TGA-FTIR study of the vapours released by volatile corrosion inhibitor model systems (2013)

Proprietary mixtures of amines and carboxylic acids are used as volatile corrosion inhibitors\n(VCIs) for the protection of steel and iron components against atmospheric corrosion during\nstorage and transportation. Interactions between amines and carboxylic acids have been\ncomprehensively reported in the literature. However, little is known about the nature of the\nvapours these mixtures emit. The present study focused on the development of the evolved\ngas analysis method which will help in the characterisation of the vapours released by VCIs.\nIn the method, the evaporation of various amine-carboxylic acid binary mixtures was\nmonitored by thermogravimetric analysis (TGA). The nature and the composition of the\nreleased vapours was followed by Fourier transform infrared (FTIR) spectroscopy. Mixtures\nconsisting of triethylamine (TEA) and acetic acid were studied as a model compound using\nTGA-FTIR at 50 °C to validate the TGA-FTIR method. As vaporisation progressed, the\ncomposition of the remaining liquid and the emitted vapour converged to a fixed amine\ncontent of ca. 27 mol %. This is just above the composition expected for the 1:3 amine:\ncarboxylic acid complex. Mixtures close to this composition also featured the lowest\nvolatility. TGA-FTIR proved to be a convenient method for studying the evaporation of\nTEA-acetic acid mixtures, and the nature and composition of the released vapours.\nAmine addition leads to the dissociation of carboxylic acid dimers in favour salt formation.\nThe formation of an ion pair between the amine and carboxylic acid was confirmed by the FTIR spectra of the liquid phase. The resulting amine-carboxylic acid mixtures showed a\nslow mass loss rate on TGA when compared with the pure amines and pure carboxylic acids.\nThis indicated that the mixtures have low volatility, hence low vapour pressure compared\nwith the pure components. The low vapour pressure of the mixtures was confirmed by the\ncalculated gas permeability values. These values were much higher for the pure amines and\nthe pure carboxylic acids. However, they dropped significantly on amine addition. The strong\namine-carboxylic acid interaction is responsible for the suppressed volatility of the mixtures.\nNo interaction is observed between amine and carboxylic acid molecules in the vapour phase\nat 230 °C.\nThe method developed was applied to characterise the model compounds simulating the\namine-carboxylic acid-based volatile corrosion inhibitors. These model systems contained the\nprimary, secondary and tertiary amines (hexylamine, morpholine and triethylamine), as well\nas carboxylic acids with different chain lengths (acetic, propanoic, hexanoic and octanoic).\nThese systems are usually employed as equimolar mixtures to protect ferrous metals against\natmospheric corrosion. The key finding of the study was that the vapours released by such\nequimolar mixtures initially contain almost exclusively free amine. After prolonged\nvaporisation, a steady-state “azeotrope”-like composition is approached. It contains excess\nacid and features impaired corrosion-inhibition efficiencies according to the Skinner test. In\npart, this behaviour can be attributed to the mismatch between the volatilities of the amine\nand carboxylic acid constituents.