In this study, iron-based coatings are deposited on stainless steel substrates by HVOF and HVAF spraying and are evaluated based on SEM examination, hardness measurements, and corrosion and wear testing and by comparison with WC-CoCr and CrC-NiCr reference coatings. The results indicate that corrosion resistance is insufficient if the coating is not fully dense and has open porosity. During spraying, the particles must be totally melted and rapidly solidified to achieve uniform coating composition. Open porosity and nonuniform distribution of alloy elements, particularly chromium, is seen to induce crevice corrosion in iron-based coatings.

This paper compares the corrosion resistance of electrolytic hard chrome plating and HVOF-sprayed WC-Co-Cr on two steel alloys used in landing gears. Test samples were evaluated by means of salt spray testing and electrochemical measurements. Post-test metallographic examinations were conducted to investigate the failure mode of the coatings. Test results are presented along with relevant findings and observations. Paper includes a German-language abstract.

The growing use of thermal spray technology in manufacturing is increasing the need for knowledge regarding the behavior of thermal spray coatings in aggressive environments. Due to the different characteristics of the spraying processes and materials used, the mechanisms of attack can vary greatly from layer to layer depending on the application. This paper presents a detailed investigation of corrosion mechanisms at room temperature and their effect on a wide range of thermally sprayed metals and ceramic-metal (cermet) composites. Paper includes a German-language abstract.

Usually small samples are used to test corrosion resistance of a coating, it is sometimes difficult to make a coating on the small samples. When a cylindrical sample is used and the coating is formed on whole of the surface, three or four times coating processes are required. Thus an abacus bead type sample is proposed in this paper. In this sample, only two times coating processes are required to make coating on whole of the surface. In this paper, the high-temperature corrosion resistance of a novel coating made of Cr-based alloys for use in waste incineration plants is tested and compared with the coating made of a Ni-based alloy. Paper includes a German-language abstract.

The corrosion resistance of thermal barrier coatings against CMAS deposit at high temperature is significantly affected by the microstructure of the coatings. Enhancing the bonding ratio between splats can reduce the inter-connected pores and then obstructs the penetration of the molten CMAS into the coatings. In this study, atmospheric plasma sprayed ZrO 2 contains 8 wt. % Y 2 O 3 (8YSZ) coating with improved lamellar bonding ratios was deposited with full-molten droplets at an enhanced deposition temperature. The microstructure of the dense 8YSZ coating and conventional 8YSZ coating before and after thermal exposure with CMAS were characterized. It was clearly revealed that by adjusting the microstructure and designing a ceramic layer with high bonding ratio, the corrosion resistance of the thermal barrier coating could be enhanced. Moreover, by designing double-ceramic-layer (DCL) TBCs composed of a porous ceramic layer and well-bonded ceramic layer, the TBCs with high CMAS corrosion resistance and low thermal conductivity can be achieved.

The bonding between flattened particles in plasma-sprayed metal coatings dominates their corrosion behavior by influencing the porosity in coatings, especially the porosity connected to the substrate for coatings used in a corrosive environment. Therefore, how to efficiently enhance the lamellar interface bonding in metallic coatings has been an important issue which has not been settled effectively. In this study, a shell-core structured powder particle designing with cladding spherical Ni20Cr powders with refractory molybdenum as alloying element is proposed to limit the evaporation of low melting point elements and subsequently raise particle temperature significantly high enough to cause impact melting. Results show that a dense coating with much low porosity was obtained due to the improved lamellar interface bonding by gas shrouded plasma spraying of the composite NiCr -Mo particles. Electrochemical method was employed to evaluate the polarization behavior of the NiCr - Mo coating to estimate its connected porosity. It was revealed that NiCr-Mo coating of excellent corrosion resistance with low connected porosity can be obtained by designing the shell-core-structured powder.

