Summary

In this lesson students will investigate the galvanic corrosion that can occur when different metals come in contact with each other in modern cars.

Grade Level

High school

NGSS Alignment

This lesson will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
• HS-PS1-7: Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
• HS-ETS1-3: Evaluate a solution to a complex real-world problem based on priorities criteria and trade-offs that account for a range of constraints, including cost, safety, reliability, and aesthetics as well as possible social, cultural, and environmental impacts.

AP Chemistry Curriculum Framework

This lesson plan supports the following units, topics, and learning objectives:

• Unit 4: Chemical Reactions
  • Topic 4.7: Types of Reactions
    • 4.7.A: Identify a reaction as acid-base, oxidation-reduction, or precipitation.
  • Topic 4.9: Oxidation-Reduction (Redox) Reactions
    • 4.9.A: Represent a balanced redox reaction equation using half-reactions.

Objectives

By the end of this lesson, students should be able to

• Describe galvanic corrosion in terms of electron flow.
• Explain galvanic corrosion in reference to the activity series of metals.
• Write half-reactions for oxidation and reduction of metals.
• Explain how electrolytes such as road salt promote corrosion.

Chemistry Topics

This lesson supports students’ understanding of

• Chemical Reactions
• Chemical Change
• Ionic compounds
• Electrolytes
• Activity Series of Metals
• Oxidation
• Reduction

Time

Teacher Preparation: 3 hours (This is a one-time event to prepare the materials). After that, repeats of the lesson should take less than 30 minutes of preparation. See Teacher Notes for additional information.

Lesson: 90 minutes

• Engage: 15 minutes
• Explore: 30 minutes
• Explain:15 minutes
• Elaborate: 45-60 minutes
• Evaluate: 30 minutes (could be several days or weeks later at end of unit)

Materials

For each group

• 1 Aluminum bar 1” x 96” x 1/8” (cut into 6-inch lengths, 4 holes pre-drilled)
  • 1 bar makes 16 fixtures
• 1 Steel bar 1” x 96” x 1/8” (cut into 6-inch lengths, 4 holes pre-drilled)
  • 1 bar makes 16 fixtures
• 500mL of 1 M solution NaCl, CaCl₂, MgCl₂, and distilled water for the control
• 4 Spray Bottles (1 for each salt solution and 1 for distilled water)
• Pan Head Bolts with matching nuts: ¼” x 20 – ¾” long.
  (Need a set of stainless steel, chrome, brass, zinc)
  • Need 1 bolt + 1 nut in each material
  • See assembly below

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.

Teacher Notes

• This resource could be used as a post-AP Chemistry exam activity.
• This lab is designed to show the different forms of corrosion that occur in modern cars. Today’s cars are made from various different metals, including aluminum, steel, brass, copper, chrome, and zinc. In particular, body panels and frames are made mainly from steel and aluminum. Although these are great materials for body parts, they don’t make good fasteners, so cars are often bolted together with bolts and rivets made from different metals, such as stainless steel, zinc, or chrome. Wherever different metals make contact with each other, galvanic corrosion can occur. Students will observe galvanic corrosion in this activity, facilitated by the presence of road salt (NaCl, MgCl₂ or CaCl₂). You can use all the salts or just the one typically used in your region. If you live near the coast, seawater would be a great electrolyte. Students will see different galvanic corrosion rates based on the salt applied, and how the salt corrodes different metals at different rates. Salt facilitates the transfer of electrons, so it should speed up the corrosion quite nicely.

• If you don’t have access to power tools or don’t feel comfortable creating the hardware pieces, consider partnering with an Industrial Tech teacher in your building and see if students taking Industrial Tech can fabricate them for you. It would also be great to have students in Industrial Tech follow the experiment or even do one on their own to see how their work can connect to science content. If this doesn’t work for you, many Home Depot and other hardware stores will cut the metal bars to size. (See Materials Section for sizing info)

• Many of the hardware pieces, including the aluminum and steel bars, can be purchased at any hardware store. Specialty metal hardware, such as chrome, aluminum, titanium, and many others are expensive, but can be found online. One good source is www.boltdepot.com.

• Standard hardware-store materials for 16 fixtures (8 in each material) can be obtained for around $60 and most of these materials can be used more than once. Also, for faster results, the corrosion reactions can be sped up significantly by alternatively spraying with the salt solutions then suspending over a hot plate to around 50°C.

• This lab can be used as part of Chemical Reactions (Single Replacement and Activity Series of Metals) or Ionic compounds (salts as electrolytes).

• Engage: Show CarWow Article about car materials. It is full of full-color photos of sports cars and includes relevant information about the metal components of modern cars. In addition, show pictures of rusted out cars (How exactly does salt rust cars?) to get buy-in regarding the topic.
  
  
• Explore: Students review Activity Series of Metals, complete the guided simulation, answer questions, and share their findings in a jigsaw activity. Assemble hardware, make solutions. Students develop hypotheses of corrosion interactions (galvanic) and predict which will corrode most on its own in the presence of road salt.

• Explain: Students explain their hypotheses based on activity series of metals and prior knowledge of electrolytes. Teacher explains RedOx chemistry that leads to corrosion. Reduction and oxidation instruction takes place over a period of days or weeks while metals react with each other and the salt solutions. Findings are collected at the end of unit.

• Elaborate: Teacher explains RedOx chemistry that leads to corrosion. Reduction/Oxidation instruction takes place over a period of days or weeks while metals react with each other and the salt solutions. Students learn half-reactions, oxidation, reduction, other aspects of electrochemistry during this time. Findings are collected at the end of unit. At the conclusion of the investigation, have students complete this simulation within student conclusion section of handout to enhance their understanding of RedOx reactions.
  
  
• Evaluate: Students collect data (qualitative and quantitative) during this time. At the conclusion of the unit, evaluate the metals and explain results. Thoughtful post-lab questions can be used to assess student learning. Students can also present their findings and conclusions to the class.
  

• Expected Results:
  • Aluminum Sample Results (no heat added)

24 hours

96 hours

7 days