Trivalent chromium conversion (TCC) coating formation on AZ91D magnesium alloy has been investigated using scanning electron microscopy, energy X-ray dispersive spectroscopy (EDS), X-ray photoelectron spectroscoy (XPS). The coating was formed in a commercial SurTec 650 bath. XPS showed that coatings contained mainly Cr, Zr, O, S and F species, which were assigned to hydrated Cr(III) (Cr(OH)3/CrOOH), ZrO2, Cr2(SO4)3, and CrF3, and lower amounts of Mg2+ and Al3+ species assigned to oxide, hydroxide and fluoride. The Cr/Zr atomic ratio was around 0.5. The coating was formed above both alpha matrix and 13 (Mg17Al12) grain boundary regions, and it was significantly thicker above the cathodic 13 phase. Raman spectroscopy evidenced the presence of Cr(VI) species, formed due to oxidation of Cr(III) by H2O2, particularly in the coating above the cathodic sites. Electrochemical impedance spectroscopy revealed that corrosion protection was provided mainly by the barrier layer at the base of the coating, which resulted in a reduction by about a factor of 2 in the anodic and cathodic current density in potentiodynamic polarization curves.

The electroassisted (EA) deposition of trivalent chromium conversion coatings on aluminium is investigated with a focus on the influence of applied potential on the chromium valence state, as determined by Raman spectroscopy. The morphology and compositions of the coatings were investigated by scanning electron microscopy and energy-dispersive spectroscopy. The EA coatings were formed in naturally-oxygenated SurTec 650 chromitAl solution at constant potentials of -1.5 and -0.5 V-SCE. The coatings contained chromium and zirconium constituents. The potentials resulted in net cathodic and anodic current densities, respectively, during the coating growth. Comparisons were made with coating formation at the open-circuit potential (OCP). The coating thickness increased in order -0.5 V-SCE < OCP < -1.5 V-SCE, a result of increasing alkalinity from the cathodic reaction that facilitates deposition of the coating constituents. Fresh coatings formed at -1.5 V-SCE revealed the presence of Cr6+ species. By contrast, Cr6+ species were not resolved in the coating formed at -0.5 V-SCE. It is proposed that less H2O2 is generated at -0.5 V-SCE to oxidize Cr3+ coating species.

Surges in the cell potential, due to an increased overpotential for hydrogen evolution, and transitions in ceramic oxide coating morphology during plasma electrolytic oxidation (PEO) of tantalum under a pulsed bipolar current regime at 1000 Hz in a silicate electrolyte are investigated using real-time imaging of gas evolution, analytical scanning electron microscopy, X-ray photoelectron spectroscopy and supplementary potential-controlled electrochemical measurements. The coatings, which contained Ta2O5, TaO and incorporated silicon species, revealed a nodular morphology that transformed with treatment time to a "pancake" type and then a "coral reef" type. The first potential surge occurred only in the cathodic potential, coinciding with an increased spark intensity, more vigorous gas evolution, emergence of "pancake" structures and a reduction in the coating porosity. The later increases in both the anodic and cathodic potential, coincided with intensification of the sparking, the establishment of silicon-rich "coral reef" structures, and formation of a comparatively thick coating. The kinetics of coating growth differed significantly between the three morphological stages. Electrochemical measurements showed that anodic discharges increased the overpotential for hydrogen evolution in the subsequent cathodic pulse, which is proposed to be due to gas impeding the coating and at and near the coating surface increasing the resistance to ionic transport.

