Influence of nanostructure on titanium corrosion resistance in fluoridated medium

ABSTRACT

Nanocrystalline titanium, mainly owing to its high corrosion resistance, mechanical strength to density ratio and biocompatibility, has a great application potential in dental implantology. However, fluoridated agents commonly used for oral hygiene could have a destructive influence on the titanium protective passive films and lead to the formation of local corrosion damages. In this work, the effect of nanostructuring on titanium corrosion resistance in the concentration of F^? which is typical for toothpastes, was evaluated by different electrochemical and surface characterisation techniques. It was found that nanostructure influences beneficially on titanium corrosion resistance in fluoride solution. Furthermore, the lower increase in nanocrystalline titanium surface roughness in corrosion solutions indicates better stability of passive film formed on its surface.

This paper is part of a thematic issue on Titanium.     

Synergism between effects of velocity,temperature, and alloy corrosion resistance in laboratory simulated fluidised bed environments

Abstract

In studies of the erosion of alloys at elevated temperature, the combined effects of velocity, temperature, and alloy corrosion resistance are not well understood. Wide variations in the effects of velocity have been observed for alloys of different corrosion resistance in various erosion–corrosion environments. There is also some evidence that temperature can affect this relationship. The object of the present work was to undertake a systematic study of the effects of erodent velocity for two alloys, mild steel and 310 stainless steel, at elevated temperatures (300 and 600°C). The velocity was controlled at values between 1·5 and 4·5 m S^?l. Weight change data and analytical scanning electron microscopy were used to characterise the degradation in the various conditions. The results showed that the ranking order of the erosion–corrosion rates of the two different alloys varied as a function of velocity. The velocity at which the ranking of the two alloys reversed increased with increasing temperature. The reasons for such behaviour are discussed in terms of the dependence of the erosion–corrosion rate on the velocity in the various erosion–corrosion regimes.

MST/3188     

Development of physical vapour deposited Mg–Zr alloys Part 2 – Characterisation of corrosion products

Abstract

The corrosion products formed on Mg–Zr physical vapour deposited (PVD) alloys immersed in 3 wt-%NaCl solution were studied using bulk and surface characterisation techniques. The alloys showed better corrosion resistance compared to other PVD Mg alloys and exhibited some of the lowest corrosion rates ever reported for Mg alloys. Their increased corrosion resistance was attributed to the formation of corrosion products containing Zr oxide/hydroxide. The microstructures of the corrosion scale are described and a corrosion mechanism is proposed.     

Gegen Fluor und Fluorwasserstoff korrosionsbeständige Werkstoffe

Corrosion Resistant Materials for Fluorine and Hydrogen Fluoride Aluminum and Duralumin are restistant against fluorine and hydrogen fluoride up to 600 and 700 K, respectively. The resistance of nickel and its alloys, particularly monel, against fluorine and hydrogen fluoride is fairly good up to 900 and 800 K. During the attack of nickel-chromium alloys by fluorine between 1000 and 1300 K, it appears an inner fluorination similarly to the inner oxidation. The resistance of titanium in water-free liquid fluorine at lower temperatures with < 0,3 mm · a^?1 is comparable to that of nickel and monel. However, the corrosion of titanium in gaseous fluorine amounts at 377 K only 0,0082 mm · a^?1. In spite of their limited resistance against fluorine and hydrogen fluoride, very pure molybdenum and tungsten are employed as construction materials in the rocket technology because of their large strength at high temperatures if fluorine-hydrogen and fluorine-hydrazine flames are used. Lanthanum and calcium borides are only little attacked by fluorine hydrazine flames between 1400 and 1800K; they are superior to all special grade alloys. The same is true in a lower temperature region (290–400 K) with fluorcarbon resins. Organic materials substitute in increasing extent metal alloys and non-renetal inorganic materials.     

