Article

Effect of PEO Coating Microstructure on Corrosion of Al 2024

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Abstract

Transient self-feedback control (TSFC) was employed to prepare plasma electrolytic oxidation (PEO) coatings on 2024 Al alloy. Compared with Al2O3 PEO coating by the custom pulse voltage (CPV) process, the PEO coating using the TSFC mode generated a thinner outer layer and more compact inner layer. In order to study the microstructure effect on the practical corrosion properties, three Al2O3 PEO coatings using the CPV and TSFC processes were studied during long term immersion in a corrosive environment by electrochemical impedance spectroscopy (EIS) and neutral salt spray test (NSST). Two groups of electrical equivalent circuits were employed based on different microstructures. During the initial immersion period a double-layer element (CPEdl /R-t) appeared due to the CPV PEO coating having a porous outer layer and thinner inner layer. Thereafter, continuous corrosion reactions happened at the metal/PEO interface, which resulted in the appearance of diffusion elements. However, for specimens prepared by TSFC, corrosion reactions only happened in the later stages of the immersion. The compact and thick inner layer mainly contributed to excellent barrier properties. stable microstructure after 380 h immersion in 3.5(wt%) NaCl solution and excellent performance in the salt spray test.

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... Numerous studies have been reported on plasma-based coating and surface activation methods on AA 2024. [20][21][22][23][24][25][26] Surface activation using atomic plasma treatments are widely reported and are well employed at various application sites. [20][21][22] Plasma electrolytic oxidation is a technique of growing an in situ ceramic layer over the metal surface to prevent the corrosion. ...
... [20][21][22] Plasma electrolytic oxidation is a technique of growing an in situ ceramic layer over the metal surface to prevent the corrosion. 23,24 Mascagni et al. 25 studied the corrosion resistance of films deposited by plasma excited chemical vapor deposition on AA 2024 alloy. In the work by Ashrafizadeh, 26 the adhesion of physical vapor deposited titanium nitride (TiN) on aluminum substrates was studied. ...
... These defects are generally detrimental to all the useful applications of PEO coatings; however, in particular, they are harmful for the anticorrosion property of the coatings. Researchers discovered that PEO-coated metals experience constant corrosion degradation during long-term immersion in chloride solution and reasoned that the penetration of the corrosive solution through the porous microstructure of the coatings was a major cause [5]. Therefore, extensive studies have been carried out in multiple directions to optimize the PEO coatings for enhanced anti-corrosion properties, e.g., coating composition and microstructure [6,7], barrier layer property [5,8], coating thickness, etc. ...
... Researchers discovered that PEO-coated metals experience constant corrosion degradation during long-term immersion in chloride solution and reasoned that the penetration of the corrosive solution through the porous microstructure of the coatings was a major cause [5]. Therefore, extensive studies have been carried out in multiple directions to optimize the PEO coatings for enhanced anti-corrosion properties, e.g., coating composition and microstructure [6,7], barrier layer property [5,8], coating thickness, etc. Thus, new electric parameters [9,10], electrolyte compositions [7,11,12] and pre-treatments [13,14] have been developed to enhance the corrosion resistance of PEO coatings. ...
Article
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In this study, a two-step Plasma Electrolytic Oxidation (PEO) method with constant primary step parameters and varying K2ZrF6 concentration in the secondary electrolyte solution was carried out to obtain a hard and dense coating on AZ91D alloy. For lower concentrations of K2ZrF6, a mixture of granular and pancake structure with higher surface porosity was obtained. Increasing the concentration up to 6 g/L caused the granular structure to disappear and a significant decrease was observed in the surface porosity as well as inner layer damage. Due to the compact inner layer structure, significant improvement in the corrosion resistance and mechanical properties of the AZ91 was observed with K2ZrF6 concentration. Highest values of hardness and corrosion resistance, i.e., 1589.45 HV and 386.30 × 103 Ω cm2, respectively, were obtained for a 6 g/L K2ZrF6 concentration.
... Microarc oxidation (MAO) is a relatively novel surface treatment technique developed from conventional anodization. 1 With this technology, a protective ceramic oxide film with high hardness, excellent wear, corrosion resistance and electrical insulation can be formed on the surfaces of some metals, such as Mg, Al, Ti, Zr or their alloys. [2][3][4][5] As an anodizing process, MAO combines electrochemical oxidation with a high-voltage sparking discharge process to form a ceramic film. The thickness of the film keeps increasing with the dielectric breakdown and sparking discharge. ...
... With the reaction time extended to over 2 h, two weak diffraction peaks can be observed corresponding to the (003) and (006) of the LDH phase. 36 The interplanar spacing of the LDH is consistent with the values in the literature for LDH materials with NO 3 − ions interlayer, indicating the NO 3 − ions have been intercalated into the interlayer spaces. 36 With the LDH treatment time increased from 2 to 24 h, the intensities of the diffraction peaks of LDH gradually increased, indicating that the crystallinity of LDH phase increases with increasing LDH treatment time. ...
Article
Microarc oxidation (MAO) film has been widely used as an anti-corrosive film for Al alloys. However, it is still a big challenge to further improve its anti-corrosion performance due to the defects that form during the MAO process. In this study, for the first time, Ni-Al layered double hydroxide (LDH) was in-situ synthesized on the MAO film to seal its defects, which contributes to the corrosion resistance enhancement of MAO treated 6061 Al alloy. The growth of the LDH on MAO film is largely determined by the specific area, chemical composition, and crystalline structures of the alumina phase of the MAO film. More importantly, LDH nanoplates can grow into the micropores and microcracks in MAO film, and then gradually seal these structural defects. The anti-corrosion performance of MAO film during long-term immersion in NaCl solution also can be improved greatly via LDH treating because the defect-sealing effects of LDH can enhance the barrier property of both the porous and dense layers of MAO film, and then restrain the penetration of corrosive agents through the MAO film to the metal substrate.
... Aluminium and its alloys have been widely used in the aerospace and automobile industries due to outstanding properties, such as a high strength-to-weight ratio, an excellent thermal conductivity and a relatively low density [1,2]. Despite these advantages, low hardness and poor wear resistance have severely restricted the industrial applications of aluminium alloys in mechanical engineering applications where surface load-bearing capacities are crucial. ...
... Oxide layers formed via PEO typically exhibit a bilayer structure comprising a porous external layer and a denser internal layer [1,2], which thickness ratio may vary in a wide range. In our case, the inner dense region was about 80 % of the overall PEO coating thickness as determined from the sample cross-section prepared by the standard metallographic polishing technique (Fig. 1c). ...
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Duplex coatings comprising MoS2/Sb2O3/graphite layer burnished on top of plasma electrolytically oxidised AA6082 alloy enhanced fretting wear resistance against alumina and steel whilst reducing friction from ~0.6 … 0.9 to ~0.08 … 0.1 in dry nitrogen and humid air. Transitions to stick-slip (~3 μm), partial slip (5–10 μm) and gross slip (>10 μm) showed an order of magnitude improvements over single-layer PEO coatings. In the latter mode, high initial contact pressures (~1.1 GPa) with steel counterpart under oxidising atmosphere increased friction due to generation of Fe/Al/Cr oxides/hydroxides in the contact region, leading micro-cracking in PEO coating followed by fatigue damage of aluminium substrate. Reducing contact pressure <1 GPa, retained topcoat lubricating behaviour, high wear resistance and eliminated fatigue damage after 10⁴ cycles.
... The reaction mechanism of the MgO in PEO process, especially at its early stage, is similar to that of conventional anodizing process, i.e. simultaneous formation at the coating/electrolyte and C/S interfaces by the outward migration of Mg 2+ from the substrate to the coating/electrolyte interface and inward migration of O 2− (H 2 O, OH − ) from the electrolyte to the C/S interface [45]. The presence of Mg 2 SiO 4 and MgSiO 3 further demonstrates that SiO 3 2− anions in the electrolyte have intensively incorporated into the formation reactions of the coatings. ...
... Sreekanth et al. [45] demonstrated that the corrosion resistance of the PEO coated samples was closely related to the compactness and the integrity of the PEO coatings, because the discharge pores and cracks in the PEO coatings act as transportation passages for the aggressive ions in the corrosion test solution. From the surface and cross-sections morphology images (Figs. 3 and 4), it can be seen that the pores and creaks of the coatings increase gradually in volume, which results in the decreasing in compactness of coatings with extended PEO time. ...
Article
Evolution processes of the corrosion behavior and structural characteristics of the plasma electrolytic oxidation (PEO) coated AZ31 magnesium alloy were investigated by using scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), potentio-dynamic polarization curves and electrochemical impedance spectroscopy (EIS) measurements. Detached coating samples were fabricated by an electrochemical method and more details of the internal micro-structure of coatings were clearly observed on the fractured cross-section morphologies of the samples compared to general polished cross-section morphologies. Evolution mechanisms of the coating corrosion behavior in relation to the evolution of micro-structural characteristics were discussed in detail.
