CORROSION PROTECTION - boote-forum.de

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CORROSION PROTECTION

90-883777 1015-0 - CORROSION PROTECTION

Table of ContentsPage

What Is Corrosion 5-1. . . . . . . . . . . . . . . . . . . . . . . . . High School Chemistry Revisited 5-1. . . . . . . . . Electrochemical Reactions 5-1. . . . . . . . . . . . . . Types of Marine Corrosion 5-1. . . . . . . . . . . . . . What to look for 5-3. . . . . . . . . . . . . . . . . . . . . . . . Stray Current Corrosion 5-3. . . . . . . . . . . . . . . . . Crevice Corrosion 5-4. . . . . . . . . . . . . . . . . . . . . .

Corrosion Protection 5-4. . . . . . . . . . . . . . . . . . . . . . . Mercury Marine Metals 5-4. . . . . . . . . . . . . . . . . Mercury Marine Multi-Step

Metal Finishing Process 5-4. . . . . . . . . . . . . Sacrificial Anodes 5-6. . . . . . . . . . . . . . . . . . . . . . Transom Mounted Anode Kit 5-7. . . . . . . . . . . . The Newest in Corrosion protection

from the Folks Who Wrote the Book 5-7. . . Additional Corrosion Protection 5-8. . . . . . . . . . MerCathode System 5-8. . . . . . . . . . . . . . . . . . . MerCruiser MerCathode System 5-9. . . . . . . . . MerCathode Monitor 5-11. . . . . . . . . . . . . . . . . . MerCathode Monitor Operation

and Testing 5-11. . . . . . . . . . . . . . . . . . . . . . . The Effect of Water Velocity on

Corrosion Rates 5-12. . . . . . . . . . . . . . . . . . . Methods of Increasing the Protection

Provided by a MerCathode System 5-12. . Stray Current Corrosion 5-13. . . . . . . . . . . . . . . . Quicksilver Galvanic Isolator 5-14. . . . . . . . . . . Anti-Fouling Paint on Drives 5-15. . . . . . . . . . . .

PageCorrosion Protection Testing and

Troubleshooting 5-16. . . . . . . . . . . . . . . . . . . . . . Testing 5-16. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Troubleshooting Corrosion 5-17. . . . . . . . . . . . . Inactive Sacrificial Anodes 5-17. . . . . . . . . . . . . Corrosion on Underwater Parts

(without MerCathode or impressed current protection) 5-17. . . . . . . . . . . . . . . . .

Continuity Devices 5-20. . . . . . . . . . . . . . . . . . . . Outboard Continuity Devices 5-20. . . . . . . . . . . MerCruiser Stern Drive Continuity

Devices 5-20. . . . . . . . . . . . . . . . . . . . . . . . . . Drive Unit Continuity Test 5-22. . . . . . . . . . . . . . Corrosion of Stainless Steel Propellers 5-24. .

Preventative Maintenance 5-25. . . . . . . . . . . . . . . . . Maintain 5-25. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spray 5-25. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Inspect 5-25. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flush 5-25. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Check 5-26. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lubricate 5-26. . . . . . . . . . . . . . . . . . . . . . . . . . . . Propellers 5-26. . . . . . . . . . . . . . . . . . . . . . . . . . . . More Lubrication 5-26. . . . . . . . . . . . . . . . . . . . . .

90-883777 101 CORROSION PROTECTION - 5-1

What Is CorrosionThere is nothing mysterious about corrosion. We’veall experienced it. It is simply any metal naturallychanging. The processes it goes through in changingare slightly complicated, but not especially complex.

High School Chemistry RevisitedTo best describe corrosion, let’s start with the mostcommon type, rust. We all know rust, but to under-stand rust, we have to go back to the very beginning.

Iron ore has a chemical composition of two ironatoms bonded with three oxygen atoms (Fe2O3). Asit is mined out of the ground, it’s a brownish-red pow-der that’s useless to us. But by refining, purifying andsmelting, we create iron, which is useful to us. Wecan use it as plain iron, or we can process it furtherand combine it with other elements to get differentkinds of steel.

Let’s say the iron is made into hinges for your back-yard fence. Everyone knows that if you leave iron (orsteel, which is mostly iron) out in the rain, it rusts. Ifit rusts long and badly enough, the metal disappearsand you’re left with a pile of brownish-red powder -rust, or iron oxide, both of which have a chemicalcomposition of Fe2O3. Yes, rust - or iron oxide - hasthe same composition as iron ore.

Here’s why. Iron atoms want to return to their naturalstate as iron ore, iron oxide or rust, which are all thesame thing. That’s the state in which iron is mostcomfortable and most stable. Left alone, it won’t turninto anything else. And most metals used inmanufactured products want to do the same - returnto their natural states.

Electrochemical ReactionsIron left out in the rain results in a specific kind of cor-rosion. It’s called an “electrochemical reaction,”meaning there is an electrical change along with achemical change. Here’s how that mouthful works:

For two iron atoms to really interlock with three oxy-gen atoms (and make Fe2O3), they have to sharesome electrons (the little particles orbiting the atom).That releases a few electrons. And since electricityis just a flow of electrons, those free electrons be-come a little bit of electricity when the chemicalchange takes place.

Remember, the iron wants to corrode into iron oxidebecause that’s its natural, most stable state. And allit needs for this to take place is oxygen. Water is asupply of oxygen, so iron rusts fastest when it gets

wet. You knew that already, but now you know why.And that same exact scenario applies to aluminumand aluminum oxide.

So those are the deep, dark secrets of corrosion asthey apply to metals. Those are also the basics of anelectrochemical reaction, which is also known as gal-vanic corrosion. (All galvanic corrosion is an electro-chemical reaction. Not all electrochemical reactions,however, are galvanic corrosion.)

Types of Marine CorrosionMetal parts under water are primarily subjected totwo basic types of corrosion: 1) galvanic corrosionand 2) stray current corrosion.

Galvanic corrosion is an electrochemical reaction be-tween two or more different (or “dissimilar”) metals.The metals must be different because one must bemore chemically active (or less stable) than the oth-er(s) for a reaction to take place. When we talk aboutgalvanic corrosion, we’re talking about electrical ex-change.

