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Chapter 5

Galvanic and Stray Current Corrosion

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Overview

• Galvanic Corrosion Understanding Galvanic Corrosion Controlling Galvanic Corrosion

• Stray Current Corrosion Understanding Stray Current Corrosion Preventing Stray Current Corrosion Testing for Stray Current

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Galvanic Corrosion

Understanding Galvanic Corrosion • Causes • Results • Galvanic Series of Metals • Additional Notes

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

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Results

New Zinc (for 1” diameter shaft)

of Galvanic Corrosion

Old Zinc after 8 months (for 1” diameter shaft)

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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

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Additional Notes • Expect corrosion with 0.25 V difference

• Most negative electrodes will decompose Magnesium @ - 1.50 V for freshwater Zinc @ - 1.03 V for saltwater Aluminum @ - 0.75 V will decompose if neither

magnesium or zinc are present

• Zinc (or magnesium) will protect Stainless steel shaft Bronze propeller Aluminum outdrive

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Signs of Galvanic Corrosion • Blistering of paint 1st Warning Sign

• Formation of powdery substance 2nd Warning Sign

• Pitting of metal Too late Severe Galvanic Corrosion

• Don’t treat the symptom, fix the problem

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Galvanic Corrosion

Controlling Galvanic Corrosion • Types of Metal • Area of Metals • Self-Destroying Metals • Use of Sacrificial Anodes • Indirect Cathodic Protection • Resistance of an Electrical Path • Between boats

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Types of Metal

• Copper, bronze and copper-nickel are compatible

• Avoid bronze propeller on plain steel shaft • Stainless steel shaft with bronze prop may

be used Need zinc washer and/or zinc prop nut Avoid graphite grease

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Area of Metal

• Good – applying a less noble metal to a large area Bronze through-hull on steel hull

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

plate

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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-magnetic If magnetic, it will corrode

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Use of Sacrificial Anodes • Made from active metals Magnesium, zinc or aluminum

• Corrosive action occurs on the expendable metal anode

• Bolted to the metal they are to protect • Never painted • Replaced when half-corroded or annually

Shaft Prop Nut Rudder

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Powerboat Zincs

6 Zincs Trim Tab

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Indirect Cathodic Protection

• Used when direct contact not possible Zinc bolted to outside of hull Inside boat connect with insulated AWG#8 to

• Rudder Post • Shaft (requires shaft brush)

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Resistance of Electrical Path

• Fresh water is less conductive than salt water Less galvanic current Use magnesium sacrificial anodes

• Salt water is more conductive than fresh water More galvanic current Use zinc sacrificial anodes

• Magnesium sacrificial anodes will not last

• Graphite grease is an excellent conductor, but is a cathode Do NOT use in stuffing boxes Do NOT use on shaft bearings

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Between Boats • Two different metals Aluminum vs steel (or other metal) Immersed in current-carrying solution

• Sea water

Interconnected by current-carrying conductor • AC ground (green) wire

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Galvanic Isolator

• Stops DC current in AC ground wire

or Isolation Transformer

Isolation Transformer Galvanic Isolator

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Stray Current Corrosion

Understanding Stray Current Corrosion • Causes • Results • Additional Notes

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Stray Current Corrosion • Requires External source of electricity From wetted metal surface (electrodes) To return circuit of lower potential

(electrolyte)

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Stray vs Galvanic Current

• Stray current corrosion is more destructive Hundreds of times stronger Galvanic potential difference 0.25 to 1.5 volts Stray current from 12 volt battery

• Sources of stray current Internal from boat’s 12 volt battery and

defective wiring External to boat from another source of DC

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Results

of Stray Current Corrosion

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Additional Notes

• Stronger than Galvanic current 100 times more destructive

• Metals can be similar or dissimilar Current flow from positive through electrolyte Positive DC terminal will corrode Both AC terminals will corrode

• Electrolyte is any moist surface Bilge water Wet wood Wet or moist surface

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Stray Current Corrosion

Preventing Stray Current • Wiring • Bonding • Battery charger • Galvanic isolators • Isolation transformers

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Wiring • Defective wiring is the most common cause Deteriorated insulation on hot wire Always use marine grade wires

• Run wires above water line Moist or wetted surfaces conduct current Moisture in loose connections will cause

corrosion

• Wires in bilge Waterproof terminals and butt spices Heat shrink tubing is 2nd choice Liquid electrical tape is also an option Electrical tape is inadequate

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Bonding • Maintain adequate bonding system All metallic bodies and surfaces at DC negative Chapter 2 (Wiring) covered bonding

• Propeller shaft bonding Recommend by some authorities Will also reduce propeller “hash” (Chapter 7) Requires a shaft brush

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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

boat • In one underwater fitting • Through bonding system • Out another underwater fitting (remember corroded prop and shaft pictures)

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Corrosion Facts

• Not all corrosion is electrical Seawater deteriorates all metals Cavitation 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

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Testing for Stray Current

• Measuring Stray Current • Corrosion Source and Mitigation

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Measuring Stray Current

• Normally AC ground and DC negative connected • To 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

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AC Stray Current Testing

• AC main circuit breaker “On” All branch circuit breakers “Off”

• Set multimeter to read AC current

• Current should be less than 1 milliampere

• Then selectively turn on each AC circuit

• If AC current exceeds 1 mA You have stray current in that circuit

• After testing Reconnect AC ground & DC negative bus bars

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DC Stray Current Testing

• DC 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 mA You have stray current in that circuit

• After testing Reconnect AC ground and DC negative bus bars

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Testing with Mitigation

• Galvanic Isolators & Isolation Transformers Stop DC current

• To check for stray current with isolator Place ammeter between DC negative bus and

green shore power wire to isolator

• To check for stray current with transformer Place ammeter between DC negative bus and

green shore power wire to transformer

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Internal DC Current Testing

• Turn off DC main and all branch breakers

• Insert ammeter in battery negative cable

• Hold down bilge pump float switch So pump will not turn on

• Turn on DC main and bilge pump breaker

• Measure stray current, if any Defective wiring or pump switch

• Test other wiring with DC devices turned off

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Summary 1 • Types of electronic corrosion Galvanic caused by dissimilar metals Stray 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 water Zincs are used to protect other metal

components

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Summary 2

• Stray current Requires an external source of current Normally 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