Galvanic andStray Current Corrosion

OverviewGalvanic Corrosion

Understanding Galvanic CorrosionControlling Galvanic Corrosion

Stray Current CorrosionUnderstanding Stray Current CorrosionPreventing Stray Current CorrosionTesting for Stray Current

Galvanic Corrosion

Understanding Galvanic Corrosion

CausesResultsGalvanic Series of MetalsAdditional Notes

CausesRequires

Two different metals (electrodes)Immersed in current-carrying solution (electrolyte)Interconnected by a current-carrying conductor

Results

New Zinc(for 1” diameter

shaft)

of Galvanic Corrosion

Old Zinc after 8 months

(for 1” diameter shaft)

Galvanic Scale of Metals

What is the voltage difference between Zinc (Zn) and Copper (Cu)?

What is more noble than Stainless Steel (Passive)?

An. 0.67v

An. Graphite

Additional NotesExpect corrosion with 0.25 V difference

Most negative electrodes will decomposeMagnesium @ - 1.50 V for freshwaterZinc @ - 1.03 V for saltwaterAluminum @ - 0.75 V will decompose if neither

magnesium or zinc are present

Zinc (or magnesium) will protectStainless steel shaftBronze propellerAluminum outdrive

Signs of Galvanic CorrosionBlistering of paint

1st Warning SignFormation of powdery substance

2nd Warning SignPitting of metal

Too lateSevere Galvanic Corrosion

Don’t treat the symptom, fix the problem

Galvanic Corrosion

Controlling Galvanic Corrosion

Types of MetalArea of MetalsSelf-Destroying MetalsUse of Sacrificial AnodesIndirect Cathodic ProtectionResistance of an Electrical PathBetween boats

Types of MetalCopper, bronze and copper-nickel are

compatible

Avoid bronze propeller on plain steel shaftStainless steel shaft with bronze prop may be

usedNeed zinc washer and/or zinc prop nutAvoid graphite grease

Area of MetalGood – applying a less noble metal to a large

areaBronze through-hull on steel hull

Bad – applying a more noble metal to a larger areaSteel screws / bolts on large bronze or monel

plate

Self-Destroying Metals

Brass (an alloy of copper and zinc)Zinc will corrode away in sea water, leaving a

copper sponge

Stainless steel hose clamps with different metal take-up screws

Stainless steel should be non-magneticIf magnetic, it will corrode

Use of Sacrificial AnodesMade from active metals

Magnesium, zinc or aluminumCorrosive action occurs on the expendable

metal anodeBolted to the metal they are to protectNever paintedReplaced when half-corroded or annually

Shaft Prop Nut Rudder

Powerboat Zincs

6 Zincs

Trim Tab

Indirect Cathodic ProtectionUsed when direct contact not possible

Zinc bolted to outside of hullInside boat connect with insulated AWG#8 to

Rudder Post Shaft (requires shaft brush)

Resistance of Electrical PathFresh water is less conductive than salt water

Less galvanic currentUse magnesium sacrificial anodes

Salt water is more conductive than fresh waterMore galvanic currentUse zinc sacrificial anodes

Magnesium sacrificial anodes will not last

Graphite grease is an excellent conductor, but is a cathodeDo NOT use in stuffing boxesDo NOT use on shaft bearings

Between BoatsTwo different metals

Aluminum vs steel (or other metal)Immersed in current-carrying solution

Sea waterInterconnected by current-carrying conductor

AC ground (green) wire

Galvanic Isolator

Stops DC current in AC ground wire

or Isolation Transformer

Isolation TransformerGalvanic Isolator

Stray Current Corrosion

Understanding Stray Current Corrosion

CausesResultsAdditional Notes

Stray Current CorrosionRequires

External source of electricityFrom wetted metal surface (electrodes)To return circuit of lower potential

(electrolyte)

Stray vs Galvanic Current

Stray current corrosion is more destructiveHundreds of times strongerGalvanic potential difference 0.25 to 1.5 voltsStray current from 12 volt battery

