Untersuchung der Spannungsrißkorrosion von Baustählen in flüssigem Zink

Investigation into stress corrosion cracking of unalloyed steels in liquid zinc This investigation has been done in order to get further informations of stress corrosion cracking in galvanized steel. The samples were made of unalloyed steel with increased contents of carbon (up to 0,24%), silicon (up to 0,30%) and copper (up to 0,4%). Welded and notched samples have been equally tested. One batch of the samples was preloaded with 70, 80, 90 and 100% of yield point and then dipped into hot zinc (450°C). Another batch was tested in liquid zinc with constant strain rates of 5 · 10^?4, 5 · 10^?5, and 5 · 10^?6 · s^?1 during 250 h. None of the tested samples have been destroyed by stress corrosion cracking.     

Einfluß der Aufkohlung auf das Kriechverhalten einer FeNiCr-Hochtemperaturlegierung

Effects of Carburization of the Creep Behaviour of a FeNiCr-High Temperature Alloy Incoloy 800 and Incoloy 800 doped with 1% Nb were carburized at 1000 °C in CH₄–H₂ to 0.83% C (mass content). The undoped alloy shows relatively coarse large M₂₃C₆ carbides at the grain boundaries, the alloy with 1% Nb has mainly fine carbides in the grains. Creep experiments were performed with the carburized and uncarburized specimens at 1000 °C, in which creep rates were attained in the range 10^?9… 10^?7 s^?1 of secondary creep. The stress dependence of the creep rate indicates two creep mechanisms: diffusion creep at low stresses and dislocation creep at high stresses. The diffusion creep is faster for both alloys after carburization. The dislocation creep is retarded by carburization for the undoped alloy. At about equal creep rate ε = 10^?7 s^?1 the carburized specimens have a longer lifetime. The fracture is brittle for Incoloy 800 in the uncarburized and carburized state, characterized by void and crack formation and poor reduction in area. The fracture of the carburized Incoloy 800 with 1% Nb is rather ductile with less void formation. The results indicate that carburization does not deteriorate the creep behaviour of the FeNiCr alloy if the reached carbon content is not too high. An addition of Nb is very favorable for the creep properties after carburization.     

Über den Einfluß des aufgenommenen Wasserstoffs auf die Wechselbiegefähigkeit von Kohlenstoffstahl bei der Beizung in Schwefelsäure

Influence of hydrogen pick-up on the alternating bending strength of carbon steel during sulfuric acid pickling Steel coupons (St 3 GBKL) were acid-pickled in 1 M sulfuric acid at 50° C. The dissolution rate (by weight), the hydrogen uptake (by high temperature vacuum extraction and gaschromatographic determination) and the alternating bending strength (by alternating bending at 25 Hz and deflection angle ± 5°) were measured as a function of pickling time. The linear corrosion rate was 21 · 10^?6 · kg · m^?2 · sec^?1, the saturation concentration of hydrogen in steel was 8,76 ppm and the diffusion coefficient of hydrogen in steel was calculated as 8,74 · 10^?7 cm² · sec^?1. The number of bendings until fracture is not only a function of the integral hydrogen concentration, but is strongly influenced by the concentration profile of hydrogen in the sample; a hypothesis is given to explain this experimental finding.     

Effect of pass deformation on microstructure, corrosion and electrochemical properties of aluminum alloy anodes for alkaline aluminum fuel cell applications

The effect of pass deformation in rolling processes on the microstructure, corrosion resistance and electrochemical activity of Al-Mg-Bi-Sn-Ga-In alloy anodes operated in an alkaline solution (85 °C, 5 mol·L^?1 NaOH with addition of NaSnO₃) with current density of 800 mA·cm^?2 was investigated. To analyze the microstructure of aluminum alloy anodes, we used scanning electron microscope, transmission electron microscope and electron backscatter diffraction techniques. We also used the hydrogen evolution method and electrochemical testing techniques to investigate the corrosion and electrochemical behavior of aluminum alloy anodes. The results showed that uniform microstructures with homogeneous distribution of fine active segregated phases as well as an excellent electrochemical performance of the aluminum alloy anodes were achieved by the dynamic recrystallization under pass deformation of 40%. We found that the aluminum alloy anodes (under pass deformation of 40%) had the lowest hydrogen evolution rate (0.092 mL·min^?1·cm^?2) and the most negative electrode potential (?1.585 V).     

