Über die transpassive Auflösung von Nickel-Basis-Legierungen und Edelstählen in sauerstoff- und chloridhaltigem Hochtemperatur-Wasser

Transpassive dissolution of nickel-base alloys and stainless steels in oxygen and chloride containing high-temperature water The corrosion behaviour of different nickel base alloys and stainless steels (2.4605 [alloy 59; NiCr23Mol6Al], 2.4633 [alloy 602 CA; NiCr25FeAlY], 2.4819 [alloy C-276; NiMol6Crl5W], 2.4856 [alloy 625; NiCr22Mo9Nb], 2.4606 [alloy 686; NiCr20Mu16], 2.4646 [alloy 214; NiCrl6AlFe] and 1.4401 [UNS S 31600; X5CrNiMol7122]) was investigated in oxygen and chloride containing high-temperature water (temperatures up to 600°C; pressures up to 38 MPa; oxygen concentration 0.48 mol/kg; chloride concentrations up to 0. 1 mol/kg). All alloys show a similar corrosion behaviour, depending on temperature. At temperatures below about 150°C, only slight intergranular corrosion was observed. At higher temperatures (between about 150 and 300°C) pitting was detected. Most of the original surface in this temperature range remained unattacked. At higher temperatures, morphology of pitting changed towards shallow pitting and the whole surface is penetrated. The high general corrosion observed in these areas can be attributed to transpassive dissolution of the alloys' protecting chromium oxide layers with following dissolution of the alloy. At supercritical temperatures, corrosion decreased drastically, and only transpassive intergranular corrosion was detected. The observed decrease of ion-induced corrosion phenomena can he attributed to the change of physical and chemical properties of water (solubility of ions). Corrosion in neutral and alkaline solution was significantly less. Both pitting and transpassive dissolution shifted towards higher temperatures or were not detected respectively.     

Pitting and crevice corrosion of superaustenitic stainless steels

Superaustenites are mainly used in offshore applications, oil production and chemical industry. Most important types of localised corrosion of these steels are pitting and crevice corrosion. Investigated materials were N08028, S31254 and three modified alloys. Chromium content of investigated alloys varied between 20 and 27%, molybdenum between 3.2 and 6.0%, nitrogen between 0.1 and 0.36% and copper between 0 and 1.1%. For means of comparison stainless steel AISI 316L has been included in the study. Pitting and crevice corrosion of these highly corrosion resistant steels has been investigated by use of standardized tests. Critical pitting temperature and critical crevice temperatures were determined according to ASTM G 48, Methods C and D, respectively. Electrochemical measurements for determination of pitting potentials were done according to ASTM G 61 as well as for determination of critical pitting temperatures according to ASTM G 150. Results are presented as function of MARC (Measure of alloying for resistance to corrosion) defined by Speidel since linear correlation coefficients were higher when compared to conventional PREN. Results obtained by different testing methods must not be compared directly. Every test however is sensitive to microstructural defects like precipitations and segregations that decrease corrosion resistance. The higher alloyed a material is, the higher is its tendency to form microstructural defects, and the more difficult is it to reach its theoretical corrosion resistance at given chemical composition.     

Effect of Cu addition on overaging behaviour,room and high temperature tensile and fatigue properties of A357 alloy

The aims of the present work are to evaluate the overaging behaviour of the investigated Cu-enriched alloy and to assess its mechanical behaviour, in terms of the tensile and fatigue strength, at room temperature and at 200 °C, and to correlate the mechanical performance with its microstructure, in particular with the secondary dendrite arm spacing (SDAS). The mechanical tests carried out on the overaged alloy at 200 °C indicate that the addition of about 1.3 wt.% Cu to the A357 alloy enables to maintain ultimate tensile strength and yield strength values close to 210 and 200 MPa, respectively, and fatigue strength at about 100 MPa. Compared to the quaternary (Al−Si−Cu−Mg) alloy C355, the A357−Cu alloy has greater mechanical properties at room temperature and comparable mechanical behaviour in the overaged condition at 200 °C. The microstructural analyses highlight that SDAS affects the mechanical behaviour of the peak-aged A357−Cu alloy at room temperature, while its influence is negligible on the tensile and fatigue properties of the overaged alloy at 200 °C.     