A modified stainless steel coating, named as M-SUS here, was prepared by the air plasma spray process (APS) and the high velocity flame spray process (HVOF) and compared with a conventional stainless steel (JIS: SUS316L). Anodic polarization tests using NaCl, HCl solutions, neutral salt spray test, and exposure test in actual tank for HCl storage were employed for the evaluation of corrosion resistance. Structure of the coatings was investigated by use of optical microscope, scanning electron microscope, electron probe micro-analyzer, and transmission electron microscope. It was found that the coating M-SUS exhibited a remarkably superior corrosion resistance by all tests mentioned above, compared with the conventional ones. Although both of the coatings compose of gamma-austenite (γ-Fe) and delta-ferrite (δ-Fe) phases, the coating M-SUS reveals much less oxide with chromium and more delta-phase enriched with molybdenum. Another exposure test using a mixed acid of 25%HNO3 and 75%HCl yielded that the δ-Fe was not etched out but the γ-Fe vanished, that is, the δ-Fe of M-SUS has a strong anti-corrosion property. It is considered that the superior corrosion resistance of coating M-SUS is attributed to the extensive formation of anti-corrosive δ-Fe and inhibition of chromium depletion resulting from oxide formation.

In the current investigation plasma spray technique was used for depositing hydroxyapatite (HA) and hydroxyapatite – silicon oxide (SiO 2 ) coatings on 316L SS substrate. In HA-SiO 2 coating, 20 wt% SiO 2 was mixed with HA. The feedstock and coatings were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) / energy-dispersive X-ray spectroscopy (EDX) analyses. The corrosion resistance of the uncoated, HA coated and HA + 20 wt% SiO 2 coated 316L SS was investigated by electrochemical corrosion testing in simulated human body fluid (Ringer’s solution). After the corrosion testing, the samples were analyzed by XRD and SEM / EDX analyses. The addition of SiO 2 reduces the crystallinity of the coating. The corrosion resistance of the 316L SS was found to increase after the deposition of the HA + 20 wt% SiO 2 and HA coatings.

Thermally sprayed Fe-based coatings reinforced by TiC particles are a cost effective alternative to carbide coatings such as WC/CoCr, Cr 3 C 2 /NiCr and hard chrome coatings. They feature a good wear resistance and, with sufficient amount of alloying elements like Cr and Ni, also a high corrosion resistance. In hydraulic systems the piston is coated for protection against corrosion and wear. New water-based hydraulic fluids require an adaption of the coating system. In order to investigate the wear and corrosion resistance of Fe/TiC a novel powder consisting of a FeCr27Ni18Mo3 matrix and 34 wt.-% TiC was applied by HVOF and compared to reference samples made of WC/CoCr (HVAF) and hard chrome. Besides an in-depth coating characterization (metallographic analyses, EMPA), wear resistance was tested under reverse sliding in a water-based hydraulic fluid. Corrosion resistance was determined by polarization in application-oriented electrolytes (hydraulic fluid at 60 °C, artificial sea water at RT).

The balanced flow type condenser is one of important part used in the air-conditioner of automobile. The manufacture needs the surface of aluminum multi-void to be coated with zinc. One of economical and efficient process is the automatic wire arc spray, in which the maintain of temperature and the in-process speed can make key effects on the quality of coatings. The relation between substrate temperature and the composition, structure and corrosion resistance of the coating of aluminum multi-void are mainly discussed to confirm the best in-process speed of aluminum multi-void, the maintain of temperature and the parameters of arc spraying. Abstract only; no full-text paper available.

316L stainless steel and Hastelloy C alloy powders were sprayed by an HVOF apparatus onto mild steel substrates. The microstructure, pore size distribution, composition and corrosion resistance of thus obtained coatings were evaluated experimentally. Corrosion resistance in sea-water was examined by monitoring the impedance and corrosion potential of samples immersed in artificial sea-water at 300 K over a period of more than 3 months and also by polarization measurement. It was found that the stainless coatings composed mainly of plastically deformed particles and some splats which were molten at the impact. By increasing the combustion pressure, the porosity as measured by mercury porosimeter could be reduced to below 1%. In comparison, Hastelloy C deposits sprayed under the standard condition were so dense that its porosity could not be measured by the porosimeter. The polarization curve and the results of impedance monitoring both exemplified that the Hastelloy C coatings possess much superior corrosion resistance to the stainless coatings in sea-water, which was attributed to the higher density and better adhesion of the Ni-base alloy coatings.

