Herein, we reveal the critical role of CrCl3 and the mechanism for the synthesis of magnesium chloride complex (MaCC), an advanced conditioning-free electrolyte for rechargeable magnesium batteries. This involves a catalytic dissolution of Mg metal by nanoscale bimetallic galvanic couples in an ethereal solution: At the initial stage, nanoscale amorphous Cr-rich ‘islands’ form on Mg surface, creating numerous Cr-Mg galvanic couples. These Cr-rich islands act as local cathodic sites due to partial electron transfer from Mg metal substrate. Furthermore, the first-principles calculation shows that Al prefers to bind at Cr-rich regions rather than Mg. These trigger a heterogeneous catalysis for the selective deposition of Al on Cr-rich islands and a dramatic increase in the dissolution rate of Mg metal on the neighboring region. This leads to an ultrahigh Mg2+-to-Al3+ concentration ratio in the resultant solution, a key property of the conditioning-free electrolyte. This study is potentially applicable to many other fields like metal-air batteries and corrosion protection of metals, where a subtle manipulation of passive layer is required.