Due to the nonsymmetric distribution of the particle plume in conventional plasma spraying, significant influence of the gun scanning pattern can appear in the structure of the coatings obtained. In this study, three scanning patterns are used to deposit YSZ powder by means of air plasma spraying. Cross-sections of the coatings are examined and interfacial fracture toughness and hot corrosion tests are conducted. Improvements in coating adhesion and corrosion resistance were obtained by modifying the scanning pattern of the gun to decrease the possibility of horizontal weak bonding between spray passes.

This study evaluates the hot corrosion behavior of NiCoCrAlY, NiCoCrAlYHfSi, NiCoCrAlTaReY, and CoCrAlYTaCSi coatings on 1.4923 stainless steel, applied by high-pressure HVOF spraying. All coatings were cycled in in an environment of Na 2 SO 4 and 82% Fe 2 (SO 4 ) 3 at 690 °C. Each cycle consisted of 1 h of heating in a silicon carbide tube furnace followed by 20 min of cooling. Weight change measurements were performed after each cycle to track corrosion kinetics, and SEM and EDS analysis were employed to analyze the corrosion mechanism. CoCrAlYTaCSi showed the best corrosion resistance of the coatings tested.

This work investigates the high-temperature oxidation kinetics of CoCrAlSiY coatings with different Si concentrations. Hot-corrosion resistance is determined at 800 and 900 °C via hot salt coating, thermal shock resistance is measured at 1050 °C, and the oxidation and corrosion products are analyzed through mineralogical and micro analysis. The results show that Si promotes the formation of an Al 2 O 3 film that improves oxidation and corrosion resistance, but excessive amounts reduce thermal shock resistance.

In this study, two coatings, one produced by HVOF spraying, the other by physical vapor deposition (PVD), were applied on a nickel superalloy substrate in order to compare their hot corrosion behavior. The coating samples were initially characterized by OM and SEM-EDS analysis, then a mixture of V 2 O 5 , Na 2 SO 4 , and NaCl was deposited on the surface and the samples were to 900 °C for 35 h. The results show that the PVD CrN coating provided better corrosion protection than HVOF-sprayed CrC-NiCrAlY.

The current investigation focuses on understanding the influence of a columnar microstructure and a sealing layer on the corrosion behavior of suspension plasma sprayed (SPS) thermal barrier coatings (TBCs). Two different TBC systems were studied in this work. First is a double layer made of a composite of gadolinium zirconate + yttria stabilized zirconia (YSZ) deposited on top of YSZ. Second is a triple layer made of dense gadolinium zirconate deposited on top of gadolinium zirconate + YSZ over YSZ. Cyclic corrosion tests were conducted between 25 °C and 900 °C with an exposure time of 8h at 900 °C. 75 wt. % Na 2 SO 4 + 25 wt.% NaCl were used as the corrosive salts at a concentration of 6 mg/cm 2 . Scanning electron microscopy analysis of the samples’ cross-sections showed that severe bond coat degradation had taken place for both TBC systems and the extent of bond coat degradation was relatively higher in the triple layer system. It is believed that the sealing layer in the triple layer system reduced the number of infiltration channels for the molten salts which resulted in overflowing of the salts to the coating edges and caused damage to develop relatively more from the edge.

Corrosion resistance of coatings deposited by thermal spraying technology HVOF (High Velocity Oxygen Fuel) requires high density in coating and good adhesion to substrate material. The majority of thermally sprayed materials meet the requirements of high corrosion resistance in terms of their composition. However, porous structure raises doubts about the performance of thermally sprayed coatings regarding sufficient protection to the base material. In fact, corrosion protection is a basic coating function. However, , no sufficient attention has been dedicated to the issue of component protection against corrosion attack using HVOF sprayed coatings. In this study, NiCoCrAlY, NiCoCrAlTaReY, NiCoCrAlYHfSi, and CoCrAlYTaCSi coatings were deposited on the substrate material 1.4923. The coatings were deposited using HP/HVOF (High Pressure / High Velocity Oxygen Fuel) thermal spraying technology. The coatings were exposed to the corrosive-aggressive environment in the form of molten salts mixture with composition of 60 % V 2 O 5 and 40 % Na 2 SO 4 at the selected temperature of 750 °C. Further, all coatings were exposed to cyclic conditions. Weight changes of individual specimens were measured after every cycle and results were recorded in diagrams. After the corrosion test, all evaluated coatings were analyzed using scanning electron microscope (SEM), analysis of elemental composition (EDS) and X-Ray diffraction. The NiCoCrAlY and NiCoCrAlTaReY coatings showed the best corrosion protection in selected corrosive aggressive environment, forming the protective oxide layer that prevented further corrosion attack. On the contrary, NiCoCrAlYHfSi and CoCrAlYTaCSi coatings were found not to be suitable for corrosion protection of components working in selected corrosive environment.

