M. Magrini

Cerium-based chemical conversion coating on AZ63 magnesium alloy

Abstract A CeCl 3 /H 2 O 2 aqueous solution treatment is assessed for the formation of conversion coatings on a AZ63 magnesium alloy. The coating composition and morphology are examined. The conversion coating appears to consist of a thin and cracked coating with ‘dry-mud’ morphology with large agglomerates over cathodic intermetallic particles. The corrosion resistance in NaCl solution has been investigated. The cerium-based conversion process improves the pitting potential of the alloy. Better corrosion resistant surfaces are obtained when the samples are submitted to repeated immersions in the conversion bath for 30 s up to 180 s of total immersion time. The thickness of the cerium conversion coating rapidly grows up in the first 30 s; afterwards it remains nearly constant. An increase of both hydrogen peroxide concentration and immersion time produces a worsening of the alloy to corrosion.

Microstructural and compositional characterization of Roman coins

Scanning electron microscopy (SEM) with energy-dispersive microanalysis (EDS) and energy-dispersive x-ray spectrometry (EDXRS) were used to investigate the composition and microstructure of some Roman coins from the Julius Caesar and Octavianus periods. The samples were Cu-based alloys with different surface conditions. The microstructural examination supported the chemical analysis data, explaining compositional changes from the surface to the bulk. Compositional data confirm the numismatic interpretation: coins issued by Julius Caesar were orichalcum (Cu-Zn alloy) coins whereas asses issued by Octavianus were Cu-Pb-Sn coins.

Effect of process parameters of plasma electrolytic oxidation on microstructure and corrosion properties of magnesium alloys

Abstract In this work, a plasma electrolytic oxidation process was applied to AZ91 and AM50 magnesium alloys and commercially pure magnesium to produce a protective surface layer. The plasma electrolytic oxidation process was carried out in an alkaline phosphate solution with a DC power supply, using relatively high current densities and short treatment times. The influence of some important process parameters such as current density, treatment time and voltage was studied. The layers were characterised by scansion electron microscopy, X-ray diffraction and X-ray photoelectron spectrometry, in order to investigate the effect of the process parameters on the microstructure and chemical composition. The corrosion resistance properties of the obtained layers were investigated by potentiodynamic anodic polarization and electrochemical impedance spectroscopy tests. The current density, applied during the treatment, influenced the morphology and the thickness of the coatings, and, consequently, the corrosion resistance. The corrosion tests evidenced that the layers obtained with plasma electrolytic process provided a good corrosion protection to the magnesium and magnesium alloys.

Microstructural evolution of Udimet 720 superalloy

The microstructural evolution of the nickel-base superalloy Udimet 720 (Special Metals Corp., New Hartford, NY) aged at 850 °C for 1000 to 2000 h is presented. After aging, the γ′ precipitates change from cubic to globular morphology. Secondary γ′ particles and topologically close-packed phases were not found. The γ′ mean diameter increases with aging times, following the Lifshitz-Wagner model. The experimented aging times have no strong effects on mechanical properties of Udimet 720.

Cu-based amorphous alloy electrodes for fuel cells

The present work describes the characteristics of Cu–Zr and Cu–Ti amorphous alloys as catalysts for oxidation of methanol and formaldehyde in alkaline solutions. Two Cu-based amorphous alloys, Cu60Zr40 and Cu60Ti40 both prepared by melt spinning were investigated. Two types of electrode were used: as-quenched amorphous alloys and surface-activated amorphous alloys. The surface-activation treatment consisted in the immersion of the ribbons in 1 M HF solution for 30 s. The determination of the oxidation activity of methanol and formaldehyde was carried out by galvanostatic and by steady-state polarization measurements and cyclic voltammetry experiments in 1 M NaOH solutions containing CH3OH or CH2O, deoxygenated by nitrogen bubbling for 30 min at 30 °C. The HF-treated amorphous alloys exhibit catalytic activity only for formaldehyde oxidation; Zr-based alloys show higher current densities than pure crystalline copper and Ti-based alloys.