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All metals have electrical potential because all atomshave electrons, which are electricity.

Galvanic corrosion of the more chemically activemetal can occur whenever two or more dissimilarmetals that are “grounded” (connected either by ac-tually touching each other, or through a wire or metalpart) are immersed in a conductive solution (any liq-uid that can transfer electricity). Anything but purewater is conductive. Salt water, fresh water with highmineral content, and polluted fresh water are veryconductive, and conductivity goes up with water tem-perature. (That’s one reason why boats in Florida ex-perience more corrosion than boats in Maine.)

The simplest example of galvanic corrosion, and themost applicable, is an aluminum lower unit with astainless steel propeller. The aluminum is the morechemically active metal (the “anode”), and the stain-less steel is the less chemically active metal (the“cathode”). Several things happen at the same time(Figure 1-1):


1. At the anode

a. Electrons flow from the anode, the metal thatis more chemically active (the aluminum driveunit), via the external conducting path to thecathode, the metal that is less chemically ac-tive (the stainless steel prop), as in the reac-tion Al→Al+++ + 3e.

b. When this happens, the more chemically ac-tive metal atoms become ions (an atom withone or more electrons either missing or add-ed) and break away into the water, wherethey can bond to oxygen ions, with which theycan share electrons and produce aluminumoxide. (This is the identical process iron ionsgo through when combining with oxygen ionsin water to form iron oxide).

c. The newly formed aluminum oxide moleculeseither drift away in the water or settle on thesurface of the aluminum. Your lower unit is lit-erally dissolving through galvanic corrosion.

2. At the cathode

a. Electrons are accepted from the anode; how-ever, they cannot simply accumulate, butreact with ions in the electrolyte.

b. The most common reaction is (as in Fig. 1-1):1 1/2 O2 + 3 H2O + 6e → 6OH-.

c. The hydroxide ion (OH-) is alkaline, andmakes the electrolyte alkaline in the area ofthe cathode. This detail is particularly impor-tant for wooden boats, as an alkaline solutionwill attack cellulose.

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It is important to understand that for each positivemetallic ion released at the anode, electrons in thecathode react to form a negative ion in the electrolyte.Electrically the anodic and cathodic reactions mustbe equivalent. Increases or decreases in the rate ofthe cathodic reaction will have a corresponding in-crease or decrease on the anodic reaction. This is abasic fact in understanding and controlling corrosion.This fact can also be demonstrated by the effect ofsize ratios between anodes and cathodes. If there isa very large anode connected to a small cathode theanode will corrode very slowly. However, if a verylarge cathode is connected to a small anode theanode will corrode very rapidly.

Marine drive components have many aluminumparts. If you do not control galvanic corrosion, overtime the aluminum will corrode away.

Galvanic corrosion can also occur without any stain-less steel components on your boat. For example,you have an aluminum drive unit and an aluminumpropeller, but you dock at a pier with steel pilings orat a steel seawall, then plug into shore power. Theground wire, which is grounded, connects your alu-minum components with the submerged steel be-cause the steel is also grounded (Figure 1-2). Con-sidering the mass of a seawall or even a single piling,your drive and propeller can sustain serious damage.(This damage can be prevented with a galvanic isola-tor, mentioned later.)

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What to look forThe first sign of galvanic corrosion is paint blistering(starting on sharp edges) below the waterline, with awhite powdery substance forming on the exposedmetal areas. As the corrosion continues, the exposedmetal areas will become deeply pitted, the metal ac-tually being eaten away (Figure 1-3).

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Galvanic corrosion of aluminum drive units - or anyunderwater aluminum on your boat - is acceleratedby attaching stainless steel components like propel-lers, trim planes (if connected to engine ground) andafter-market steering aids. In doing this, you haveintroduced a dissimilar metal to which electrons fromyour drive unit will flow. Another condition that will in-crease the speed or intensity of galvanic corrosion isremoval or reduction in surface area of sacrificialanodes. (Much more about sacrificial anodes later.)But you don’t need stainless steel components forgalvanic corrosion to take place. Galvanic corrosioncontinually affects all underwater aluminum, but at areduced rate when no dissimilar metals are con-nected to your aluminum parts. When in contact withan electrolyte, most metals form small anodes andcathodes on their surfaces due to such things as alloysegregation, impurities, or cold working.

We have used stainless steel (cathode) and alumi-num (anode) in this discussion as an example; how-ever, other metals coupled with aluminum also pro-duce galvanic corrosion cells. For example, zincconnected to aluminum will form a corrosion cell, butin this case, the aluminum becomes the cathode andthe zinc (anode) corrodes. One of the worst coupleswith an aluminum drive would be connecting it withcopper or a copper alloy (bronze).

Another cause of galvanic corrosion is the shorepower hookup. When you plug in, you tie your alumi-num drive unit to other boats using shore powerthrough the green grounding lead. Your aluminumdrive unit is now part of a large galvanic cell (a bat-

tery) interconnected with onshore metal that is in thewater - as well as other boats - and corrosion may begreatly accelerated.

Stray Current CorrosionWe’ve discussed what galvanic corrosion can do, us-ing just the electrical potential in metals. Imaginewhat happens if you add more electricity. That’s ex-actly the basis for stray current corrosion.

Stray current corrosion occurs when metal with anelectrical current flowing into it is immersed in waterthat is grounded (such as in any lake, river or ocean).The current can leave the metal and flow through thewater to ground. This will cause rapid corrosion of themetal at the point where the current leaves. Stray di-rect current (or battery current) is particularly de-structive. Stray current corrosion can cause rapid de-terioration of the metal. If the metal in questionhappens to be an aluminum part like your drive unit,it can be destroyed in a matter of days.

Stray current corrosion is different from galvanic cor-rosion in that galvanic corrosion is caused by con-nections between dissimilar metals of your boat’sdrive components, and utilizes the electrical potentialof those dissimilar metals. Electrons flow from onedissimilar metal (the anode) to another dissimilarmetal (the cathode). In stray current corrosion, elec-tricity from an outside source flows into your boat’smetal components and out through the water for aground.