Plasma electrolytic oxidation (PEO) has been carried out under unipolar and bipolar pulsed current regimes to investigate the formation of black ceramic coatings on AZ31 magnesium alloy in an aluminate-tungstate electrolyte at frequencies of 100, 1000 and 2000 Hz. The coatings were characterized by scanning electron microscopy (SEM) assisted by energy dispersive X-ray spectroscopy (EDS), and micro-discharges were investigated by real-time imaging and photomultiplier methods. Coating color was quantified by the CIELAB method. Blackness of coatings increased with frequency and tungsten incorporation, with blackest coatings obtained under unipolar conditions. Blacker coatings also revealed increased roughness and large internal pores, which were attributed to stronger plasma discharges and greater gas evolution. Continuing light emission after termination of each anodic current pulse suggested that sites of high coating temperature may facilitate discharge initiation in the subsequent anodic pulse when the cycle time is short. Large porosity and increased voltage indicated cathodic discharges also occur at high frequency when the coating is sufficiently thick. However, it seems that the cathodic discharges have no effect on the blackness of the coating.

The prospective use of nanotechnology for medical devices is increasing. While the impact of material surface nanopatterning on the biological response is convincing, creating a large surface area with such nanotechnology remains an unmet challenge. In this paper, we describe, for the first time, a reproducible scale-up manufacturing technique for creating controlled nanotubes on the surfaces of Ti and Ti alloys. We describe an average of approximately 7.5-fold increase in cost and time efficiency with regards to the generation of 20, 50, and 100 nm diameter nanotubes using an anodisation technique. These novel materials have great potential in the medical field through their influence on cellular activity, in particular, protein absorption, focal adhesion, and osteoinduction. In this paper, we provide a step-by-step guide to optimise an anodisation system, starting with design rationale, proof of concept, device upscaling, consistency, and reproducibility check, followed by cost and efficiency analysis. We show that the optimised device can produce a high number of anodised specimens with customisable specimen shape at reduced cost and time, without compromising the repeatability and consistency. The device can fabricate highly uniform and vertically oriented TiO 2 nanotube layer with desired pore diameters.

The effects of applied current density, anodizing time, and electrolyte temperature on the cell and pore morphology of anodic films and the voltage‐time response obtained during galvanostatic anodizing of AA2024‐T3 alloy in sulphuric acid electrolytes have been studied. Scanning electron microscopy was employed to observe the film morphology. Sponge‐like porous structure was promoted by anodizing at relatively low current density and high electrolyte temperature. In contrast, linear porous structure was favoured under the converse conditions. Intermediate conditions resulted in films containing either sequential layers of the 2 morphologies or a morphology incorporating features of the 2 types; such conditions were associated with anodizing voltages in the range 25 to 35 V. The reasons for the morphological differences are proposed to be due to interactions between film growth stresses and stresses arising from oxygen evolution on the development of the alumina cells.

Copper-enriched layers were developed onto aluminum-copper alloys using alkaline etching in sodium hydroxide, for both, sputter deposited and bulk conditions. Enriched alloys were evaluated by potentiodynamic polarization in sodium chloride solution in order to determine the effect of the enriched layers on the pitting potential of the alloys. Rutherford backscattering spectroscopy was employed to quantify the enrichments and their locations just beneath the alumina-based oxides remaining from the etching. For the sputter deposited aluminum-copper alloys, the results show some scattering of the pitting potential data, and no correlation between pitting potential and the alloy enriched layer. In the case of bulk Al-2wt.%Cu alloy, with the copper in solid-solution, the pitting potential increased for the enriched specimens, indicating also a different pit morphology, with respect to the non-enriched alloy.

The porosity within plasma electrolytic oxidation (PEO) coatings strongly affects their properties, such as the wear resistance. Two typical types of coatings, i.e. bi-layered coatings with large internal pores, which show low wear resistance, and single-layered coatings with excellent wear performance, have been found following PEO of aluminium alloys under pulsed current regimes with an aluminate electrolyte. In this paper, the mechanisms of formation of the different coatings are investigated and discussed based on a thorough investigation of PEO of an Al–Cu–Li alloy using systematic variations of 1000Hz pulsed current waveforms and electrolyte concentration. Both parameters have important roles in determining the structure of the resultant coatings. The coatings formed in a dilute electrolyte, containing 5gl−1 NaAlO2, were bi-layered and contained both large pores and pancake structures irrespective of the application of a negative pulse. In contrast, the application of negative current pulse favored the formation of single-layered coatings that contained fewer pancake structures in an electrolyte 32gl−1 NaAlO2. A more concentrated electrolyte, containing 56gl−1 NaAlO2, resulted in relatively compact, single-layered coatings independently of a negative current pulse. Similar observations for the coating morphologies were made if the alloy was replaced by high purity aluminium and also for the alloy when the frequency was reduced to 100Hz. Two different models for the growth of single- and bi-layered coatings are proposed.

