Corrosion Frequently Asked Questions

For stern drive owners - What do I need to know about corrosion?

The good news is that corrosion can be prevented! Marine corrosion is a fact of boating life, but you can protect your investment with diligence & proper maintenance. As the boat owner, YOU are the most important key to effective corrosion protection. Here are some things you should know:

1. All MerCruiser sterndrives come standard with aluminum sacrificial anodes & 3-year warranty against certain types of corrosion (see your owner's manual for more details)
2. Bravo sterndrives are also equipped with a MerCathode, optional on Alphas.
3. Magnesium anodes are suggested for fresh water use ONLY & can be purchased from your authorized dealer.
4. Anodes & sterndrives should be inspected on a regular basis for signs of corrosion. Do not paint anodes.
5. Not all anodes are created equal. Don't sacrifice your sterndrives protection with the use of poor quality after-market anodes.
6. The MerCathode should be tested annually by an authorized dealer for proper operation. Do not paint or power-wash the MerCathode.
7. Your particular boating environment may require that you install additional anodes or other protective devices.
8. Whenever you add stainless steel accessories (for example like props or trim tabs) it's a good idea to have the corrosion resistance checked to make sure it is still adequate
9. If you use shore power, you should have an approved galvanic isolator installed to protect against stray current corrosion (electrolysis) from your dock, from your boat's accessories or from other nearby boats.
10. Corrosion can be caused by many factors, such as the wrong type or improper application of anti-fouling paint, marine growth, stray electrical current (electrolysis), an improperly wired or damaged MerCathode, etc. These types of conditions are not covered by the warranty.
11. If you notice signs of corrosion, contact your authorized MerCruiser dealer to arrange for an in-the-water hull potential test. This test will help the dealer evaluate your protection level & sources of corrosion.
12. See your owner's manual for more information.
13. A very informative booklet on corrosion "Marine Corrosion Protection Guide" (#90-88181301) is available through your dealer.

What is the length of the corrosion warranty?

The corrosion warranty is for three years from the original date of purchase of the complete power package for 1988 and newer MerCruiser product that have a one-year limited product warranty. See the complete corrosion warranty under the section titled Warranties or in your owner's manual. Protection against certain types of corrosion damage must be provided by using such systems as the Mercury Precision MerCathode system and/or galvanic isolator. These certain types of corrosion damage include damage due to stray electrical currents (on-shore power connections, nearby boats, submerged metal), etc. It is the owner's responsibility to ensure that all necessary anti-corrosion devices are in good working order and are providing adequate protection.

What is a MerCathode and what does it do?

The Mercury Precision MerCathode system provides automatic protection against galvanic corrosion. It is a solid-state device that operates off a boat's 12-volt battery and provides protection by impressing a reverse blocking current that stops the destructive flow of galvanic currents. If your power package does not come already equipped with a MerCathode system, Mercury strongly recommends that you install one, particularly if your boat is equipped with a stainless steel propeller or other submerged stainless steel hardware.

What is a galvanic isolator and what does it do?

The Mercury Precision Galvanic Isolator is a solid-state device that is series connected in line into the boat's green safety grounding lead ahead of all grounding connections on the boat. This device functions as a filter, blocking the flow of destructive low voltage galvanic (DC) currents, but still maintaining the integrity of the safety grounding circuit. A galvanic isolator is only necessary on boats utilizing a shore power connection.

My sterndrive paint is peeling and/or blistering. What is going on?

One of the first signs of galvanic corrosion is paint blistering (starting on sharp edges) below the waterline, with a white powdery substance forming on the exposed metal areas. As the corrosion continues, the exposed metal areas will become deeply pitted, with the metal actually being eaten away. Galvanic corrosion of aluminum sterndrive units, or any other underwater aluminum on your boat, is accelerated by attaching stainless steel components like propellers, trim planes (if connected to engine ground) and after-market steering systems. Whenever you add stainless steel accessories, it's a good idea to have the corrosion resistance checked to make sure it is still adequate.

Why is my stainless steel prop rusty?

Stainless steel propellers can discolor and form areas of light rust. Brushed finish stainless props are more susceptible than brightly finished stainless steel props. Rust stains can occur from polluted water, galvanic corrosion from the boat or marina, or from oxygen depletion. The propeller can be easily cleaned with rubbing compound or Scotchbrite. Do not use steel wool. After cleaning, use chrome polish to protect the finish. Stainless steel props can also turn white from calcium or lime in the water. In this case, clean the prop with household mineral bath and tile cleaner. Again, do not use steel wool. Follow-up with chrome polish to protect the finish.

Can I use anti-fouling paint?

Anti-fouling paint may be applied to boat hull and boat transom, but you must observe the following precautions:

• IMPORTANT: DO NOT paint anodes or MerCathode system reference electrode and anode, as this will render them ineffective as galvanic corrosion inhibitors.
• IMPORTANT: If anti-fouling protection is required for boat hull or boat transom, copper or tin based paints can be used if not prohibited by law. When using copper or tin based anti-fouling paints, you should avoid any electrical interconnection between the MerCruiser product, anodic blocks, or MerCathode system and the paint. Allow a minimum of 1-1/2 in. (40 mm) UNPAINTED area on the transom of the boat around these items.
• Sterndrive unit and transom assembly should be painted with a good quality marine paint or an anti-fouling paint that DOES NOT contain copper, tin, or any other material that could conduct electrical current. Do not paint drain holes, anodes, MerCathode system, and items specified by the boat manufacturer.