Abstract: A mixture of magnesium-iron alloy and salt is confined within a nonwoven cellulose covering and forms an elongated cartridge which is wrapped with multiple layers of absorbent paper. The cartridge and paper are enclosed within a pouch of perforated plastic film to form a heating element. One or more heating elements are attached by adhesive tape to the inside of a generally triangular polyethylene film envelope, and peripheral portions of the envelope are heat sealed to form an enclosed chamber. An overlying mating sheet of polyethylene coated nylon film and a pad of thermoplastic foam material are heat sealed to peripheral portions of the envelope, and parallel strips of double-sided adhesive tape, with removal protective strips, are attached to the outer surface of the envelope to form a flexible heater device. The device is used by being wrapped around either a vertical or horizontal pipe joint containing a heat activated epoxy.