High-temperature mechanisms of hydrogen evolution in Ni–P coated titanium hydride (TiH2) powder

• Ni–P compounds were deposited on TiH2 through an electroless plating method.
• The potential phase transformations were monitored at a wide temperature range.
• The effects of hydrogen gas evolution on the Ni–P coating were characterized.
• Finally, the active mechanisms of dehydration were determined at high temperatures.

In this work, Ni–P compounds were deposited on titanium hydride (TiH2) by the electroless plating method and the possible chemical reactions were studied in a broad temperature range of 100–1200 °C. The microstructure evolution and thermal decomposition mechanisms of Ni–P coated TiH2 were considered using characteristics of the coated powder samples treated at different temperatures. The characterization was carried out by X-ray diffraction analysis (XRD), energy dispersive spectroscopy (EDS) and field-emission scanning electron microscopy. It was shown that, up to 900 °C, no potential chemical reaction or deterioration of the Ni–P film occurs. However, the coating is partially swollen due to high back pressure of hydrogen gas behind the surface layer and some turgid islands are formed. At 1000 °C, the active mechanism of hydrogen release process begins to switch from internal diffusion to chemical reaction. At temperatures higher than 1000 °C, the coating becomes oxidized, some intermetallic or oxide compounds such as NiO, TiO2, NiTiHO0.95, TiNiH and Ti17P10 are formed and the hydrogenation state is reduced down to 1. Finally, it was found that in the temperature ranges of 100–900 °C, 900–1000 °C and 1000–1200 °C, the active mechanisms are internal diffusion, a combination of chemical reaction and internal diffusion, and chemical reaction, respectively.

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