Which extinguishing agent is effective on Class D fires by forming a crust and depleting oxygen?

The effective extinguishing agent for Class D fires is dry powder, which is specifically designed for combustibles such as metals (like magnesium, sodium, and potassium). Class D fires are characterized by burning metals that can react violently with water or other common extinguishing agents.

Dry powder extinguishing agents work by smothering the fire and disrupting the combustion process. When applied, the dry powder forms a protective crust over the burning material, which isolates it from the oxygen in the environment. This crust formation also helps prevent the release of flammable vapors, thereby reducing the oxygen available to sustain the fire.

In contrast, options like foam, water, and carbon dioxide are not suitable for Class D fires. Water can react with certain metals explosively, while foam is designed for Class A and B fires and is ineffective against burning metal. Carbon dioxide is more suitable for Class B and C fires, particularly for electrical equipment and flammable liquids, but does not form a crust and is not effective for the unique chemistry of Class D materials. Thus, dry powder stands out as the safe and effective choice for this particular class of fire.