For example, your boat may be sitting between aboat leaking DC current and the best ground for thatcurrent. Rather than the DC current moving exclu-sively through the water to ground, your boat couldprovide a path of lower resistance. The DC currentcould enter a thru-hull fitting, travel through the bond-ing system, and leave via your drive to ground. Re-member that corrosion occurs at the location whereDC current leaves metal and enters water.

Stray current can come from an outside source eitherinternal or external to your boat. Internal sources in-volve a short in your boat’s wiring system, such as apoorly insulated wire in the bilge, any electrical ac-cessory that may be improperly wired, or a wire withweak or broken insulation that is intermittently wet.

External sources are almost always related to shorepower connections. A boat with internal stray currentproblems can cause accelerated corrosion to otherboats plugged into the same shore power line if theyprovide a better ground. The stray current would betransmitted to other boats through the commonground wire, but can (and should) be blocked byinstalling a galvanic “isolator” (discussed later).


A much more subtle - but potentially more damaging- cause of stray current corrosion can occur withoutany electrical problems. Suppose you cruise back toyour marina after a weekend on the water, and plugin to shore power to recharge batteries using your au-tomatic trickle charger. Then you go to work for theweek. On Monday, a large steel-hulled boat (withscratched and scraped paint) ties up next to yourboat. He also plugs into shore power and goes visit-ing onshore for a few days. A battery has just beenformed - the large steel hull and your small aluminumdrive connected by the shore power ground wire. De-pending on the proximity, relative sizes and how longyour neighbor is ashore, when you try to go out thenext weekend you may find your drive highly deterio-rated. This unfortunate scenario can also be pre-vented by the installation of a galvanic isolator.

Crevice CorrosionThere is a form of corrosion that affects many metals,particularly stainless steel, called crevice corrosion.A crevice may be formed under a deposit (such assand or silt), under a plastic washer, under a fibrousgasket, under tightly wrapped fishing line or any-where moisture can get in and not get back out, andform a stagnant zone. Stainless steel is an imp-based alloy containing chrome and nickel. The quali-ty that causes it to be stainless (non-rusting) is itsformation of a thin, tightly adhering surface layer ofchrome oxide. If this surface is deprived of oxygen,the oxide layer breaks down and the stainless steelwill rust just like plain steel. In other words, stainlesssteel is only stainless when it has access to oxygen.In a crevice where there is moisture depleted of oxy-gen, stainless steel rusts. The simplest prevention forthis condition is to seal out the moisture or clean offany deposits.

Corrosion Protection

Mercury Marine MetalsSome metals are more resistant to corrosion thanothers. Gold is the most resistant of all, but solid goldisn’t very appropriate for use in drive units due to itsextreme softness and, of course, cost. The fact is, tobuild an outstanding drive unit with maximum perfor-mance and durability, a variety of metals and alloysmust be used. Therefore, a variety of special mea-sures must be taken to protect them against corro-sion.

The most common metal used in Mercury Marinedrive units is a strong, lightweight aluminum alloy.High-stress parts are made of hardened steel. Themoving parts exposed to water - drive shaft, shiftshaft, and prop shaft - are made from corrosion resis-tant stainless steel.

XK-360 aluminum alloy is one of the most corrosion-resistant aluminum alloys on the market today, andit’s a Mercury Marine exclusive. No other manufac-turer offers this special corrosion resistant alloy. De-veloped specifically for Mercury Marine products byMercury Marine engineers, this unique, lightweightalloy is used to cast gearcases, drive shaft housings,outboard cylinder blocks, and other components ex-posed to corrosion.

Mercury Marine Multi-Step Metal Finishing ProcessHow Mercury Marine finishes metal parts dependson what metal the part is made of, and where the fin-ished part is going to be used. Aluminum alloys arethe primary material of construction, but there aremany steel parts. In the early steps of the finishingprocess, these parts are treated differently for en-hanced protection.

All aluminum alloy parts are completely cleaned in achemical bath (Figure 2-1). Then they are chromate-converted This involves immersing them in a solutioncontaining chromate ions.

The chromate ions react with the aluminum alloy toform a surface layer of aluminum and chromate. Thisaluminum chromate surface provides an excellentbase for later coatings to adhere to, as well as a layerof corrosion protection. If the paint and primer arescratched completely through, the chromate will offera last line of defense against corrosion.


Steel parts, including some stainless steel parts, arecleaned, then coated with electro-deposited paint(EDP). In this process, the object being painted re-ceives an electrical charge, and the paint particles re-ceive an opposing charge. The paint particles areelectrostatically drawn to the object being painted,resulting in more complete coverage (Figure 2-2).The EDP serves as a primer, or base, for an acrylicfinish coating.

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All parts, whether chromate converted or coated withelectro-deposited paint, are then painted with a chro-mate-enriched epoxy primer (Figure 2-3), followedby a high-solids acrylic enamel. Both the epoxy prim-er and acrylic enamel are thermally cured by bakingat a high temperature. Through the entire process,the paint equipment is computer-controlled to pro-vide uniform coverage. Also, all the parts for each in-dividual motor are painted at the same time in orderto ensure color matching.

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All of the paints used are formulated to provide ahard, chip-resistant elastic finish.

One note: from the viewpoint of corrosion, all thestainless steel parts in the water connected to thedrive should be painted, and the aluminum un-painted. This may sound like a crazy statement, butthink back to the discussion of anodes and cathodes.With a very large anode (aluminum) and a very smallcathode (stainless steel), corrosion is greatly re-duced. If the aluminum is painted and the stainlessunpainted, the aluminum is protected by the paint -as long as the paint surface is perfectly intact. In thereal world of sandbars, rocks and logs, the paint willget scratched, and that scratched bare aluminum isnow a very small anode. As in most cases, aestheticswins over science, so the aluminum is painted andthe stainless steel is polished.


Sacrificial AnodesFor additional protection from galvanic corrosion,Mercury Marine drive units are equipped with inex-pensive and easy-to-replace sacrificial anodes (Fig-ure 2-4). These anodes are galvanically very activeand, therefore, corrode first, protecting the othermore expensive drive components. But becausethey are self-sacrificing, the anodes must be in-spected often and, when 50% consumed, replaced,

In the past, a special zinc alloy was the only anodematerial in use (in fact, anodes are often referred toas “zincs”), but zinc must be processed in virtuallysterile conditions to avoid contamination. Contami-nation will eliminate all of zinc’s sacrificial properties.

