Underlying Corrosion Mechanism Research Articles

Enhanced high-temperature corrosion resistance of Cr-modified phosphate/aluminum powder composite coatings: Mechanisms and thermodynamic insights

Phosphate composite coatings are regarded as one of the tangible solutions for surface protection in salt spray environments, benefiting from their cost-effectiveness and prominent structure stability upon high-temperature exposure. However, rapid curing of these coatings often leads to crack formation, and the underlying corrosion mechanisms in service conditions remain insufficiently understood, limiting their practical applications. In this study, we developed chromium-incorporated phosphate/aluminum powder composite (Cr-P/Al) coatings and evaluated their corrosion resistance under alternating conditions of salt spray corrosion and oxidation at 450 °C. By integrating experimental results, ab initio calculations, and thermodynamic analysis, we demonstrate that the incorporation of Cr significantly enhances the performance of the P/Al coatings. The structural evolution was first revealed that Cr’s modification reduced tensile stress and prevented cracks in the coating. A mixed layer of amorphous oxides and phosphates forms on the surface of the Al powder, providing robust binding strength. A novel corrosion mechanism was identified that Cl2 and volatile chlorides facilitated the removal of Cl-. This process is more ineffective with Cr-P/Al coating, as amorphous Cr oxide has low reactivity at a high log[p(Cl2)] and low log[p(O2)] compared with amorphous Mg and Fe oxide, thereby enhancing the corrosion resistance of the coating. These coating densification methods and corrosion protection mechanisms provide a critical foundation for the future application of phosphate composite coatings.

Hot corrosion behavior of Mo–Si–Ti alloys

AbstractThe hot corrosion behavior of two ternary Mo–Si–Ti alloys (eutectic Mo–20.0Si–52.8Ti (at.%) and eutectoid Mo–21.0Si–34.0Ti (at.%)) was investigated at C and C for up to 100 h. Both Mo‐based alloys evidenced a hot corrosion attack, resulting in a uniform attack of the substrate surface. The higher Ti content of the eutectic alloy significantly reduced the formation of solid and volatile Mo oxides compared with the eutectoid alloy. After 24 h at C, the corrosion products formed on the eutectic alloy were only one‐tenth the thickness of the layers formed on the eutectoid alloy. The corrosion products were examined with optical and electron microscopy. Semiquantitative electron probe micro analysis and X‐ray diffraction measurements were used to identify the formed phases. The underlying corrosion mechanisms comprising not only hot corrosion‐induced fluxing but also oxidation‐induced pesting are discussed in detail.

Effect of solidification pressure on the corrosion behavior and mechanical properties of Al-7Si-3Cu-(0.4 Mg)-(0.5Ge) alloys

Developing practical microstructural solutions that simultaneously enable excellent corrosion resistance and mechanical properties in Al-Si-Cu-based alloys has been increasingly in demand. In this study, we attempt to tailor the microstructural evolution to manipulate the mechanical performance and corrosion resistance of Al-7Si-3Cu-(0.4 Mg)-(0.5Ge) alloys fabricated by GPa-level pressure combined with solution and/or aging treatments. It was found that as the solidification pressure increased, the dendritic growth tendency of the α-Al phase became less pronounced, and the modification of eutectic Si became increasingly significant. Complete solid solution alloys were even achieved under 6 GPa. The corresponding kinetic and thermodynamic mechanisms of pressure-induced microstructural evolution were explored in detail. Upon aging treatment, dense irregular Si precipitates emerged from the supersaturated matrix in Al-7Si-3Cu alloys solidified at 5 GPa and 6 GPa, while denser and finer Si precipitates growing along the 〈100〉Al directions dominated the matrix of Al-7Si-3Cu-0.4 Mg-0.5Ge alloys, which resulted in considerable enhancement of mechanical properties. The electrochemical corrosion behavior was evaluated in the as-cast and heat-treated conditions. The underlying corrosion mechanisms were discussed in terms of pressure, supersaturated solutes and precipitates. These results reveal the feasibility of designing high-strength and corrosion-resistant Al-Si-Cu series alloys via the manipulation of pressure and heat treatment.

