Fun Science: Two metals in contact do fun stuff

Have you ever made lasagna, and later discovered black spots or holes on the tin foil you used to cover it? Those spots are due to bimetallic or galvanic corrosion. Galvanic corrosion is an electrochemical process that occurs when two different metals contact through an electrolyte. Any two metals or alloys can experience galvanic corrosion, but pairs with dissimilar potentials will experience more. The potential of a metal is an inherent property of that metal, like density or hardness. Galvanic corrosion can be a very destructive force, or it can be exploited to make electrical current in a battery. In the case of the lasagna, the lasagna functions as the electrolyte, the pan as one metal, and the tin foil as the second metal.

How to make a simple battery at home

The first battery was invented in 1800 by Alessandro Volta. It was called the voltaic pile, and it was composed of a stack of zinc and copper disks.

A voltaic pile, the earliest kind of battery. Voltaic piles were used to discover many elements and to study electricity (credit: wikimedia commons)

If you have coins, you can make a battery. US pennies are zinc coated with pure copper and US nickels are 75% copper.

Battery 1 (weak, but easy): You can make a weak battery by stacking pennies alternated with nickels. Just separate the coins with paper towels soaked in vinegar, which will serve as the electrolyte. Here’s a great summary of some experiments you can do with this system. If you have a multimeter, you can measure the voltage of your system; the more alternating sets of coins, the higher the voltage. This battery won’t be powerful enough to light an LED, but if you keep it wet for a few days, you will be able to see the effects of the corrosion on the coins.

Battery 2 (strong, but more work): If you’re more ambitious, you can sand the copper off one side of the pennies, and create a battery from just pennies. A few pennies like this can easily light LEDs.The video below shows how to make battery 2.

Battery 2 is much more powerful because the metals in battery 2 (the zinc of the penny’s core and the copper of the penny’s surface) have a higher difference in potential than those in battery 1 (the 75% copper of the 5 cent coin and the pure copper of the penny surface). The farther apart two substances are on the galvanic series, the more voltage there will be.

Galvanic corrosion and the Statue of Liberty

The Statue of Liberty has an iron skeleton covered by a thin layer of copper. It was built with insulators between the copper and iron to prevent corrosion, but these insulators broke down. The Statue of Liberty was extensively renovated in the 1980s to repair damage from this corrosion.

Galvanic corrosion occurs in a lot of systems. If you use washers that are a different kind of metal than your screw, galvanic corrosion will occur. Galvanic corrosion can get even trickier: alloys that contain more than one kind of metal are composed of crystal grains that may vary slightly in composition. Galvanic corrosion can occur in an alloy between grain boundaries!

The bolts are a different kind of stainless steel, which has led to corrosion (credit: wikimedia commons)

Fortunately, we have methods for combatting corrosion. Corrosion only eats away at the lower potential metal. So engineers often design less critical pieces out of lower potential metals, so that they are sacrificial. Galvanic and other kinds of corrosion are major topics of research, relevant to boat construction, bridges, high temperature processing, and more. And thanks to galvanic corrosion, you can power a light with just pennies.

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