... Despite these outstanding properties, the application of aluminum alloys in some aqueous environments may be restricted due to corrosion problems. Thus, many efforts have been made to improve corrosion properties with the help of various surface treatment methods such as surface cladding [2], conversion and smart composite coatings [3][4][5], thermal spraying [6,7], chemical and physical vapor deposition [8,9], surface hydrophobization [10] and surface anodizing [10][11][12] and plasma electrolytic oxidation (PEO) [13][14][15][16][17][18][19][20][21][22][23][24][25][26]. ...
... PEO coating grows by establishing plasma microdischarges due to the dielectric breakdown phenomenon under high electrical field. However, it should be noted that plasma microdischarges do not necessarily lead to coating growth and may propagate some microdefects in the coating, hence providing suitable paths for the corrosive species to reach and attack the underlying substrate [13][14][15][16][17][18][19][20][21][22][23][24][25][26]. ...
Article
Plasma electrolytic oxidation processes were performed on aluminum substrates in a solution composed of KOH, Na4P2O7 and NaAlO2 compounds at constant applied potential, i.e. 520V DC, with and without incorporating multiwall carbon nanotubes (MWCNTs). The morphology and chemical composition of PEO coatings was studied using SEM and XRD techniques. The SEM characterizations revealed that MWCNTs were incorporated into the PEO coating with the detrimental effect on the coating compactness, particularly in the inner barrier layer. In addition, the characteristic peaks corresponding to the γ-Al2O3 phase get more intensified by loading MWCNTs. The corrosion protection performance of the PEO coatings was also studied by the Electrochemical Impedance Spectroscopy (EIS) and Scanning Kelvin Probe (SKP) techniques. The results revealed that protective properties of the PEO coating are deteriorated with incorporating MWCNTs by reducing the compactness of the inner and outer layer of the PEO coating. The obtained results also demonstrated that both types of PEO coatings exhibit a similar mechanism of protection by barrier effects.
... Rs is the resistance of the solution between the working and reference electrodes; CPE1 and R1 are the outer layer capacitance and resistance, respectively; CPE2 and R2 are the inner layer capacitance and resistance, respectively; and CPE3 and R3 are the double-layer capacitance and charge-transfer resistance, L in the equivalent circuit represents the negative loop of the Nyquist curve, respectively. Further, the choice of the suitable equivalent circuit was selected using various combinations of these elements and the circuit which best fit the experimental curve [33,34]. Here, a constant phase element (CPE) which reflects the surface distribution activity, roughness of the coating surface, electrode porosity, and distribution of current and potential on the electrode, is used [35,36]. ...
... R s is the resistance of the solution between the working and reference electrodes; CPE 1 and R 1 are the outer layer capacitance and resistance, respectively; CPE 2 and R 2 are the inner layer capacitance and resistance, respectively; and CPE 3 and R 3 are the double-layer capacitance and charge-transfer resistance, L in the equivalent circuit represents the negative loop of the Nyquist curve, respectively. Further, the choice of the suitable equivalent circuit was selected using various combinations of these elements and the circuit which best fit the experimental curve [33,34]. Here, a constant phase element (CPE) which reflects the surface distribution activity, roughness of the coating surface, electrode porosity, and distribution of current and potential on the electrode, is used [35,36]. ...
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Coatings prepared by different electrolyte additives were investigated on AZ31 magnesium alloy by plasma electrolytic oxidation. In this study, scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction analysis were employed to assess the morphologies, chemical and phase compositions of the plasma electrolytic oxidation (PEO) coatings, respectively. Furthermore, electrochemical impedance spectroscopy was used to evaluate the corrosion behavior of the composite coating. The investigation of the effect of electrolyte additives in the base electrolyte showed that the PEO specimens exhibit different surface and cross-sectional morphologies, and phase compositions. The results showed that SiO32− was conducive to the growth of the ceramic layer, and the ceramic layer developing in the electrolyte which contained AlO2− showed a typical double-layer structure. The corrosion resistance of coating formed in a phosphate bath was higher than that of the coating formed in silicate bath and coating formed in an aluminate bath. Moreover, the corrosion resistance of the coating formed in the fluoride bath was the highest.
... In addition to other factors, the reduced porosity in the presence of Ce might be related to its involvement in plasma discharges, as it happens with other metal atoms (Na, K, Al), creating a higher energy environment that promotes water decomposition [39]. Flash-PEO coatings reveal a porous morphology due to oxygen evolution and electrolyte vaporization at the location of micro-discharges [11,42,43]. Some of the pores correspond to the remnants of the discharge channels as seen in the cross-section in Figure 3b. ...
... Journal Pre-proof comparison with conventional PEO coatings, the oxide layers developed in the present work do not show α-Al 2 O 3 (characteristic peak at 25.6º) [48,51]. This is due to differences in the applied electrical parameters and intensity of discharges during the process: α-Al 2 O 3 is normally formed above 450 V, whereas γ-Al 2 O 3 forms at lower values (350-400 V) [19,37,42,52]. No crystalline Ce or W containing phases were identified possibly due to their low amount. ...
Article
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Flash plasma electrolytic oxidation (flash-PEO) with in situ incorporation of inhibitors is explored as a strategy for corrosion protection of 2024 aluminium (Al) alloys. Thin (~5 μm) and energy-efficient coatings were successfully developed on the Al substrate in 100 s treatment time using different electrolytes based on molybdate, stannate, tungstate, vanadate lanthanum or cerium salts additives with and without complexing agent (EDTA). Screening of the optimum combination in search of minimum thickness and best corrosion protective performance was performed using electrochemical impedance spectroscopy (EIS). Coatings based on EDTA-Ce and WO4²⁻ were selected and investigated further in comparison with the inhibitor-free phosphate coating in terms of morphology, composition, corrosion behaviour and paint adhesion. EDTA-Ce coating showed excellent paint adhesion and the highest impedance modulus at short immersion times in 3.5 wt% NaCl aqueous solution. Neutral salt spray testing of this coating as a full system comprising an epoxy primer showed no signs of corrosion after 1000 h of exposure.
... Plasma electrolytic oxidation (PEO) is a relatively novel surface treatment method, developed for the growth of functional crystalline oxide layers on valve metals, particularly when improving the corrosion resistance, tribological properties, thermal stability or dielectric properties are demanded [1][2][3][4][5][6]. Plasma discharge is a key feature of this process that occurs at the metal/electrolyte interface as the applied voltage exceeds a certain critical breakdown value (typically several hundreds of volts). ...
Article
Plasma electrolytic oxidation (PEO) was performed on commercially pure aluminum substrate at a constant applied potential of 520 V in a solution composed of 0.05 M KOH, 0.04 M Na4P2O7 and 0.04 M NaAlO2 compounds. Orthogonal discrete wavelet transform (ODWT) was employed to analyze plasma currents at different time stages of the PEO process. The results of wavelet transform were presented in the form of energy distribution (ED) and standard deviation of partial signal (SDPS) plots. The variations of PEO coating morphology with processing time were also studied by scanning electron microscope (SEM). The comparison of the ED plots at different processing time stages indicated that the current transient shapes remained unchanged during the whole PEO process. Similarly, a detailed comparison of the SDPS plots indicated that the intensity and lifetime of plasma discharges increased and their frequentness decreased with processing time. The SEM micrographs of the surface and cross section of the coating supported the findings of wavelet transform in case of both pore size and population. It was also shown that the lifetime of B-type plasma discharges which were responsible for the dielectric breakdown phenomena and coating thickness enhancement, increased from ~ 80 μs to ~ 160 μs with time. In addition, increasing the intensity and lifetime of B-type plasma discharges led to regeneration of the interfacial voids and discontinuities, particularly in the last stages of the process.
... It has also been implemented on Al and Al alloys and the corrosion resistance lifetime determined by salt spraying test has been observed to be 336 h and longer than that achieved by other methods [26] including anodic oxidation [27,28]. The corrosion resistance can be improved further by reducing the cavities in the MAO coatings [29,30]. The phosphating process is used industrially to protect steels from corrosion by forming a compact and insoluble layer of metal phosphate on the surface. ...
Article
The corrosion resistance of aluminum and its alloys requires improvement in some applications. In this work, a 20 μm thick compact alumina coating doping Fe and P is obtained on the substrate of LY12 Al alloy by combining micro-arc oxidation (MAO) and phosphating to improve the corrosion resistance. The reaction mechanism is investigated and proposed. The coating shows a relatively small surface roughness (Ra = 0.5 μm) and a high Vickers hardness (1200 HV). The corrosion resistance is evaluated by the salt spraying test and Tafel polarization and a lifetime of up to 1800 h and a passivation potential of above 10 V are observed.
... To develop biodegradable magnesium implants with acceptable biodegradation rate, PEO coatings have been produced on various magnesium alloys, such as AM50 [3], AZ91D [4], WE43 [5], AZ31 [6], Mg-Zn-Ca [7], ZX50 [8] and ZK60 [9]. ...
... Many researchers also point out the possibility to obtain the hardest alumina phasecorundum (hexagonal, α-phase) [24][25][26]. Hardness [27,28] combined with excellent chemical stability [26,[28][29][30] of the coatings shows great potential for use in many practical applications where high-quality protective layers are required. In addition, it was demonstrated that the composition of the coating can be modified by particle addition in the electrolyte [31] which opens great possibilities for development of functionalized PEO coatings. ...
