1. Stray Current Corrosion
Chapter 5
Galvanic and
Stray Current Corrosion
PowerPoint slides by
Stf/C Harl Porter, SN
Marine Electronics
Rear Commander for Electro-Mechanical Systems is
R/C Gene Danko, SN
Left is a 4-month old prop badly corroded by stray current corrosion. Shaft was badly pitted and also required replacement

2. Overview Galvanic Corrosion Stray Current Corrosion
Understanding Galvanic Corrosion
Controlling Galvanic Corrosion
Stray Current Corrosion
Understanding Stray Current Corrosion
Preventing Stray Current Corrosion
Testing for Stray Current
Major sections in this chapter

3. Galvanic Corrosion Understanding Galvanic Corrosion Causes Results
Galvanic Series of Metals
Additional Notes
Major topics in this section

4. Causes Requires Two different metals (electrodes)
Immersed in current-carrying solution (electrolyte)
Interconnected by a current-carrying conductor
Fig 5-1

5. Results of Galvanic Corrosion New Zinc Old Zinc after 8 months
(for 1” diameter shaft)
Zinc did what was expected of it, protected the shaft and prop
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
Fig 5-2
Note that values are for Salt water 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
(no notes)

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
Don’t treat the symptom, fix the problem
Fix the problem, at first or second warning sign,
Else will have to eventually replace the severely corroded metal part

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
Major topics in this section

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
(no notes)

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
(no notes)

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
(No notes)

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
Don’t mix types of metal in sacrificial anodes;
Should all be zinc or all magnesium
Magnesium is only used in fresh water
Shaft
Prop Nut
Rudder

14. Powerboat Zincs
6 Zincs
Trim Tab

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)
Shaft brush illustrated latter

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
(No notes)

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
Prevention is Galvanic Isolator (next slide)

18. Isolation Transformer
Galvanic Isolator
or Isolation Transformer
Stops DC current in AC ground wire
Galvanic Isolator
Isolation Transformer
Illustrations from Chapter 4 on AC
Now know why required by ABYC if have steel boat or aluminum outdrive

19. Stray Current Corrosion
Understanding Stray Current Corrosion
Causes
Results
Additional Notes
Major topics in this section

20. Stray Current Corrosion
Requires
External source of electricity
From wetted metal surface (electrodes)
To return circuit of lower potential (electrolyte)
Different from Galvanic Corrosion
Metal (electrodes) can be of same metal
Requires external source of power (battery)

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
(no notes)

22. of Stray Current Corrosion
Results
of Stray Current Corrosion
Fig 5-5
Four months of stray-current corrosion on new 13” bronze prop and 1” diameter stainless steel shaft
Boat with wiring problem was two boats away
Current entered boat via an underwatter fitting, transited the boat via its bonding system, and departed via shaft and prop. Corrosion is at points of departure

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
Stray current corrosion worse than Galvanic corrosion
DC current corrosion worse than AC current corrosion

24. Stray Current Corrosion
Preventing Stray Current
Wiring
Bonding
Battery charger
Galvanic isolators
Isolation transformers
Major topics in this section

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
Best defense is well maintained wiring, especially in / through bilge

26. Bonding Maintain adequate bonding system Propeller shaft bonding
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
Is carbon “brush” at end of metal strip. This carbon brush makes the electrical contact with the shaft, while minimizing shaft wear.

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)
(no notes)

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
(no notes)

29. Testing for Stray Current
Measuring Stray Current
Corrosion Source and Mitigation
Topics in this section

30. Measuring Stray Current
Normally AC ground and DC negative connected
To measure current, insert ammeter in series To
Battery
Negative
DC Neg
AC Gnd
Shore
Power
Bus
Bar
ABYC Req
Temporary
break wire
to insert
Ammeter A
Must temporary disconnect wire between AC ground bus bar and DC negative bus bar and put ammeter between these two bus bars

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
Step by step directions for testing

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
Step by step directions for testing

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
Temporarily disconnect either Galvanic Isolator or Isolation transformer to get and measure stray current
After testing reconnect green AC ground wire to either Galvanic isolator or Isolation 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
(no notes)

35. Summary 1 Types of electronic corrosion Galvanic current
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