The anodic dissolution kinetics of Mg alloys in water based on ab initio molecular dynamics simulations

AbstractThe corrosion susceptibility of magnesium (Mg) alloys presents a significant challenge for their broad application. Although there have been extensive experimental and theoretical investigations, the corrosion mechanisms of Mg alloys are still unclear, especially the anodic dissolution process. Here, a thorough theoretical investigation based on ab initio molecular dynamics and metadynamics simulations has been conducted to clarify the underlying corrosion mechanism of Mg anode and propose effective strategies for enhancing corrosion resistance. Through comprehensive analyses of interfacial structures and equilibrium potentials for Mg(0001)/H2O interface models with different water thicknesses, the Mg(0001)/72 H2O model is identified to be reasonable with −2.17 V vs. standard hydrogen electrode equilibrium potential. In addition, utilizing metadynamics, the free energy barrier for Mg dissolution is calculated to be 0.835 eV, enabling the theoretical determination of anodic polarization curves for pure Mg that aligns well with experimental data. Based on the Mg(0001)/72 H2O model, we further explore the effects of various alloying elements on anodic corrosion resistance, among which Al and Mn alloying elements are found to enhance corrosion resistance of Mg. This study provides valuable atomic‐scale insights into the corrosion mechanism of magnesium alloys, offering theoretical guidance for developing novel corrosion‐resistant Mg alloys.

Electrochemical characterization of localized corrosion mechanism of ALD Al2O3-coated copper for microelectronic application

To achieve performance and reliability in microelectronics devices, copper interconnections must be protected from aggressive environment during the manufacturing process and throughout the life of the devices. Atomic Layer Deposition Al2O3 thin films has been used as a compatible layer with key integration process steps, as well as being corrosion-resistant in neutral media. However, electrochemical tests in aggressive environment have revealed that the alumina prematurely failed to protect copper from localized corrosion. In this study, local and global electrochemical techniques have been employed to evaluate the reactivity of copper at multiple scales and to elucidate the underlying corrosion mechanisms.

Effect of V Content on Corrosion Behavior of Al-V Alloys Produced by Mechanical Alloying and Subsequent Spark Plasma Sintering

Al-V alloys produced via high-energy ball milling have been reported to show simultaneous improvement of corrosion resistance and mechanical properties compared to traditional Al alloys. In these alloys, V content plays a crucial role in increasing or decreasing the corrosion resistance. Therefore, the effect of V and microstructure on corrosion of high-energy ball milled and subsequently spark plasma sintered Al-xV alloys (x = 2, 5, 10 at%) has been studied. Cyclic potentiodynamic polarization tests and electrochemical impedance spectroscopic analysis revealed the increment of V content up to 5 at% enhanced the corrosion resistance of the alloy. However, highly heterogeneous microstructure in Al-10 at%V resulted in significant localized corrosion over the immersion time. The electrochemical impedance spectroscopy studies over 14 days of immersion revealed underlying corrosion mechanisms.

Unraveling the CMAS corrosion mechanism of APS high-yttria-stabilized zirconia thermal barrier coatings

In this study, the CMAS corrosion behavior of two high-yttria-stabilized zirconia thermal barrier coatings (38YSZ and 55YSZ) deposited by air plasma spraying is investigated and compared with a typical 8YSZ coating at 1250 °C to unravel the underlying corrosion mechanism. The 8YSZ coating undergoes severe CMAS attack driven by dissolution-reprecipitation and intergranular corrosion. However, the 38YSZ and 55YSZ coatings can react with CMAS melt vigorously to form a dense reaction layer with CMAS-resistant apatite phase at the CMAS/coating interface, thereby resisting the CMAS infiltration. The apatite phase can be formed only when the yttria content in c-ZrO2 precipitated from CMAS attains the critical value (∼20 at%). Moreover, compared to the 38YSZ coating, apart from the high yttria content, the low zirconia content in the 55YSZ coating also allows higher free Y3+ available to form apatite phase via inhibiting the formation of c-ZrO2, thus facilitating the formation of more apatite phase.

Corrosion behavior of 7050 and 7075 aluminum alloys processed by reactive additive manufacturing

In this work, high-strength 7075 and 7050 aluminum alloys were processed by the laser powder bed fusion (LPBF) technique. Titanium and boron carbide particles were added to base 7050 and 7075 alloy feedstocks. These reacted in the melt to form inoculants in situ, resulting in a crack-free, refined, and textureless microstructure. X-ray diffraction, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Energy Dispersive Spectroscopy (EDS) techniques were used to characterize the process-induced microstructure of the alloys. Reactive Additive manufactured (RAM) alloys showed precipitation of various sub-micron to nanoscale phases such as Zn-Cu, Cu-Fe, Mg-Cu, Mg-Zn, and pure Zn. The corrosion response of RAM alloys was analyzed and compared with respective wrought alloys by various electrochemical measurements such as potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and atomic spectroelectrochemistry (ASEC) techniques. A reduction in corrosion resistance was observed in RAM alloys as compared to wrought counterparts due to thepresence of unreacted Ti and B4C particles and a higher cathodic activation attributed to the underlying microstructure. Time evolution surface imaging and post-corrosion microstructures were analyzed to support the understanding of underlying corrosion mechanisms.

