This article reports a new method of forming protective coatings on metal surfaces. The process is very simple and environmentally friendly. Polymerization occurs spontaneously on the metal surface after the immersion of a cleaned metal substrate into a dominantly aqueous monomer solution. A layer of coating with uniform thickness is formed in situ. The coating thickness can be controlled from 1 to 50 mu m. This article will describe the spontaneous polymerization process as it occurs specifically on steel and for a 4-carboxyphenyl maleimide-styrene monomer system. The polymerization propagation mechanism is free radical, and alternating copolymers are always obtained in this system, even with significant changes in the monomer feed composition. This could be attributed to the formation of a charge transfer complex between the monomer pair. The incorporation of the rigid imide ring into the copolymer backbone significantly improves the coating’s thermal properties, and styrene enhances the electrical properties. These properties are relatively insensitive to the monomer feed composition, supporting the formation of alternating copolymer. The effects of several process variables have been evaluated, including solution pH, monomer concentration, solution temperature, and monomer feed composition. The proposed initiation mechanism involves the direct electrochemical reduction of the monomer by the steel surface to generate the initiating free radicals.