By Andrew Spaulding
Last week in “Marine Corrosion – Part 1” we discussed some marine corrosion terms and how sacrificial anodes work to protect your boat’s underwater metals. This week, I want to cover the corrosion survey process. The goal of a corrosion survey is to measure the potential of the individual underwater metal units and the boat as a whole without any outside influence with a reference cell. A reference cell can be one of several types, the most common being silver/silver chloride or zinc. The metal that makes up these reference cells is pure or a very specific alloy. The potentials between these reference cells and many different metals and alloys have been recorded in tables by laboratories.
Once these measurements are recorded for the boat in question, the values are compared to the reference cell tables. The potential between a fiberglass-hulled boat and a silver/silver chloride reference cell should be from -550mV to -1100mV. If the boat as a whole is in this range, you boat has the proper amount of sacrificial anode. If your boat is less negative than this, you are under protected against galvanic corrosion. If your boat is more negative than this, you are over protected.
You may think that over protected is ok, but unfortunately, being over protected has its own set of problems. Under water coatings can be literarily blown off the bottom by gas bubbles forming on the surface of the metal. Also, an alkali solution can form on aluminum which will eat into the metal. Neither of these issues are good and their symptoms can look like galvanic corrosion. The reason we use precise alloys for our reference cell and a volt meter that is accurate measuring 10s of millivolts is so that we can know what is going on with the boat, not make guesses, nor assume that the dock chat about corrosion is correct.
Once we have the boat’s hull potential recorded and the underwater metals are surveyed, the corrosion survey can progress. While measuring the hull potential, we turn on and off every direct current circuit. If the boat has a problem, one or more of the circuits will change the measured hull potential. This is an indication that that circuit has a fault that needs to be addressed.
If the boat lives at the dock plugged into shore power, this process needs to be repeated with the AC power cord and all of the AC circuits. Typically, AC power is not a contributor to long term corrosion issues, but since some AC circuits are intermittently used it is important to test them as part of a corrosion survey. AC power can be the source of serious corrosion, although it is usually so severe that it gets noticed quickly.
Corrosion due to a faulty electrical circuit is called stray current corrosion. Stray current corrosion severely damage underwater metals in a very short period of time. The pictures above show a propeller and shaft that were damaged to the point of replacement in a few weeks.
Last week, I went out to start a corrosion survey. We didn’t get a chance to finish the survey due to thunderstorms, but we did find some suspect DC circuits. We also measured the hull potential at the maximum for galvanic corrosion protection. Any more protection and the boat would be over protected. This condition is okay at the beginning of the season since the anodes are at their maximum potential. I would expect as the anodes do their work over the season, the hull potential of the boat will settle into the middle of the acceptable range.