Gegen die Spannungsrißkorrosion beständige nichtrostende Manganchromstähle

Stress-corrosion resistant stainless manganese chromium steels The following conclusions may be drawn from the results of investigations into the stress corrosion cracking of austenitic and austeno-ferritic MnCr steels (19–22Mn, 13–18Cr, additions of Mo, V, Nb, Ti, N, B): Addition of nitrogen gives rise to a decrease of stress corrosion cracking resistance in magnesium chloride, sodium chloride with potassium dichromate and water at high temperatures. The same applies to the influence if nickel on corrosion in magnesium chloride and water, and for molybdenum in magnesium and sodium chlorides. From among laboratory melts the type 05 Mn 19Cr 13 had the highest resistance, followed by its modifications with additions of boron, vanadium, molybdenum, titanium, niobium and nitrogen. From among the semi-technical melts the nitrogen containing steels turned out to be least resistant, too. During further investigations the chromium level of 13% turned out to be insufficient to prevent pitting in sodium chloride solutions including seawater.     

Die Oxydation von als Umhüllungen für Brennstoffelemente in schnellen Reaktoren verwendeten Metallen durch sauerstoffhaltiges flüssiges Natrium

Oxidation of metals and alloys for cladding of fast reactor fuel elements by oxygen containing liquid sodium Fuel element cladding of stainless steels and vanadium alloys require the use of high purity sodium. Steels have advantages in this respect because they are not sensitive to oxygen in sodium, while V alloys – though having high mechanical strength and good behaviour under irradiation – have but insufficient corrosion resistence. Kinetic laws in the case of V alloys strongly depend upon oxygen concentration; alloying with Ti and Zr, however, can produce a definite improvement in corrosion resistance.     

The hot corrosion of cobalt-base alloys in a modified Dean's rig - III. CoCrMo alloys

A range of CoCrMo alloys have been exposed at 900°C to salt-bearing atmospheres in a modified Dean's rig. The atmosphere consisted of air containing vapours of, respectively, Na₂SO₄, NaSO₄ + NaO, and Na₂SO₄ + NaCl. Both isothermal and 24-h cyclic exposures were used. In general, the presence of molybdenum in the Cr₂O₃-forming alloys caused accelerated and sometimes catastrophic corrosion. The influence of 2.5 Mo addition to the alloys was observed to be minimal. The presence of 10% Mo in the CoO-forming alloys caused acidic fluxing in the pure Na₂SO₄, while the basic salt caused sulphidation corrosion.     

Electrochemical characteristics of titanium based biomaterials in artificial saliva

In the last decade, new titanium alloys have been developed in different areas of dentistry, such as Ti6Al7Nb, Ti6Al2Nb1Ta1Nb, and Ti5Al2.5Fe. The aim of this study was to compare the Ti6Al7Nb, Ti6Al2Nb1Ta1Nb, Ti5Al2.5Fe, and Ti6Al4V alloys with the commercial titanium, regarding the corrosion resistance in artificial saliva. In the electrochemical estimations the polarization data are converted into instantaneous corrosion rate values (I_corr). The passivation properties were comparable for the four alloys. The EIS spectra are best fitted using an equivalent circuit (EC), which corresponds to the model of a two‐layer structure for the passive film. High impedance values (in the order of 10⁶ Ω cm²) were obtained from medium to low frequencies for all materials suggesting high corrosion resistance in artificial saliva. The electrochemical and corrosion behavior of Ti6Al4V is not affected on substituting vanadium with niobium, iron, molybdenum, and tantalum.     

Corrosion resistant Ti alloy for sulphuric acid medium: Suitability of Ti–Mo alloys

The corrosion resistance of Ti–Mo (5, 10, 15 and 25 wt% molybdenum) alloys in 5–25% sulphuric acid was evaluated. The Ti–Mo alloys offered a better corrosion resistance than commercially pure titanium (CP‐Ti). The higher impedance values, higher phase angle maximum, ability to reach the phase angle maximum at relatively lower frequencies, ability to exhibit a constant phase angle maximum over a wider range of frequencies, higher phase angle values at 0.01 Hz, have confirmed the formation of a stable passive oxide film on Ti–Mo alloys. The study recommends the use of Ti–Mo alloys, particularly Ti–25Mo alloy, as a suitable material of construction for sulphuric acid medium.     