Corrosion behaviors according to the welding process of superduplex stainless steel welded tubes: Gas tungsten arc welding vs. laser beam welding

The corrosion behaviors of superduplex stainless steel welded tubes in an acidic brine environment were examined by comparing the welds formed by the two welding processes of tungsten inert gas (TIG) welding and laser beam (LB) welding. In contrast to TIG-welds with a larger bead size, higher oxide inclusion, and lower N content, the poor corrosion resistance of as-welded LB-weld sample recovered after post-weld heat treatment (PWHT) by restoring the phase balance and dissolving the deleterious inhomogeneities around the fusion line. The key metallurgical factor controlling the corrosion behavior of LB-welded steel tubes, and underlying corrosion mechanism were also provided.

Effect of the carbon on the electrochemical performance of rechargeable Zn-air batteries

Carbon materials as catalyst substrates play key roles in Zn-air batteries which not only construct abundant tri-phase interfaces for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) to take place but also enable the diffusion of reactants. Carbon corrosion is known to occur in the aqueous electrolyte which leads to catalysts dissolution, electrode flooding, and rapid performance degradation. In this study, rechargeable Zn-air batteries with MnO2 as the bifunctional catalysts and different carbon as catalyst carriers, such as carbon black, CNTs, and graphene have been assembled with their electrochemical performance systematically evaluated. The correlation between the graphitization, surface, structure properties of the carbon, and the electrochemical performance of air-electrodes has been elucidated. The electrolyte composition change during cycling and the underlying corrosion mechanism of carbon have been explored. CNTs with high crystallinity and less edge exposure is an excellent candidate over activated carbon and graphene as a catalyst carrier for metal-air batteries.

Role of zirconium conversion coating in corrosion performance of aluminum alloys: An integrated first-principles and multiphysics modeling approach

A variety of chromate-free conversion coatings are being actively investigated to improve the corrosion performance of light-weight alloys for aerospace and defense applications. Advancing conversion coating, however, requires an in-depth understanding of the underlying corrosion mechanisms in order to rationally design sustainable coatings. Here, we present a multiscale modeling approach to predict corrosion performance of metallic materials, with a focus on localized corrosion of Cu-containing aluminum alloys coated with ZrO2 layer. First-principles and transition-state theory are used to implement the kinetics model, which includes electrolyte-metal interfacial reactions. The modeling framework systematically characterizes and couples multiple electrochemical and physical (e.g., transport) phenomena to explore interrelationships between pit morphology, surface chemistry, and local environment. This multiscale model can quantitatively link the corrosion rate of ZrO2-coated aluminum alloys with the evolution of interfacial reactions during immersion, which is very difficult to establish using in situ experiments. We have evaluated the presented multiscale model using available experimental data. The rate of corrosion and pit stability were quantitatively assessed for various environmental parameters and applied potentials. Results show that Zr-based conversion coating strongly enhances the corrosion performance of aluminum alloys due to zirconium involvement in interfacial kinetics.

(Digital Presentation) Evaluating the Corrosivity of Liquid LiPF6 Electrolytes with Nickel-Coated Mild Steel Used in the Manufacturing of Li-Ion Cells for Energy Storage

Effects of electrolyte degradation on cell performance and corrosion of cell components, e.g. current collectors, are topics that have been reported in the literature.1,2 Recently, it has been reported that water contamination of LiPF6 liquid electrolytes can lead to salt degradation, generating difluorophosphoric acid (DFPH) and hydrofluoric acid (HF).3 With strict quality control of the moisture content in individual components and the use of dryroom conditions for cell assembly the risk of moisture contamination can be practically eliminated. When moisture is allowed to enter the cell in controlled research samples, dark stains indicative of pitting type corrosion have been observed on the inside of mild steel cans that are used to house cylindrically wound electrodes (see Figure 1 inset). Motivation to understand the underlying corrosion mechanism and the influential factors is important to help lower the risk of occurrence in manufacturing further.The use of a simple corrosion plate cell to study the susceptibility of a mild steel sample immersed in a LiPF6/EC/DMC (15/25/60 wt%) electrolyte will be discussed, with a focus on measured corrosion potentials, E corr, and current densities, j corr, extracted from the Tafel region of a potentiodynamic scan. Observations on the influence of acid degradation products in the electrolyte on the corrosion susceptibility of the mild steel will be discussed and applied to study select electrolyte additives previously reported in literature. Early results have shown that additives that scavenge HF and/or water directly can effectively suppress j corr, as shown in Figure 1, whereas additives that increased DFPH generation had no apparent effect.

