ch 5 corrosion slides 042909
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MElec-Ch5 - 1
Chapter 5
Galvanic and
Stray Current Corrosion
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MElec-Ch5 - 2
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 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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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 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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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 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
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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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