Allport Marine SurveyTony Allport, SAMSŪ AMSŪPrincipal Marine Surveyor Steilacoom, Washington, USA Serving the Puget Sound Region Including Olympia, Tacoma, & Gig Harbor |
by Tony Allport, SAMSŪ AMSŪ When should zincs be replaced? The short answer is when they are about 50% gone; ideally this state of decay will coincide with your regular haul out interval. The trick is finding the right amount of zinc to protect the boat's underwater metal for that period. Keep in mind that the greater the surface area of your zinc the more protection it will provide; and the greater the weight, the longer it will last. The correct ratio of the surface area of the cathodic (protected) metal to the surface area of the anode (zinc) is key to achieving this balance. There are formulas and electronic measurements that can help size the zinc anodes on your boat, but there are also many variables affecting the rate at which your boat will use zinc. It basically comes down to trial and error. As you haul your boat, if the zincs look the same as they did when you put them on, you have a problem. If they don't wear away they aren't working. You may have too much zinc. This can be harmful to wooden boats and it can cause bottom paint adhesion problems on aluminum hulls. It may be indicated by blistering of the bottom paint and excessive hard growth such as barnacles and mussels encrusting the protected metal. Inactive zinc may also indicate that it is not electrically bonded to the metal it is supposed to be protecting. If your boat is kept in fresh water, zinc is less effective. You should be using aluminum or even magnesium anodes. Always make sure they have not been painted. When the zincs look half as big as the ones you put on last time, then you are doing fine. Replace them with new zincs of the same size. Working zinc should have a clean, coarse, freshly exposed metallic surface; looking as though it has just been sandblasted. This is evidence of millions of microscopic craters created as electrons are ejected from the surface of the zinc in an effort to maintain its inherent galvanic potential. If your zincs are gone, or nearly so, then your boat is under protected. Increase the number or size of the zincs you are using or reduce the exposed surface area of the metals below the waterline that need cathodic protection. Several factors influence the rate of zinc corrosion. These include the temperature and salinity of the water, the flow of water past the boat, and electrical anomalies on the boat or in the marina. Be aware of changes in the rate of zinc usage since it may be a clue to other problems. A snap shot of the level of protection that the zinc anodes are providing can be taken at any time while the boat is in the water. This is done by measuring the hull potential with a multimeter and a silver chloride reference cell. Reference cells are made for this purpose with long lead wires attached to them which should be connected to the negative terminal of your multimeter. The other end of the wire with the reference cell attached is then dangled over the side of the boat with the reference cell under water. With the multimeter set to read DC volts the positive probe should be connected to any part of the DC ground or DC negative wiring. The reading you get should be negative and in the range of a fraction of a volt. There are recommended ranges for hull potential readings for different hull materials. Compare your reading with the recommendations found in ABYC standard E-2, or other texts on marine corrosion. Metal boats should have fairly high hull potentials for adequate cathodic protection, - 850 to - 1100 millivolts for steel hulls and - 950 to - 1100 for aluminum. Fiberglass boats have a wide range of acceptable hull potentials ranging from - 550 to - 1100 millivolts. Wooden boats have a very small window of acceptable hull potentials, between - 550 and - 600 millivolts. This is because byproducts of the galvanic reaction will destroy wood fibers. I generally recommend that serious wooden boat owners install an impressed current system to more easily maintain this narrow range of protection over time. A single hull potential reading only tells you if the boat is adequately protected at the moment, but a series of measurements taken over time can indicate when the cathodic protection has dropped below the acceptable level. Corrosion monitors are available that constantly indicate your hull potential. They are cheaper than impressed current systems and are very useful on metal hulls or boats with aluminum drives. Keep in mind that the presence of any kind of stray current can wildly skew these readings. As an example: I once checked the hull potential on a decrepit old Chris Craft and found it to be very high. I commented to the broker that it appeared to be seriously over zinced for a wooden boat. He doubted my assumption saying that the boat hadn't been hauled in several years. When it was hauled we discovered that there was no zinc at all on it and that there appeared to be third degree burns on the starboard rudder. It was blistered to the point that sheets of bronze foil could be peeled away. This was clearly a case of stray current skewing the hull potential reading. Good electrical continuity is essential between the zinc and the metal it is protecting. You cannot hang a zinc anode over the side on a string and expect it to do any good. Clean, tight, metal to metal contact is best; or provide a low resistance connection with a wire. Since zincs are frequently replaced while a boat is hanging in the slings for survey it is easy for the surveyor to check the quality of the new electrical connection with a multimeter set to the ohms scale. Simply place one probe on the zinc and the other on the metal that the zinc is attached to. The reading should be less than one ohm. Never paint a zinc anode as its exposed surface area is key to its effectiveness. Painting the metal that the zinc is protecting, however, can be beneficial in so far as it reduces its exposed surface area. For further reading on this subject refer to ABYC E-2 and Everett Collier's, The Boatowner's Guide To Corrosion. |
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