Einfluß einiger Legierungselemente auf die Vanadium-Korrosion von Nickel und Nickellegierungen

Influence of some alloying elements on the vanadium corrosion of nickel and nickel alloys With the exception of Cr all other alloying constitutents (Fe, Cu, Mo, W) increase the corrosion of Ni in vanadium pentoxide melts. Mo (in concentrations > 5% ) has the greatest effect at all temperatures while the influence of > 5% W comes to bear at the higher temperatures (900°C). Addition of Cr produces a definite increase in the resistance of Ni, in particular at concentrations exceeding 20% Cr. Sulphates respectively increase corrsion, on the one hand because of the higher solubility of oxygen in the melt, on the other hand because of the melting point depression produced. In this case, too, Cr has a positive effect on the corrsion behaviour. In sodium sulfate:sodium chloride mixtures (90:10) the corrosion of NiCr aloys is considerably lower than in the presence of vanadium lower than in the presence of vanadium pentoxide. at 800 and 900°C in particular there is a definite influence attributable to the presence of the chloride ion, while at 700°C this effect can be compensated by the formation of corrosion resistant scales on the metal surface.     

An investigation of thin oxide films thermally grown in situ on Fe24Cr and Fe24Cr11Mo by auger electron spectroscopy and X-ray photoelectron spectroscopy

AES depth profiling and XPS have been used for the characterization of thin oxide layers thermally grown in situ in the UHV-analysis chamber on pure iron, chromium and the alloys Fe24Cr and Fe24Cr11Mo at a temperature of 384°C. The apparent oxide film thickness and the film composition were monitored as a function of oxygen exposure. The oxidation rate of the Fe24Cr alloy was found to lie in between that of pure iron and chromium. The films formed have a duplex structure, the outer part being iron oxide, the inner part mostly chromium oxide. Alloying with molybdenum decreases the rate of oxidation by a mechanism involving the formation of a barrier layer rich in molybdenum at the oxide-metal interface. No molybdenum is found in the outer part of the oxide film.     

Performance of Corrosion-Resistant Alloys in Concentrated Acids

Nickel alloys containing optimum amounts of chromium(Cr),molybdenum(Mo) and tungsten(W) are widely used in the chemical processing industries due to their tolerance to both oxidizing and reducing conditions.Unlike stainless steel(SS),Ni-Cr-Mo(W) alloys exhibit remarkably high uniform corrosion resistance in major concentrated acids,like hydrochloric acid(HCl) and sulfuric acid(H_2SO_4).A higher uniform corrosion resistance of Ni-Cr-Mo(W) alloys,compared to other alloys,in concentrated acids can be attributed to the formation of protective oxide film of Mo and W in reducing acids,and Cr oxide film in oxidizing solutions.The localized corrosion resistance of Ni-Cr-Mo(W) alloys,containing high amount Cr as well as Mo(or Mo + W),is also significantly higher than that of other commercially available alloys.The present study investigates the role of alloying elements,in nickel alloys,to uniform corrosion resistance in concentrated acids(HCl,HCl + oxidizing impurities and H2SO4) and localized corrosion performance in chloride-rich environments using ASTM G-48 test methodology.The corrosion tests were conducted on various alloys,and the results were analyzed using weight loss technique and electrochemical techniques,in conjunction with surface characterization tools.     

Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>-Deposit-Induced Corrosion of Mo-Containing Alloys

Disk alloys used in advanced gas turbine engines often contain significant amounts of Mo (2 wt% or greater), which is known to cause corrosion under Type I hot corrosion conditions (at temperatures around 900 °C) due to alloy-induced acidic fluxing. The corrosion resistance of several model and commercial Ni-based disk alloys with different amounts of Mo with and without Na₂SO₄ deposit was examined at 700 °C in air and in SO₂-containing atmospheres. When coated with Na₂SO₄ those alloys with 2 wt% or more Mo showed degradation products similar to those observed previously in Mo-containing alloys, which undergo alloy-induced acidic fluxing Type I hot corrosion even though the temperatures used in the present study were in the Type II hot corrosion range. Extensive degradation was observed even after exposure in air. The reason for the observed degradation is the formation of sodium molybdate. Transient molybdenum oxide reacts with the sodium sulfate deposit to form sodium molybdate which is molten at the temperature of study, i.e., 700 °C, and results in a highly acidic melt at the salt alloy interface. This provides a negative solubility gradient for the oxides of the alloying elements, which results in continuous fluxing of otherwise protective oxides.     

