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This equation, accounting for several important effects including electromigration, diffusion, diffusion-potential, electroneutrality, and chemical and electrochemical reactions, defines the dynamic concentration distribution of each chemical species in a crevice solution.

Past research into the mechanism governing the time to active crevice corrosion-the incubation period-of a passive metal crevice has produced theoretical models coupled with the B-dot model, the Debye-Hückel limiting law, and other activity models to correct for nonideal behavior at moderately high concentrations. In this research, the transport model of Watson and Postlethwaite is coupled with the ionic interaction model of Pitzer to predict the effect of the crevice gap on the iR drop and chemical activity of the crevice solution. Two cathodic reactions, crevice external oxygen reduction and crevice internal hydrogen ion reduction, are assumed to balance metal dissolution. To validate the model, the experimental Type 304 (UNS S30400) stainless steel crevice of Alavi and Cottis is simulated. Model predictions improve upon predictions of past models and match observations of this experimental work within experimental uncertainty. The effect of crevice gap on a titanium crevice immersed in 0.5 M aqueous sodium chloride (NaCl) solution at 25°C also is predicted. The iR drop, electrical conductivity, and chemical activity of the solution increases as the crevice gap decreases. The relationship between iR drop and deviation from charge electroneutrality of the solution is investigated.

E stimated to account for five percent of the gross national product of an industrialized nation, corrosion is a major problem. Occurring in small occlusions, crevice corrosion, a form of localized corrosion, is especially dangerous because it can weaken high stress structural members causing sudden, often catastrophic, failure. An understanding of the crevice corrosion mechanism enables prediction of the passivity of crevices formed by different metals. This capability is vital for the proper selection of alloys for corrosive duty.In an effort to provide this understanding, a generic mathematical model of crevice corrosion has been developed. Capable of predicting the incubation period, the time from immersion to active crevice corrosion, of passive metals or alloys in a suitable electrolytic solution, this model could be used in the selection of alloys for corrosive duty. A key parameter in the selection of a passive metal, the incubation period is the time required to destroy the protective passive film and develop a critical crevice solution. Once attained, corrosion in a critical crevice will rapidly propagate throughout the cell.No assumptions are made upon the nature of the metal or alloy in the implementation of this model. All information is read from specially formatted text files at run-time adding unmatched flexibility to the model. The user may design these text files using an application developed as a complement to the computer model. This model features an innovative algorithm developed to adjust crevice solution composition for chemical and electrochemical equilibrium. Equilibrium expressions are assembled at run-time from a generic equilibrium description. The lack of assumptions in this algorithm allow for any complete set of chemical and electrochemical reactions to be solved.A rigorous approach to model the tendency for the system to approach electroneutrality has been developed. Based on theoretical principles, the Poisson Equation is solved to bring the system near electrically neutral conditions.The complete corrosion simulation package features several pre-assembled alloy models, and the creation of new alloy models is simple. An alloy builder, embedded in the software, creates user-specified alloys from existing metal models. The user need only specify the metals to include in the alloy and their respective weight fractions. A set of formatted text files is automatically created and the new alloy is ready to be simulated.

This paper presents a model of carbon dioxide corrosion in a steel straight pipe under turbulent fl ow, which includes protective iron carbonate fi lm formation, fi lm undermining, and fi lm erosion. The fl ow fi eld was discretized using a fi nite volume technique, with mass transport of dissolved species accounted for with an infi nitely dilute solution model. The experimental results of Nesic and Lee, Corrosion 59, 616-628 (2003) were accurately predicted and transient corrosion trends for several temperatures and carbon dioxide partial pressures were determined. The model calculates thinner but denser fi lms at higher fl uid fl ow rates, which corresponds to the experimental trend obtained by Schmitt and Mueller, NACE International, Paper 44 (1999).On présente dans cet article un modèle de corrosion au gaz carbonique dans une conduite d'acier droite dans des conditions d'écoulement turbulent, qui inclut la formation de fi lms de carbonate de fer protecteur, le soulèvement des fi lms et l'érosion de fi lms. Le champ d'écoulement a été discrétisé à l'aide d'une technique de volumes fi nis, où le transport de matière d'espèces dissoutes prend en compte un modèle de solutions infi niment diluées. Les résultats expérimentaux de Nesic et Lee (2003) ont été prédits avec précision, et des tendances de corrosion pour plusieurs températures et pressions partielles de gaz carbonique ont été déterminées. Le modèle calcule des fi lms plus minces mais plus denses à des débits de fl uides plus élevés, ce qui corrobore la tendance expérimentale obtenue par Schmitt et Mueller (1999).