GENERAL INFORMATION
Electrolyte The water in which a boat operates acts as the electrolyte for the galvanic corrosion process. The better a conductor the electrolyte is, the more severe and rapid the corrosion. Cold, clean freshwater is the poorest electrolyte. As water temperature increases, its conductivity increases. Pollutants will increase conductivity; brackish or saltwater is also an efficient electrolyte. This is one of the reasons that most manufacturers recommend a freshwater flush for marine equipment after operation in saltwater, polluted or brackish water.
PROTECTION FROM GALVANIC CORROSION Because of the environment in which marine equipment must operate, it is practically impossible to totally prevent galvanic corrosion. There are several ways by which the process can be slowed. After taking these precautions, the next step is to "fool" the process into occurring only where you want it to occur. This is the role of sacrificial anodes and impressed current systems.
Slowing Corrosion Some simple precautions can help reduce the amount of corrosion taking place outside the hull. These are not a substitute for the corrosion protection methods discussed under Sacri$cial Anodes and Impressed Current Systems in this chapter, but they can help these protection methods do their job. Use fasteners of a metal more noble than the part they are fastening. If corrosion occurs, the larger equipment will suffer but the fastener will be protected. Because fasteners are usually very small in comparison to the equipment being fastened, the equipment can survive the loss of
material. If the fastener were to corrode instead of the equipment, major problems could arise. Keep all painted surfaces in good condition. If paint is scraped off and bare metal exposed, corrosion will rapidly increase. Use a vinyl- or plastic-based paint, which acts as an electrical insulator. Be careful when using metal-based antifouling paints. These should not be applied to metal parts of the boat, outboard motor or stem drive unit or they will actually react with the equipment, causing corrosion between the equipment and the layer of paint. Organic-based paints are available for use on metal surfaces. Where a corrosion protection device is used, remember that it must be immersed in the electrolyte along with the rest of the boat to have any effect. If you raise the power unit out of the water when the boat is docked, any anodes on the power unit will be removed from the corrosion cycle and will not protect the rest of the equipment that is still immersed. Also, such corrosion protection devices must not be painted because this would insulate them from the corrosion process. Any change in the boat's equipment, such as the installation of a new stainless steel propeller, will change the electrical potential and could cause increased corrosion. Keep in mind that when you add new equipment or change materials, you should review your corrosion protection system to be sure it is up to the job.
Sacrificial Anodes Anodes are usually made of zinc, a far from noble metal. Sacrificial anodes are specially designed to do nothing but corrode. Properly fastening such pieces to the boat will cause them to act as the anode in any galvanic reaction that occurs; any other metal present will act as the cathode and will not be damaged.
