Scanning Vibrating Electrode Technique Research Articles

Roles of Al and Mg on the Microstructure and Corrosion Resistance of Zn-Al-Mg Hot-Dipped Coated Steel.

In this work, a novel zinc-aluminum-magnesium (Zn-Al-Mg, ZM) coated steel was prepared using the hot-dip method. The microstructure and corrosion resistance of the ZM-coated steel were investigated. Compared to the conventional galvanized steel (GI), the ZM coating demonstrated a distinctive phase structure, consisting of Zn phase, binary eutectic (Zn/MgZn2), and ternary eutectic (Zn/Al/MgZn2). The corrosion resistance of the ZM-coated and GI-coated steels was evaluated by neutral salt spray test (NSST), polarization and electrochemical impedance spectroscopy (EIS). The results indicated that ZM-coated steel provided superior long-term corrosion protection in a NaCl environment compared to GI-coated steel. The scanning vibrating electrode technique (SVET) proved to be an effective method for investigating the evolution of the anodic and cathodic on the local coating surface. GI-coated steel exhibited a potential and current density distribution between the cathodic and anodic sites nearly three orders of magnitude higher than that of ZM-coated steel, suggesting a higher corrosion rate for GI-coated steel.

Co-deposition of natural particles in the acidic zinc electroplating process

The co-deposition of natural particles was studied in zinc electroplating on mild steel in the acidic sulfate bath aiming a composite coating. Despite most papers reported the effect of natural extracts in such process, the evaluation of natural particles adhered to the coating was not explored yet. The process efficiency, surface morphological changes, and corrosion resistance were evaluated in different concentrations of avocado seed powder (ASP, Persea americana), cocoa bean shell powder (CBSP, Theobroma cacao), and garlic peel powder (GPP, Allium sativum). In general, the current efficiency was not significantly influenced by the presence of any natural particles, reaching values over 90 %. The presence of natural particles impacted the active sites on the substrate surface, modifying the grain growth assessed by scanning electron microscopy (SEM) analysis. The GPP samples showed better visual aspect and higher compactivity. By Fourier transform infrared spectroscopy analysis, such natural particles are constituted by molecules of long carbon chains with chemical groups – amines, carboxylic acids, and phenols. Hence, the presence of natural particles in the coating layer led to different responses in the contact angle test, where GPP samples showed higher wettability (θ < 90°), followed by ASP and CBSP samples (θ > 100°). Such behavior indicates a greater bath penetration during the electroplating process in the presence of more hydrophilic particles, which may have resulted in the refined deposits of the GPP samples. The concentration of 0.330 g/L GPP presented higher impedance modulus (6548.3 Ω.cm2 at 0.03 Hz) in the electrochemical impedance spectroscopy (EIE) and reduced the anodic ionic current density at least four times in the scanning vibrating electrode technique (SVET). The same was observed in the salt spray test (SST) adapting ASTM B117, which revealed to be sensitive enough to indicate that the 0.330 g/L GPP sample showed greater corrosion resistance.

Highly efficient anti-corrosion polyetherimide (PEI) coating encapsulated with aluminum di-hydrogen tri-polyphosphate

Protective composite coatings based on polyetherimide (PEI) incorporated with aluminum di-hydrogen tri-polyphosphate (ATP) particles for the protection of carbon steel are reported here. The ATP particles were used as an inhibiting pigment. The layers were applied by the dip coating method leading to a full uniform coverage as confirmed by microscopic and Raman spectroscopy techniques. The anti-corrosion characteristics of the PEI-coated samples were investigated in a 3.5 wt.% NaCl solution by electrochemical impedance spectroscopy. The results revealed that PEI-ATP composite coating exhibited higher impedance values above 109 Ω.cm2 (up to 30 days) compared to a Blank-PEI coating. The active protection was studied by the scanning vibrating electrode technique revealing clear signs of inhibition in the artificial defects. The proposed mechanistic insights into the protective properties of PEI-ATP coating was further supported using X-ray photoelectron spectroscopy.

