Enhancement of Service Life of Steam Generating Tubes in Oil-Fired Boiler for Power Generation Employing Plasma Spray Technology

The effects of Ni-50 mass% Cr alloy coating, that is plasma-sprayed onto the fire-side of steam generating tubes in a heavy oil-fired boiler, on the high temperature corrosion resistance were examined. One of the severe environments in the industrial manufacturing facilities, where thermal sprayed coatings are employed, is the high temperature corrosion such as the oxidation, sulfidation, and low melting fuel ash corrosion in the fire-side of boiler tubes. In the fossil fuel-fired steam generating boiler facilities, the degradation or failure of steam generating tubes that were derived from the contaminants in a lower grade fuel have often occurred. The situation of degradation of the water evaporator and superheater tubes and corrosion-preventing effects of plasma sprayed coating are described. The enhanced effects of plasma sprayed Ni-50 mass% Cr alloy coating for the suppression of hot corrosion failure of the steam generating tubes of boiler are summarized.     

Studies on the Sliding Wear Performance of Plasma Spray Ni-20Cr and Ni3Al Coatings

Two metallic powders namely Ni-20Cr and Ni₃Al were coated on AISI 309 SS steel by shrouded plasma spray process. The wear behavior of the bare, Ni-20Cr and Ni₃Al-coated AISI 309 SS steel was investigated according to ASTM Standard G99-03 on a Pin-on-Disc Wear Test Rig. The wear tests were carried out at normal loads of 30 and 50 N with a sliding velocity of 1 m/s. Cumulative wear rate and coefficient of friction (μ) were calculated for all the cases. The worn-out surfaces were then examined by scanning electron microscopy analysis. Both the as-sprayed coatings exhibited typical splat morphology. The XRD analysis indicated the formation of Ni phase for the Ni-20Cr coating and Ni₃Al phase for the Ni₃Al coating. It has been concluded that the plasma-sprayed Ni-20Cr and Ni₃Al coatings can be useful to reduce the wear rate of AISI 309 SS steel. The coatings were found to be adherent to the substrate steel during the wear tests. The plasma-sprayed Ni₃Al coating has been recommended as a better choice to reduce the wear of AISI 309 SS steel, in comparison with the Ni-20Cr coating.     

Resistance of thermal-sprayed duplex coating composed of aluminum and 80Ni-20Cr alloy against aqueous corrosion

The development of corrosion-resistant sprayed coatings without sealing is required to increase the reliability of the thermal spray coating method and to expand the field of application for wet corrosion environments. The conventional wire flame-sprayed aluminum coating on steel without sealing has poor resistance against aqueous corrosion and has restricted practical use. A duplex coating composed of sprayed aluminum on an 80Ni-20Cr alloy undercoat exhibited sufficient resistance in a hot, near-neutral aqueous environment through a trial use in a vegetable oil process. In this paper, the mechanism of corrosion resistance of the duplex coating is investigated by electrochemical polarization measurements and electron probe microchemical analysis (EPMA) to examine the individual role of each layer and the change of the microstructure with time.     

Analytical Studies on the Behavior of Nickel- and Cobalt-Base Shrouded Plasma Spray Coatings at Elevated Temperature in Air

Nickel- and Cobalt-base coatings were developed on boiler-tube steels by the argon-shrouded, plasma spray process. The cyclic behavior of bare and coated boiler-tube steels has been studied in air at 900 °C. Four types of coatings were used: Ni-22Cr-10Al-1Y (NiCrAlY), Ni-20Cr, Ni₃Al and Stellite-6 (St-6). The NiCrAlY has also been used as a bond coat of approximately 150 μm thick before the final coating in all the cases. Oxidation products analyzed in the scale are mostly oxides of the elements present in the coatings and substrate steels. NiCr₂O₄ and CoCr₂O₄ spinels are the most-common observed phases in the scale for nickel- and cobalt-base coatings. The internal oxidation of coatings and diffusion of iron from the base steel to the upper scale occurred during the study. Cracks formed in the scale and coating and may be due to differences in composition of coatings, bond coat, substrate and oxides formed.     

