This is the most common alloy for sacrificial
anodes. It is over 99% 'special high grade' zinc with
a small addition of aluminium and cadmium
The specification is to MIL-A-18001
This 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 but anodes are also available with an alloy that is 95% aluminium.
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.