Corrosion Monitoring in Atmospheric Conditions: A Review

A variety of techniques are available for monitoring metal corrosion in electrolytes. However, only some of them can be applied in the atmosphere, in which case a thin discontinuous electrolyte film forms on a surface. In this review, we describe, evaluate and compare both traditional and state-of-the-art real-time corrosion monitoring techniques to identify those suitable for atmospheric conditions. For atmospheric corrosion monitoring (ACM), electrochemical impedance spectroscopy (EIS), electrochemical noise (EN), electrical resistance (ER) probes, quartz crystal microbalance (QCM), radio-frequency identification sensors (RFID), fibre optic corrosion sensors (FOCS) and respirometry, the underlying principles, characteristics and application examples are described, and their advantages and drawbacks outlined. Finally, the techniques are compared in terms of their sensitivity, ease of setup, data processing, ability to identify underlying corrosion mechanisms and applicability in different fields of atmospheric corrosion protection and research.

Recent insights in corrosion science from atomic spectroelectrochemistry

AbstractA fundamental understanding of corrosion mechanisms is the key to developing suitable corrosion protection approaches, and for the prediction of service life of metallic structures. However, conventional corrosion testing methods such as mass loss and electrochemical testing do not guarantee estimation of "true" corrosion rate and often mask the underlying mechanisms, due to either low sensitivity or a lack of element‐resolved information. Relatively recent work in corrosion science has led to the development of a new class of corrosion testing approaches, namely atomic spectroelectrochemistry; whereby direct insight of dissolution and corrosion mechanisms can be obtained during electrochemical testing. Atomic spectroelectrochemistry provides real‐time and element resolved dissolution rate of material via coupling electrochemical flow cell with inductively coupled plasma – atomic spectroscopy. This concise review discusses the basic working principle of atomic spectroelectrochemistry and its recent applications in corrosion science to understand the true underlying corrosion mechanisms of a range of metallic materials.

The impact of corrosion-stress interactions on the topological features and ultimate strength of large-scale steel structures

Aged marine structural analysis often relies on simplified corrosion modelling. Empirical or statistical methods are used to predict a uniform thickness reduction over time. Although convenient, this approach cannot incorporate the corrosion evolution or the rough surfaces in the damaged area. This is fundamentally due to the lack of representation of the underlying corrosion mechanisms in service environments. To better understand how structural response changes based on corrosion under service loads, this paper presents a series of finite element analyses which consider the coupling relationship between the surface mechanical stresses and the resulting change of corrosion rate. The coupling provide complex corrosion-stress interaction depending on the experimental datasets. The quantification of this interaction is based on in situ experimental measurements of corrosion kinetics at different stress levels. The simulations show the stress effect results in the generation of more realistic corrosion patterns on the structural surface, based on a two-bay/two-span large-scale panel model subject to uniaxial compression. In addition, the incorporation of corrosion experiments allows the modelling of corrosion evolution based on physical observations instead of empirical assumptions. The irregular surface damage leads to a change in structural buckling mode, and up to 8% reduction in ultimate strength compared to models without considering the stress effect.

A first-principles analysis of the charge transfer in magnesium corrosion

Magnesium is the lightest structural engineering material and bears high potential to manufacture automotive components, medical implants and energy storage systems. However, the practical use of untreated magnesium alloys is restricted as they are prone to corrosion. An essential prerequisite for the control or prevention of the degradation process is a deeper understanding of the underlying corrosion mechanisms. Prior investigations of the formation of gaseous hydrogen during the corrosion of magnesium indicated that the predominant mechanism for this process follows the Volmer–Heyrovský rather than the previously assumed Volmer–Tafel pathway. However, the energetic and electronic states of both reaction paths as well as the charge state of dissolved magnesium have not been fully unraveled yet. In this study, density functional theory calculations were employed to determine these parameters for the Volmer, Tafel and Heyrovský steps to gain a comprehensive understanding of the major corrosion mechanisms responsible for the degradation of magnesium.