Article
This study presents a novel approach to produce phosphorescent coatings on metal surfaces. Strontiumaluminates are themost popular modern phosphorescent materials exhibiting long afterglow at room temperature and a broad spectral distribution of luminescence in the visible range. However, despite a large amount of research done, methods for synthesis of such materials remain relatively energy inefficient and environmentally unfriendly. A long-afterglow luminescent coating containing SrAl2O4:Eu2+, Dy3+ is prepared by the plasma electrolytic oxidation on the surface of commercial aluminum alloy Al6082. During the electrical discharges in this process, the strontium aluminate is formed in a similar way to the solid-state reaction method. X-ray powder diffraction analysis confirms that the monoclinic SrAl2O4 phase is present in the coating. Optical properties of the obtained coating were analyzedwith luminescencemethods classically used for studies of luminophores. The performance of the coating was compared with commercially available strontium aluminate powder. The proposed method of coating synthesis may be of value for the development of energy-efficient and longlasting automotive and public safety infrastructure.
... Whereas very limited numbers of corrosion pits and cracks on the B6 coating imply its good corrosion resistance. Since the corrosion resistance of the PEO coating is strictly associated with its porosity density [72,73], the deep penetrating pores become the shortcuts for the corrosive solution penetrating into the metal substrate and thus degrades its anticorrosion performance [2,74]. In addition to the porosity measurement by the direct calculation on the surface morphologies in Fig. 4, the surface porosity of the PEO coating was also measured from the EIS values [75]. ...
Article
Full-text available
Plasma electrolytic oxidation (PEO) is a widely applied surface treatment method which can improve the corrosion resistance of valve metals and their alloys. In this study, PEO treatment was conducted on pure Al using a power supply under a bipolar pulsed mode at fixed duty cycle and frequency. The effect of different concentrations of borax in the electrolyte on the corrosion resistance of PEO grown oxide coatings in 3.5 wt% NaCl solution was comprehensively studied by a potentiodynamic polarization test and electrochemical impedance spectroscopy method. We found that the average pore size and porosity on the PEO grown oxide coating decreased with increasing borax concentration. The dense microstructure of oxide coating can be seen when the borax concentration in the electrolyte was high. The corrosion resistance of PEO coating increased with increasing borax content. The corrosion resistance of pure Al was greatly improved up to 23,058 times higher by the formation of a PEO layer treated in the electrolyte containing 6 g L⁻¹ borax due to its ability to grow a dense γ-Al2O3 phase with less defects.
... More complicated circuit modes have been used by other scientists to describe the corrosion performance of PEO coatings [351][352][353][354][355]. For example, Sung group [351] prepared a composite coating composed of TiO 2 , Al 2 TiO 5 , and Al 2 O 3 on 6061 Al alloy substrate via PEO utilizing a silicate-electrolyte with TiO 2 powder. ...
Article
In recent years, considerable research has long been devoted to the development of metallic materials with excellent surface properties through various surface modification techniques. A plasma electrolytic oxidation (PEO), one of the electrochemical coatings, has considered the eco-friendly wet coating in alkaline-based electrolytes where the surface characteristics of metal would be altered significantly by electrochemical reactions assisted by plasma discharges, resulting in the formation of hard, conformal, adhesive inorganic layer on the metal substrate. This review described a couple of the scientific principles including transient discharge behavior at breakdown, nucleation and growth of inorganic layer, and electrophoresis for incorporating inorganic particle. It outlined the essential microstructural features, which were related to defect structure, plasma-induced microstructural transformation, phase transition, and roles of inorganic agents. The protective nature of the present coating was highlighted by considering structural reliabilities, such as tribological and corrosion performances. In addition, the emerging applications arising from functional properties of the present coating, such as biomedical, catalysis, light, and energy performances, were reviewed. The benign approaches used to improve the structural and functional properties of coating layers are described utilizing pre- and post-treatments of PEO.
... To develop biodegradable magnesium implants with acceptable biodegradation rate, PEO coatings have been produced on various magnesium alloys, such as AM50 [3], AZ91D [4], WE43 [5], AZ31 [6], Mg-Zn-Ca [7], ZX50 [8] and ZK60 [9]. ...
... To develop biodegradable magnesium implants with acceptable biodegradation rate, PEO coatings have been produced on various magnesium alloys, such as AM50 [3], AZ91D [4], WE43 [5], AZ31 [6], Mg-Zn-Ca [7], ZX50 [8] and ZK60 [9]. ...
... To develop biodegradable magnesium implants with acceptable biodegradation rate, PEO coatings have been produced on various magnesium alloys, such as AM50 [3], AZ91D [4], WE43 [5], AZ31 [6], Mg-Zn-Ca [7], ZX50 [8] and ZK60 [9]. ...
... To develop biodegradable magnesium implants with acceptable biodegradation rate, PEO coatings have been produced on various magnesium alloys, such as AM50 [3], AZ91D [4], WE43 [5], AZ31 [6], Mg-Zn-Ca [7], ZX50 [8] and ZK60 [9]. ...
Article
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Different microstructures of the Al2O3 plasma electrolytic oxidation (PEO) films were obtained by controlling the frequency of applied high- frequency square voltage waveform (HFSVW) under different transient self- feedback control (TSFC). The morphology and compactness of the PEO films were characterized by scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). Experimental results indicated that uniform, compact, small-grained TSFC PEO films can be produced by applying high- frequency HFSVWs.
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This study investigated the effect of the incorporation of ZrO2 particles on the electrochemical response of low-carbon steel processed by electrochemical plasma coating (EPC). The EPC process was carried out in an alkaline electrolyte with and without ZrO2 particles under AC condition. The structure and constituent phases of the EPC-treated samples were analyzed through SEM and XRD. The electrochemical impedance spectroscopy analyses of the coating layer with ZrO2 particles were performed in 3.5 wt.% NaCl solution to evaluate the electrochemical behavior in comparison to that without ZrO2 particles. The corrosion resistance of the EPC-treated sample incorporating ZrO2 particles was explained in relation to the equivalent circuit model.
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A galvanostatic test panel design consisting of a coated and scribed 7075‐T6 Al alloy as a working electrode and an uncoated through‐hole 316 stainless‐steel fastener as a counter electrode has been used to accelerate galvanic corrosion and compare various primer coatings and pretreatments and different impressed passed currents in an ASTM G85‐A2 environment. The relative humidity, chloride concentration, and impressed current magnitude were changed to investigate their effects on galvanic corrosion. During the chamber exposure, the electrochemical measurements showed fluctuations in the measured current and potential. Various corrosion morphologies, depths, and volume loss were recorded for panels with different coating systems. Electrochemical measurements, optical profilometry, and photography were used to characterize and measure the corrosion performance of different coating systems. Also, electrochemical impedance spectrometry was used to compare the corrosion resistance and porosity of different coating systems. A galvanostatic test panel design consisting of a coated and scribed 7075‐T6 Al alloy as a working electrode and an uncoated through‐hole 316 stainless‐steel fastener as a counter electrode has been used to accelerate galvanic corrosion and compare various primer coatings and pretreatments and different impressed passed currents in an ASTM G85‐A2 environment.
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The effects of corrosion products on corrosion behaviors of AZ31 magnesium alloy with a plasma electrolytic oxidation (PEO) coating were investigated under the salt spray corrosion test (SSCT). The surface morphology, cross-sectional microstructure, chemical and phase compositions of the PEO coating were determined using scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction analysis (XRD), respectively. Further, the corrosion process of the samples under the SSCT was examined in a non-aqueous electrolyte (methanol) using electrochemical impedance spectroscopy (EIS) coupled with equivalent circuit. The results show that the inner layer of the coating was destroyed firstly and the corrosion products have significant effects on the corrosion behaviors of the coating. The results above are discussed and an electrochemical corrosion model is proposed in the paper.
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This work reports that the filiform corrosion of a AZ31B Mg alloy can be inhibited by compact plasma electrolytic oxidation (PEO) under a transient self-feedback control mode. Corrosion protection of the C-PEO film was investigated through an immersion test of 3.5 wt% NaCl solution and a simulative service environment test of NSST, respectively. The results revealed that compactness of the film significantly influenced the corrosion protection property of the C-PEO-coated Mg alloy, and the results are summarized as follows: (1) good compactness of the PEO film prevented the soluble ions from diffusing into the scratch; (2) the high insulation of the compact PEO film can slow down cathodic current; (3) the corrosion products could deposit on the scratch and block the active sites on the Mg alloy surface.
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Results show the possibilities of pore filling approach to modify alumina coatings with various materials in order to enhance coating optical (or other) properties and develop new functional materials; as well as demonstrate novel alumina phase transition detection approach. Luminescent PEO coatings were produced on aluminum surface using pore-filling method. Three stage process was developed to modify alumina coating in order to enhance its luminescent properties. Eu³⁺ recharging to Eu²⁺ followed by significant (up to 10 times) total luminescence intensity increase was observed, Eu ion presence evaluated in final coating by measuring fast decay kinetics. Structure of obtained coatings was analyzed using XRD and FTIR spectroscopy indicating presence of ≠-alumina phase.
