Corrosion behaviour of investment cast and friction stir processed Ti-6Al-4V

The corrosion behaviour of investment cast and friction stir (FS) processed Ti-6Al-4V alloy was studied in HCl solution. FS processing was performed with the peak temperatures both above and below the β transus. All of the samples exhibited active-passive transitions in deaerated 5% HCl at room temperature, but the β FS processed samples exhibited superior corrosion behaviour. The corrosion morphology after immersion in 20% HCl was rationalized on the basis of a difference in partitioning of the alloying elements, which controls the composition of the α and β phases.     

Electrochemical study of growth behaviour of plasma electrolytic oxidation coating on Ti6Al4V: Effects of the additive

The growth behaviour of plasma electrolytic oxidation (PEO) coating on Ti6Al4V was studied by scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization test, focusing on effects of the electrolyte additive - calcium hypophosphite. The EIS analysis of the outer layer of the PEO coating provided insight into the structure of the coating, which was confirmed by SEM results. The EIS analysis of the inner layer of the PEO coating provided information matching well with the results of the potentiodynamic polarization test.     

Corrosion resistance properties of plasma nitrided Ti-6Al-4V alloy in nitric acid solutions

The corrosion resistance of plasma nitrided Ti-6Al-4V titanium alloy in nitric acid solutions was investigated using polarisation curves, cyclic voltammetry, XRD analysis and surface microhardness. For comparison, untreated alloy samples were tested under the same conditions. While the untreated alloy shows a passive behaviour, the compound layer obtained by glow-discharge nitriding treatment, which is composed of an outer TiN and an inner Ti₂N layer, shows a high electrochemical inertia; however, if the polarisation potential is higher than a threshold or the period of immersion in high oxidising acid solution is sufficiently long, then the compound surface layer will be attacked and removed with corrosion rate higher than in the case of the untreated Ti-6Al-4V alloy. The corrosion resistance of Ti₂N layer results higher in comparison with that of TiN layer.     

Investigation of the plasma electrolytic oxidation of Ti6Al4V under single-pulse power supply

Plasma electrolytic oxidation (PEO) was studied using electrochemical impedance spectroscopy (EIS) and transient waveform analysis. First, EIS measurements on coatings prepared under constant current density were conducted in the working electrolyte. The EIS analysis showed that the coating structure changed with the termination voltage. Second, the pulse waveform of the power supply was recorded during the PEO process. It showed that a charging process occurred across the breakdown coating at the beginning of the pulse. The present study analysed this charging process in detail.     

The effects of hydrogen on fatigue crack growth behaviour of Ti-6Al-4V and Ti-4.5Al-3V-2Mo-2Fe alloys

The fatigue crack growth rates (FCGRs) of Ti-6Al-4V and Ti-4.5Al-3V-2Mo-2Fe alloys were determined in gaseous hydrogen, air, and a soft vacuum. In hydrogen and at a stress ratio of 0.1, the deflected crack path associated with the localized brittle fracture of the α-phase could account for the reduced FCGR of Ti-6Al-4V. At a higher stress ratio of 0.5, hydrogen embrittlement enhanced cracking and alleviated the effect of crack deflections in Ti-6Al-4V specimens, resulting in the FCGR in hydrogen similar to that in air. In contrast, the FCGR of Ti-4.5Al-3V-2Mo-2Fe was insensitive to both the environment and stress ratio.     

Stress corrosion cracking facet crystallography of Ti-8Al-1Mo-1V

The spatial and crystallographic orientations of facets formed during stress corrosion cracking of Ti-8Al-1Mo-1V have been characterized using quantitative fractography and electron backscatter diffraction. The results indicate that most facets are formed nearly perpendicular to the loading direction on irrational {h k i l} planes. The facets were imaged with high resolution scanning electron microscopy and were found to contain evidence of localized plastic flow despite their “brittle” appearance at moderate magnification. Some fracture planes were related to titanium hydride habit planes, however, the mechanism of faceted growth does not appear to involve hydride nucleation, growth, and fracture, but rather hydrogen enhanced localized plasticity.     

Synergistic corrosion behavior of coated Ti60 alloys with NaCl deposit in moist air at elevated temperature

In the present paper, the corrosion behavior of Ti60 alloys with an aluminide, TiAlCr, and enamel coatings in moist air containing NaCl vapor at 700-800 °C were studied. The results showed that the TiAlCr and aluminide coatings failed to protect the substrate from corrosion due to the cyclic formation of volatile products during corrosion at 800 °C. However, an uneven continuous protective Al₂O₃ scale could form on the aluminide coating during corrosion at 700 °C. And the enamel coating could protect Ti60 from corrosion due to its high thermochemical stability and matched thermal expansion coefficient with substrates of Ti-base alloys during corrosion.     

