Questions for the metallurgists in the house regarding alloys...

MrEureka

How would the addition of a small amount of silver - let's say 3% of the total weight - affect the properties of aluminum?

How about if a little copper was added to tin?

Just trying to understand postage currency patterns a little better.

lasvegasteddy

that's metallurgy not chemistry

it would add a little to strength characteristics of such alloy
corrosion resistance would be improved some

MrEureka

that's metallurgy not chemistry

Oops. Fixed it.

MrEureka

corrosion resistance would be improved some

Interesting that the aluminum pieces that contain a bit of silver are the ones usually found corroded. Probably just a fluke, since there aren't many of these coins in existence.

lasvegasteddy

aluminum is prone to oxidation issues
it's a weird alloy and almost never found in pure form

most of it has zinc alloyed in for strength...which is where the corrosion originates and becomes accelerated
corrosion of aluminum eats below surface

surface contamination with silver is usually found above surface
it forms on or above it's surface

it's not the silver causing corrosion
it's the other alloys in the aluminum composition base

aluminum is very soft like gold
they need alloys to strengthen them

silver isn't exactly hard...it's in soft metal class too
so if they added it...it was for strengthening/corrosion address of this aluminum alloy

<<< never was a metallurgist but very close with great metal knowledge of tool n die requirements of being a machinist
most of my knowledge is waning the further i'm removed from it

hopefully an active in the field metallurgist does chime in...but what i've offered is of a sound base there

ProfLiz

Adding a little silver to aluminum makes it much stronger. The process is called "precipitation hardening" or "age hardening", and it's the same alloying trick used to strengthen aluminum in the rivets used on airplane wings. (Aircraft aluminum uses magnesium and silicon instead of silver, but the metallurgical principle is the same.) Back in the 1860's, aluminum was a precious metal, but pure aluminum is very soft, and can be easily bent. Adding silver would strengthen the material without adulterating it with a "base" metal. As an added bonus, age hardened aluminum alloys start out soft (good for striking the coin) and become harder over time (good for using the coin) - although I'm not sure that behavior was known in the 1860's.

Aluminum is susceptible to pitting corrosion, especially under moist or salty conditions. The silver is probably not the culprit.

Tin alloyed with a few percent copper is considered to be pewter. (Pewter is a class of alloys that start from tin, and may contain varying amounts of copper, lead, antimony, etc.) The copper is added for the same reason as above: Tin is soft, and the copper addition strengthens the alloy. The effect is not as pronounced as in aluminum/silver, but still significant. The copper addition should not worsen corrosion.

In the link you posted, I was interested to see that a 75% silver - 25% aluminum composition was reported, though no examples are known to exist. I would think that alloy would be difficult to work with due to the potential for brittleness. I wonder if those coins ever got made!

Hope that helps.

P.S. Yes, I'm a metallurgist. That's the "Prof" in ProfLiz! ;-)

drwstr123

Home
Aluminium
Aluminium alloys
Properties of aluminium alloys

Pure aluminium is relatively soft. To overcome this, the metal can be alloyed with other metals (alloying elements). Most of the aluminium reaching the marketplace has been alloyed with at least one other element.

Based on the type of alloying element, the aluminium alloys are divided into 8 groups
Alloy code
Charakteristics
1xxx Series
Contains no alloying elements. The proportion of aluminium is 99.3 – 99.9% and the rest is formed by tiny impurities. The combination of material properties, especially superior conductivity makes these alloys suitable for applications mainly in electrical and heat-power industry. Materials of this series are considered non-hardenable alloys and have tensile strengths of 40 – 60 MPa.
2xxx Series
The alloying element is copper. Alloys of this series are high strength alloys. The strength is achieved by the heat treatment process. The tensile strength is about 400 MPa on completion of hardening. Alloys of this series are considered not suitable for surface treatments and poor for welding.
3xxx Series
The alloying element is manganese. Alloys of this series are moderate in strength, they have good formability and they are suitable for anodizing and welding.
4xxx Series
The alloying element is silicon.
5xxx Series
The alloying element is magnesium. Alloys of this series are moderate in strength (200 – 350 MPa). The strength is achieved by the heat treatment processing or forming. Alloys of this series have excellent resistance to corrosion in aggressive atmosphere and seawater.
6xxx Series
The alloying elements are magnesium and silicon. Alloys of this series are moderate in strength (200 – 350 MPa). The strength is achieved by the heat treatment processing or forming. Alloys of 6xxx series can be easily anodized.
7xxx Series
The alloying element is zinc. Alloys of this series have the highest strength among all series. The tensile strengths ranging between 450 – 500 MPa may exceed 600 MPa in some cases. These alloys prone to stress corrosion, especially when welded.
8xxx Series
The alloying element is other than for the other series (including lithium).