Characterization of Intergranular Corrosion in AA 5xxx Al-Mg Alloys

Abstract Intergranular corrosion refers to a selective corrosion attack at the grain boundaries of polycrystalline materials. In engineering Al-Mg alloys of the AA 5xxx series it is caused by the formation of chemicallyless noble β-Al8Mg5 phases along the grain boundaries. The sensitivity of a material to intergranular corrosion can be assessed based on the mass losses in the so-called nitric acid mass loss test (NAMLT) according to ASTM G67. However, a detailed investigation of the underlying corrosion mechanisms requires that the β phases are made directly visible in the microstructure by metallographic methods. In the present work, the NAMLT mass losses of three AA 5xxx alloys with different Mg contents are compared with the results from two different etching methods. On the one hand, the alloys are etched in diluted phosphoric acid, a substance routinely used to examine the grain boundary occupancy in AA 5xxx materials. On the other hand, a newer etching method using a dilute ammonium persulfate solution is tested which etches the β-Al8Mg5 phases in the microstructure in a way that they can be examined at higher magnifications; even examinations in the scanning electron microscope are possible.

Molybdenum – A biodegradable implant material for structural applications?

Molybdenum as a potentially new biodegradable material was investigated. Degradation behavior of commercially high purity molybdenum was observed in simulated physiological salt solutions (Kokubo's SBF with/without TRIS-HCl, Cu2+ addition and 0.9% NaCl solution). Potentiodynamic polarization, immersion mass loss and ion concentration measurements paired with REM/EDX analysis reveal gradual dissolution of molybdenum in the proper order of magnitude for stent application, associated with formation of thin, non-passivating corrosion products. The underlying corrosion mechanism is discussed as well as a comparison to literature data. However, formation of calcium phosphates (CaP) in SBF significantly decreases corrosion rates. In-situ polarization was found to be a potential way for overcoming this problem and simultaneously enhancing corrosion above the benchmark for a degradable stent material. Statement of SignificanceBiodegradable metals have the potential to overcome severe complications common to orthopedic and cardio-vascular implants. However, the need for a material with moderate and predictable degradation, high strength and toughness as well as MRI suitability must be satisfied. Molybdenum as potential new biodegradable material may just fulfill these requirements. An overall positive picture of molybdenum as an interesting alternative to recently discussed metallic biodegradable materials can be concluded from the herein presented results and from literature data, showing directions for future research on the topic.

Long‐term exposure of austenitic steels and nickel‐based alloys in lignite‐biomass cofiring

AbstractThe aim of this study was to assess the long‐term impact that the addition of biomass provokes on superheater materials exposed to fireside corrosion environments. Alloys covering a broad range of commercially available materials were investigated. Their corrosion kinetics under different corrosive deposits and atmospheres was evaluated, and their corrosion products analyzed to deepen understanding of the underlying corrosion mechanisms. Therefore, three nickel‐based alloys and three austenitic steels containing 20–24 wt.% Cr were tested at 650°C for 7,000 hr. The long‐term exposure shows new mechanistic aspects of Type II hot corrosion that were revealed by accelerated material depletion. The formation of Ni–NiS eutectic and the formation of a Cr depleted zone close to the substrate corrosion product interface are indicative of the breakaway occurrence. Differences in the corrosion behavior are related to the balance of Ni, Mo, Co, and Cr and can serve as the material selection argument. The evaluation concluded with the finding that alloys presenting Mo and Ni might be preferentially used in fireside corrosion in the presence of biomass, whereas the use of austenitic steels suffer less corrosion if no biomass is present in the corrosive atmosphere.

Effect of different aging processes on the corrosion behavior of new Al–Cu–Li–Zr–Sc alloys

AbstractThe intergranular corrosion and exfoliation corrosion behaviors of Al–Cu–Li–Zr–Sc alloys under different aging effects, such as single‐stage aging, strain aging, and double‐stage aging, were studied. Among the three aging treatments, single‐stage aging resulted in the best resistance to corrosion, followed by double‐stage aging; strain aging resulted in the worst corrosion resistance. A 3.5% precooling strain could increase the dislocation density, which promoted the precipitation of corrosion‐prone T1 phase and increased the corrosion driving force of the alloy. Double‐stage aging made the precipitated T1 phases finer and more uniform and reduced the number of equilibrium phases at grain boundaries, thus improving the corrosion properties of the alloy. The corrosion susceptibility of the alloy was attributed to the T1 phase and precipitate‐free zone (PFZ), and the underlying corrosion mechanism was revealed as preferential dissolution of the equilibrium phase at grain boundaries and its surrounding distortion zone, followed by expansion of the PFZ along the grain boundaries, resulting in the development of corrosion from the grain boundaries to the intragranular regions.