Utilizing a dedicated micro-sized three-electrode cell, this study systematically investigates early-stage electrochemical properties and corrosion behavior of pure iron under single droplets. Various volumes and NaCl concentrations were considered during the evaporation-driven shape and concentration evolution of single droplets. The measurements disclosed that reducing the droplet size from 5 µL to 1.5 µL at 0.01 M NaCl concentration, increased noise resistance (R_n) and polarization resistance (R_p) values. However, at 0.1 M and 0.2 M NaCl concentrations, reducing droplet size led to the domination of relatively high chloride ion concentration over oxygen diffusion, resulting in a very low R_n and R_p and hence enhanced localized corrosion.

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The corrosion inhibiting effect of lithium carbonate has been studied over the pH range relevant for atmospheric corrosion conditions at the polymer/metal interface. It is demonstrated that lithium carbonate offers optimum corrosion protection under moderately alkaline conditions attributed to the formation of a protective layer identified as aluminum-lithium layered double hydroxide. It is proposed that an active-to-passive transition occurs in the presence of lithium carbonate under moderately alkaline conditions where anodic dissolution of the aluminum matrix is an important step prior to the formation of the protective layer. After prolonged exposure, the protective layer is uniformly formed on the surface of AA2024-T3 and lithium containing AA2198-T8 covering both the matrix and active intermetallic particles thus hindering further corrosion reactions.@en

The formation process of a lithium-based conversion layer on AA2024-T3 and its corrosion protective behavior are studied using electrochemical noise (EN). Wavelet transform, as well as noise resistance analysis, have been employed to interpret the EN data. The EN data confirmed five different stages during the conversion layer growth, accompanied by anodic dissolution, increasing corrosion protection of the conversion layer, and adsorption, growth and desorption of hydrogen bubbles simultaneously. The detachment of hydrogen bubbles, localized and uniform corrosion generate different features in the EN signals with energy maxima in high, intermediate and low frequency bands, respectively. In addition, EN results show that the lithium-based conversion layer still provides efficient protection after re-immersion in a corrosive environment, even though localized damage occurs. Moreover, the EN data corresponds well with the morphological layer formation and breakdown observed with microscopy techniques. The results demonstrate that EN is a powerful tool to provide continuous time- and frequency-resolved information about inhibition efficiency.

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Electroless nickel (Ni) immersion gold (Au), commonly referred to by the acronym ENIG, is the most common protective coating applied on the exposed copper (Cu) traces of printed circuit boards (PCBs). In this work, we elucidate the local corrosion mechanism of the ENIG-Cu system by applying microscopic, surface analysis and electrochemical techniques with high spatial resolution to provide a comprehensive understanding of the complex local corrosion mechanism of the ENIG-Cu system. The corrosion initiation is highly localised and associated with pores or micro-defects in the Au layer. The corrosion initiates by the dissolution of the underlying Ni layer, being less noble than Au. The dissolution propagates in lateral and perpendicular directions relative to the surface in an elliptical fashion. With time, the direction of corrosion propagation changes to a predominantly lateral attack of the Ni layer. The corrosion process is governed by the cathode/anode ratio of the Au/Ni galvanic couple.

