Oxidation resistance and corrosion behavior of hot-dip aluminized coatings on commercial-purity titanium

Aluminizing is an effective method to protect alloys from oxidation and corrosion. In this article, the microstructure, morphology, phase composition of the aluminized layers and the oxide films were investigated by SEM, EDS and X-ray diffraction. The high temperature oxidation resistance and electrochemical behavior of hot dip aluminizing coatings on commercial-purity titanium had been studied by cyclic oxidation test and potentiodynamic polarization technique. The results show that the reaction between the titanium and the molten aluminum leads to form an aluminum coating which almost has the composition of the aluminum bath. After diffusion annealing at 950 °C for 6 h, the aluminum coating transformed into a composite layer, which was composed of an inner layer and an outer layer. The inner layer was identified as Ti₃Al or Ti₂Al phase, and the outer layer was TiAl₃ and Al₂O₃ phase. The cyclic oxidation treatment at 1000 °C for 51 h shows that the oxidation resistance of the diffused titanium is 13 times more than the bare titanium. And the formation of TiAl₃, θ-Al₂O₃ and compact α-Al₂O₃ at the outer layer was thought to account for the improvement of the oxidation resistance at high temperature. However, the corrosion resistance of the aluminized titanium and the diffused titanium were reduced in 3.5 wt.% NaCl solution. The corrosion resistance of the aluminized titanium was only one third of bare titanium. Moreover, the corrosion resistance of the diffused titanium was far less than bare titanium.     

Development of sintered Ti-30mass%Mo alloy and its corrosion properties

Ti-30mass%Mo alloy is characterized by far better corrosion resistance than pure titanium in non-oxidizing acid such as 35% HC1 solution. This alloy was newly developed in this work by a powder metallurgical process, because the conventional melting-casting process had difficulties in manufacturing the alloy owing to its heavy gravity segregation. The corrosion behavior of sintered Ti-30Mo alloy was studied by both immersion test and electro-chemical measurement. The corrosion rate of 1.3X10⁻²[mm/yearxxx] was obtained by immersion test in 35% HC1 solution, which was 10³ times superior to that of pure titanium. The potentio-dynamic polarization curve of this alloy in 35% HC1 solution showed more noble corrosion potential and lower anodic current density than those of pure titanium. The passive film formed on the alloy, in which Mo was enriched by preferential dissolution of titanium, thought to suppress the anodic reaction and improve its corrosion resistance.     

Crevice corrosion monitoring of titanium and its alloys using microelectrodes

Crevice corrosion of titanium and its alloys in 10% sodium chloride was investigated at 100°C with the aid of microelectrodes. Potential, pH and chloride ion concentration inside the crevice were monitored using an Ag/AgCl electrode, a tungsten microelectrode and a Ag/AgCl chloride ion selective microelectrode, respectively. The pH and Cl^? concentrations within the crevice were calculated from the standard potential‐pH and potential‐log[Cl^?] calibration curves. The effect of Mo on the crevice corrosion of titanium was also studied. The passivation behavior on the titanium and Ti‐15%Mo alloy was studied using electrochemical impedance studies. There was no apparent change in pH and Cl^? ion activity inside the crevice for the alloy at 100°C, whereas a marginal decrease in pH and increase in Cl^? ion concentration were observed for pure titanium. Thus pure titanium is susceptible to crevice corrosion in hot 10% NaCl solutions at 100°C. The chloride ion activity was found to be reduced for the alloy so that the pH inside the crevice increased. The corrosion reaction resistance (R_t) was found to increase with the addition of Mo as an alloying element. It also increases with externally applied anodic potential. Hence, Mo is an effective alloying element, which enhances the crevice corrosion resistance of titanium.     

Low-Temperature Interface Reaction Between Titanium and the Eutectic Silver-Copper Brazing Alloy

Reaction zones formed at 790 °C between solid titanium and liquid Ag-Cu eutectic alloys (pure and Ti-saturated) have been characterized. When pure Ag-Cu eutectic alloy with 40 at.% Cu is used, the interface reaction layer sequence is: αTi/Ti₂Cu/TiCu/Ti₃Cu₄/TiCu₄/L. Because of the fast dissolution rate of Ti in the alloy, the reaction zone remains very thin (3-6 μm) whatever the reaction time. When the Ag-Cu eutectic alloy is saturated in titanium, dissolution no longer proceeds and a thicker reaction zone with a more complex layer sequence grows as the reaction time increases. Four elementary chemical interaction processes have been identified in addition to Ti dissolution in the liquid alloy. These are growth of reaction layers on Ti by solid state diffusion, nucleation and growth from the liquid of TiCu₄, isothermal solidification of silver and, finally, chemical conversion of the Cu-Ti compounds by reaction-diffusion in the solid state. A mechanism combining these processes is proposed to account for the constitution of Ti/Ag-Cu/Ti joints brazed at 780-800 °C.     

