This document discusses two types of corrosion that can affect boats: galvanic corrosion and stray current corrosion. Galvanic corrosion occurs when two dissimilar metals are immersed in a conductive solution and are electrically connected. It can be controlled by proper selection of compatible metals. Stray current corrosion is caused by an external electric current and is more destructive. It can be prevented through proper wiring and isolation devices to stop electric current flow in the boat.

1. Type of Corrosion
STUDENT :VASAVA HARSH KANCHANBHAI
BRANCH:MECHANICAL 4 SEM
1 SHIFT

2.  Galvanic Corrosion
 Understanding Galvanic Corrosion
 Controlling Galvanic Corrosion
 Stray Current Corrosion
 Understanding Stray Current Corrosion
 Preventing Stray Current Corrosion
 Testing for Stray Current

3. Understanding Galvanic Corrosion
 Causes
 Results
 Galvanic Series of Metals
 Additional Notes

4.  Requires
 Two different metals (electrodes)
 Immersed in current-carrying solution (electrolyte)
 Interconnected by a current-carrying conductor

5. New Zinc
(for 1” diameter shaft)
of Galvanic Corrosion
Old Zinc after 8 months
(for 1” diameter shaft)

6. 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.  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

8.  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

9. 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.  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.  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.  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.  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. 6 Zincs
Trim Tab

15.  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)

16.  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.  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.  Stops DC current in AC ground wire
or Isolation Transformer
Isolation TransformerGalvanic Isolator

19. Understanding Stray Current Corrosion
 Causes
 Results
 Additional Notes

20.  Requires
 External source of electricity
 From wetted metal surface (electrodes)
 To return circuit of lower potential (electrolyte)

21.  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

22. of Stray Current Corrosion

23.  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. Preventing Stray Current
 Wiring
 Bonding
 Battery charger
 Galvanic isolators
 Isolation transformers

25.  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.  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. • 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.  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.  Measuring Stray Current
 Corrosion Source and Mitigation

30.  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

31.  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 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.  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.  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.  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.  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