Ion beam analyses of surface films formed on amorphous Fe12Mo18C alloy in 1 N HCl

The Rutherford backscattering with 2 MeV He⁺ and the ¹⁶O(d,p)¹⁷O¹ nuclear reaction with 1.8 MeV D₂⁺ were utilized to determine the concentration profiles of iron, molybdenum and oxygen in the surfaces of an amorphous Fe12Mo18C alloy polarized in 1 N HCl at potentials from ? 0.17 to 1.6 V(SCE). The thickness of the surface film was estimated as 20–200 nm. In the primary active region, selective dissolution of iron and carbon causes enrichment of molybdenum ions in the corrosion product film and of metallic molybdenum in the topmost part of the underlying alloy. A further potential increase led to a decrease in the molybdenum content in the film by transpassive dissolution. Molybdenum ions were not concentrated in the film formed in the stable passive region of 0.5–1.5 V(SCE). The high passivating ability of the amorphous alloy even in the aggressive HCl was ascribed to the homogeneity of the alloy structure as well as to the effect of molybdenum.     

An x-ray photo-electron spectroscopy study of the passivity of amorphous Fe-Mo alloys

The increase in molybdenum content in amorphous FeMoPC alloys facilitates the passivation in 1 N HCl in contrast to crystalline iron base alloys without chromium. An attempt was made to clarify the effect of molybdenum through the composition of surface film analyzed by XPS. The passive film consists mainly of ferric oxy-hydroxide, while a large amount of hexavalent molybdenum species is found in the surface film formed in the active region. The effect of molybdenum is interpreted as follows: the active dissolution necessary for proceeding the passive film formation leads to the precipitation of hexavalent molybdenum species on the alloy surface and as a result, the active dissolution is stifled. This fact leads to the passivation with the aid of iron ions which are already accumulated in the alloy/solution interface during the active dissolution.     

Beitrag zum physikalischen und korrosionschemischen Verhalten von IVa-Metallegierungen

The physical and corrosion-chemical behaviour of IVa metal alloys The stability of titanium in respect of non-oxidant acids can be considerably increased by alloying it with Zr, Hf, Nb, Mo or Re. In the case of Zr, Hf, Nb and Ta, a decisive improvement of the corrosion behaviour can only be attained if the At-percentage of these metals in the alloys exceeds 50 pC. In the case of molybdenum, a content from 20 pC upwards has already a markedly passivating effect. But the improvement in corrosion properties must be bought at the price of poorer machinability. A particularly effective alloying metal, even in small quantities, is Rhenium. An excellent corrosion resistance and, at the same time, good machinability can be attained with ternary Ti-Mo-Ta alloys (with Mo + Ta accounting for some 20 At-pC) and with a Ti-12Mo-1Re alloy. The alloying of titanium with Zr, Hf, V, Cr or Mo is apt to cause, in some cases, a considerable deterioration in the scaling resistance of titanium. Improvements can be obtained by using small quantities of Nb, W and particularly Ta. Ti-Ta alloys with Ta contents up to 10 At-pC still have a good scaling resistance at 900° C.     

The effect of water on the anodic behaviour of FeMo alloys in organic solvents

The presence of molybdenum in FeMo alloys impedes the anodic dissolution process of these alloys in organic solutions of sulphuric and hydrochloric acids. The inhibiting action of molybdenum increases with a lower water concentration in the solution. As the solutions are dehydrated, there is a noticeable increase in the stability of molybdenum compounds occurring on the alloy surface as a result of anodic dissolution.     

Removal of vanadium from ammonium molybdate solution by ion exchange

The separation techniques of vanadium and molybdenum were summarized, and a new method of removal V(Ⅴ) from Mo(Ⅵ) by adsorption with chelate resin was presented. Nine kinds of chelate resins were used to investigate the adsorbent capability of V(Ⅴ) in ammonium molybdate solution with static method. The test results show that DDAS, CUW and CW-2 resins can easily adsorb V(Ⅴ) in ammonium molybdate solution, but hardly adsorb Mo(Ⅵ). The dynamic experimental results show more than 99.5% of V(Ⅴ) can be adsorbed, and the adsorption rate of Mo(Ⅵ) is less than 0.27% at 294-296 K for 60 min at pH 7.42-8.02. The mass ratio of V to Mo decreases to l/5 0000 in the effluent from 1/255 in the initial solution. The loaded resin can be desorbed by 5% NH3·H2O solution, and the vanadium desorption rate can reach 99.6%. The max concentration of vanadium in desorbed solution can reach 20 g/L, while the concentration of molybdenum is less than 0.8 g/L.     

