What should not be used to extinguish a Class D fire involving combustible metals such as magnesium?

In extinguishing a Class D fire, which involves combustible metals like magnesium, water should not be used because it can react violently with the burning metal. When water is applied to magnesium, it can cause the magnesium to ignite even more vigorously and create explosive hydrogen gas, worsening the fire rather than extinguishing it.

Class D fires require specialized extinguishing agents that can safely deal with the unique characteristics of combustible metals. Dry powder extinguishers, typically filled with agents like sodium chloride, are designed specifically for these types of fires and can smother the flames effectively. Other options, such as foam and carbon dioxide extinguishers, are not suitable for Class D fires as they do not possess the necessary properties to suppress metal fires without risking a dangerous reaction. Therefore, avoiding water is crucial in effectively managing a Class D fire.