The emergence of multiphysics modeling tools that span molecular interactions, solid-state physics, and materials microstructure through to thermodynamics, fluid mechanics and electrochemical kinetics provides new opportunities for the construction of predictive modeling tools for corrosion science and engineering. One particular field in which models have been actively developed from the atomistic to macroscopic levels includes the problem of the prediction of performance and the molecular design of chemical corrosion inhibitors. Herein we provide a concise review of these historical and contemporary approaches. Afterwards, a general outline for a multiphysics model is presented for the prediction of corrosion inhibitor efficiency (i.e. % reduction in corrosion rate) as a function of environment, material, inhibitor concentration, and the molecular identity of the inhibitor. Applications to experimental design and analysis, lifetime prediction and inhibitor design are then discussed.

Organic corrosion inhibitors can provide an effective means to extend the life of equipment in aggressive environments, decrease the environmental, economic, health and safety risks associated with corrosion failures and enable the use of low cost steels in place of corrosion resistant alloys. To guide the construction of advanced models for the design and optimization of the chemical composition of organic inhibitors, and to develop predictive tools for inhibitor performance as a function of alloy and environment, a multiphysics model has been constructed following Staehle's principles of "domains and microprocesses". The multiphysics framework provides a way for science-based modelling of the various phenomena that impact inhibitor efficiency, including chemical thermodynamics and speciation, oil/water partitioning, effect of the inhibitor on multiphase flow, surface adsorption and self-assembled monolayer formation, and the effect of the inhibitor on cathodic and anodic reaction pathways. The fundamental tools required to solve the resulting modelling from a first-principles perspective are also described. Quantification of uncertainty is significant to the development of lifetime prediction models, due to their application for risk management. We therefore also discuss how uncertainty analysis can be coupled with the first-principles approach laid out in this paper.

Quantum chemistry is a powerful tool for computing the properties of molecules and their interactions with one another in a variety of environments. In this second paper of a two-part series, the technique is applied in this work to calculate fundamental properties of inhibitor molecules important to the overall corrosion inhibitor performance. The study focuses on the issue of oil/water partitioning as quantified by the partition coefficient (log P) and the important issue of inhibitor speciation according to the acid dissociation constant (pKa). pKa and log P values are then calculated from first-principles for a series of imidazole derivatives and integrated into a model for inhibitor availability as a function of the water cut. Applications to lifetime prediction and inhibitor design are then discussed.

The breakdown of passivity of iron in alkaline sulfide solution is a two-step process. The first step involves the nucleation of mackinawite patches at faults in the oxide film and their subsequent growth by dissolution of the oxide. The second step is associated with the growth of pits under these patches. At higher positive potentials, mackinawite may be oxidized to F%O3 and sulfur with simultaneous formation of pyrite. The resistance of passive films to breakdown is affected not only by its thickness but also by its composition (expressed as deviation from stoichiometry). Both are determined by the potential and time of growth. Emphasis was given to the kinetics and mechanism of mackinawite deposition on passive iron, and its relationship with the passivity breakdown process.

Results are presented of experiments performed using a new method to measure the naphthenic acid corrosion potential. The method consists of adding pure iron powder into a small autoclave containing the crude or oil sample. The test is then performed at a given temperature for one hour, after which the oil sample is filtered and the remaining liquid is sent for iron content determination (ppm). The tests are run at 7 different temperature levels, 3 more are run as repeated tests. A best-fitted curve is drawn through these 10 experimental points and the maximum point is thus determined. This becomes the main outcome of the test and it is used to give a measure of the naphthenic acid corrosion potential. The same general trends as observed in the past using the neutralization number or TAN (Total Acid Number) is obtained. However, this new test seems capable oí detecting anomalous cases where oil samples having larger values of TAN exhibit less corrosivity than others having much lower values of TAN or where they show completely different corrosivity despite having similar or the same TAN. KeywordsNaphthenic acid corrosion. Total Acid Number (TAN). The Fe powder test. Palabras claveCorrosión por ácidos nafténicos. Número de Acidez Total (TAN). Ensayo del polvo de hierro.