Answer: Choose metals and alloys that resist the environment and avoid dissimilar-metal contact; design to avoid crevices, stagnant water, sharp corners and small-anode/large-cathode joints, with easy drainage and access for inspection.
A. Proper selection of material
- Use noble or corrosion-resistant metals where affordable (Ti, Ni, Cu alloys) and pure metals rather than impure ones.
- Use corrosion-resistant alloys such as stainless steel (≥ 12% Cr forms a protective Cr₂O₃ film), monel, brass.
- Avoid contact of two metals far apart in the galvanic series; if unavoidable, choose metals close together in the series.
- Choose the material for the actual medium: e.g. stainless steel resists oxidising acids but suffers pitting in chloride-rich water.
- Non-metallic materials (plastics, ceramics, rubber linings) can be used where suitable.
B. Proper design (principles, with figures described)
- Avoid galvanic contact or insulate it: figure: a bolt joining a steel plate to a copper plate with an insulating washer and sleeve between the metals ("good"), versus direct metal contact ("bad").
- Large anode, small cathode: if dissimilar metals must touch, the more active metal should have the larger area (e.g. copper rivets in a steel plate, not steel rivets in copper).
- Avoid crevices and gaps: figure: a riveted lap joint with a narrow gap where moisture collects ("bad") next to a butt-welded joint with a smooth surface ("good"). Welded joints are preferred to riveted ones.
- Allow complete drainage: figure: a tank with a flat bottom and the outlet a little above the base, leaving stagnant liquid ("bad"), next to a tank with a sloping bottom and the outlet at the lowest point ("good"). Angle sections should be placed so that water drains off.
- Avoid sharp corners and bends: use rounded corners and smooth bends in pipes, which reduce turbulence and erosion-corrosion and avoid stagnant pockets.
- Uniform stress and temperature: avoid stressed regions (stress corrosion) and hot spots; anneal after cold working.
- Accessibility: parts should be easy to inspect, clean, repaint and replace; avoid designs that trap dust and moisture.
