The role of alloying elements in commercial alloys for corrosion resistance in oxidizing‐chloridizing atmospheres. Part II: Experimental investigations

In an extensive study the role of the alloying elements in commercial alloys for corrosion resistance was studied in air without and with 0.1 and 2 vol.% Cl₂, respectively. In the first part of this paper [1] the thermodynamic fundamentals were discussed on the basis of the new concept of the quasi‐stability diagrams. The second part which is presented here reports the results from investigations at 650, 800 and 1000°C and testing times up to 1000 hrs where 14 commercial alloys were tested with regard to their corrosion behavior. The materials were selected so that the role of the alloying elements Mo, C, Si, Al, N, Fe, Ni and Cr would be evident from the results. The exposure tests were followed by extensive microstructural analyses of the corrosion scales and the metal subsurface zones so that type, mechanism and extent of corrosion could be characterized in great detail. At the end a ranking was possible of the different materials and with regard to the detrimental or beneficial role of the different alloying elements. The present results thus provide a much deeper insight into materials resistance in oxidizing‐chloridizing environments at high temperatures.     

Corrosion of furnace wall materials in waste-wood fired power plant

Abstract

Corrosion tests were performed with four different materials exposed at the furnace wall in a power boiler burning recycled wood, with the aim of evaluating coatings to reduce the corrosion. The nickel base Alloy 625 and the iron–chromium–aluminium alloy Kanthal APMT had the lowest corrosion rates followed by the stainless steel 310S. The low alloy steel 16Mo3, from which the walls are constructed, had the highest rate. Different corrosion mechanisms were found to occur according to the alloy type. Thermodynamic modelling showed that chlorine gas exists at extremely low levels under the prevailing conditions and the hydrated form is thermodynamically favoured.     

Structure of Al-modified silicide coatings on an Nb-based ultrahigh temperature alloy prepared by pack cementation techniques

In order to prepare Al-modified silicide coatings on an Nb-based ultrahigh temperature alloy, both a two-stage pack cementation technique and a co-deposition pack cementation technique were employed. The two-stage process included siliconizing a specimen at 1150 °C for 4 h followed by aluminizing it at 800-1000 °C for 4 h. The coating prepared by pack siliconization was composed of a thick (Nb,X)Si₂ (X represents Ti, Cr and Hf elements) outer layer and a thin (Nb,X)₅Si₃ transitional layer; after the siliconized specimens were aluminized at or above 860 °C, a (Nb,Ti)₃Si₅Al₂ phase developed at the surface of the coating, and furthermore, when aluminizing was carried out at 860 °C, a new (Nb,Ti)₂Al layer formed in the coating between the (Nb,X)₅Si₃ layer and the substrate, but when aluminizing was performed at 900-1000 °C, the new layer formed was (Nb,Ti)Al₃. The co-deposition process was carried out by co-depositing Si and Al on specimens at 1000-1150 °C for 8 h under different pack compositions, and it was found that the structure of co-deposition coatings was more evidently affected by co-deposition temperature than pack composition. An Al-modified silicide coating with an outer layer composed of (Nb,Ti)₃Si₅Al₂, (Nb,X)Si₂ and (Nb,Ti)Al₃ was obtained by co-depositing Si and Al at 1050 °C.     

CeO2颗粒对铌合金表面镀渗复合涂层抗氧化性能的影响

为了提高铌合金的高温抗氧化性,采用化学镀结合包埋渗技术在铌合金表面制备了含有CeO_2颗粒的复合涂层,研究了复合涂层的微观结构和高温抗氧化性能。结果表明,不含CeO_2的Al/Ni涂层以NiAl相为主,Al/Ni-CeO_2涂层则含NiAl、NiAl_3、Al3Nb和CeO_2等相。经1000℃氧化测试,Al/Ni复合涂层氧化50 h后增重为8.0 mg/cm~2,表面主要生成Al_2O_3、AlNbO_4相;Al/Ni-CeO_2复合涂层50 h后氧化增重为4.0 mg/cm~2,表面以Al_2O_3、CeO_2、NiAl、NiAl_3、Al3Nb、AlNbO_4相为主。高温氧化后,2种涂层样品表面均生成连续致密的Al_2O_3膜,涂层与基体结合良好;含CeO_2的涂层,其稀土氧化物主要在Ni膜拖拽力作用下富集于涂层互扩散区。稀土氧化物颗粒的添加细化涂层组织,降低涂层中Al元素的消耗,填补涂层中的孔洞,增强了氧化膜与涂层的粘附力,有效提高了涂层的抗氧化性。     

Mechanism of Degradation of Titanium Alloy IMI 834 and Its Protection Under Hot Corrosion Conditions

The excellent combination of high-temperature strength and lightweight properties makes titanium-base alloys attractive for high-temperature applications in aircraft engines. However, more hot corrosion of titanium alloys is a life-limiting factor, particularly when aircraft fly at low altitudes across the sea. In the present paper, an attempt has been made to understand the degradation mechanism of titanium alloy, IMI 834 under hot corrosion conditions at elevated temperatures. The hot corrosion studies were carried out by determining weight loss at different temperatures and in salts of pure Na₂SO₄, 90% Na₂SO₄+10% NaCl and 90% Na₂SO₄+5% NaCl+5% V₂O₅. Subsequently, the rate constants were evaluated. The depth of attack due to hot corrosion was compared with oxidation data. Finally, the degradation mechanism of the titanium alloy that leads to degradation of mechanical properties in aggressive environments has been discussed and suitable coatings suggested to enhance the operational life of engines by effectively preventing both oxidation and hot corrosion.     

