Periodic aggregation patterns of oxide particles on corroding metals: chemical waves due to solution feedback processes

Chemical waves that produce periodic patterns are common occurrences in nature. The underlying processes involved have been studied in many disciplines of science, but rarely reported in the chemistry of corrosion. In this study of carbon steel corrosion, iron oxide crystals are observed to deposit in concentric wave patterns or in discrete bands, known as Liesegang patterns. We demonstrate that oxide growth in these patterns is preceded by the formation of a hydrogel network, which consists of a semi-stationary phase of loosely connected metal-hydroxide colloids and a mobile phase of solution saturated with metal cations. Once the hydrogel network covers the metal surface, a metal cation produced by corrosion reactions at the metal surface must diffuse through the layer into the bulk solution. While diffusing through the porous network, the metal cation undergoes adsorption-precipitation as metal-hydroxide colloids which later can dissolve back into the solution. When the kinetics of precipitation and dissolution of the metal cation can be effectively coupled with the transport flux of the dissolved metal cation, the precipitation-dissolution-diffusion cycles can be sustained over time which can lead to periodic aggregation patterns of metal-hydroxide colloids at a specific time. We also establish that for transition metal cations the precipitation-dissolution-diffusion process can couple with reversible redox reactions between the soluble and less soluble metal cations, which can affect the overall transport of banded aggregates of metal-hydroxide colloids and the growth and transformation of metal-hydroxides into crystalline oxides. If systemic feedback between different elementary processes is sustained over long durations, iron-oxide crystals of different chemical compositions and shapes aggregate in Liesegang patterns. This work demonstrates unequivocally that non-uniform deposition of metal oxides during corrosion can occur via strongly coupled solution reactions and transport processes, and not simply as a result of metallurgical non-uniformity and/or localized solution environments.

Using recurrence plots for the analysis of the nonlinear dynamical response of iron passivation-corrosion processes

Recurrence plots (RPs) and recurrence quantification analysis (RQA) are used in this work to study different nonlinear dynamical regimes emerging in an electrochemical system, namely, the electrodissolution-passivation of iron in chloride-containing sulfuric acid solutions. Current oscillations at different applied potentials and chloride concentrations exhibit bifurcations from periodic to complex (bursting) periodic and aperiodic or chaotic behaviors, associated with different dissolution states of iron. The clarification of these transitions is essential to understand the type of corrosion (uniform or localized) taking place as well as the underlying mechanisms governing the stability of the metal. The RQA reveals that the predictability of the chloride-perturbed Fe|0.75M H₂SO₄ system strongly depends on the chloride concentration and the applied potential. At relatively low chloride concentrations, RQA measures, based on vertical and diagonal structures in RPs, display a decrease upon the breakdown of the passivity on iron and the initiation of localized corrosion (pitting). Phases of pitting corrosion (propagation/growth and unstable pitting) that followed pit initiation are discerned by keen changes of complexity measures upon varying the applied potential. At higher chloride concentrations, the evolution of RQA measures with the potential signifies a transition from the passive-active state dissolution to the polishing state dissolution of iron inside pits. The increase of the applied potential at late stages of pitting corrosion increases the nonlinear correlations and thus the complexity of the system decreases, which corroborates the RQA.

First-principles study on the atomistic corrosion processes of iron

The corrosion of iron presents an important scientific problem and a serious economic issue. It is also one of the most important subjects in materials science because it is basically an electrochemical process and closely related to other topics such as the electrocatalysis of the oxygen reduction reaction. So far, many studies have been conducted to address the corrosion of iron, a very complicated process that occurs when iron is exposed to oxygen and water. An important question is, at which site of the iron surface the corrosion starts and how it results in the final stage of the corrosion. In the present study, as an example of superficial defects, Fe dimers sticking out of Fe(100) surfaces are considered in order to understand the iron corrosion process from first-principles using density functional theory. We found that the Fe dimers spontaneously react with O₂ and H₂O to form Fe₂(OH)₄ + 4OH^-. Here, it is interesting to note that the Fe dimer plays the role of a water splitting catalyst, because the space above it is always vacant and can accept oxygen molecules many times for reacting with the surrounding water molecules. Then, if the Fe₂(OH)₄ molecules are detached from the surface, they react with O₂ to form Fe₂O(OH)₄ without an activation barrier, and, in turn, the Fe₂O(OH)₄ and H₂O molecules react to form Fe₂(OH)₆ complexes with an activation energy of 0.653 eV. If these complexes further dissociate into Fe(OH)₃ molecules, they react with each other to form Fe₂O₃·2H₂O with an activation energy of 0.377 eV. This work may provide useful information on possible iron corrosion processes by water in the air.