Effect of chromium on the protective properties of aluminide coatings

Aluminide and chromaluminide diffusion coatings on nickel and a nickel-base superalloy, EI 867, were subjected to different corrosion tests consisting of oxidation under isothermal and thermal cycling conditions, and oxidation in the presence of fused sodium sulfate. It was found that chromium present in the surface layers of aluminide coatings has a beneficial effect on their resistance to oxidation and hot corrosion.     

The hot-corrosion behavior of novel CO-deposited chromium-modified aluminide coatings

This paper reports the successful co-deposition of inclusion-free chromiummodified aluminide coatings using a pack-cementation process. The substrate used was the nickel-base superalloy, René 80H. The coatings were of the outward-diffusion type; however, unlike the usual outward-diffusion coatings, the present coatings were relatively free of pack inclusions. The coatings consisted of -Cr precipitates in a matrix of -NiAl. The morphology and distribution of the -Cr precipitates could be adjusted to the extent that two types of coating structures could be obtained. The Type I coating structure contained lamellar -Cr precipitates situated in the surface region of the coating, whereas the Type II coating structure contained small, spheroidal -Cr precipitates distributed throughout the outer of a two-layered coating. Both coating types exhibited significantly improved hot-corrosion resistance in a 0.1% SO₂-O₂ environment at 900°C compared to a commercial aluminide coalting. A study of the corrosion behavior of Type I coatings containing pack inclusions showed that the inclusions were deleterious to the corrosion resistance of the coatings. The corrosion behavior of chromium-aluminide coatings was dependent on both the distribution and amount of -Cr precipitates in the coating.     

Structure and cyclic oxidation resistance of Pt, Pt/Pd-modified and simple aluminide coatings on CMSX-4 superalloy

The coatings were prepared by the means of Pt and Pt/Pd galvanizing, followed by vapor phase aluminizing at 1050 °C. Microstructural and phase analysis revealed that all the investigated coatings consisted mainly of β-NiAl phase, however the Pt-modified aluminide coating also contained PtAl₂ phase and pure platinum precipitates. The cross-sectional microstructure of the coatings was zonal and composed of β-NiAl phase zone and the diffusion zone. The Pt modified aluminide coating's cross-section also incorporated an outermost zone consisting of β-NiAl and PtAl₂ phases. The concentration profiles proved that both Pt and Pd contents decrease gradually inwards the modified coatings. Cyclic oxidation tests at 1100 °C proved that Pt/Pd-modified aluminide coatings exhibit the best performance under cyclic conditions. The analysis of oxidation kinetics curves showed that the course of simple aluminide coating's oxidation is slightly different from that of Pt- and Pt/Pd-modified aluminide coatings.     

Comparison between field and laboratory steam oxidation testing on aluminide coatings on P92

Steam oxidation has become an important issue for steam power plants as operating temperatures increase from the current 550 to 600–650 °C. For the last 10 years several groups have been carrying out steam oxidation testing of both uncoated substrates and coatings in the laboratory. On the other hand, field testing results are very scarce. In this paper, a comparison of laboratory steam oxidation testing with field test results carried out by Alstom at the Kraftwerk Westfalen power station located in Hamm, Germany will be presented. Both slurry deposited aluminide coatings and uncoated P92 steel have been included in the study. Under steam (atmospheric pressure) and isothermal conditions in the laboratory at 650 °C, spallation of oxides formed on ferritic steels occurs after significantly longer time when compared to exposure to real operating conditions. Oxide spallation results in serious damage in steam power plants by obstructing heat exchanger tubes, erosion of valves and turbine blades, etc. Moreover, the thickness of the oxide scales formed under field testing conditions is significantly higher after similar exposure. On the other hand, aluminide coated P92, which exhibit thickness through cracks, have shown to be stable in the laboratory for up to 60 000 h at 650 °C under steam, without evidence of crack propagation. However, field test results indicate that some degree of crack propagation occurs but without causing substrate attack up to 21 700 h of exposure. Moreover, the aluminium oxide observed in both laboratory and field tested specimens is different.     

