Which Metal Is the Most Resistant to Corrosion?
AbstractHere's a practical engineering challenge: you need to build an enclosure for your dogs, using material that they can't chew through. It's going to be a lot of work to build, so you want to do it right. What material should you choose for the fencing? This project uses the scientific method to evaluate materials.
Andrew Olson, Ph.D., Science Buddies
This project is based on:
- Burns, P.T., 2003. Rust Busters, California State Science Fair Abstract. Retrieved October 24, 2006.
The purpose of this project is to determine which metal would be the most corrosion-resistant.
Corrosion is what happens to metals when they are exposed to water and oxygen in the environment. When iron or steel corrodes, the iron forms reddish brown colored oxides and hyrdoxides: what we commonly refer to as "rust."
Rusting of iron is an electrochemical process. The iron atoms lose electrons (the chemical process of oxidation), which break down water into oxygen and hydroxide ions (the chemical process of reduction). The hydroxide ions react with the oxidized iron and the dissolved oxygen in the water to form iron oxide.
Iron oxide is permeable to water and oxygen, so the chemical reaction can continue beneath the surface layer. For other metals, such as copper and alumnium, an oxidized layer on the surface actually protects the metal underneath from further corrosion.
In this project, you will measure the corrosion rate of different metals when exposed to fresh and salt water.
Terms and Concepts
To do this project, you should do research that enables you to understand the following terms and concepts:
- stainless steel,
More advanced students should also study:
- Why are roofing nails zinc-coated?
- What chemical reaction occurs when iron rusts?
- Wikipedia contributors, 2006. Rust, Wikipedia, The Free Encyclopedia. Retrieved October 24, 2006.
- NJScuba.net, 2006. Artifacts & Shipwrecks: Iron, Steel & Rust, New Jersey Scuba Diver. Retrieved October 24, 2006.
- Asato, R., date unknown. Corrosion, Internet Chemistry, Leeward Community College. Retrieved October 24, 2006.
- Corrosion Doctors, date unknown. Aluminum Corrosion, Corrosion-Doctors.org. Retrieved October 24, 2006.
Materials and Equipment
To do this experiment you will need the following materials and equipment:
- short lengths (about 10 cm) of solid wire,
- made of a different metals, e.g.:
- steel or iron,
- zinc-coated steel,
- you should be able to find these at your local hardware store,
- you will need 3 lengths of each type of wire,
- match wire diameter as closely as you can;
- made of a different metals, e.g.:
- 2 pencils,
- 2 jars,
- lab notebook,
- graph paper and colored pencils,
- camera (optional).
- Do your background research so that you are knowledgeable about the terms, concepts, and questions.
- Cut three 10 cm lengths of each type of wire.
- 1 length of each type of wire will be immersed in plain tap water,
- 1 length of each type of wire will be immersed in salt water,
- 1 length of each type of wire will remain dry in air.
- Fill one jar 2/3 full of plain tap water.
- Fill the other jar 2/3 of salt water.
- Take one sample of each type of wire and wrap the end of the wire (2–3 turns) around a pencil. Leave some space between the wires, but make sure that they will fit into the jar. Repeat for a second set of wires on the second pencil.
- Immerse one set of wires in plain tap water, and the other set in the salt water. The pencil should rest across the top of the jar.
- Observe the wires at least once a day for two weeks. Take notes of your observations in your lab notebook. Examine the entire length of each wire. Are the changes you notice the same along the whole length? Why or why not?
- If you have a camera, take photographs of the wires at the start of the experiment and whenever you notice an interesting change. Use your camera's date function (if it has one) to mark the picture; otherwise, be sure to keep good notes about when you took the pictures in your lab notebook.
- At the end of the experiment, compare the 3 sets of wire samples:
- the set kept in plain tap water,
- the set kept in salt water,
- the set kept in air.
- Develop a graded rating scale (1–5 or 1–10) to describe the changes you observed. Each number in your scale should have clear rules for distinguishing it from the other numbers.
- If you took pictures, use your photographs to illustrate your rating scale on your display board.
- Use your rating scale to make graphs that show what happened to the different metals in each of the three conditions.
- Which combination (of metal and environmental conditions) showed the greatest amount of oxidation?
- Which combination (of metal and environmental conditions) showed the least amount of oxidation?
Ask an Expert
- In addition to your visual observations, you could also measure the weight of the wires at the beginning and end of the experiment. Do you think the weight of any of the wires will change? Why or why not? What actually happens?
- How does temperature affect the oxidation rate of different metals? Design an experiment to find out.
- You could try different concentrations of salt water. Does more salt in the water speed up the oxidation reaction? Why or why not?
- What happens if there is very little oxygen in the water? Use boiled water (to remove oxygen), and fill the jar all the way to the top with water (pour carefully so that you don't re-oxygenate the water). Attach the wires to the bottom of the jar lid (e.g., with hot glue), and close the jar lid tightly. Compare to a similar jar set-up with water that wasn't boiled.
- What happens if the pH of the water is changed? You could make the water acidic by adding vinegar, or basic by adding baking soda.
- Have you seen pictures of the Titanic or other underwater shipwrecks? What happens to artifacts like iron cannonballs when they are brought to the surface from a shipwreck? Design an experiment to see what happens to metal wires immersed in fresh or salt water and then exposed to air.
- Compare painted and unpainted wires.
If you like this project, you might enjoy exploring these related careers: