A practical lesson in marine metals

This week Alyssa Demko from the Jenkins Lab and I dove to make repairs on our sample pump intake on the SIO pier.  Very soon the pump will supply water to our membrane inlet mass spectrometer and biological sampling manifold, so we’re eager to keep things in good working condition.  Our pump intake is secured to the pier by a very heavy stainless steel metal bracket.  When we first installed the metal bracket we opted for silicon bronze hardware; silicon bronze is pricey but among the most corrosion resistant alloys available.  When we last dove I noticed the hardware was corroding very rapidly, to the point that a good storm would have ripped the whole contraption off the pier.  Fortunately it’s summer!  Here’s some of the hardware that we recovered from our dive:

When installed these were 5 x 0.5 inch bolts, the head was 3/4 inch.  This is some serious corrosion for a 4 month deployment!  Silicon bronze is supposed to be corrosion resistant, so what happened?

The problem is that when two or more metals are in contact in the presence of a electrolyte (like seawater) they interact.  Specifically, some metals like to donate electrons to other metals.  The metal doing the donating (called the anode) corrodes more quickly than the metal that receives them (called the cathode).  Because this transfer replaces electrons that the cathode loses to seawater, the presence of the anode actually slows the corrosion of the cathode.  This is a well known process that we planned for when we designed the system, and we included a zinc plate called a sacrificial anode that serves no purpose other than to donate electrons to the stainless steel bracket.  Enter silicon bronze.

How readily one metal donates electrons to another metal is indicated by their location on the galvanic series.  The further apart two metals are, the more readily electrons flow from the anodic to the cathodic metal.  Silicon bronze is more anodic that stainless steel, particularly the 316 alloy we are using, but I figured it was close enough.  Apparently not, however, and we didn’t account for other factors that can influence the rate of electron transfer.  An important one is the surface area of the cathode relative to the anode.  Remember that the cathode is losing electrons to seawater, this is what is driving the flow of electrons from the anode to the cathode in the first place (if you put them in contact in say, air, mineral oil, or some other non-conductive medium nothing will happen).  So the more surface of the cathode is in contact with seawater, the more electrons will flow from the cathode to seawater, and from the anode to the cathode.  In our system the relatively small bronze bolts were attached to the a very large stainless steel bracket, and I think this accounts for the rapid corrosion.

There is one thing that I still don’t understand, which is why the zinc anode in the system didn’t protect the bronze and stainless steel.  Bronze will sacrifice itself for stainless steel (as we’ve clearly demonstrated), but zinc should sacrifice to bronze and stainless steel.  However, our sacrificial zinc anode looks almost as good as it did when I installed it.  In the meantime here’s a video of the impressive lobster party taking place on the piling where our sampling system is located (don’t even think about it, it’s a no take zone!). At the end of the video you can see our shiny new 316 stainless steel bolts. Hopefully these last!