Sebacate intercalated Ca[sbnd]Al layered double hydroxide pigments for corrosion protection of low carbon steel: Anion exchange and electrochemical properties

Sebacate (SB) molecules were incorporated into CaAl layered double hydroxides (CaAL-LDH) using a hydrothermal method. The resulting products underwent characterization through X-ray diffraction, infrared spectroscopy, and scanning electron microscopy. To investigate the release kinetics and thermodynamics of the corrosion inhibitor, total organic carbon analysis (TOC) was employed. A comparison was made with CaAl-LDH prepared without corrosion inhibitors, in which nitrates were the primary anions within the layered clay structure. An assessment of anion exchange between nitrates and chlorides was conducted using total nitrogen analysis (TN). The SB anion comprised approximately 49.2% of the total dried powder synthesized, with a maximum release efficiency of around 86.7% fitting a Langmuir model. For the evaluation of the corrosion inhibition effect on steel surfaces, electrochemical impedance spectroscopy, potentiodynamic polarization, and scanning vibrating electrode technique were employed. The corrosion inhibition impact was ascribed to the liberation of SB anions from the CaAl-LDH pigments, accompanied by alkalinization stemming from partial particle dissolution. Additionally, the incorporation of organic molecules notably enhanced the stability of pigments in aqueous solutions compared to similar ones hosting nitrates. These findings suggested that CaAl-LDH pigments have potential as nanocontainers for organic inhibitors, showing promising prospects in the field of corrosion research.

Bimetallic lattice doped fillers with durable oxygen vacancies for corrosion resistance of coated steel in hostile environments

A series of Ca-doped Co3O4 nano-sheets (Ca-Co3O4) rich in oxygen vacancies were prepared using the alkali precipitation method at different pH values and added to the coating. Scanning electron microscopy and X-ray diffraction were performed to confirm the successful synthesis of the nano-fillers. The electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) spectroscopy determined that the nano-filler has abundant oxygen vacancies. The corrosion resistance test results showed that the epoxy coating containing the Ca-Co3O4 (pH = 12.5) filler exhibited the best corrosion resistance. After 48 h of corrosion in a high-temperature and high-pressure oxygen-rich (HTHP-O2) environment, the |Z|0.01 Hz is 5 orders of magnitude higher than that of the pure Co3O4 coating. Furthermore, the scanning vibrating electrode technique and O2-adsorption-desorption test indicated that Ca-Co3O4 with durable and stable oxygen vacancies could provide a complete, stable and dense passive film for the substrate (Q235B steel). Because oxygen vacancies can selectively transport O2 to the coating-substrate interface ahead of other corrosive media.

Study on the Microstructure and Properties of FeCoNiCrAl High-Entropy Alloy Coating Prepared by Laser Cladding-Remelting

Laser remelting technology effectively repairs defects such as pores and cracks in the coating. To investigate the impact of laser remelting on high-entropy alloy coatings, this study used Q235 steel as the substrate and employed laser cladding technology to prepare FeCoNiCrAl high-entropy alloy coatings, followed by laser remelting treatment. The phase composition and microstructure of the coatings were extensively characterized using equipment such as optical microscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Additionally, the wear resistance and corrosion resistance of the coatings were tested using a multifunctional material surface performance tester, an electrochemical workstation, and SVET (Scanning Vibrating Electrode Technique). The results indicate that following laser remelting treatment, the atomic proportion of Fe elements on the coating surface decreased from 33.21% to 26.03%, while the atomic proportion of Al elements increased from 12.56% to 20.31%. The phase composition of the coating underwent a marked transformation, shifting from a structure composed of FCC, A2, and B2 phases to a singular BCC structure characterized by the presence of A2 and B2 phases. Concurrently, the grain morphology on the coating surface transitioned from elongated plate-like grains to equiaxed grains. Laser remelting enhanced the wear resistance of the coating. Laser remelting had no significant impact on the corrosion resistance of the non-cracked regions of the coating.

