The　effect　of　the　Ca2Mg6Zn3　phase　on　the　corrosion　behavior　of　biodegradable　Mg-4.0Zn-0.2Mn-xCa　(ZM-xCa,　x　=　0.1,　0.3,　0.5　and　1.0　wt.%)　alloys　in　Hank＇s　solution　was　investigated　with　respect　to　phase　spacing,　morphology,　distribution　and　volume　fraction.　With　the　increase　in　Ca　addition,　the　volume　fraction　of　the　Ca2Mg6Zn3　phase　increased　from　2.5%　to　7.6%,　while　its　spacing　declined　monotonically　from　43　mu　m　to　30　mu　m.　The　Volta　potentials　of　secondary　phases　relative　to　the　Mg　matrix　were　measured　by　using　scanning　kelvin　probe　force　microscopy　(SKPFM).　The　results　show　that　the　Volta　potential　of　the　intragranular　spherical　Ca2Mg6Zn3　phase　(+109　mV)　was　higher　than　that　of　the　dendritic　Ca2Mg6Zn3　phase　(+80　mV).　It　is　suggested　that　the　Ca2Mg6Zn3　acted　as　a　cathode　to　accelerate　the　corrosion　process　due　to　the　micro-galvanic　effect.　The　corrosion　preferred　to　occur　around　the　spherical　Ca2Mg6Zn3　phase　at　the　early　stage　and　developed　into　the　intragranular　region.　The　corrosion　rate　increased　slightly　with　increasing　Ca　content　from　0.1　wt.%　to　0.5　wt.%　because　of　the　enhanced　micro-galvanic　corrosion　effect.　The　decrease　in　the　phase　spacing　and　sharp　increase　in　the　secondary　phase　content　resulted　in　a　dramatic　increase　in　the　corrosion　rate　of　the　ZM-1.0Ca　alloy.