Rusting in a Coastal Workshop: Choosing a Six-Month Storage Method
A workshop in a coastal town stores 5 000 iron nails and 2 000 steel screws in open cardboard boxes on a concrete floor. The air is 30 °C with 85% relative humidity and carries salt spray from the sea 800 m away. Within three weeks the nails show reddish-brown flaking and the cardboard is stained. The nails must stay usable for at least six months, and the screws must keep their exact diameter because they are used in precision assemblies. The manager considers four options:
- A. Spray the nails with oil: RM 120 per month; the oil must be reapplied every time a box is opened.
- B. Keep opened stock in sealed plastic containers with silica gel: RM 400 for the containers plus RM 60 per month for gel.
- C. Paint the nails: RM 250, but the paint must be removed from every nail before use.
- D. Hot-dip galvanise the screws: RM 900, adds about 0.08 mm to the diameter.
1. Using the conditions given, explain why the nails rust so quickly. Refer to the requirements for rusting and the specific figures in the data.
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Rusting requires iron, oxygen and water - and it is an electrochemical process, so dissolved ions speed it up.
- Water: 85% relative humidity at 30 °C means a thin film of moisture is present on the metal almost continuously; 30 °C also raises the rate of reaction compared with a cool store.
- Oxygen: the nails sit in open cardboard boxes, so air circulates freely over the surface.
- Electrolyte (the key factor here): salt spray from the sea 800 m away deposits sodium chloride on the metal. Salt water conducts electricity far better than pure water, so the tiny electrochemical cells on the iron surface complete their circuit more easily and iron ions leave the metal faster. Salt also absorbs moisture from the air, keeping the surface wet for longer.
- Cardboard: it absorbs moisture and holds it against the nails, so the nails sit in a damp, salty layer rather than drying out.
Product: hydrated iron(III) oxide, which is flaky and porous - it does not seal the surface, so the iron underneath keeps corroding until it is used up. That is why three weeks is enough to see visible flaking.
2. Compare options A, B and C for the nails over six months and recommend one. Support your answer with calculations and with the practical constraints in the situation.
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Cost over six months:
- A: 120 × 6 = RM 720, plus labour.
- B: 400 + (60 × 6) = RM 760.
- C: RM 250 plus the labour of removing paint from 5 000 nails.
Judging each option:
- A is competitive on cost but fails on performance: the oil coating is wiped off each time a box is opened, and it is exactly the opened boxes that are exposed. Protection is therefore intermittent, and reapplication depends on someone remembering.
- C looks cheapest but is unusable: paint must be removed from every single nail before use - hours of labour, and scraping damages the surface, creating fresh sites for rust. Paint also fills the threads of any screw.
- B costs about the same as A but attacks the actual cause: sealing removes the humid air, and silica gel absorbs the moisture that enters when the box is opened. It needs no repeated application, the containers are reusable beyond six months, and the gel can be dried in an oven and reused.
Recommendation: B, because it removes the water and the salt-laden air rather than relying on a coating that is destroyed by handling. On a per-month basis B becomes cheaper the longer the stock is kept, and it protects the screws too.
3. Explain why option D is unsuitable for the screws, and suggest one alternative that would keep the screw diameter unchanged.
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Why D fails: galvanising adds about 0.08 mm to the diameter, meaning roughly 0.04 mm on each side of the thread. The screws are for precision assemblies, so this is the wrong direction of error - a screw that is 0.08 mm oversize will not enter a matching tapped hole, and forcing it damages the thread. Galvanising also risks hydrogen embrittlement in high-strength steel, and the zinc layer can flake where threads are cut.
Alternative that keeps the diameter unchanged: store the screws in sealed containers with volatile corrosion inhibitor (VCI) paper or with silica gel, exactly as in option B. The VCI vapour coats the metal with a molecular layer only a few nanometres thick, which is far below the tolerance and needs no removal. Other acceptable answers: use stainless steel screws (the material itself resists rust, though at higher cost and different strength), or apply a thin film of oil that is wiped off before assembly without changing dimensions.
General principle to state: a protection method must not alter the dimensions or the function of the part; the cheapest coating is useless if the component no longer fits.