High Temperature Oxidation and Wear Resistance of Y-modified Hot Dipping Aluminized Coating on SCH12 Steel

Microstructures,high-temperature oxidation and wear resistance of hot dipping Al-Si-Y coating on SCH12 heat resistant cast steel were investigated in this study.The aluminized coating was characterized by scanning electron microscopy(SEM),energy dispersive X-ray spectrometry(EDX)and X-ray diffraction(XRD).The results showed that the coating was composed of the Al-rich outer layer and the intermetallics inner layer.In the Al-rich layer,some Y-rich precipitates and Fe-Al-Si-Cr precipitates could be observed.The intermetallics layer presented three layers induced by the increase of Fe,Cr,Ni content and the corresponding decrease of Al,Si content.The oxidation tests were conducted in still air at 850℃for up to 100 h.After oxidation,a top oxide scale composed of mainlyα-Al2O3,Al5Y3O12 was formed on the steel surface.The intermetallics beneath the oxide scale consisted of mainly FeAl and small amount of Fe2Al5 and Cr3Si phase.The mass gain of the coated and uncoated SCH1steel is 0.45 mg/cm 2 and 0.57 mg/cm 2,respectively.The wear resistance was investigated using a high-temperature pin-on-disc tribometer at 650℃.The wear rate for the coated and uncoated steel is 0.45μm 3 /μm.N and 3.01μm 3 /μm.N,respectively.The high temperature wear tests and oxidation tests results demonstrated that the yttrium-modified aluminized specimen had significantly improved high-temperature wear resistance and equivalent oxidation resistance compared with the original SCH12 specimen.     

Pack Cementation Coatings for High-Temperature Oxidation Resistance of AISI 304 Stainless Steel

Aluminum and titanium are deposited on the surface of steel by the pack cementation method to improve its hot-corrosion and high-temperature oxidation resistance. In this research, coatings of aluminum and titanium and a two-step coating of aluminum and titanium were applied on an AISI 304 stainless steel substrate. The coating layers were examined by carrying out scanning electron microscopy (SEM) and x-ray diffraction (XRD). The SEM results showed that the aluminized coating consisted of two layers with a thickness of 450???m each, the titanized coating consisted of two layers with a thickness of 100???m each, and the two-step coatings of Al and Ti consisted of three layers with a thickness of 200???m each. The XRD investigation of the coatings showed that the aluminized coating consisted of Al₂O₃, AlCr₂, FeAl, and Fe₃Al phases; the titanized layers contained TiO₂, Ni₃Ti, FeNi, and Fe₂TiO₅ phases; and the two-step coating contained AlNi, Ti₃Al, and FeAl phases. The uncoated and coated specimens were subjected to isothermal oxidation at 1050?°C for 100?h. The oxidation results revealed that the application of a coating layer increased the oxidation resistance of the coated AISI 304 samples as opposed to the uncoated ones.     

Influence of microarc oxidation and hard anodizing on plain fatigue and fretting fatigue behaviour of Al-Mg-Si alloy

The present study compares the performance of microarc oxidation (MAO) and hard anodizing (HA) treated Al-Mg-Si alloy (AA6063) test samples under cyclic loading in uniaxial tension with a stress ratio of 0.1 (plain fatigue) and fretting fatigue loading. Fatigue test specimens were treated using MAO and HA techniques. MAO coated specimens were ground to reduce the surface roughness comparable with that in HA coated specimens. In that process the porous outer layer was removed. Characterization of coated and uncoated specimens was done with reference to the coating morphology, microhardness, surface roughness and residual stress. The specimens were tested under plain fatigue and fretting fatigue loading at ambient temperature. While the ground MAO coating exhibited relatively less amount of porosity, HA coating had through thickness cracks. MAO coating had compressive residual stress and it was very hard compared with HA coating. Both types of coated samples exhibited slightly higher friction force than that experienced by the uncoated specimens. Fretted region of the HA coated samples was rougher than that of the MAO coated specimens. Plain fatigue lives of both coated samples were inferior to those of the uncoated specimens. The inferior plain fatigue lives of MAO coated specimens compared with those of the substrate may be attributed to the tensile residual stresses supposedly present in the substrate leading to an early crack initiation in the substrate adjacent to the coating. As friction force of MAO coated samples was higher than that experienced by uncoated specimens, the fretting fatigue lives of MAO coated samples were slightly inferior to those of uncoated samples. As the anodized layer had preexisting through thickness cracks and strong adhesion with the substrate, cracks propagated from HA coating through the interface into the substrate easily. This may be the reason for the HA coated samples exhibiting inferior plain fatigue and fretting fatigue lives compared with MAO coated and uncoated samples.     

