Proprietary mixtures of amines and carboxylic acids are used as volatile corrosion inhibitors
(VCIs) for the protection of steel and iron components against atmospheric corrosion during
storage and transportation. Interactions between amines and carboxylic acids have been
comprehensively reported in the literature. However, little is known about the nature of the
vapours these mixtures emit. The present study focused on the development of the evolved
gas analysis method which will help in the characterisation of the vapours released by VCIs.
In the method, the evaporation of various amine-carboxylic acid binary mixtures was
monitored by thermogravimetric analysis (TGA). The nature and the composition of the
released vapours was followed by Fourier transform infrared (FTIR) spectroscopy. Mixtures
consisting of triethylamine (TEA) and acetic acid were studied as a model compound using
TGA-FTIR at 50 °C to validate the TGA-FTIR method. As vaporisation progressed, the
composition of the remaining liquid and the emitted vapour converged to a fixed amine
content of ca. 27 mol %. This is just above the composition expected for the 1:3 amine:
carboxylic acid complex. Mixtures close to this composition also featured the lowest
volatility. TGA-FTIR proved to be a convenient method for studying the evaporation of
TEA-acetic acid mixtures, and the nature and composition of the released vapours.
Amine addition leads to the dissociation of carboxylic acid dimers in favour salt formation.
The 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
slow mass loss rate on TGA when compared with the pure amines and pure carboxylic acids.
This indicated that the mixtures have low volatility, hence low vapour pressure compared
with the pure components. The low vapour pressure of the mixtures was confirmed by the
calculated gas permeability values. These values were much higher for the pure amines and
the pure carboxylic acids. However, they dropped significantly on amine addition. The strong
amine-carboxylic acid interaction is responsible for the suppressed volatility of the mixtures.
No interaction is observed between amine and carboxylic acid molecules in the vapour phase
at 230 °C.
The method developed was applied to characterise the model compounds simulating the
amine-carboxylic acid-based volatile corrosion inhibitors. These model systems contained the
primary, secondary and tertiary amines (hexylamine, morpholine and triethylamine), as well
as carboxylic acids with different chain lengths (acetic, propanoic, hexanoic and octanoic).
These systems are usually employed as equimolar mixtures to protect ferrous metals against
atmospheric corrosion. The key finding of the study was that the vapours released by such
equimolar mixtures initially contain almost exclusively free amine. After prolonged
vaporisation, a steady-state “azeotrope”-like composition is approached. It contains excess
acid and features impaired corrosion-inhibition efficiencies according to the Skinner test. In
part, this behaviour can be attributed to the mismatch between the volatilities of the amine
and carboxylic acid constituents.