Chapter 5 Galvanic and Stray Current Corrosion MElec-Ch5 - 1 Overview • Galvanic Corrosion Understanding Galvanic Corrosion Controlling Galvanic Corrosion • Stray Current Corrosion Understanding Stray Current Corrosion Preventing Stray Current Corrosion Testing for Stray Current MElec-Ch5 - 2 Galvanic Corrosion Understanding Galvanic Corrosion • • • • Causes Results Galvanic Series of Metals Additional Notes MElec-Ch5 - 3 Causes • Requires Two different metals (electrodes) Immersed in current-carrying solution (electrolyte) Interconnected by a current-carrying conductor MElec-Ch5 - 4 Results of Galvanic Corrosion New Zinc (for 1” diameter shaft) Old Zinc after 8 months (for 1” diameter shaft) MElec-Ch5 - 5 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 MElec-Ch5 - 6 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 MElec-Ch5 - 7 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 MElec-Ch5 - 8 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 MElec-Ch5 - 9 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 MElec-Ch5 - 10 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 MElec-Ch5 - 11 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 MElec-Ch5 - 12 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 MElec-Ch5 - 13 Powerboat Zincs Trim Tab 6 Zincs MElec-Ch5 - 14 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) MElec-Ch5 - 15 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 MElec-Ch5 - 16 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 MElec-Ch5 - 17 Galvanic Isolator or Isolation Transformer • Stops DC current in AC ground wire Galvanic Isolator Isolation Transformer MElec-Ch5 - 18 Stray Current Corrosion Understanding Stray Current Corrosion • Causes • Results • Additional Notes MElec-Ch5 - 19 Stray Current Corrosion • Requires External source of electricity From wetted metal surface (electrodes) To return circuit of lower potential (electrolyte) MElec-Ch5 - 20 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 MElec-Ch5 - 21 Results of Stray Current Corrosion MElec-Ch5 - 22 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 MElec-Ch5 - 23 Stray Current Corrosion Preventing Stray Current • • • • • Wiring Bonding Battery charger Galvanic isolators Isolation transformers MElec-Ch5 - 24 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 MElec-Ch5 - 25 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 MElec-Ch5 - 26 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) MElec-Ch5 - 27 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 MElec-Ch5 - 28 Testing for Stray Current • Measuring Stray Current • Corrosion Source and Mitigation MElec-Ch5 - 29 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 MElec-Ch5 - 30 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 MElec-Ch5 - 31 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 MElec-Ch5 - 32 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 MElec-Ch5 - 33 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 MElec-Ch5 - 34 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 MElec-Ch5 - 35 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 MElec-Ch5 - 36