Cr 3 C 2 -25%NiCr, Stellite 6, NiCrBSi and Hastelloy C-276 coatings were deposited on substrate material P91 by HP/HVOF (High Pressure / High Velocity Oxygen Fuel) thermal spraying technology. The resistance against high temperature corrosion was evaluated exposition of coatings to corrosive-aggressive environment in the form of molten salts mixture with composition of 60% V 2 O 5 and 40% Na 2 SO 4 at temperature of 750 °C. Further, coatings were exposed to cyclic conditions. After the corrosion tests, all coatings were analyzed using scanning electron microscope (SEM), and analysis of elemental composition (EDX). Alloys-based coatings showed very similar corrosion mechanism in the selected aggressive environment and the same can be stated about cermet coatings. The obtained results prove that HVOF deposited coatings can replace current surface protection of components in power equipment such as nitriding.

This study investigates the corrosion resistance Gd 2 Zr 2 O 7 /YSZ coatings and a YSZ layer of similar thickness. All coatings were produced by suspension plasma spraying, resulting in a columnar structure. Corrosion tests conducted at 900 °C for 8 h in a molten salt bath show that Gd 2 Zr 2 O 7 is not as corrosion resistant as YSZ. Molten salts react with Gd 2 Zr 2 O 7 producing GdVO 4 along the surface as well as between the columns of the coating. The formation of GdVO 4 between the columns, in combination with the low fracture toughness of Gd 2 Zr 2 O 7 , is likely responsible for the lower corrosion resistance. Furthermore, the presence of another layer of Gd 2 Zr 2 O 7 on top of the Gd 2 Zr 2 O 7 /YSZ coating, to prevent salt infiltration, did not improve corrosion resistance.

In this research project a hybrid technology is developed to repair turbine blades. This technology incorporates procedural and manufacturing aspects like raising the degree of automation or lowering the effort of machining and includes materials mechanisms (e.g. diffusion processes) as well. Taking into account these aspects it is possible to shorten the process chain for regenerating turbine blades. In this study the turbine blades of the high pressure turbine are considered and therefore nickel-based alloys are regarded. To repair or regenerate turbine blades the following methods are employed: welding and brazing and a subsequent aluminizing CVD-process. The focus in this work lies on the brazing method and the required filler-metal is applied together with the hot-gas corrosion protective coating by means of thermal spraying and represents the first stage of this hybrid technology. In the second stage of this hybrid technology the brazing process is integrated into the aluminizing CVD-process and a first effort is presented here.

The aim of the research project is to combine repair brazing with protective coating against hot-gas corrosion into a common integrated process. Both the braze-metal as well as the hot-gas corrosion protection coating is applied by means of thermal spraying. The material layout is to be realized as far as possible to the near net shape by using thermal spraying. The processes are to be performed in such a way that the brazing is integrated into the CVD diffusion annealing process as a transient liquid phase bonding (TLP bonding) process which, as a consequence, can then be eliminated as a separate processing step. The thermal spraying processes of atmospheric plasma spraying (APS), high velocity oxygen fuel spraying (HVOF) and cold gas spraying (CGS) are to be qualified for this purpose. Thus the project working hypothesis is to be able to transform thermal coating and joining processes into a common integrated hybrid process and, in doing so, obtain both high-quality and economic advantages. The importance of combining these processes lies in reducing the effort of grinding as well as economizing on the vacuum brazing, which is currently a separate process step, and consequently lowering the production costs.

