Effect of plasma electrolytic oxidation coating on the stress corrosion cracking behaviour of wrought AZ61 magnesium alloy

An attempt was made to understand the effect of silicate based plasma electrolytic oxidation (PEO) coating on the stress corrosion cracking (SCC) behaviour of an AZ61 wrought magnesium alloy. The SCC behaviour of untreated and PEO coated specimens was assessed using slow strain rate tensile tests at two different nominal strain rates, viz. 1 × 10⁻⁶ s⁻¹ and 1 × 10⁻⁷ s⁻¹, in ASTM D1384 test solution at ambient conditions. The PEO coating was found to improve the general corrosion resistance to a significant extent; however, the improvement in the resistance to stress corrosion cracking was only marginal.     

Mullite-rich plasma electrolytic oxide coatings for thermal barrier applications

A study has been undertaken of the characteristics exhibited by mullite-rich plasma electrolytic oxide coatings grown on aluminium alloys by using silicate-rich electrolytes. It is found that they can be grown at a higher rate, and to a greater thickness, than alumina PEO coatings on aluminium. The thermal conductivity of these coatings has been measured using a steady-state method. It is shown to be of the order of 0.5 W m^− 1 K^− 1, which may be compared with ∼ 1.5 W m^− 1 K^− 1 for pure alumina PEO coatings and ∼ 10-15 W m^− 1 K^− 1 for dense polycrystalline mullite. Coupled with excellent substrate adhesion and good mechanical properties, this relatively low conductivity makes these coatings attractive for thermal barrier applications. Furthermore, they are shown to exhibit a relatively low global stiffness (∼ 40 GPa), which will reduce the magnitude of thermally-induced stresses and improve the resistance to spallation during temperature changes.     

Optical emission spectroscopy studies of discharge mechanism and plasma characteristics during plasma electrolytic oxidation of magnesium in different electrolytes

The discharge mechanism of the plasma electrolytic oxidation (PEO) process in different electrolytes was investigated by examining the variation of the optical emission spectra (OES). The spectrum of active species existed in the bubble layer. The bubble layer was initially broken down, followed by the breakdown of the dielectric barrier layer. Breakdown is the initial stage of discharge. A micro-discharge formation model, which assumes that the discharge ignition in the bubble layer developed at the oxide/electrolyte interface, was proposed. The active plasma species that appeared in different electrolytes during the PEO process were also studied. The appearance order of the excited active plasma species depended on the energy that the orbit transition of the species needed, but was not related to the anion concentration in the electrolyte. The anions in the electrolyte, except the OH⁻, also had little influence on the composition of the active plasma species during the PEO process. The active plasma species were mainly composed of metal atoms, metal cations, and gases produced by water decomposition. The electron temperature of the excited hydrogen was between 6 × 10³ and 3 × 10⁴ K. The high temperature provided the possibility of ceramic film melting and sintering. The source and transition of the active plasma species were also studied. They were found to undergo dissociation, ionization, and excitation processes.     

Substitution of hexavalent chromate conversion treatment with a plasma electrolytic oxidation process to improve the corrosion properties of ion vapour deposited AlMg coatings

In this paper plasma electrolytic oxidation (PEO) is examined as a potentially beneficial post-deposition treatment for Al-5 wt.%Mg coatings deposited onto Ti alloy substrates using Ion Vapour Deposition (IVD), with a view to replacing toxic hexavalent chromate conversion treatments and at the same time enhancing the barrier protection properties of such IVD coatings. The aqueous corrosion behaviour of PEO layers formed on IVD coatings was evaluated by means of potentiodynamic polarisation scans, open circuit potential measurements and electrochemical impedance spectroscopy. Normally, IVD aluminium-based coatings require a post-coat shot peening treatment to densify the coating structure; however it was found that PEO layers could be formed successfully on as-deposited IVD coatings deposited onto Ti alloy substrates, providing a cost effective process to improve corrosion behaviour. PEO treatment provides unique improvements in the corrosion resistance of IVD coatings; the PEO layer possesses effective anticorrosion properties in aqueous environments containing Cl⁻ ions. A more positive corrosion potential, lower corrosion current and increased polarisation resistance were recorded for PEO layers, compared to traditional chromate conversion treatments.     