Hot deformation behavior of TiAl alloys prepared by blended elemental powders

A nearly full dense Ti-45Al-7Nb-0.4W (at.%) alloy billet with dimension of 120 mm in diameter and 50 mm in height was fabricated by reactive sintering of blended elemental powders. The high temperature deformation behavior was investigated by isothermal compressive tests, performed at temperature in 1000–1200 °C with strain rates from 1 × 10^?3 s^?1 to 1 × 10^?1 s^?1. Results indicate that the dependence of flow stress on temperature and strain rate is well fit for a hyperbolic-sine relationship using the Zener–Hollomon parameter. The measured apparent activation energy Q and stress exponent are determined as 420 kJ mol^?1 and 3.7, respectively. High oxygen content, high Nb content and fine grain size are main reasons for the high activation energy and high strength of PM TiAl alloy. An appropriate set of deformation processing parameters of 1200 °C and 1 × 10^?3 s^?1 are recommended for the present TiAl alloy.     

Corrosion and deposition of magnetite in ferritic steel steam generator tubes under acid chloride conditions

Abstract

Corrosion of a tube made up of 9Cr–1Mo, 2·25Cr–1Mo, and 2·25Cr–1Mo–Nb ferritic steels and containing artificial defects has been investigated under realistic steam generator conditions (355°C, 17·6 MPa) with acid chloride fault water chemistry (2 mg/kg HCl). Four regions of corrosion and magnetite deposition behaviour were observed on the tube surface. In non-heat flux regions, magnetite deposition was affected by mass transfer and probably also by surface potential. In low heatflux regions (<660 kW m^?2) observed increases in the rates of magnetite deposition and corrosion wereprobably due to rises in the degree of iron supersaturation and HCl concentration, brought about by boiling. Enhanced HCl concentrations in the normal heat flux region (660 kW m^?2) prevented magnetite deposition and caused an increase in corrosion of the three steels. Increases in corrosion and magnetite deposition were also observed at the weld between the 9Cr–1Mo and 2·25Cr–1Mo steels. In defects, accelerated corrosion was seen only in the 9Cr–1Mo steel and was confined to the top 0·15 mm. It is concluded that the corrosion behaviour observed in this work is determined by the residence time and concentration of solutions of HCl on tube and defect surfaces.     

Über den Einfluß der Strömungsgeschwindigkeit und der NaCl-Konzentration auf die Sauerstoffkorrosion unlegierten Stahls in Wässern

Influence of flow velocity and sodium chloride concentration on the oxygen corrosion of unalloyed steels in water Corrosion of mild steel in aerated water normally leads to pitting, and to an enrichment of anions within the pits. Even HCO₃^? ions can support localized corrosion. The intensity of this is remarkably increased by chloride ions. In equilibrium water with Ca²⁺ and HCO₃^? ions and about 40 ppm free CO₂ protective rust layers are formed after about 500 hs exposure not depending on flow rate and salt concentration. These layers decrease the mean corrosion rate to about 0.1 mm/a, but do not prevent localized corrosion. Protective passive layers with extremely low corrosion rates are formed in flowing water with Ca²⁺ and HCO₃^? ions and about 40 ppm free CO₂. The minimum flow rate can be estimated for a 3/4 inch pipe to lie between 0.35 and 1.5 m/s. Investigations with water containing 40 ppm free CO₂ without Ca²⁺ at pH 4.7 and with NaHCO₃ (pH 7.2) show at low pH high corrosion rates – as expected –, and at pH 7.2 formation of protective layers with poor reproducibility in comparison with the situation in equilibrium water. Thus, Ca²⁺ ions support the protective properties of rust layers. HCO₃^? ions are necessary for the formation of protective layers because these are not formed in pure NaCl solutions.     

Evaluation of the corrosion and oxide microstructural characteristics of pure Zr and the Zr−1.5Nb alloy

The corrosion behavior and the oxide microstructural characteristics of pure Zr and the Zr-1.5Nb alloy were investigated. The crystalline structure and morphology of zirconium oxide formed in a water at 360°C and 18.9 MPa was analyzed using a High Voltage Electron Microscope (HVEM). The corrosion rate increased more in pure Zr than in the Zr-1.5Nb alloy after the transition of the corrosion rate. From a HVEM oxide analysis and an imaging process using the DigitalMicrograph^™ program for both oxides formed on pure Zr and the Zr−1.5Nb alloy, it was found that most of the oxide amorphology consisted of both equiaxed and columnar grains. They formed in the outer and middle region of the oxide layer and were identified as a monoclinic phase. However, a complex oxide structure formed near the metal/oxide interface in the Zr−1.5Nb alloy, which showed good corrosion resistance. Thus, this implies that the Nb affected the oxide characteristics of the crystal structure and changed the corrosion behavior.     