Corrosion resistance of the high‐strength filler metal NiCr20Fe14Mo11WN (alloy 50 PLUS)

The Nb‐free NiCr20Fe14Mo11WN alloy has been developed as a new filler metal which combines high strength and excellent corrosion resistance in wet environments at ambient temperatures. The alloy is characterized by exceptional resistance to pitting and crevice corrosion, no sensitiveness to chloride induced stress corrosion cracking and shows high mechanical properties, e.g. 0.2 %‐yield strength. Because the alloy is practically free of stable nitride formers like niobium, titanium etc., no nitride formation during welding of super austenitic, duplex and super duplex steels is possible, which proves the alloy to be superior to alloy 625.     

Effect of alloying elements on the behaviour of TiAl-based alloys

TiAl-based alloys containing Nb, Mo and Cr have been produced by induction melting both under an Ar atmosphere and in vacuum. Fracture toughness and isothermal oxidation tests at 900 °C have been performed on a number of alloys in order to evaluate the effect of alloying elements on both fracture toughness and hot corrosion. The results highlighted that there is an alloy composition range that could allow to obtain a good compromise between high temperature oxidation resistance and fracture toughness. In fact only for a selected Al/Ti ratio and a limited content of alloying elements it is possible to attain the alloy microstructural control and, for Mo free alloys, good protectiveness against oxidation.     

On the substitution of conventional corrosion tests by an electrochemical potentiokinetic reactivation test

Nickel-based alloy 625 was investigated by means of a conventional corrosion test (Streicher-test according to ASTM G28A [1]) and an electrochemical potentiokinetic reactivation test (according to ASTM G108 [1], with some modifications). Susceptibility to intergranular corrosion after various heat treatments was examined by using both test methods and the results were compared. Dependent on the type of heat treatment applied, both methods showed partially different results. Possibilities and limitations of replacing Streicher-test with EPR-test are pointed out and commentated. In conclusion, the EPR-test presents itself as a promising and quick investigation technique for detecting susceptibility to intergranular corrosion in nickel-based alloys.     

压下率对冷轧铝/钢(4A60/08Al)复合带材组织性能的影响

结合金相组织观察、显微硬度测量及室温拉伸实验, 对压下率分别为30%、40%、50%及60%时的4A60/08Al冷轧复合带的组织与力学性能进行了研究。采用模拟钎焊后拉剪实验对复合带的界面结合强度进行了评价。结果表明: 随着压下率的增加, 复合带钢层的硬度和屈服强度明显增加, 而铝层的硬度变化不大; 压下率从30%增加到50%时, 界面平均抗剪强度从52 MPa增加到65 MPa, 拉剪断裂位置也从铝钢界面断裂过渡到铝层断裂, 因此把压下率50%定义为4A60/08Al冷轧复合带的稳定压下率; 另外, 当压下率超过50%时, 复合带材的钢层在520℃×24 h退火后的组织基本为等轴的再结晶组织, 综合力学性能良好。     

镁合金表面冷喷涂层防护研究进展

镁合金作为最轻质的金属结构材料,由于其密度低和比强度高等优良的物理和力学性能,在航空、航天、汽车以及电子等领域引起广泛关注。然而,镁合金化学性质活泼、耐腐蚀和耐磨损性差等缺点严重制约其进一步应用。近些年发展起来的冷喷涂技术,在固态下制备涂层,涂层致密且与基体结合良好,因此可为镁合金表面防护提供一种新的有效方法。主要综述了镁合金表面冷喷涂耐腐蚀涂层(纯铝、铝合金和复合材料涂层)和耐磨损涂层(合金和复合材料涂层),论述了影响冷喷涂层耐腐蚀、耐磨损以及其他力学性能(硬度和涂层/基体结合强度)的主要因素,包括杂质元素含量、合金种类以及复合材料涂层中陶瓷颗粒含量、尺寸和形貌等。对比了几种常用表面处理技术制备的纯铝涂层的耐腐蚀性能,并阐述了冷喷涂技术在镁合金表面防护方面的优势。此外,还分析了热处理对冷喷涂纯铝和复合材料涂层耐蚀性的影响。最后提出了目前冷喷涂技术在镁合金防护方面的局限性以及发展难题,对未来研究趋势进行了展望。     

