In order to study the effect of defects on the corrosion behavior of MAO coatings, global electrochemical method including potentiodynamic polarization (PDP) and electrochemical impedance ...spectroscopy (EIS) was used to measure the corrosion resistance of MAO coatings. Local electrochemical measurements including local impedance spectroscopy (LEIS) and scanning vibrating electrode technique (SVET) were used to study the local corrosion mechanism. The confocal microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to observe the microstructure and phases of the samples. Results of global electrochemical measurement demonstrate that the scratches cause the reduction of the impedance and the increase of the corrosion current density. LEIS results show the impedance value distribution of the coating with the scratch. With the changes over time, the minimum impedance value of the scratch decreases first and then increases from the third hour. SVET results show the corrosion current density distribution of the scratched coating. The maximum corrosion current density at the scratch increases first and then decreases. The results of local electrochemical measurements show that the corrosion rate at the scratch decreases when the reaction lasted 3 h. Confocal microscopy found that the scratch is expanding with increasing time, and the depth and cross-sectional area of the coating is 5 times deeper and 10 times larger than that of 7075Al with scratch at the first 6 h. The 6-day immersion test proved that the scratch coating still has the protective effect on the substrate. The main corrosion product is Al(OH)3. A physical model reflecting the mechanism of the scratch propagation and the formation of the corrosion product film is proposed.
•Global and local electrochemical corrosion of scratched MAO coating on Al Alloy was studied.•The electrochemical impedance and current distribution of scratches were analyzed by LEIS and SVET.•A phenomenological model for the influence of scratch on the local corrosion of the MAO coated Al alloy was proposed.
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•Synthesis of microcapsules (5−20 μm) containing isophorone diisocyanate using tris (p-isocyanatophenyl) thiophosphate as a shell forming agent.•Microcapsules show high encapsulation ...efficiency and high thermal and chemical resistance.•Microcapsules show high compatibility with epoxy coating formulations.•Electrochemical studies confirmed important healing ability of the microcapsules- containing coating.
This work aims at investigating the self-healing ability of epoxy coatings, modified with microcapsules containing highly reactive isocyanate in their core. Highly efficient, thermally and chemically stable isophorone diisocyanate microcapsules were prepared via emulsification followed by interfacial polymerization at the surface of oil droplets of the oil-in-water (O/W) emulsion. The microcapsules were incorporated into an epoxy coating to protect carbon steel from corrosion. Scanning Electron Microscopy (SEM) was used to assess the microcapsules̕ and coating morphology. The physico-chemical characterization of the microcapsules was studied by Fourier Transformed Infrared Spectroscopy (FTIR) and Thermogravimetric Analysis (TGA). Electrochemical Impedance Spectroscopy (EIS) was employed to evaluate the protective performance of coated steel samples and results confirmed that the barrier properties of modified coatings increased over time. The self-healing ability was studied via Localized Impedance Spectroscopy (LEIS), Scanning Vibrating Electrode Technique (SVET) and Scanning Ion-Selective Electrode Techniques (SIET) on coated steel samples containing artificial defects. This comprehensive study confirmed the ability of the capsules to heal damaged areas in the coating and to mitigate corrosion thanks to the formation of a protective polymeric barrier layer.
The effect of cerium salts on the cut-edge of zinc-based sacrificial coated steel was investigated by theoretical approach (based on individual polarization curves on the top surface and by Scanning ...Vibrating Electrochemical Technique (SVET). In both approaches, cerium salts increase the galvanic corrosion of zinc-based sacrificial layer coupled with steel. Furthermore, the anion parts of cerium salts exhibited a synergistic accelerated effect of galvanic activity on the cross-section. The SVET results indicated that the addition of Al, Mg alloying elements promotes localized corrosion and influences the formation and distribution of corrosion products.
•Effect of CeCl3 and Ce(NO3)3 on the cut-edge of steel coated by zinc alloyed (HDG, ZnAl, and ZAM).•Cerium salts accelerated the corrosion of zinc coated steel on the cut-edge.•NO3- ion exhibited a synergistic accelerating effect on galvanic corrosion.•The presence of Al and Mg alloying elements promotes localized corrosion.
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•pH-dependent electrochemical behaviour of Al3Mg2 has been confirmed.•Galvanic corrosion currents versus pH of coupled Al3Mg2/Al were studied by SVET.•Dealloying element changes from ...Al to Mg as pH deviates from strong alkaline.•Three models were proposed to depict the corrosion process of Al3Mg2 at pH 2–12.