The Corrosion of Titanium Aluminides in H2/H2S/H2O Atmospheres at 800–1000°C

The corrosion behavior of three Ti–Al intermetallics containing 20, 30, and 40 wt.% Al was studied over the temperature range 800–1000°C in a H₂/H₂S/H₂O gas mixture. Ti–20Al and Ti–40Al alloys had the single-phase structure of Ti₃Al and TiAl, respectively, while Ti–30Al was a two-phase mixture of Ti₃Al+TiAl. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants increased with increasing temperature, but decreased with increasing Al content. The Ti–40Al alloy exhibited the best corrosion resistance among all alloys studied. The scales formed on Ti–Al intermetallics were heterophasic and duplex, consisting of an outer-scale layer of pure -TiO₂ and an inner layer of -TiO₂ with minor amounts of -Al₂O₃ and Ti_l-xS. The amount of -Al₂O₃, which increased with increasing Al content, is responsible for the reduction of the corrosion rates as compared with those of pure Ti oxides.     

Corrosion of Ni-Ti alloys in the molten (Li,K)2CO3 eutectic mixture

The corrosion of pure Ni and of binary Ni-Ti alloys containing 5, 10, and 15 wt.% Ti respectively in molten (0.62Li,0.38K)₂CO₃ at 650°C under air has been studied. The corrosion of the single-phase Ni-5Ti alloy was slower than that of pure Ni, forming an external scale composed of NiO and TiO₂. The two-phase Ni-10Ti and Ni-15Ti alloys underwent much faster corrosion than pure Ni, producing an external scale containing NiO and TiO₂, and a thick internal oxidation zone of titanium mainly involving the intermetallic compound TiNi₃ in the original alloys. The rates of growth of the external scales for the Ni-Ti alloys were reduced with the increase of their titanium content, while the internal oxidation was significantly enhanced. The corrosion mechanism of the alloys is also discussed.     

Low-melting-point titanium-base brazing alloys—part 1: Characteristics of two-, three-, and four-component filler metals

The melting point, microstructure, phase, and electrochemical behavior of Ti-21Ni-15Cu alloy, together with two-, three-, and four-component low-melting-point titanium-base brazing alloys, are presented in this paper. Five filler metals were selected for the study, in which melting points were measured by differential thermal analysis, phases identified by x-ray diffractometry, and corrosion behaviors tested by potentiodynamic polarization. The experimental results show that the three-component Ti-15Cu-15Ni and the newly developed Ti-21Ni-14Cu alloys exhibit the combination of lower melting point and superior corrosion resistance compared to the two-and four-component titanium alloys, 316L stainless steel, and a Co-Cr-Mo alloy in Hank’s solution at 37 °C. On a short time basis, the presence of Ti₂Ni and Ti₂Cu intermetallics in the Ti-15Cu-15Ni and Ti-21Ni-14Cu alloys should not be preferentially dissolved in galvanic corrosion with respect to the dissimilar Ti-6Al-4V alloy.     

Performance studies of bare and Co-plated titanium alloy as cathode current collector in Molten Carbonate Fuel Cell (MCFC)

The corrosion characteristics of bare, heat treated and cobalt coated titanium alloys were studied and compared with that of SS 316 in molten carbonates (Li/K = 62/38 vol.%) at 650 °C under oxygen atmosphere using electrochemical and surface characterization techniques. Immersion test of titanium alloys conducted in cathode environment followed by atomic absorption spectroscopy (AAS) indicated leaching of molybdenum from the alloy. Coating the alloy with Co was found to decrease the molybdenum dissolution rate. X-ray diffraction results showed the formation of LiTiO₂ and Li₂TiO₃ on the surface of the titanium alloys and formation of LiFeO₂ and Fe₂O₃ in the case of SS 316. SEM and EDAX analysis of the post-test samples revealed the loss of Mo, Sn and Zr from the titanium alloys and loss of Cr and Ni from SS 316. Electrochemical studies showed that the conductivity of the corrosion scale was higher for cobalt electroplated alloy when compared to other titanium alloys and lower than that of SS 316. Cobalt coated titanium alloy exhibited higher polarization resistance than other alloys. The present study confirmed that the surface modification of titanium alloy lead to the formation of a protective layer with better corrosion barrier properties and better electronic conductivity in molten carbonate fuel cell cathode operating conditions.     