Microstructure and creep properties of a near-eutectic directionally solidified multiphase Mo–Si–B alloy

Due to their excellent creep behavior and acceptable oxidation resistance at ultrahigh temperatures multiphase Mo-based alloys are potential candidates for applications in aerospace engines and the power generating industry. The resulting materials properties, as well as the microstructure of Mo–Si–B materials, strongly depend on the manufacturing process. In the following paper we report on a new Mo–Si–B alloy which was processed by crucible-free zone melting (ZM) from cold pressed elemental powders. SEM investigations of the zone molten microstructure showed well-aligned arrangements of a three-phase microstructure consisting of a Mo solid solution (Mo_SS), and the two intermetallic phases Mo₃Si and Mo₅SiB₂. First, high temperature mechanical properties, such as the compressive strength and creep strength at about 1100 °C, were evaluated and compared with a commonly used Ni-based superalloy and a PM processed Mo–Si–B material. In comparison to the PM processed reference alloy, the creep resistance of ZM materials was found to be substantially improved due to the relatively coarse directionally solidified microstructure. Thus, ZM alloys show great potential for applications at targeted application temperatures of around 1200–1300 °C.     

Mechanical and corrosion behaviour of a Ti‐Al‐Nb alloy after deformation at elevated temperatures

The mechanical properties of Ti6Al7Nb alloy deformed and heat treated at elevated temperatures were correlated with its microstructure and corrosion behaviour in Ringer (of different pH values: 2.49, 6.9 and 8.9) and Ringer–Brown solutions. Microstructural analysis revealed a Widmanstatten structure for the alloys deformed at 1100 °C (β field) and structure with α grains at 930 °C (α + β field). The thermo‐mechanical processing improved the electrochemical behaviour of Ti6Al7Nb alloys, especially their passive state. Open circuit potential variations in time reflected more compact, stable, resistant passive films on the surface of the treated alloys. Open circuit potential gradients simulating the non‐uniformities of pH along the implant surface have very low values that cannot generate galvanic corrosion. Corrosion rates and ion release are very much reduced. Impedance spectra were fitted with a two time‐constants equivalent circuit for some alloys and with three time‐constants equivalent circuit for other alloys.     

Effect of Fe additions on microstructure and mechanical properties of a multi-component Nb–16Si–22Ti–2Hf–2Al–2Cr alloy at room and high temperatures

The phase constitutions, microstructural evolutions, and mechanical properties of Nb–16Si–22Ti–2Hf–2Al–2Cr–xFe alloys (where x = 1, 2, 4, 6 at.%, hereafter referred to as 1Fe, 2Fe, 4Fe and 6Fe alloys, respectively) prepared by arc-melting were investigated. It was observed that the nominal Fe content affected the solidification path of the multi-component alloy. The as-cast 1Fe alloy primarily consisted of a dendritic-like Nb_SS phase and (α+γ)-Nb₅Si₃ silicide, and the as-cast 2Fe and 4Fe alloys primarily consisted of an Nb_SS phase, (α+γ)-Nb₅Si₃ silicide and (Fe + Ti)-rich region. In addition to the Nb_SS phase, a multi-component Nb₄FeSi silicide was present in the as-cast 6Fe alloy. When heat-treated at 1350 °C for 100 h, the 1Fe and 6Fe alloys almost exhibited the same microstructures as the corresponding as-cast samples; for the 2Fe and 4Fe alloys, the (Fe + Ti)-rich region decomposed, and Nb₄FeSi silicide formed. The fracture toughness of the as-cast and heat-treated Nb–16Si–22Ti–2Hf–2Al–2Cr–xFe samples monolithically decreased with the nominal Fe contents. It is interesting that at room temperature, the strength of the heat-treated samples was improved by the Fe additions, whereas at 1250 °C and above, the strength decreased, suggesting the weakening role of the Nb₄FeSi silicide on the high-temperature strength. As the nominal Fe content increased from 1 at.% to 6 at.%, for example, the 0.2% yield strength increased from 1675 MPa to 1820 MPa at room temperature; also, the strength decreased from 183 MPa to 78 MPa at 1350 °C.     