Influence of Tris in simulated body fluid on degradation behavior of pure magnesium

In current paper, influence of tris-hydroxymethyl-aminomethane (tris) in simulated body fluid (SBF) on degradation behavior of pure magnesium is investigated using electrochemical tests as well as degradation measurement. Our results shows that tris mainly affects earlier degradation behavior of pure magnesium alloy. Tris and HCl used in preparation of SBF will form Tris–HCl which only lowers corrosion potential of magnesium slightly but accelerates degradation rates of pure magnesium by teens times. Consumption of OH^? generated during magnesium dissolution by Tris–HCl progressively promotes transformation from Mg to Mg²⁺, which is the main reason for quite high degradation rate of pure magnesium in SBF. Pure magnesium is also more sensitive to pitting corrosion due to inclusion of Tris–HCl in SBF. This study deepens the understanding on degradation mechanism of biomedical magnesium alloys.     

Effect of small alloying additions on behaviour of rapidly solidified Cu–Cr alloys

Abstract

The mechanical properties of as well as microstructural changes in rapidly solidified ternary Cu–Cr based alloys were studied for various ternary additions. The flow stresses of the binary and various ternary alloys are explained in terms of Orowan strengthening mechanisms. Zirconium, magnesium, and, to a lesser extent, silicon affected the age hardenability of the alloys, refining the chromium dispersion by modifying the precipitation sequence described by Tang. The coarsening kinetics was insensitive to the presence of a third alloying element, showing that these additions did not affect chromium transport within the copper matrix. Nevertheless, these additions influenced the morphology of the chromium particles during coarsening; zirconium tended to keep the particles spherical, while titanium additions increased their aspect ratio. Titanium reduced dramatically the age hardenability of the alloy by provoking heterogeneous nucleation of bcc chromium on small Ti0₂ particles present in the as cast structure, resulting in a coarser particle size distribution at peak strength.

MST/1171     

Simulation of road salt corrosion in austenitic alloys for automotive exhaust systems

Abstract

Cyclic oxidation tests in air with intermittent salt spraying have been performed to simulate the conditions of road salt (NaCl–CaCl₂) enhanced corrosion in automotive exhaust systems.

Tests were carried out at 600 and 700°C on three austenitic alloys, including two stainless steels currently employed for exhaust components (AISI 316 Ti and AISI 302B) and a higher nickel heat resisting alloy.

The presence of salt causes internal corrosion, both along a regular front beneath an outer oxide scale and down alloy grain boundaries. An increase in temperature accelerates the corrosion rate and particularly enhances intergranular penetration.

The results of micrographic and microanalytical investigations are in general agreement with an active oxidation mechanism in which Na_xCl_x vapour species, and not only chlorine, appear to play an important role. The regions directly affected are depleted in chromium and iron and enriched in nickel.

Although internal oxidation of silicon is observed, a high silicon content (2%) does not necessarily ensure effective protection against this type of attack.     

Development of physical vapour deposited Mg–Zr alloys Part 3 – Comparison of alloying and corrosion behaviour in Mg–V and Mg–Zr physical vapour deposited alloys

Abstract

The microstructures and corrosion products of Mg–V and Mg–Zr physical vapour deposited alloys were compared. Factors affecting the alloying behaviour of Mg with V or Zr as well as with other alloying additions such as Ti, Cr, Mn, and Si were considered. The greater thermodynamic stability of the Zr oxide in air and Zr hydroxide in 3 wt-%NaCl were the main reasons for the enhanced corrosion resistance of Mg–Zr alloys compared to Mg–V alloys.     

The oxidation of binary alloys of chromium with metals of the first long period

Binary alloys of chromium with titanium, vanadium, manganese, iron, cobalt and nickel respectively have been oxidation-tested in air up to 1100° C. From the results the temperature corresponding to a weight change of 1 mg cm^–2 over 4 h was determined by interpolation, and this temperature has been plotted against composition.With the exception of a 50% cobalt alloy, alloys of chromium with either iron, cobalt or nickel withstand air oxidation at much higher temperatures than the constituent metals. Oxidation resistance in the other systems is generally lower.An additional note describes the determination, during oxidation, of comparative electrical resistance of scales on iron, chromium, and an iron-20% chromium alloy.     