Effects of alloying elements on the mechanical properties and corrosion behaviors of 2205 duplex stainless steels

The effects of alloying elements on the microstructure, mechanical properties, and corrosion behaviors of duplex stainless steels (DSSs) have been investigated in this study. Experimental alloys were prepared by varying the concentrations of the constituent elements in DSSs. Hot ductility test, tensile test, charpy impact test, and corrosion test were performed to evaluate the properties of the experimental alloys. The results showed that the extent of edge cracking of DSSs increased with the increasing value of the crack sensitivity index (CSI). The higher the hot ductility index (HDI) was, the better the hot ductility of DSSs achieved. Austenite (γ) stabilizer generally caused a decrease in the strength and an increase in the charpy impact absorbed energy of the stainless steel. On the contrary, ferrite (α) former exerted its beneficial effect on the strength but became detrimental to the toughness of DSSs. The presences of sulfur and boron also caused a decrease in the impact energy, but nitrogen and carbon hardly affected the toughness within the concentration range tested in this study. The value of pitting nucleation potential (E _np ) of different nitrogen contents in 3.5 wt.% NaCl solution at room temperature was almost the same, but the value of pitting protection potential (E _pp ) among these alloys was increased with increasing the content of nitrogen. The susceptibility to stress corrosion cracking (SCC) of DSSs was high when tested in boiling 45 wt.% MgCl₂ solution. On the other hand, the time to failure of the experimental steels in 40 wt.% CaCl₂ solution at 100 °C was longer than that in MgCl₂ solution. Nitrogen could affect the SCC behavior of DSSs in CaCl₂ solution through the combinative effects by varying the pitting resistance and the slip step dissolution. An optimum nitrogen (N) content of 0.15 wt.% was found where the highest SCC resistance could be obtained. Although γ phase exhibited better resistance to SCC, cracks were found to penetrate through α and γ grains or to propagate along the α/γ interface. As a result, a mixed transgranular plus intergranular mode of fracture surface was observed.     

Optimization of Oxidation Temperature for Commercially Pure Titanium to Achieve Improved Corrosion Resistance

Thermal oxidation of commercially pure titanium (cp-Ti) was carried out at different temperatures, ranging from 200 to 900 °C to achieve optimum corrosion resistance of the thermally treated surface in simulated body fluid. Scanning electron microscopy, x-ray diffraction, Raman spectroscopy and electrochemical impedance spectroscopy techniques were used to characterize the oxides and assess their protective properties exposed in the test electrolyte. Maximum resistance toward corrosion was observed for samples oxidized at 500 °C. This was attributed to the formation of a composite layer of oxides at this temperature comprising Ti₂O₃ (titanium sesquioxide), anatase and rutile phases of TiO₂ on the surface of cp-Ti. Formation of an intact and pore-free oxide-substrate interface also improved its corrosion resistance.     

Effect ofβ volume fraction on the dynamic grain growth during superplastic deformation of Ti3Al-based alloys

The superplastic deformation behavior of Ti₃Al based(α ₂+β alloy was studied with respect to the volume fraction of α₂/β. Three alloys containing 21, 50 and 77% in volume fractions ofβ exhibited large tensile elongations of over 500% at 970°C with a strain rate of 2.5x10^-4 sec^-1. The largest elongation was observed in the alloy with 21% ofβ. As the volume fraction ofβ phase increased, the flow stress and correspondingly, the strain-rate sensitivity values decreased. Due to the higher diffusivity of Ti in,β phase than in α₂ phase, the increase inβ volume fraction from 21 % to 77% accelerated the dynamic grain growth, and degraded the superplasticity of the Ti₃Al-based alloys. The strain-based grain growth behavior was quantitatively analyzed and incorporated into a constitutive equation. The calculated flow curves are in agreement with the experimental ones in the stable deformation region.     

