EIS investigation and patina characterization of weathering steel exposed to each of the four seasons in the São Paulo metropolitan area

Transition between growth of dense and porous films: theory of dual-layer SEI

The formation of passivating films is a common aging phenomenon, for example in weathering of rocks, silicon, and metals. In many cases, a dual-layer structure with a dense inner and a porous outer layer emerges. However, the origin of this dual-layer growth is so far not fully understood. In this work, a continuum model is developed, which describes the morphology evolution of the solid-electrolyte interphase (SEI) in lithium-ion batteries. Transport through the SEI and a growth reaction governed by the SEI surface energies are modelled. In agreement with experiments, this theory predicts that SEI grows initially as a dense film and subsequently as a porous layer. This dynamic phase transition is driven by the slowing down of electron transport as the film thickens. Thereby, the model offers a universal explanation for the emergence of dual-layer structures in passivating films.

Evolution and Evaluation of Aesthetic Properties in Weathering Steel Accelerated Patinas: The Role of Lepidocrocite

Weathering steels are widely used in civil engineering, architecture and contemporary art due to their mechanical properties, their enhanced resistance to atmospheric corrosion as well as their aesthetic properties. Artists and blacksmiths often apply chemical treatments to obtain the appealing colors of a patina in a shorter period of time. However, the development of an accelerated patina may have an effect on the final appearance and color of the surface. With the aim of evaluating differences in color and studying the evolution of the surface, eight accelerated patinas were made and exposed to the atmosphere for periods of time of up to 24 months and were compared to a natural patina. The characterization studies showed the presence of lepidocrocite on the surface. A close inspection of the X-ray diffraction patterns showed the displacement of the (020) lepidocrocite reflection and asymmetric broadening of selective lines of this phase that were associated to stacking and twins faults, respectively. These faults decrease with the exposure time and are related to a maximum at 630 nm in the reflectance spectrum and the stabilization of the b* coordinate (yellow color). The colors of the accelerated patinas differ from the natural patina at short exposure times. However, they tend to converge at longer exposure times.

The Holes of Zn Phosphate and Hot Dip Galvanizing on Electrochemical Behaviors of Multi-Coatings on Steel Substrates

The aim of this investigation is focused on the evaluation of distinctive coatings commonly applied in the automotive industry. The resulting corrosion behavior is analyzed by using electrochemical impedance spectroscopy (EIS), equivalent circuit (EC) and potentiodynamic polarization curves. The novelty concerns a comparison between tricationic phosphate (TCP), cataphoretic electrodeposition (CED) of an epoxy layer, TCP + CED and HDG (hot-dip galvanized) + TCP + CED multi-coatings. Both the naturally deposited and defect-induced damage (incision) coatings are examined. The experimental impedance parameters and corrosion current densities indicate that multi-coating system (HDG + TCP + CED layers) provides better protection. Both planar and porous electrode behaviors are responsible to predict the corrosion mechanism of the majority of samples examined. Although induced-damage samples reveal that corrosion resistances decreased up to 10×, when compared with no damaged samples, the same trend of the corrosion protection is maintained, i.e., TCP < CED < TCP + CED < HDG + TCP + CED. It is also found that the same trend verified by using electrochemical parameters is also observed when samples are subjected under salt spray condition (500 h). It is also found that porous electrode behavior is not a deleterious aspect to corrosion resistance. It is more intimately associated with initial thickness coating, while corrosion resistance is associated with adhesion of the CED layer on TCP coating. The results of relative cost-to-efficiency to relative coating density ratios are associated with fact that a CED coating is necessary to top and clear coating applications and the TCP + CED and the HDG/TCP + CED coating systems exhibit the best results.

EIS Investigation of the Corrosion Behavior of Steel Bars Embedded into Modified Concretes with Eggshell Contents

This investigation is focused on evaluation of the corrosion behavior of embedded steel bars (SB) into concretes. Conventional and modified concretes with eggshell are prepared. Although the effect of calcium carbonate on mechanical behavior is recognized and reported, their effects as eggshell (ES) particles replacing portions of sand and cement contents are reasonably scarce. Corrosion behavior is evaluated by electrochemical impedance spectroscopy (EIS) and the potentiodynamic polarization technique. Equivalent circuit and porous electrode behavior are also considered. The novelty concerns a promising use of concrete with ES content to maintain corrosion resistance concatenated with reasonable structural properties. For this purpose, three distinct concrete mixtures are proposed, i.e., a reference and two modified concretes. One replaces 10 wt.% with cement and another 10 wt.% with sand content. It is found that porous electrode behavior helps to predict the corrosion mechanism. Finer ES particles in concrete mixture provides a rapidly passivation on rebar. This reflects positively in corrosion current density after long-term immersion. Additionally, an environmentally friendly aspect associated with economical factor constitutes a promise use of the concrete.