Monitoring of oxidation of metals in gas environments for energy production

Monitoring of oxidation in gas environments typical of energy production is essential for determination of lifetime and condition of various components operating at high temperatures. One of the most important phenomena to be monitored is the oxide growth kinetics and its changes during the exposure. Three different monitoring methods have been presented in this paper. First, a conventional method called thermogravimetry based on the weight change measurement during oxidation is reviewed. Second, a novel monitoring technique, HT-CER technique, where contact electric resistance is measured as a function of time and distance of the measuring tip is presented. The third approach described involves a method under development, based on the change of capacitance over two growing oxide surfaces. Simple oxidation in air at a typical temperature for superheater tubes in boilers, 600 °C, has been selected as the reference condition for all of the monitoring methods.     

载荷对镍基合金涂层高温磨损特性的影响

研究了一种镍基合金涂层在不同载荷下的高温干摩擦滑动磨损特性。结果表明,涂层磨损的主要方式是微犁削;室温磨损和低载下的高温磨损因涂层保持较高硬度、单位面积磨损体积小,高温高载下磨损因涂层磨损表层形成白亮层,磨损体积显著提高;磨损过程中涂层磨损表面覆盖的氧化物磨屑特别是致密的氧化物层对涂层起到保护作用。     

Identification of degradation mechanisms in coatings for supercritical steam applications

The development and qualification of coatings for materials used in modern steam power plants stems from the increased demand for higher efficiency, and hence higher operating temperatures. Within the EU funded project ‘SUPERCOAT’, several coatings, both overlay and diffusion type, were investigated. Seven different coatings are presented in this work. They included two commercially available HVOF coatings (Ni–20Cr and Ni–50Cr), an aluminium‐based slurry coating (IPCOTE), together with two further variations of this slurry coating containing sputter‐coated inter‐layers. An overlay slurry coating consisting of silica particles embedded in a matrix of alumina and chromia was also examined. The final coating to be investigated was a pack‐aluminised sample of P92. All the coating systems examined showed superior oxidation resistance compared to the 9%Cr steel substrate (P91 or P92) in extended exposures to a steam environment at 650 °C. However, in service component lifetime will be limited by degradation of the coating, therefore it is essential that the mechanisms controlling this behaviour are understood. This paper reviews several degradation mechanisms that have been observed during long‐term exposure of these coatings. The mechanisms that have been observed include depletion of active alloying elements, diffusion of aluminium into the substrate from the coating, formation of Kirkendall porosity and mechanical failure of the coatings. Examples of each of these mechanisms will be presented. Possible processing routes to avoid these degradation mechanisms will also be discussed.     

成分和结构对Ni—W—P镀层抗冲刷腐蚀性能的影响

研究了不同成分和结构的ＮｉＷＰ合金镀层的硬度、耐蚀性及在液固双相流中的抗冲刷腐蚀性能,并对镀层表面膜进行了ＸＰＳ分析,结果表明：镀层中钨含量的增加有利于提高镀层的硬度、耐蚀性和抗冲刷腐蚀性能。由Ｎｉ（ＯＨ）２,ＷＯ３,Ｎｉ３（ＰＯ４）２组成的表面膜具有较强的修复能力,这是使镀层具有优异抗冲刷腐蚀性能的主要因素之一。４００℃以下较低温度的热处理提高了镀层的硬度,但削弱了镀层的耐蚀性,然而对抗冲刷腐蚀性能却略有提高。     

镍基变形高温合金的氧化研究

对某镍基时效强化高温合金在750-950℃,100 h进行静态恒温氧化研究,结果表明,合金在750-950℃氧化100 h的氧化动力学均近似服从抛物线规律。合金900℃和950℃发生内氧化。合金外氧化膜组成：Cr2O3、NiCr2O4、少量NiO和微量TiO2,内层氧化物为少量的Al2O3和TiO2。     

高温度梯度定向凝固振荡性微观结构的特征和形成机制

利用新型高温梯度（ＧＬ＝２５６～１３００Ｋ／ｃｍ）定向凝固装置，研究了籽晶法制备含细亚结构的镍基高温合金ＤＤ３单晶及二元模型合金Ａｌ０．８５％Ｃｕ晶体生长过程的组织演化与选择特征中的非稳态组织特性，特别是定向胞状振荡性微观结构的组织特征及其与定向凝固工艺条件的关系，并将其与在晶体生长透明有机物模拟装置Ｊａｃｋｓｏｎ温度平台（ＧＬ可达１５０Ｋ／ｃｍ）上原位观察到的ＣＢｒ４定向生长结果相对比，从而分析其形成机制，探索有效控制和利用该现象的工艺途径。     

铸造镍基高温合金K447的高温氧化行为

用静态增重法测定了铸造镍基高温合金K447在700℃～950℃空气中的恒温氧化行为,其氧化动力学符合抛物线规律.在900℃以下为完全抗氧化级,在900℃～950℃为抗氧化级.X射线衍射、扫描电镜和能谱分析表明,K447氧化膜分为3层：外层是疏松的Cr2O3和TiO2的混合物,并含有少量的NiO及NiCr2O4尖晶石;中间层是Cr2O3;内氧化物层是Al2O3,并含有少量TiN.     

激光熔覆镍基合金涂层及其应用研究

采用激光熔覆技术在４５钢表面获得镍基合金涂层。研究了激光熔覆涂层工艺、熔覆层的组织结构、硬度和腐蚀－冲蚀磨损特性,并用２Ｃｒ１３不锈钢进行了腐蚀－冲蚀磨损对比试验。激光熔覆镍基合作层已经成功地应用于矿用渣浆泵的平衡盘等零件。