Filiform corrosion as a pressure-driven delamination process.

Filiform corrosion produces long and narrow trails on various coated metals through the detachment of the coating layer from the substrate. In this work, we present a combined experimental and theoretical analysis of this process with the aim to describe quantitatively the shape of the cross-section, perpendicular to the direction of propagation, of the filaments produced. For this purpose, we introduce a delamination model of filiform corrosion dynamics and show its compatibility with experimental data where the coating thickness has been varied systematically.

Filament dynamics in confined chemical gardens and in filiform corrosion.

Two reaction systems that are at first sight very different produce similar macroscopic filamentary product trails. The systems are chemical gardens confined to a Hele-Shaw cell and corroding metal plates that undergo filiform corrosion. We show that the two systems are in fact very much alike. Our experiments and analysis show that filament dynamics obey similar scaling laws in both instances: filament motion is nearly ballistic and fully self-avoiding, which creates self-trapping events.

Compositional analysis of copper-silica precipitation tubes.

Silica gardens consist of hollow tubular structures that form from salt crystals seeded into silicate solution. We investigate the structure and elemental composition of these tubes in the context of a recently developed experimental model that allows quantitative analyses based on predetermined reactant concentrations and flow rates. In these experiments, cupric sulfate solution is injected into large volumes of waterglass. The walls of the resulting tubular structures have a typical width of 10 microm and are gradient materials. Micro-Raman spectroscopy along with energy dispersive X-ray fluorescence data identify amorphous silica and copper(ii) hydroxide as the main compounds within the inner and outer tube surfaces, respectively. Upon heating the blueish precipitates to approximately 150 degrees C, the material turns black as copper(ii) hydroxide decomposes to copper(ii) oxide. Moreover, we present high resolution transmission electron micrographs that reveal polycrystalline morphologies.

Bubble guidance of tubular growth in reaction-precipitation systems.

Numerous types of reaction-precipitation systems involve the growth of tubular structures similar to those formed in "silica gardens". As a model case for this phenomenon, we investigate the rapid growth of hollow tubes in the reaction between sodium silicate and cupric sulfate. The latter solution is injected hydrodynamically at constant flow rates of 1-20 mL h(-1) into a large reservoir of waterglass. In this study, the growth is templated and guided by single, buoyant gas bubbles. The resulting tubes can be several decimetres long and have constant radii in the range of 100-600 microm. Systematic measurements show that bubble size governs the tube radius. According to this radius, the system selects its growth velocity following volume conservation of the injected solution. Moreover, scanning electron microscopy reveals intricate ring patterns on the tube walls. We also show evidence for the existence of a minimal and a maximal tube radius. Finally, we report an intriguing collapse of tubes created at high silicate concentrations, which yields twisted ribbon-like structures. Critical radii and tube collapse are discussed in terms of simple competing forces.

Oscillatory growth of silica tubes in chemical gardens.

We report distinct growth regimes of hollow silica fibers formed by hydrodynamic injection of cupric sulfate into silicate solution. The tubes grow either steadily along a continuous jet of buoyant solution or through relaxation oscillations that are governed by chemo-mechanical processes. The dependence of the oscillation period on flow rate and copper concentration is explained in the framework of a simple model. Tailored flow patterns allow the directional control of the tubes and their use as miniature connectors. Our findings are applicable to the understanding of chemical gardens, promise a wealth of nonlinear phenomena, and offer possible applications in microfluidics.

Traveling fronts of copper deposition.

We report the experimental observation of traveling fronts during the electroless deposition of copper on passive steel substrates. The low-carbon steel samples are passivated in nitric acid prior to the plating experiment, thus creating a thin, protective oxide layer on the steel surface. The deposition experiments are carried out from slightly acidic (pH 3.2) copper sulfate solution and copper nitrate solution with the latter showing front propagation only in the presence of chloride ions. For up to 30 s, fronts propagate with constant velocities in the range from 0.5 to 5 mm/s depending on the experimental conditions. This phase of constant-speed propagation gives way to accelerating fronts and very rapid, spatially unstructured deposition. Front-mediated plating is observed over a wide range of cupric ion concentration and constitutes a striking and unexpected example for pattern formation in electrochemical systems.