Method to improve corrosion resistance of AA 5083 by cerium based conversion coating and anodic polarisation in molybdate solution

Abstract The corrosion resistance of aluminium alloys can be improved by the formation of chemical conversion coatings. Usually, chromate pretreatment processes are used. However, because of their high toxicity, alternative methods have been studied in the last years. In the present paper, a process to improve the corrosion resistance of AA 5083 was investigated. The main process consisted of serial immersion in two solutions containing two different cerium salts, followed by a further step involving anodisation in Na2MoO4. The corrosion resistance was investigated by potentiodynamic anodic polarisation tests and electrochemical impedance spectroscopy. The morphology and composition of the oxide layers were characterised by scanning electronic microscopy equipped with an energy dispersive X-ray detector. Serial treatment in the two solutions with different cerium salts and H2O2 produced a more uniform and thicker layer with a larger amount of deposits above the alloy cathodic areas than the single two treatments. In this way, the alloy corrosion resistance in chloride environments was improved and decreased the anodic reaction rate. When anodisation in molybdate salts was added, even better results were found. The corrosion resistance was higher due to a thicker and less conductive passive layer.

Structural characterization and electrochemical behaviour of Fe-Ni-Mo-B amorphous alloys

Abstract A series of amorphous alloys obtained by the “melt-spinning” technique, with variable nickel and molybdenum contents, have been examined. Potentiodynamic anodic and cathodic polarization curves have been obtained for each of these amorphous alloys in various environments (1N HCl, 1N H 2 S0 4 , 3% NaCl, 1N NaOH). Their electrochemical properties were correlated with the amorphous structure and with their composition, characterized by means of X-ray diffraction and Mossbauer spectroscopy. Surface morphology has been observed by Scanning Electron Microscopy (SEM). The nickel content, in amorphous Fe-Ni x -Mo 4 -B 13 (balance iron) alloys enhanced the corrosion resistance in acid (IN H 2 SO 4 , 1N HCl), neutral (3% NaCl) and alkaline (1N NaOH) environments. The molybdenum content (from 4 to 8.5%) in the amorphous Fe-Ni 20 -Mo x -B 13 (balance iron) alloys increased the corrosion resistance only in alkaline solutions. In the various environments tested, the corrosion potentials tend to ennoble on increasing the nickel content. While the alloys are in the active state in acid and neutral environments, they are passive in alkaline environments. A correlation has been found between the short range order in the alloys of the Fe-Ni-Mo-B type, of similar chemical composition, and the electrochemical behaviour.

Structural characterization and electrocatalytic properties of Au30Zr70 amorphous alloy obtained by rapid quenching

The present work describes the characteristics of Au30Zr70 amorphous alloy as a precursor of a catalyst for hydrogen evolution both in acid and in basic environments and for oxidation of methanol and formaldehyde in alkaline solutions. Amorphous Au–Zr ribbons were prepared by melt spinning methods and were characterized by X-ray diffraction, optical and scanning electron microscopy. In order to obtain an active electrocatalyst, the surface of the amorphous ribbons was treated by immersion in 1 M HF solution. The results of electrocatalytic tests were compared to those obtained with untreated ribbons and also with electrodes made in polycrystalline pure gold. Moreover, some untreated samples were aged in air for 30 days before the electrochemical measurements, in order to investigate the effect of surface oxidation on the electrochemical behaviour of the alloys. The HF treatment yielded a porous structure rich in nanocrystalline gold particles which had better electrocatalytic activity than untreated ribbons or polycrystalline gold electrodes. Ageing in air produced a duplex phase structure, comprising of ZrO2 scales and nanocrystalline gold which had also improved electrocatalytic activity.

Effects of atmospheric pressure plasma JET treatment on aluminium alloys

Abstract In this paper, the effect of surface treatments carried out with an atmospheric pressure plasma jet, generated by air, on the passive film of AA1071 and AA2024 was investigated. The samples of the aluminium alloys, before and after the surface plasma treatments, were analysed by secondary ions mass spectrometry depth profiling and by X-ray photoelectron spectroscopy. Potentiodynamic anodic polarisation tests and electrochemical impedance spectroscopy measurements were carried out to study the influence of the surface plasma treatments on the corrosion resistance properties of the Al alloys. The plasma treatments induced the oxidation of the surface and the formation of a thicker and denser passive layer, which exhibited higher corrosion resistance than the native film.

Surface hardening of Ti – 6Al – 4V alloy by diffusion treatment of electroless Ni – B coatings

Abstract Electroless nickel – boron coatings on Ti – 6Al – 4V alloy were obtained by immersion in nickel chloride aqueous solutions with both dimethylamine amine borane and sodium borohydride as reducing agents. On some samples, a graphite layer was deposited by physical vapour deposition before the electroless deposition. Heat treatments at 800°C for 40 h were performed to obtain surface hardening of the titanium alloy. The effect of different deposition agents on the depth of hardening was studied by XRD, SEM, EDS and glow discharge optical spectrometry. Surface hardness >1000 HV and depth of hardening >500 mm were achieved by diffusion of layers obtained with sodium borohydride as reducing agent.