Effect of Hf and Y alloy additions on aluminide coating performance

Iron- and Ni-base alloys, with and without Hf or Hf and Y alloy additions, were aluminized by chemical vapor deposition to study the potential for minor alloy additions to improve oxidation resistance of coated alloys. Compared to uncoated specimens, the coated specimens showed improved cyclic oxidation resistance at 1100° and 1150 °C. However, alumina scale spallation was observed at relatively short times and, particularly for the Ni-base alloy X, the aluminized lab-cast alloy with Hf tended to have poor coating performance compared to the commercial alloy without Hf. Internal oxidation of Hf at 1150 °C and rapid Al depletion in the relatively thin aluminide coatings contributed to the observed detrimental Hf effect. For the Ni-base alloys, the increased scale spallation could be attributed to much higher S contents (10-50 ppma) in the laboratory-cast alloys. Oxide scale spallation from the coating surface was minimized when Hf and Y were added to a casting and the [Y]/[S] content ratio was ∼ 1.     

Protective Aluminide Coatings for Refractory Metals

Refractory metals are promising materials for high-temperature applications. However, these materials exhibit low oxidation resistance at elevated temperatures. To overcome this problem, aluminum diffusion layers were applied to molybdenum, niobium, tantalum, and tungsten using a pack cementation process. The coated samples were characterized using EPMA, optical microscope, and XRD. Homogeneous diffusion layers of different intermetallic phases were identified. The observed phases were in agreement with phase predictions made using thermodynamic calculations. Oxidation tests at 1300 \(^{\circ }\)C for up to 100 h in synthetic air were carried out on uncoated as well as aluminized samples. The oxidation kinetics were analyzed via TGA measurements. After oxidation, the formed oxides and the remaining aluminum diffusion layers were characterized. It was found that an additional halogen treatment can significantly reduce oxidation attack of the substrate and support the formation of a continuous protective Al\(_2\)O\(_3\) layer.     

Effect of trivalent rare earth dopants in nanocrystalline ceria coatings for high-temperature oxidation resistance

Nanocrystalline ceria (NC) and La³⁺-doped nanocrystalline ceria (LDN) particles synthesized by the micro-emulsion method were coated onto AISI 304 stainless steel (SS) in order to study their high-temperature oxidation resistance properties at 1243 K in dry air for 24 h. Results were compared with those for micro-ceria (MC) coatings. The coated samples showed 90% improvement in oxidation resistance compared to uncoated and MC coated steels as seen from SEM cross-sectional studies. X-ray diffraction (XRD) analysis showed the presence of chromia in both NC and 20 LDN (NC doped with 20 at.% La³⁺) samples which is absent in uncoated steels. From secondary ion mass spectrometry (SIMS) depth profiles, Fe, Ni depletion zones were observed in LDN-coated samples, indicating diffusion through the oxide layer. The role of oxygen vacancies in the nanoceria coatings on the formation of protective chromia layers is discussed.     

Corrosion protection of commercial steel using stainless steel coatings deposited by Cathodic Arc Plasma Deposition technique

The commercial steel AISI 1010 was coated with AISI 316L steel using Cathodic Arc Plasma Deposition (CAPD) technique. The coatings were deposited in vacuum and in the presence of nitrogen, acetylene and mixture of the two as reactive gases. The coatings were deposited as a function of time while other parameters remained constant. The coatings 0.75 to 1.3 μm thick were adherent and amorphous. The aqueous corrosion properties of the coated samples in 3.5 wt % NaCl solution were studied by Tafel, cyclic and potentiodynamic polarization measurements. The derived corrosion parameters were then compared among the various uncoated and coated samples. The study revealed that the coated samples were more corrosion resistant than the uncoated one. Similarly, the samples coated in the nitrogen + acetylene mixture atmosphere were more corrosion resistant than the samples coated in only nitrogen and acetylene atmospheres. The corrosion parameters were also compared as a function of coating time to ascertain best coating thickness.     