Microstructure and oxidation behaviour of Ir-rich Ir–Al binary alloys

In the current study, alloys of Ir–11Al, Ir–23Al, Ir–30Al, Ir–41Al and Ir–45Al (at.%) were prepared to investigate the microstructure and oxidation behaviour of Ir-rich Ir–Al alloys. Ir(Al)_ss and/or β-IrAl intermetallic phases were found to exist in the prepared alloys. During isothermal oxidation at 1100 °C, the Ir(Al)_ss and β-IrAl individually changed to porous and dense Al₂O₃. The microstructure of the oxide scale formed on Ir–23Al was similar to that of its former alloy which possessed a dendrite-like configuration. It was found that the mass change of Ir–45Al followed a parabolic law, showing the best oxidation resistance among the Ir–Al alloys.     

Microstructure and mechanical properties of Cu/Al joints brazed using (Cu,Ni, Zr,Er)-modified Al—Si filler alloys

To design a promising Al—Si filler alloy with a relatively low melting-point, good strength and plasticity for the Cu/Al joint, the Cu, Ni, Zr and Er elements were innovatively added to modify the traditional Al—Si eutectic filler. The microstructure and mechanical properties of filler alloys and Cu/Al joints were investigated. The result indicated that the Al—Si—Ni—Cu filler alloys mainly consisted of Al(s,s), Al₂(Cu,Ni) and Si(s,s). The Al—10Si—2Ni—6Cu filler alloy exhibited relatively low solidus (521 °C) and liquidus (577 °C) temperature, good tensile strength (305.8 MPa) and fracture elongation (8.5%). The corresponding Cu/Al joint brazed using Al—10Si—2Ni—6Cu filler was mainly composed of Al₈(Mn,Fe)₂Si, Al₂(Cu,Ni)₃, Al(Cu,Ni), Al₂(Cu,Ni) and Al(s,s), yielding a shear strength of (90.3±10.7) MPa. The joint strength was further improved to (94.6±2.5) MPa when the joint was brazed using the Al—10Si—2Ni—6Cu—0.2Er—0.2Zr filler alloy. Consequently, the (Cu, Ni, Zr, Er)-modified Al—Si filler alloy was suitable for obtaining high-quality Cu/Al brazed joints.     

Corrosion behavior of novel Cu–Ni–Al–Si alloy with super-high strength in 3.5% NaCl solution

A novel Cu–6.5Ni–1Al–1Si–0.15Mg–0.15Ce alloy with super-high strength was designed and its corrosion behavior in 3.5% NaCl solution at 25 °C was investigated by the means of SEM observation, TEM observation and XPS analysis. The alloy after solution treatment, 80% cold rolling and aging at 450 °C for 1 h had the best comprehensive properties with hardness of HV 314, electrical conductivity of 19.4% IACS, tensile strength of 1017 MPa, and average annual corrosion rate of 0.028 mm/a. The oxides and chloride products formed at first, followed by the formation of dyroxides products. The alloy showed super-high strength, good electrical conductivity and corrosion resistant because Ni₂Si hindered the precipitation of large NiAl at the grain boundary and the denickelefication of the alloy.     

Electrochemical study of the corrosion of different alloys exposed to deaerated 80 °C geothermal brines containing CO2

Corrosion of metallic engineering materials accounts for problems during geothermal operation in the Upper Rhine Graben (URG). Herein, we study the electrochemical behaviour of various metal alloys in an 80 °C simulated geothermal environment by using potentiodynamic polarisation and open-circuit potential measurements. Two different natural geothermal waters from URG geothermal sites were used for the experiments. The measurements reveal spontaneous passivation to be a key process for all alloys. This ennoblement protects more noble alloys from significant corrosion (e.g. titanium gr. 2, alloy 625) and brings less noble alloys to failure, mostly due to pitting corrosion (e.g. 316L).     

