Galvanic and Stray Current Corrosion

Transcription

1 Chapter 5 Galvanic and Stray Current Corrosion MElec-Ch5-1

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 MElec-Ch5-2

3 Galvanic Corrosion Understanding Galvanic Corrosion Causes Results Galvanic Series of Metals Additional Notes MElec-Ch5-3

4 Causes Requires Two different metals (electrodes) Immersed in current-carrying solution (electrolyte) Interconnected by a current-carrying conductor MElec-Ch5-4

5 Results of Galvanic Corrosion New Zinc (for 1 diameter shaft) Old Zinc after 8 months (for 1 diameter shaft) MElec-Ch5-5

6 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

7 Additional Notes Expect corrosion with 0.25 V difference Most negative electrodes will decompose V for freshwater V for saltwater 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

8 Signs of Galvanic Corrosion Blistering of paint 1 st 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

9 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

10 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

11 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

12 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

13 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

14 Powerboat Zincs 6 Zincs Trim Tab MElec-Ch5-14

15 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

16 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

17 Two different metals Between Boats 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

18 Galvanic Isolator or Isolation Transformer Stops DC current in AC ground wire Galvanic Isolator Isolation Transformer MElec-Ch5-18

19 Stray Current Corrosion Understanding Stray Current Corrosion Causes Results Additional Notes MElec-Ch5-19

20 Stray Current Corrosion Requires External source of electricity From wetted metal surface (electrodes) To return circuit of lower potential (electrolyte) MElec-Ch5-20

21 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

22 Results of Stray Current Corrosion MElec-Ch5-22

23 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

24 Stray Current Corrosion Preventing Stray Current Wiring Bonding Battery charger Galvanic isolators Isolation transformers MElec-Ch5-24

25 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 2 nd choice Liquid electrical tape is also an option Electrical tape is inadequate MElec-Ch5-25

26 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

27 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

28 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

29 Testing for Stray Current Measuring Stray Current Corrosion Source and Mitigation MElec-Ch5-29

30 Measuring Stray Current Normally AC ground and DC negative connected To measure current, insert ammeter in series AC Gnd DC Neg To Shore Power Bus Bar ABYC Req A Temporary break wire to insert Ammeter Bus Bar To Battery Negative MElec-Ch5-30

31 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

32 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

33 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

34 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

35 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

36 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