The carbon coating in carbon-encapsulated magnetic nanoparticles is considered as a tight and impermeable barrier which should perfectly protect the magnetic core material from external chemical environment. To study the integrity of the coating, carbon-encapsulated iron nanoparticles were subjected to corrosion tests, in which various corrosion agents were used. Several mineral and organic acids, as well as active gaseous environments with various oxidation potential, were applied. Additionally, the corrosion resistance was studied under the so-called galvanic corrosion, using two metal ions (copper and silver) which have higher redox potential than the zero-valent iron. The release of iron from the core as well as the morphology, structural features, chemical composition, and magnetic properties of carbon-encapsulated iron nanoparticles was systematically monitored at each stage of the corrosion process. The largest release of Fe from the encapsulate core was observed when nitric acid was used as the corrosion agent.