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Titanium alloy produced by selective laser melting (SLM) requires surface treatment to improve its bioactivity. In this study, Micro-arc oxidation (MAO) was applied on TC4 alloy produced by SLM with a homogeneous coating of microporous TiO 2 and significant amounts of Ca and P. The microstructure, thickness, roughness, and composition of the MAO coating with different oxidation times were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD), respectively. The adhesion strength of the MAO coating was obtained by scratch testing. The corrosion behavior of the MAO coatings was evaluated by potentiodynamic polarization test and electrochemical impedance spectroscopy. The bioactivity of the coatings was carried out by immersion test in simulated body fluid (SBF). The results showed that the thickness of the MAO coatings, roughness and pore size gradually increase as a function of the oxidation time, which at the same time increases the rutile phase constituent. The coating prepared by 15 min exhibited the best adhesive strength, which mainly consists of TiO 2 , CaO, CaHPO 4 and Ca 3 (PO4) 2 . The corrosion resistance and bioactivity of MAO coatings could be improved by changing the oxidation time, and MAO coating formed at 15 min has the best corrosion resistance and bioactivity. The whole MAO process on SLM-produced TC4 alloy can be divided into four stages through “anodic oxidation → film puncturing → micro-arc oxidation → arc light stage” according to voltage change and microstructure evolution. In addition, a large amount of grain boundary interfacial area in SLM-produced TC4 alloy may participate in the MAO reaction to form a large number of discharge channels and share the total energy to decline energy density.
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Plasma electrolytic oxidation (PEO) coatings were fabricated on the aluminum in silicate electrolyte. Microstructures and elemental compositions of these PEO coatings were examined by scanning electron microscopy (SEM) equipped with an energy-dispersive x-ray spectroscopy (EDS). Corrosion resistances of the coated samples were evaluated using the measurements of open-circuit potential (OCP), repetitive polarization and electrochemical impedance spectroscopy (EIS) in 3.5 wt.% NaCl solution. The results indicated that the PEO coatings not only act as a physical shield limiting the penetration of corrosive solution, but also establish an anodic location with high concentration of H⁺ ions and other product ions as pitting corrosion begins. With the development of pitting corrosion, the accumulation of corrosion products inside the pits and pores of the coatings decreases the corrosion process effectively.
Article
Various plasma electrolytic oxidation (PEO) ceramic coatings were fabricated on aluminum in the electrolytes with different concentrations of Na2SiO3·9H2O. The morphology of fractured cross-section and coating/substrate (C/S) interface, phase compositions, and corrosion behavior of these ceramic coatings were characterized employing field emission scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), X-ray diffractometer (XRD), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements. The results showed that adding Na2SiO3·9H2O in the electrolyte could reduce the decrease amplitude of the voltage-time curve, and it can affect the types of the anodic film before the beginning of plasma discharges. A novel phenomenon was recorded as PEO treatment was carried out in the electrolyte with 1.85 g l⁻¹ Na2SiO3·9H2O that the substrate was first covered by a barrier-type anodic film, and then a porous-type anodic film appeared after the barrier-type anodic film was broken down. The anodic film continued to grow for a long duration in the PEO process after being broken down. Characteristics of PEO ceramic coatings with a similar thickness acquired in various electrolytes were also evaluated. XRD results indicated that these ceramic coatings of similar thickness were mainly composed of α-Al2O3 and γ-Al2O3, and the α-Al2O3 proportion decreased with increasing Na2SiO3·9H2O concentration. The electrochemical data showed that the corrosion resistance of these PEO ceramic coatings of similar thickness decreased by adding Na2SiO3·9H2O in the electrolyte.
Article
Humidity sensor based on thin micro-arc oxidation alumina (MOA) film was fabricated by a transient self-feedback control (TSFC) technique for the first time. The MOA film consisted of a polycrystalline nature of α and γ-Al2O3 phases, while high surface/volume ratio led to remarkable adsorption of the MOA film. Moreover, the humidity performance of the MOA sensor indicated good sensitivity of 8045.55 at 98% RH. Fast response and recovery times of the MOA sensor were 90 s and 30 s, respectively, at 20 °C and 1000 Hz. Exposing the MOA sensor to different humidities for longer durations indicated an excellent stability due to the α-Al2O3 phase in the MOA film. The mechanism in the sensor, to humidity, was explained by using equivalent circuits of (RC) and (C(RZ)). In the low RH range, the process was mainly dominated by the migration of protons through the adsorbed water molecule layer on the surface of the MOA film; while, in the high RH range, the process was due to the faster diffusion of protons in the single or multiple layers that were formed on the materials surface.
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The possibilities for manufacturing metal foams or other porous metallic structures are reviewed. The various manufacturing processes are classified according to the state of matter in which the metal is processed-solid, liquid, gaseous or ionised. Liquid metal can be foamed directly by injecting gas or gas-releasing blowing agents, or by producing supersaturated metal-gas solutions. Indirect methods include investment casting, the use of space-holding filler materials or melting of powder compacts which contain a blowing agent. If inert gas is entrapped in powder compacts, a subsequent heat treatment can produce cellular metals even in the solid state. The same holds for various sintering methods, metal powder slurry foaming, or extrusion and sintering of polymer/powder mixtures. Finally, electro-deposition or metal vapour deposition also allow for the production of highly porous metallic structures. The various ways for characterising the properties of cellular metals are reviewed in second section of this paper. Non-destructive as well as destructive methods are described. Finally, the various application fields for cellular metals are discussed. They are divided into structural and functional applications and are treated according to their relevance for the different industrial sectors. (C) 2001 Elsevier Science Ltd. All rights reserved.
Article
The water content of 12 paints on steel substrate, exposed to 0.1M sodium chloride at 25 C for 24 h, was calculated from impedance measurements and compared with gravimetric determinations. Brasher's empirical expression was compared with five mixture formulas derived from the theory of dielectric properties of heterogeneous substances by Boettcher, Bruggeman, Looyenga, Wagner, and Rayleigh. The 12 paints were based on two grades of red lead and iron oxide in epoxy, chlorinated rubber, alkyd, and linseed oil binders. The significance of the results for cyclic wet/dry paint tests according to (1) the difference between the water absorption and desorption rates of a single paint and (2) the testing of different types of paints in the same cycle test program are exemplified. A possible explanation is given for the poor correlation often found between the results of cyclic wet/dry tests and those obtained in practical applications.
Article
A study was made to investigate the influence of carbon nanotubes (CNTs)-incorporated oxide layer on corrosion resistance and microstructure of 7075 Al alloy prepared by plasma electrolytic oxidation (PEO). To protect 7075 Al alloy from corrosion, the present work deals with the electrochemical response of the oxide layer on 7075 Al alloy specimen prepared by PEO method under AC condition in an electrolyte containing CNTs. Because of the strong zeta potential of the functionalized CNTs, the incorporation of CNTs was achieved through the combined effects of electrophoresis and physical mixing in molten oxide. From the results of potentio-dynamic tests in 3.5 wt % NaCl solution, the PEO-treated sample containing CNTs shows better corrosion properties than that without CNTs. This was attributed to the fact that significant decrease of pore density by the incorporation of CNTs into the oxide layer. High corrosion resistance of the PEO-treated 7075 Al alloy was also revealed by impedance tests and analyzed in relation to the equivalent circuit model.
Article
This work for the first time reports a monolayer plasma electrolytic oxidation (PEO) coating on 2024 Al alloy substrate by transient self-feedback control (TSFC) mode. The monolayer coating without outer porous layer exhibits rather excellent compactness and corrosion protection attributed to extensive distribution of scattered ceramic particles with small size and less porosity. A possible mechanism is proposed to explain the formation of TSFC coating. Due to uniform surface distribution of the filming-formative particles and the discharge energy under high frequency square voltage (HFSV) waveform, the formation of the outer porous layer has been prevented effectively.
Article
The permeation and sorption of water in eight coatings below the glass transition temperature was monitored using capacitance measurements. For each coating deviations from ideal-fickian diffusion were observed. The diffusion rate was found to decrease considerably. This anomalous (or non-fickian) behaviour is known in literature as pseudo-fickian diffusion or two-stage sorption. To analyse the measurements further a new method is developed. The data were transformed to a time-dependent diffusion rate. The analysis resulted in a new pseudofickian diffusion model, which could also be represented by a spectral distribution of diffusion rates. The results agree well with the relaxation model of Berens and Hopfenberg. It is shown that the Berens-Hopfenberg model, in particular, and pseudo-fickian behaviour, in general,can be understood as diffusion influenced by free-volume relaxation.
Article
An ac/dc spark anodization method was used to deposit an oxide film (6±3μm in thickness) on the Al–Cu alloy AA2219. The oxide films were formed at 10mA/cm2 for 30min in an alkaline silicate solution, showing three main stages of growth. Scanning electron microscopy and electron microprobe analysis revealed that the oxide films are not uniform and consist of three main layers, an inner Al-rich barrier layer (∼1μm), an intermediate Al–Si mixed oxide layer (∼2±1μm), and an outer porous Si-rich layer (∼3±3μm). In addition, microscopic analysis showed that the Al2Cu intermetallics present in the alloy have not been excessively oxidized during the anodization process and thus are retained beneath the oxide film, as desired. The coating passivity and corrosion resistance, evaluated using linear sweep voltammetry (LSV) in pH 7 borate buffer solution and electrochemical impedance spectroscopy (EIS) in 0.86M NaCl solution, respectively, were both significantly improved after spark-anodization.