MrEureka

Great input, thanks!!!

ProfLiz

drstrw123 is absolutely right: Pure aluminum is rarely used in any application, and alloying with other elements is the rule. The list of he posted gives the major classes of engineering alloys.

What I find fascinating, though, is that at the time these pattern coins were made, none of these alloys had yet been discovered. Aluminum was a rare and precious metal, more expensive than gold. There was little metallurgical knowledge about how to alloy it - and certainly no systematic exploration of how its properties changed with different alloying elements.

It was generally known that alloying pure metals usually makes them stronger. I imagine that the metallurgist at the mint wanted to strengthen the aluminum, and in the absence of other information, chose a precious metal that (1) he had on hand and (2) is the same color as aluminum. After all, it was important for the aluminum to look like aluminum, and not gold or copper!

MrEureka

Aluminum is susceptible to pitting corrosion, especially under moist or salty conditions. The silver is probably not the culprit.

The aluminum available in those days contained silicon and iron impurities. Could the silver have reacted with the impurities to cause the corrosion?

Hydrant

I had a slightly different theory but I'm not a metallurgist, so I'll just go with ProfLiz on this one. I think it was the part about airplane rivets etc. Anyway it is an interesting topic.

ProfLiz

Iron and silicon are the two most common unintentional impurities in aluminum even today. And some aluminum alloys are intentionally enriched with one or the other or both. (Most usually, silicon.) Neither one degrades the corrosion resistance of aluminum.

Silver does not react significantly with iron or silicon. That is, if you mix silver with silicon, they do not interact with each other. Instead they stay separate, like oil and water. The same is true for silver and iron. So interactions between silver, iron, and silicon are not to blame for the corrosion seen in those coins.

Salt is a major culprit in pitting corrosion of aluminum. If an aluminum coin gets even a small amount of salt on its surface - from being near the ocean, or from being touched by a sweaty finger - pitting corrosion can occur. The worst part is that the salt doesn't get "used up" when the corrosion happens. Unless the salt is removed, corrosion will progress whenever there is sufficient ambient humidity.

Aluminum has a great reputation for corrosion resistance, but that's really only in dry air. Have you ever noticed that you almost never find really old aluminum cans by the side of the road? They literally corrode away in salty or acidic conditions.

LoveMyLiberty

You look for old aluminum cans by the side of the road?

BAJJERFAN

<< <i>Adding a little silver to aluminum makes it much stronger. The process is called "precipitation hardening" or "age hardening", and it's the same alloying trick used to strengthen aluminum in the rivets used on airplane wings. (Aircraft aluminum uses magnesium and silicon instead of silver, but the metallurgical principle is the same.) Back in the 1860's, aluminum was a precious metal, but pure aluminum is very soft, and can be easily bent. Adding silver would strengthen the material without adulterating it with a "base" metal. As an added bonus, age hardened aluminum alloys start out soft (good for striking the coin) and become harder over time (good for using the coin) - although I'm not sure that behavior was known in the 1860's.

Aluminum is susceptible to pitting corrosion, especially under moist or salty conditions. The silver is probably not the culprit.

Tin alloyed with a few percent copper is considered to be pewter. (Pewter is a class of alloys that start from tin, and may contain varying amounts of copper, lead, antimony, etc.) The copper is added for the same reason as above: Tin is soft, and the copper addition strengthens the alloy. The effect is not as pronounced as in aluminum/silver, but still significant. The copper addition should not worsen corrosion.

In the link you posted, I was interested to see that a 75% silver - 25% aluminum composition was reported, though no examples are known to exist. I would think that alloy would be difficult to work with due to the potential for brittleness. I wonder if those coins ever got made!

Hope that helps.

P.S. Yes, I'm a metallurgist. That's the "Prof" in ProfLiz! ;-) >>



Aluminum is readily dissolved by lye i.e. sodium or potassiun hydroxides. It also reacts readily with alcohols like isopropyl [rubbing alcohol] and ethanol.

ProofArtworkonCircs

57 years ago I was tasked with making some silicon diodes. The connection and diffusion into the silicon had to be PURE aluminum. Our resident expert told me to use aluminum foil. He said it had to be pure to be processed so thin. Another assignment was using 94% gold, 6 % aluminum. With age that makes a pretty purple color.

ProfLiz

<< <i>You look for old aluminum cans by the side of the road? >>



Well, no, because they've all corroded away. Saves me a lot of trouble. ;-)

derryb

Think of the metallurgist as a chef who deals in metals. Alloys are concocted to bring the properties of additional metals into the final product.