Corrosion characteristics of the wrought Ni‐Cr‐Mo alloys

This paper concerns the wrought, nickel‐chromium‐molybdenum (Ni‐Cr‐Mo) alloys, a family of materials with a long history of use in the chemical process industries. Their attributes include resistance to the halogen acids and resistance to pitting, crevice attack, and stress corrosion cracking in hot, halide salt solutions. The purpose of this paper is to characterize the performance of the Ni‐Cr‐Mo alloys in several key chemicals, using iso‐corrosion diagrams. These indicate the expected corrosion rates over wide ranges of concentration and temperature. Furthermore, the differences between individual Ni‐Cr‐Mo alloys, and their behavior relative to the stainless steels are defined. The data indicate benefits of both a high chromium content and a copper addition, as used in Hastelloy® C‐2000® alloy.     

On the Possibility of Replacing Vanadium and Molybdenum in Alloy VT16 with Iron

The mechanical properties of alloys of the Ti – Al – V – Mo system equivalent in phase composition to alloy VT16 are studied with the aim of determining the possibility of replacing the expensive alloying elements with iron. It is shown that the ductility of an alloy of type VT16 in which molybdenum has been replaced with iron is somewhat higher than in the case of replacement of vanadium with iron. The conclusion is that vanadium and molybdenum in alloy VT16 can be partially replaced with iron without a deterioration in the mechanical properties.     

The corrosion behavior of amorphous nickel base alloys in a hot concentrated phosphoric acid

The corrosion behavior of more than 50 melt-spun alloys was examined in an 87 wt% H₃PO₄ solution at 433 K. Ni-(30–50)Ta alloys possess the highest corrosion resistance due to the formation of a hydrated tantalum(V) oxyhydroxide film containing a small amount of phosphate. Corrosion rates of Ni-(1–10)Ta-P, Ni-(15–20)Cr-P and Ni-(10–20)Mo-P alloys are comparable to that of crystalline tantalum metal. The corrosion resistance of the former two alloy families is based on the formation of tantalum(V) phosphate and chromium(III) phosphate, respectively. The surface film on the Ni-Mo-P alloys consists of molybdenum(VI) and Ni(II) oxyhydroxides and phosphates, and nickel oxyhydroxide seems more protective than molybdenum oxyhydroxide.     

Corrosion behaviour of low-cost Ti–4.5Al–xV–yFe alloys in sodium chloride and sulphuric acid solutions

ABSTRACT

The electrochemical behaviour of low-cost α+β Ti–4.5Al–xV–yFe (where x?+?y?=?4; x?=?1–3; and y?=?1–3) alloys and commercial Ti–6Al–4V alloy was compared in sodium chloride and sulphuric acid solutions. The low-cost alloys were developed by partial substitution of vanadium with iron and the reduction in aluminium content from 6 to 4.5?wt-%. The influence of iron addition and reduction in aluminium content on the corrosion performance of the experimental alloys was assessed via open circuit potential, potentiodynamic polarisation measurements and scanning electron microscopy. The results show that partial replacement of vanadium with up to 3?wt-% iron and the reduction in aluminium content yielded superior corrosion resistance in some of the low-cost alloys when compared with the commercial Ti–6Al–4V alloy. The Ti–4.5Al–1V–3Fe experimental alloy could serve as an alternative low-cost alloy to commercial Ti–6Al–4V alloy in a number of land-based applications such as marine, chemical and petrochemical industries.     

The potentiodynamic screening of materials for high efficiency gas-fired appliances

As part of an overall programme to select materials for secondary heat exchangers in gas-fired condensing appliances, the corrosion characteristics of a number of commercially available austenitic stainless steels, and wrought and cast aluminium alloys were determined with a potentiostat. Results indicated that austenitic stainless steels performed generally better than the wrought or cast aluminium alloys evaluated. Wrought aluminium alloys showed better corrosion resistance than the various aluminium ‘LM’ casting alloys investigated. Of the stainless steels tested, type 316 showed the best overall characteristics. Amongst the wrought aluminium alloys assessed, AlMgSi showed the most corrosion resistance, closely followed by Al1SiMgMn and Al1Mn. Alloys Al7Si1Mg and Al12Si showed superior corrosion resistance to that of Al5Si3Cu.     

The corrosion and oxidation resistance of iron- and aluminium-based powder metallurgical alloys

The corrosion performance of both iron- and aluminium-based powder metals is reviewed and compared, where possible, to similar conventional alloys. Iron-based alloys produced from consolidated powder or compacted amorphous chips have generally been found to have improved corrosion resistance compared to their wrought counterparts, whereas sintered iron-based alloys generally exhibit poorer corrosion resistance. The corrosion performance of aluminium-based alloys produced from consolidated powder is frequently better than that of their conventional counterparts. Aluminium alloys are considered in four groups: AlZnMgCu, AlMg, AlLi and AlCu alloys. The current understanding of the corrosion behaviour of powder metals is assessed and suggestions are made for future investigations.