When fighting metal/Class D fires, what is recommended regarding the protective crust?

• A. Break the crust to expose fuel
• B. Avoid breaking the protective crust because doing so can cause flashback
• C. Break the crust only with water
• D. Break the crust with Halon to prevent flashback

Answer: (B)

In metal/Class D fires, a crust of oxide forms on the burning surface and acts as a heat- and oxygen-impeding layer. Disturbing that crust by breaking it exposes fresh, hot metal to air, which can trigger a rapid oxidation and a flashback or re-ignition of the fire. Because of that risk, the prudent approach is to keep the crust intact and use a proper metal-fire extinguishing method to smother and cool the material without breaking the crust. Water or Halon is not appropriate for breaking the crust on metal fires, as water can react violently with reactive metals and Halon is not effective for this class of fire.