Article
Optical emission spectroscopy and fast video imaging of the microarc oxidation process of aluminium alloys are carried out.Optical measurements allow the establishment of relationships between the sparks aspect and the aluminium emission line intensity. The transition from spark to microarc is quantified through the decay of the aluminium line intensity. The characteristic time of that decay is shown to be strongly dependent on the current density. Scanning electron microscopy characterization shows that the oxide layer structure and growth rate are strongly dependent on the process duration and on the applied current density. Finally, the correlations that are drawn between the material analyses and the plasma measurements allow proposing some ways to enable the process control through optical measurements.
Article
Alumina coatings were deposited on Al alloy substrates using an electrolytic plasma technique, based on a dielectric barrier discharge created during anodic oxidation in an aqueous electrolyte. The substrate material (BS Al 6082) was biased anodically with an unbalanced AC high voltage. During processing, a plasma current density of 100 mA/cm2 was used, at which a coating deposition rate of 1.67 μm/min was achieved. Coating abrasive wear and corrosion properties were assessed by conducting dry and wet rubber wheel abrasive tests and potentiodynamic polarization experiments, respectively. X-Ray diffraction (XRD) and transmission electron microscopy (TEM) were used to investigate the coating microstructure, and the coating/substrate interface. The property test results show that the coatings possess excellent abrasive wear and corrosion resistance. XRD analyses indicate that the coatings consist of α- and γ-Al2O3. An amorphous+nanocrystalline inner layer (1.5-μm thick) and a nanocrystalline (50–60 nm) intermediate layer in the coating were observed by TEM. The higher resistance to wear and corrosion can in part be attributed to the presence of these interlayers.
Article
Using electrochemical impedance spectroscopy (EIS) technique, the water uptake process and the evolution of impedance models of polypropylene coated mild steels and LY12 Al alloys immersed in 3.5% (weight fraction) NaCl solution were studied. The equivalent electrical circuit (EEC) fitting results suggested that there was one single semi-circle in the impedance spectra of immersed samples in the initial period of immersion, which was corresponded to one time constant. Along with the immersion time, the number of time constant in EEC for metal/coating electrode system increased. The diffusion process towards solution of corrosion products forming on the metal/coating interface might be restrained due to the influence of coating barrier, resulting in the initiation of diffusion component in EEC. This diffusion did not act as a Warburg behavior, leading to a deviation of dispersive number, n, from 0.5. The studies revealed that the water transport in polypropylene clear varnish followed the Fickian diffusion during the initial time of exposure to the solution. After a certain time of immersion, the water uptake reached a saturation value. According to the variation of coating capacitance with time, the water diffusion coefficient was deduced. The experimental results indicated that the water diffusion coefficients in polypropylene coatings were independent of substrate materials used and coating thickness. Relative dielectric constant of dry coatings (εd) and water volume fraction of the coatings at saturation (φ∞) were also calculated. The similar results were found in time dependence of coating capacitance calculated from the impedance imaginary component at a fixed high frequency and that obtained from the fitting result by EEC, which indicated that the coating capacitance from the impedance imaginary component was reliable to evaluate barrier performance of coatings against water.
Article
The aim of this article is to outline the theory of diffusion of small molecules through polymer films and to describe some techniques for the measurement of water vapour, oxygen and carbon dioxide permeation through paint coatings. The effect of polymer type and pigmentation on permeability will be discussed, together with some practical applications where permeability is an important factor governing performance
Article
Dense alumina ceramic coatings of 7μm thickness were fabricated on 2024 aluminium alloy by microarc oxidation (MAO). The corrosion behaviour of the MAO coated alloys was evaluated using potentiodynamic polarisation and EIS measurements. The results show that the corrosion process of the coated alloy can be divided into three stages: (1) the initial stage (the first 2–6h of immersion): penetration of corrosion medium into the aluminium alloy was inhibited by coating; (2) the second stage (after 24h of immersion), corrosion medium penetrated to attack the interface between the substrate and the coating; (3) the final stage (after about 96h): corrosion process was controlled by the diffusion of corrosion products.
Article
An attempt was made to produce calcium containing plasma electrolytic oxidation (PEO) coatings on AM50 magnesium alloy using an alkaline electrolyte. This study was performed in three alkaline electrolytes containing calcium hydroxide and sodium phosphate with three different mass ratios viz., 1:2.5, 1:5 and 1:7.5. All the three coatings produced were found to contain Ca and P in appreciable amounts. The concentration of P was found to be higher in the coatings obtained in the electrolytes with higher concentration of phosphate ions. Even though all the three coatings were found to be constituted with magnesium oxide and magnesium phosphate phases, X-ray diffraction analyses revealed that the phase composition was influenced by the phosphate ion concentration/conductivity of the electrolyte. Further, the PEO coating obtained in the 1:7.5 ratio electrolyte was found to contain di-calcium phosphate (monetite) and calcium peroxide phases, which were absent in the other two coatings. Potentiodynamic polarization studies performed in 0.1M NaCl solution showed that the coatings obtained from the 1:5 ratio electrolyte possessed a superior corrosion resistance, which is attributed to the combined effect of thickness, compactness and phase/chemical composition of this coating.
Article
Electrochemical impedance spectroscopy (EIS) is a common technique used in the assessment of the thickness of anodic oxide films formed on Al and its alloys. However, it is shown here that the resistance (R) values obtained from EIS data are highly dependent on the solution in which the measurements are made, while the capacitance (C) values are independent of the solution conditions and are therefore a much more reliable means of determining oxide film thickness and other properties. We also show that the experimental EIS data for unsealed porous anodic oxides formed on top of a barrier Al oxide film in acidic solutions exhibit only a single time constant, with very little influence seen from the overlying porous oxide layer on the apparent R and C values. The reason for this is demonstrated by utilizing the Laplace equation to calculate the electric field induced in the oxide, which confirms that the EIS response of these films can generally be assumed to reflect the properties and coverage of the underlying barrier oxide film.
Article
The formation of alumina-based coatings on aluminium by AC plasma electrolytic oxidation (PEO) has been investigated using a silicate electrolyte with selective additions of fine zirconia particles. The coatings comprised an amorphous barrier layer, a relatively dense intermediate layer and a more porous outer layer that contained silicon species. Zirconia was incorporated non-uniformly into the outer layer and, to a limited extent, the intermediate layer, as both particles and a component of cellular microstructures. Following treatments firstly in zirconia-containing electrolyte and secondly in zirconia-free electrolyte, the zirconia did not extend beyond about the middle of the intermediate layer, indicating its limited inward mass transfer during microdischarges. The coating efficiency decreased at oxidation times in excess of 40min due to dissolution of either the substrate or the coating, or physical loss of coating material. The oxidation of aluminium consumed on average ∼29% of the anodic charge; the remainder was used mainly in generation of oxygen gas.
Article
Impedance and gravimetric methods have been applied to study the uptake of water over a range of solution chloride ion concentration in free films (i.e. paint films not attached to a substrate). The paint films were 25 μm experimental silicone modified polyester. Comparisons have been made between the impedance and gravimetric methods to highlight advantages/disadvantages of each technique, and between the attached and free film results. Water uptake on the free films, calculated from gravimetric tests, increases with immersion time and with decreasing solution chloride ion concentration, in agreement with similar tests on attached films. The calculation of water uptake by the 1 kHz capacitance method was unreliable due to other processes occurring, such as paint film degradation and changing modes of distribution of water uptake at longer times. Sample failure time calculated by the single frequency (1 kHz) impedance test is a better measure of performance life than that calculated from wide frequency range tests. Both the free and attached film results show failure time decreasing with increasing solution chloride ion concentration. This suggests that the paint film degradation is caused predominantly by the solution and not by secondary effects such as build-up of alkalinity in the accumulated water layer at the metal/film interface on the attached films caused by cathodic reduction of oxygen. The marked drop in paint film resistance or the large rise in the 1 kHz capacitance with time is thought to be a measure of the extent of degradation of the paint film, caused by hydrolysis of the silicone modified polyester paint film. Water uptake, calculated by either the 1 kHz capacitance or gravimetric method, is greater but sample failure time is longer for the attached films than for the free films. Initially, water appears to enter the paint film rapidly, but slows down after a few hours when paint degradation becomes increasingly significant, and dependent on the solution chloride ion concentration.
Article
The microstructure and morphology of the surface of aluminium alloys strongly affect the nucleation, growth, and protective properties of chromate conversion coatings. Samples of AA 2024-T3 cladded with AA1230 were pretreated with two different procedures before they underwent a chromating process, which consisted of dipping in a commercial Alodine 1200 solution. In one procedure, an extra step, consisting of immersion in nitric–hydrofluoric acid solution for 30 s (desmutting bath), was carried out after immersion in sulfuric –phosphoric acid solution (acid pickling bath). Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and electrochemical investigations highlighted the fact that this extra step in the pretreatment route gave rise to a more etched, but less reactive surface, due to the removal of a large amount of cathodic sites, mainly related to AlFeSi precipitates present in the clad layer. A similar effect, but on a smaller scale, was caused also by the increase of the time of immersion in sulfuric –phosphoric acid solution. The decrease of galvanic coupling gives rise to a more homogeneous nucleation of the chromate conversion coating on the surface of the Alclad 2024-T3. Moreover, the chromate film tends to be denser and with fewer defects.
