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Abstract The effects of the Mg content on the microstructure and hydrogen generation kinetics of Mg NdNiMg 15 composites by hydrolysis reaction in seawater (3.5 wt% NaCl) was investigated. The presence of NdNiMg 15 in Mg matrix promotes galvanic corrosion. Thus, a galvanic coupling was observed between the NdNiMg 15 phase (cathode) and the Mg phase (anode). An acceleration of the corrosion rate of Mg phase was observed due to this galvanic coupling. Electrochemical tests confirm that pure Mg has a corrosion rate in salt water up to 750 times lower than that of the Mg phase in the Mg NdNiMg 15 90% composite. The best hydrolysis performance (100% of the theoretical hydrogen generation yield in slightly more than 15 min) observed for Mg NdNiMg 15 90% composite, was explained by a combined effect of galvanic corrosion, intergranular corrosion and pitting corrosion. Highlights • Hydrolysis of Mg NdNiMg 15 composites are studied in NaCl solution. • Galvanic coupling between NdNiMg 15 phase (cathode) and the Mg phase (anode) exists. • Corrosion rate of composite is 750 times higher than the one of pure Mg. • NdNiMg 15 promote the galvanic and intergranular corrosion. [ABSTRACT FROM AUTHOR]