Aluminum is a very attractive material for bipolar plates in proton exchange membrane (PEM) fuel cells due to its low weight, excellent thermal and electrical conductivity, as well as good recyclability. However, to achieve the necessary low contact resistance and corrosion stability, a suitable surface coating is needed. In the present study, two different material systems were evaluated, electroless deposited NiP based coatings and high-power impulse magnetron sputter-deposited (HiPIMS) Ti based coatings. The addition of a 100 nm amorphous carbon (a-C) top-layer on the NiP and Ti based coatings was also evaluated. The electrochemical corrosion behavior of the coatings was evaluated in simulated PEM environments. Potentiodynamic and potentiostatic polarization experiments revealed that the HiPIMS Ti coatings performed better than NiP coatings. The addition of the a-C top-layer on the other hand was found to be detrimental to the Ti based coatings resulting in a corrosion current density of 3.6 µA/cm2. The opposite was observed in the case of the NiP based coatings where the addition of the a-C layer decreases the corrosion current thereby increasing corrosion resistance. Further analysis showed that defects in the coating as well as the presence of a Fe-Si rich particles in the Al substrate were the initiation points for the corrosion attacks to occur.

Transition metal oxides like Co, Ni, and Mn are promising alternatives to noble metals such as Pt for oxygen electrocatalysis in green energy. Alloying these metals forms multicomponent catalysts with compelling properties. In this study, CoCrFeNi and MnCrFeNi thin films are synthesized using High-Power Impulse Magnetron Sputtering (HiPIMS) and their catalytic activity for the Oxygen Reduction Reaction (ORR), the Oxygen Evolution Reaction (OER), and corrosion resistance in 1 molar (1 M) potassium hydroxide (KOH) are evaluated. MnCrFeNi films exhibit a fine-grained single face-centered cubic (FCC) phase, while CoCrFeNi films have larger grains and multiple phases. ORR on CoCrFeNi follows a 2+1 electron transfer pathway, producing hydroxide radicals, while MnCrFeNi exhibits a 2-electron pathway, yielding hydrogen peroxide. Anodization reduces the CoCrFeNi overpotential from 0.9 to 0.5 V versus the reversible hydrogen electrode (RHE), comparable to platinum and iridium catalysts (Pt/C, Ir/C). Anodization also shifts CoCrFeNi ORR to a 2-electron pathway. In situ Raman spectroscopy detects no ORR intermediates, but nickel oxyhydroxide (NiOOH) appears during OER. Substituting Mn for Co increases corrosion resistance by raising the corrosion potential. All films show passive behavior during polarization, demonstrating their potential for corrosion protection and electrocatalysis in green energy applications. 

Oxygen electrocatalysts play a key role in renewable and fossil-free energy production. Bifunctional catalysts active for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) allow use of the same material system for both energy production (ORR) and fuel generation (OER). However, optimizing the performance of bifunctional catalysts requires in depth understanding of the catalyst structure, its surface chemistry in terms of active sites and the underlying catalytic mechanism. Here, the catalytic performance of CoCrFeNi thin films is investigated, synthesized using high-power impulse magnetron sputtering, as bifunctional oxygen electrocatalysts. The film crystal structure and morphology, and thereby the catalytic performance, can be tuned by the ion acceleration (bias) to the substrate. To further enhance the catalytic activity, anodization is used to electrochemically modify the films, forming a thicker oxide layer enriched in Co and Ni cations which significantly improves the ORR performance. Anodization improves the catalyst stability during OER, with an OER potential of 1.45 V versus the reversible hydrogen electrode (RHE) at 10 mA cm−2 for more than 24 h. While the corrosion resistance is high both before and after anodization, in terms of catalytic activity the anodized films outperformed the as-deposited ones. This makes anodized films excellent electrocatalyst candidates in corrosive alkaline environments such as fuel cells and electrolyzers. 

An approach involving the quantification of microstructure characterized by different techniques such as SEM, EDS, and SKPFM is statistically treated to provide a deeper insight into the influence of recycling AA6063 on localized corrosion susceptibility. Particularly, the intermetallic particles and the two forms of localized corrosion – pitting and intergranular corrosion are systematically documented, measured, and analyzed. Even trace amounts of Cu and Zn introduced into the alloy from recycling had a remarkable effect on the localized corrosion susceptibility. The study found that the initiation and early evolution of the two localized corrosions are in competition, and the predominance of one over the other is closely linked to the composition of the alloy, and microstructure. Recycled variants with higher trace Cu made the alloy more susceptible to pitting attack whereas higher trace Zn is linked with greater IGC susceptibility. The trace amount of higher Zn addition has a particularly beneficial effect on pitting susceptibility as it reduces the likelihood of pitting even in alloys with a higher trace Cu content. The SKPFM results obtained in this study provided a basis for the circumferential pitting susceptibility around intermetallic particles, as a higher volta potential difference (∆V) implied a higher driving force for corrosion. ∆V differences between the different variants were further explained based on trace recycled element distribution in the microstructure. 

A mechanistic model of atmospheric bimetallic corrosion with a simplified empirical approach to the onset of localized corrosion attacks is presented. The model was built for a typical bimetallic sample containing aluminum alloy 1050 and stainless steel 316L sheets. A strategy was developed that allowed the model to be calibrated against the measured galvanic current, geometrical corrosion attack properties, and corrosion products. The pitting-onset simplification sets all pits to be formed at a position near the nobler metal and treated all pits as being of the same shape and size. The position was based on the location of the highest pitting events and the pit attributes on an average of the deepest pits. For 5 h exposure at controlled RH (85%, 91%, and 97%) and salt load (86 μg NaCl/cm2), the model was shown to be promising: both for analysis of local bimetallic corrosion chemistry, such as pH and corrosion products, and for efficient assessment of pitting damage by computing a single largest pit depth. Parametric studies indicated that the pitting-onset approximation deviated the most at the beginning of exposure and when RH was below 91%. © 2023 by the authors.