TiAl合金表面机械研磨渗铝涂层的制备及高温性能

利用机械研磨渗在TiAl合金表面制备了铝化物涂层。在600℃经过150 min的振动处理后,TiAl合金表面形成了约为30μm的涂层。该涂层均匀致密,Al和Ti元素沿涂层均匀分布。XRD分析表明该涂层的相结构为Al3 Ti相。在900℃下,经过300 h的循环氧化后,该铝化物涂层表面形成了一层连续均匀的Al2 O3膜,表现出良好的抗氧化和抗剥落性能。相比之下,TiAl合金空白样表面形成了分层结构的氧化层,主要由Al2O3和TiO2混合氧化物组成,因此表现出较差的高温性能。     

Preparation of aluminide coatings at relatively low temperatures

1 Introduction Protective coatings by pack aluminizing are frequently applied to metals to protect them from high temperature oxidation and hot corrosion attack [1, 2]. Pack aluminizing consists of heating the parts to be coated in a closed or vented pac…     

Microstructure and stresses in HVOF sprayed iron aluminide coatings

The microstructure and state of stress present in Fe₃Al coatings produced by high velocity oxygen fuel (HVOF) thermal spraying in air at varying particle velocities were characterized using metallography, curvature measurements, x-ray analysis, and microhardness measurements. Sound coatings were produced for all conditions. The microstructures of coatings prepared at higher velocities showed fewer unmelted particles and a greater extent of deformation. Residual stresses in the coatings were compressive and varied from nearly zero at the lowest velocity to approximately −450 MPa at the highest velocity. X-ray line broadening analyses revealed a corresponding increase in the extent of cold work present in the coating, which was also reflected in increased microhardness. Values of mean coefficient of thermal expansion obtained for assprayed coatings using x-ray analysis were significantly lower than those for powder and bulk alloy.     

Comparison of inward and outward grown Pt modified aluminide diffusion coatings on a Ni based single crystal superalloy

Two commercial Pt modified aluminide coatings (RT22 and MDC150L) on the same single crystal Ni-based superalloy (CMSX-4) were studied by: scanning electron microscopy; transmission electron microscopy; energy dispersive X-ray spectrometry; and gravimetry. The RT22 coating is an inward grown coating (high activity), while MDC150L is produced by outward growth (low activity). Samples were oxidised in still laboratory air at 1050 °C for various times up to 2000 h. It was found that the outward grown coating produced a slower growing oxide that was more spallation resistant. Several possible reasons for this were identified including: coating purity; coating surface topography; and ductile to brittle transition temperature. The microstructural differences between the two coatings in the as-coated condition were investigated and the development of their microstructure during heat treatment was described. A model for coating growth during heat treatment was proposed.     

Factors affecting the microstructure of platinum-modified aluminide coatings during a vapor phase aluminizing process

Platinum (Pt)-modified aluminide coatings were developed by electroplating a thin layer of Pt followed by an industrial vapor phase aluminizing process. The goal of this work was to systematically investigate the effect of critical coating process parameters (such as the electroplated Pt thicknesses, Al contents in Cr-Al nuggets, diffusion heat treatments) and substrates on the final Pt-modified aluminide coatings. Surface morphology and cross-section microstructure of the developed coatings were inspected and compared by using Optical Microscope, Scanning Electron Microscope (SEM) equipped with energy dispersive spectroscopy (EDS). Experimental results showed that the Al and/or Pt increase shall favor the formation of ξ-PtAl₂ phase; transformation of ξ-PtAl₂ into β-(Ni,Pt)Al phase can be obtained via a heat treatment process; Cr, Co elements in the studied Ni-base superalloy substrates did not show significant influence on coating outer layer microstructure; while substrate elements affect the microstructure of the coating interdiffusion layer.     

Microstructure evolution and corrosion behavior of Fe-Al-based intermetallic aluminide coatings under acidic condition

Two Fe-Al-based intermetallic aluminide coatings were fabricated on 430-SS (Fe-Cr) and 304-SS (Fe-Cr-Ni) substrates by pressure-assisted solid diffusion bonding with coating on pure Fe as control. The microstructure and intermetallic phases of the coatings were characterized by SEM, EDS and EBSD. A network of Cr₂Al₁₃ with matrix of Fe₄Al₁₃ was formed by inter-diffusing of Al with the substrates. The corrosion behavior of intermetallic coatings was investigated in 0.5 mol/L HCl solution by mass-loss, OCP, Tafel plot and EIS. It was found that corrosion resistance was greatly enhanced by dozens of times after the addition of Cr and Ni compared with that on pure Fe. The presence of cracks in the coating on 430-SS provided a pathway for corrosion media to penetrate to the substrate and accelerated the corrosion rate. Moreover, the corrosion product was analyzed by XRD, demonstrating that the addition of Cr and Ni facilitated the formation of more corrosion resistant phases, and therefore improved corrosion resistance.     

