Marine Surveyors Lexicon


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Corrosion Survey

By placing the silver:silver chloride half cell of the Corrosion Test Meter in the water and probing the immersed metal inside the hull (thru-hulls, shafts, etc.) with the probe of the meter, the test meter will show whether or not the metal is freely eroding, being protected or excessively protected, or if there is a problem with the bonding wire (loose, broken or not making good contact with the thru-hull). The readings will be low if the metal is unprotected. The readings for all the different pieces will be slightly different if the boat is not Bonded. When the boat is bonded, the readings should all be exactly the same.

To conduct a survey of a boat for galvanic corrosion problems, the necessary tools are a Corrosion Test Meter and a notepad.

Draw a simple outline of the boat. Then probe each metal fitting connected to the water. Write the test meter readings at the location of each on your outline and identify each with symbols. Read each fitting even though the boat is already bonded. A different reading at any fitting means the bonding wire is not correct. Remember the engine has its own body of water and protection. The half-cell is not in its body of water so engine readings do not reflect conditions inside the engine. Check the engine zincs visually. A shaft brush is needed because oil in the transmission often isolates the engine from the prop shaft.

Unbonded bronze fittings will read in the freely eroding range.

After bonding, the readings at each piece should be the same.

After a sacrificial anode is installed to the bonded boat, the voltage should be considerably above the freely eroding range and will continue to rise for 5 to 24 hours depending on the size of the zinc and the area of metal to be protected, until the boat has polarized.

Corrosion Survey Report - Example No. 1

Name of Boat:Foxtail
Hull Material: Wood
Reason for Survey: Owner reports snow-like foam at shaft logs, struts and rudder posts.

Survey Findings:

  • No bonding
  • Separate zincs on shafts and rudders
  • Wood burning from uncontrolled over zincing
  • Wood wet

A.C. Stray Current Present? No
D.C. Stray Current Present? No

Recommendations:: Bond the boat and install shaft brushes. Remove individual zincs. Install zinc at stern. Keep bilge dry. Boat A.C. is not grounded to hull, install Zinc Saver and ground AC green wire to bonding system.

Surveyor's Notes:

  1. Readings on rudder shafts indicates they are being protected by their own zincs (Bronze 500 to 700 protected)
  2. Reading on rudder shaft log means there is a poor connection to the rudder shaft. This should be the same voltage if the connections are good.
  3. The props and shafts have their own zincs and are being protected.
  4. The shaft logs are being protected, but have a poor connection to the shaft. Higher than necessary voltage is burning wood.
  5. The engine has its own body of water and its own zincs. Check zincs.
  6. All bronze thru-hulls are "freely eroding" and should be protected.G. Shorepower pins on boat receptacle have no adverse readings on corrosion test meter, but AC green wire is not grounded to the hull as recommended by ABYC.

Corrosion Survey Report - Example No. 2

Name of Boat:Shockwave
Hull Material: Fiberglass
Reason for Survey: Corrosion on Thru-hulls and heavy corrosion

Survey Findings:

  1. No bonding
  2. Hot wire on A.C. shore cord connected to engine, prop and shaft.

A.C. Stray Current Present? Yes
D.C. Stray Current Present? No


  • Trace and correct A.C. wiring.
  • Replace shaft and prop.
  • Install zinc shaft brush and bond.
  • Leave off zinc collar from shaft.
  • Connect A.C. ground bonding per ABYC.
  • Install Zinc Saver.

Surveyor's Notes:

  1. No protection on bronze rudder and shaft.
  2. No protection on bronze thru-hulls.
  3. Meter flickers on shaft and engine indicating a Stray Current problem.
  4. Pull shore cord and read boat receptacle pins. Find and remove the connection between hot wire and DC ground immediately. Connect green AC ground to bonding system.

Corrosion Survey Report - Example No. 3

Name of Boat:Caramia
Hull Material:Fiberglass
Reason for Survey:Diver reports all thru-hulls forward of engine are corroding.

Survey Findings:

  • Bonding not soldered or sealed and heavily corroded.
  • Bonding wire broken between both engines and cooling intakes.
  • Starboard shaft log leaking. Eight inches of water in bilge. No automatic bilge pump.

    A.C. Stray Current Present?No
    D.C. Stray Current Present?No


    • Rebond boat.
    • Solder and seal all connections.
    • Tighten shaft packing.
    • Wire AC ground to bonding system.

    Surveyor's Notes:

    1. Boat bonded but bonding damaged by corrosion and stress resulting from poor installation. (Readings not the same at each bonded piece).
    2. Forward thru-hulls are not bonded.

    Corrosion Survey Report - Example No. 4

    Name of Boat:Going Home
    Hull Material:Fiberglass
    Reason for Survey:Excessive zinc use. Diver reports corrosion on all thru-hulls and prop.

    Survey Findings:

    • AC "on" readings 320.
    • AC "off" readings 770.
    • AC receptacle on docks reads 30V AC between neutral and ground (should be 0).

    A.C. Stray Current Present?Yes
    D.C. Stray Current Present?No

    Recommendations:: Install Zinc Saver in AC to green wire to engine block. Leave shore cord unplugged. Report dock problem to dockmaster. Renew wasted thru-hulls and propeller.

    Surveyor's Notes: Boat well bonded with all readings the same (320) freely eroding zone. With shore cord connected, all readings are 770. Serious AC stray current problem coming from dock.

    Corrosion Survey Report - Example No. 5

    Name of Boat:Fun Time
    Hull Material:Fiberglass
    Reason for Survey:Diver reports corrosion on port side thru-hull.

    Survey Findings:

    1. Bonding connections not sealed
    2. Galley sink drain corroding.
    3. Six inches water in bilge.
    4. No shaft brush.
    A.C. Stray Current Present?(No AC)
    D.C. Stray Current Present?No


    • Re-bond galley sink thru-hull.
    • Install shaft brush.
    • Seal all connections.
    • Keep bilge dry.

    Surveyor's Notes: Boat well bonded except for port side bronze thru-hull fitting and no shaft brush. Port side thru-hull, prop and shaft corroding.

    Corrosion Survey Report - Example No. 6

    Name of Boat:Lot o Fun
    Hull Material:Fiberglass
    Reason for Survey:Engine cooling seacock does not close. New battery loses charge rapidly.

    Survey Findings:

  • No bonding
  • Radio ground wire attached to positive battery terminal and to cooling water intake.
  • AC green wire not wired to engine and bonding system as recommended by ABYC.

    A.C. Stray Current Present?No
    D.C. Stray Current Present?Yes

    Recommendations:: Remove ground wire from positive terminal and attach to negative terminal. Replace intake vale. Bond boat. Install shaft brush. Connect AC green wire to bonding system.

    Surveyor's Notes:

    1. Boat not bonded and all bronze freely eroding.
    2. Shaft has zinc collar.
    3. Extremely serious DC stray current problem when +8 volts discovered. Valve will quickly corrode away and cause rapid flooding of the boat.
    4. Readings indicate no AC wiring is connected to the DC ground.