The Thermal Behavior of Two Magnetic States in Smart Anticorrosion Coatings

Compounds of nitrilotrimethylenephosphonic acid (NTP) with transition metals are often used as corrosion inhibitors for steels. An FeNTP anticorrosion coating is formed on the steel surface. Doping with certain metals (for example, Zn or Cd) greatly improves anticorrosion properties. Despite the fact that FeNTP, FeZnNTP, and FeCdNTP are completely isostructural compounds, substantial changes in their properties occur upon doping: (i) according to the X-ray electron spectra, Fe atoms in FeNTP are in a high-spin (HS) state, while FeZnNTP and FeCdNTP contain Fe atoms with zero spin (LS); (ii) the difference in the quadrupole splitting of the Mössbauer spectra is specific to the ratio between the HS and LS states. Quantum mechanical calculations of the FeNTP system showed two solutions with the properties coincideing with the experimentally found LS and HS states for these systems. In this study, we tested the hypothesis about the coexistence of two states. To study possible thermal redistribution between two magnetic states, the Mössbauer spectra of FeNTP, FeZnNTP, and FeCdNTP were recorded at various temperatures (77, 300, and 373 K). The Mössbauer data indicate that, with an increase in the temperature, a second component occurs in FeNTP (the ground state is HS; LS occurs already at room temperature and its fraction increases with an increase in the temperature) and FeZnNTP (the ground state is LS; HS occurs at 373 K). At all investigated temperatures, FeCdNTP has only one LS component.