Evaluation of hydrogen absorption behaviour during acid etching for surface modification of commercial pure Ti, Ti-6Al-4V and Ni-Ti superelastic alloys

The hydrogen absorption behaviour during acid etching for the surface modification of commercial pure Ti, Ti-6Al-4V and Ni-Ti superelastic alloys has been investigated on the basis of the surface morphology, electrochemical behaviour and hydrogen thermal desorption analysis. To simulate the conventional acid etching for the improvement of the biocompatibility of Ti alloys, the specimens are immersed in 1 M HCl, 1 M H₂SO₄ or 0.5 M HCl + 0.5 M H₂SO₄ aqueous solution at 60 °C. Upon immersion, commercial pure Ti absorbs substantial amounts of hydrogen irrespective of the type of solution. In H₂SO₄ or HCl + H₂SO₄ solutions, the hydrogen absorption occurs for a short time (10 min). For Ti-6Al-4V alloy, no hydrogen absorption is observed in HCl solution, whereas hydrogen absorption occurs in other solutions. For Ni-Ti superelastic alloy, the amount of absorbed hydrogen is large, resulting in the pronounced degradation of the mechanical properties of the alloy even for an immersion time of 10 min, irrespective of the type of solution. The hydrogen absorption behaviour is not necessarily consistent with the morphologies of the surface subjected to corrosion and the shift of the corrosion potential. The hydrogen thermal desorption behaviour of commercial pure Ti and Ni-Ti superelastic alloy are sensitively changed by acid etching conditions. The present results suggest that the evaluation of hydrogen absorption is needed for each condition of acid etching, and that the conventional acid etching often leads to hydrogen embrittlement.     

Surface characterization of passive film formed on nitrogen ion implanted Ti-6Al-4V and Ti-6Al-7Nb alloys using SIMS

The aim of this paper is to study the effect of N⁺ ion implantation on corrosion and phase formation on the implanted surfaces of Ti-6Al-4V and Ti-6Al-7Nb alloys. Nitrogen ion was implanted on Ti-6Al-4V and Ti-6Al-7Nb alloys at an energy of 70 and 100 keV, respectively using a 150 keV accelerator at different doses ranging from 5 × 10¹⁵ to 2.5 × 10¹⁷ ions/cm². Electrochemical studies have been carried out in Ringer’s solution in order to determine the optimum dose that can give good corrosion resistance in a simulated body fluid condition. The implanted surfaces of such modified doses were electrochemically passivated at 1.0 V for an hour. Secondary ion mass spectroscopy was used to study and characterize titanium oxide and titanium nitride layers produced on implanted surface and to correlate them with the corrosion resistance. The nature of the passive film of the implanted-passivated specimen was compared with the unimplanted-passivated as well as as-implanted specimens.     

Influence of microstructure and alloying elements on corrosion behavior of Ti-13Nb-13Zr alloy

This work reports the effect of heat treatment on the corrosion behavior of Ti-13Nb-13Zr alloy in Ringer’s solution. The microstructural evolution of various phases after beta solution treatment (βST) and alpha + beta solution treatment (α+β ST), is studied using optical microscope, electron probe microanalysis and XRD techniques. Corrosion behavior of the solution treated samples is studied in Ringer’s solution using open circuit potential-time measurements and cyclic polarization. Corrosion studies reveal that, the heat-treated samples with depletion of Nb to very high levels in alpha phase exhibit inferior corrosion behavior. Amongst all the heat-treated samples investigated, the water quenched α+β ST specimen exhibit superior corrosion resistance due to the even distribution of the alloying elements in the three phases, namely α,α″ and β.     

Fretting corrosion behaviour of thermally oxidized CP-Ti in Ringer’s solution

The fretting corrosion behaviour of thermally oxidized (at 650 °C for 48 h) commercially pure (CP) titanium in Ringer’s solution was evaluated and reported for the first time in this paper. A cathodic shift in free corrosion potential (FCP) with the onset of fretting occurs very rapidly for untreated CP-Ti whereas it occurs over the entire 18,000 cycles for thermally oxidized (TO) CP-Ti and the extent of cathodic shift in FCP is relatively less for TO CP-Ti. The tribocorrosion performance of TO CP-Ti is better than that of untreated CP-Ti.     

Electrochemical corrosion behaviour of nanotubular Ti-13Nb-13Zr alloy in Ringer’s solution

Nanotubular oxide layer formation was achieved on biomedical grade Ti-13Nb-13Zr alloy using anodization technique in 1 M H₃PO₄ + 0.5 wt.% NaF. The as-formed and heat treated nanotubes were characterized using SEM, XRD and TEM. Corrosion behaviour of the nanotubular alloy was investigated employing potentiodynamic and potentiostatic polarization. The alloy after nanotubular oxide layer formation exhibited significantly higher corrosion current density than the bare alloy. The lower corrosion resistance of the nanotubular alloy was suggested to be associated with the distinctly separated barrier oxide/concave shaped tube bottom interface. A heat treatment at 150 °C appreciably enhanced the corrosion resistance property.     

Corrosion behaviour of laser gas nitrided Ti–6Al–4V in HCl solution

Laser surface nitriding of Ti–6Al–4V alloy was carried out with a Nd:YAG pulsed laser. The microstructure and corrosion behaviour of the nitrided samples were examined, using SEM, XRD, XPS, and anodic polarization tests in 2 M HCl solution. Laser nitriding produced a thin continuous TiN layer followed by TiN dendrites and TiN_0.3 needles. The laser nitrided specimen exhibited less corrosion current density, passivated more readily and also, maintained a lower current density over the duration of the experiment. This was correlated with the formation of very thin, continuous TiN_xO_y film, which could retard chloride ions ingress into the substrate.     

