Rust, Tarnish, and Verdigris

OK, so you certainly know what rust looks like, and may have seen tarnished silver, but what is that other thing? And what does it have to do with rust and tarnish?

Rust, tarnish, and verdigris (and the oxide coating on old pennies) all result from the chemical reaction of a bare metal surface with something in the air. That something may be oxygen, water, carbon dioxide, another chemical, or some combination. Rust forms when iron is exposed to air and moisture (water). If a piece of iron is left outside, it will eventually rust away to nothing. If you don't want an iron object to rust, you need to protect its surface by painting it or coating it with another metal, such as zinc (called galvanizing). You might think that silver tarnish also forms from a reaction with oxygen, but tarnish actually needs both oxygen and sulfur to form. Tarnished silver may look ugly, but the tarnish actually protects the silver underneath from further reaction!

Verdigris (VUR-di-gree) is the green coating that forms on copper or other copper-containing metals, such as bronze or brass. It forms from the reaction of copper with oxygen, carbon dioxide, and chloride (near seawater). Perhaps the most recognizable American example of verdigris is the Statue of Liberty. She is made of copper, and was originally brown! But over time, a verdigris coating has formed, giving her the familiar green color. You can sometimes see other examples of verdigris on copper roofs on old buildings, or on outdoor statues. Keep your eyes open, and you may spot some verdigris in your own neighborhood!

Copperplating

Some metal items are not made from just one metals, but are composed of a thin layer of one metal on another. This is called plating, and very often, the base metal is cheaper than the coating. Silverplate is a common example of this. Plating is done by first dissolving the coating metal, then placing the base object in the solution. Sometimes electricity is needed to create the coating, but sometimes the reaction just happens spontaneously.

Here's an experiment you can perform to plate copper onto a nail, screw, or other iron or steel object.

What You'll Need:

20-30 pennies

1/4 cup vinegar

1 teaspoon salt

Water

Plastic or glass bowl

Spoon

1 or 2 nails or screws

Pour the vinegar and salt into the bowl, stir, and add water until the salt dissolves. Put the pennies into the liquid and let them sit for 10-15 minutes. What happens to the pennies? Remove the pennies from the solution and add the nails or screws. Let them sit for 30 minutes. Have they changed color? If not, check back in an hour. You will eventually notice a brown coating of copper metal forming on the nail or screw.

Steel Away!

One of the most common metals used in today's world is steel. Steel is mostly iron, but it also contains other elements. As we've already seen, this kind of metal mixture is called an alloy. Most steel contains carbon in addition to iron. Stainless steel also has some chromium, and it doesn't rust. Other types include elements such as molybdenum, vanadium, tungsten, nickel, silicon, manganese, or boron. Different types of steel are used for different purposes. Some are good at high temperatures; others are hard and good at cutting.

The next time you're in a hardware or home-improvement store, take a look at the tools. How many different kinds of steel can you find?

All About Alloys

There are some familiar metals that are actually mixtures of several different types of metals - they are called alloys (AL-oise). Here are a few you've probably heard of (and some that are likely new to you):

• Steel: mixture of iron and one or more other metals (there will be a separate post about steel later)
• Brass (copper and zinc): a shiny yellow metal that looks a little like gold
• Bronze (copper and tin): a dark gold to brown metal that gets darker in color as it ages
• Pewter (copper, tin, and lead): a dark gray metal used for bowls, flatware, jewelry and other decorative items
• Solder (lead and usually tin): an alloy that melts at a relatively low temperature; used to make electrical circuits
• Nitinol (nickel and titanium): this alloy remembers its original shape; used to make eyeglass frames
• Electrum (gold and silver): also known as white gold; used in jewelry
• Alnico (aluminum, nickel, and copper): used to make magnets
• Amalgam (silver, tin, copper, zinc, and mercury): a soft alloy that has been used to fill dental cavities
• Nichrome (nickel and chromium): an alloy that glows orange-red and gives off heat when an electrical current is passed through it; used in toasters and electric burners

Take a look around your house and see how many examples of alloys you can find!

Fun Facts 11

And finally...

• In the early 1900's, glow-in-the-dark watches were made with paint that contained radium (Ra, #88). Many of the young women who made these watches became very sick because of their exposure to this radioactive element.
• Lutetium (Lu, #71) is probably the most expensive element you've never heard of! It costs more than gold or even platinum!
• Did you know that your pencil doesn't actually contain lead? The stuff you write with is carbon (C, #6) in the form of graphite.
• Humans have been using the metal iron (Fe, #26) for thousands of years. There is a 1500-year old iron pillar in Delhi, India that is still rust-free, probably because it is located in an area with a dry climate.
• The nibs (tips) of very expensive pens may be made with rhodium (Rh, #45) because it does not wear down easily.
• Gas camping lanterns often use a compound of thorium (Th, #90) which glows bright white when heated.
• Rechargeable batteries often contain both nickel and cadmium (Cd, #48). They are called nicad batteries.
• Tin (Sn, #50) slowly disintegrates in temperatures below freezing (32^oF or 0^oC). In the 1700's, organs in unheated churches could be destroyed because their tin-containing pipes would turn brittle and crumble.
• Barium (Ba, #56) X-rays can be used to detect problems in the stomach or digestive tract. Barium sulfate does not let X-rays pass through, so after you drink a barium cocktail, those parts of your body become visible.
• The catalytic converter in your car contains palladium (Pd, #46) which helps to reduce the amount of pollution that comes out of the exhaust pipe.