Al含量和烧结温度对Ti_(50)Ni_(50)与Ti_(47)Ni_(47)Al_6合金耐蚀性的影响

采用粉末冶金方法制备了Ti_(50)Ni_(50)与Ti_(47)Ni_(47)Al_6合金,通过极化曲线、配备能谱分析的扫描电镜测试手段研究了Al含量和烧结温度对烧结合金耐蚀性的影响。结果表明:1080℃烧结Ti_(50)Ni_(50)合金表现为钝化特征,蚀孔尺寸较小,弥散分布;Al含量为6%时,合金表现为活性溶解,蚀孔尺寸及腐蚀区域面积显著增加,耐蚀性降低;烧结温度提高至1180℃时,合金重新表现为钝化,蚀孔尺寸及腐蚀区域面积显著减小,耐蚀性最佳。     

Corrosion behaviour of thermally sprayed Al and Al/SiCp composite coatings on ZE41 magnesium alloy in chloride medium

Thermally sprayed Al and Al/SiCp composite coatings have been deposited on ZE41 magnesium alloy and mechanical compaction at room temperature was applied to the Al and Al/SiCp coatings to reduce their porosity. Corrosion behaviour of coated samples was evaluated and compared to that of uncoated substrate in 3.5 wt.% NaCl solution using electrochemical measurements. Al and Al/SiCp composite coatings reduced the corrosion current density of Mg-Zn alloys by three and two orders of magnitude, respectively, and reductions up to four orders of magnitude were obtained after mechanical compaction.     

Magnetron-sputtered oxidation protection coatings for Mo–Si–B alloys

Oxidation protective Mo–70Al, Mo–37Si–15B and Mo–46Si–24B (at.%) coatings with 5–10 μm thickness were deposited on Mo–9Si–8B alloys by magnetron sputtering; and their oxidation behavior was studied at 800, 1000 and 1300 °C in air. On the Mo–70Al layer a dense aluminum borate scale grew at 800 °C; however, this coating rapidly degraded at 1000 °C linked to substrate oxidation at uncoated areas. The Mo–37Si–15B and Mo–46Si–24B layers provided oxidation protection to the Mo–Si–B alloy at 800 and 1000 °C for up to 100 h due to formation of a borosilicate scale. The latter coating was protective for short times even at 1300 °C.     

Oxidationsverhalten nitridverfestigter Nickelbasislegierungen

Oxidation behaviour of nitride-strengthened Ni-base alloys Nitride-strengthened Ni-base alloys of the type NiCr7030, produced by DESU-technology, provide promising mechanical properties up to temperatures of 1200°C. This paper provides data on the cyclic and isothermal oxidation behaviour of these alloys in the temperature range of 1000–1200°C. The results show, that depending on the actual composition of the alloys tested even at 1200°C corrosion rates below 0.3 g/m² h can be achieved. It was found that the silicon content must be limited to a maximum of 1.00 wt.-% to guarantee the excellent oxidation resistance even at temperatures higher than 1100°C.     

Effect of carbon addition on solidification behavior,phase evolution and creep properties of an intermetallic β-stabilized γ-TiAl based alloy

Improving mechanical properties of advanced intermetallic multi-phase γ-TiAl based alloys, such as the Ti-43.5Al-4Nb-1Mo-0.1B alloy (in at.%), termed TNM alloy, is limited by compositional and microstructural adaptations. A common possibility to further improve strength and creep behavior of such β-solidifying TiAl alloys is e.g. alloying with β-stabilizing substitutional solid solution hardening elements Nb, Mo, Ta, W as well as the addition of interstitial hardening elements C and N which are also carbide and nitride forming elements. Carbon is known to be a strong α-stabilizer and, therefore, alloying with C is accompanied by a change of phase evolution. The preservation of the solidification pathway via the β-phase, which is needed to obtain grain refinement, minimum segregation and an almost texture-free solidification microstructure, in combination with an enhanced content of C, requires a certain amount of β-stabilizing elements, e.g. Mo. In the present study, the solidification pathway, C-solubility and phase evolution of C-containing TNM variants are investigated. Finally, the creep behavior of a refined TNM alloy with 1.5 at.% Mo and 0.5 at.% C is compared with that exhibiting a nominal Ti-43.5Al-4Nb-1Mo-0.1B alloy composition.