Mercury Marine has developed a new aluminumalloy that provides better protection (a higher galvan-ic potential) and lasts longer than zinc. But, as in mostchanges, there is a compromise, and in the case ofaluminum anodes, higher galvanic potential comeswith lower mechanical strength. This means that forsome designs where the anode bears a load orserves some structural purpose, zinc is still the pref-erable material.

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The anodes that are included as standard equipmenton a drive unit will provide adequate protection underlight to moderate service conditions. For many mod-erate to heavy conditions, additional anodes can beused to achieve adequate protection. For severeconditions, such as when using underwater stainlesssteel components, Mercury Marine recommendsthat a Quicksilver MerCathode System or a Tran-som-Mounted Anode Kit be installed. If the boat isequipped with shore power, we strongly recommendthat a Quicksilver Galvanic Isolator be installed.

Sacrificial trim tabs help compensate for propellertorque and also act as sacrificial anodes (Figure 2-5).Sacrificial trim cylinder anodes and sacrificial drive-mounted anodes provide protection when stainlesssteel components are installed (Figures 2-6 and 2-7).

To determine if your boat needs additional corrosionprotection, a hull-potential test should be performed(refer to Corrosion Protection Testing, starting onpage 16). On dual drive installations, or on boats withconsiderable amounts of metal immersed below thewaterline, more than one MerCathode System orAnode Kit may be necessary.

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Transom Mounted Anode KitThe Transom-Mounted Anode Kit (Figure 2-8) is aninexpensive means of providing additional corrosionprotection. A large anode is included in the kit, alongwith the necessary mounting hardware to attach itand ground it to the drive unit. The anode must begrounded (making good electrical contact) to thedrive unit in order for the anode to provide corrosionprotection. Because they are self-sacrificing, theanodes must be inspected periodically and replacedwhen 50% or less remains.

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! CAUTIONDue to the location of the sacrificial trim tab, thedrive unit MUST BE kept in the “In” position whenthe boat is moored. If the drive unit is raised, thetrim tab may be out of the water and, therefore,unable to act as a galvanic corrosion inhibitor.

DO NOT paint anodes. Painting will render theminoperative.

When replacing anodes, be sure to scrape theanode mounting surface down to bare metal andto tighten anodes securely. Anodes MUST makegood electrical contact with the drive in order toprovide protection.

The anodes will not provide corrosion protectionwhen the boat is removed from the water; there-fore, the drive unit should be flushed with freshwater to remove salt water or pollutants prior toplacing the boat in storage. For example, driedsalt deposits can react with moisture in the air tocreate a cell, and corrode metal.

The Newest in Corrosion protectionfrom the Folks Who Wrote the BookNew from Quicksilver is a magnesium sacrificialanode (Figure 2-9), developed specifically to providemore protection in fresh water than zinc or aluminumanodes. In fact, this new magnesium alloy is so effec-tive it even offers protection in moving fresh water.

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Also new is the Quicksilver Prop Nut Anode and theQuick’Defender Anode. The Prop Nut Anode (Figure2-10) is offered in Quicksilver’s exclusive lightweightaluminum. It attaches to the engine’s propshaft andoffers an added measure of protection.

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The Quick’Defender Anode (Figure 2-11) is de-signed to attract galvanic corrosion away from yourengine or drive when your boat is moored. TheQuick’Defender is the only port or starboard hanginganode offered in lightweight aluminum.

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! WARNINGDO NOT ATTEMPT TO USE MAGNESIUMANODES IN SALT WA TER. They will provide over-protection. Overprotection will result in a differ-ent electrochemical reaction that will create hy-drogen on the metal surface of the drive, underthe paint. The paint will blister and peel com-pletely off the surfaces of the overprotecteddrive.

There is a common misconception that you can over-protect your drive by using too many zinc or sacrificialaluminum anodes. This is not true. The corrosion po-tential of any metal is a voltage that can be measuredby a reference electrode. Such measurements in wa-ter commonly are made with a silver/silver chloridereference electrode. The corrosion potential of a sac-rificial anode is a characteristic value for that metal,and it does not matter if you have one piece of themetal or 100 pieces. The corrosion potential staysthe same. Of course, 100 anodes would be expen-sive, heavy and a considerable drag under water.Only by increasing the corrosion potential by using adifferent anode material (such as magnesium in sea-water) can you overprotect your drive.

Additional Corrosion Protection

! CAUTIONMercury Marine recommends a MerCathode Sys-tem or Anti-Corrosion Anode Kit be installedwhenever using a stainless steel propeller, or ifa boat is equipped with stainless steel compo-nents (immersed below the waterline) that areconnected into the engine ground system. If aboat is equipped with stainless steel after-planes, a large anode should be installed on eachto handle the increased galvanic corrosion po-tential.

MerCathode SystemThe Quicksilver MerCathode System provides auto-matic, permanent protection against galvanic corro-sion. A solid-state device that operates off a boat’s12-volt battery, the MerCathode System providesprotection by impressing a reverse blocking currentthat stops the destructive flow of galvanic currents.

Once again, this mouthful has a simple translation. Inthe first chapter we learned that the process of gal-vanic corrosion occurs when two dissimilar metalsare grounded (connected) and immersed in a con-ductive liquid. Electrons flow from the more chemi-cally active metal directly to the less chemically ac-tive metal through the external connection. Positivelycharged ions (as Al+++) move from the anode andnegatively charged ions (as OH-) move from thecathode through the electrolyte. The result of thisprocess is the dissolving of the anode. But by send-ing an opposing current through the conductive liq-uid, the MerCathode System basically blocks theions from leaving the more chemically active metal.

The MerCathode System consists of a controller, ref-erence electrode and anode. The reference elec-trode senses the corrosion potential of the drive in thewater, and regulates the controller to keep the protec-tive current within a prescribed range for optimumblocking and, hence, optimum corrosion protection.The protective current (from the battery) is emittedinto the water via the controller and anode. The sur-face of the anode is platinum-coated so that it will notcorrode due to the current flow, like sacrificialanodes. The MerCathode System automatically ad-justs itself to compensate for changes in corrosionpotential caused by variations in water temperature,velocity and conductivity (such as salt content). Iteven compensates for changes in the condition of thepaint on the drive unit.


