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
of Galvanic Corrosion
New Zinc
(for 1” diameter shaft)
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)?
An. 0.67v
What is more noble
than Stainless Steel
(Passive)?
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
Trim Tab
6 Zincs
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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
or Isolation Transformer
• Stops DC current in AC ground wire
Galvanic Isolator
Isolation Transformer
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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
DC
Neg
AC
Gnd
To
Shore
Power
To
Battery
Negative
ABYC Req
A
Bus
Bar
Temporary
break wire
to insert
Ammeter
Bus
Bar
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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
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