An investigation of the corrosion behaviour of a FeNiCoAlTa shape memory alloy in 3.5 wt-% NaCl solution

In this study, the corrosion behaviour of an FeNiCoAlTa (NCAT) shape memory alloy in 3.5% (w?w^?1) NaCl solution was evaluated. Linear polarisation resistance and potentiodynamic polarisation tests were conducted at 25°C. An open circuit potential (OCP) of ?381?mV (vs. saturated calomel reference electrode) and a corrosion rate (CR) of 0.0174?mm?y^?1 were obtained. The NCAT alloy did not show any passivation in the 3.5% (w?w^?1) NaCl solution. Its corrosion behaviour was very similar to that of G10180 (AISI 1018) carbon steel with the NCAT showing a lower CR and a less active OCP. Its corrosion in the solution was a combination of general dissolution and localised attack (pitting). Aging heat treatment caused β-Ni₃Al phase, and tantalum to precipitate on the grain boundaries, resulting in chemical species segregation between the grain boundaries and the grains. The chemical segregation caused intergranular corrosion of the alloy.     

Microstructure and corrosion resistance of tantalum after nitrogen ion implantation

This paper investigates the effect of nitrogen ion implantation on surface structure as well as resistance against tantalum corrosion. Bulk Ta surface was implanted with 30?keV nitrogen ions at a temperature of 100°C with doses between 1?×?10¹⁷ and 1?×?10¹⁸?ions/cm². The implanted samples were characterised by atomic force microscopy, X-ray diffraction analyses and the corrosion test to identify structural, compositional and electrochemical changes at various doses. The experimental results indicate the formation of hexagonal tantalum nitride (TaN_0.43), in addition to the fact that by increasing the ion dose, nitrogen atoms occupy more interstitial spaces in the target crystal, a case which can significantly improve corrosion resistance. The maximum extent in the improvement of the micro hardness was 75% and the reduction in the corrosion current was 83%. According to scanning electronic microscopy and corrosion results, in the dose of 1?×?10¹⁸?ions/cm² the highest corrosion resistance was received against the H₂SO₄ corroding media.     

On the Stress corrosion cracking behaviour of aluminium alloy sheet in an aqueous solution of 3% NaCl + 0.3% H2O2

The stress corrosion cracking (SCC) behaviour of aluminium alloy sheet was investigated in the long transverse direction using the slow strain rate testing technique. The synthetic environment used was an aqueous solution of 3% NaCl + 0.3% H₂O₂. No indications of SCC sensitivity are observed for the alloys 2024-T351, 8090-T81, and 2091 CPHK-T8X. The alloys 2091 T8X and 6061-T4 are found to be susceptible to intergranular stress corrosion cracking. At strain rates below 4 · 10^?7 s^?1, the slow strain rate testing technique indicates a slight SCC sensitivity with alloy 6013-T6. Fractography reveals transgranular stress corrosion cracking. Transgranular stress corrosion cracking is also observed with 6061-T4 specimens which are dynamically strained at strain rates below 5 · 10^?7 s^?1. Aqueous 3% NaCl solution with hydrogen peroxide addition promotes pitting and intergranular corrosion. The loss of ductility caused by these corrosion processes interferes with the evaluation of the results of the slow strain rate testing technique.     

Electrochemical behavior of Ni3Al-based intermetallic alloys in NaOH

The corrosion behavior of Ni₃Al-based intermetallic alloys in a 0.5 M NaOH solution was studied at 25 °C. The open circuit potential, cathodic and anodic potentiodynamic polarization, Tafel plots and linear polarization resistance measurements were used to characterize the corrosion behavior. For the Ni₃Al(B, Zr) alloy, potentiodynamic polarization curves showed a wide passive region that can be found between about ?0.220 V_SCE and 0.520 V_SCE. On the other hand, a narrow passive region, in the range of potentials from about ?0.180 V_SCE to 0.180 V_SCE, was observed for the Ni₃Al(B, Zr, Cr, Mo) alloy. Chromium, as an alloying element in the Ni₃Al(B, Zr, Cr, Mo) alloy, contributes to transpassive dissolution of the passive film at much lower anodic potentials and remarkably reduces the passivation region. The experiments indicated also that damaged passive films on alloys repairs itself and pits do not initiate. The surface of both alloys and passive films possess extremely high corrosion resistance in a studied solution. However, Tafel and linear polarization tests revealed that freshly exposed surfaces of the Ni₃Al(B, Zr) alloy exhibited better corrosion resistances than the Ni₃Al(B, Zr, Cr, Mo) alloy. Both methods, used for the determination of corrosion rates gave very similar results. The calculated corrosion rates are about 2.8 ·10^?3 and 6.0·10^?3 mm year^?1 for the Ni₃Al(B, Zr) alloy and B, respectively.     

