308A Metal Types and Properties 2 — Questions and Answers
Question 1: What is 'galvanic corrosion' and why must sheet metal workers be aware of it?
- Corrosion caused by water only
- Accelerated corrosion that occurs when dissimilar metals are in contact in the presence of an electrolyte (Correct answer)
- Corrosion caused by sunlight
- Corrosion that only affects painted surfaces
Correct answer: Accelerated corrosion that occurs when dissimilar metals are in contact in the presence of an electrolyte
Galvanic corrosion occurs when two different metals are in contact with each other and exposed to moisture. The more reactive (anodic) metal corrodes faster than it would alone. Sheet metal workers must isolate dissimilar metals.
Question 2: What is the thickness in inches of 24-gauge galvanized steel?
- 0.0598 inches
- 0.0239 inches (Correct answer)
- 0.1000 inches
- 0.5000 inches
Correct answer: 0.0239 inches
24-gauge galvanized steel has a thickness of approximately 0.0239 inches (0.607 mm). This is a common gauge used for residential and light commercial ductwork.
Question 3: What is copper sheet metal primarily used for in the sheet metal trade?
- Structural framing
- Roofing, flashing, gutters, and architectural features due to its durability and aesthetic patina (Correct answer)
- HVAC main trunk ductwork
- Heavy industrial exhaust systems
Correct answer: Roofing, flashing, gutters, and architectural features due to its durability and aesthetic patina
Copper is prized for architectural applications including roofing, flashings, gutters, downspouts, and decorative elements. It develops a distinctive green patina over time and has excellent corrosion resistance.
Question 4: What is 'work hardening' and how does it affect sheet metal during fabrication?
- The metal gets softer with more bending
- The metal becomes harder, stronger, and more brittle with repeated bending or forming, eventually cracking if overworked (Correct answer)
- The metal changes colour
- The metal becomes heavier
Correct answer: The metal becomes harder, stronger, and more brittle with repeated bending or forming, eventually cracking if overworked
Work hardening (strain hardening) occurs when metal is repeatedly deformed — bent, hammered, or formed. The crystal structure becomes distorted, making the metal harder but more brittle. Annealing reverses this process.
Question 5: What is the purpose of 'annealing' sheet metal?
- To make it harder
- To soften work-hardened metal by heating it to a specific temperature and cooling it slowly, restoring ductility (Correct answer)
- To add a protective coating
- To increase its thickness
Correct answer: To soften work-hardened metal by heating it to a specific temperature and cooling it slowly, restoring ductility
Annealing heats work-hardened metal to a temperature that allows the crystal structure to recrystallize, then cools it slowly. This restores the metal's ductility and formability, allowing further work without cracking.
Question 6: What is the difference between 'ferrous' and 'non-ferrous' metals?
- Ferrous metals are always stainless
- Ferrous metals contain iron as their primary element; non-ferrous metals do not contain significant iron (Correct answer)
- Non-ferrous metals are always magnetic
- There is no practical difference
Correct answer: Ferrous metals contain iron as their primary element; non-ferrous metals do not contain significant iron
Ferrous metals contain iron (steel, stainless steel, cast iron) and are generally magnetic and susceptible to rust. Non-ferrous metals (aluminum, copper, brass, zinc) do not contain significant iron and are typically non-magnetic.
308A Metal Types and Properties 2
What is 'galvanic corrosion' and why must sheet metal workers be aware of it?