The influence of the deformation speed in C-ring tests in Ringer's solution on crack initiation and propagation of extruded Mg10Dy and Mg10Dy1Nd is investigated. Deformation speeds varying from 2 to 0.012 mm/min allow corrosion times from a few minutes to hours. Both the crack initiation force (higher for Mg10Dy1Nd) and displacement (higher for Mg10Dy) increase with decreasing deformation speed up to a corrosion time of 1 h and then decrease, more for Mg10Dy1Nd and slightly more for the displacement compared to the force at higher corrosion times. The decrease is associated with the higher corrosion times—corrosion pits become visible at a test time of 1 h on the tensile side. In Mg10Dy1Nd the fracture toughness increases with decreasing deformation speed, and no clear picture is seen for Mg10Dy. Sub-cracks often initiate at corrosion pits, which show a correlation to twinned grains at the tensile side—increasing fracture toughness.