Importance of coating adhesion in a corrosive environment was studied experimentally. Tensile adhesion strength of HVOF sprayed 316L stainless steel and Hastelloy C coatings were tested in as-sprayed condition as well as after immersion in seawater. It was found that the adhesion strength of the stainless steel coatings degraded rapidly whereas that of the Hastelloy coatings remained almost intact. Specimens with an artificial defect were also immersed in seawater. The cross sectional observation after the test revealed that the corrosion at the coating-substrate interface proceeded much faster with the stainless steel coating as compared to the Ni-base alloy coating. A model experiment to simulate the galvanic corrosion of a coating-substrate couple was carried out and no significant difference in the galvanic current density was found between the two coatings when coupled with the steel substrate. The tightness of the coating-substrate interface was then tested with a fluorescent dye penetration test. The dye could penetrate the boundary between the stainless steel coating and the substrate whereas the boundary between the Ni-base alloy coating and the substrate was so tight that no penetration occurred. The size of the micro-gaps at the coating-substrate boundary was discussed from the viewpoint of classical Washburn-Ridiel theory. It was concluded that such micro-gaps between the coating and substrate must be eliminated for these barrier-type coatings to be used in corrosive environments. Heat treatment was highly effective for suppressing the preferential corrosion at the coating-substrate boundary.

The increasing demand of the industry for high quality coatings with a low cost aspect has led to the development of ultra thin, nearby finishing free carbide coatings. The R&D work performed for this publication was focused on the tailoring of carbide powders together with the development of a new generation of HVOF-Systems working on higher combustion chamber pressures. An as-sprayed surface roughness of less than 1,5µm has to be envisaged for a thin nearly finishing free coating. Therefore, the starting powder has to have fine particles size with a homogenous distribution of carbides and matrix metals. To ensure the corrosion resistance, the matrix metals have to be completely alloyed and the coatings have to be dense. The used HVOF-System must be able to feed and spray these fine powders without any blockage of the powder feeder or nozzle clocking. The coating quality has to fulfil the requirements of hardchrome plating as wear and corrosion resistance is concerned. The low cost level of thin hard-chrome coatings shall be matched. The suitability for using this coating on applications like hydraulic cylinders has been proven. Generally, this technique of thermal spraying can be used as a hard-chrome alternative as well as for new applications where thermal spraying was not put into account due to cost and technical reasons. The aim was to fill the gap between thin film technologies as PVD or CVD techniques and conventional thermal spraying. The new technology enables the industry to coat large components with nearly no restrictions in size and for competitive prices.

A study was carried out on the corrosion resistance of the JP- 5000 coatings of Superalloy 625, WC-12%Co, WC-17%Co, WC-10%Co-4%Cr and Cr 3 C 2 -25NiCr materials and comparisons made with chrome plating. The coating corrosion resistance was evaluated using the standard salt fog test (ASTM B117) and also by submersion tests in 10% FeCl 3 solution (ASTM G48), in 10% HNO 3 and in 0.1M H 2 SO 4 + 0.2M HCl solutions. The coating corrosion mechanisms in relationship with the coating microstructure and phase constitutions are discussed. The test results show that the HP/HVOF coating corrosion resistance can be significantly improved through spray parameters and coating structure optimization. All coatings tested herein having optimum microstructure and phase constitutions have shown improved corrosion resistances in comparison with chrome plating.

Two kinds of Fe-16Cr-30Mo-(C,B,P) alloy powders of different composition were thermal sprayed onto mild steel using HVOF and APS processes. Perfect amorphous coating was formed by HVOF and APS processes in both powders. The corrosion potential of the amorphous coatings sprayed by HVOF process was higher and the passive current density was lower than that of the amorphous coatings sprayed by APS process in 1N HCl solution. The coatings of perfect amorphous phase were little corroded after one week immersion tests and the corrosion rates of the amorphous coatings showed a fairly less value of 0.03 mm/y compared with Inconel 625 plate in 5N HCl solution at room temperature.