The influence of carbonaceous surface impurities on excrescence initiation in the rimming steel/CO2/CO system

The influence of surface carbon contamination in determining the sites of excrescence growth has been studied, for the reaction of rimming steel in carbon dioxide containing 1·5%CO + 1000 ppm H₂O + 10 ppm CH₄ at 17·25 × 10⁶ N m^?2 and 450°C. Patterns visible in the oxide layer in the early stages of oxidation can be attributed to hydrocarbon impurities remaining on surfaces not rigorously cleaned. The impurity can accumulate at specific areas on the surface and can then accelerate oxidation such that excrescences start growing early in the normally protective stage. Surface cleaning procedures and their limitations for corrosion studies are discussed.     

Electrochemical study on corrosion behaviour of zircaloy-2 in aqueous medium containing permanganate ion

Abstract

The corrosion compatibility of zircaloy-2 was studied in strong oxidising permanganate based acidic (HMnO₄) medium using impedance spectroscopy and potentiodynamic anodic polarisation methods at room temperature (28°C) and elevated temperatures (65 and 90°C). The average corrosion rate of zircaloy-2 in HMnO₄ medium (0·417–8·33 mM) was found to be 1·32×10^?4 μm h^?1 at 28°C, 2·59×10^?4 μm h^?1 at 65°C and 3·53×10^?4 μm h^?1 at 90°C. At 28°C, the polarisation resistance R_p values obtained from impedance spectrum were higher (in kΩ) when compared to R_p values at 65 and 90°C, indicating a lower corrosion rate at lower temperature. Comparative studies with 2·5 mM H₂SO₄ and platinum (as a working electrode) showed the effects of oxidising nature and participation of additional redox reactions in HMnO₄ medium. Cyclic polarisation studies showed the absence of pitting attack on zircaloy-2.     

Thermal expansion and dimensional stability of unidirectional and orthogonal fabric M40/AZ91D composites

Unidirectional (60%, volume fraction) and orthogonal (50%, volume fraction) M40 graphite fibre reinforced AZ91D magnesium alloy matrix composites were fabricated by pressure infiltration method. The coefficients of thermal expansion (in the temperature range of 20-350 ℃) and dimensional stability (in the temperature range of 20-150 ℃) of the composites and the corresponding AZ91D magnesium alloy matrix were measured. The results show that coefficients of thermal expansion of the composites in longitudinal direction decrease with elevating temperature. The coefficients of thermal expansion (CTE) for unidirectional M40/AZ91D composites and orthogonal M40/AZ91D composites are 1.24×10-6 ℃-1 and 5.71×10-6 ℃-1 at 20 ℃, and 0.85×10-6 ℃-1 and 2.75×10-6 ℃-1 at 350 ℃, respectively, much lower than those of the AZ91D alloy matrix. Thermal cycling testing demonstrates that the thermal stress plays an important role on residual deformation. Thus, a better dimensional stability is obtained for the AZ91D magnesium alloy matrix composites. More extreme strain hysteresis and residual plastic deformation are observed in orthogonally fabric M40 reinforced AZ91D composite, but its net residual strain after each cycle is similar to that of the unidirectional M40/AZ91D composite.     

Application limits of Ta and Ta-2.5% W for sulfur acid handling

Long term corrosion test with welded coupons were carried out to determine the application limits of Ta and Ta-2.5% W in sulfuric acid (H₂SO₄). The program included the following two fields of investigation:

• • Determination of corrosion behavior in 90 through 100% H₂SO₄ at temperatures up to 200°C.
• •Determination of corrosion behavior in 96 wt % H₂SO₄ comparing recovered nitration-spent acid and technical grade between 150 and 230°C.

Mainly immersion tests were performed. A comparison of Ta and Ta-2.5 % W showed that in technical H₂SO₄ the alloy performed better than the metal. Regardless of which material was considered, the higher the H₂SO₄ concentration, the lower the temperature necessary to achieve acceptable corrosion behavior. In technical H₂SO₄, the following application limits were revealed: Ta: 96 wt %/200°C – 97 wt %/150°C Ta-2.5% W: 96 wt %/210°C – 97.5 wt %/175°C. Above 97.5 wt % the corrosion resistance decreased rapidly. The recovered nitration-spent acid led to remarkably lower corrosion rates due to small amounts of nitric acid. In these types of acidcontaining oxidizing compounds. 230°C seemed to be satisfactory, provided the wall of the heat exchanger was sufficiently thick.     

