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Preventing Corrosion

Out on the water, your engine is constantly exposed to elements that can cause corrosion. Combating its damaging effects is critical.

Preventing Corrosion

Galvanic Corrosion

Galvanic corrosion occurs when two dissimilar metals, such as an aluminum gearcase and a stainless steel propeller, are immersed together in a conductive liquid. The resulting chemical reaction eats away at metal components, creating costly damage. Mercury Marine® designs a first line of defense into its products by using corrosion-resistant alloys, a multi-layer paint system and additional corrosion-resistant materials throughout the powertrain. By following a few easy additional steps, you can do even more to keep the effects of corrosion at bay, especially if you boat in saltwater.

Pro Tip

Starting your engine when it is not in the water or connected to a water source, even briefly, can damage the seawater pump impeller. The soft impeller relies on water for lubrication. If the impeller is damaged the engine can overheat.

Flush, Rinse and Repeat

An outboard or sterndrive engine operated in saltwater should be flushed and rinsed after each use. This is easy to do and is an important way to inhibit corrosion. 

Flush it First

Flushing clears the cooling system of corrosive saltwater, and rinsing removes salt residue before corrosion can begin. Flushing after use in brackish, polluted or very silty freshwater is also recommended. MerCruiser® engines equipped with the optional SeaCore® freshwater cooling should be flushed to clear saltwater from the heat exchanger system and the exhaust.

There are several methods used to flush the cooling system. Check your owner’s manual for the flushing options for your engine.

Hose Fitting

Many late-model Mercury® outboards have a dedicated flush fitting that can be used to connect a garden hose directly to the motor when the boat is on a trailer or in the water. Follow your operation and maintenance manual instructions to use the fitting.

Flush Muff

A flush muff device can be used to flush an outboard that is not equipped with a dedicated hose fitting. A muff is also required for flushing a MerCruiser engine unless it has been fitted with an accessory Engine Flush Kit (see a Mercury Authorized Dealer for more information). The muff fits around the gearcase and covers the cooling water inlets with rubber cups. A garden hose attached to the muff supplies freshwater. Follow your operation and maintenance manual instructions to use the flush muff.

Rinse Away Salt


Rinse the outboard cowl, mounting bracket and trim ram area. Always lower the outboard to a vertical position before rinsing the cowl. Rinsing the cowl while the outboard is tilted can allow water to enter the air intake at the rear of the cowl. Next remove the cowl and the plastic flywheel cover and carefully rinse salt residue from the powerhead. Do not use high-pressure water or a spray nozzle, which can force water into electrical connections. Just let the water flow from the hose and take care to keep water away from the engine air intake and the alternator. Also rinse off the cowl latches, then allow the engine to dry.


Rinse the outdrive unit with freshwater, including the trim ram area. Occasionally rinse salt residue from the engine using a hose and low water pressure. Remove the boat drain plug before rinsing to allow water to escape the bilge.

Mercury Corrosion Guard Engine Protect

Once the outboard powerhead or sterndrive engine is dry, apply a light coat of Mercury Corrosion Guard Engine Protect to sterndrive and inboard engine blocks and to under-cowl areas of outboard engines. Corrosion Guard forms a water-resistant barrier on painted and non‑painted metal surfaces. The clear film coating quickly dries to the touch and won’t become sticky over long-term use. Take care to keep Corrosion Guard away from the air intake, alternator, and belts and pulleys. Spray Mercury Corrosion Guard Heavy Duty on the outdrive, gearcase, tilt and trim motor, exposed electrical connections and other surfaces that operate in the water. Avoid covering anodes. Use Mercury All-in-One Spotless Shine to remove unsightly water spots from fiberglass, metal, plastic and rubber surfaces, which helps keep your boat and engine shining like new.

Sacrificial Anodes

Anodes, often called “zincs” and usually located on the gearcase, transom bracket or trim rams of a Mercury outboard or MerCruiser sterndrive, are designed to protect aluminum components from galvanic corrosion, especially when using a stainless steel propeller. A sacrificial anode is made of a more-vulnerable material and is intended to corrode more easily, or “sacrifice” itself, to protect the adjacent components. A common misconception among some boat owners is that non-corroded anodes are a good sign that corrosion is under control. In fact, the opposite is true. If your anodes show no signs of wear or corrosion, that may be an indication that damaging corrosion is occurring on other more critical parts.

Check your operation and maintenance manual for the location of anodes on your Mercury outboard or MerCruiser sterndrive.

It’s good practice to frequently inspect sacrificial anodes, especially if you operate your boat in saltwater.

Anodes look like a small gray block and are usually secured with a single bolt.