Influence of Si,Ca and Ag addition on corrosion behaviour of new wrought Mg–Zn alloys

Abstract

The development of new wrought magnesium alloys for automotive industry has increased in recent years owing to their high potential as structural materials for low density and high strength/weight ratio demands. However, the poor mechanical properties and low corrosion resistance of the magnesium alloys have led to searching a new kind of magnesium alloys for better strength, ductility and high corrosion resistance. The main objective of the present research is to investigate the mechanical properties and the corrosion behaviour of new magnesium alloys, Mg–Zn–Ag (ZQ) and Mg–Zn–Si–Ca (ZS) alloys. The ZQ6X and ZS6X–YCa alloys were prepared by using hot extrusion method. Hardness AC and DC polarisation tests were carried out on the extruded rods, which contain different amounts of silver or silicon and calcium. The potential difference in air between different phases and the matrix was examined using scanning Kelvin probe force microscopy. The microstructure was examined using optical and electron microscopy (TEM and SEM), X-ray analysis and EDS. The results showed that the silver addition improved the mechanical properties but decreased the corrosion resistance. The addition of silicon and calcium also affected both mechanical properties and corrosion behaviour. These results can be explained by the effects of alloying elements on microstructure of Mg–Zn alloys such as grain size and precipitates caused by the change in precipitation and recrystallisation behaviour.     

Reduction of titanium and other metal oxides using electrodeoxidation

Abstract

Many reactive and refractory metals are currently produced industrially by reducing their compounds, including oxides, using a more reactive metal. In some cases, where there is substantial oxygen solubility in the metal, the oxygen is first removed by carbochlorination followed by reduction. Titanium and zirconium are made by reduction of the volatile tetrachlorides by magnesium. The processes consist essentially of two reduction steps: reducing magnesium chloride to magnesium metal and then reduction of the metal compound; this makes the overall reduction process relatively expensive. Electrodeoxidation is very simple in that the oxide to be reduced is rendered cathodic in molten alkaline earth chloride. By applying a voltage below the decomposition potential of the salt, it has been found that ionisation of oxygen is the dominant cathode reaction, rather than alkaline earth metal deposition. In the laboratory, this technique has been applied to reduce a large number of metal oxides to the metals, including titanium, zirconium, chromium, niobium, tantalum, uranium and nickel. Furthermore, when mixed oxides are used as the cathode, alloys or intermetallic compounds of uniform composition are obtained. This may offer advantages over conventional technology for those alloys that are difficult to prepare at present, owing to differences in either density or vapour pressure.     

Effect of the chromium content on the corrosion of nickel based alloys in primary water of pressurised nuclear reactors

Abstract

Stress corrosion cracking (SCC) is a damaging mode of alloys used in pressurised water reactors. Those damages led to the replacement of Alloy 600 (15% Cr) by alloy 690 (30% Cr) but the mechanism responsible for the SCC and the reason for the positive effect of chromium are not yet very well understood. In this paper, we studied the corrosion of synthetic alloys – with controlled chromium content varying from 5 to 30 wt%. Characterisation was done using SEM and TEM observations together with chemical analysis and mapping using EDX, EFTEM and SIMS. The outer oxide scale is composed of crystallites, beneath it the presence of a continuous chromium oxide is accompanied with a Cr depleted zone for alloys that contain more than 10% Cr. The penetration of oxygen over very large distances (several microns) on triple junctions is demonstrated, as well as the role of plastic deformation that modifies strongly the overall structure of the oxide scale.     

Preparation and Performance of Electroless Nickel on AZ91D Magnesium Alloy

The corrosion of magnesium alloy in different plating solutions was researched. The results demonstrated that corrosive condition of the alloy immersed in nickel chloride solution and nickel sulfate solution is serious and in nickel acetate solution and nickel nitrate solution is less. Magnesium alloy was handled with four acid pickling formulas and two activation formulas and the effects of different pickling formulas and activation formulas were researched through comparative experiment. The experimental results indicated that after handed with pickling formula about 500 mL L^?1 H₃PO₄ (85%), 110 mL L^?1 HNO₃ (68%), room temperature for 30 s and activation formula about 375 mL L^?1 HF (40%), room temperature for 10 min, magnesium alloy could realize electroless nickel plating directly and the performance of the prepared plating was much better. The properties of the nickel-plating coating were researched by electrochemical workstation, scanning electron microscope, and X-ray diffraction. The results demonstrated that this Ni–P coating was very uniform and meticulous; the structure of Ni–P coating was amorphous; and comparing with magnesium alloy, the corrosion potential of this plating increased about 799 V and the corrosion current density declined obviously. The nickel-plating coating effectively improved the anticorrosion performance of magnesium alloy.     

