Martyr magnesium anode designed to protect the drive from corrosion in fresh water lakes and rivers. Volvo Penta Magnesium Cavitation Plate Anode | Martyr CM3855411M Volvo Penta cavitation plate anode Magnesium anode for fresh water applications Replaces Volvo Penta 3855412 For anti-spray anode see Volvo p/n 3857858 For aluminum see p/n CM3855411A For zinc see p/n CM3855411Z