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.

The HVOF sprayed coatings of SUS316L stainless steel and Hastelloy C nickel-based alloy were studies with respect to corrosion behaviour in seawater. Corrosion took place at the small crack, which was formed by insufficient filling up of sprayed particles. The corrosion mechanism of the sprayed film was similar to the crevice corrosion. Some treatments for decrease of the crack resulted in considerable improvement of the corrosion resistance.

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.

The application of HVOF spraying to deposit high quality coatings of corrosion resistant alloys for protecting an underlying steel substrate against corrosion in seawater has received much interest over the past few years. Despite the attainment of low levels of porosity and oxide, the coatings to not appear to offer the same level of corrosion resistance as the corresponding bulk materials. The aim of the work reported here is to demonstrate the level of corrosion performance that can be expected from coatings of corrosion resistant alloys deposited using the HVOF spraying process. Three alloy types are considered, a stainless steel with a composition similar to 316L, a nickel alloy with a composition similar to 625 alloy, and commercially pure titanium.

In recent years Norwegian industries together with SINTEF and NTNU have run several projects dealing with development of thermally sprayed ceramic-metallic coatings for corrosion and wear applications. The mechanisms of erosion, corrosion and combined actions have been studied. The coatings studied are of the metal-carbide / metallic binder type sprayed by the High Velocity Oxygen Fuel (HVOF) process. The influence of powder properties like carbide particle size, powder grain size distribution, chemical composition (i.e. type of carbide and composition of the metallic binder) together with the influence of spray systems and parameters have been studied. The studies have demonstrated how optimization of powder and spray parameters improve corrosion and wear behavior of the coatings. Attention has also been paid to methods used for manufacturing powders for thermal spraying. This is very important from a corrosion point of view. The manufacturing methods should make sure that metallic binders are sufficiently alloyed to achieve the necessary corrosion resistance. The results from the work done in these projects are valuable for suppliers of thermal spray powders, spray companies and end users of coatings.

Thermal spray coatings as Cr 3 C 2 -NiCr obtained by high velocity oxy-fuel spraying (HVOF) are mainly applied due to their behaviour against aggressive erosive-abrasive and corrosive atmospheres and their thermal stability at high temperatures. In order to increase the corrosion protection that it offers to the substrate trying to close the interconnected pores, it is possible to apply a thermal treatment with the gun during the spraying of the coating. This treatment could be applied in different ways. One of these ways consists of spraying only a few layers of coating followed by thermal treatment and finally the spray of the rest of layers. This thermal treatment on spraying is studied related to the corrosion properties of the system. The study comprises the electrochemical characterisation of the system by open circuit potential (OC), polarisation resistance (R p ), cyclic voltammetry (CV) and impedance spectroscopy measurements (EIS). Optical and scanning electron microscopy characterisation (OM and SEM) of the top and cross-section of the system has been used in order to justify the electrochemical results.

The SUS316L stainless steel rod specimen coated with plasma-sprayed Al 2 O 3 deposits has been fatigued in a physiological saline solution (0.9 % NaCl solution) to evaluate the potential of its application to prosthetic implant materials. Push-pull loading fatigue tests were conducted at the stress ratio of R = -1, and at the frequency of 2 Hz. Pure titanium powder was selected for undercoat. Fatigue damage was examined on longitudinal section of the specimen and fracture surface by optical and electron microscopy from the microstructural viewpoints. The plasma spraying of Al 2 O 3 powder has significantly improved fatigue properties of the substrate metal in the longer range of fatigue lives, compared with the results of the non-coated steel specimen. It was found from electrochemical experiments that titanium for undercoat metal has acted as sacrificial anode to protect the substrate metal from corrosive attack and under lower stress amplitudes the plasma sprayed Al 2 O 3 coating kept the solution out at an early stage of fatigue lives. Fatigue cracks preferentially originated from flaws, which had been caused on the substrate metal surface through grit blasting, and extended into the bulk of substrate metal. Fatigue cracks appear not to develop into plasma-sprayed deposits while the deposits could accommodate themselves to the crack opening displacement at the surface of substrate metal. It was understood that the plasma sprayed coating has enhanced fatigue properties in the solution both by keeping the solution out during the early stage of fatigue lives and by electrochemical effects of the undercoat metal when the topcoat was cracked in macroscopic scale.