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We study microstructure, mechanical, and corrosion properties of Zr_1-xCr_xB_y coatings deposited by hybrid high-power impulse/DC magnetron co-sputtering (CrB₂-HiPIMS/ZrB₂-DCMS). Cr/(Zr + Cr) ratio, x, increases from 0.13 to 0.9, while B/(Zr + Cr) ratio, y, decreases from 2.92 to 1.81. As reference, ZrB_2.18 and CrB_1.81 layers are grown at 4000 W DCMS. ZrB_2.18 and CrB_1.81 columns are continual from near substrate toward the surface with open column boundaries. We find that the critical growth parameter to achieve dense films is the ratio of Cr⁺-dominated ion flux and the (Zr + B) neutral flux from the ZrB₂ target. Thus, the alloys are categorized in two groups: films with x < 0.32 (low Cr⁺/(Zr + B) ratios) that have continuous columnar growth, rough surfaces, and open column boundaries, and films with x ≥ 0.32 (high Cr⁺/(Zr + B) ratios) that Cr⁺-dominated ion fluxes are sufficient to interrupt continuous columns, resulting in smooth surface and dense fine-grain microstructure. The pulsed metal-ion irradiation is more effective in film densification than continuous Ar⁺ bombardment. Dense Zr_0.46Cr_0.54B_2.40 and Zr_0.10Cr_0.90B_1.81 alloys are hard (>30 GPa) and almost stress-free with relative nanoindentation toughness of 1.3 MPa√m and 2.3 MPa√m, respectively, and remarkedly low corrosion rates (∼1.0 × 10^-6 mA/cm² for Zr_0.46Cr_0.54B_2.40 and ∼ 2.1 × 10^-6 mA/cm² for Zr_0.10Cr_0.90B_1.81).

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For many decades, corrosion and corrosion inhibition of high-strength aluminium alloys have been studied indirectly and through traditional and separately performed electrochemical, spectroscopic and microscopic techniques. These approaches employed to date commonly lack sufficient lateral and time resolution to unravel early-stage events which is controlled at the nanoscopic levels at which microstructural heterogeneities actually steer local and dynamic electrochemical activities. Besides, techniques with appropriate resolution like transmission electron microscopy (TEM) have been applied to the field, but carried out ex-situ, normally providing no detailed on-site time-resolved information to investigate distinctive-but-consecutive stages of corrosion and corrosion inhibition phenomena. That is why theories of relevance are established through bridging and linking separately-obtained information and therefore are described in rather stochastic than deterministic terms. This is particularly the case for the legacy alloy AA2024-T3 which is prone to complicated forms of local corrosion resulting from extremely complex and heterogeneous local microstructures. Local corrosion in AA2024-T3 is site-specific where complicated local degradation events predominantly take place at surface intermetallic particles (IMPs) dispersed in the alloy matrix and eventually lead to pitting and intergranular forms of corrosion. Thus, the detailed understanding of space- and time-resolved local corrosion mechanisms of engineered microstructures is of pivotal importance to developing reliable and active protection strategies. However, despite the high demand for time- and space-resolved mechanistic information of local corrosion, it has not yet been possible to unambiguously define the morphological and micro-electrochemical characteristics during local corrosion and corrosion inhibition owing to extremely demanding experimental challenges. Nevertheless, this thesis put efforts into carrying out dedicated TEM experimental approaches including in-situ liquid-phase, quasi in-situ and ex-situ TEM to provide time-resolved and direct nanoscopic evidence of local corrosion and corrosion inhibition processes from early surface initiation to an advanced stage of propagation. @en

Liquid-phase transmission electron microscopy (LP-TEM) has provided corrosion scientists with a unique opportunity to directly correlate nanoscopic morphological and compositional evolutions to the corresponding electrochemical response of corroding thin TEM specimens. Electrochemical liquid cell designs are key components of a LP-TEM study towards an implementation which is representative for realistic exposure conditions of bulk samples. However, the application of commercially available liquid cells in corrosion studies brings along an important shortcoming of galvanic coupling effects due to the inevitable connection of the TEM specimens with Pt patterned electrodes. Here, we introduce an approach of fabricating electrochemical liquid cells to alleviate the current cell design challenge for corrosion studies. Besides, we present a protocol for preparing thin specimens to be electrochemically investigated with our home-made electrochemical liquid cell. We finally confirm the effectiveness of this methodology by electrochemically evaluating thin specimens of AA2024-T3 in an open-cell configuration through open circuit potential and potentiodynamic polarisation measurements.