The influence of a low concentration of strontium chromate on the corrosion inhibition of superpure aluminium and AA2014‐T6 aluminium alloy in 0.6 m chloride solution has been investigated to simulate the leaching process of inhibitors from coatings. The potential‐time and polarisation behaviour show influences on both the anodic and cathodic kinetics on the superpure aluminium surface. However, predominant cathodic inhibition was observed for the AA2014‐T6 aluminium alloy. It was evident that chromium species were reduced at cathodic second phase particles, forming a thin passive film at the cathodic sites, which blocks the oxygen reduction reaction and, consequently, provides effective corrosion inhibition. Copyright © 2015 The Authors Surface and Interface Analysis Published by John Wiley & Sons Ltd.

Plasma electrolytic oxidation (PEO) is of increasing interest for the formation of ceramic coatings on metals for applications that require diverse coating properties, such as wear and corrosion resistance, low thermal conductivity, and biocompatibility. Porosity in the coatings can have an important impact on the coating performance. However, the quantification of the porosity in coatings can be difficult due to the wide range of pore sizes and the complexity of the coating morphology. In this work, a PEO coating formed on titanium is examined using high resolution X-ray computed tomography (X-ray CT). The observations are validated by comparisons of surface views and cross-sectional views of specific coating features obtained using X-ray CT and scanning electron microscopy. The X-ray CT technique is shown to be capable of resolving pores with volumes of at least 6 μm(3). Furthermore, the shapes of large pores are revealed and a correlation is demonstrated between the locations of the pores, nodules on the coating surface, and depressions in the titanium substrate. The locations and morphologies of the pores, which constitute 5.7% of the coating volume, indicate that they are generated by release of oxygen gas from the molten coating.

Self-organized porous anodic oxide films are produced when reactive metals such as aluminum and titanium are electrochemically oxidized in baths that dissolve the oxide. The role of oxide stress in the initiation of pores in anodic aluminum oxide was investigated, for constant current anodizing in phosphoric acid. Through-thickness profiles of the in-plane stress in the oxide were measured by in-situ monitoring of stress change during open circuit dissolution following anodizing. During barrier oxide growth prior to pore formation, compressive stress accumulated to several GPa within a 3–5nm thick layer at the oxide surface, while stress in the interior of the oxide was relaxed. Oxide composition measurements revealed elevated concentrations of incorporated phosphate ions in the same region, indicating that stress is generated by field-driven anion incorporation. Pores initiate when surface stress reaches a maximum, and is accompanied by oxide flow establishing the pore shape. It is suggested that pores are created by a flow instability caused by spatially nonuniform near-surface compressive stress.

This study demonstrates the formation of uniform barrier-type anodic films on magnetron-sputtered magnesium films at high current efficiency in ethylene glycol electrolytes containing 0.1 mol dm−3 NH4F and various concentrations (0.1–28 mol dm−3) of H2O. The anodic films containing a crystalline MgF2 phase develop both at the metal/film and film/electrolyte interfaces due to simultaneous migrations of anions inwards and cations outwards, respectively. When a Mg −1.2 at% Au/Mg bilayer film is anodized, initial prior oxidation of magnesium proceeds with gold atoms accumulating in a thin layer beneath the anodic film. The accumulated gold atoms are incorporated into the anodic film as a band when the alloy layer is completely anodized. Fluoride-containing gold species are formed by the incorporation and the gold species migrate outwards at a rate of 0.4 times the rate of Mg2+ ions. The addition of phosphate in the electrolyte results in the formation of an amorphous anodic film, and the phosphate incorporated into the anodic film is distributed throughout the film thickness. The transport number of cations is also influenced by the phosphate incorporation.