Article
Two different mathematical formulas for estimating effective capacitance from Constant-Phase-Element (CPE) parameters, taken from the literature, are associated unambiguously with either surface or normal time-constant distributions. Application to different systems are used to illustrate the importance of using the correct formula that corresponds to a given type of distribution. Experiments and simulations are used to show that the effective capacitance obtained for a normal distribution yields correct values for the film thickness under conditions where the local resistivity does not vary significantly. When the local resistivity varies considerably over the thickness of a film, the experimental frequency range may preclude observation of the capacitance contribution of a portion of the film, resulting in under prediction of the film thickness.
Article
We report on abnormal behavior in anodic oxidation of Al in mechanically confined structures for formation of horizontal nanoporous anodic alumina oxide (H-AAO). Instead of smooth pore walls, periodic dendrite inner pore structures form, the growth rate is suppressed to 5% of its value during bulk anodization under the same conditions, and a steady-state is never reached. These anomalies associated with H-AAO originate from suppressed volume expansion and a plastic flow of confined by the hard mask. By determining new anodization conditions leading to zero volume expansion, dendritic H-AAO can be avoided and kinetic retardation minimized.
Article
The photoelectrochemical behavior of anodic films on Al alloys, containing titanium, tantalum, and tungsten (valve metals), has been studied as a function of alloy composition and anodizing conditions. Photocurrent spectroscopy has been used to get information on bandgap and the flatband potential values of different mixed oxides. Both insulator-like and semiconducting behavior has been observed for anodic oxides grown on and alloys dependent on alloy initial composition. Optical bandgap values, , of different oxides are in accordance with predictions based on the correlation between and the difference of electronegativities of the oxide constituents, indicating potential for tailoring solid state properties of ternary oxides.
Article
The effects of adding Na2 WO4 . 2H2O and SiC powder to the electrolyte on the characteristics of ceramic coatings on Al based alloys formed by microarc oxidation were studied. The ceramic coating quality was improved by the two additives, but the addition of Na2 WO4 . 2H2O was more effective, as it improved the formation of Al2O3 phase in the ceramic coating, increased the ratio of the internal dense region to the whole ceramic coating, optimised the ceramic coating phase structure, and improved its wear resistance. The SiC powder was able to enter into the ceramic coating and improve its wear resistance, at the same time increasing the friction coefficient of the coating.
Article
The electrochemical impedance spectroscopy (EIS) of epoxy-coated aluminum alloy LY12 has been investigated during exposure to 3.5% NaCl solution. Using the continuous simulation of EIS by expanded general electrical model, the time-dependent impedance model of the alloy/coating/solution system was deduced. The results shown that the composite electrode displayed a barrier behavior before water and oxygen penetrated to alloy base. After water and oxygen reached the base, the impedance associated with corrosion of alloy base changed with the immersion time as following: (i) active corrosion period at the beginning (double-layer capacitance, Cdl, in parallel to the charge transfer resistance of electrochemical corrosion Rct), (ii) impeding of the diffusion of corrosion production at the intermediate period as a result of the presence of coated film (a constant phase element Zdiff was additionally in series with Rct), and (iii) appearance of the characteristic impedance related to Cl− ion-participating reaction with alloy base at the later stage. From the linear part of ln Cc–t0.5 curve in the early immersion stage, the apparent diffusion coefficient of water was obtained. The diffusion coefficient of water and Cl− ion through the coating was also calculated by the required time for diffusion of permeation species through the coating to the metal interface obtained from the simulation of EIS data by which the occurrence of characteristic impedance element(s) corresponding to special species arrival can be determined.
Article
A.C. impedance measurements carried out on epoxy-resin elcctrocoated steels were used to determine the pore resistance of the organic film as a function of time of exposure to 3% sodium chloride solution. The thermogravimctric analysis of coatings after various exposure times enabled determination of the quantity of electrolyte inside the pores, while gravimetric liquid sorption experiments were used to evaluate water uptake inside the coating. The results were interpreted in terms of a model in which water rapidly penetrates macrocapillaries (macropores) present as defects in the external portion of the coating and extra fine capillaries formed by the polymer net, followed by water and ions penetration through the macropores. The number, dimensions and shape of the pores through the film, estimated by optical microscopy, additionally confirmed the proposed model of electrolyte penetration. It was shown that conduction through the coating depends only on conduction through the macropores, although the quantity of electrolyte inside the macropores is only one tenth of that inside the polymer net.
Article
Three commercial paint systems, based on water-borne acrylic, chlorinated rubber and alkyd resins, were used to evaluate the water uptake in 5% NaCl solution (wt.%) and partially deionised water. Impedance spectroscopy allows following the coating capacitance evolution and, from this, water uptake versus immersion time can be tracked. Its analysis leads to the conclusion that diffusion process seems to be an ideal Fickian one just in one type of paint. The two others have important deviations from this ideal behaviour. In those conditions, an apparent diffusion coefficient can be estimated. The study of the electrolyte flux with exposition time was analysed and fitted to an expression, which includes four exponential terms with four time constants, associated to different processes. Three time constants behave as exponential decay functions and the fourth one has an exponential growth shape. A physical meaning for the expression is suggested. The values of all the studied parameters indicate that the barrier properties of the water-borne acrylic paint are the worst.
Article
The corrosion resistance of anodized Al 6061 produced by two different anodizing and sealing processes was evaluated using electrochemical impedance spectroscopy (EIS). The scanning electron microscope (SEM) was employed to determine the surface structure and the thickness of the anodized layers. The EIS data revealed that there was very little change of the properties of the anodized layers for samples that were hard anodized in a mixed acid solution and sealed in hot water over a 365 day exposure period in a 3.5 wt% NaCl solution. The specific admittance As and the breakpoint frequency fb remained constant with exposure time confirming that the hard anodizing process used in this study was very effective in providing excellent corrosion resistance of anodized Al 6061 over extended exposure periods. Some minor degradation of the protective properties of the anodized layers was observed for samples that were hard anodized in H2SO4 and exposed to the NaCl solution for 14 days.
Article
Anodic film growth on 2014-T4 aluminium alloy at 60 V in 50 g l−1 di-sodium tetraborate at 60 °C has been examined by transmission electron microscopy and Rutherford backscattering spectroscopy. Initial film growth proceeds at relatively high efficiency on the initially etched and desmutted alloy. During the subsequent period of current decline, the reactive electrolyte species penetrate the outer film at preferred regions, establishing conditions for pore development by field-assisted dissolution. In the alkaline electrolyte, such field-assisted dissolution also appears to proceed locally, probably through mechanical disruption of the film, giving rise to a feathered film morphology. The oxidation of copper from the alloy, in the presence of an enriched layer of copper, developed largely by initial etching, also influences film morphology through parallel oxygen gas generation, creating oxygen-filled voids. Such gas-filled voids may rupture or be removed from the alumina film material through field-assisted dissolution at the pore base. In the former case, cracking allows access of the anodizing electrolyte to the enriched alloy/film interface, with subsequent dissolution of the enriched layer and local film growth; these give rise to lateral porosity in addition to that from pores passing perpendicularly to the alloy surface. The efficiency of anodizing is about 12%, with losses from Al3+ ion ejection, field-assisted dissolution, oxygen gas generation, film rupture, interface dissolution and local film repair.
Article
Ceramic coatings are produced on aluminum alloy by autocontrol AC pulse Plasma Electrolytic Oxidation (PEO) with stabilized average current. Transient signal gathering system is used to study the current, voltage, and the transient wave during the PEO process. SEM, OM, XRD and EDS are used to study the coatings evolution of morphologies, composition and structure. TEM is used to study the micro profile of the outer looser layer and inner compact layer. Polarization test is used to study the corrosion property of PEO coatings in NaCl solution. According to the test results, AC pulse PEO process can be divided into four stages with different aspects of discharge phenomena, voltage and current. The growth mechanism of AC PEO coating is characterized as anodic reaction and discharge sintering effect. PEO coating can increase the corrosion resistance of aluminum alloy by one order or two; however, too long process time is not necessarily needed to increase the corrosion resistance. In condition of this paper, PEO coating at 60 min is the most protective coating for aluminum alloy substrate.
Article
Permeability of a PVC coating attached to a metal substrate and as a free film was assessed by electrochemical impedance spectroscopy. The results were compared with those from gravimetric tests. The impedance spectra of applied and free films have distinct behaviour. The concentration of the outer solution is determinant for the coating capacitance and for the water uptake of the film. The results revealed important differences between the two techniques.
Article
Electrochemical impedance spectroscopy (EIS) was used to elaborate the characteristic of waterborne and high solid epoxy coatings. The clear coats were investigated under long-term immersion condition. The coating capacities and the coating resistances of the two classes of materials revealed important influences of the coating formulations, like internal or external emulsification, degree of crosslinking and type of resin and hardener.The high solid systems showed a higher water up-take for 100% crosslinking compared to 80%, which was effected by an increased number of free amine groups. For waterborne coatings the chemical nature of the hardener and the choice of emulsifier dominated the effects of different degrees of crosslinking. Waterborne coatings had always higher dielectric constants, whereas after a few hours of immersion the resistance of high solids were 1–2 order of magnitude higher.