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Lithium salts have been proposed as promising environmentally friendly alternatives to carcinogenic hexavalent chromium-based inhibitors for the corrosion protection of aerospace aluminium alloys (AAs). Incorporated into organic coatings, lithium salts are released at damaged locations to establish a conversion layer in which distinct sublayers have different barrier characteristics. Thus, detailed knowledge on the sequence of formation events from the early stages of nucleation towards the final multi-layered arrangement is essential for developing and optimising lithium-leaching technology for protective coatings. Here, liquid-phase-transmission electron microscopy (LP-TEM) is employed to observe nanoscopic morphological evolutions in situ during the lithium-based conversion process of AA2024-T3. Thanks to dedicated preparation of delicate sandwiched TEM specimens allowing us to explore the events cross-sectionally, we provide real-time direct mechanistic information on the conversion process from the initiation to an advanced growth stage. In parallel, we perform supplementary ex situ SEM and TEM investigations to support and validate the LP-TEM findings. The unprecedented experimental approach developed and executed in this study provides an inspiring base for studying also other complicated surface conversion processes in situ and at the nanoscopic scale.

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Lithium leaching coatings have recently been developed as eco-friendly active corrosion protection technology for aerospace aluminium alloys (AAs) by the formation of a conversion layer at coating defects. While general conversion layer formation characteristics were studied and reported before, here we study the local layer formation process with sub-micron resolution at and around intermetallic particles (IMPs) in AA2024-T3. Top- and cross-sectional-view morphological electron micrograph observations along with open circuit potential (OCP) measurements are performed, mimicking coating defect conditions upon lithium carbonate leaching from the coating matrix. The results revealed five stages of the conversion process in which the alloy matrix and different IMPs evolve morphologically, compositionally, and electrochemically. Besides, we found a correlation between the OCP response of the AA2024-T3 system and the morphological and compositional evolutions of the alloy matrix and IMPs at different stages of exposure. Passive layer and alloy matrix dissolution leading to surface Cu-enrichment and S-phase dealloying occur at early stages of exposure. They precede the formation of a columnar layer on the alloy, followed by the establishment of a dense-like layer at the final stage. Dealloying of Al₂CuMg can assist the conversion process by providing local supersaturation. Through complementary experiments in a sodium carbonate solution and besides X-ray diffraction analysis, we found out that lithium plays a critical role in stabilising the corrosion product throughout the conversion process.

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Cerium-based compounds have been studied for decades as non-toxic candidates for the protection of aerospace aluminium alloys (AAs) like AA2024-T3. However, the complex heterogeneous microstructure of these alloys has hindered a thorough understanding of the subsequent stages of corrosion protection provided by this class of inhibitors. Thus, this work is devoted to unravelling the interaction mechanisms of different intermetallic particles (IMPs) in AA2024-T3 with cerium nitrate at the nanoscopic scale. This has been fulfilled through detailed top-view and cross-sectional analytical TEM investigations along with electrochemical evaluations. In line with our recent findings, we here report dealloying of IMPs as the main factor governing the rate of local cerium precipitation in contrast to micro-galvanic corrosion between IMPs and the surrounding matrix. Furthermore, we discuss a connection between the electrochemical response of the AA2024-T3 system and the morphological and compositional evolutions of individual IMPs including Al2CuMg, Al2Cu, Al7Cu2Fe(Mn) and Al76Cu6Fe7Mn5Si6 at different stages of a 96-h exposure.