Contribution of Oxygen Reduction to Mg Corrosion: Experimental Evidence By Electrochemical Microprobes

Unravelling the mechanisms of magnesium corrosion is vital for establishing reliable research methodologies, developing new alloys and predicting magnesium degradation behaviour. Hydrogen evolution reaction is widely regarded as by far the main cathodic process during corrosion of magnesium. Hence, tracing the amount of released hydrogen was thought to be a reliable measure of magnesium degradation rate. Recently, experimental evidences obtained by different methods have been presented by several groups that oxygen reduction (ORR) is important secondary cathodic reaction during Mg corrosion [1-8]. For model galvanic couple Al-Cu-Mg, oxygen consumption on the surface of Mg along with that on Cu was observed by using oxygen sensing microoptode [1]. A simultaneous measurement of local current density by scanning vibrating electrode technique (SVET) and local oxygen concentration by oxygen sensing microoptode found a strong oxygen consumption at the active corrosion sites on the surface of commercially pure Mg in NaCl electrolyte [2]. Then, the evidence for the varying contributions of ORR to the total cathodic process on Mg alloys during atmospheric and aqueous corrosion was presented based on newly developed respirometric methods [3-5]. While around 10% of the total cathodic current was ascribed to ORR during aqueous corrosion, up to 60% of the total corrosion process was shown to be due to the ORR contribution during simulated atmospheric corrosion with wet-dry cycles. Nanoparticle-based O2 imaging was used to map decreased O2 concentration at the interface of several Mg alloys exposed to simulated body fluids [6].High ORR rate on a slowly corroding ultra-high-purity Mg (UHP-Mg) in NaCl electrolyte was shown in our recent work [7]. Comparing that with lower ORR rate for faster corroding commercial purity Mg (CP-Mg), the rate of ORR was found to depend on the growth rate of Mg(OH)2 , which is the main corrosion product of Mg in the simple saline solution. The formation of Mg(OH)2 impeded the diffusion of oxygen from bulk electrolyte to metal interface, preventing metallic Mg substrate from the interaction with oxygen. The distribution of local concentration of dissolved gaseous H2 and O2 assessed by amperometric Clark-type micorprobes at the interface of UHP-Mg and CP-Mg demonstrated the inverse relationship between HER and ORR, and supports the finding that diffusion-controlled ORR highly depends on the barrier property of corrosion products on the surface [8]. The contribution of ORR current to total cathodic current reached 29.1% for UHP-Mg, while only 0.9% for CP-Mg after 1 hour of immersion in NaCl solution. The contribution of ORR decreases with immersion time, due to the impeded access of dissolved O2 to Mg interface caused by thickening of Mg(OH)2 layer. A numerical model was developed considering the mixed potential diagram and measured local oxygen concentration. REFERENCES: [1] D. Snihirova, M. Taryba, S.V. Lamaka, M.F. Montemor, Corrosion inhibition synergies on a model Al-Cu-Mg sample studied by localized scanning electrochemical techniques, Corrosion Science, 112 (2016) 408-417.[2] E.L. Silva, S.V. Lamaka, D. Mei, M.L. Zheludkevich, The Reduction of Dissolved Oxygen During Magnesium Corrosion, ChemistryOpen, 7 (2018) 664-668.[3] M. Strebl, S. Virtanen, Real-Time Monitoring of Atmospheric Magnesium Alloy Corrosion, Journal of The Electrochemical Society, 166 (2019) C3001-C3009.[4] M. Strebl, M. Bruns, S. Virtanen, Editors’ Choice—Respirometric in Situ Methods for Real-Time Monitoring of Corrosion Rates: Part I. Atmospheric Corrosion, Journal of The Electrochemical Society, 167 (2020) 021510.[5] M. Strebl, M.P. Bruns, G. Schulze, S. Virtanen, Respirometric In Situ Methods for Real-Time Monitoring of Corrosion Rates: Part II. Immersion, Journal of the Electrochemical Society, (2021).[6] B. Zeller-Plumhoff, A.R. Akkineni, H. Helmholz, D. Orlov, M. Mosshammer, M. Kühl, R. Willumeit-Römer, M. Gelinsky, Oxygen-sensitive nanoparticles reveal the spatiotemporal dynamics of oxygen reduction during magnesium implant biodegradation, NPJ Materials Degradation, 6 (2022) 95.[7] C. Wang, D. Mei, G. Wiese, L.Q. Wang, M. Deng, S.V. Lamaka, M.L. Zheludkevich, High rate oxygen reduction reaction during corrosion of ultra-high-purity magnesium, NPJ Materials Degradation, 4 (2020) 42.[8] C. Wang, W. Xu, D. Höche, M.L. Zheludkevich, S.V. Lamaka, Exploring the contribution of oxygen reduction reaction to Mg corrosion by modeling assisted local analysis, Journal of Magnesium and Alloys, 11 (2023) 100-109. ACKNOWLEDGEMENTS: C. Wang, Wen Xu, thank China Scholarship Council for the funding, grants no. 201806310128 and 201908510177