TiN reinforced Ti coatings were produced by using the arc spraying process. TiN hard phases were synthesized during spraying using Ar/N 2 reactive atomizing gas. The spray process was realised in an air atmosphere with a shrouded gun and in a chamber with closed loop Ar/N 2 -gas atmosphere. The content of TiN phases in Ti-coatings was increased by rising the N 2 -amount in atomizing gas during spraying. Sprayed coatings obtained a graded hardness ranging from 450HV 0.1 near the substrate up to 650HV 0.1 near the top (mean values). TiN-particle precipitations exhibited micro hardness up to 1350HV 0.1 . Structure investigations of manufactured coatings proved that they consist of pure titanium, titanium nitrides and small amounts of titanium oxides. Wear resistance of the coatings, tested by Taber Abraser and Pin on Disc, decreases from the coating surface to the substrate. According to Kesternich test, Ti/TiN-arc sprayed coatings exhibit good corrosion resistance.

In a BRITE-EURAM Project gas shroud devices were developed for arc-wire and air plasma spraying processes to obtain cost effective coatings with high density, low defect contents and high corrosion resistance. To test the two devices FeCrAl, NiCrAl and NiCrTi coatings sprayed by shrouded arc were studied for boiler application; AMDRY 995 coatings sprayed by shrouded plasma were analysed for gas turbine blades. In this paper the results are presented of the extensive characterization performed by CESI on the coatings produced with the innovative techniques developed by the other partners. The coatings were sectioned and analysed by metallography and image analysis to determine oxide and porosity contents (total defect levels were less than 5%). Hardness and adhesion were measured following the ASTM standard procedures. On FeCrAl, NiCrAl and NiCrTi coatings sprayed by shrouded arc-wire technique both thermal cycling and corrosion tests were performed. A sulphidizing gaseous atmosphere was used to simulate boiler environment burning sulphur rich fuel as Orimulsion. The corrosion behavior after 1000h at 450°C resulted to be as good as that of an HVOF sprayed similar coating. Moreover coated tubes survived to thermal cycling tests without any delamination. Hot corrosion "Dean tests" were performed on AMDRY995 coatings sprayed by shrouded plasma onto two Ni-base superalloys (Ud 529 and In738). The environment was typical of hot parts in gas turbines; the corrosion resistance results after 1750h at 850°C were comparable to those typically obtained on the same material sprayed by vacuum plasma spray technique. The applicability of the shrouded plasma technique to the field of gas turbines was confirmed.

Microstructural features such as cracks, pores and delaminations are known to significantly affect the properties of thermal spray ceramic coatings. As an example, tensile properties depend mostly on the size, the shape and on the degree of clustering of pores since highly clustered porosity generates regions of poor mechanical properties from where cracks easily propagate through the entire volume. Another example concerns the corrosion resistance of coated components which is significantly degraded when pores are connected. To circumvent such disadvantages, several post-treatments may be implemented. The most commonly used is sealing of pores by impregnation using either low melting temperature metallic alloys or organic materials. In addition to be a costly additional step to the deposit manufacturing process, the coating working temperature limit is significantly reduced. Therefore, numerous works along the twenty past years have been devoted to find ways to densify ceramic coatings. Among these treatments, the use of high energy laser beams was very often considered. This paper aims to bring a contribution to those efforts by presenting results concerning the laser treatment of zirconia thermal spray coatings.

Thermal spray processes are widely used to deposit high-chromium nickel-chromium coatings to improve high temperature oxidation and corrosion behaviour. However, in spite of the efforts made to improve the present spraying techniques, such as HVOF and plasma spraying, these coatings may still exhibit certain defects such as unmelted particles, oxide layers at splat boundaries, porosity and cracks, which are detrimental to corrosion performance in severe operation conditions. Due to low process temperature only mechanical bonding is obtained between the coating and substrate. Laser remelting of the sprayed coatings was studied in order to overcome the drawbacks of sprayed structures and to markedly improve the coating properties. The coating material was high-chromium nickel-chromium alloy, which contains small amounts of molybdenum and boron (53.3%Cr- 42.5%Ni - 2.5%Mo - 0.5%B). The coatings were prepared by high-velocity oxy-fuel spraying onto mild steel substrates. High power fiber coupled continuous wave Nd-YAG laser equipped with large beam optics was used to remelt the HVOF sprayed coating using different levels of scanning speed and beam width (10 mm and 20 mm). Coating remelted with the highest traverse speed tended to suffer cracking during rapid solidification inherent to laser processing. However, choosing appropriate laser parameters, non-porous, crack-free coatings with minimal dilution between coating and substrate were produced. Laser remelting resulted in the formation of dense oxide layer on top of the coatings and full homogenization of the sprayed structure. The coatings as-sprayed and after laser remelting were characterized by optical and electron microscopy (OPM, SEM). Dilution between coating and substrate was studied with EDS. The properties of the laser remelted coatings were directly compared with properties of as-sprayed HVOF coatings.