of spec and the parts don’t function correctly. I have some 10-50 changer parts that looked fine in 2002 that are now swollen and cracked 22 years later! This is evidence that the internal corrosion and swelling never stops. Any repair or crack filling is only temporary.

The biggest hurdle when making the new 10-50 changer parts was that there were no reliable existing parts to replicate. Dimensions or a mold taken from a non working and swollen part were not useful. Through the use of the patent drawings and six different first style changers and many trials we were able to work out the correct dimensions. The patent contains the Victor engineering drawings but of course they are without dimensions and duplication has distorted the drawings. We were unable to reliably correlate the physical measurements of known steel parts in the design with the published drawings. The design of the first style changer, while robust, allows little room for error. With the newly made correct parts and careful adjustment the changer can be restored to work like new. Some parts sets are available for those that may need them.

I’ll let you know if the modern casting company has any further information about the lead-containing release agent. A first hand account backed up with information about the actual material would be great information. Thanks again, Paul.

Mark

[JerryVan]

For what it's worth, I found this...
While this article suggests that simple impurities in die casting materials were at fault, Paul's assertion that a lead mold release agent was to blame is probably not without merit. If indeed a manufacturer used lead as a release, it could be very plausible that the outcome would be the same as using lead contaminated ingredients.

https://alliedmetalcompany.com/wp-conte ... lletin.pdf

INTERGRANULAR CORROSION
The introduction of 99.99% pure zinc (SHG – Special High Grade) as the base for zincaluminum casting alloys, effectively and completely eliminated intergranular corrosion under
normal service conditions. However, intergranular corrosion can still occur under wet or damp
conditions if zinc-aluminum alloys are exposed to temperatures above about 700C (1580F).
When present above normal specification levels, Sn, Pb, In, Cd, Bi, Hg, and Th can promote
intergranular corrosion, so every effort must be made to ensure that all zinc castings meet the
appropriate ASTM standard. Copper and Magnesium are present in the alloys to help prevent
intergranular corrosion. Zamak 7 is not recommended in hot and humid environments due to
low quantities of CU and mg present in this alloy.
Impact strength can decrease though intergranular corrosion. At 600C (1400F) in high humidity,
the loss of impact strength is modest. At 950C (2030F) in high humidity, intergranular attack is
ten times greater and a significant loss of impact strength is possible.
6
In practice, caution should be exercised in the use of Zamak and ZA alloys in humid
environments above 700C (1580F) and impurities must be controlled to within specified limits.
When in doubt, tests should always be conducted.
Lead, Cadmium, and tin at levels exceeding the limits shown in Table 1 can cause die cast parts
to swell, crack, or distort. These defects can occur within 1 year. The maximum limit for lead,
which can promote the occurrence of subsurface network corrosion, is 0.006%. Cadmium is
detrimental in its effect at some concentrations and is neutral at others. As such, the maximum
limit for cadmium is set at 0.005%. Tin, like lead, can promote subsurface network corrosion,
and therefore is also restricted to the maximum safe limits of 0.005%.
Some tests such as those by ASTM (1961) have, measured loss of strength with time; both
AG41A and AG40A were tested, and between 10 and 20 years of exposure, impact strength
decreased rapidly for total losses of 78 and 69% respectively, in the outdoor industrial
atmosphere and 33 and 38% in the outdoor rural atmosphere. Indoors, AC41A lost 52% (unlike
AG40A, which remained unchanged). These decreases in mechanical properties were probably
caused by Intergranular corrosion, to which die casting alloys produced 50 years or more ago
were often very susceptible. Intergranular attack can reduce cross-sectional areas and create
stress, raising notches, while not reducing the overall specimen dimensions.
When certain impurities are present and segregate to give phases that are very different
electrochemically, corrosion will proceed rapidly along the boundaries of the phases. Cadmium,
tin, lead, indium, and thallium are among the impurities that can be present unless controlled
and are particularly harmful, but iron and nickel also must be controlled at low levels.
Magnesium additions were developed early on as a beneficial addition for casting alloys,
although other additions can be used; for example, the 5% aluminum alloy used for coating
steel contains cerium and lanthanum.
Early die casters had great problems in avoiding brittle castings until the cause of this defect
was determined. Nowadays, the regular production of 99.99+ % zinc gives die casters a good
starting point, but to ensure that there is no pick up of impurities, they must still practice very
good housekeeping and avoid metal purchases from unknown or less reputable sources. The
current high purities of zinc used to make alloys have also enabled the magnesium content of
the alloys to be reduced to about 0.04% while still preventing hot shortness of the alloys