The MerCathode System is extremely economical tooperate. Even under the most severe corrosionconditions and in continuous use, an average batterywill last three weeks or more before recharging isnecessary. The system automatically shuts off whenthe boat is removed from the water.

There are two types of MerCathode Systems avail-able. Figure 2-12 shows a transom-mounted two“button” system. This system is mounted with theanode and reference electrode on (and through) thetransom of the boat on opposite sides of the drive.The black controller is mounted inside the boat ashigh as possible (Figure 2-13). Do not mount the con-troller so that the terminals can be exposed to water.

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When using a MerCathode, all underwater metalparts to be protected must be connected to the nega-tive (-) battery terminal (ground). Although the Mer-Cathode System provides protection to all the ex-posed metal surfaces that are grounded (electricallyconnected) to the drive, it does not protect internalsurfaces which pocket moisture and dirt. We recom-mend that the drive unit be flushed with fresh waterbefore storage to remove salt water or other pollut-ants. (Dried salt can absorb moisture from the air andcreate an active cell.)

We also strongly recommend that the MerCathodeSystem be tested at least once a year to ensure thatit is providing adequate corrosion protection. (Referto Corrosion Protection Testing, following.) The testshould be conducted in the water where the boat ismoored.

MerCruiser MerCathode SystemQuicksilver has developed a second type of MerCa-thode System specifically for MerCruiser sterndrives. It features an integrated one-piece anode andreference electrode as shown in Figure 2-14. Thisanode/reference electrode is mounted directly on thedrive unit (gimbal housing) of a MerCruiser sterndrive, eliminating the need to drill holes through thetransom as with the old system. The blue controlleris mounted in the engine compartment, usually ontop of the engine (Figure 2-15). Installation is veryquick and easy. Again, do not mount the controller sothat the terminals can be exposed to water.

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For proper operation of the Quicksilver MerCruiserMerCathode System, the following information mustbe observed.

• Do not paint the electrode assembly. This will ren-der the system inoperative.

• Do not replace plastic caps (Figure 2-16) withzinc anode heads. (Newer models do not haveexternal gimbal housing caps. See Figure 2-17.)If you do, the anode caps will be too close to theMerCathode reference electrode and will cause itto incorrectly gauge the corrosion potential of thedrive in water and, therefore, cause the controllerto think that the drive is protected, and shut off.Sacrificial anodes should not be installed withinten inches of the reference electrode. (Sacrificialanodes may be installed as long as they are fur-ther away from the reference electrode than thegimbal housing caps.)

• The power supply for the MerCathode System isprovided by the engine wiring harness. If yourboat is equipped with a battery switch, the switchmust be left in the “On” position when the boat ismoored in order for the system to provideprotection.

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NOTE: To allow the battery switch to be placed in the“Off” position while boat is moored, remove and dis-card the red purple power supply lead from the “+”terminal of controller and connect a separate wire be-tween the “+” terminal on the controller and the posi-tive (+) battery terminal. This lead must tee fitted witha three-amp fuse, placed within six inches (15 cm) ofthe positive battery terminal.

• Do not substitute a black controller for a blue con-troller or vice-versa. They are not interchange-able.

• We recommend testing your MerCathode Sys-tem at least once each year to ensure it is provid-ing the proper corrosion protection. Refer to Cor-rosion Protection Testing on the following pages.

! CAUTIONNever exchange the blue and the black control-lers.Never paint the MerCathode System referenceelectrode or anode. This will prevent the systemfrom functioning properly.

Never use abrasives or sharp tools to clean theanode. This could break through the platinumplating and corrosion of the anode will result.

MerCathode System should be attached directlyto boat battery with no switches that could possi-bly deactivate the system.

Protect the reference electrode of a power wash-er is used to clean the boat and/or drive.


MerCathode MonitorThe optional MerCathode Monitor (Figure 2-18) al-lows you to check the operation of the MerCathodeSystem with the push of a button.

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MerCathode Monitor Operation andTestingWhen a MerCathode System and a MerCathodeMonitor are both installed on a boat equipped with anew stem drive, the Monitor may initially indicate theprotective current is not being supplied through theMerCathode anode. This condition is normal and, insuch a case, the green light will not illuminate whenthe red button is depressed. This is only a temporarysituation caused by the sacrificial anodes and/or newpaint on the drive unit providing complete protection.

After the boat has been in the water for a while,scratches and abrasions expose aluminum surfacesto the water. Water can also work its way into directcontact with the aluminum in seams and joints.

This is when the MerCathode System begins protect-ing the drive. The green light on the Monitor will beginto glow when the red button is pushed, but during thistransition period the green light may only flicker. Thegreen light will become steady as soon as the MerCa-thode is called upon to provide continuous automaticprotection. If the stern drive is equipped with a stain-less steel propeller, the MerCathode System will usu-ally be activated immediately and there won’t be a“waiting period” before the Monitor shows a steadygreen light.

The following test can be used to check the MerCa-thode Monitor for proper operation.

1. Connect a jumper wire between the “R” and “-”terminals on the MerCathode controller (Figure2-19).

Jumper Wire

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2. If the Monitor light illuminates, the Monitor is ingood condition, but the MerCathode System mayor may not be providing protective current. Per-form the MerCathode test procedure, on pages18-19, to determine if the MerCathode is opera-tional.

3. If the light does not come on, disconnect the Mon-itor lead from the “A” terminal on the MerCathodecontroller and connect the lead to a 12-voltsource (Figure 2-20).

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If the light now illuminates, the Monitor is operatingproperly, but the MerCathode System may or maynot be providing protection current. Again, performthe MerCathode test procedure, on pages 18-19, todetermine if the MerCathode is operational.

If the light does not illuminate, check the Monitor wir-ing for damage or loose connections. If the wiring isgood, the Monitor is faulty and must be replaced.

4. Disconnect the Monitor lead from the 12-voltsource immediately after performing the test andreattach to the “A” terminal to prevent corrosiondamage.

