Definition of kinetic characteristics of decomposition reactions of nitrilo-tris(methylenephosphonato)triaqua-iron(II) [Fe$^{\mathrm{II}}$(H$_2$O)$_3\{\mu$-NH(CH$_2$PO$_3$H)$_3\}$]$_n$ by non-isothermal kinetics method with use of thermogravimetry

F. F. Chausov^abc, V. G. Petrov^b, V. A. Aleksandrov^b, M. A. Shumilova<sup>b

a</sup> Udmurt State University, Izhevsk, Russia
^b Institute of Mechanics, Ural Branch of the Russian Academy of Sciences, Izhevsk, Russia
^c Physical and Technical Institute, Ural Branch of the Russian Academy of Sciences, Izhevsk, Russia

Abstract: Main stages of thermal decomposition of nitrilo-tris(methylenephosphonato) triaqua-iron(II) [Fe$^{\mathrm{II}}$(H$_2$O)$_3\{\mu$-NH(CH$_2$PO$_3$H)$_3\}$]$_n$ by non-isothermal kinetics method are investigated. This coordination polymer is formed at interaction of corrosion inhibitor — nitrilotrismethylenephosphonic acid — with Fe$^{2+}$ ions and is one of components of protective anticorrosive layers at corrosion inhibition of iron alloys in neutral water environments. The analysis of the thermogravimetric data obtained when heating samples [Fe$^{\mathrm{II}}$(H$_2$O)$_3\{\mu$-NH(CH$_2$PO$_3$H)$_3\}$]$_n$ in the range of 30–500 $^{\circ}$C in the argon atmosphere has shown that process of thermal decomposition three main stages includes: an eliminating of two water molecules from the iron coordination sphere (30–109 $^{\circ}$C), an eliminating of one water molecule from the iron coordination sphere (109–175 $^{\circ}$C), disintegration of a ligand with eliminating of nitrogen and methylphosphonic acid (175–228 $^{\circ}$C). Processing of the thermogravimetric data and the data of differential thermal analysis by Horovitz-Menger's method have allowed evaluating of activation energy and preexponential kinetic coefficient for all three stages, respectively: Е$_{а1}$ = 54.129 kJ/mol, k$_{01}$ = 5.83$\cdot$10$^6$ s$^{-1}$; Е$_{а2}$ = 69.355 kJ/mol, k$_{02}$ = 1.54$\cdot$10$^7$ s$^{-1}$; Е$_{а3}$ = 151.650 kJ/mol, k$_{03}$ = 2.22$\cdot$10$^{14}$ s$^{-1}$. The received results demonstrate not equal bonding force of three coordinated water molecules with the central iron atom, showing thereby influence of Jahn-Teller's distortion of the Fe(II) coordination sphere. Data on thermal decomposition kinetic constants of a nitrilotris(methylenephosphonato) triaqua-iron(II) are of practical interest to problems of corrosion inhibition in neutral water environments at the raised temperatures, for example, for anticorrosive protection of a steam and hot water boilers.

Keywords: nitrilo-tris(methylenephosphonato)triaqua-iron(II), thermal decomposition, thermogravimetry, kinetic characteristics, corrosion inhibitors.

UDC: 547.1'546.722+543.(572.3+573)