Year 9 Science · Unit 2 · Lesson 03
Challenge Worksheet
Learning Goals
1. A student claims: “Stainless steel doesn’t corrode because it has no chemical properties.” Explain what is wrong with this claim. Is the forming of stainless steel’s protective oxide layer a physical or a chemical change? Explain.
Someone claims…
“Our garden stakes are coated with a revolutionary non-reactive polymer skin, making them rustproof: the iron cannot corrode because it has no contact with oxygen or water.” After two summers in a NSW garden, customers report that the stakes are heavily rusted around the base, where the coating has chipped. The company says this is “misuse”, not a product failure.
2. Which part of the company’s claim is scientifically accurate? Explain why an unbroken polymer coating would prevent rusting.
3. Explain why the stakes rusted fastest near the base, where the coating was chipped and the stake stands in moist soil.
Data: how fast do metals corrode outdoors?
ISO 9223 sorts outdoor places into corrosivity categories. Table 1 gives the thickness lost from bare metal in its first year outdoors (1 µm = 0.001 mm).
| Category | Typical place | Carbon steel | Zinc |
|---|---|---|---|
| C2 low | Dry inland country town | 1.3 to 25 | 0.1 to 0.7 |
| C3 medium | City, or coast with little salt | 25 to 50 | 0.7 to 2.1 |
| C4 high | Industrial area, or coast with some salt | 50 to 80 | 2.1 to 4.2 |
| C5 very high | Surf coast with heavy salt spray | 80 to 200 | 4.2 to 8.4 |
Source: ISO 9223:2012, corrosion rates for the first year of exposure.
4. A fence post has a zinc coating 84 µm thick and stands in a C4 place. Using the fastest zinc rate for C4, calculate how many years it will take for the zinc to be used up.
Answer years
5. A student says: “This table must be wrong. Zinc is higher in the reactivity series than iron, so zinc should corrode faster than steel.” Evaluate the student’s reasoning. Hint: think about what the lesson says about aluminium and copper.
6. A report recommends gold water pipes because “gold is totally non-reactive and safe.” Give one scientific advantage of gold pipes and one major reason they would not be used.
7. Suggest a better way to protect iron garden stakes that keeps working even if the surface is scratched. Name the method, explain the science, use Table 1 to estimate how long an 84 µm coating would last in a C3 garden, and compare the method with a simple paint coating, including one trade-off.