Active/passive protection and anti-UV waterborne epoxy coatings based on low defect functionalized PDA-GO-CeO2 material for excellent corrosion control

In the marine environment, besides external damage and internal defects, ultraviolet (UV) radiation is also a pivotal factor affecting the stable properties of coatings. Improving the anti-UV aging and active protection properties is beneficial to prolong the service life of coatings. Herein, the PDA-GO-CeO2 composite material is obtained through a facile stepwise growth of polydopamine (PDA) and CeO2 on GO nanosheets and further preparing a multifunctional anti-corrosion PDA-GO-CeO2-EP coating. As expected, composite coating displays strong adhesion force (6.9 MPa) and tensile strength (21.1 MPa), profiting from the enhanced crosslinking density and interface structure caused by PDA-GO-CeO2 and resin matrix interaction. Since low defect functionalized PDA-GO-CeO2 constructs the reinforced physical barrier, the anti-corrosion ability of PDA-GO-CeO2-EP coating (109.7 Ω·cm2) is more dependable than those of EP (106.7 Ω·cm2), GO-EP coating (108.8 Ω·cm2) and PDA-GO-EP (109.1 Ω·cm2) coatings after immersing in 3.5 wt.% NaCl solution for 100 d. Thanks to the synergistic effect of PDA/CeO2, the active protection performance of PDA-GO-CeO2-EP coating is endowed, which is further proved through the in situ scanning vibrating electrode technique (SVET). Following the UV aging treatment, PDA-GO-CeO2-EP coating shows minor gloss loss (20.4 %) and better adhesion force (4.3 MPa). Furthermore, the resistance value of PDA-GO-CeO2-EP coating is 7.03 × 106 Ω cm2 after 30 d immersion in saline solution, which is 242 times greater than that of EP coating (2.90 × 104 Ω cm2). This dependable performance of PDA-GO-CeO2-EP coating comes from the reflection, refraction, and absorption of UV rays by nanoscale CeO2 particles. Consequently, this work offers a simple and promising strategy for fabricating multifunctional long-term protection coatings, thus hopefully extending the service life of marine engineering equipment.

Pretreatment of Carbon Steel with Hybrid Film Modified with Different Natural Inhibitors (Garlic and Cocoa)

One way to protect carbon steel from corrosion is through pretreatment and corrosion inhibitors, such as the use of silane layers that can protect against corrosion. In this study, the corrosion resistance of the tetraethyl orthosilicate/3-glycidoxypropyltrimethoxysilane hybrid film modified with garlic peel powder or cocoa shell powder on carbon steel was evaluated. By electrochemical impedance spectroscopy (EIS) it was observed that the incorporation of inhibitors promoted higher values of impedance modulus compared to the bare metal substrate or in the absence of inhibitor, and the concentration of 1.77 g/L provided the best performance for both natural inhibitors. The Fourier transform infrared spectroscopy showed that the addition of inhibitors to the hybrid film promoted better hydrolysis in the formation of the films. By the scanning electron microscopy and energy dispersive spectroscopy it was noted that there was the formation of the hybrid film on the metal surface, and the insertion of the inhibitor to the silane film enabled the formation of grains that precipitate from the deposition of the inhibitor on the substrate surface. The scanning vibrating electrode technique shows that the inhibitors improved corrosion resistance of the hybrid film, corroborating with the EIS analysis. The contact angle shows that the presence of the inhibitor in the silane film makes it less hydrophilic. The roughness profile shows that the hybrid film with cocoa has the highest roughness represented by a higher average value of average roughness (Rz).