5. Remove the jumper wire from the MerCathodecontroller.

The Effect of Water Velocity onCorrosion RatesIn general, an increase in water velocity through cur-rents or tides increases the corrosion rate of metals.This is because the flowing water puts more water incontact with the metal and, therefore, more oxygenin contact with the metal. For example, the corrosionrate of zinc in still salt water is less than one mil (0.001in.) per year. In the same salt water with a velocity ofsix ft. per second (just four miles per hour), the samezinc has a corrosion rate of over eight mils per year.

It is also more difficult to provide corrosion protectionin flowing water. If sacrificial anodes are used, addingmore anodes and distributing them on the drive isnecessary. If an impressed current system (MerCa-thode) is used, increased output is necessary. In thecase of a MerCathode System, the output is limitedby the controller to prevent draining the battery; insome cases (depending on both water velocity andconductivity), the output may not be enough to pro-vide protection. For example, if it takes 45 milliampsper square inch to provide protection in still water, itcan take 370 milliamps per square inch in water mov-ing at ten ft. per sec (approx. seven mph).

Methods of Increasing the ProtectionProvided by a MerCathode SystemBackground:

1. The MerCathode controller output is limited, bydesign, to approximately 200 mA to avoid a rapiddrain of the boat’s battery.

2. The MerCathode can put out the full 200 mA insalt water.

3. In fresh water, the output is limited by the area ofthe anode and the conductivity of the water, usu-ally to less than 25 mA.

4. If the boat is equipped with a gimbal ring MerCa-thode (blue controller), adding a transom mountMerCathode (black controller) will not add muchto the protection level. When the blue controllerturns on, it will shut off the black controller.

Solutions:

Fresh Water: Two or more additional MerCathodeanodes can be installed and connected to the anodeterminal of the controller (Figure 2-21).

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Put the anode lead (orange wire) on the anode termi-nal of the blue controller. Attach the second anodelead wire (orange) to the anode terminal on the bluecontroller. Attach additional anodes. This results inextra anodes operating off the terminal. If the boathappens into salt water there will not be a problem,as the controller is limited to 200 mA.

The black controller and transom reference can alsobe used.

! CAUTIONDo not place the anodes near the reference.

In Salt Water: Add another MerCathode Systemcontroller, wired in parallel (Figure 2-22). (The addi-tional MerCathode controller must be of the sametype as the original one; i.e., black plus black or blueplus blue.) This will double your protection to 400 mA.(Keep in mind that this will also increase the drain onyour battery.) If you take your boat into fresh water,the conductivity of the water will limit the output ofyour MerCathode Systems.

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Stray Current CorrosionThere is a greater danger for boats that connect toAC shore power: destructive, low-voltage galvaniccurrents (DC) passing through the shore powerground wire (Figure 2-23). Normally, AC is not a cor-rosion problem, but because the boat, pier and wireare all connected, or due to a leakage, there can bea direct current (DC) also present. This is potentiallyvery damaging and requires additional protection.

! WARNINGNever disconnect or place a switch in the shorepower green safety grounding lead, as this couldcreate an electrical shock hazard.

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Safety regulations require a three-wire cable for car-rying shore power aboard any boat, and that one ofthese leads (the green lead) grounds all electricaland propulsion equipment to shore. This safety pro-cedure reduces the danger of shock, but also con-nects the underwater metal components on yourboat with metal parts on neighboring boats usingshore power, steel piers and metal objects on shorethat are grounded and extend into the water. This in-terconnecting of dissimilar metals allows destructivegalvanic currents to flow between them. If these cur-rents are allowed to continue, your drive unit will ex-perience severe corrosion damage in a very shorttime (as little as days).

In most cases, sacrificial anodes and even the Mer-Cathode System (if equipped) will not be able to con-trol or counteract the increased corrosion potential.

The Quicksilver Galvanic Isolator (Figure 2-24) or anisolation transformer can be used to galvanically iso-late the AC shore power ground from the boat.

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Quicksilver Galvanic IsolatorThe Quicksilver Galvanic Isolator is a solid-state de-vice that is series connected in line into the boat’sgreen safety grounding lead ahead of all groundingconnections on the boat (Figure 2-25). This devicefunctions as a filter, blocking the flow of destructivelow voltage galvanic (DC) currents, but still maintain-ing the integrity of the safety grounding circuit (Fig-ure 2-26).

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The Quicksilver Galvanic Isolator provides inexpen-sive protection when compared to a similarly rated120-volt AC, 50-amp isolation transformer. TheQuicksilver Galvanic Isolator measures just 8” x 4” x3” and weighs less than 10 lbs. It’s much smaller andlighter than a transformer, which is about four timesthe size and six-to-eight-times the weight. (A 220-voltAC, 50-amp transformer is 18- to 24-times the size,weight and cost.)

Isolator installation is easy, requiring only four mount-ing screws. The isolator is designed for use in 120-and 240-volt AC, 60-Hertz circuits with up to a60-amp rating and is ABYC certified and UL-ap-proved. The Quicksilver Galvanic Isolator is the onlyproduct of this type approved by Underwriters Labo-ratories at the time of this publication.

An important performance note: The QuicksilverGalvanic Isolator contains both diodes and a largecapacitor, whereas most competitive products con-tain only diodes. If there is even the smallest AC cur-rent leakage through the ground circuit, diodes canbe “biased,” becoming conductive and allowing thedestructive galvanic current (DC) to pass along withthe AC Any AC current leakage renders useless anyisolators with only diodes. An isolator with a capaci-tor, like the Quicksilver Galvanic Isolator, solves thisproblem.

Anti-Fouling Paint on DrivesFouling is a major concern in many situations. Marineanimals (barnacles, mussels, etc.) and vegetationcan make life miserable for boaters. There are anti-fouling paints available, but some can affect corro-sion protection or even accelerate corrosion.

In the past, tributyltin- (sometimes referred to as“TBT” or “organotin”) based anti-fouling paints con-trolled fouling and did not cause corrosion problemsfor aluminum drives (Figure 2-27). Recently, environ-mental concerns and legislation have restricted orprohibited the use of tributyltin paints. Presently, tri-butyltin based paints must be applied by a state-li-censed repair shop. In the U.S. and Canada, tributyl-tin is prohibited for vessels less than 25 meters withan exemption for aluminum hulls, fittings and drives.If TBT paint can be obtained, it is still recommendedfor drives.

