The effects of the Mg content on the microstructure and hydrogen generation kinetics of MgNdNiMg15 composites by hydrolysis reaction in seawater (3.5 wt% NaCl) was investigated. The presence of NdNiMg15 in Mg matrix promotes galvanic corrosion. Thus, a galvanic coupling was observed between the NdNiMg15 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 MgNdNiMg15 90% composite. The best hydrolysis performance (100% of the theoretical hydrogen generation yield in slightly more than 15 min) observed for MgNdNiMg15 90% composite, was explained by a combined effect of galvanic corrosion, intergranular corrosion and pitting corrosion.