In-situ measurement of electrochemical activity related to filiform corrosion in organic coated steel by scanning vibrating electrode technique and scanning micropotentiometry

A new approach to studying the mechanism of filiform corrosion in organic coated steel based on the scanning vibrating electrode technique (SVET) and micropotentiometry (potentiometric SECM) is presented. The electrochemical activity of the process under the coating is evaluated by mapping the ionic current densities emanating from some artificial defects made in specific locations of the filament identifiable thanks to the transparency of the coating. In addition, antimony tips are employed to investigate the pH changes associated to the different corrosion reactions at the metal-paint interface. Local pH levels associated to anodic and cathodic regions are determined along the filament.

A comparative study of microstructure and corrosion resistance in DP980 steel joints under different laser welding method

Three different laser welding methods were used to welding 2 mm DP980 steel butt joint, which were laser welding, laser-TIG hybrid welding and laser powder feeding (CoCrNi) welding respectively. An observational analysis was carried out in terms of microstructure, mechanical properties and corrosion resistance. It was discovered that a central microstructure of laser welding and laser-TIG hybrid welding the weld was dominated by slatted martensite. The laser powder feeding welding has resulted in a remarkable reduction of cylindrical crystals and an increased number of isometric crystals in weld center due to the addition of CoCrNi powder. Microstructure was further refined in incomplete phase transition zone by three different laser welding methods. The tempered martensite was also formed in the tempering zone, which softened the joint. The hardness values decreased considerably and tensile samples were also fractured at this location. Comparison with corrosion resistance of their joints through polarization curves and impedance spectra shows that laser powder feeding joint > laser joint > laser-TIG hybrid joint. The corrosion resistance of laser and laser-TIG hybrid joints in the base metal (BM) > heat affected zone (HAZ) > weld zone (WZ) is shown based on scanning vibrating electrode technique (SVET) localized area testing data at different time intervals. The laser powder feeding welding enhances the density of passivation film by reacting Cr to form Cr2O3, thereby improving corrosion resistance of entire joint.

Facile self-assembly fabrication of anticorrosive imine-based composite material for excellent protective coatings

Designing suitable additive materials for high-performance anti-corrosion coatings continues to be a formidable obstacle. Although imine molecules are a promising family of organic compounds due to the π-conjugated aromatic structure containing N heteroatoms, their research as the anticorrosive additives is still in its early stages. Herein, we have prepared a rod-like imine molecule, Hexaazatrinaphthalene (HATN), which was further assembled with reduced graphene oxide (rGO) to obtain a novel HATN/rGO composite via a facile electrostatic self-assembly. As such, the HATN/rGO composite as additive material was incorporated into epoxy resin to develop a corrosion-resistant HATN/rGO epoxy coating, showing exceptional passive and active anti-corrosion properties with a low corrosion rate of 5.75 × 10−6 mm a−1 and a high corrosion inhibition efficiency of 99.83 %. Remarkably, the resistance at 0.01 Hz value of HATN/rGO epoxy coating is superior to those of rGO-based epoxy coatings previously reported. Scanning vibrating electrode technique (SVET) and salt spray measurements further demonstrate long-term performance of the HATN/rGO epoxy coating with excellent corrosion resistance. Additionally, theoretical calculation proves that the obvious electron transfer from rGO to HATN molecule promotes charge-transfer capability and electrochemical activity of the HATN/rGO composite, suggesting its great potential as the additive material for high-performance corrosion protection.