铁铝含量对Fe-Cr-Ni合金渗铝涂层及其防结焦性能的影响

采用固体粉末包埋渗铝法,在Fe-Cr-Ni合金表面制备渗铝涂层,并研究了其因渗剂中铁铝含量的不同对Fe-Cr-Ni合金渗铝、氧化及结焦行为的影响。通过对各组渗铝以及渗铝并氧化后的试样表面及截面微观形貌、相组织进行研究,并结合结焦增重曲线发现,铁铝比例的增加能够促使合金表面形成平整致密的渗铝涂层,提高氧化膜的质量,但也会降低渗层的厚度,影响表面氧化膜的连续性。选用合适的铁铝含量可以获得较高质量的氧化铝涂层,进而达到防结焦效果。     

Fundamental investigation of the degradation laws of aluminide coatings during high‐temperature oxidation

During high‐temperature oxidation, the aluminide coating degrades by two mechanisms, one is the oxidation of the outer surface, and the other is Al diffusion into the substrate. The relevant laws are investigated for coatings on Co‐base superalloys oxidised in air at high temperatures. The driving force mainly comes from the oxidation reaction for the outer degradation layer, and the chemical potential for the inner degradation layer. By an approximation approach, it can be concluded that the life of the outward‐diffusion coating is roughly directly proportional to the second power of the main‐phase thickness of the coating.     

Deposition and corrosion resistance of HVOF sprayed nanocrystalline iron aluminide coatings

The microstructure and corrosion properties of nanocrystalline Fe–40Al coatings obtained by thermal spraying of milled powder were investigated. The coatings were sprayed under similar high-velocity oxy-fuel (HVOF) conditions and were varied by the size of the starting feedstock powder. The coatings have complex microstructure consisting essentially of a mixture of well-flattened splats and non-fully melted powder particles within which an equiaxed nanometer-scale structure is retained. Amorphous Al₂O₃ and nanocrystalline Fe-rich oxides together with Fe₃Al (resulting from Al depletion and reaction in the flame) were present at intersplat boundaries. The amount of these phases and porosity, as well as the presence of unmelted powder particles, has been quantified. It is shown that the feedstock powder size has a strong effect on the coating hardness by modifying the amount of hard unmelted powder particles. The electrochemical response of the coatings shows the same general type of active–passive–transpassive behaviour than the microcrystalline bulk Fe–40Al but with poorer corrosion resistance parameters. Analysis of corrosion damage shows a prevalent localized attack at intersplat boundaries or around unmelted powder particles, probably enhanced by galvanic phenomena, that is likely responsible for the poorer corrosion properties of the coatings. To a lesser extent, corrosion takes place by a more global form of attack within splats containing ultrafine grains. If the amount of unmelted powder particles controls the overall hardness of the coatings, it appears to have limited direct effect—if any—on the corrosion behaviour. Thus, the hardness/corrosion balance can be optimized by a good selection of powder size feedstock.     

电脉冲沉积铝化物微晶涂层的高温腐蚀行为

采用自行设计的振动式电脉冲沉积装置在Q235钢表面沉积厚度达100μm的铝化物微晶涂层,并研究涂层的高温腐蚀行为。采用振动式电脉冲沉积技术沉积涂层时,铝电极与工件瞬时接触放电,可以达到很高的温度,从而形成了铁铝金属间化合物涂层,而且涂层与基体为冶金结合;涂层冷却速度很快,由此获得微晶结构涂层;流动的氩气可有效地保护涂层的质量。将沉积铝化物微晶涂层的Q235钢分别在600℃空气中氧化200h和600℃99.98%SO2气氛中硫化50h,然后进行SEM、EDS和XRD分析。结果表明,沉积铝化物微晶涂层后Q235钢的抗氧化和硫化性能均大幅度提高。     

Low-temperature formation and oxidation resistance of ultrafine aluminide coatings on Ni-base superalloy