Article
Micro-arc oxidation (MAO) is an effective approach to improve the properties of aluminum and its alloy by forming ceramic films on the surface. However, the oxide layers often have a porous surface structure, which exhibits relatively high friction coefficients. In this work, in order to enhance the surface and mechanical properties of the films produced by micro-arc oxidation, Al 2 O 3 coatings embedded with Fe micrograins of different thicknesses were produced on aluminum alloys by adding Fe micrograins into the electrolyte during MAO. Compared to the Al 2 O 3 coatings without Fe micrograins, the MAO Al 2 O 3 coatings with Fe micrograins are much denser and harder, and the wear resistance is also improved significantly. The enhancement can be attributed to the enhancement of the surface structure and morphology of the MAO Al 2 O 3 coatings with embedded Fe micrograins.
Article
The phase distribution for ceramic coatings formed by microarc oxidation (MAO) on 2024 aluminum alloy was investigated using X-ray diffraction. The results showed that the ceramic coatings mainly consisted of α-Al2O3 and γ-Al2O3 phases. The percentage of α-Al2O3 gradually increased from the external surface to the interface between the coating and the substrate of samples. The surface layer of coatings mainly contained the γ-Al2O3 phase, and its fraction of the composition remained almost constant with oxidation time. It is believed that the difference in the amounts of α-Al2O3 and γ-Al2O3 phases in the different layers of coatings was caused by the various cooling rates of molten Al2O3, which temporarily existed in the microarc zone.
Article
Primary aliphatic amines (diethylenetriamine, triethylenetetramine, tetraethylenepentamine) and super acids (CF3SO3H, HPF6) have been investigated as room temperature curing agents for hybrid organic–inorganic thin films. CP/MAS NMR analysis indicates that the curing mechanisms promoted by amines and super acids are different, producing Ormosils (organically modified silicates) with different structures. Amine-cured Ormosils were found to be highly crosslinked, dense, and adherent to the underlying aluminum alloy (AA) substrate; analogous super acid-cured Ormosils were oligomeric, exhibiting poor adhesion properties to the underlying metal substrate. The choice of the curing agent was found to affect the Ormosil film corrosion resistance characteristics as indicated by results of potentiodynamic polarization curves and accelerated corrosion tests. Amine-cured films provided corrosion protection, with Rcorr=37–157 kΩ cm2, the magnitude of which depended on several processing parameters including stir time prior to amine addition and amine chain length. Super acid-cured films provided poor corrosion resistance, Rcorr=1–2 kΩ cm2.
Article
Spark anodizing of aluminium at 5 A dm−2 in sodium metasilicate/potassium hydroxide electrolytes is studied, with particular emphasis on the mechanism of coating growth, using transmission electron microscopy and surface analytical techniques, with coatings typically 10 μm, or more, thick. Two-layered coatings develop by deposition of an outer layer based on amorphous silica, associated with low levels of alkali-metal species, at the coating surface and growth of an inner, mainly alumina-based, layer, with an amorphous region next to the metal/coating interface. Formation of crystalline phases in the inner layer, mainly γ-Al2O3, with some α-Al2O3 and occasional δ-Al2O, is assisted by local heating, and possibly also by ionic migration processes, arising from the rapid coating growth at sites of breakdown. Due to local access of electrolyte species in channels created by breakdown events, the silicon content in the inner coating regions varies widely, ranging from negligible levels to about 10 at.%. Silica deposition at the coating surface and formation of Al2SiO5 and Al6Si2O13 phases is promoted by increased time of anodizing and concentration of metasilicate in the electrolyte. However, at sufficiently high concentration of metasilicate and pH, when more extreme voltage fluctuations accompany breakdown, the two-layered nature of coatings is replaced by a mixture of aluminium-rich and silicon-rich regions throughout the coating thickness.
Article
The inherent reactivity of the Al–Cu alloys is such that their use for structural, marine, and aerospace components and structures would not be possible without prior application of a corrosion protection system. Historically these corrosion protection systems have been based upon the use of chemicals containing Cr(VI) compounds. Organic–inorganic hybrid silane coatings are of increasing interest in industry due to their potential application for the replacement of current toxic hexavalent chromate based treatments. In the present study, a hybrid epoxy–silica–alumina coating with or without doped cerium nitrate has been prepared using a sol–gel method. The hybrid coatings were applied by a dip-technique to an Al–Cu alloy, Al 2024-T3, and subsequently cured at room temperature. The anticorrosion properties of the coatings within 3.5% NaCl were studied using electrochemical impedance spectroscopy (EIS), and conventional DC polarisation. An exfoliation test method involving immersion in a solution of 4 M NaCl, 0.5 M KNO3 and 0.1 M HNO3 was also used. The cerium nitrate doped sol–gel coating exhibited excellent anticorrosion properties providing an adherent protection film on the Al 2024-T3 substrate. The resistance to corrosion of the sol–gel coating was also evaluated by analysing the morphology of the coating before and after corrosion testing using scanning electron microscopy.
Article
An improved process has been developed for the spontaneous deposition of cerium oxide conversion coatings for corrosion protection of aluminum alloy 7075-T6. Approximately 80% of panels prepared using the improved process inhibited corrosion up to two weeks (336 h) in ASTM B117 salt fog testing, compared to approximately 20% for previous processes. Coatings were deposited from water-based solutions of CeCl3 and other compounds. Coating thickness, surface morphology, and performance in salt fog testing were sensitive to process parameters including surface preparation prior to coating, immersion time in the coating solution, and post-treatment. Surface preparation of the alloy was a critical first step in the deposition process. Desmutting followed by degreasing in a water solution of a commercial alkaline cleaner at a specific temperature was found to be an acceptable pre-treatment. Coating thickness, as determined by Auger electron spectroscopy depth profiling, varied depending on the surface pre-treatment and time that the panel was immersed in the coating solution. Immersion of desmutted, degreased panels for 15 min produced coatings 200 nm thick. Post-treatment, which converted Ce4+ in the as-deposited coating to Ce3+ as shown by X-Ray photoelectron spectroscopy, consisted of immersion in a phosphate sealing solution. Transmission electron microscopy analysis indicated that the as-deposited coatings were composed of a heretofore unidentified nano-crystalline cerium compound, presumably a hydrated Ce4+ oxide or Ce4+ hydroxide.
Article
The plasma electrolytic oxidation process of aluminium alloys is investigated for two different current waveforms. It is shown that particular conditions may be established which strongly reduce the arcing that usually cause detrimental defects in the oxide layer for treatment time greater than typically 40–50 min. This results in a “softer” process. As a consequence thick homogenous layers may be grown with no large discharge channels. Through the presented results, the importance of the negative charge density relative to the positive one is evidenced thus pointing out the need of using a pulse bipolar current supply.
Article
Micro-arc discharge oxidation (MDO) is a cost-effective plasma electrolytic process which can be used to improve the wear resistance of aluminium alloy parts by creating a hard thick alumina coating on the component surface. However, for sliding wear applications, such alumina coatings often exhibit relatively high friction coefficients against many counterface materials. Therefore, a duplex treatment, combining a load-supporting MDO alumina layer with a low friction diamond-like carbon (DLC) coating, produced by a modified plasma-immersion ion implantation (PI3) process, has been investigated. PI3 provides a flexible method of implanting ions into complex-shaped parts using a low temperature, low voltage plasma onto which high voltage pulses are superimposed. It can also be used to enhance the adhesion and growth characteristics of films formed under plasma conditions. In this work, a weakly-ionized, hot-filament supported low-voltage argon–acetylene plasma (with C2H2/Ar ratios from 1.0 to 0.15) was used, in combination with a low-frequency dc pulse voltage PI3 system (in this case 100 μs, 5 kV pulses at 850 Hz) to deposit a low friction DLC top layer onto MDO-treated Al alloy coupons. Microhardness measurements and pin-on-disc sliding wear tests were performed to evaluate the mechanical and tribological properties. Ball-on-plate impact tests were also carried out to assess coating layer adhesion/cohesion. Scanning electron microscopy (SEM) was used to observe coating morphology, and to examine wear scars from pin-on-disc tests and crater scars from impact tests. The work demonstrates that a hard and uniform DLC coating, with good adhesion and a low coefficient of friction, can be successfully deposited on top of an alumina intermediate layer, which provides excellent load support; such that the coating can withstand much higher contact stresses than would normally be the case with aluminium-alloy substrate materials. The C2H2/Ar ratio significantly influences the interfacial adhesion between the DLC and alumina layers, but has no significant effect on coating hardness. It is suggested that the C2H2/Ar ratio should be selected in the range of 0.25–0.35 to obtain a hard a-C:H carbon film with low-hydrogen-content and excellent adhesion. The investigations indicate that a duplex combination of micro-arc oxidation and PI3 represents a promising technique for surface modification of Al-alloys for tribological applications in which high contact loads are anticipated.