Inhibition of Metastable Alumina Formation on Fe–Cr–Al–Y Alloy Fibers at High Temperature Using Titania Coating

Fe–Cr–Al–Y fine fibers suffer from intensive whisker formation, which is due to fast-growing meta-stable phases such as gamma or theta alumina at about 800–900°C. To inhibit the fast growth of meta-stable alumina phases, a thin layer of sol–gel titania was deposited on the fibers. The oxidation kinetics were recorded and compared between uncoated and coated samples. The scale morphology was studied by Scanning Electron microscopy (SEM). The scale cross section were examined using focused-ion beam (FIB). The phases of the titania coating were examined by Raman spectroscopy. It was found that at 915°C, titania greatly inhibits the formation of meta-stable alumina. No whiskers were found on the surface of the fibers compared to uncoated samples. The weight gain for titania coated sample is almost one half during the first 50 hr compared to the uncoated sample at 915°C. The scales on the coated sample is much thinner and fewer cracks were found at the scale and alloy interface. The inhibition mechanism is discussed briefly.     

Hot Corrosion Studies of HVOF-Sprayed Coating on T-91 Boiler Tube Steel at Different Operating Temperatures

The aim of the present work is to investigate the usefulness of high velocity oxy fuel-sprayed 75% Cr₃C₂-25% (Ni-20Cr) coating to control hot corrosion of T-91 boiler tube steel at different operating temperatures viz 550, 700, and 850 °C. The deposited coatings on the substrates exhibit nearly uniform, adherent and dense microstructure with porosity less than 2%. Thermogravimetry technique is used to study the high temperature hot corrosion behavior of uncoated and coated samples. The corrosion products of the coating on the substrate are analyzed by using XRD, SEM, and FE-SEM/EDAX to reveal their microstructural and compositional features for the corrosion mechanisms. It is found that the coated specimens have shown minimum weight gain at all the operating temperatures when compared with uncoated T-91 samples. Hence, coating is effective in decreasing the corrosion rate in the given molten salt environment. Oxides and spinels of nickel-chromium may be the reason for successful resistance against hot corrosion.     

铝—硅涂层防护性能的研究

采用静态高温氧化、热腐蚀试验以及多种物理分析方法研究了镍基合金上的铝—硅涂层的防护性能。揭示了硅在涂层中的分布形式及其在高温曝置期间的变化情况。指出铝—硅涂层的防护性能明显优于渗铝涂层,而且在本试验范围内随硅含量的增加而提高。这是由于: (a) 铝—硅涂层减轻或防止涂层中“MC碳化物缺口”的出现; (b) 含硅的γ′-相具有优良的抗氧化性能; (c) 1100℃短时间曝置后在Al—Si涂层与基体界面处形成的连续的富硅M_6C“隔层”起扩散屏障的作用。     

The effects of a CeO2 coating on the corrosion parameters of type 304 stainless steel

Coatings of CeO₂ derived from inorganic sol-gel dispersions were applied to type 304 bright annealed stainless steel (British Standard 1449 type 304S31) coupons and subjected to heat treatments at 450 and 550 °C. The coatings, 0.5 μm thick, were transparent, adherent, and stable. The aqueous corrosion properties of the coated coupons in 1 molar NaCl were then studied by potentiodynamic polarization measurements and electrochemical impedance spectroscopy. The derived corrosion parameters were then compared with similarly heat-treated, uncoated 304SS coupons. The results indicate that the application of a CeO₂ coating, heat treated at 550 °C, improves the corrosion rate as compared to an uncoated heat-treated coupon. The polarization curve for the 450 °C heat-treated, coated coupon showed passive behavior compared to transpassive for the uncoated specimen. The impedance spectrum data for the uncoated, as-received coupon could be modeled using a resistor in series with a parallel capacitor resistor combination “Randles” equivalent circuit. A coated coupon, heat treated at 550 °C, could be modeled using a more complex equivalent circuit involving a constant phase element, due to the CeO₂ coating. Modeling of the impedance characteristics for the oxidized coupons required the addition of a second series element consisting of a parallel resistor capacitor combination to give a Chi square statistic fit of X² < 5 X 10^-4.     

Oxidation of Uncoated and Aluminized 9-12% Cr Boiler Steels at 550-650?°C

In this paper, oxidation behavior of 9-12% Cr steels P91 and HCM12A is studied in air and in a mixture of air and water vapor. Comparison is made between these steels in uncoated condition and coated with aluminum diffusion coating by a slurry method. Oxidation tests were carried out at 550, 600, and 650 °C for a discontinued duration of 1000 h; every 250 h the specimens were slowly cooled to room temperature and weighed. SEM + EDS and XRD characterization were performed after 500 and 1000 h. The results showed that oxidation rate of uncoated P91 and HCM12A was significantly higher in the mixture of air and water vapor than in air. Oxidation resistance of the studied materials improved substantially when they were aluminized.     

