The effect of nanoroughness on contact angles and pinning is investigated experimentally and numerically for low-energy surfaces. Nanoroughness is introduced by chemical vapor deposition of tetraethoxysilane and was quantified by scanning force microscopy. Addition of a root-mean-square roughness of 2 nm on a flat surface can increase the contact angle after fluorination by a semifluorinated silane by up to 30°. On the other hand, nanoroughness can improve or impair the liquid repellency of superhydrophobic surfaces that were made from assembled raspberry particles. Molecular dynamics simulations are performed in order to gain a microscopic understanding on how the length and the surface coating density of semifluorinated silanes influence the hydrophobicity. Solid-liquid surface free energy computations reveal that the wetting behavior strongly depends on the density and alignment of the semifluorinated silane. At coating densities in the range of experimental values, some water molecules can penetrate between the semifluorinated chains, thus increasing the surface energy. Combining the experimental and numerical data exhibits that a roughness-induced increase of the contact angle competes with increased pinning caused by penetration of liquid into nanopores or between neighboring semifluorinated molecules.

The electrochemical stability of the buried metal-organic coating interface of painted metal is crucially governed by how effectively the oxygen reduction reaction at the interface is inhibited. As this interface is not directly accessible for study by conventional electrochemical techniques, a new non-destructive method has been developed wherein hydrogen permeation is used to quantitatively measure the oxygen reduction kinetics underneath the coatings. Presented here are results obtained with an adaptation of the Devanathan-Stachurski cell where the oxygen reduction reaction kinetics on the coated exit is probed by monitoring the dynamic electrochemical equilibrium potential established as a result of oxygen reduction and hydrogen oxidation reactions. From the hydrogen uptake on the entry side thus a full current-potential relationship curve (I (U)) can be constructed. First tested on non-coated Palladium, the results show very good concurrence of the oxygen reduction currents from the permeation experiments and that from standard cyclic-voltammetry (CV) measurements in buffered and non-buffered acidic electrolyte and under alkaline condition. Initial results obtained on coated samples are also presented. (C) 2015 The Authors. Published by Elsevier Ltd

Corrosion driven delamination of coatings is crucially determined by the rate of the cathodic oxygen reduction reaction at the buried metal-organic coating interface. Quantitative measurement of this rate at such interfaces by conventional techniques is impeded due to the blocking of ion transport by the coating. A new approach where hydrogen permeation is used as a tool to measure the oxygen reduction rate underneath coatings has been recently introduced. This permeation based potentiometry approach measures the rate of the oxygen reduction reaction by correlating the open circuit potential established as a consequence of the dynamic electrochemical equilibrium between hydrogen oxidation and oxygen reduction reactions on the coated exit side with the hydrogen uptake rate on the entry side. In this work, the interfacial reaction kinetics of the oxygen reduction reaction causing delamination of the coating are investigated by this hydrogen permeation based potentiometric approach. Moreover, thus performed prolonged cathodic polarizations of the palladium/coating interface have been found to destroy the interface, just like it is the case in the cathodic delamination process. Initial results obtained on ultra-thin films of iron on palladium are also presented, showing that the technique is applicable also on technically more relevant metal surfaces. Corrosion driven degradation of the buried metal/organic coating interface is of utmost interest in the contemporary space of coatings science. Earlier studies have shown conclusively that the oxygen reduction reaction (ORR) plays a fundamental role in the destruction of this buried metal/organic coating interface. The role of the interface between organic coating and metal has been object of intense research over last decades. Leidheiser et al.1 pointed out the key role of the cathodic ORR for the degradation process and also showed that diffusion processes such as water and oxygen transport through the coating can influence this delamination rate. Stratmann and co-workers 2-4 proposed a detailed delamination mechanism for a basic polymer coating on steel where galvanic coupling between the defect site of metal dissolution (local anode) and delamination site of electrochemical oxygen reduction (local cathode) supported by cation migration along the polymer/metal interface lead to the progressive deadhesion of the polymer from the underlying metal based on comprehensive analysis of the local electrode potential at the metal/polymer interface using primarily the Scanning Kelvin Probe (SKP) technique along with complementary Auger electron spectroscopy and mechanical deadhesion tests. Fürbeth and Stratmann 5-7 extended their studies to coated zinc (or galvanized steel), which also involves anodic processes at the interface. Also other works by Brewis et al. 8 and Dickie et al. 9 show the importance of interfacial electrochemical reactions on metal/polymer adhesion. Up to now our knowledge about the reactions and how they are determined by the interface itself is too rest...

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Magnesium (Mg) alloys are a very attractive material of construction for biodegradable temporary implants. However, Mg alloys suffer unacceptably rapid corrosion rates in aqueous environments, including physiological fluid, that may cause premature mechanical failure of the implant. This necessitates a biodegradable surface barrier coating that should delay the corrosion of the implant until the fractured/damaged bone has healed. This review takes a brief account of the merits and demerits of various existing coating methodologies for the mitigation of Mg alloy corrosion. Since among the different coating approaches investigated, no single coating recipe seems to address the degradation control and functionality entirely, this review argues the need for polymer-based and biodegradable composite coatings.