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Other anti-fouling formulations (tin-free) are beingtested. Copper and copper oxide formulations thatare electrically non-conducting have had minimal ef-fect on drive corrosion. If this type of paint is used onthe hull, an unpainted gap of at least 1.5 in. shouldbe left around the drive. We will continue to test fur-ther developments of tin-free anti-fouling paints andwe will announce successful formulations in servicebulletins.


Corrosion Protection Testingand Troubleshooting

TestingFor diagnostic tests, a simple digital volt/ohm meter(multimeter) is necessary. An analog version may beused, but it must be a high-impedance model. (Eventhe most inexpensive digital volt/ohm meter has highimpedance.)

One of the most useful methods for determining ifcorrosion below the waterline is occurring is throughthe measurement of the “hull potential” (Figure 3-1).This is done by immersing a reference electrode,usually silver/silver chloride (a silver wire with a coat-ing of silver chloride), into the water about six inchesbehind the drive. This electrode is connected to thepositive (red) terminal of a digital volt/ohm meter. Thenegative (black) lead from the meter is attached tothe battery (or system) ground. With the meter set ona two-volt DC scale (if using a silver/silver chloridereference), the “hull potential” is displayed.

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The values displayed will be discussed in the chartslater in this chapter. However, please note that thevalues given do not indicate a line, one side of whichis “protected” and another side of which is “corrod-ing.” From an “ideal” potential to “active” corrosion isa graduated scale. Local water conditions may alsocause a lower or higher potential to be “ideal.”

IMPORTANT: Be sure to observe the followingwhen performing tests:

• If the unit is equipped with a MerCathode Sys-tem, make sure that the battery is fullycharged (12.6 volts or above).

• New boats will usually produce higher read-ings than normal. This is because the driveunit is being protected by a new finish andnew sacrificial anodes. To obtain an accuratediagnosis, the test should be performed afterthe boat has been used at least one or twoweeks. This will give the paint a chance to“soak” and acquire minor abrasions andscratches, which will result in a more accu-rate reading.

• Boats should be moored (without being oper-ated) for at least eight hours before perform-ing the test. This is necessary to allow theMerCathode System and/or sacrificialanodes to polarize the water molecules in di-rect contact with the drive. Be careful not torock the boat excessively while boarding toperform the test, as this will alter the testreading.


Troubleshooting CorrosionThe first signs of corrosion below the waterline arepaint blistering, usually on sharp edges, and theformation of powdery white corrosion material on ex-posed aluminum surfaces. If the corrosion is allowedto continue, pitting of the aluminum will occur. Thecharts included later in this chapter may help you de-termine the cause of the corrosion and the correctiveaction needed to prevent its continuance.

First, some basic observations on corrosion and cor-rosion prevention must be considered.

• Loosely adhering, powdery white material thatforms on sharp edges or near fasteners shouldnot be confused with hard, tough, white or off-white calcareous (calcium carbonate) deposits,which form uniformly on well-protected surfaces(painted or unpainted). These deposits are pri-marily a result of the calcium and magnesium inthe water, and heat in the area of the deposit.

• There must be electrical continuity from the bat-tery (or system) ground through all parts of theoutboard or drive.

Inactive Sacrificial AnodesIf the underwater portion of the drive unit shows signsof corrosion but the sacrificial anodes are not beingconsumed, the problem may be due to the following:

• The sacrificial anodes may not be making goodelectrical contact with the drive unit. Remove theanode, scrape the mounting surfaces on the partto be protected down to bare metal, and reinstallanodes.

• Zinc sacrificial anodes may have a protectivecoating of a very dense oxide film on their surface(which usually has a charcoal gray appearance).This condition usually occurs in fresh water, but itcan also happen in saltwater areas.

To confirm this condition, test for continuity betweenthe anode and the drive using a multi-meter set to“ohms” on the R x 1 scale. If the anode must bescraped with a knife in order to get a conductive read-ing, the anode is oxidized and should be replaced.Sanding the surface with coarse sandpaper providesa temporary solution, but the oxide will form again.

Corrosion on Underwater Parts(without MerCathode or impressed current protection)

Cause or Observed Condition Corrective Action

Sacrificial anode(s) consumed Replace anode(s) when 50% consumed

Stainless steel prop installed Add MerCathode (impressed current protection) or addi-tional sacrificial anodes

Sacrificial anode(s) not grounded to drive Remove anode(s), clean contact surface, reinstall, check con-ti nuity

Loss of continuity between underwater parts & ground Provide good ground connections

Shore power causing overload of anode(s) and/or MerCa-thode

Disconnect shore power or install Quicksilver Galvanic Iso-la tor

Paint on drive heavily worn (exposing more metal) Prime and repaint, and/or install additional anode(s)

Sacrificial anode(s) painted Remove paint or replace anode(s)

Drive tilted so far that anode(s) are out of water Leave drive down, install additional anode (below water-line), or transom mount a MerCathode system

Power trim cylinders only corroded Provide good ground to drive. All parts must be grounded.

Corrosion in area of exhaust outlets Exhaust deposits cancause corrosion

Remove deposits with marine or auto wax

Corrosion occurring after unit is removed from salt water Wash exterior and flush interior with fresh water

Corrosion and/or salt buildup between mating parts Exclude moisture from between mating parts with Quicksil-ver 2-4-C with Teflon

Stainless steel parts corroding: 1. Tightly wrapped fishline or foreign material excludes oxygen, causing corrosion 2. Iron particles, such as from a wire brush, cause rusting 3. Propeller pitting can occur if electrical continuity is lost

Clean parts, remove foreign material, insure continuity

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Continuity DevicesGalvanic corrosion, as we’ve discussed, is an elec-trochemical reaction. Being such, all underwatermetal components on the drive unit can only tee pro-tected if they maintain electrical continuity. Severaldifferent methods are used on outboards and sterndrives.

Outboard Continuity DevicesOutboards 35-horsepower and above use braidedstainless steel continuity straps between drive shafthousing and swivel bracket (Figure 3-2).

It was found that a ground strap was required be-tween the hydraulic system and the mid-section ofthe outboard (Figure 3-3).