Surface characterisation of DC plasma electrolytic oxidation treated 6082 aluminium alloy: Effect of current density and electrolyte concentration

Plasma electrolytic oxidation (PEO) is a specialised but well-developed process which has found applications in aerospace, oil/gas, textile, chemical, electrical and biomedical sectors. A novel range of coatings having technologically attractive physical and chemical properties (e.g. wear- and corrosion-resistance) can be produced by suitable control of the electrolyte as well as electrical parameters of the PEO process. Oxide ceramic films, 3 to 40 μm thick, were produced on 6082 aluminium alloy by DC PEO using 5 to 20 A/dm² current density in KOH electrolyte with varied concentration (0.5 to 2.0 g/l). Phase analysis (composition and crystallite size) was carried out using X-ray diffraction and TEM techniques. Residual stresses associated with the crystalline coating phase (α-Al₂O₃) were evaluated using the X-ray diffraction Sin²ψ method. Nanoindentation studies were conducted to evaluate the hardness and elastic modulus. SEM, SPM and TEM techniques were utilised to study surface as well as cross-sectional morphology and nano features of the PEO coatings. Correlations between internal stress and coating thickness, surface morphology and phase composition are discussed. It was found that, depending on the current density and electrolyte concentration used, internal direct and shear stresses in DC PEO alumina coatings ranged from − 302 ± 19 MPa to − 714 ± 22 MPa and − 25 ± 12 MPa to − 345 ± 27 MPa, respectively. Regimes of PEO treatment favourable for the production of thicker coatings with minimal stress level, dense morphology and relatively high content of α-Al₂O₃ phase are identified.     

Plasma characterization and technological application of a hollow cathode plasma source with an axial magnetic field

An efficient plasma source has been established by arranging a hot hollow cathode electron emitter in a strong axial magnetic field, allowing for a reduction of working gas flow by one order of magnitude without loss of discharge stability. Moreover, with the reduction of gas flow not only an increase of the discharge impedance was observed, but also a multiplication of ion current density together with a highly expanded volume of the plasma plume.By means of spatially resolved Langmuir probe measurements, combined with the usage of an energy-resolved mass-spectrometer, plasma density profiles and energy distribution functions of electrons and ions have been measured. Generally, with an increase of the magnetic field and with the reduction of the working gas flow ion energy distribution functions shift from mean values of a few eV to 10 eV and more, while charge carrier densities increase from 10⁹ cm^− 3 to more than 10¹² cm^− 3. A strongly increased ability to dissociate and ionize reactive gases was observed.Two promising applications related to the coating of tools and components are discussed: the sputter etching with argon ions and the reactive pulse magnetron sputter deposition of wear-resistant chromium nitride layers. Whereas the first mentioned process provides pre-heating and etching rates higher than all actually used in tool coating industry, the second one offers advantages for film growth kinetics leading to significant improvements in composition, structure, surface morphology, and hardness of the deposited layers.     

Spectroscopic characterization of plasma during electrolytic oxidation (PEO) of aluminium

We present results of an optical emission spectroscopic study of Plasma during Electrolytic Oxidation (PEO) of aluminium in citric acid and in sodium-tungstate water solutions. The line shape analysis of the first two hydrogen Balmer lines indicates presence of two PEO processes characterized by relatively low electron number densities N_e ≈ 0.8 × 10¹⁵ cm^− 3 and at 2.5 × 10¹⁶ cm^− 3. Apart from these two N_e values, N_e ≈ 6.0 × 10¹⁶ cm^− 3 is determined from the width of the Al II 704.2 nm line. Three considerably different N_e values imply presence of three types of discharge during PEO of aluminium. The electron temperature T_e is determined from relative line intensities of the O II (T_e ≈ 38,300 K) and W I (T_e ≈ 3300 K) lines. The use of two databases of transition probabilities for O II lines introduces systematic difference of reported T_e results.     