Preparation and corrosion behavior of Mg(Li)-M layered double hydroxides films on the surface of magnesium-lithium alloy

Different Mg(Li)-M (M: Al, Fe, Cr) layered double hydroxides (LDHs) films were successfully prepared on Mg-Li alloy with in-situ growth method. Further, the corrosion inhibitor was screened and the tartaric acid-loaded LDHs (Mg(Li)-M LDHs-TA) films were prepared. The morphologies, compositions, structures and corrosion resistance of the LDHs films changed before and after tartaric acid loading. The Mg(Li)-Cr LDHs-TA film exhibits the most excellent corrosion resistance with a current density icorr 1.5 × 10–9 A cm–2. Moreover, corrosion mechanisms of the Mg(Li)-M LDHs and Mg(Li)-M LDHs-TA films were discussed in combination with Scanning Vibrating Electrode Technique.

Localized Corrosion and Passive Film Characteristics of Additive Vs. Wrought Al-Zn-Mg-Cu-Based Alloy

Al-Zn-Mg-Cu alloys of the 7xxx series have proven to be excellent candidates for the aerospace, defense, and automotive industries due to their high strength to weight ratio, high corrosion resistance, and fatigue performance. For additively manufactured alloys manufactured using laser powder bed fusion (LPBF), the non-conventional microstructures formed as a result of one or more factors like alloy modification, LPBF process, and subsequent thermal treatments can lead to change in corrosion behavior and mechanisms.Using scanning vibrating electrode technique (SVET) and immersion tests in chloride environments, this work concentrates on understanding the localized corrosion behavior of additively manufactured Al-Zn-Mg-Cu based alloy in comparison with its wrought counterpart. To understand the corrosion mechanisms, microstructures including the precipitates are characterized at different length scales using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and correlated with the corrosion behavior. The difference in passive film characteristics of additive vs. wrought Al-Zn-Mg-Cu based alloy is studied using electrochemical impedance spectroscopy (EIS) and X-Ray photoelectron spectroscopy (XPS) characterization. Overall, the LPBF-based alloy studied shows better corrosion performance than the wrought counterpart in chloride environments. The microstructural differences and mechanisms responsible for this behavior is proposed.

Novel Low Temperature Galvanic Coatings

A major influence on performance and lifetime of steel-based structures is corrosion. Some such structures include ground support equipment (GSE). GSE plays a vital role in the safety and efficiency of aircrafts and airports worldwide. GSE encompasses all support equipment at airports, necessary for the function and logistical operation of airports, air bases and other aviation facilities. According to the IMPACT study undertaken by the National Association of Corrosion Engineers (NACE) in 2013, the global cost of corrosion was estimated to be US$2.5 trillion, equivalent to 3.4% of the global GDP at the time. The corrosion of GSE can be attributed to a number of factors present including, but not limited to, moisture in air, salt particles in coastal winds and industrial pollutants from fuel. For these reasons, corrosion prevention through means of protective coatings is of utmost importance; as well as a strict programme of corrosion inspection and repair. Traditional means of protection against corrosion centre around Hot-Dipped Galvanising (HDG). This is the process of applying a protective zinc layer to steel in order to prevent corrosion of a component or structure. One of its major issues is cost. This is primarily due to the need to outsource galvanising to external contractors, especially for manufacturers that do not have the capacity or capability to undertake the process in-house. HDG also happens at high temperatures of ~450°C. This requires a great amount of energy and would be impossible to implement in-house for manufacturers. The repair of HDG sections is also a concern. This often sees the protective Zn coating removed and the section welded to repair. In this instance, the steel substrate is left unprotected and susceptible to corrosive attack. The common solution here is Cold Galvanising Coatings (CGCs) or Zn Rich Paint (ZRP). ZRPs are, however, laden with issues; poor adhesion, UV degradation and poor galvanic contact with the substrate. These shortcomings mean the service life of ZRPs and CGCs are limited.The current work focuses on developing a lower-temperature alternative centred around fusible alloys. The aim is to provide a corrosion-resistant coating that can bond with the substrate at lower temperatures, be applied on-site and offer galvanic protection. The fusible alloy is mixed with zinc powder to create a novel corrosion resistant coating. The coating may also be used for repair of galvanised GSE, which often sees welded areas left susceptible to corrosive attack. Microstructural analysis was carried out as a function of Zn loading weight as well as curing time and temperature. A systematic study on curing time and temperature identified the optimal processing parameters for the novel coating. Heating of below 250°C was used to melt the fusible alloy to envelop the zinc. This can be achieved using a conventional oven, while Near Infrared (NIR) heating was explored as a rapid heating option. The resulting optical and scanning electron microscope (SEM) images show that the fusible alloy acts as a matrix, while “islands” of Zn are dispersed therein to offer galvanic protection. The use of a metal matrix in the microstructure allows for superior electrical contact between the sacrificial Zn and the structural steel substrate, unlike that given by most CGCs and ZRPs. Electrochemical studies, such as Zero Resistance Amplitude (ZRA) and Open Circuit Potential (OCP), and accelerated corrosion tests, implementing the Scanning Vibrating Electrode Technique (SVET), have shown that the coating offers excellent galvanic protection to steel in a 1% NaCl electrolyte. ZRA galvanic corrosion tests demonstrated that current flow between components was negligible. This suggests no obvious self-corrosion in the coating, despite the nobility of fusible alloy components. Current OCP results show that adding Zn to the fusible alloy shifts the OCP from -425mV to -970mV in a 24hr test in 1%NaCl. This is comparable to that of Zn which is stable at -1V and shows promise when compared to bare steel at -700mV, meaning the coating offers galvanic protection to the steel. This is reinforced by SVET maps of coated steel samples, confirming that steel remains cathodic to the coating throughout tests, showing that performance improves as a function of Zn content. Adhesion tests, using the 0t method, show the peak performance comes at a known Zn wt% with cracks appearing and propagating as Zn wt% is increased. Overall; peak microstructure, corrosion protection and adhesion are achieved and demonstrated. The resulting product is a robust, protective, and dynamic coating; a step forward in the fight against corrosion of steel and protection of HDG.