Characterisation and corrosion resistance of Zn–Mn coatings electrodeposited from acidic chloride bath

Zn–Mn alloy coatings were galvanostatically electrodeposited from an acidic chloride bath. Effects of deposition current density, pH and temperature on surface morphology, microstructure and corrosion resistance of Zn–Mn coatings were studied. The coatings deposited at 10, 50 and 100 mA cm^?2 had a single η-Zn phase structure. However, a dual phase structure of η-Zn and ?-Zn–Mn with higher Mn content was found for the coatings deposited at 200 mA cm^?2. The dual structure degraded the corrosion resistance of the coatings. The highest corrosion resistance was achieved for the Zn–Mn coating deposited at 100 mA cm^?2, pH 4·9 and 25°C. This coating contained 4·1 wt-%Mn and showed a unique surface morphology consisting of randomly arranged packs of very thin platelets, laid perpendicular to the surface and provided a high compactness deficient free structure.     

Environmental corrosion resistance of porous TiAl intermetallic compounds

Porous TiAl intermetallic compound, as a novel substitute for current inorganic porous material, offsets the shortages of both ceramics and metals. The environmental corrosion resistance of porous TiAl intermetallic compound was investigated. The kinetic equation for the cyclic oxidation of porous TiAl alloy at 600 ℃ is determined to be Δm2=1.08×10-5t. After total oxidation of 140 h, porous TiAl intermetallic compound shows more stability of pore structure and the mass gain of TiAl alloy is 0.042 g/m2, which is only 10.6% that of porous 316L stainless steel. The kinetic equation for the cyclic corrosion behavior of porous TiAl alloy in hydrochloric acid with pH=2 at 90 ℃ is determined to be Δm2=5.41×10-5t-2.08×10-4. After 50 h exposure, the mass loss of TiAl alloy is 0.049 g/m2, which is only 14.8% and 5.57% that of porous Ti and stainless steel, respectively. The kinetic equation in hydrochloric acid with pH=3 is determined to be Δm2=2.63×10-6t-3.72×10-6.     

Corrosion stability of electrodeposited cyclic multilayer Zn–Ni alloy coatings

Abstract

This paper reports on a study of electrodeposition and characterisation of cyclic multilayer coatings of Zn–Ni alloy from a sulphate bath. Cyclic multilayer alloy coatings were deposited on mild steel through the single bath technique by appropriate manipulation of cathode current densities. The thickness and composition of the individual layers of the CMA deposits were altered precisely and conveniently by cyclic modulation of the cathode current during electrodeposition. Multilayer deposits with sharp change in composition were developed using square current pulses, using thiamine hydrochloride and citric acid as additives. Laminar deposits with different configurations were produced and their corrosion behaviours were studied by AC and DC methods in 5%NaCl solution. It was observed that the corrosion resistance of the CMA coating increased progressively with the number of layers (up to certain optimal numbers) and then decreased. The decrease in corrosion resistance at high degree of layering was attributed to interlayer diffusion due to less relaxation time for redistribution of metal ions at cathode during deposition. The coating configurations have been optimised for peak performance of the coatings against corrosion. It was found that CMA coating developed at cyclic cathode current densities of 3·0/5·0 A dm^?2 with 300 layers showed the lowest corrosion rate (0·112×10^?2 mm/year) which is ～54 times better than that of monolithic Zn–Ni alloy, deposited from the same bath. The protection efficacy of CMA coatings is attributed to the difference in phase structure of the alloys in successive layers, deposited at different current densities, evidenced by X-ray diffraction analysis. The formation of multilayers and corrosion mechanism were examined by scanning electron microscopy.     

Inhibition of iron corrosion from scratched lacquered tinplate by stannous ions

Abstract

The inhibition of iron corrosion from damaged lacquered tinplate in oxygen free pH 4 citrate buffer solution by the addition of stannous tin has been demonstrated using an accelerated simulated can corrosion test. The inhibition was total for tinplate of nominal coating mass 2·8 g m^?2 for additions of 5–15 ppm of stannous ion, but was only partial for tinplate of nominal coating masses 1·4, 1·0, and 0·5 g m^? up to a 60 ppm addition. The extent of undermining of the lacquer from around a standard scratch was shown to be dependent on the tin coating mass remaining on the tinplate, rather than the presence of stannous ions in solution.