Effect of fluoride on the corrosion behavior of nanostructured Ti-24Nb-4Zr-8Sn alloy in acidulated artificial saliva

The surface of titanium dental implants is highly susceptible to aggressive fluoride ions in the oral environment. Nanotechnology has proven an effective approach to improve the stability and corrosion resistance of titanium by applying a passive film. In this study, we investigated the effects of fluoride on the corrosion behavior of nanostructured(NS) Ti-24 Nb-4 Zr-8 Sn(Ti2448) alloy in acidulated artificial saliva(AAS)at 37 ℃, and then conducted comparisons with its coarse grained(CG) counterpart. Electrochemical techniques, such as potentiodynamic polarization and electrochemical impedance spectroscopy(EIS), as well as surface analysis including X-ray photoelectron spectroscopy(XPS) with argon ion sputtering, and scanning electronic microscopy(SEM) were employed to evaluate the effects of fluoride on sensitivity to pitting and the tolerance of Ti2448 to fluoride in AAS solution. The results demonstrate that corrosion current density increased with F-concentration. In all respects, the NS Ti2448 alloy presented corrosion resistance superior to that of its coarse grained(CG) counterpart at low F-concentrations(0.1%).Furthermore, a high content of F-(1%) was shown to promote the active dissolution of both alloys by increasing the rate of corrosion. Following immersion in the fluoridated AAS solution for 60 days, a tissuefriendly compound, Ca_3(PO_4)_2, was detected on the surface of the NS when F-= 0.01% and Na_2 TiF_6 was identified as the main component in the corrosion products of the CG as well as NS Ti2448 alloys when F-= 1%. High concentrations of F-produced pitting corrosion on the CG alloy, whereas NS Ti2448 alloy presented general corrosion in the form of lamellar separation under the same conditions. These findings demonstrate the superior corrosion resistance of the NS Ti2448 alloy as well as lower pitting sensitivity and higher tolerance to fluoride due mainly to grain refinement.     

Coatings and hardfacing alloys for corrosion and wear resistance in diesel engines

Abstract

The potential for surface engineering solutions to increase the corrosion and wear resistance of diesel valves is examined with regard to operation in residual fuel oil combustion products. The environment in which the exhaust valve operates is discussed. The corrosion test is used to sort the 27 coating treatments and 16 hardfacing alloys examined. H ardfacing materials showing the greatest potential are also evaluated in hot wear tests. As reference materials, En 52 steel, Nimonic 80A, and Stellite 6 are included. Corrosion tests over the temperature range 500–700°C have shown that the surface treatments can be ranked in terms of their composition and expected metal surface temperature. Good corrosion resistance to residual fuel oil combustion products reflects primarily the content of the alloy, the base metal (Fe, Ni, or Co) being of secondary importance. Refractory metals (Ti, Nb, Ta, Mo, and W) and other minor alloying additions do not significantly alter the corrosion behaviour, unless they are present in large quantities. Hot wear tests (650°C, 80 MN m^?2 contact pressure) of candidate hardfacing alloys have also demonstrated the importance of the Cr content of the alloy. This probably results from the ease of formation of a protective glaze, lowering the coefficient of friction and therefore the wear rate. Additions of refractory metals were beneficial. Since optimum wear resistant alloys contained high levels of Mo and vv: they were poor in terms of corrosion resistance. It is therefore necessary to balance wear resistance against corrosion resistance in the selection of materials for residual fuel oil service. Two coating systems, vacuum plasma sprayed Ni–50Cr and a chromised diffusion treatment appeared to have potential as valve surface treatments for improved corrosion resistance; and two hardfacing alloys, Tristelle TS2 and Colmonoy 8, offered balanced behaviour as seat materials with acceptable wear resistance and improved corrosion performance. These materials have been evaluated in the engine tests of three different diesel engine manufacturers.

MST/3157     

The effect of alloying additions on the oxide morphologies of super-heater tubes at elevated temperatures

Abstract

This paper considers the oxidation of boiler tube alloys; viz. 321H which contains titanium, and E1250 which contains niobium and vanadium, at temperatures of 650 and 750°C, under laboratory conditions, for 720 hours. The resulting oxides have been characterised using advanced techniques including scanning electron microscopy, energy dispersive X-ray spectroscopy and site specific crosssections using focused ion beam milling.

