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Engineer’s Angle: Corrosive forces unavoidable, but manageable

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Engineer’s Angle: by Rich Merhige

Corrosion happens. On yachts, there’s a perfect storm of different metals, forces and particles that water contains, depending on the environment. If you’re dealing with an aluminum or steel hull, corrosion can cause it to deteriorate. It can also destroy tanks, pipes, valves and many other critical components. Fortunately, with proper preventative maintenance, corrosion can be manageable.

Corrosion is the process of deterioration or breakdown of a material’s critical properties; this most frequently affects metal. It occurs when metal or other materials erode because of environmental factors such as oxygen concentration, pH, water temperature and the presence of dissolved salts.

Yachts are among the structures most exposed to environmental corrosion because of the sea. The sea is a corrosive environment because of the salt present, which makes it a good conductor of electricity. This creates a lot of free ions that accelerate corrosion. Some of the most common types of corrosion found in the marine industry are galvanic and stray current corrosion.

Galvanic corrosion is an electrochemical reaction between different metals when they are connected. For example, a boat might experience galvanic corrosion if the hull is aluminum with a stainless-steel propeller, aluminum being highly chemically active and stainless steel being less chemically active. Add water to the mix and the aluminum will start to dissolve.

Stray current corrosion is different from galvanic corrosion because galvanic corrosion is caused by connections between different metals of a vessel’s components, and utilizes the electrical currents generated by the metals. Stray current corrosion occurs when electricity from an external source (whether inside or outside the boat) flows into a yacht’s hull and out through the water for a ground. The electricity will cause rapid corrosion at the point where it leaves the boat. This also can occur on metal components of a fiberglass boat.

Whether galvanic or stray current, corrosion negatively affects whatever material it comes in contact with. Corroded metal parts that are under the waterline, such as stern tubes, exposed propeller shafts, propellers and other exposed metals, will ultimately fail, causing loss of operation whether docked or at sea. If ignored, corrosion can eventually occur inside your engine room, causing machinery and equipment to fail.

There are multiple methods to slow down corrosion using cathodic protection techniques. The most prevalent marine method is the use of sacrificial anodes, which are materials that get corroded first, protecting the base metal underneath. An anode is a metal casting that comes in different shapes, based on their applicability.

The selection of the form of an anode is based on various factors – the shape of the surface being protected, availability of space, accessibility and ease of installation. For example, flat anodes are used for large, flat surfaces such as the hull. Bracelet anodes are used to protect a vessel’s pipelines and propeller shaft. There are many methods of securing an anode to the surface it is protecting, including welding, bolting and using studs or brackets. It is important when installing anodes to make sure the attachment points are free of paint, dirt or marine growth so the anode makes direct contact with the metal it is protecting.

One of the most effective ways to slow down corrosion is by applying coatings to parts of the vessel that are susceptible, such as the vessel’s hull. When a vessel’s surface has two coats of an electrically insulating coating (epoxy, polyurethane or vinyl-based), it provides added protection against corrosion and reduces the demand on anodes.

Also, try to avoid stagnant water within the vessel – any place water lies stagnant is at risk for corrosion.

Once you replace corroded parts with new ones, use anodes and coatings on the new metal parts to ensure it doesn’t happen again. Check the anodes and coatings periodically to ensure they are in good shape and capable of doing their job. If not, replace them or reapply protection as soon as possible.

Rich Merhige is owner of Advanced Mechanical Enterprises and Advanced Maintenance Engineering in Fort Lauderdale (AMEsolutions.com). Comments are welcome below.

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