Characterization of AC PEO coatings on magnesium alloys

Optical emission spectroscopy, fast video imaging and coating characterization are employed to investigate AC plasma electrolytic oxidation (PEO) of magnesium alloys. The findings revealed initiation and gradual increase in the number of discharges after 2-4 ms of each anodic pulse once a critical voltage was reached. No discharges were observed during the cathodic half-cycles. The lifetimes of discharges were in the range of 0.05-4 ms. A transition in the voltage-time response, accompanied by a change in the acoustic and optical emission characteristics of discharges, was associated with the development of an intermediate coating layer with an average hardness of 270-450 HV_0.05. The coatings grew at a rate in the range 4.0-7.5 µm min^− 1, depending on the substrate composition. Regardless of the substrate, the coatings consisted of MgO and Mg₂SiO₄, with incorporation of alloying element species. Electrolyte species were mainly present in a more porous layer at the coating surface, constituting 20-40% of the coating thickness. A thin barrier layer consisting of polycrystalline MgO was located next to the alloy. The corrosion rate of the magnesium alloys determined using potentiodynamic polarization in 3.5 wt.% NaCl was reduced by 2-4 orders of magnitude by the PEO treatment.     

Corrosion resistance of TiO2 films grown on stainless steel by atomic layer deposition

Titanium dioxide (TiO₂) films have been deposited onto stainless steel substrates using atomic layer deposition (ALD) technique. Composition analysis shows that the films shield the substrates entirely. The TiO₂ films are amorphous in structure as characterized by X-ray diffraction. The electrochemical measurements show that the equilibrium corrosion potential positively shifts from − 0.96 eV for bare stainless steel to − 0.63 eV for TiO₂ coated stainless steel, and the corrosion current density decreases from 7.0 × 10^− 7 A/cm² to 6.3 × 10^− 8 A/cm². The corrosion resistance obtained by fitting the impedance spectra also reveals that the TiO₂ films provide good protection for stainless steel against corrosion in sodium chloride solution. The above results indicate that TiO₂ films deposited by ALD are effective in protecting stainless steel from corrosion.     

The effect of cysteine on the corrosion of 304L stainless steel in sulphuric acid

The effect of cysteine on the corrosion of 304L stainless steel in 1 mol l⁻¹ H₂SO₄ was studied using open-circuit potential measurements, anodic polarization curves, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). All the electrochemical measurements obtained in the presence of low cysteine concentration (10⁻⁶-10⁻⁵ mol l⁻¹) presented the same behaviour as those obtained in the absence of cysteine, a passivated steel surface. However, for higher cysteine concentrations (10⁻⁴-10⁻² mol l⁻¹), a different behaviour was observed: the corrosion potential stabilized at a more negative value; an active region was observed in the anodic polarization curves and the electrochemical impedance diagrams showed an inductive loop at lower frequencies and a much lower polarization resistance. These results show that the presence of cysteine at high concentration turns the surface of 304L stainless steel electrochemically active, probably dissolving the passivation layer and promoting the stainless steel anodic dissolution. SEM experiments performed after immersion experiments at corrosion potential were in good agreement with the electrochemical results.     

