MElec-Ch5 - 1

1
Chapter 5

• Galvanic and
• Stray Current Corrosion

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

3
Galvanic Corrosion

• Understanding Galvanic Corrosion
• Causes
• Results
• Galvanic Series of Metals
• Additional Notes

4
Causes

• Requires
• Two different metals (electrodes)
• Immersed in current-carrying solution
  (electrolyte)
• Interconnected by a current-carrying conductor

5
Results
of Galvanic Corrosion
New Zinc (for 1 diameter shaft)
6
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
7
Additional Notes

• Expect corrosion with 0.25 V difference
• Most negative electrodes will decompose
• Magnesium _at_ - 1.50 V for freshwater
• Zinc _at_ - 1.03 V for saltwater
• Aluminum _at_ - 0.75 V will decompose if neither
  magnesium or zinc are present
• Zinc (or magnesium) will protect
• Stainless steel shaft
• Bronze propeller
• Aluminum outdrive

8
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
• Dont treat the symptom, fix the problem

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

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

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

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

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
14
Powerboat Zincs
15
Indirect Cathodic Protection

• Used when direct contact not possible
• Zinc bolted to outside of hull
• Inside boat connect with insulated AWG8 to
• Rudder Post
• Shaft (requires shaft brush)

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

17
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

18
Galvanic Isolator
or Isolation Transformer

• Stops DC current in AC ground wire

19
Stray Current Corrosion

• Understanding Stray Current Corrosion
• Causes
• Results
• Additional Notes

20
Stray Current Corrosion

• Requires
• External source of electricity
• From wetted metal surface (electrodes)
• To return circuit of lower potential (electrolyte)

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 boats 12 volt battery and
  defective wiring
• External to boat from another source of DC

22
Results
of Stray Current Corrosion
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

24
Stray Current Corrosion

• Preventing Stray Current
• Wiring
• Bonding
• Battery charger
• Galvanic isolators
• Isolation transformers

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 2nd choice
• Liquid electrical tape is also an option
• Electrical tape is inadequate

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

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)

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

29
Testing for Stray Current

• Measuring Stray Current
• Corrosion Source and Mitigation

30
Measuring Stray Current

• Normally AC ground and DC negative connected
• To measure current, insert ammeter in series

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

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

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

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

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

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