Bob F Posted March 11, 2005 Report Share Posted March 11, 2005 Some members have recently been asking what sacrificial anodes are, why they are needed and where they are fitted. So, here's my attempt at trying to "simply??" explain them... The batteries fitted in your cars or boats are made of two different metal rods placed in water (distilled water. also called an electrolyte). When your boat and/or engine is placed in water you effectively have the same thing - two or more different metals placed in an electrolyte (sea water). So you have virtual batteries which cause a small electric current to flow. Part of the chemical reaction in a battery causes atoms from one of the metals rods to be removed, causing the metal to slowly be eaten away. This is always the metal rod which is the "anode" in the battery. The other metal rod (cathode) remains unaffected and does not get eaten away. So, when you have two different metals in sea water, one of those metals will assume the role of the anode and will slowly be eaten away. The other metal remains intact. It is extremely difficult to stop this reaction taking place, so instead of stopping it we use the reaction to protect the metals that we don't want to corrode. If you fit a type of metal that will always be the anode, it will corrode thus preventing the other metals corroding. When comparing two different metal types, the anode will always be the metal that is the least "Noble" in the "Periodic Table". This will only mean something to you if you remember any of your Physics or Chemistry at school. It's not important to know. Zinc is one of the least Noble metals, so it will tyically always assume the role of anode when compared with other metals that are fitted to our boats and engines. The "sacrificial anode" that is deliberately fitted on our boats is a piece of Zinc which will always assume the role of the anode compared to the other metals, so the Zinc slowly corrodes thus preventing the other metal pieces on your boat corroding. There, that was simple, wasn't it!!!??? So, what do these anodes look like and where are they fitted? There are many different shapes and sizes, but essentially they look like a small block of lead (it's Zinc of course). On outboards they are typically fitted on the underside of the mounting bracket and on the sides of the engine by the anti-cavitation plates. I'm not sure of the location of the anodes on inboard engines, but I believe they are placed near the propeller, near the shaft drive, and at various places around and on the engine block. You will often see the zinc metal blocks fitted with metal straps which connect to other metal areas. Needless to say, if your anodes have "rotted" away you need to replace them before other parts of you boat or engine start to disappear. If your boat is left in the water all the time, they typically need replacing every year. That's my take on it, anyway. cheers Bob F. Quote Link to comment Share on other sites More sharing options...
Newboy Posted March 11, 2005 Report Share Posted March 11, 2005 If your boat is moored in a mariner with shore power, you can get a little electronic machine which stops/slow down the effect. Quote Link to comment Share on other sites More sharing options...
Guest fish finger Posted March 11, 2005 Report Share Posted March 11, 2005 So if you have one of these anodes on your outboard and it is being kept on a mooring, is it best to lift the outboard up and out of the water or to leave it down ? (given the mooring is deep enough for the engine not to ground). The anode will not stop corrosion if it is not in the water. Alex Quote Link to comment Share on other sites More sharing options...
Maverick Martin Posted March 11, 2005 Report Share Posted March 11, 2005 Alex You would not leave your engine in the down position but always tilt it up clear of the water. I guess one reason for this would be less metal in the water less electrolysis action. You are correct in your assumption that the sacrificial anodes on your outboard would not offer any protection in this position. You would fit sacrificial anodes to other areas on the boat. When my boat was kept on a pontoon I had 2 largish sacrificial anodes fitted on my trim tabs and these would as Bob has said need replacing on an annual basis. The sacrificial anodes on the engine were replaced about once every two or three years. I believe the guys with outdrives or inboards are in a worse situation as they cannot lift the outdrives or props clear of the water so the annodes of their legs need more frequent replacing and it aint cheap This hobby gets more expensive by the day Martin Quote Link to comment Share on other sites More sharing options...
Jack the Lad Posted March 11, 2005 Report Share Posted March 11, 2005 Click these guys, they are the experts and give helpful info for leisure and commercial anodes M.G.DUFF Note: If you buy an anode and it feels much 'lighter' than normal it may not be the bargain you expected - it's a fresh water anode. Fresh water and sea water anodes are different. Jack Quote Link to comment Share on other sites More sharing options...
Manic Moore Posted March 11, 2005 Report Share Posted March 11, 2005 Nicely done Bob Quote Link to comment Share on other sites More sharing options...
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