Imidazolines have been used in the area of corrosion inhibition since at least the mid 1940's when it was shown that long chain organic compounds with polar functional groups had corrosion inhibition properties when applied to oil-field environments. Part I. of this paper reported the synthesis, separation/isolation and performance characteristics of various imidazoline species synthesised from oleic acid (OA) and diethylenetriamine (DETA). These techniques have been employed in order to yield relatively 'pure' imidazolines for more detailed study.
This paper presents the preliminary results of performance prediction calculations of various imidazoline species made using a previously developed semi-empirical QSAR prediction model. The data was then 're-modelled' with the new compounds to increase the base set of data the model draws on. As in the previous study, corrosion rate data for mild steel corrosion inhibitors in carbonated brine media have been fitted to the Temkin adsorption isotherm.
This study indicates that the models, even at this early stage, are capable of approximating performance of structurally similar imidazolines, although not refined enough to differentiate the effect of slight structural changes. Ideally, experimental and predicted concentrations required to give a specific performance would be equal, although, considering areas of possible error and the limited sample size still involved, this approach provides a promising approach to the prediction of corrosion inhibition efficiency.
The selection of an appropriate corrosion inhibitor for a particular system depends not only on the structure of the inhibitor, but also on parameters related to the system such as the electrolyte, the metal, amount and type of corrosion product, temperature, type of oil and the economics of the inhibition program. The design of corrosion inhibitors has traditionally involved an empirical process, although recent research has focussed on the use of theory to inform the design and selection process. The development of modem surface analytical techniques such as Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy as well as secondary ion mass spectrometry (SIMS) have enabled the study (albeit ex-situ) of inhibitor films on metal surfaces.
Through the use of the aforementioned techniques for elucidation of inhibitor adsorption mechanisms and a detailed structure-activity study (similar to those conducted by the pharmaceutical industry), steps are being made toward an appropriate methodology for the prediction of corrosion inhibitor efficiency in different environments. There have been two main approaches to the construction of structure activity relationships for corrosion inhibitors. The first, an empirical method in which each functional group in an inhibitor molecule is assumed to contribute a unique, independent and additive increment on the level of corrosion protection. Corrosion rates reflecting inhibitor performances are correlated with the molecular fragments 1. The second, a semi-empirical method, involves the correlation of molecular properties, (i.e., quantum chemical properties) with inhibitor performance. The quintessential component of the semi-empirical approach is the determination of the correct molecular descriptors.
The selection criteria employed commonly in the choice of corrosion inhibitors for oil fields are generally based on extensive laboratory and field testing, although some theories and principles have been utilised for the selection of efficacious corrosion inhibitors. A survey of the recent literature for corrosion in