等离子喷涂 NiCoCrAlY / Al2 O3 高温固体润滑耐磨涂层的抗氧化性能研究

目的研究等离子喷涂NiCoCrAlY/Al2O3高温固体润滑耐磨涂层在850℃时的高温抗氧化性能和抗氧化机理。方法采用喷雾造粒、化工冶金包覆技术制备NiCoCrAlY/Al2O3复合粉体,并采用等离子喷涂技术在45#钢表面制备NiCoCrAlY/Al2O3复合涂层。采用SEM和XRD研究粉体和涂层的显微结构和物相组成,并采用马弗炉研究复合涂层在850℃的恒温氧化动力学曲线,通过研究氧化96 h以后涂层表面的组织形貌,探讨NiCoCrAlY/Al2O3复合涂层的抗氧化机理。结果 NiCoCrAlY合金层均匀致密地包覆在Al2O3颗粒的表面,包覆层厚度约为3~5μm。复合粉体的主要组成为Al2O3相和NiCoCrAlY合金相,没有其他杂质相的存在。等离子喷涂NiCoCrAlY/Al2O3复合涂层氧化动力学曲线分为大斜率直线、抛物线和系数几乎为0的抛物线等3个阶段。氧化96 h以后,涂层的氧化质量增量为4.9 mg/cm2左右,表面形成了一层连续的氧化物保护膜,经EDX分析,氧化膜层主要由Al,O,Cr和Ni组成。结论等离子喷涂NiCoCrAlY/Al2O3复合涂层具有良好的高温抗氧化性能,涂层中Ni,Cr,Al的氧化以及硬质相Al2O3的加入是涂层抗氧化的主要原因。     

Mold manufacture with plasma spraying

A process has been developed to produce molds or tooling using a steel or chrome- plated steel model. The investigation examined the effect of coating and model materials, model temperature and spray angle on the coating separation from the model surface, coating delamination, and surface quality. A polished model disk was heated and then plasma sprayed with iron, nickel, Ni- Al, or Ni- Cr- B- Si. It was found that the minimum temperature to facilitate entire coating removal was lower for steel models and varied be tween 200 and 450 °C depending on the material. However, at higher temperatures the higher bond strength produced by oxidation on the steel resulted in significant coating pullout. A chrome- plated model, heated to 600 to 700 °C, is required to produce a defect- free coating. The effect of substrate angle on open porosity is most critical for the Ni- Cr- B- Si alloy and least important for Ni- Al coatings. The sur face roughness of the plasma- sprayed molds is comparable to the corresponding models, permitting good surface detail reproducibility. Several molds and tools were produced for use in the glass, rubber, and plastics industries.     

Erosion-corrosion of thermally sprayed coatings in simulated splash zone

The damage of marine steel structures in the splash zone is very severe. Applying thermally sprayed metal coatings is among the most important protective technologies, but the service life of current coatings is limited. In this paper, arc spray was used to prepare four types of metal coatings, that is, aluminum (Al), zinc bottom coating combined with aluminum topcoat (Zn + Al), aluminum-zinc pseudo alloy (Al/Zn) and aluminum-titanium pseudo alloy (Al/Ti) coatings. These metal coatings were sealed with epoxy priming and aliphatic polyurethane topcoat. Erosion-corrosion experiments were carried out on self-made device by simulating the splash zone working environment of steel structures and protective coatings. The results show that all the sealed coatings could improve the steel resistance to erosion-corrosion, and the aluminum-zinc pseudo alloy is the most excellent coating. In the coating failure process, mechanical erosion rather than corrosion is the key factor causing coating erosion-corrosion in splash zone. Improving the anti-cutting properties could help to prolong the coating life.     

Performance of Al‐rich oxidation resistant coatings for Fe‐base alloys

The oxidation resistance of Al‐rich coatings made by chemical vapor deposition and pack cementation was examined on representative ferritic‐martensitic (FM, e.g. Grade 91, Fe‐9Cr‐1Mo) and austenitic steel substrates at 650°‐800 °C. To evaluate the potential benefits and problems with these alumina‐forming coatings, oxidation exposures were conducted in a humid air environment where the uncoated substrates experience rapid oxidation, similar to steam. Exposure temperatures were increased to accelerate failure by oxidation and interdiffusion of Al into the substrate. The difference in the coefficient of thermal expansion (CTE) between coating and substrate was found to cause cracking and coating failure during rapid thermal cycling on thicker coatings with Fe‐Al intermetallic phases. Therefore, thinner coatings with less Al and a ferritic Fe(Al) structure were evaluated more extensively and tested to failure at 700° and 800 °C on FM steels. The remaining Al content at failure was measured and used to improve a previously developed coating lifetime model. At 700° and 800 °C, thin coated austenitic specimens continue to exhibit protective behavior at more than double the lifetime of a similar coating on FM steel. The longer lifetime was attributed to the ferritic coating‐austenitic substrate phase boundary inhibiting Al interdiffusion.     