Anodes may become coated with a thin film of scum in some water conditions. This film can be removed with a stiff brush.

Mercury recommends replacement when about half of the anode has been lost to corrosion.

To get the best protection, avoid inexpensive aftermarket anodes. You can count on a genuine Mercury replacement anode to be made with the highest-quality, most-effective alloy.

Anodes must be in direct contact with the water to work properly, so never paint or apply other coatings over them.

Mercathode System

The Mercury MerCathode® system for outboards, inboards and sterndrives connects to the boat’s 12-volt battery and provides protection against galvanic corrosion by impressing a reverse blocking current that stops the destructive flow of galvanic currents. By sending an opposing current through the conductive liquid, the system blocks the ions from leaving the more chemically active metal. The system also adjusts itself to compensate for changes in corrosion potential caused by variations in water temperature and conductivity, including the salt content of the water. It even compensates for changes in the condition of the paint on the drive unit.

Galvanic Corrosion

Galvanic corrosion is an electrochemical reaction that occurs when two or more dissimilar metals, for example an aluminum lower unit and a stainless steel propeller, are connected together and immersed in a conductive solution, known as an electrolyte, such as saltwater or brackish water. Electrons flow from the more chemically active metal (anode) directly to the less chemically active metal (cathode) through the external connection. Positively charged ions move from the anode and negatively charged ions move from the cathode through the electrolyte. The result of this process is the dissolving of the anode. Conductivity increases with water temperature, which is one reason boats in Florida experience more corrosion than boats in colder climates such as Maine or Alaska.

Signs of Galvanic Corrosion and How to Prevent It

One of the first signs of galvanic corrosion is paint blistering, usually beginning on sharp edges below the waterline. Galvanic corrosion appears as a white powdery substance forming on the exposed metal areas. As corrosion advances, exposed metal areas may become deeply pitted, as metal is eaten away. Galvanic corrosion of aluminum sterndrive units, or any other underwater aluminum on your boat, is accelerated by proximity to stainless steel components like propellers, powered trim tabs and aftermarket steering systems.If this happens to your motor, check the anodes immediately. They may be dissipated, coated with scum or marine growth, or otherwise not functioning and should be replaced with a new set of quality Mercury anodes. Use Mercury paint and finish products to renew the finish and protect the underlying aluminum components.

How to Touch Up Paint

An annual touch-up with Mercury paint products provides important protection for your engine’s lower unit. As with any painting project, a quality result begins with careful preparation of the surface before any paint is applied.

Remove the propeller to protect it from paint overspray and cover the prop shaft with a rag or newspaper.

Clean the area to be touched up to remove all oils, wax, grease, salts and other marine build‑up. Use an appropriate marine cleaner, a solvent-type wax remover, grease remover or naphtha.

If the exposed aluminum is pitted or if paint blisters are present, start by sanding the affected area with 180 or 220 grit sandpaper.

Prime the area with Mercury Light Gray Primer following instructions on the can. After the primer is dry, scuff lightly with a medium maroon Scotch‑Brite™ pad.

Apply Mercury Phantom Black paint, following instructions on the can. Apply several thin, even coats rather than a single heavy coat. Let the paint set up between coats.

Before the final coat of paint dries, apply a coat of Mercury Leveler to fill in any remaining scratches or sanding marks for a perfect finish.

  • Never paint over sacrificial anodes (“zincs”), as the anode material must be in contact with water to provide corrosion protection. Either mask off or remove the anode when painting near it.
  • Scratches in the paint that don’t penetrate through the original black primer can be quickly touched up using a Mercury Phantom Black Paint Pen. Its brush-like tip makes it easy to fill in scratches, and the color is a perfect match to the original factory paint.

Next Topics

Maintenance: What's In It For You?

Boating is an activity that fuels pride of ownership, and for many owners a strong do-it-yourself attitude. Knowing you’ve covered the basics of maintenance gives you confidence that your boat is in tip-top shape every time you leave the dock.

Fuel Basics

Using the correct fuel in your Mercury Marine® engine and properly maintaining your fuel system can help ensure many seasons of reliable service. For more guidance on fuel requirements for your engine, see the fuel section of your Mercury® operation and maintenance manual.

Battery Basics

Properly maintaining your batteries helps ensure your engine starts up easily and your other electronic accessories are properly powered when you’re ready to head out for fun and adventure on the water.

The Mercury Dealer Advantage

No one knows how to keep a Mercury engine performing at its peak better than the experts at your local Mercury Authorized Dealer. You can trust them to have the parts, expertise and factory training to handle all of your maintenance or repair needs.

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