Outboards 75-horsepower and above also use wavewashers between clamp brackets and swivel bracket(Figure 3-4) and another wave washer betweenswivel bracket and bottom yoke to ensure electricalcontinuity.

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MerCruiser Stern Drive ContinuityDevicesStainless steel trim hoses (Figure 3-5) help ensurecontinuity to trim cylinders. (Hoses on older Mer-Cruiser models were also equipped with starwashersunder hydraulic connector attaching nuts. Trim cylin-ders on older MerCruiser models were also equippedwith spiral springs.)

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The transom assembly and stern drive unit areequipped with a continuity circuit to ensure goodelectrical continuity between engine, transom as-sembly and stern drive components. Good electricalcontinuity is essential for the anodic trim tab and Mer-Cathode System to function effectively.

A Continuity Circuit Kit is also available to retrofit old-er MerCruiser I, TR and TRS models with this sys-tem. The continuity circuit should be periodically in-spected to ensure that there are no looseconnections or damaged wires. Isolated corrosion ononly one or two components on the drive unit couldindicate improper grounding of those components(see Figures 3-6 through 3-14).


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Drive Unit Continuity TestFor all underwater components on the drive unit to beeffectively protected by the MerCathode Systemand/or sacrificial anodes, electrical continuity mustbe maintained. If the unit is equipped with a MerCa-thode System, all underwater components must beelectrically grounded to the negative (-) battery termi-nal to be protected.

To help ensure proper grounding of underwater com-ponents, current MerCruiser models are equippedwith a continuity circuit.

The following test can be used to check if the driveunit is properly grounded (see Figure 3-15).

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1. Boat must be in the water when performing thistest.

2. Set DC volt meter on 0-2 volt (0-2000 millivolt)scale.

3. Connect negative (black) meter lead to negative(-) battery terminal.

4. Suspend end of positive (red) meter lead in thewater within 6” (15 cm) of drive unit. Do not allowit to contact drive unit. Reading should be above3 millivolts.

5. Connect end of positive meter lead to each me-tallic component on stern drive. Be sure there isgood electrical contact to each metal surface.Reading should drop below 2 millivolts.

6. A reading higher than 2 millivolts indicates im-proper grounding.


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Corrosion of Stainless SteelPropellersGood continuity must be maintained between a stain-less steel propeller and propeller shaft to prevent cor-rosion damage to the propeller. The corrosion dam-age will appear in the form of deep pits or holes in themetal (Figure 3-16).

To help ensure good continuity, the propeller shouldbe removed periodically and all mating surfaces onpropeller, propeller-attaching parts and propellershaft should be cleaned. A liberal coat of Quicksilver2-4-C with Teflon, Special Lubricant 101, or PerfectSeal should be applied to propeller shaft before rein-stalling the propeller. Be sure to retorque the propel-ler nut to 55 lbs. ft. (75 N·m).

On MC-I, Black Max and TRS Cleaver Propellers(with square rubber drive hub), a continuity washercan be installed between the spline washer and thepropeller to help ensure continuity (Figure 3-17). Thiswasher is included as standard equipment with thesepropellers.

Other Mercury Marine propellers with Flo-Torq

molded rubber hubs or plastic hubs have continuitydevices built in.

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Preventative MaintenanceThere are several simple service procedures you canperform to maintain the quality of corrosion resis-tance built into your Mercury Drive Unit. A little pa-tience, some simple hand tools, and a few Quicksil-ver products are all you need.

MaintainMaintain a complete paint covering on the lower unit(Figure 4-1). Check the finish regularly, and primeand paint nicks and scratches. If any bare metal is ex-posed, sacrificial anodes will be eaten away rapidlyand corrosion of the drive unit will quickly occur. Useonly tin anti-fouling paint on or near aluminum sur-faces below the waterline. Never use paints contain-ing copper or mercury. It’s a good idea to check thelower unit frequently for sand abrasion, nicks andscratches. If bare metal is showing, apply two coatsof paint to prevent corrosion.

SpraySpray the entire powerhead, all electrical connec-tions, and everything under the cowl with QuicksilverCorrosion Guard (Figure 4-2).

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InspectInspect the sacrificial trim tab at regular intervals andreplace it before it’s half gone (Figure 4-3). Neverpaint trim tabs or the mounting surface on the gear-case. Additional anodes or a MerCathode Systemwill be required if a stainless steel prop is installed.

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FlushFlush the motor with fresh water after each saltwaterexcursion, and wash down exterior of motor withfresh water as well (Figure 4-4). Attachments forflushing are available for all Mercury Drive Units andare designed for easy use. Use them!

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CheckCheck the prop shaft for fish line (Figure 4-5). If astainless steel shaft is wrapped with line, oxygen iseliminated from the surface, allowing corrosion. Re-move the fish line. Fish line cutters are standard onall MerCruiser Stern Drives and Mercury and MarinerOutboards, 9.9- to 125-horsepower.

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LubricateLubricate everything according to your owner’smanual (Figure 4-6). In salt water, more frequent lu-brication is required. Do not use lubricants that con-tain graphite on or near aluminum. Graphite causesrapid corrosion to aluminum when the two are com-bined in salt water. Additional grease fittings have re-cently been added to many Mercury Drive Units in avariety of areas. The more lubrication, the better per-formance and corrosion resistance you’ll get. Re-member, saltwater operation requires lubricationmuch more often!

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PropellersThe propeller should be removed every 60 days andQuicksilver 2-4-C with Teflon, Special Lubricant 101,Anti-Corrosion Grease or Perfect Seal applied to theprop shaft (Figure 4-7). When re-installing the prop,be sure the prop nut is tightened sufficiently (see yourowner’s manual). Have an authorized dealer lubri-cate the cover nut and bearing carrier spool at leastonce each season.

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More LubricationOther lubrication points are: thumbscrews, startermotor pinion shaft, upper shift shaft, swivel tube,steering tube, throttle and shift linkages, reverse locklever and cam, and tiller handle hinges. The lowerunit must be checked and lubrication added as nec-essary (Figure 4-8).

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By following these simple maintenance steps, yourMercury Drive Unit will remain corrosion free . . . justas it was designed!

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