Effect of particle state on the adhesive strength of HVOF sprayed metallic coating

NiCrBSi and Ni-50Cr coatings were deposited using the high velocity oxygen fuel (HVOF) spray process under different spray parameters with two powders of different sizes to clarify the influence of the melting state of spray particles on the adhesive strength of the coating. The adhesive strength of the coating was estimated according to the American Society for Testing and Materials (ASTM) C633-79. The melting state of the spray droplet was examined from the coating microstructure. It was found that the melting state of spray particles had a significant effect on the adhesive strength of HVOF sprayed Ni-based coatings. The significant melting of the spray particle did not contribute to the increase in the adhesion of HVOF metallic coatings. On the other hand, the deposition of a partially melted large particle contributed to the substantial improvement of adhesive strength of the HVOF coating. The subsequent coating presented a dense microstructure and yielded an adhesive strength of more than 76 MPa, which was double that of the coating deposited with completely molten particles. It can be suggested that the good melting of the spray particle is mainly related to the mechanical interlocking effect, which reaches the limited and approximately defined adhesive strength up to 40–50 MPa.     

电弧喷涂Zn-Al伪合金涂层耐腐蚀性能研究进展

通过在钢基体表面制备涂层可以很好地延长钢铁材料的服役时间,减少因腐蚀造成的重大事故和人员伤亡。相较于传统的纯Zn涂层、纯Al涂层以及Zn-Al合金涂层,Zn-Al伪合金涂层能够为基体材料提供长久有效的腐蚀防护,在钢铁材料的腐蚀防护中具有巨大的应用潜力。简述了Zn-Al伪合金涂层电弧喷涂制备工艺的特点;介绍了Zn、Al、Zn-Al合金及Zn-Al伪合金涂层在模拟海洋环境下的腐蚀防护原理;在此基础上从组分、喷涂工艺参数(喷涂距离、喷涂电流和喷涂电压)、元素掺杂(Mg、Si及Re)及后处理工艺(封孔、激光重熔)等角度,论述了其对Zn-Al伪合金涂层耐蚀性的影响;讨论了Zn-Al伪合金涂层防腐体系在桥梁、海洋钢结构件、地下运输管道中的应用现状;最后总结了目前研究工作中存在的挑战,提出了电弧喷涂Zn-Al伪合金涂层尚需深入研究的重点问题,为提高钢铁材料使用寿命提供了参考。     

High Temperature Corrosion Behavior of Cold Spray Ni-20Cr Coating on Boiler Steel in Molten Salt Environment at 900 °C

Cold spray is an emerging technology that produces high density metallic coatings with low oxide contents and high thermal conductivity, which makes them ideal for high temperature corrosion resistance. In the current investigation, Ni-20Cr alloy powder was deposited on SA 516 boiler steel (0.19C-1.07Mn-0.020S-0.25P-0.010Si-balance Fe) by cold spray process. Oxidation kinetics was established for the uncoated and cold spray Ni-20Cr coated boiler steel in an aggressive environment of Na₂SO₄-60%V₂O₅ at 900 °C for 50 cycles by the weight change technique. X-ray diffraction, FE-SEM/EDAX, and x-ray mapping techniques were used to analyze the oxidation products. Uncoated steel suffered corrosion in the form of intense spalling and peeling of its oxide scale, which may be due to the formation of unprotective Fe₂O₃ oxides. The Ni-20Cr coating was successful in reducing the weight gain of the steel by 87.2% which may be due to the formation of oxides of nickel and chromium.     

Al-Cr涂层低温扩散制备及其相组成的研究

通过水溶液镀Cr后再用AlCl₃-EMIC (氯化1-甲基3-乙基咪唑)离子液体镀Al, 制备Cr/Al复合镀层, 其后通过低温扩散热处理制备Al-Cr涂层. 用OM, SEM, BSE, EDS和XRD研究了短时间热处理时, 热处理温度对Al-Cr涂层组成和结构的影响. 结果表明, Cr/Al复合镀层在540 ℃时即可发生明显的互扩散, 形成Al-Cr合金层. 通过对复合镀层中Cr和Al镀层厚度的控制, 可以得到不同组成的Al-Cr合金涂层. 6.5 μm Cr/15 μm Al的复合镀层经640 ℃/960 min热处理, 表面形成以Al₈Cr₅为主的合金涂层; 1.6 μm Cr/15 μm Al的复合镀层经 600 ℃/30 min热处理, 可得到以Al₄Cr为主的Al-Cr涂层.     

