Corrosion　behaviour　of　AZ31　magnesium　during　the　initial　six　exposure　periods　in　a　tropical　marine　atmosphere　is　investigated.　The　results　reveal　that　corrosion　process　of　magnesium　is　dominated　by　pitting　corrosion　which　consists　of　initiation　of　new　pits,　propagation　of　small　scale　pits　and　coalescence　of　neighbouring　pits.　There　exists　a　critical　depth　above　which　the　pits　cease　to　grow　down,　resulting　in　the　fluctuation　of　the　mean　pit　depth.　Different　exposure　conditions　are　found　to　be　crucial　for　the　different　pit　characters.　Pits　on　the　skyward　surface　are　in　deep-hole　shape,　while　the　groundward　surface　is　covered　with　shallow　dish　pits.