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• Porous Ag-enriched phase forms during corrosion via de-alloying of Mg-Ag phase. • Dissolved Ag mainly redeposits to Mg surface forming Ag nanoparticles. • Both of the formed porous Ag-enriched phase and re-deposited Ag nanoparticles can result in evident cathodic activation. • The cathodic activation ability is significantly affected by Ag content and existing form of Ag. The effect of Ag on cathodic activation and corrosion behaviour of Mg-Mn-Ag alloys are investigated in the present work. During corrosion, Mg-Ag intermetallic phase undergoes dealloying to form porous Ag-rich remnant whereas Ag in the alloy matrix is dissolved to form Ag+ ions that subsequently redeposit on the alloy surface via displacement reaction with Mg to form Ag nanoparticles, which lead to significant cathodic activation. The cathodic activation is a self-catalysed process since enhanced cathodic activity promotes anodic dissolution which, consequently, results in further enhancement of cathodic activity. Thus, the corrosion rate of Mg-2 Mn-4Ag alloy increases significantly with immersion time. [ABSTRACT FROM AUTHOR]