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Identifying corrosion initiation events in metals and alloys demands techniques that can provide temporal and spatial resolution simultaneously. Transmission electron microscopy (TEM) enables one to obtain microstructural and chemical descriptors of materials at atomic/nanoscopic level and has been used in corrosion studies of many metal-electrolyte combinations. Conventionally, ex situ and quasi in situ TEM studies of pre- and post-corroded samples were performed, but possible experimental artifacts such as dehydrated surfaces might not fully represent the real interfacial conditions as compared to those when actually immersed in the electrolyte. Recent advances in liquid cell transmission electron microscopy (LC-TEM) allows for in situ monitoring morphological and even compositional evolutions in materials resulting from interaction with gas or liquid environments. Corrosion science, as a challenging field of research, can benefit from this unparalleled opportunity to investigate many complicated corroding systems in aqueous environments at high resolution. However, “real life” corrosion with LC-TEM is still not straightforward in implementation and there are limitations and challenging experimental considerations for conducting reliable examinations. Thus, this study has been devoted to discussing the challenges of in situ LC-TEM wherein state-of-the-art achievements in the field of relevance are reviewed.@en

This work focuses on the cross-sectional characterization of the protective conversion layer formed on AA2024-T3 by lithium-leaching from a polyurethane coating in a corrosive environment. The layer shows a multi-layered arrangement comprising nanoscopic local phases. Transmission electron microscopy (TEM) and complementary high-resolution secondary ion mass spectroscopy (SIMS) were employed to observe the cross-sections of the entire layer formed at different locations of a 1-mm-wide scribe, in terms of morphology, structure and chemical composition. The conversion layer was comprised of two ubiquitous sublayers; a thin dense layer (i.e. 150 nm) adjacent the alloy substrate and a porous layer. The former represents an amorphous lithium-containing pseudoboehmite phase, Li-pseudoboehmite, whereas the latter is composed of amorphous and crystalline products; an outer columnar layer merely seen on the peripheral region is also crystalline. Through a sandwich structure and the d₍₀₀₃₎ basal spacing, the crystalline phases were identified as Li-Al layered double hydroxide. Although lithium was found uniformly spread within different regions, the local phases with no/low concentration of lithium were revealed with energy filtered TEM and confirmed with SIMS analysis.

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Nanoscopic characterization of heterogeneous intermetallic particles (IMPs) which microstructurally and compositionally evolve during local corrosion is crucial in unravelling the mechanisms and sequence of initial and local corrosion events. Herein, we study site-specific initiation events focused on microscopic constituent intermetallic compounds and nanoscopic dispersoids in AA2024-T3 at the nanoscale using a combined quasi in-situ and ex-situ analytical TEM approach. Our findings show a dealloying-driven local corrosion initiation at the studied IMPs that have been considered as cathodic phases traditionally. Besides, local degradation which is a result of galvanic interactions between dealloyed regions of IMPs and their adjacent alloy matrix is largely governed by the intrinsic electrochemical instability of intermetallic compounds.

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Dealloying is involved in materials science responsible for fabrication of nanoscale structures beneficially but for corrosion degradations detrimentally. Detailed understanding related to the latter is critical for designing corrosion-resistance alloys and dedicated inhibition systems. Thus, direct nanoscopic observations of nano-structural and compositional evolutions during the process are essential. Here using liquid phase-transmission electron microscopy (LP-TEM), for the first time, we show dynamic evolution of intricate site-specific local corrosion linked to intermetallic particles (IMPs) in aerospace aluminium alloys. To thoroughly probe degradation events, oxidation direction is controlled by purposefully masking thin specimens, allowing for observing top-view surface initiation to cross-sectional depth propagation of local degradations. Real-time capturing validated and supported by post-mortem examinations shows a dealloying-driven process that initiates at IMPs and penetrates into the depth of the alloy, establishing macroscopic corrosion pits. Besides, controversial mechanisms of noble-metal redistribution are finally elucidated.