Inhibition of the localised corrosion of AA2024 in chloride solution by 2-mercaptobenzimidazole and octylphosphonic acid

The scanning vibrating electrode technique was employed to monitor the in situ localised electrochemical activity on aluminium alloy 2024-T3 at the free corrosion potential in aggressive NaCl solutions with and without corrosion inhibitors 2-mercaptobenzimidazole (MBI) and octylphosphonic acid (OPA). MBI is a very effective inhibitor against pitting corrosion, even in a chloride environment without the reservoir of MBI. In contrast, localised corrosion was observed at several points at the early stage of immersion of AA2024 in NaCl solution containing OPA. Ex situ energy-dispersive X-ray spectroscopy analysis of the Al2CuMg particle at the cross-section revealed that MBI does not entirely prevent its dissolution but reduces the dissolution rate and the rate of oxygen reduction by forming an insoluble complex compound Cu–MBI on the partially dealloyed AlCu-based particles, i.e., on the porous copper remnants. The MBI’s action is an efficient option for mitigating the corrosion of Cu-based aluminium alloys.

Lithium salts as active corrosion inhibitors for aluminum substrates

As an effort to replace carcinogenic chromate from protective coatings, researchers are continuously searching and proposing new non-toxic chemicals sharing similar properties: leachable, responsive, and film-forming. Lithium (Li), in the form of either a salt additive in coatings or alloyed with aluminum (Al), is observed to be able to form insoluble protective layers and thus reduce the corrosion rate of the Al substrates. The protective film formed is often composed of Li, Al, carbonate (CO3), hydroxide (OH) etc., sharing similar structure to a layered double hydroxide (LDH) as indicated by several mechanistic studies. The successful formation of the irreversible/insoluble protective film requires oxygen, correct pH, and the right combination of available ion species. In this study, electrochemical impedance spectroscopy (EIS) results show that these leachable inhibitor coatings yield poor barrier properties before and after artificial weathering. Scanning electron microscopy pictures show the growth of an inorganic layer between the coating and aluminum substrate, indicative of the formation of a layered double hydroxide (LDH) structure. Current mapping generated by scanning vibrating electrode technique (SVET) indicates that aerated lithium salt solutions can cause pitting corrosion over bare aluminum, while organic coatings pigmented with lithium were able to keep the surface relatively passive, with and without the presence of a defect. An anodic shift of the surface of the organic coatings pigmented with lithium containing a defect is seen while keeping the surface relatively passive, indicating anodic passivation of the metal surface.

