Galvanic and Stray Current Corrosion - Amazon Simple ... · PDF filedifference between Zinc...
Transcript of Galvanic and Stray Current Corrosion - Amazon Simple ... · PDF filedifference between Zinc...
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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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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 Transformer Galvanic 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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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 Power
Bus 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