Copper Coating

Many metal objects around you are plated - they have only a thin coating of the metal you see covering another metal (usually less expensive). In particular, a lot of things are gold-plated or silver-plated. Gold and silver cost too much to experiment with, but here's an easy way to copper-plate a nail.

What you'll need:
1/4 cup (50 mL) of lemon juice or vinegar
Plastic or paper cup
10 to 20 dull pennies
Salt
2 small iron nails (not the galanized kind)
Steel wool scouring pad

Pour the lemon juice into the cup and drop the pennies in. Add a pinch of salt. What do you see happening? Wait three minutes. While you're waiting, scrub one of the nails with the scouring pad to clean off the surface. Rinse the nail well. Just put the second nail off to the side. Drop the cleaned nail into the cup, and wait about an hour. After that time has passed, take the nail out and compare it to the one you set aside. What do you see? When you're done, make sure to pour the lemon juice or vinegar down the drain and to throw the cup away.

As you may know, pennies are coated with the element copper (Cu). When you dropped them into the lemon juice or vinegar, bubbles formed. Metals like copper often react with acids (like the citric acid in lemons or the acetic acid in vinegar) to form bubbles of hydrogen (another element!) gas. You may have noticed that this solution changed color as some of the copper dissolved. When you added the iron (yes, yet another element!) nail, the dissolved copper collected on its surface, giving the nail a brown coat of copper.

The instructions said that you should not use a galvanized nail (which is coated with zinc - these elements are everywhere!), but you might want to try one, just to see what happens!

Yummy Iron!

It is important that you get plenty of iron in your food, because it helps make sure that your blood can carry oxygen to your muscles. Sometimes, iron is added to foods, like cereal, that doesn't naturally have a lot. You might be surprised to learn that the iron is actually added as tiny bits of metal! And you might be even more surprised to learn that you can actually see them! Here's how:

What you need:
1 cup (250 mL) iron-fortified cereal, such as Total
Gallon-sized (4 L) plastic zipper bag
Rolling pin
Sheet of white paper
Magnet (not the flexible type)

Put the cereal into the plastic bag and seal it closed. Crush the cereal with your fist to break up the flakes, then put the bag on a flat surface and really pulverize the cereal with the rolling pin. It should be like a fine powder when you're done.

Pour a little bit of the cereal powder onto the sheet of paper and spread it out. Put the magnet under the paper and move it around. Do you see any black bits that move? Those are tiny pieces of iron metal!

You might worry about eating metal, but it's OK. The pieces are too small to harm your throat or stomach. And your stomach contains a strong acid that dissolves the metal pieces so that they can get into your body.

Have you ever seen a metallic sheep??

What happened to the temperature in the last experiment? And how did the steel wool look at the end? You should have seen that the temperature went up by several degrees, and that the steel wool looked rusty after the experiment was done. What happened?

First of all, what exactly is steel wool? Is it the fleece from metallic sheep? Of course not. Steel wool is just very thin wires made of iron that is sold in clumps that looks something like sheep's wool. It is often used instead of sandpaper for removing paint or smoothing surfaces.

So, where did the heat come from? When iron comes in contact with the oxygen (O₂) and water (H₂O) in air, a chemical reaction called oxidation occurs. Rust is actually a mixture of several compounds of iron, oxygen, and hydrogen, with names like iron oxide and iron hydroxide. Do you remember learning about thermal energy in "Where Does Heat Come From?"? We said that a wood fire feels hot because the thermal energy of the wood was higher than that of the ashes and other material left over from the fire. The same thing applies to the iron. The thermal energy of rust is lower than that of the original iron, and you feel the energy lost as heat.

But why didn't the steel wool rust before you put it into the jar? Normally, steel wool is coated with oil or some other material that protects it from air. The vinegar you rinsed the steel wool with took the coating away, and the bare iron was exposed. So, the vinegar wasn't actually involved in the reaction, but it did allow the reaction to happen.

If you go to the drugstore, you can actually find a product that uses the oxidation of iron to generate heat! ThermaCare heat wraps contain iron (and some other chemicals) that react with air and produce warmth that helps relieve the pain of arthritis and sore muscles.