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Galvanic Corrosion

This might seem technical, but this is a post about galvanic corrosion.

Let's simplify it.

When certain (dissimilar) metals touch one can cause the other to corrode.  This process is called, variously, galvanic corrosion, dissimilar metal corrosion, and electrolytic corrosion.

Essentially, electrical current can pass between two dissimilar metals.

Some metals are more "noble," meaning they are more electro positive and therefore protected, also called cathodic.  These metals include copper, bronze and brass.

Other metals are less "noble," meaning they are more "anodic" toward those further up the galvanic scale.  Anodic means they are more capable of corrosion because when joined to those more noble metals they are electro negative.  Electrical current passes more easily from them.  Their metal is corroded and carried toward the more noble.  These less-noble metals include zinc, galvanized steel, aluminum, steel and iron.

When more and less noble metals, or "dissimilar metals," touch, the generated electrical current passes from the less noble (anodic) to the more noble metal (cathodic).  The anodic metal corrodes and deposits itself onto the cathodic metal.   This corrosion process is more likely in the presence of salt water or acidic solutions, but even moisture in the form of condensation can cause corrosion, though it is less likely.

However, it is always a best practice to keep dissimilar metals from touching.

Yes, home inspectors look for this stuff!  The most frequent place we usually see dissimilar metal corrosion is at water heater connections.  Leaking is often the result.

Recently, in brand-new construction, and a final walk through inspection, seeing this over a basement AC unit, I thought I should mention it.

You might think it pedantic of me to do so (pedantic means overly fussy or finicky), but I did anyway.  Look at the photo on the right.  That is copper touching steel.  There is the potential for condensation from the AC line.  Could it be acidic?  I don't know!  But it is easy to separate the metals now.  No touch, no problem.

Many years ago I installed a whole-house water filter in my house.  Yes, we are on public water, and it's amazing how cruddy the filter gets in two months!  See the sand at the bottom now?  That filter is one month old.

But I digress.  The left photo shows what I used to better attach that plumbing, and additional weight, to the floor joist above. 

The plumbing is copper.  The hanger is copper.  Similar metals!

And yes, you can see my bonding cable connecting the pipes left to right.

My recommendation:  when in doubt, it is always best to keep metals separated.  Even the slightest possibility over time may cause a problem.  So, why not eliminate that possibility now?  Yep, the picky, picky, pedantic home inspector.  That's me...

 

 

Jay Markanich Real Estate Inspections, LLC  

Based in Bristow, serving all of Northern Virginia.

Office (703) 330-6388   Cell (703) 585-7560

www.jaymarinspect.com


Comment balloon 14 commentsJay Markanich • July 30 2015 12:35AM
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