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.
•Copper protection in 3.5 wt.%NaCl by 5-(4-pyridyl)-1,3,4-oxadiazole-2-thiol (PyODT).•High inhibitive efficacy of PyODT on Cu corrosion was reached at long exposure.•A uniform and compact inhibitor ...film was formed on copper surface.•Interaction between Cu and PyODT proceeded through thiol and oxadiazole groups.•Machine learning algorithms and Raman data were firstly applied in corrosion study.
An oxadiazole derivative with functional groups favouring its adsorption on copper surface, namely 5-(4-pyridyl)-1,3,4-oxadiazole-2-thiol, has been explored as potential inhibitor of copper corrosion in 3.5 wt.% NaCl. Electrochemical evaluation by electrochemical impedance spectroscopy, potentiodynamic polarization and SVET reveals high inhibition efficiencies. Surface microscopy inspection and spectroscopic analysis by Raman, SEM-EDX and XPS highlight the formation of a compact barrier film responsible for long-lasting protection, that is mainly composed of the organic molecules. Machine learning algorithms used in combination with Raman spectroscopy data were applied successfully for the first time in corrosion studies to allow discrimination between corroded and inhibitor-protected metal surfaces. Quantum Chemistry calculations in aqueous solution and Molecular Dynamic studies predict a strong interaction between copper and the thiol group and an extensive coverage of the metal surface, responsible for the excellent protection against corrosion.
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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.
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.
•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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Corrosion of AlZnMg coated steel in chloride media is a complicated process, which progress is determined not only by the electrode reactions but also by the local pH and formation of soluble and ...insoluble corrosion products. In order to adequately describe and predict the corrosion behavior of steel/AlZnMg cut‐edge, a multi‐ion transport, and reaction model (MITReM) is built. Such model takes into account the transport of species in the solution together with their production/consumption in electrochemical and chemical reactions. The main aim of this work is the validation of a numerical model for describing and predicting corrosion of steel/AlZnMg cut‐edge in immersed conditions. This first part describes the experimental work done to obtain model input parameters and validation measurements. The metallic coatings are characterized experimentally with respect to their composition and microstructure, global and local electrochemical behavior (polarization curves, AESEC, and SVET). Local distribution of corrosion products and local pH are also characterized. No blocking effect of the precipitated corrosion products has been observed and dedicated measurements are performed to validate this observation. Based on the experimental input from this part, numerical models of cut‐edge corrosion with different levels of details are developed in Part II.
A combination of electrochemical and surface analytical techniques is applied to study the corrosion process of cut edges of aluminized steel with different coating alloy compositions. The outcome are qualitative (rate constants, pH distributions, ..) and quantitative (position and composition of corrosion products) insights. The result of this work is used as input for numerical corrosion modelling and validation of simulation results in part II.
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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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