Ultrafine aluminide coatings were successfully produced on Ni-18Fe-17Cr superalloy at 540-600 °C in a modified pack-aluminizing process. Repeated ball-impacts accelerated the formation of the aluminide coatings by a surface refining process, resulting in atomic diffusion occurring at a relatively low temperature. The effects of the operation temperature and the treatment duration on the formation of the coatings have been investigated. The coatings possessed a two-layer structure. The top layer, approximately 5 µm in thickness, exhibited equiaxial coarse grains and was dominated by NiAl₃, with small amounts of Fe₂Al₅ and CrAl₅. The bottom layer showed high density, homogeneous, ultrafine grains with diameters approximately 30-50 nm. High-temperature oxidation tests were carried out at 1000 °C. The oxidation kinetics and microstructure of the oxide scale were studied. The experimental results indicated that the coatings greatly enhanced the high-temperature oxidation resistance of Ni-18Fe-17Cr superalloy.     

Creep and corrosion testing of aluminide coatings on ferritic-martensitic substrates

Pack and chemical vapor deposited (CVD) aluminide coatings on commercial ferritic-martensitic Fe-9Cr-2W steel are being investigated by creep and corrosion testing at 650 °C. Results from different coating thicknesses show that the coated region makes no contribution to the creep strength. The creep behavior of uncoated material was studied after various heat treatments to simulate the coating process and typical secondary heat treatments. Alternating creep and corrosion exposures showed little effect on the creep strength of uncoated material but coated materials became progressively weaker. The coatings were protective in wet air at 650 °C after creep testing.     

钴基超合金铝化物涂层的高温氧化寿命

通过分析钴基超合金外扩散型铝化物涂层的高温氧化及退化行为 ,并采用近似方法推算 ,得出了这类涂层氧化和退化与时间的关系。研究表明 ,涂层外侧退化主要由氧化反应驱动力所控制 ,涂层的外侧退化速率正比于其氧化速率 ;而涂层的氧化寿期近似正比于其主体层厚的平方。运用氧化退化与时间的关系式 ,进行简便的辅助实验 ,即可预测出这类涂层的高温氧化寿命。     

稀土元素Y对TiNi形状记忆合金基体上的渗铝涂层恒温氧化行为影响

采用粉末法,在不同Y含量的TiNi形状记忆合金基体上制备了650℃渗铝涂层,并对Y是如何影响涂层的形成以及涂层700℃恒温氧化性能进行了研究。结果表明渗铝涂层由外层TiAl₃和内层NiAl₃构成,涂层的生长主要由Al的内扩散控制。当Y含量低于1at.%时,稀土元素Y的添加促进TiAl₃外层的生长,抑制NiAl₃ 内层的生长。恒温氧化实验表明：添加0.5 at.%Y能明显降低渗铝涂层的氧化速度,但添加1 at.%Y和5 at.%Y却加速涂层的氧化。文中对Y是如何影响涂层的形成以及恒温氧化性能进行了分析。     

Iron aluminide coatings by an in-situ reaction process

Iron aluminide (Fe₃Al) coatings were prepared by a novel reaction process. In the process, the iron aluminide coating is formed by an in-situ reaction between the aluminum powder fed through a plasma transferred arc (PTA) torch and the steel substrate. Subjected to the high temperature within an argon plasma zone, aluminum powder and the surface of steel substrate melt and react to form the iron aluminide coating. The prepared coating was found to be phase-pure iron aluminide that is porosity-free and metallurgically bonded to the steel substrate. It is expected that the principle demonstrated in this process can be applied to the preparation of other intermetallic and alloy coatings.     

Effect of high-density current electropulsing on corrosion cracking of titanium aluminide intermetallic

The effect of electropulsing on the corrosion cracking of titanium aluminide produced by self-propagating high-temperature synthesis has been investigated. The electropulsing treatment led to improved corrosion resistance in sodium fluoride solution and also eliminated corrosion cracking at the α₂/γ interface during corrosion in a solution of nitric and hydrofluoric acids. This behavior was attributed to thermal and athermal effects resulting from electropulsing and leading to the interaction of conduction electrons with the defect structure. The effect of magnetic field accompanying electropusling on depinning of dislocations also has been discussed. Support for this is provided on the basis of X-ray diffraction analysis and microhardness testing.