Structure and water-barrier properties of vanadate-based corrosion inhibitor films

Uniform, crack-free vanadium-based corrosion inhibitor films were grown on aluminum alloy 2024. Neutron and X-ray reflectivity were utilized to investigate the structure, water-barrier properties and speciation of the inhibitor film. The top part of the alloy participates in the formation of vanadate film. The similarity of films prepared from H₂O and D₂O implies the vanadate film is not hydrated. The film has a layered structure with vanadium enriched at the alloy interface. The film behaves as an effective water barrier when the thickness is greater than 800 Å. Films grown without K₃Fe[CN]₆ accelerator are found to be extremely thin.     

A facile approach to fabricate superhydrophobic Zn surface and its effect on corrosion resistance

A superhydrophobic Zn surface was prepared by the following facile approach. First, a simple method, water-only hydrothermal oxidation, was employed to fabricate loose-stacked ZnO nanorods with hexagonal cross-sections, resulting in a superhydrophilic Zn surface. Then the superhydrophilic Zn was modified with a low-surface-energy compound, changing the surface character from superhydrophilicity to superhydrophobicity. The superhydrophobic surface reduced the Zn corrosion rate to about one-eighth of that with an untreated surface. Such effective impact on corrosion resistance could be attributed to the obstructive effect and capillary effect caused by the surface superhydrophobicity.     

Biodegradation and corrosion behavior of manganese oxidizer Bacillus cereus ACE4 in diesel transporting pipeline

The degradation problem of petroleum products arises since hydrocarbon acts as an excellent food source for a wide variety of microorganisms. Microbial activity leads to unacceptable level of turbidity, corrosion of pipeline and souring of stored product. The present study emphasizes the role of Bacillus cereus ACE4 on degradation of diesel and its influence on corrosion of API 5LX steel. A demonstrating bacterial strain ACE4 was isolated from corrosion products and 16S rRNA gene sequence analysis showed that it has more than 99% similarity with B. cereus. The biodegradation and corrosion studies revealed that B. cereus degraded the aliphatic protons and aromatic protons in diesel and is capable of oxidizing ferrous/manganese into oxides. This is the first report that discloses the involvement of manganese oxidizer B. cereus ACE4 on biodegradation of diesel and its influence on corrosion in a tropical country pipeline.     

Point defect model and corrosion of anodic oxide coatings on Ti-6Al-4V

Electrodeposited anodic oxide coatings were produced on Ti-6Al-4V substrates using (i) phosphoric acid and (ii) aqueous electrolyte containing dissolved calcium and phosphorus. Different coatings were produced by varying the time periods. The coatings contained lower oxides of titanium and small quantities of phosphorus and/or calcium deposited from the electrolyte. Coatings produced from shorter duration had higher resistance to corrosion by simulated body fluid. Point defect model is obeyed by all the coatings. As postulated by this model, coatings containing calcium are attacked more by simulated body fluid than coatings which do not contain calcium.     

Detrimental effect of particle sol-gel coating on the corrosion behavior of A380-SiC composite

In this study, the corrosion susceptibility of aluminum matrix composites reinforced with artificially oxidized SiO₂ and sol-gel Fe/TiO₂ coated silicon carbide particles (SiC_p) has been investigated. Corrosion behavior of the composites, fabricated by the liquid metal infiltration technique, was established in chloride containing alkaline environments by cyclic polarization (CP) and electrochemical impedance spectroscopy (EIS) techniques. It has been found that, sol-gel coating of SiC particles with Fe/TiO₂ has a detrimental effect on the corrosion characteristics of A380-SiC metal matrix composite.     

The effect of atmospheric corona treatment on AA1050 aluminium

The effect of atmospheric corona discharge on AA1050 aluminium surface was investigated using electrochemical polarization, SEM-EDX, FIB-SEM, and XPS. The corona treatment was performed with varying time (1, 5, and 15 min) in atmospheric air. A 200 nm oxide layer was generated on AA1050 after the 15 min air corona treatment. A significant reduction in anodic and cathodic reactivities was observed starting from 1 min exposure, which further decreased with prolonged exposure (15 min) and after delayed testing (after 30 days). The reduction in surface reactivity is due to the formation of thicker and denser oxide film.     

Microstructure, spallation and corrosion of plasma sprayed Al2O3–13%TiO2 coatings

The electrochemical corrosion behaviours of the steel substrates coated with three different plasma sprayed Al₂O₃–13%TiO₂ coatings were studied in this paper. The three kinds of Al₂O₃–13%TiO₂ coatings were conventional ME coating, nanostructured NP coating and NS coating. There were micro cracks, laminar splats and straight columnar grains in ME coating. For the two nanostructured coatings, the laminar microstructure and columnar grains were not obvious. The NP coating had the highest hardness and spallation resistance. Electrochemical corrosion behaviour of the three coatings was mainly investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in aqueous Na₂SO₄ solution.