Corrosion Evolution of Nickel Aluminum Bronze in Clean and Sulfide-Polluted Solutions

Nickel aluminum bronze (NAB) alloys are reported to suffer accelerated local corrosion in sulfide-polluted seawater. In this work, the real-time in situ scanning vibrating electrode technique (SVET) was employed to monitor the evolution of the corrosion product film of a typical NAB alloy immersed in the clean and sulfide-polluted 3.5% NaCl solutions. In the sulfide-free condition, the corrosion current peak surged at the individual point of the NAB surface and receded to calm in 2 h. In the presence of the sulfide, however, multiple active points on the measured metal surface released high corrosion current for a long time, indicating that intense corrosion had occurred. The corrosion mass loss was more than four times the former. Global electrochemical techniques, scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were adopted to perform a comprehensive analysis of the composition of the corrosion product films. The results show that a dense layer of aluminum and cuprous oxide forms on the NAB surface in the sulfide-free solution, while a loose mixture of cuprous sulfide and cuprous oxide is detected in the sulfide-contaminated solution. This finding is believed to account for the observed distinction between the corrosion behavior of NAB in the two solutions.

Study on formation mechanism and corrosion resistance of an environmentally protective insulating coating on non-oriented electrical steel

In the present work, we developed an environmentally protective insulating coating for non-oriented electrical steel, which was composed of phosphate film-forming agent, rare-earth passivating agent, silane coupling agent and polyurethane additive. The morphology, composition, electrochemical properties and corrosion resistance of the coating were characterized by scanning electron microscopy (SEM), confocal Raman microscopy (CRM), scanning vibrating electrode technique (SVET), and electrochemical measurements. The formation mechanism of the coating revealed that the synergistic effect of rare-earth passivating agent and silane coupling agent made the distribution of rare-earth oxides uniform and hindered the agglomeration of phosphate. Further addition of polyurethane formed cross-linked network structures with the original coating to enhance corrosion resistance. The electrochemical results indicated a relatively low corrosion current density (0.12 μA cm−2) and a relatively high polarization resistance (143 kΩ cm2). The SVET results illustrated a good passivation effect of the coating, contributing to a continuous protection to the substrate. In addition, the coating could withstand the annealing test for 2 h and resist the neutral salt spray test for 8 h. Consequently, the comprehensive properties of the coating were found to be higher than industrial standards, satisfying application requirements for non-oriented electrical steel.

Evaluation of the Influence of Eggshell (ES) Concentration on the Degradation Behavior of Mg-2.5Zn Biodegradable Alloy in Simulated Body Fluid.

Currently, permanent vascular stents are fabricated using titanium and stainless steel implants that are nondegradable and offer high stability, but they have certain disadvantages. For example, the prolonged exposition of aggressive ions in the physiological media and the existence of defects in the oxide film create conditions for corrosion to occur, thus triggering unwanted biological events and compromising the mechanical integrity of the implants. Moreover, when the implant does not need to be permanent, there is the need to submit the patient for a second surgery for implant removal. As a solution for nonpermanent implants, biodegradable magnesium alloys have been deemed a promising substitute, for example, for cardiovascular-related applications and the construction of orthopedic devices. A biodegradable magnesium alloy (Mg-2.5Zn) reinforced by zinc and eggshell was employed in this study as an environment-conscious magnesium (eco) composite (Mg-2.5Zn-xES). Disintegrated melt deposition (DMD) was used to fabricate the composite. Experimental studies were conducted to investigate the biodegradation performance of Mg-Zn alloys containing 3 and 7 wt % eggshell (ES) in simulated body fluid (SBF) at 37 °C. Different corrosion techniques were used to study the corrosion behavior of the Mg-2.5Zn-xES composites, including weight loss measurements, hydrogen evolution, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and scanning vibrating electrode technique (SVET). Scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) were employed to scrutinize the corroded surfaces' morphology and composition. The outcomes indicated that Mg-2.5Zn-3ES possesses the lowest degradation activity.