How To Avoid Aluminum Corrosion in Boat Engines
August 1, 2009 by Chris Wethered
Filed under Boat Engines And Sailboat Hardware, Questions Visitors Are Asking And Answers
Boat engines, both inboard and outboards use aluminum for a range of reasons. Here a few that come to mind:
- Outdrive casings
- Protective tubing or piping
- Radiators / heat exchange – DIY projects
- Holding tanks
There are other uses for the alloy, but they all share the same problem – Salt water erosion.
Basically any metal when put in salt water erodes, the rate of erosion depends on how reactive that metal, or alloy is – remember your chemistry experiments!
Electrolysis occurs and there is a flow of direct current created. The more metals in the salt water the quicker the erosion. The solution lies in prevention rather than cure. Cure can be very expense and prevention not so.
Does your lower unit look this?
Regularly checks
To avoid corrosion, you must regularly check your aluminum components. Some advice says weekly, some say monthly. It’s up to you. It’s best to monitor more often than not, until you have an idea of the rate of attrition.
So start frequently and then lengthen your checks. The individual environment of your components dictates the corrosive nature of salt water.
Outboard checks
- All wires, particularly the junction box. Which serves two functions. First the charging system of the engine, second to power the electrical choke. Look out for green erosion and loose wires
- The cylinder gasket head. Get under the hood. If the head is weakened and water gets in then you will suffer a hydraulic lock. This can easily be avoided. Depending on the make of your outboard you may need to have the sacrificial anode replaced when the gasket is decarbonized by your local shop.
- Replacing sacrificial anodes. Whether zinc or aluminum alloy, these should be replaced when one third of the original anode has disappeared. Never use two different metals for your
anodes. You must get your outdrive casing and your anodes electrically bonded. This balances the charge between the two metals. Otherwise the corrosion that follow will cause damage very quickly to your lower unit. That’s the reason not to use different metals as anodes too.
These bolts are subject to galvanic erosion
Inboard checks
- Replace your anodes as described above. You can use an ohmmeter to determine the charge differential between your metals, for the electrical bonding.
- Use corrosion inhibitors. They are designed to protect and extend the life of boat engine components. Counteracting the aggressive nature of salt water erosion.
- Tightening up loose wires, bolts and screws. Due to the vibration of the engine
Long or short term cure?
The trick with inhibitors is this; if your looking to seal and protect your metal work for an extended period of time then use a thicker one. Conversely, if you check back more often then use a lighter one. The leading products on the market are galvanic isolators. When your hooked up to shore power, they maintain AC current. Which prevents zinc damage.
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