The results have shown that small concentrations of titanium and vanadium have a pronounced effect on the morphologies of the oxide layers. Where titanium is present, as in alloy 321H, titanium rich nodules form leading to the development of “whisker” growths from the nodules. On the E1250 alloy, vanadium rich, gem like, crystals form on the surface of the chromium/manganese rich scale. Cross-sectional analysis has shown a possible relationship between the surface features and underlying niobium particles.     

Acid corrosion behaviour of amorphous and crystalline Ti75Ni20Si5 alloys

The corrosion resistance of amorphous and crystalline Ti₇₅Ni₂₀Si₅ alloy as well as that of pure crystalline titanium as a reference material were investigated in sulphuric and hydrochloric acids. Potentiokinetic polarization curves were obtained for each of these alloys in order to determine their corrosion properties. It was observed that in these media the Ti₇₅Ni₂₀Si₅ alloy in the amorphous state shows a higher corrosion resistance than titanium whereas in the crystalline state its corrosion resistance is comparable with that of titanium.     

Influence of remelting on microstructure,hardness and corrosion behaviour of AlCoCrFeNiTi high entropy alloy

Abstract

High entropy alloys are a newly developed class of alloys, which tend to form a single solid solution or a mixture of solid solutions with simple crystal structures. These alloys possess excellent mechanical properties, thermal stability and corrosion resistance. In the present paper, an AlCoCrFeNiTi high entropy alloy was obtained by induction melting, and the influence of the remelting process on the mechanical and corrosion resistance characteristics of the alloy was investigated. Thus, optical and scanning electron microscopy revealed less phase segregation and a fine dendritic structure for the remelted alloy, while corrosion tests indicated that present alloy, in remelted state, has better corrosion resistance than as cast alloy and stainless steel. The Vickers microhardness measurements demonstrated an improvement of the alloy microhardness by remelting process due to the decrease in phase segregation and the increase in dendrite refinement level.     

Effect of vapour phase hydrogen peroxide,as a decontaminant for civil aviation applications,on microstructure,tensile properties and corrosion resistance of 2024 and 7075 age hardenable aluminium alloys and 304 austenitic stainless steel

Abstract

A response to the chemical or biological contamination of aircraft requires the use of a suitable decontaminant. Among possible chemical decontaminants, vapour phase hydrogen peroxide appears to be a likely candidate in terms of a combination of efficacy, low environmental impact and potential for materials compatibility. The present paper examines the effect of hydrogen peroxide, both in the vapour phase and as a liquid concentrate on two common structural materials used in aviation, namely 2024 and 7075 age hardenable aluminium alloys and on 304 austenitic stainless steel, the latter as employed in galley and lavatory surfaces. The present paper characterises both the effects of hydrogen peroxide on the microstructure of the materials and the impact that decontamination has on the tensile properties and corrosion resistance of these materials. Microstructural effects are both relatively small in magnitude and confined to a region immediately beside the exposed surface. No systematic effect is found on either the tensile properties or the post-exposure corrosion resistance of the three alloys examined. These observations are encouraging in terms of the use of vapour phase hydrogen peroxide for decontamination applications.     

Laser cladding of Mg-Al alloys

Among rapid solidification processing methods, laser cladding is a unique and promising technique which can be used to increase the corrosion resistance of materials. This paper describes the improvement of the laser-cladding process for magnesium-based alloys and an investigation of the effect of the laser-cladding technique upon the microstructure and the corrosion resistance of magnesium-based alloys. The cladding apparatus and techniques have been adapted for magnesium cladding to overcome the oxidation and high vapour pressurerelated problems. Laser-clad Mg₂₇Al₇₃, Mg₅₃Al₄₇, and Mg₇₂Al₂₈ have been obtained. Eutectic phases were observed in the two magnesium-rich alloys and in the interface of the aluminiumrich alloy. Polycrystalline structure was formed in Mg₂₇Al₇₃. Laser-clad Mg₂₇Al₇₃ was found to be superior to laser-clad Mg-5 wt% Zr, Mg-2 wt% Zr, cast AZ91 B and cast magnesium in corrosion properties.