Ch 5 Corrosion Slides 042909

8/13/2019 Ch 5 Corrosion Slides 042909

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Chapter 5

Galvanic and

Stray Current Corrosion


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Overview

Galvanic Corrosion

Understanding Galvanic Corrosion

Controlling Galvanic 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 voltagedifference betweenZinc (Zn) and Copper(Cu)?

What is more noblethan 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 saltwaterAluminum @ - 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

1stWarning Sign

Formation of powderysubstance

2nd Warning Sign

Pitting of metal

Too late

Severe Galvanic Corrosion

Dont 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 arecompatible

Avoid bronze propeller on plain steel shaft

Stainless steel shaft with bronze prop maybe used

Need zinc washer and/or zinc prop nut

Avoid graphite grease


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Self-Destroying Metals

Brass (an alloy of copper and zinc)

Zinc will corrode away in sea water, leaving acopper sponge

Stainless steel hose clamps with differentmetal 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 expendablemetal 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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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 acathode 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 conductorAC 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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Understanding Stray Current Corrosion Causes

Results

Additional Notes


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Requires

External source of electricity

From wetted metal surface (electrodes)

To return circuit of lower potential

(electrolyte)


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Stray vsGalvanic 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 boats 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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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 wireAlways 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 2ndchoice Liquid electrical tape is also an option Electrical tape is inadequate


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Bonding

Maintain adequate bonding systemAll 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 groundand a nearby boat has stray current Your boat will be damaged, unless

Stop DC current in AC ground wire

Galvanic Isolators & Isolation Transformersbut

Stray current may flow through yourboat

In one underwater fitting Through bonding system

Out another underwater fitting

(remember corroded prop and shaft pictures)


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Testing for Stray Current

Measuring Stray Current Corrosion Source and Mitigation


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AC Stray Current Testing

AC main circuit breaker OnAll branch circuit breakers Off

Set multimeter to readAC current

Current 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 testing Reconnect AC ground & DC negative bus bars

DC S C T i


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DC Stray Current Testing

DC main circuit breaker OnAll 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 testing Reconnect AC ground and DC negative bus bars

T i i h Mi i i


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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 andgreen shore power wire to isolator

To check for stray current with transformer

Place ammeter between DC negative bus andgreen shore power wire to transformer

I t l DC C t T ti


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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

S 1


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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 metalcomponents


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Ch 5 Corrosion Slides 042909