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The soil corrosion of widely applied galvanized steel structures, such as power transmission towers, must be considered to prevent harm to their structural integrity and to mitigate the high costs associated with early failure. A full two-level factorial design was used to evaluate the relative significance of various influencing factors on the underground corrosion of hot-dip galvanized steel. Experiments were performed in simulated soil solutions. The effects of temperature and the concentrations of chloride, sulfate, bicarbonate and citric acid were evaluated using statistical analysis of the results. Using analysis of variance, temperature, citric acid and chloride were found to be individually significant. Also, temperature/citric acid and temperature/chloride significantly interacted to increase the corrosion rate. The lead-in pencil electrode technique was used to further evaluate the impact of the above mentioned factors on the dissolution behavior of the Zn coating. The results revealed that chloride and citric acid affect salt film formation at the pit bottom, while temperature alters the dissolution kinetics by changing the diffusion coefficient of the dissolving Zn(II) species. Moreover, the effect of bulk solution dissolved oxygen concentration on the corrosion rate of the galvanized steel was modeled. It was found that oxygen concentration does not have a dominant effect on the overall corrosion behavior of galvanized steel. Rather, the effect of temperature is dominant.@en

Aluminum alloy (AA) 3003 is widely used as fin and tube material in heat exchangers. In these heat exchangers, the tube and fin material are brazed together by means of an AA4xxx alloy. In this study, the effect of a simulated brazing step on the microstructure and corrosion behavior of a twin roll casted (TRC) AA3003 was studied. In particular, the effect of simulated brazing on the corrosion properties of the centre line segregation (CLS) of alloying elements and the interaction of the intermetallic particles with the surrounding matrix is explored. It is shown that the cross sections are significantly more active than the surface of the TRC AA3003, revealing the importance of the CLS on the corrosion behavior. Furthermore, it is shown that the number of pits and their depths decreased considerably on the exposed cross sections after simulated brazing, especially near the CLS. Scanning Kelvin probe force microscopy measurements show that after the simulated brazing step the Volta potential difference between the intermetallic particles and the matrix reduced significantly. This change in the Volta potential could be caused by the slight increase of Si content in the matrix.@en

Two types of austenite morphologies, equiaxed and Widmanstätten, were produced through different phase transformation routes to evaluate the critical factors affecting the intergranular corrosion susceptibility in a 2205 duplex stainless steel. These distinct austenite morphologies behaved quite differently in secondary phase precipitation on exposure to sensitization temperature. Although the Widmanstätten microstructure was found to have a larger degree of coherent ferrite/austenite interface area compared with the equiaxed one, it showed a higher degree of sensitization. It was clarified that, in addition to the ferrite/austenite interface coherency, the extent of an interface area and presence of un-stable ferrite also play prominent roles in intergranular corrosion susceptibility.

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In this study, we report on a combined microscopic, analytical and electrochemical characterization of the nanoscopic passive layer on a tungsten‑molybdenum-containing super duplex stainless steel. We used scanning transmission electron microscopy/energy dispersive X-ray spectroscopy, scanning Kelvin probe force microscopy, scanning tunneling spectroscopy, and Mott–Schottky electrochemical impedance spectroscopy analysis to correlate the local chemical composition and electronic properties of passive layers on austenite and ferrite phases. The passive layer on the ferrite phase contains a higher amount of Mo, W, and Cr, which accommodates a higher nobility of ferrite and a higher local energy of the band gap compared to those on the austenite. The two aforementioned phases exhibit a different composition and semi-conductive properties of their passive layers leading to dissimilar local corrosion susceptibility. These findings are of pivotal importance in further studies of austenite and ferrite phase resolved corrosion resistance of duplex stainless steel demanding a dedicated alloying strategy.

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This work investigates the synergetic effect of zinc aluminum polyphosphate (ZAPP) and 2-mercaptobenzimidazole (MBI) on the corrosion protection of mild steel coated with a solvent-borne epoxy-polyamide layer. The magnitude and trend of electrochemical impedance spectroscopy data over 70-d immersion in 3.5 wt.% NaCl solution indicate superior corrosion protection of the combined inhibitors compared to those containing either just ZAPP or MBI. Pull-off tests show that the combined inhibitor system provides an improved adhesion strength. The enhanced corrosion performance is correlated to precipitation of a protective layer at the coating/metal interface verified by SEM and electrochemical studies upon exposure to electrolytes.

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