Martyr driveshaft housing anode protects the upper housing from corrosion in fresh water lakes and rivers. Mercruiser Magnesium Driveshaft Housing Anode | Martyr CM821630C2M Mercruiser driveshaft housing anode For all Mercruiser Bravo I, II, II X, XR and XZ drives Magnesium anode for use in fresh water applications Magnesium replacement for Mercury 821630A2 For aluminum see p/n CM821630C2A For zinc see p/n CM821630C2Z