Corrosion and Corrosion Inhibition Studies of Aerospace Aluminium Alloys at the Nanoscale using TEM Approaches

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. Team Yaiza Gonzalez Garci

Corrosion resistance of hot-dip galvanized steel in simulated soil solution: A factorial design and pit chemistry study

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.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.(OLD) MSE-

Localised aqueous corrosion of electroless nickel immersion gold-coated copper

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.Team Yaiza Gonzalez GarciaTeam Arjan Mo

Challenges to electrochemical evaluation of nanometric sandwiched thin specimens using liquid cells designed for application in liquid-phase TEM corrosion studies

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.Team Yaiza Gonzalez GarciaQN/Zandbergen LabQN/AfdelingsbureauTeam Arjan Mo

Dealloying-driven local corrosion by intermetallic constituent particles and dispersoids in aerospace aluminium alloys

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.(OLD) MSE-6QN/AfdelingsbureauQN/Zandbergen La

Laterally-resolved formation mechanism of a lithium-based conversion layer at the matrix and intermetallic particles in aerospace aluminium alloys

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.Team Yaiza Gonzalez GarciaQN/Afdelingsbureau(OLD) MSE-6QN/Zandbergen LabTeam Arjan Mo

Characterization of the passive layer on ferrite and austenite phases of super duplex stainless steel

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.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.(OLD) MSE-6QN/Zandbergen La

Application of in-situ liquid cell transmission electron microscopy in corrosion studies: a critical review of challenges and achievements

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.Accepted Author Manuscript(OLD) MSE-6QN/Zandbergen LabQN/Afdelingsburea

Enhanced corrosion protection of mild steel by the synergetic effect of zinc aluminum polyphosphate and 2-mercaptobenzimidazole inhibitors incorporated in epoxy-polyamide coatings

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.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.(OLD) MSE-

Evaluation of the formation and protectiveness of a lithium-based conversion layer using electrochemical noise

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.Team Arjan MolTeam Yaiza Gonzalez Garci