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Chapter 5

Galvanic andStray 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 TransformerGalvanic 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

PowerBus

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