Small signal frequency response studies for plasma electrolytic oxidation

Plasma assisted electrochemical treatments provide new possibilities in surface modification of various materials including light weight alloys. However, their large-scale application is still restricted, mainly due to poor understanding of the process mechanisms and consequent limitations in process control and automation. This problem can be resolved if the frequency response (FR) of the system is known and applied for process diagnostics. Our previous work has shown the effectiveness of the large signal mode for FR studies of plasma electrolytic oxidation (PEO) of aluminium. This research is dedicated to FR measurements during PEO of Al in the small signal mode corresponding to small perturbations of voltage signal around large DC values. The study was carried out during PEO of Al at DC voltages which were varied from 450 to 600 V. The FR obtained is a frequency dependent admittance of the PEO electrolyser; this complex number is represented by a modulus and a phase angle. Under potentiostatic conditions, the modulus evolution strongly correlates with the average current value; therefore, it bears insufficient amounts of independent information. The FR phase angle measured within this study was never obtained before. Depending on the frequency, it varies between 0° and 70 ° in the capacitive domain. One of the most notable features of this characteristic is low values at 500-5000 Hz when microdischarges appear during PEO. The other feature is a correlation with the coating growth. As a result, a new diagnostic tool was developed and shown to be effective for evaluation of microdischarges and surface properties during the treatment, thus decreasing the uncertainty in the system.     

Low-temperature plasma carburizing of AISI 420 martensitic stainless steel: Influence of gas mixture and gas flow rate

Low-temperature plasma carburizing was studied aiming to determine the effect of the gas mixture and flow rate on the surface properties of AISI 420 martensitic stainless steel samples. Plasma carburizing was carried out for gas mixtures of 20% Ar + 80% H₂ comprising CH₄ contents between 0.25 and 1.00%, and gas flow rates ranging from 1.67 × 10^− 6 to 6.68 × 10^− 6 Nm³ s^− 1. The modified layers were characterized by confocal laser scanning microscopy, X-ray diffractometry and microhardness measurements. The plasma was also characterized by optical emission spectrometry. Results indicate the presence of a hard and thin outer layer and a carbon-enriched martensite diffusion layer. It is shown that gas mixture composition plays an important role in the process kinetics. Spectroscopic characterization of the glow discharge shows that the variation of the CH₄ content in the gas mixture leads to a variation of the emission lines intensity but does not significantly alter the relative peak intensities. It suggests a variation on the plasma density and no significant variation on the active species. It also indicates that, for the studied conditions, the emission spectroscopy cannot be applied as a tool for process control.     

Surface modification of PET films by atmospheric pressure plasma-induced acrylic acid inverse emulsion graft polymerization

In order to make polyester film surface hydrophilic, atmospheric pressure plasma jet (APPJ) is used to activate the film surface first followed by inverse emulsion grafting polymerization of acrylic acid (AA). The graft ratio and wettability test reveal that the effectiveness of APPJ on initiating graft polymerization increases with increasing plasma treatment duration and helium flow rate. As the jet-to-substrate distance increases, the graft efficiency goes through a maximum at 2 mm. When oxygen is added to the plasma treatment gas, the graft efficiency decreases substantially. Fourier transform infrared spectrometry (FTIR) analysis shows new bands appearing at 2500-3600 cm^− 1 and 1546 cm^− 1 for the plasma-grafted samples. Scanning electron microscopy (SEM) results show that the grafted layer is built with a large number of spherical particles at submicron or even nanoscale. With a high graft ratio, agglomeration of neighboring particles becomes more pronounced, and eventually a relatively continuous graft layer is obtained with a corresponding surface contact angle of 5° which is considered superhydrophilic.     

Influence of pH on the deterioration of plasma electrolytic oxidation coated AM50 magnesium alloy in NaCl solutions

