Zinc anodes
The most common alloy for sacrificial
anodes is over 99% 'special high grade' zinc with
a small addition of aluminium and cadmium
The specification is to MIL-A-18001
It has very low iron and other impurities.
Zinc anodes are suitable for all applications in sea
water but are not effective in fresh water.
Change to Magnesium provided you are not at
sea for more than a week or so in a year.
Zinc is expensive at present but anodes are also available with an alloy that
is 95% aluminium. They
are just as effective at sea, slightly more active in
brackish water and can be cheaper as a poured
anode. They are not suitable for fresh water.