Nickel-chromium alloys have been used as coatings to deal with oxidation environments at high temperature. The present work is a comparative study of HVOF and cold sprayed Ni-20Cr coating on a boiler steel (SAE 213-T22) in a molten salt environment of Na 2 SO 4 -60%V 2 O 5 at 900°C under cyclic conditions. The weight change technique was used to establish the kinetics of corrosion. X-ray diffraction, surface and cross-sectional FE-SEM/EDS techniques were used to analyse the corrosion products. The hot corrosion resistance of both the coatings was better than the uncoated steel. This may be attributed to the formation of oxides and spinels of nickel and chromium in the coated steels. These oxides might have blocked the pores and splat boundaries, and acted as diffusion barriers to the inward diffusion of corroding species. Based upon the overall results and subsequent analysis of hot-corrosion data the cold spray process may be recommended as a better choice for the deposition of the Ni-20Cr coating on Mo-containing T22 steel in comparison with the HVOF spray process for hot corrosion protection.

In this investigation, Ni-20Cr alloy powder was deposited on SA 516 boiler steel by cold spraying. The oxidation kinetics of both coated and uncoated samples, evaluated in molten salt, followed a parabolic rate. The rate constant of the Ni-20Cr coated steel was much lower than that of the bare boiler steel. X-ray diffraction, SEM-EDX, and X-ray mapping were used to analyze the oxidation products of the coated and uncoated boiler steel. The uncoated steel exhibited intense spalling and peeling of its oxide scale, which may be due to the formation of Fe 2 O 3 oxides. The Ni-20Cr coating reduced the weight gain of the steel by more than 87% possibly due to the formation of nickel and chromium oxides.

Detonation-gun spray technology is a novel coating deposition process which is capable of achieving very high gas and particle velocities approaching 4-5 times the speed of sound. This process provides the possibility of producing high hardness coatings with significant adherence strength. In the present study, this technique has been used to deposit WC-Co coatings on T22 boiler steel. Investigations on the oxidation performance of detonation-sprayed WC-Co coatings in air and in molten salt Na 2 SO 4 -82Fe 2 (SO 4 ) 3 at 700°C under cyclic conditions have been carried out. The thermogravimetric technique was used to establish the kinetics of corrosion. The uncoated boiler steel suffered a catastrophic corrosion in the form of intense spalling of its oxide scale during air, as well as, molten salt induced oxidation. The WC-Co coated specimens showed lesser overall weight gains in comparison to their uncoated counterparts in both the environments. The oxidation kinetics for the coated specimens followed nearly the parabolic rate law. The overall weight gain has been found to be higher in the case of air oxidation as compared to that in salt environment for all the cases. Scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS) techniques were used to analyse the corrosion products, which indicated the formation of W and O as main elements in the oxide scales of the coated steel in both the environments.

Fe-Al based composite coatings were deposited on Grade 20 steel substrates by High Velocity Arc Spraying (HVAS) technology. Hot salt corrosion properties of the coatings were studied at the temperatures of 450 °C,650 °C and 800 °C . Results showed that the corrosion resistance of the Fe- Al/Cr 3 C 2 composite coatings changed little with the increase of temperature, contrasted to the substrate and Fe-Al coatings. The excellent corrosion resistance of the Fe-Al/Cr 3 C 2 composite coatings was attributed to the oxidation of the iron, aluminum and chromium to form protective scales. In hot salt, there are mainly iron oxides at the outer corrosion surface; aluminum and chromium oxides are found at the inner corrosion surface, which can effectively protect the Fe- Al/Cr 3 C 2 composite coatings from corrosion.

Cr 3 C 2 -NiCr, NiCr, WC-Co and Stellite-6 alloy coatings were sprayed on ASTM SA213-T11 steel using the HVOF process. Liquid petroleum gas (LPG) was used as the fuel gas. Hot corrosion studies were conducted on the uncoated as well as HVOF sprayed specimens after exposure to molten salt at 9000C under cyclic conditions. The thermo-gravimetric technique was used to establish the kinetics of corrosion. XRD, SEM/EDAX and EPMA techniques were used to analyse the corrosion products. All these overlay coatings showed a better resistance to hot corrosion as compared to that of uncoated steel. NiCr Coating was found to be most protective followed by the Cr 3 C 2 -NiCr coating. WC-Co coating was least effective to protect the substrate steel. It is concluded that the formation of Cr 2 O 3 , NiO, NiCr 2 O 4 , and CoO in the coatings may contribute to the development of a better hot corrosion resistance. The uncoated steel suffered corrosion in the form of intense spalling and peeling of the scale, which may be due to the formation of unprotective Fe 2 O 3 oxide scale.