The corrosion deterioration process of plasma electrolytic oxidation (PEO) coatings on AM50 magnesium alloy prepared from two different based electrolytes, i.e., an alkaline phosphate electrolyte and an acidic fluozirconate electrolyte, were investigated using electrochemical impedance spectroscopy (EIS) in a 0.1 M NaCl solution with pH of 3, 7 and 11, respectively. It was found that the PEO coating formed in alkaline phosphate electrolyte, which was composed mainly of MgO, suffered from rapid chemical dissolution and lost its protection capability very quickly in acidic NaCl solution (pH 3). The chemical dissolution of this PEO coating was retarded in neutral NaCl solution (pH 7) and the corrosion damage was localized in this environment. On the other hand, in the alkaline NaCl solution (pH 11), the MgO coating underwent only slight degradation. The PEO coating produced in acidic fluozirconate electrolyte, the failure was marked by the flaking-off of the large areas of coating in acidic NaCl solution (pH 3). However, in the neutral and alkaline NaCl solutions, the coating underwent only a slight degradation without any observable corrosion damage in the 50 h test. The results showed that the deterioration process of PEO coated magnesium alloy was governed mostly by the pH of NaCl solution and it was also strongly related to the microstructure and composition of the PEO coatings.     

Modification of polystyrene with a highly reactive cold oxygen plasma

Samples of biaxially oriented polystyrene (PS) were exposed to an extremely non-equilibrium oxygen plasma. The neutral gas kinetic temperature was 70 °C, the electron temperature was about 3 eV, the plasma density was about 2 × 10¹⁵ m^− 3, and the neutral oxygen atom density was 4 × 10²¹ m^− 3. The plasma was created with an electrodeless radiofrequency discharge and the samples were kept at a floating potential. The plasma treatment time was between 1 and 50 s. The samples were characterized by high-resolution X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), microbalance, and water contact-angle (WCA) measurements. The etching rate was found to be relatively independent of the treatment time, at about 9 nm/s. The corresponding probability for oxidation with O atoms was calculated from the known flux of O atoms onto the surface, and was close to 6 × 10^− 4. The AFM did not show any appreciable increase in the surface roughness due to the plasma treatment. Both the XPS and the WCA measurements showed a well-activated surface after just a second of plasma treatment. The rapid functionalization was explained by the huge flux of O atoms onto the sample surface. The tilting of the samples during the XPS characterization made it possible to conclude that the oxygen-containing functional groups were concentrated at the surface of the film, at a depth much smaller than that examined by XPS. The functional groups found using XPS were the standard C-O, O-C-O, C = O, O-C═O as well as -O-CO-O-. The appearance of functional groups with a high degree of oxidation, like carboxyl and carbonate, was attributed to the oxygen's interaction with the aromatic phenyl rings. The ageing results showed that the concentration of the oxygen functional groups did not change much, but the contact angle of the water drop increased moderately during the first day and remained almost unchanged after subsequent ageing.     

Low temperature growth of nanocrystalline TiO2 films with Ar/O2 low-field helicon plasma

TiO₂ thin films were deposited on silicon wafer substrates by low-field (1 < B < 5 mT) helicon plasma assisted reactive sputtering in a mixture of pure argon and oxygen. The influence of the positive ion density on the substrate and the post-annealing treatment on the films density, refractive index, chemical composition and crystalline structure was analysed by reflectometry, Rutherford backscattering spectroscopy (RBS) and X-ray diffraction (XRD). Amorphous TiO₂ was obtained for ion density on the substrate below 7 × 10¹⁶ m^− 3. Increasing the ion density over 7 × 10¹⁶ m^− 3 led to the formation of nanocrystalline (~ 15 nm) rutile phase TiO₂. The post-annealing treatment of the films in air at 300 °C induced the complete crystallisation of the amorphous films to nanocrystals of anatase (~ 40 nm) while the rutile films shows no significant change meaning that they were already fully crystallised by the plasma process. All these results show an efficient process by low-field helicon plasma sputtering process to fabricate stoichiometric TiO₂ thin films with amorphous or nanocrystalline rutile structure directly from low temperature plasma processing conditions and nanocrystalline anatase structure with a moderate annealing treatment.     