Article
Using electrochemical impedance spectroscopy (EIS) technique, the analyses of impedance models and the water uptake processes of epoxy-coated mild steels and LY12 aluminum alloys immersed in 3.5% (weight fraction) NaCl solution were studied. The fitting results using suitable electrical equivalent circuits (EEC) suggested that there were four kinds of impedance models in the course of immersion. In the initial period of immersion, an EEC containing the coating capacitance and the coating resistance was used to fit the EIS data. Along with the immersion time, the number of time-constant of impedance spectra for metal/coating systems increased, indicating that corrosion reaction took place. After a certain time's attacking of metal substrates, the diffusion process towards solution of corrosion products forming at the metal/coating interface might be restrained due to the presence of coating barrier, resulting in the appearance of diffusion elements. This diffusion did not act as an ideal Warburg behavior, leading to a deviation of dispersive number, n, from 0.5. After longer immersion times, the Cl− ions reached the surface of metal through the coating and participated in chemical reactions with corrosion products to form salt film on aluminum alloy surface. The formation of chloride ion-incorporation salt film resulted in the change of impedance spectrum characteristics. The studies revealed that the water transport in epoxy varnish coatings followed the Fickian diffusion law during the initial time of exposure to the testing solution. After immersion for a certain time, the water uptake reached saturation and the coating capacitance tended to a relatively steady value. According to the variation of coating capacitance with time, the water diffusion coefficient was deduced. The experimental results indicated that the water diffusion coefficients in epoxy coatings were slightly dependent on coating thickness and were to some extent influenced by substrate type. The similar surface chemical structures of epoxy coatings on different metals were found from the Fourier transform infrared spectroscopy (FT-IR)–attenuated total reflection (ATR) measurements. The glass transition temperature (Tg) of epoxy coatings prepared on different metal substrates before and after immersion was measured by using differential scanning calorimetry (DSC). Lower diffusion rate of water in Al alloy based epoxy films than that in steel based ones was interpreted by the higher absolute value and smaller decrease of Tg of organic coatings painted on the former substrate.
Article
Plasma electrolytic oxide coatings have been produced on both aluminium and magnesium substrates. Their microstructures have been studied and deductions made about formation conditions. The thermal conductivities of the coatings have been measured using a simple steady state method. The values obtained are relatively low (∼1 W m−1 K−1). This is explained in terms of the microstructure, which exhibits an extremely fine grain size and a significant proportion of amorphous phase. The porosity levels are low, so the low conductivity is not due to the presence of pores. It is noted that, even with a thickness limit of the order of 100 μm, coatings with such low conductivity may prove useful as thermal barrier layers, particularly since they exhibit excellent adhesion characteristics.
Article
A study has been made of the electrical characteristics and optical emission spectra exhibited when discharge events take place during plasma electrolytic oxidation processing. Both conventional and small area experimental arrangements have been employed, allowing detailed measurement of durations, and temporal distributions, as well as such characteristics as charge transfer, and power. Individual discharges are of short duration, typically tens to hundreds of microseconds, but there is a strong tendency for them to occur in cascades that commonly last between several ms and several tens of ms. The composition, temperature and electron density of the plasma formed during PEO processing are inferred from characteristics of the emission spectra. This confirms that there are two distinct regions of plasma; a lower density peripheral region at ~ 3500 K, and a higher density core at ~ 16,000 ± 3500 K. The implications of these results are considered in terms of the interpretation of different types of experimental measurement, and attention is also briefly given to how such behaviour might relate to the mechanisms of growth.
Article
In the paper, a modified Plasma Electrolytic Oxidation process (PEO) to produce ceramic coatings on Al alloys, suitable for tribological applications, is discussed. The process utilises bipolar current pulses in the kHz frequency range, providing better control over plasma discharges occurring at the sample surface. The coatings, formed on a 2024 series Al alloy, are characterized by means of optical microscopy, SEM, EDX, XRD and surface profilometry. Microhardness and scratch adhesion tests are performed to evaluate the coating mechanical performance. The effects of current pulse frequency on both the layer growth kinetics and the process energy efficiency are discussed. It is found that in the 1 to 3 kHz frequency range, the layer growth rate can be increased from 0.5 to 1 to 1.6 to 3.2 μm min−1 and the volume fraction of the porous outer layer can be reduced from 25 to 20% to 15 to 10% of the total layer thickness, compared to the conventional 50 Hz AC PEO process. The inner layer, despite a slight increase in porosity, preserves a relatively high hardness of 1200 to 1500 HK25 and good adhesion (LC2=60 N), which should be sufficient for many tribological applications.
Article
The problem of technical and economical optimization of the process of micro-arc discharge oxidation of high-strength aluminium for the fabrication of oxide ceramic layers for tribotechnical purposes is considered in terms of experimental design. To estimate the effectiveness of the process, a generalized parameter is used which accounts for oxide mass yield as a principal parameter, and mechanical and geometrical characteristics of the layer as restricting parameters. The methods of chemical weight, scanning electron microscopy, optical and durometric analyses are used. The influence of the silicate–alkali electrolyte composition and the amount of electricity carried through the cell on the layer properties is discussed. The response surface of the generalized parameter is plotted with the aid of desirability functions. The area of regimes corresponding to 2–3 g l−1 KOH and 2–3 g l−1 Na2SiO3 electrolyte composition and (2.50–3.33)×103 C m−2 of carried electricity is outlined for the most effective fabrication of uniform oxide layers with 165–190 μm thickness and 18–23 GPa hardness.
Article
This paper overviews the relatively new surface engineering discipline of plasma electrolysis, the main derivative of this being plasma electrolytic deposition (PED), which includes techniques such as plasma electrolytic oxidation (PEO) and plasma electrolytic saturation (PES) processes such as plasma electrolytic nitriding/carburizing (PEN/PEC). In PED technology, spark or arc plasma micro-discharges in an aqueous solution are utilised to ionise gaseous media from the solution such that complex compounds are synthesised on the metal surface through the plasma chemical interactions. The physical and chemical fundamentals of plasma electrolysis are discussed here. The equipment and deposition procedures for coating production are described, and the effects of electrolyte composition and temperature on ignition voltage, discharge intensity and deposited layer thickness and composition are outlined. AC-pulse PEO treatment of aluminium in a suitable passivating electrolyte allows the formation of relatively thick (up to 500 μm) and hard (up to 23 GPa) surface layers with excellent adhesion to the substrate. A 10–20 μm thick surface compound layer (1200HV) and 200–300 μm inner diffusion layer with very good mechanical and corrosion-resistant properties can also be formed on steel substrates in only 3–5 min by use of the PEN/PEC saturation techniques. Details are given of the basic operational characteristics of the various techniques, and the physical, mechanical and tribological characteristics of coatings produced by plasma electrolytic treatments are presented.
Article
The water uptake of a commercial alkyd paint system is studied using the impedance spectroscopy technique. The first part of the paper is devoted to establish the experimental setup to accurately measure capacitance values in the pF range, characteristic of the studied system. It was concluded that electrochemical instrumentation is not adequate to perform this task. The second part of the paper discusses the water uptake of the paint system under two different immersion conditions: deionized water and 5% NaCl solution. This analysis leads to the conclusion that the diffusion process seems to follow the ideal-Fickian behaviour in both systems, the corresponding diffusion coefficients were calculated.
Article
In this paper, the sorption/diffusion of water from an aqueous electrolyte into a commercial epoxy paint coating was compared using gravimetry, electrochemical impedance spectroscopy (EIS) and attenuated total reflection infra-red spectroscopy (ATR-IR) techniques as a function of electrolyte composition. All three techniques show an apparent two-stage diffusion process. Diffusion coefficients for water sorption obtained by ATR are consistent with those obtained using (conventional) gravimetric methods and impedance spectroscopy (EIS). We discuss the methods used to derive diffusion coefficients, and concentration of the sorbed species, from capacitance measurements. As such, we postulate that the cause of anomalous diffusion behaviour is due to the fact that the penetrant enters the polymer because of classical diffusion down the concentration gradients, which are modified a result of the time dependence of the surface concentration. Hence, an apparent two-stage sorption profile is not necessarily significant in a heterogeneous coating and Brasher’s equation remains the most appropriate for data analysis.
Article
Constant-phase elements (CPE) are used extensively in equivalent electrical circuits for fitting of experimental impedance data. The CPE behavior is generally attributed to distributed surface reactivity, surface inhomogeneity, roughness or fractal geometry, electrode porosity, and to current and potential distributions associated with electrode geometry. In this work, different electrochemical systems showing the CPE dependence in the high-frequency range for the overall impedance were considered. Local electrochemical impedance spectroscopy was found to provide a good means for assessing the influence of local variations on the CPE behavior seen in global impedance measurements. A separation between 2D and 3D distributions could be easily observed. In the case of a 2D distribution (AZ91 Mg alloy), the origin of the CPE behavior was the distribution of high-frequency resistance associated with the geometry of the disk electrode; whereas, the capacitance was independent of position. In the case of the aluminium electrode, the CPE behavior could be attributed to a combination of 3D and 2D distributions. Geometric distributions can play a significant role in the impedance response of electrochemical systems, and these distributions can lead to CPE behavior.
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