Corros. Mater. Degrad., Volume 6, Issue 1 (March 2025) – 4 articles

Biomedical alloys in general, except for the biodegradable type, exhibit passive behavior in neutral chloride solutions. Two commonly used biomedical alloys are nitinol (NiTi) and Co-35Ni-20Cr-10Mo (CoNiCrMo). In this work, the passive behavior of electropolished NiTi and CoNiCrMo in a simulated physiological solution (phosphate-buffered saline) was compared using data largely obtained from our previous studies involving potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The potentiodynamic results showed a marked difference in passive behavior between the alloys, with NiTi remaining completely passive up to the oxidation of water and CoNiCrMo, in contrast, undergoing solid-state oxidation and then transpassive dissolution. Both alloys exhibited Tafel-type behavior over the initial part of the passive range. A small but distinct difference in the apparent Tafel slopes was found between the two alloys and can be attributed to the difference in their predominant oxide; that is, TiO₂ versus Cr₂O₃. The EIS results also showed marked differences between the alloys in terms of the oxide thickness and resistivity. The thickness was greater for NiTi—consistent with surface analytical results—and differed in potential dependence between the two alloys in the passive region. The oxide resistivity, conversely, was substantially lower for NiTi and showed a similar potential dependence for the two alloys. Full article

Corrosion is one of the causes of failure in reinforced concrete structures, and forming a passive film on the steel is essential for protection. Although several studies have looked at passive film formation in concrete pore solutions, few have considered its formation in hardened concrete and the influence of silica fume (SF) in the binder composition. This study aims to evaluate the influence of the SF content on passive film formation time in concrete. Periodic measurements assessed the electrical resistivity and corrosion current density of concrete samples containing 5%, 10%, 15%, and 20% SF. The alkalinity of the mixtures and the kinetics of the pozzolanic reaction were also monitored by XRD and titration tests. The control mixtures exhibited susceptibility to corrosion, regardless of the curing age evaluated. In contrast, the partial replacement of cement with SF accelerated the formation of the passive film on the steel surface, suggesting a delayed onset of corrosion due to modifications in the physical properties of the concrete. Also, the portlandite content and pH can predict passive film formation, with SF significantly accelerating this process. Full article

Corrosion investigations of steel-reinforced concrete structures are often based on half-cell potential measurements, in which the diffusion potentials can be a significant source of measurement errors. Therefore, the diffusion potentials must be taken into account in order to enable accurate half-cell potential measurements. This study covers the measurement of the diffusion potentials in cement mortars with pH differences due to carbonation and various mortar moisture conditions. The effect of chloride exposure of the mortars on the diffusion potentials is outside of the scope of this study. The mortars consisted of ordinary Portland cement (OPC) and blast furnace cement (BFC) with water–cement ratios of 0.5–0.7. The use of color indicators allows for the observation of the pH drop around the carbonation front, which propagates as the carbonation progresses. The diffusion potentials in the mortars under study have measurement values between 10 and 240 mV. The measured diffusion potentials seem to correlate with the magnitude of the pH drop rather than the progress of the carbonation depth. The moisture condition of the mortars significantly affects the magnitude of the arising diffusion potentials. Full article

Seawater ballast tanks in vessels are subject to severe service conditions caused by repeated filling/emptying, as well as temperature variation. Consequently, relatively thick, barrier-type coatings are used for corrosion protection of their internals. These are generally formulated with solvent-based epoxy binders and contain a range of flake pigments designed to limit environmental entry. Here, we report on a detailed study of damage processes in order to understand the mechanisms of failure after hygro-thermal cyclic corrosion testing. Similar formulations were cured using variant phenalkamine cross-linkers. Visual observation after corrosion testing shows minimal changes and no sign of corrosion damage. However, high-resolution analytical microscopy and nanoscale tomography reveal the onset of microstructural and chemical damage processes inside the coating. Thus, kaolin and talc pigments in the coating remained stable under hygro-thermal cycling; however, dolomite and barium sulphate dissolved slightly, causing voids. Galvanic protection of the substrate by aluminium flake pigments was disproven as no electrical connection was evident. Vibrational spectroscopy revealed a decrease in residual epoxy functionality after exposure for the coating cured with the more stable phenalkamine. This was correlated with an increase in glass transition temperature (Tg) and no observable corrosion of aluminium flakes. In contrast, the less stable phenalkamine cross-linker caused the binder Tg to decrease and aluminium flakes and substrate corrosion to become evident. Full article