308A Metal Types and Properties

Question 1

What is the most commonly used metal for HVAC ductwork fabrication?

Accepted Answer

Galvanized steel

Answer

Galvanized steel (steel coated with zinc) is the most widely used material for HVAC ductwork. The zinc coating provides corrosion resistance, and the steel provides structural strength at a reasonable cost.

Question 2

What is the purpose of the zinc coating on galvanized steel?

Accepted Answer

To provide sacrificial corrosion protection — the zinc corrodes before the base steel

Answer

Zinc acts as a sacrificial anode, corroding preferentially before the underlying steel. Even if the coating is scratched, the zinc continues to protect nearby exposed steel through galvanic (cathodic) protection.

Question 3

What gauge system is used for sheet metal thickness in Canada, and how does the gauge number relate to thickness?

Accepted Answer

Lower gauge numbers indicate thicker metal — 16 gauge is thicker than 24 gauge

Answer

The gauge system is inversely related to thickness. Lower numbers are thicker (16 gauge = 0.0598") and higher numbers are thinner (26 gauge = 0.0179"). This system is used throughout North America.

Question 4

What type of stainless steel is most commonly used in sheet metal work and what makes it 'stainless'?

Accepted Answer

Type 304 (18-8), containing at least 18% chromium and 8% nickel that form a passive oxide layer

Answer

Type 304 (also called 18-8) stainless steel contains minimum 18% chromium and 8% nickel. The chromium forms a thin, self-healing chromium oxide layer on the surface that resists corrosion.

Question 5

What is the primary advantage of using aluminum sheet metal for certain applications?

Accepted Answer

It is lightweight (about 1/3 the weight of steel) and naturally corrosion-resistant

Answer

Aluminum weighs approximately one-third as much as steel and forms a natural oxide layer that provides excellent corrosion resistance. These properties make it ideal for architectural, marine, and food-processing applications.

Red Seal Sheet Metal 308A Practice Test

Red Seal Sheet Metal 308A Practice Test

308A Metal Types and Properties