Microstructure and wear properties of silicide based coatings over Mo–30W alloy

The paper deals with the microstructure and wear properties of the Mo-30W alloy, coated with silicide based oxidation resistant coatings using pack cementation technique. The microstructure of the coating revealed that the coating was free of cracks or pores, adherent to the substrate and comprised of either single layer or double layer depending the coating temperature. Scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) was used to determine diffusion profiles for Mo, W and Si. Reciprocating sliding wear and friction experiments were performed on the uncoated and coated alloy. Double layer coating showed an improved friction coefficient as compared to base alloy as well as a single layer coated alloy. The wear tests also showed a marked improvement of wear resistance of coated alloy as compared to uncoated alloy. The coating was found to be wear resistant at 7 N.     

Effect of Aluminizing and Nitridation on the Oxidation Behavior of AISI 316LN SS

The application of stainless steel at temperatures above 973 K is rendered difficult by the chromium loss due to generation of volatile CrO₃ species and the consequent reduction in the capacity to maintain protective scale. We have attempted a method to circumvent this problem by modifying the surface. The surface was modified by three techniques each suiting a particular application. The first two methods involved aluminizing the surface by physical vapor deposition as the initial step. The pre-treatments were carried out in two ways viz. (i) high temperature diffusion-annealing and (ii) laser annealing of the surface. In the third method the specimen was modified by ion-nitriding. The oxidation behavior of these modified steels was compared with stainless steel in the as-received condition to exactly gauge the effect of surface modification. The oxidation experiment was carried out at 1123 K for 3.6 Ms. The oxidation was interrupted at specific time intervals to examine the mass change of the specimen and mass of the spalled oxide. The results indicated that aluminizing followed by heat-treatment and laser-treatment showed significant improvement in the adherence of the scale. On the other hand the bare and nitrided specimens showed similar behavior characterized by intermittent spallation and poor adherence. Nitriding resulted in the increase in the surface hardness. The post-oxidation examinations were carried out using SEM, EDAX and GIXRD. The uncoated specimen showed the presence of uniform oxide layer and contained oxides of iron and chromium. The aluminum-coated specimen showed the presence of adherent scale. The surface showed a nodular structure due to the formation of pits and zones of enhanced oxidation. The major part of the oxide was alumina with small fractions of chromium and iron oxides. The nodular region was enriched in the oxide of iron. The analysis of the surface by GIXRD revealed the nature of different phases formed on the surface. The oxide on the oxidized bare metal was Cr₂O₃, aluminized (non-oxidized) AlFe and Al₂Fe, aluminized and oxidized Al₂O₃ and FeCr₂O₄, aluminized and laser-treated Al₂O₃, FeCr₂O₄, Fe₂O₃ and Fe₃O₄, nitrided Fe₂O₃ and FeCr₂O₄. In the light of the above results the mechanism of scale failure has been proposed.     

Effect of Micro Arc Oxidation Coatings on Corrosion Resistance of 6061-Al Alloy

In the present study, the corrosion behavior of micro arc oxidation (MAO) coatings deposited at two current densities on 6061-Al alloy has been investigated. Corrosion in particular, simple immersion, and potentiodynamic polarization tests have been carried out in 3.5% NaCl in order to evaluate the corrosion resistance of MAO coatings. The long duration (up to 600 h) immersion tests of coated samples illustrated negligible change in weight as compared to uncoated alloy. The anodic polarization curves were found to exhibit substantially lower corrosion current and more positive corrosion potential for MAO-coated specimens as compared to the uncoated alloy. The electrochemical response was also compared with SS-316 and the hard anodized coatings. The results indicate that the overall corrosion resistance of the MAO coatings is significantly superior as compared to SS316 and comparable to hard anodized coating deposited on 6061 Al alloy.     