Assessment of duplex coating combining plasma electrolytic oxidation and polymer layer on AZ31 magnesium alloy

Mechanical and corrosion tests were performed on a polymer-coated AZ31 magnesium alloy pre-treated by plasma electrolytic oxidation (PEO). Results were compared with a fluorotitanate–zirconate conversion coating pre-treatment. Mechanical performance was assessed by standardized adhesion (ISO 2409:2007), impact (ISO 6272‐1:2004) and impact + adhesion (ISO 6272/ASTMD2794) pass/fail tests. Corrosion behavior was monitored using electrochemical impedance spectroscopy (EIS), ac/dc/ac measurements and continuous exposure to salt fog per ASTM B117 and cyclic exposure per VDA 621‐415 (VDA). The PEO + polymer coating revealed lower impact resistance but better corrosion resistance than the Ti/Zr + polymer coating. The ac/dc/ac procedure demonstrated to be faster than EIS measurements for evaluation of the corrosion performance of studied coatings.     

Modification of graphite oxide nanoparticles prepared via electrochemically oxidizing method

Graphite oxide nanoparticles (GONPs), prepared via electrochemically oxidizing technique, were chemically reduced by hydrazine hydrate in water medium. The electrical conductivity of the samples showed an improvement by three orders of magnitude from 1.38 × 10⁻⁶ s cm⁻¹ (before reduction) to 1.02 × 10⁻³ s cm⁻¹ (after reduction). UV–vis measurement was used to explained the restoration of the conducting conjugated networks in graphite nanoparticles. The hydrophilic GONPs or their reduced samples could be further organically modified, using cetyltrimethylammoniumbromide (CTAB) as a modifier, turned to a hydrophobic nanoparticles which could form homogeneous and stable dispersion in toluene, and other organic media.     

Effect of Gd doping on magnetic, electric and dielectric properties of MgGdxFe2−xO4 ferrites processed by solid state reaction technique

MgGd_xFe_2−xO₄ (x = 0.0, 0.05, 0.1 and 0.15) ferrites, with improved dc resistivity, initial permeability, saturation magnetization, and extremely low relative loss factor, have been synthesized by solid state reaction technique. The microstructures, electric, dielectric and magnetic properties have been investigated by means of X-ray diffraction, Keithley 2611 system, impedance analyzer and VSM respectively. The addition of Gadolinium in Mg ferrite has been shown to play a crucial role in enhancing the electric, dielectric and magnetic properties. The dc resistivity is increased by two orders of magnitude as compared to Mg ferrite. Saturation magnetization has been increased by two times and remnant magnetization has been increased by more than three times due to the doping of Gd³⁺ ions in Mg ferrite. The relative loss factor was found to have very low values and is of the order of 10⁻⁴-10⁻⁵ in the frequency range 0.1-30 MHz. The variations of electric, dielectric and magnetic properties of the samples have been studied as a function of frequency and Gd³⁺ ions concentration measured at room temperature. High resistivity and improved magnetic properties can be correlated with better compositional stoichiometry and the replacement of Fe³⁺ ions by Gd³⁺ ions. The mechanisms responsible to these results have been discussed in this paper.     

Polypyrrole microstructure deposited by chemical and electrochemical methods on cotton fabrics

Electrochemically and chemically coated cotton fabrics with polypyrrole are comparatively evaluated and characterized in order to produce the conducting fabrics/textiles. The polypyrrole coated fabric is obtained electrochemically by constant current electrolysis (2 mA cm⁻²) at room temperature for 4 h. The stability, electrical conductivity and electrochemical behaviour of such composite coating are evaluated by means of SEM, FTIR, TGA, DSC, four-probe conductivity, impedance spectroscopy and cyclic voltammetry. When compared to chemical method thicker films of polypyrrole with globular microstructure could be obtained by electrochemical technique and the conductivity of the polypyrrole film was also high (1.9 × 10⁻² to 3.3 × 10⁻¹ S cm⁻¹). The weight uptake and the electrical conductivity of the coated fabric increase with concentration of pyrrole and time of electrolysis. Many other physico-chemical properties of the polypyrrole films obtained by the two methods were found to be qualitatively similar.