工艺参数对高速电弧喷涂Al/1Cr13复合涂层组织结构的影响

用高速电弧喷涂制备Al/1Cr13复合涂层,采用3因素3水平正交试验法系统研究了电弧电流、电弧电压和喷涂距离对复合涂层的组织结构、孔隙率和氧含量的影响规律。采用扫描电镜对复合涂层的显微组织和孔隙率进行表征,采用氧氮含量分析仪测得涂层的氧含量。结果表明,在第9组喷涂参数即电弧电流为240A,电压为32V,喷涂距离为150mm的条件下制备的高速电弧喷涂Al/1Cr13复合涂层组织较致密,Al和1Cr13涂层的孔隙率最低分别为1.6%和2.2%。Al涂层氧含量显著低于1Cr13涂层,最低约为2%。     

Electrochemical Characteristics in Seawater for Cold Thermal Spray-Coated Al-Mg Alloy Layer

For corrosion protection of carbon steel in a marine environment,cold arc thermal spray coating was applied to the surface with Al and Al-Mg alloy wires.The surface hardness of Al and Al-Mg thermal spray coatings increased with Mg content.And the various electrochemical experiments were carried out to evaluate corrosion damage characteristics of the thermal spray coating layers.The Al and Al-Mg thermal spray coating layers presented negative potentials compared to carbon steel in corrosion potential measurements.And an anodic polarization experiment revealed a tendency of activation polarization with no passivation.Furthermore,the corrosion damage of the thermal spray coating layer in galvanostatic experiment was observed mainly at the defect area,and the Al-3Mg thermal spray coating layer presented less surface damages than others.In addition,the Al-3Mg thermal spray coating layer showed the lowest corrosion rate while having a sufficient driving voltage for cathodic corrosion protection.Therefore,it is an optimal thermal spray material for sacrificial anode.     

Application of High Temperature Corrosion-Resistant Materials and Coatings Under Severe Corrosive Environment in Waste-to-Energy Boilers

Corrosion-resistant materials (CRMs) and coatings are key technologies to increase power generation efficiency and reduce maintenance in waste-to-energy (WTE) plants. Corrosion environment became severe as steam temperatures have increased. The steam condition of more than 400 °C/3.9 MPa became possible in WTE boilers by using highly durable corrosion-resistant coatings, such as thermal spray of Al/80Ni20Cr alloy, HVOF-sprayed NiCrSiB alloy, Alloy 625 weld overlay for waterwall tubes and also superheater tubes. Also, the use of 310S type stainless steels and high Cr-high Mo-Ni base and high Si-Cr-Ni-Fe alloys have progressed because of a better understanding of corrosion mechanisms. Furthermore, high durability coatings using cermet and ceramic materials were applied to high temperature superheaters. This paper describes the major developments and the application of CRMs and coating technologies in the last 30 years in WTE plants, the corrosion mechanisms of alloys, the deterioration mechanisms of spray coating layers, and future subjects for the development of corrosion-resistant materials and coatings.     

High‐temperature corrosion behavior of some post‐plasma‐spraying‐gas‐nitrided metallic coatings on a Fe‐based superalloy

The objective of the present study is to propose a cost‐effective process for modifying commercially available coatings by gas nitriding using commonly available equipment and starting materials. Al–Cr and Ti–Al metallic coatings were deposited on Superfer 800H (Fe‐based superalloy) using a plasma spray process. Then the gas nitriding of the coatings was done in the lab and the parameters were optimized after conducting several trials on plasma‐sprayed‐coated specimens. Characterization and high‐temperature corrosion behavior of coatings after exposure to air and molten salt at 900°C were studied under cyclic conditions. Techniques like XRD, SEM/EDX, and X‐ray mapping analysis were used for the characterization of the coatings and analysis of the oxide scale. Both the coatings successfully protected the substrate and were effective in decreasing the corrosion rate when subjected to cyclic oxidation (Type‐I hot corrosion) at 900°C for 50 cycles in air and molten salt (a salt mixture of Na₂SO₄–60%V₂O₅ dissolved in distilled water). Based on the findings of the present study, the coatings under study are recommended for tapplications to super‐heater and reheater tubes of boilers and all those surfaces that face fireside corrosion, such as fluidized beds, industrial waste incinerators, internal combustion engines, gas turbines or steam turbines, to provide protection against degradation in these environments. The cost of the product/process is approximately Rs. 0.62 per mm² in case of Al–Cr coating and Rs. 1.86 per mm² in case of Ti–Al coating.     