Effect of solid solution treatment on in vitro degradation rate of as-extruded Mg-Zn-Ag alloys

The degradation behaviors of the as-extruded and solution treated Mg-3Zn-xAg (x=0, 1, 3, mass fraction, %) alloys, as well as as-extruded pure Mg, have been investigated by immersion tests in simulated body fluid (SBF) at 37 °C. The as-extruded Mg-Zn(-Ag) alloys contained Mg₅₁Zn₂₀ and Ag₁₇Mg₅₄. While the quasi-single phase Mg-Zn(-Ag) alloys were obtained by solution treatment at 400 °C for 8 h. The quasi-single phase Mg-Zn(-Ag) alloys showed lower degradation rate and more homogeneous degradation than corresponding as-extruded Mg alloys. Degradation rate of solid-solution treated Mg-3Zn-1Ag and Mg-3Zn-3Ag was approximately half that of corresponding as-extruded Mg alloy. Moreover, the degradation rate of solid-solution treated Mg-3Zn and Mg-3Zn-1Ag was equivalent to that of as-extruded pure Mg. However, heterogeneous degradation also occurred in quasi-single phase Mg-Zn-Ag alloys, compared to pure Mg. So, preparing complete single-phase Mg alloys could be a potential and feasible way to improve the corrosion resistance.     

Tribological properties of titanium aluminides coatings produced on pure Ti by laser surface alloying

Titanium aluminides coatings were in-situ synthesized on a pure Ti substrate with a preplaced Al powder layer by laser surface alloying. The friction and wear properties of the titanium aluminides coatings at different normal loads and sliding speeds were investigated. It was found that the hardness of the titanium aluminides coatings was in the following order: Ti₃Al coating > TiAl coating > TiAl₃ coating. Friction and wear tests revealed that, at a given sliding speed of 0.10 m/s, the wear volume of pure Ti and the titanium aluminum coatings all increased with increasing normal load. At a given normal load of 2 N, for pure Ti, its wear volume increased with increasing sliding speed; for the titanium aluminides coatings, the wear volume of Ti₃Al coating and TiAl coating first increased and then decreased, while the wear volume of TiAl₃ coating first decreased and then increased with increasing sliding speed. In addition, the friction coefficients of pure Ti and the titanium aluminides coating decreased drastically with increasing sliding speed. Under the same dry sliding test conditions, the wear resistance of the titanium aluminium coatings was in the following order: Ti₃Al coating > TiAl coating > TiAl₃ coating.     

Untersuchungen über das Korrosionsverhalten von Zirkoniumlegierungen. III. Untersuchungen an Zirkonium-Titan-Legierungen

Investigations into the corrosion behaviour of zirconium alloys III. Investigations on zirconium titanium alloys Investigations on zirconium alloys containing up to 4OO% titanium and, eventually, up to 10% Nb or Mo and small amounts of Fe, Ni and Cr (together 1.5% maximum) have revealed that titanium increases the dissolution power of zirconium for other alloying elements without tending to form two-phase structures. As to corrosion behaviour the better alloys are somewhat superior to other alloys. While titanium impairs the corrosion resistance of pure zirconium with respect to boiling mineral acids (20% HCl, sulphuric or nitric acids) titanium additions up to 10% of improve the corrosion behaviour of ZrNb and ZrMo alloys; alloys of the type Zr10Nb10Ti or Zr10Ti10Ta are comparable, with respect to the corrosion resistance, to pure zirconium. The scaling resistance of zirconium passes through a minimum at a five to ten percent Ti and arrives at its maximum value with 40% Ti. Further improvements may be obtained by addition of 10% Nb.     

Electrochemical impedance spectroscopic studies of titanium and its alloys in saline medium

The influence of potential on electrochemical behaviour of pure Ti, Ti‐6Al‐7Nb, and Ti‐6Al‐4V ELI alloy under saline conditions were investigated by electrochemical impedance spectroscopy (EIS). All measurements were carried out in saline solution (0.9% NaCl) at different impressed potentials (corrosion potential (E_corr), 0 mV (SCE) and + 500 mV (SCE)) for 1 h. The experimental results were compared with those obtained by potentiodynamic polarization curves. The corrosion current densities obtained for the titanium alloys showed lower values than for pure Ti, indicating the formation of a stable passive film with time. Electrochemical impedance spectroscopic studies indicated that the resistance of the passive film increased with the impressed potential. The impedance spectra were fitted using a non‐linear least square (NLLS) fitting procedure. The magnitude of the corrosion resistance of titanium alloys under saline solution was compared and results are presented.     