Sources of stray currentInternal from boat’s 12 volt battery and

defective wiringExternal to boat from another source of DC

Resultsof Stray Current

Corrosion

Additional NotesStronger than Galvanic current

100 times more destructive

Metals can be similar or dissimilarCurrent flow from positive through electrolytePositive DC terminal will corrodeBoth AC terminals will corrode

Electrolyte is any moist surfaceBilge waterWet woodWet or moist surface

Stray Current Corrosion

Preventing Stray CurrentWiringBondingBattery chargerGalvanic isolatorsIsolation transformers

WiringDefective wiring is the most common cause

Deteriorated insulation on hot wireAlways use marine grade wires

Run wires above water lineMoist or wetted surfaces conduct currentMoisture in loose connections will cause

corrosionWires in bilge

Waterproof terminals and butt spicesHeat shrink tubing is 2nd choiceLiquid electrical tape is also an optionElectrical tape is inadequate

BondingMaintain adequate bonding system

All metallic bodies and surfaces at DC negativeChapter 2 (Wiring) covered bonding

Propeller shaft bondingRecommend by some authoritiesWill also reduce propeller “hash” (Chapter 7)Requires a shaft brush

AC Ground Isolation•If your boat has the better ground… and a nearby boat has stray current

Your boat will be damaged, unless…Stop DC current in AC ground wire

Galvanic Isolators & Isolation Transformers but

•Stray current may flow through your boatIn one underwater fittingThrough bonding systemOut another underwater fitting (remember corroded prop and shaft pictures)

Corrosion FactsNot all corrosion is electrical

Seawater deteriorates all metalsCavitation also erodes props

Stray current corrosion can be eliminated

Galvanic corrosion can be reduced and controlled

DC current is 100 times worse than AC current

Testing for Stray CurrentMeasuring Stray Current Corrosion Source and Mitigation

Measuring Stray CurrentNormally AC ground and DC negative

connectedTo measure current, insert ammeter in series

To

Battery

Negative

DC Neg

AC Gnd

To

Shore

Power

Bus

Bar

Bus

Bar

ABYC Req

Temporary

break wire

to insert

Ammeter

A

AC Stray Current TestingAC main circuit breaker “On”

All branch circuit breakers “Off”

Set multimeter to read AC currentCurrent should be less than 1 milliampere

Then selectively turn on each AC circuit

If AC current exceeds 1 mAYou have stray current in that circuit

After testingReconnect AC ground & DC negative bus bars

DC Stray Current TestingDC main circuit breaker “On”

All branch circuit breakers “Off”

Set multimeter to read DC current

Current should be less than 0.01 milliampere

Then selectively turn on each DC circuit

If DC current exceeds 0.01 mAYou have stray current in that circuit

After testingReconnect AC ground and DC negative bus bars

Testing with Mitigation

Galvanic Isolators & Isolation TransformersStop DC current

To check for stray current with isolatorPlace ammeter between DC negative bus and

green shore power wire to isolator

To check for stray current with transformerPlace ammeter between DC negative bus and

green shore power wire to transformer

Internal DC Current Testing

Turn off DC main and all branch breakers

Insert ammeter in battery negative cable

Hold down bilge pump float switchSo pump will not turn on

Turn on DC main and bilge pump breaker

Measure stray current, if anyDefective wiring or pump switch

Test other wiring with DC devices turned off €

Summary 1Types of electronic corrosion

Galvanic caused by dissimilar metalsStray current requires external current

Galvanic current

Requires Different metals Immersed in current carrying solution Connect together by current carrying conductor

Brass will disintegrate in sea waterZincs are used to protect other metal

components

Summary 2Stray current

Requires an external source of currentNormally is caused by defective wiring

Especially in / through bilge Make sure any connections are waterproof

DC is 100 times more destructive than AC Over 1 mA AC Over 0.01 mA DC