Improvement of 9% Ferritic Steel Against Cyclic Oxidation by CVD-FBR Al–Mn Coating

Cyclic-oxidation tests were performed on uncoated and Al–Mn coated P92 stainless steels in air at 650 °C. SEM and X-ray analyses revealed the formation of iron–chromium spinel, magnetite and hematite on uncoated steel, which spalled dramatically. The aluminized substrate promoted the formation of a continuous, adherent alpha-alumina scale. The formation of this protective alumina layer depended mainly on the characteristics of the initial aluminide coating. A coating with high aluminum content allowed the formation of an alumina oxide, which cracked easily during thermal cycling. The improvement of the cyclic-oxidation resistance was provided by the alumina scale obtained on the coating with lower Al content. COSP modeling showed good performance by surface coating. Aluminum coating is able to extend metal life by the delaying time to reach crossover.     

Complex Boronized Layer on the Hot-dip Aluminized Steels and Its Surface Performances

DIFFUSION treatment could eliminate the porous inthe hot-dipped aluminum layers and promote thecombination of the profile with the matrix.On the otherhand,boron atoms could decrease the brittleness of theferro-aluminum alloyed layer.More luckily,thetemperature range for boronizing is consistent with thatfor diffusion treatment.Therefore,complex boronizingwas carried out on the hot-dip aluminized steels bydiffusion-treating at high temperature in order to furtherraise wear-resistance,anti-ox…     

Fatigue performance of a SAE 1045 steel coated with a Colmonoy 88 alloy deposited by HVOF thermal spraying

The present investigation has been conducted to study the fatigue behavior of a SAE 1045 steel both uncoated and coated with a Colmonoy 88 alloy (NiCrBSiW) of approximately 410 μm thick, deposited by HVOF thermal spraying. Previously to deposition the samples were grit-blasted with alumina particles of approximately 1 mm in equivalent diameter. Tensile and fatigue tests were carried out with the uncoated and coated specimens. Fatigue tests were conducted under rotating bending conditions (R = − 1) at a frequency of 50 Hz. The samples tested were in three different surface conditions, including polished, grit-blasted and coated. The fatigue limit was determined by means of the staircase method employing a stress step of 5 MPa. The results indicate that the presence of the coating gives rise to a reduction in the fatigue life of the coated samples tested in air in comparison with the uncoated specimens. On the contrary, when the coated samples were tested in a NaCl solution at alternating stresses less than 350 MPa, these showed an increase in fatigue life in comparison with the polished uncoated samples. The analysis of the fracture surfaces of the specimens tested in air revealed that alumina particles present on the surface of the grit-blasted samples acted as stress concentrators, inducing the nucleation of fatigue cracks at the substrate-coating interface, which explains the reduction in fatigue life. However, under corrosive conditions and low alternating stresses, the presence of the coating provides an effective protection against corrosion-fatigue failures, giving rise to an improvement of the corrosion-fatigue performance of the coated system. On the contrary, at elevated alternating stresses, the coating was observed to delaminate from the substrate, leading to an impairment of the corrosion-fatigue behavior of the coated samples.     

热浸镀Al-Si-Y对S304不锈钢高温性能的影响

通过热浸镀铝方法,对S304奥氏体不锈钢在熔融Al-Si-Y中进行处理。采用SEM,XRD,高温氧化试验及摩擦磨损试验对热浸镀涂层的组织结构及高温性能进行研究。结果表明,热浸镀涂层由富铝层及化合物层组成,富铝层中分布有富Y的Al-Fe-Si-Ni和Al-Si-Fe析出相,化合物由外到内由主要物相为(Fe,Cr)2SiAl7,含Si的(Fe,Cr)Al3以及含Si的(Fe,Cr)Al2的3个亚层组成。高温氧化和磨损试验表明,热浸镀铝后S304不锈钢与原始S304样品相比,抗氧化性能有所改进,耐高温磨损性能明显提高。     

The Effect of Deformation Substructure on the High-Temperature Oxidation of Inconel 625

The high-temperature, isothermal-oxidation behavior of a superalloy was studied in the as-rolled and deformed conditions. The microstructural changes occurring during the oxidation of samples were examined using optical, scanning electron microscopy (SEM), fine-probe EDS microanalysis, and X-ray diffraction techniques. The topography of the oxide layers formed in the as-rolled and cold-deformed specimens exposed at various temperatures and time intervals is also examined. The kinetics and microstructural results are presented for the comparative study of the structural changes occurring during high-temperature oxidation. It was found that a Cr₂O₃ external layer was adherent and uniform on the rolled specimens in comparison to the scattered and preferential oxide developed on the deformed specimens. The latter can be attributed to the concurrent dynamic changes occurring in the deformed substructure that subsequently lead to breaking and spallation of the oxide.