Titanium has an excellent corrosion property in chloride containing environments such as seawater. A modified HVOF spray process was developed by introducing a mixing chamber between the combustion chamber and the powder feed port. Nitrogen gas was fed into the mixing chamber to control the temperature of the combustion gas generated in the combustion chamber. By controlling the flow rate of nitrogen, various Ti coatings with different degree of oxidation and porosity could be fabricated. The densest coating produced by this process with surface polishing treatment maintained excellent corrosion protection over a steel substrate in artificial seawater in a laboratory test over 1 month.

Coatings of HastelloyC fabricated by HVOF spraying with a gas shroud (GS) have shown the superior barrier characteristic and corrosion resistance in seawater environment. During immersion of these coatings in artificial seawater, however, vibrational behaviour of the corrosion potential was observed. Some types of surface modification of the sprayed coatings and changing of the spray condition were carried out and their effect on the corrosion potential was investigated. It was revealed that the vibration is related to surface oxides formed during the spray process. Surface modification of the sprayed coatings and changing of the spray condition could reduce the vibration effectively.

For 6 years, we have developed corrosion resistant coatings for the marine structural steels by using a thermal spray technique. Such a coating requires primarily impermeability and secondarily cleanliness. In order to make denser and highly corrosion resistant coatings, we selected spray materials and improved fabrication processes. In addition, some new methods were designed to evaluate the sprayed particle’s state and coating properties with high accuracy and sensitivity. An inert gas shroud system was attached with the commercial HVOF apparatus and this attachment enabled inflight spray particles to be accelerated over 750 m·s-1 and simultaneously to avoid to be oxidized. The coating of HastelloyC nickel base alloy by this process had zero through porosity and 0.2 mass% of oxygen content, leading to be comparable to the bulk material of HastelloyC in terms of corrosion resistance. This coating, formed on steel, demonstrated an excellent protective performance over 10 months in the marine exposure test.

This paper examines the effects of spark plasma sintering (SPS) on flame-sprayed zirconia coatings. It describes how the zirconia layers were produced, treated, and tested. The combination of heating and loading increased coating hardness and adhesion strength by a factor of three and caused a significant reduction in porosity. It also led to phase transformations which, in some cases, had an offsetting effect on coating properties. Paper includes a German-language abstract.

This study examines stainless steel and Hastelloy C coatings sprayed using commercial HVOF equipment. Porosity and oxygen content have been measured under various spray conditions and laboratory corrosion tests have been carried out using electrochemical techniques. The report first summarizes major results obtained in the laboratory evaluation, then presents the results of marine exposure testing for up to six months. It concludes with detailed comparisons between the two. Paper includes a German-language abstract.

Previous studies have shown that gas shrouding is an effective means for controlling oxidation during HVOF spraying. In this present work, the authors attach a gas shroud to an oxyfuel torch with a longer barrel to further investigate the correlation between the state of HVOF sprayed particles and the density and oxygen content of the resulting layers. It is shown that with gas shielding, extended barrel length, and optimized spraying parameters, it is possible to accelerate powder particles to a velocity of over 750 m/sec with maintaining a high molten fraction, thereby producing very dense (zero porosity) stainless steel layers with oxygen contents less than 0.2% by weight. Paper includes a German-language abstract.

Oxidation of HVOF sprayed 316L stainless steel coatings was studied experimentally. Oxygen content in the sprayed coatings was analyzed and its dependence on several spray parameters such as spraying distance, mixture ratio of fuel to oxygen, and composition of atmospheric gas on the substrate was studied. The oxygen content in the original powder was about 0.03 wt%, which typically increased to 0.3 % in the HVOF sprayed coatings under the standard spraying conditions. Reduction of spray distance significantly increased the oxygen level due to the excessive heating of substrates by the flame. The sprayed deposits were analyzed by XRD and the oxides within the coatings were identified as magnetite Fe 3 O 4 or chromite FeCr 2 O 4 . By using a nitrogen-gas shield attached to the substrate, it was revealed that the oxidation during flight is around 0.2 wt%. Control of oxidation by attaching a gas shroud to the HVOF nozzle has been attempted and oxygen content below 0.15 % has been achieved so far while maintaining deposition efficiency over 73 %.

A new method to capture and evaluate the condition of thermal sprayed particles has been developed by using an agar gel target. HVOF sprayed Hastelloy C particles were collected by a gel target placed at the substrate position, i.e., 380mm downstream from the spray nozzle exit. In the surface layer of the target, a large number of fine particles were observed, whereas in the deeper part, globular particles were trapped. The ratio of particles in the surface layer with respect to the deeper part changed by spraying parameters. Furthermore, particles in the target were separated by cutting the gel, collected after resolving the agar, and then observed by SEM. Particles collected from the target's surface layer were fine particles (under 10µm) and fragments of dendritic crystals. These collected from the deeper part were mostly unmelted particles, some of which exposed dendrites. Results obtained by other techniques such as splat observation and in-flight diagnostics are compared with these results. It was concluded that by capturing thermal sprayed particles with a gel target, it is possible to visualize and quantify the melting condition of HVOF sprayed particles.

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 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.

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.