Magnetic nanoparticles embedded into anti-corrosion primer coatings are demonstrated as potential sensors for the depletion of corrosion inhibitor molecules. In corrosive environments these nanoparticles are chemically transformed to species that have different structural and magnetic properties. By monitoring changes in their magnetic state in such environments, strong correlations between the particles’ magnetic state and the amount of remaining inhibitor molecules can be established, enabling a novel sensing approach that is both non-destructive and non-contact. Herein we demonstrate the transformation of the magnetic particles via characterisation of their magnetic and material properties after aqueous ageing at different pH, by X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, and vibrating sample magnetometry (VSM). Furthermore, a non-destructive approach for monitoring the magnetic particles that have been placed in coatings, through their interactions with alternating magnetic fields, is also proposed.