Anodic oxidation of reactive metals such as Al and Ti produces oxide films with self-organized arrangements of nanoscale pores. Stress-driven mass transport of oxide is considered to play an important role in pore formation and self-ordering. Using in situ stress monitoring during both anodizing and subsequent open-circuit oxide dissolution, distributions of in-plane residual stress were measured in anodic alumina films formed by galvanostatic anodizing in phosphoric acid. Anodizing produced significant stress both in the oxide and at the metal-oxide interface. For oxides grown to 20nm thickness, the oxide stress was tensile below 3mA/cm(2) and compressive above this current density, while the interface stress exhibited the opposite dependence. Stress generation correlated with interfacial volume change due to reactions and transport processes: oxide or interface stress was compressive when interfacial volume was created, and vice versa. Compressive stress buildup in the oxide is apparently required for self-ordered pore formation by flow-assisted mechanisms. From the present results, a simple criterion was derived specifying the conditions for compressive stress and pore formation in terms of parameters governing film composition, ionic transport and interfacial reaction kinetics. (C) 2015 Elsevier Ltd. All rights reserved.

The present study reports, for the first time, the highly efficient formation of barrier-type anodic films, with flat and parallel metal/film and film/electrolyte interfaces, on magnesium in ethylene glycol electrolytes containing ammonium fluoride and water. The anodizing voltage increases linearly with time during galvanostatic anodizing at 10Am−2 up to 350V. The anodic film formed to 200V is 247nm thick, containing a crystalline MgF2 phase. Analysis by Rutherford backscattering spectroscopy discloses the film composition of MgF1.8O0.1 and Pilling–Bedworth ratio (PBR) of 1.67. The PBR value greater than unity and the formation of chemically stable fluoride-based films may contribute to the film growth at high current efficiency.

Anodic oxide growths on commercially pure titanium in 1 M sulphuric and phosphoric acids have been investigated in the present study. Ruptures and blisters of the oxide layers were found as result of a huge pressure of oxygen bubbles developed within the film. The ruptures were more pronounced when higher anodic voltages are reached for both acid anodizations. The oxygen evolution was probably inhibited more significantly for phosphoric acid anodization compared to sulphuric acid anodization. The anodic oxides also showed more ruptures after anodized titanium specimens were immersed for 60 days in naturally aerated 3.5% NaCl electrolyte in higher anodic voltage compared to the immediately 'just' immersed conditions in the NaCl electrolyte. The corrosion behaviours of the anodic oxides were studied in the 3.5% NaCl by potentiodynamic polarization and electrochemical impedance spectroscopy. Increased corrosion resistance of the anodic oxides on titanium in NaCl were observed with increase in anodic voltages. The anodic oxide produced in the phosphoric acid showed a higher corrosion resistance than that of the sulphuric acid at each anodizing condition; indicates that the corrosion protection by anodizing for titanium in phosphoric acid should probably be considered in priority compared with sulphuric acid.

Aircraft coatings are generally multi-layered with both barrier properties and are corrosion inhibitor filled to prevent corrosion. A new technique, automated in situ block face ultramicrotome imaging using an FEG-SEM has been employed for high resolution 3-D imaging of such aircraft coatings. A pristine and a cracked aged coating were visualised by this method clearly showing the coatings microstructure and subsequent crack growth properties. Filler particles were segmented and labelled allowing the coating to be more clearly visualised and enabling statistical particle analysis and meshing. Finally the method was used to investigate the inhibitor leaching properties of four aircraft primers. Crown Copyright (C) 2013 Published by Elsevier Ltd. All rights reserved.