Hot corrosion behavior of Thermal Barrier Coatings (TBCs) has been studied by comparison between double layer coatings and graded coatings. Two types of oxide ceramics, 8 mass % Y 2 O 3 -ZrO 2 (8YZ) and 2CaOSiO 2 -15mass% CaOZrO 2 (C 2 S-15 CZ), with a bond coating of NiCrAlY were applied to metallic substrates in this study. After a hot corrosion test by V 2 O 5 -Na 2 SO 4 corrosive ashes, hot corrosion behavior of TBC has been investigated by visual inspection, metallography, X-ray diffraction and EPMA. The C 2 S-15%CZ coating reacted with V 2 O 5 only where it was in direct contact with the material. The affected area from the reaction was limited to the coating surface where V 2 O 5 existed. The coating showed adequate hot corrosion-resistance. It was found on the 8YZ coating that Y 2 O 3 , the stabilizing component, particularly reacts with V 2 O 5 and loses its function; this led to partial spalling of the coating. It was observed that the durability of the double layer TBC was largely influenced by the performance of a corrosion resistant NiCrAlY undercoat which provided protection against corrosive components penetrating through the ceramic topcoat. It was observed that the graded coating degraded due to oxidation of NiCrAlY particles which independently existed near the coating surface and affected the durability of TBC.

The goal of this work is to develop a flame sprayed NiCr-Cr 3 C 2 coating that resists oxidation, corrosion, and erosion in boiler environments and to assess the influence of Cr 3 C 2 fraction and sealing on coating performance. In the experiments, NiCr cermet alloys with different proportions of chromium carbide were applied to boiler steel substrates by means of subsonic velocity flame spraying and some of the coatings were sealed with a silicon resin. The paper describes the various tests performed and presents the results. Paper includes a German-language abstract.

This paper evaluates the effect of bond coat oxidation on the coating life of graded, duplex, and triplex TBC systems deposited on Inconel 617 and 738 substrates by vacuum plasma spraying. In oxidation experiments, the triplex system had the lowest number of cycles to failure. The graded system was found to be superior to the duplex system in terms of oxidation resistance, but inferior in terms of hot corrosion resistance. It was also found that the various TBC systems are more durable on Inconel 738 than on Inconel 617, particularly the graded systems. Paper includes a German-language abstract.

The gas turbine blades in a severe environment are overlaid with MCrAlY coatings by Low Pressure Plasma Spray (LPS) process for protection against hot corrosion. However, the service life of each blade is limited by damage due to embrittlement layer, which is formed by reaction diffusion at the interface between the coating and the substrate. Reaction diffusion behavior between the CoNiCrAlY coatings and substrates was investigated. In addition, high-temperature oxidation behavior of the CoNiCrAlY coating by LPS was evaluated. The CoNiCrAlY coatings for reaction diffusion test were carried out by 3 kinds of spray process (LPS, High Velocity Oxygen Fuel Spray: HVOF, Atmospheric Plasma Spray: APS) on 2 kinds of substrate (Directional Solidification and Single Crystal Ni-based super alloys). It has been found that the CoNiCrAlY coating by APS inhibited the reaction diffusion at the boundary of the coating and the base material as compared with LPS coating. It was also confirmed that the protective dense layer of aluminum oxide against hot corrosion was formed in the surface of the CoNiCrAlY coatings by LPS.

It is pointed out that properties corresponding to 3 to 5 % of gross domestic products are lost by corrosion in every year in advanced countries. Corrosion including oxidation is still one of the biggest technical problems which human beings are facing. The application of thermal spray coatings is one of the strongest weapons to prevent corrosion of steel and iron structures. The thermal spray coatings, however, are not panaceas to prevent corrosion. They involve many problems and it is important to understand the proper ways to apply the thermal spray coatings for corrosion resistance. In this paper, a state-of-the-art review on the science and technology against corrosion, oxidation and hot corrosion by the thermal spray coatings is presented.