How To Tell Zinc, Aluminum, and Magnesium Apart

A question we're often asked: how can I tell if this anode is zinc, aluminum, or magnesium?

Boaters may find random anodes after the purchase of a boat or tucked away in the garage or tool chest.  They may take off an anode the yard put on for them last haul-out and have no knowledge of what it is.  Most folks know that mixing metals is to be avoided (we'll speak in a future post about why and what happens when it cannot be helped).  So the discovery of a mystery anode is usually taken with an understandable degree of caution.  Should I use this on my boat?  Is this the right alloy?  What is the alloy here anyway?

The most effective way to determine the alloy of an anode is with a reference cell.  A simple test will reveal the anode's potential allowing for a doubtless conclusion.  But say a reference cell isn't to hand (here's that link again, just in case).  What then?

Well, there will be clues.  The most obvious one will be weight.  A brand new magnesium anode will weigh significantly less than zinc and a good deal less than aluminum.  Customers are often surprised at how light magnesium actually is and are surprised at how heavy zinc is.  A full-sized (as in, "not corroded") ZC-406 in zinc weighs 7.5 lbs.  The same anode cast in aluminum comes in at 3 lbs.  And the magnesium version falls even lower on the scale at 1.6 lbs.  So, if you've found a full anode, the weight of it is likely to be all you need to know.

Note: our product pages offer accurate weights for every product in our inventory, so it will always be possible to compare the three alloys.

There are also visual differences if you know what to look for.  Starting again with magnesium, you'll find that freshwater anodes are often a very dark gray with a dull finish to them.  There can be some exceptions to that, with magesium cast in very controlled environments sometimes turning out bright and shiny.  But in general, its more common for magnesium to be darker and rougher.  Your fingers will not glide across magnesium, they'll catch and stop on the surface. 

Zinc and aluminum are trickier.  In fact, the majority of these questions are more specifically asking "Is this zinc or aluminum?".  Both metals will be lighter in color.  Both are likely to be shiny (depending on the casting process, they may be dull, but never so dull as magnesium).  Zinc is going to generally be darker than aluminum.  Sometimes, more noticeably, aluminum may have a very slight gold tint to it, a side effect of aluminum's extremely high casting temperatures reacting with the air of its environment.

That's all assuming you have a new mystery anode.  How about one that's already been below the waterline for a while?  Assuming they were used in the appropriate type of water, you can count on a few things in broad strokes.

• Magnesium is likely to turn some shade of yellow and become porous.
• Aluminum will chalk up into sharp, white, craggy peaks.
• Zinc tends to corrode more uniformly, leaving behind a fine, gray dust.

These clues, combined with a reliable reference point for weights, should get you close to 100% certainty.  Whenever that isn't possible, we're always a phone call or an email away to help.