The effects of pH on the electrochemical behaviour of β-phase (Al3Mg2) and its role in corrosion of AA5000 series alloys in NaCl solution were investigated. The galvanic corrosion behaviour of synthesized Al3Mg2 intermetallic particle coupled with Al in 0.01 M NaCl solution over a wide pH window of 2–12 was studied using the scanning vibrating electrode technique (SVET). The results show that bulk Al3Mg2 acts as anode at pH 2–10.5, while cathode at pH 12. The corrosion resistance properties of bulk Al3Mg2 were then evaluated via potentiodynamic polarization and electrochemical impedance spectroscopy tests. Surface characterization were carried out by confocal laser scanning microscope (CLSM), scanning electron microscope (SEM) and X-ray photoelectron spectrometry (XPS). These results indicate that Al3Mg2 exhibits active dissolution of both Al and Mg elements at pH 2, while selective dissolution occurs at pH 3.5–12. At pH 3.5–10.5, any deviation from neutral pH causes a significant increase in the selective dissolution rate of Mg element, which results in a rough outer layer and a relatively compact inner layer of corrosive surface. At pH 12, the selective dissolution of Al element becomes dominant and the formation of compact Mg(OH)2 achieves complete passivity on the surface of Al3Mg2. This work contributes to further understanding of the electrochemical behaviour of β-phase in AA5000 series alloys.
•Microelectrochemical characterization of the electrochemical activity on welded zones in FSW 2098-T351 Al-Cu-Li alloy.•Imaging of galvanic coupling effects and local electrochemical activity on ...welded zones using amperometric and potentiometric probes.•Galvanic interactions within and between the coupled welded zones were visualized by SVET and SECM analyses.•Pt and Sb/Sb2O3 probes used to map reactive sites developed on the coupled welding joint/heat affected zones (WJ/HAZ).•Spatially-resolved images of anodic and cathodic sites associated with oxygen consumption and local pH changes were monitored and studied.
Localized electrochemical methods supported by surface analytical characterizations were employed to investigate galvanic coupling effects and local electrochemical activity developed along the welded zones in Friction Stir Welded 2098-T351 Al-Cu-Li alloy. The investigation was carried out in the coupled weld joint/heat affected zones (WJ/HAZ) for both, the retreating (RS) and the advancing (AS) sides. The correlation between the surface chemistry, the microstructural characteristics and the electrochemical activity of these welded areas was studied. The results showed the development of galvanic interactions within and between the WJ and HAZ regions, which were imaged using the scanning vibrating electrode technique (SVET), and scanning electrochemical microscopy (SECM). SVET analyses showed that HAZ was more susceptible to the development of anodic sites than WJ. SECM in amperometric operation mode showed that WJ coupled to HAZ exhibited higher oxygen consumption and higher cathodic activity compared to HAZ. Furthermore, SECM in potentiometric operation showed alkalinization around WJ and increased acidity in HAZ, mainly at sites of severe localized corrosion (SLC). Based on the SVET and SECM results in combination with the surface analyses, it is proposed that the microgalvanic cells formed within these welded zones are due to the presence of secondary phases in the 2098-T351 alloy and their interactions with the adjacent matrix.
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The corrosion behavior of solid solution Mg-xSn (x = 1, 5, 10 wt%) alloys is explored as a function of Sn content in chloride-containing conditions. A suite of in situ electrochemical techniques and ...an in situ scanning vibrating electrode technique (SVET) is utilized to assess free corrosion rates and the extent of cathodic activation. The latest advances in improving the corrosion resistance of Mg alloys have demonstrated that micro-alloying with As or Ge can greatly lower corrosion rates compared to pure Mg and retard cathodic activation to a substantial degree. To broaden the options for suitable non-toxic alloying elements beyond Ge, the current article demonstrates a decreasing corrosion rate by 77%, 85% and 95% for Sn additions of 1%, 5% and 10% (wt%) respectively, when compared to HP Mg freely corroding in 0.6 M aqueous NaCl. A corrosion film formed on Mg-10Sn which displays superior barrier properties. Polarization resistance (Rp) values consistently one order of magnitude greater than that obtained on HP Mg, and the other Mg-Sn alloys, over a 24 h immersion period is demonstrated. Furthermore, the extent of cathodic activation for Mg-10Sn is shown to be reduced by 94% relative to HP Mg. The work presented herein provides advancements in the understanding of corrosion resistant Mg alloys and is pertinent to the potential use of Mg-Sn alloys in transport applications, battery electrode materials and as a candidate sacrificial anode for the cathodic protection of Mg alloy AZ31B-H24. Prospects for protection are discussed.