Steam oxidation resistance and thermal stability of chromium aluminide/chromium hybrid coating on alloy steels formed at low temperatures

This study investigated the feasibility of forming a hybrid coating with a structure consisting of a top Cr-aluminide layer and an inner Cr layer on alloy steels by using a two step process: electro-Cr plating and then pack aluminising at low temperatures. The oxidation resistance of the coating so formed was tested in ultrasupercritical steam of 650 °C and 30 MPa. The factors affecting the oxidation kinetics of the coating were studied by comparing its oxidation behaviour with that of the pack Fe-aluminide coating tested in the same ultrasupercritical steam. The thermal stability of the coating at 650 °C was investigated by a series of isothermal annealing experiments in argon atmosphere. It was demonstrated that the outer Cr-aluminide layer of the coating can improve the steam oxidation resistance of the steel substrate. The inner Cr layer can function as an effective barrier preventing the outward diffusion of Fe from the steel substrate; it can also act as a buffer zone, substantially reducing the rate of the inward Al diffusion process.     

EFFECT OF MICROSTRUCTURE CHANGE ON OXIDE FORMATION OF SPUTTERED Ni-3Cr-10Al NANOCRYSTALLINE COATING

1.IntroductionThesputterednanocrystallinecoatingswiththesameconlpositionasthesubstrateweresuccessfullyappliedtoK38G[ll,Kl7F[2]lLDZc5[3]Ni-basetlsuperalloys.Themaincharacterizati0nisthatnan0crystallizati0ncouldimpr0vetheselectiveoxidationofAl,therefore,thetransiti0nfr0minternalt0externaloxidati0nofAlisrealizedandanex-ternalAl2O3scaleisfOrn1ed.Niisthemainingredient0fNi-basedsuperal1oysandtheeffectivenessofthesesuperal1oysdependsgreatly0ntllepresellceofAlorCrtoformaprotectivelayerofAl2O:jo…     

高速电弧喷涂不锈钢涂层的旋转回归试验研究

对比研究了３Ｃｒ１３不锈钢高速电弧喷涂涂层和普通电弧喷涂层的孔隙率、氧化物含量、涂层结合强度和滑动磨损性能,并采用多因素正交旋转回归设计和单因素对比试验的方法了高速电弧涂工艺参数对３Ｃｒ１３不钢涂层显微硬度的影响,用微机回归得到了喷涂工艺参数与涂层硬度之间的定量关系式。结果表明,和普通电弧喷涂涂层相比,高速电弧喷涂涂层组织致密、孔隙度低、涂层氧化物含量增多,涂层－基体界面结合强度大于４０ＭＰａ,比     

Cyclic oxidation behavior of some plasma-sprayed coatings in Na2SO4-60%V2O5 environment

Cyclic oxidation behavior of plasma-sprayed NiCrAlY, Ni-20Cr, Ni₃Al, and Stellite-6 coatings was investigated in an aggressive environment of Na₂SO₄-60%V₂O₅ by thermogravimetric techniques for 50 cycles. These coatings were deposited on a Ni-base superalloy, namely Superni 600; 10Fe-15.5Cr-0.5Mn-0.2C-Bal Ni (wt.%). X-ray diffraction, scanning electron microscopy/energy dispersive x-ray (SEM/EDX), and electron probe microanalyzer (EPMA) techniques were used to analyze the oxidation products. The uncoated superalloy suffered accelerated oxidation in the form of intense spallation of its oxide scale. After deposition of the NiCrAlY coating, the superalloy showed a minimum mass gain, whereas after application of the Stellite-6 coating, a maximum mass gain was observed among the coatings studied. All of the coatings were found to be useful in reducing the spallation of the substrate superalloy. Moreover, the coatings were successful in maintaining continuous surface contact with the base superalloy during the cyclic oxidation. The phases revealed for the oxidized coatings were mainly the oxides of chromium and/or aluminum and the spinels containing nickel-chromium/cobalt-chromium/nickel-aluminum mixed oxides, which are reported to be protective against high-temperature oxidation/hot corrosion.