The present work evaluates the oxidation and hot corrosion resistance of high velocity oxy-fuel (HVOF) sprayed WC-NiCrFeSiB coating deposited on Ni-based superalloy (Superni 75) and Fe-based superalloy (Superfer 800H). The coated as well as uncoated specimens were exposed to air and molten salt (Na 2 SO 4 -25%NaCl) environment at 800 °C under cyclic conditions. The thermogravimetric technique was used to establish the kinetics of corrosion. The corrosion products were characterized using the combined techniques of X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe micro analyser (EPMA). The WC-NiCrFeSiB coating provides necessary resistance against oxidation and hot corrosion to both the nickel and iron based superalloys in the given environmental conditions at 800 °C. The oxides of active elements of the coatings, formed in the surface scale as well as at the boundaries of nickel and tungsten rich splats, have contributed for the oxidation and hot corrosion resistance of WC-NiCrFeSiB coatings, as these oxides act as barriers for the diffusion/penetration of the corrosive species through the coatings.

Al-based coating was prepared with Al-based cored wire by arc spraying. The properties of high-temperature oxidation resistance and corrosion resistance of Al-based coating were compared with those of Fe-Al coating and 45CT coating. The property of H 2 S corrosion resistance of Al-based coating was compared with that of A304 stainless steel and Fe-Al coating. The results suggest that the Al-based coating has better properties of high-temperature oxidation resistance and corrosion resistance than Fe-Al coating and 45CT coating. Al-based coating has better property of H 2 S corrosion resistance than A304 stainless steel and Fe-Al coating.

High temperature protection requires full coating density, high adhesion, minor oxide inclusions, and preferably fine grains, which is not achievable in most thermal spray processes. High velocity oxygen-fuel (HVOF) thermal spray process has been applied extensively for making such coatings with the highest density and adhesion strength, but the existence of not or partially melted particles are usually observed in HVOF-formed coatings because of relative low flame temperature and short particle resident time in the process. This work has investigated the development of an innovative HVOF process using a liquid-state suspension/slurry containing small alloy powders. The advantages of using small particles in a HVOF process include uniform coating, less defective microstructure, higher cohesion and adhesion, full density, lower internal stress and higher deposition efficiency. Process investigations have proven the benefits for making alloy coatings with full density and high bond strength attributing to increased melting of the small particles and the very high kinetic energy of particles striking on the substrate. High temperature oxidation and hot corrosion tests at 800°C have demonstrated that the alloy coatings made by the novel process have superior properties to conventional counterpart coatings in terms of oxidation rates and corrosion penetration depths.

Several model Fe- and Ni-based alloys with increased content (up to 30 at.-%) of protecting scale forming elements were developed. High temperature corrosion resistance of bulk alloys as well as thermally sprayed coatings and welded overlays were investigated under the waste power plants simulated atmosphere (500 °C, 100 hours, 75N 2 -20O 2 -4.9Ar-0.1Cl 2 ). Arc and HVOF spraying as well as PTA overlay welding were used to produce the coatings. After an exposure the samples were examined with electron probe micro-analysis (EPMA). It was shown that the protection behaviour of overlay welds depends on the content of alloying elements, although the last is limited because of weldability decrease by high alloying level. High temperature corrosion resistance of thermally sprayed coatings is determined by their porosity, which can be varied over a very broad range depending on the applied spray method. The arc sprayed coatings need an additional post-treatment to close a porosity. Two methods were applied, pre-oxidation treatment in the air and sealing with the commercial sealant. Newly developed iron-based coatings with increased aluminium content (< 20 wt- %) sprayed with HVOF-spraying with powders obtained by means of high energy ball milling demonstrate high corrosion resistance. Selected coatings were evaluated for 1000 h exposure under chlorine-containing salt deposits at the higher temperature (600 °C).

In this study, Cu-based bulk metallic glass coatings were deposited by atmospheric plasma spraying with different hydrogen flow rates. The crystallization and oxidation of the coatings is assessed along with corrosion resistance. As thermal energy in the plasma jet increases, the melting fraction and oxidation of particles in the coating increases as does porosity. All of these factors have an effect on the corrosion resistance of Cu-based bulk metallic glass coatings and their relative impact is discussed.