Aluminium vs. zinc anodes
Which is better; aluminium or zinc anodes in salt water environments? Both will work. Aluminium, however has many benefits over zinc.
Whether you choose zinc or aluminium, the grade and the composition are key to the effectiveness of the anode.
While anodes may appear to be fairly simple pieces of metal, they are actually a result of sophisticated anti-corrosion engineering. Cathwell anode alloys complies with NORSOK M-503 and DNV GL RP-B401.
Advantages of aluminium anodes
Weight: Aluminium is significantly lighter than zinc, by a factor of 2.5. Al anodes are lighter to ship and to fit.
Capacity: The electrochemical capacity is more than 3 times higher than of the same mass of zinc (you can protect more with less). See data sheet for aluminium alloyed sacrificial anodes.
Driving voltage: Aluminium anodes has a relatively high driving voltage. This means that it provides better distribution of the current, compared with zinc.
Environment: Aluminium anodes carry a better environmental footprint than zinc anodes. Aluminium anode alloys do not contain cadmium, which is harmful to the marine population.
Cost: Aluminium anodes are less expensive, taking into account the significantly reduced weight requirement compared with zinc.
(Note that there are certain restrictions for use of aluminium alloyed anodes in ballast and cargo tanks)
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Advantages of zinc anodes
Availability: Traditionally used by the maritime industry, hence zinc anodes are widely available. See data sheet for zinc alloyed sacrificial anodes.
Geometry: Zinc anodes can be produced in rather complex geometry, as opposed to aluminium. This is particularly important for slender designs, such as rope guard anode rings.
No restrictions for use in tanks: Zinc anodes are not subject to the same class restrictions as aluminium for use in tanks with possible explosive atmosphere.
The anode surface corrodes more evenly: Zinc anodes tend to dissolve more evenly and completely; while typical aluminium anodes erode unevenly with visible “craters”.
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While commonly referred to simply as “zinc,” anodes are in fact available in several different alloys, including aluminum and magnesium.