Abstract: An immersion heater (1) for use in a chemically corrosive environment. An electrically conductive heating element (7) is immersible in the environment (C) and is capable of a watt-density of at least 30 watts per square inch when current flows through the element. A metallic sheath (9) covers the heating element. A coating (11) is applied over the sheath. The coating is capable of maintaining its coating properties not only when the heater is immersed in the environment, but also when the watt-density of the heating element exceeds 30 watts per square inch, and the temperature of the environment exceeds the boiling point of water.