The present research examined the effects of inclusions and microstructure on the initial marine corrosion and evolution of corrosion products in weathering steels by using first-principle modeling ...and various highly-sensitive analytical techniques including in-situ scanning vibrating electrode technique (SVET), scanning electron microscope/energy-dispersive X-ray spectroscopy (SEM/EDS), x-ray diffraction (XRD), and electrochemical workstation. The results demonstrated that CaS in the (Al, Mg)Ox-CaS inclusion formed in both Q500qE and Q370qE steels preferentially dissolved and triggered the initial corrosion. The acidic environment created between the inclusions and the iron matrix further promoted the dissolution of the inclusions. Moreover, due to the discrepancy in corrosion tendency, galvanic couples generated between the bainite ferrite (BF) phase and martensite/residual austenite (M/A) island in the Q500qE steel as well as the ferrite phase and pearlite phase in the Q370qE steel, accelerating the initial corrosion. In addition, pearlite facilitates a faster spread rate of local corrosion compared to bainite. Furthermore, with prolonged exposure, Q500qE steel exhibited more uniform and dense structure of the corrosion products layer, demonstrating a higher corrosion resistance than Q370qE steel. Finally, the mechanistic model was established to illustrate the influence of inclusions and microstructure on corrosion initiation and propagation of weathering steels.
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•Mg-Gd PVD coatings have been deposited onto Mg5Gd magnesium alloy pins.•The coating is effective in corrosion protection of Mg5Gd magnesium pins.•The protection mechanism involved ...sacrificial anodic dissolution of the PVD coating.•Galvanic current, OCP and SVET measurements supported galvanic protection mechanism.•The polished pin and PVD coating displayed similar VPD levels measured by SKPFM.
In this work metallic pins made of Mg5Gd alloy were coated with magnetron sputtered PVD Mg5Gd layers. Immersion tests in Hank’s balanced salt solution (HBSS) were performed. The PVD coating drastically reduced the corrosion rate of the Mg5Gd pin. The corrosion mechanism of the coating was studied in model electrochemical measurements by Scanning Vibrating Electrode Technique and galvanic current measurements. The tests revealed the preferential anodic dissolution of the Mg5Gd coating galvanically coupled with the bulk Mg5Gd pin. Local measurements of Volta potential difference revealed the VPD contrast between the pin and the coating surfaces only after immersion in HBSS.
In order to analyze the effect of scratch on the corrosion behaviour of a calcium phosphate conversion coating (CPCC) on AZ80, the electrochemical testing, scanning vibrating electrode technique ...(SVET), immersion test and hydrogen evolution experiment were performed to study the corrosion resistance of AZ80, AZ80 with CPCC and coated AZ80 with scratch. The results show that the coating improves the corrosion resistance of the AZ80 from a current density of (85±4) to (4±1)μA/cm2. When the coating was damaged, its protection on substrate would be reduced. The scratch with a length of around 12 mm on the coating reduced the corrosion resistance to a current density of (39±1)μA/cm2. In addition, the corrosion occurred initially in the scratch area and the corrosion site first occurred at the junction of the scratch and the coating. Besides, the micro corrosion mechanism of the specimen containing scratch was clarified.
The surface modification of NAB alloys was developed by thermal diffusion with the gradient outer layer of Ni-Cu solid solution and the inner layer of Ni-Al-Cu intermetallic. The Ni-Cu layer ...exhibited local corrosion with uniform corrosion along with grain boundaries and pitting corrosion. The pits were initiated by the attack of Cl− at the surface defects. The galvanic corrosion between outer passive film and the inner bare Ni-Cu substrate promoted the growth of the pits. The gradient Ni-Cu layer showed decreased interface potential drop in the pits, thus relieving the driving force for galvanic corrosion and depressing the propagation of the pits.
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•The NAB alloy was surface modified with gradient Ni-Cu layer.•The gradient Ni-Cu layer improved the corrosion resistance due to passive films.•The gradient Ni-Cu layer showed local corrosion with fine pitting corrosion.•The gradient Ni-Cu layer decreased interface potential drop in the pits.•The pits growth was depressed due to the relive of galvanic corrosion.
The gradient Ni-Cu layer on nickel aluminum-bronze alloy was obtained by the thermal diffusion process. The corrosion resistance was improved due to the formation of protective film, consisted of Ni(OH)2 and Cu2O. Although the Ni-Cu layer exhibited local corrosion with uniform corrosion and pitting corrosion simultaneously during the immersion test, the pitting growth was depressed because of the gradient distribution of Ni and Cu. The relieving driving force for galvanic corrosion between outer passive layer and bare Ni-Cu in the pitting was the main factor, which was caused by the decreasing interface potential drop.
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