The work aims to study failure of coating on sintered NdFeB. Aided by SEM and electrochemical test methods including polarization curves, AC impedance and zero charge potential, main reason was testified to be NdFeB corrosion inside the coating, which reduced the coating adhesion. Phytic acid derivatives could efficiently inhibit corrosion inside the coating. The was 27.93 kJ/mol. The was mainly physical adsorption and partly chemical adsorption. The corrosion inhibition mechanism is the adsorption of phytic acid derivatives on NdFeB, which prevents the occurrence of corrosion.