The anomalies of the properties of titanium alloys with β stabilizing elements

Conclusion From measurements of the resistivity, thermal conductivity, and thermo-emf of titanium alloys with vanadium, niobium, tantalum, and molybdenum it was established that compounds Ti₃V, Ti₃Nb, Ti₃Ta, and Ti₇Mo can exist at temperatures below 600°C, which is confirmed by the anomaly in the variation of the physical properties of the alloys with compositions matching the given stoichiometric ratio of components. Under certain conditions these compounds can be precipitated in pure form.TsNIIChERMET. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 52–54, April, 1968.     

Air Oxidation of Ni–Ti Alloys at 650–850°C

The oxidation of pure Ni and three Ni–Ti alloys containing 5, 10, and 15 wt.% Ti over the temperature range 650–850°C in air was studied to examine the effect of titanium on the oxidation resistance of pure nickel. Ni–5Ti is a single-phase solid solution, while the other two alloys consisted of nickel solid solution (-Ni) and TiNi₃. The oxidation of Ni–Ti alloys at 650°C follows an approximately parabolic rate law and produces a decrease in the oxidation rate of pure Ni by forming an almost pure TiO₂ scale. At higher temperatures, Ni–Ti alloys also follow an approximately parabolic oxidation, and their oxidation rates are close to or faster than those of pure Ni. Duplex scales containing NiO, NiTiO₃ and TiO₂ formed. Some internal oxides of titanium formed, especially at 850°C. In addition, the two-phase structure of Ni–10Ti and Ni–15Ti was transformed into a single-phase structure beneath the scales.     

Electrochemical stability of TiO2 nanotubes with different diameters in artificial saliva

Recently, titanium and titanium alloys with nanotube layers by anodizing process have gained great interests as surgical implant materials. In this present paper, their electrochemical stability of self-organized TiO₂ nanotubue layers prepared by anodization of pure Ti in 0.5 wt.% hydrofluoric acid has been investigated in simulated biological environment by use of open-circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests. The electrochemical testing results indicate that the nanotubular Ti with the diameter of TiO₂ nanotube lower than 86 nm shows a better corrosion resistance in artificial saliva than that of the mechanically polished Ti. Moreover, the electrochemical stability of Ti nanotubes 22 to 59 nm in diameter is improved but that of Ti nanotubes larger than 86 nm decreases. Besides, the corrosion attack of the nanotubular Ti is shown by the collapse of TiO₂ nanotubue layer. The results suggest that the electrochemical corrosion behavior of nanotubular Ti in artificial saliva is related to the diameter of the nanotubes and thickness of the barrier layer.     

Elastic modulus and in vitro biocompatibility of Ti−xNb and Ti−xTa alloys

The effect of niobium and tantalum on the elastic modulus and the in vitro biocompatibility in binary titanium alloys was studied. The Young's modulus of titanium was effectively lowered with additions of Nb or Ta, depending significantly upon the microstructure. Martensitic microstructures such as α' and α" decreased the elastic modulus, while the ω phase increased it. Ti−10%Nb, Ti−30%Nb and Ti−30%Ta alloys exhibited very low elastic moduli of 74, 80, and 58 GPa, respectively. The corrosion resistance of Ti−xTa was slightly higher than that of Ti−xNb, which was comparable to that of CP Ti or Ti−6A−4V. No ion release was detected in Hank's solution, while Ti ions were released in 0.1% lactic acid ranging from 0.03 to 0.11 μg/ml for both the Ti−xNb and Ti−xTa alloys. MG63 osteoblast-like cell proliferation on Ti−30%Ta was less active compared with Ti−30%Nb, CP Ti or Ti−6Al−4V.     

Effect of Ti3SiC2 content on the corrosion behavior of Al/Ti3SiC2 composite coatings

The purpose of this study was to investigate the electrochemical corrosion properties of flame-sprayed Al and Al/(5, 10, 15)% Ti₃SiC₂ coatings in a 3.5% NaCl solution. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used for analyzing the microstructural characteristics of the coatings. For examining the corrosion behavior of the coatings, a potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) were used. After the potentiodynamic polarization test, the SEM micrograph of coatings indicated that Ti₃SiC₂ particles played a significant role in pitting corrosion. The results of potentiodynamic polarization tests revealed that Al/Ti₃SiC₂ coating is nobler than that of the pure aluminum coating. On the contrary, the addition of Ti₃SiC₂ particles reduced the process of thickening the passive layer. The results of the EIS tests demonstrated that the presence of Ti₃SiC₂ particles significantly enhances the corrosion resistance of the coatings.