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Chapter 5. Galvanic and Stray Current Corrosion. Overview. Galvanic Corrosion Understanding Galvanic Corrosion Controlling Galvanic Corrosion Stray Current Corrosion Understanding Stray Current Corrosion Preventing Stray Current Corrosion Testing for Stray Current. Galvanic Corrosion.

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chapter 5
Chapter 5

Galvanic and

Stray Current Corrosion

  • Galvanic Corrosion
    • Understanding Galvanic Corrosion
    • Controlling Galvanic Corrosion
  • Stray Current Corrosion
    • Understanding Stray Current Corrosion
    • Preventing Stray Current Corrosion
    • Testing for Stray Current
galvanic corrosion
Galvanic Corrosion

Understanding Galvanic Corrosion

  • Causes
  • Results
  • Galvanic Series of Metals
  • Additional Notes
  • Requires
    • Two different metals (electrodes)
    • Immersed in current-carrying solution (electrolyte)
    • Interconnected by a current-carrying conductor

Old Zinc after 8 months

(for 1” diameter shaft)


of Galvanic Corrosion

New Zinc

(for 1” diameter shaft)

galvanic scale of metals
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

additional notes
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
signs of galvanic corrosion
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
galvanic corrosion9
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
types of metal
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
area of metal
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
self destroying metals
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
use of sacrificial anodes
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


Prop Nut


indirect cathodic protection
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)
resistance of electrical path
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
between boats
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
galvanic isolator

Isolation Transformer

Galvanic Isolator

Galvanic Isolator

or Isolation Transformer

  • Stops DC current in AC ground wire
stray current corrosion
Stray Current Corrosion

Understanding Stray Current Corrosion

  • Causes
  • Results
  • Additional Notes
stray current corrosion20
Stray Current Corrosion
  • Requires
    • External source of electricity
    • From wetted metal surface (electrodes)
    • To return circuit of lower potential (electrolyte)
stray vs galvanic current
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

of Stray Current Corrosion

additional notes23
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
stray current corrosion24
Stray Current Corrosion

Preventing Stray Current

  • Wiring
  • Bonding
  • Battery charger
  • Galvanic isolators
  • Isolation transformers
  • 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
  • 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
ac ground isolation
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


  • Stray current may flow through your boat
    • In one underwater fitting
    • Through bonding system
    • Out another underwater fitting

(remember corroded prop and shaft pictures)

corrosion facts
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
testing for stray current
Testing for Stray Current
  • Measuring Stray Current
  • Corrosion Source and Mitigation
measuring stray current

DC Neg

AC Gnd










break wire

to insert






Measuring Stray Current
  • Normally AC ground and DC negative connected
  • To measure current, insert ammeter in series
ac stray current testing
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
dc stray current testing
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
testing with mitigation
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
internal dc current testing
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
summary 1
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
summary 2
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