capt. david rifkin (usn, ret.) june 2008 madrid, spain

MDY ’08INTERNATIONAL SYMPOSIUM ON

YACHT DESIGN AND CONSTRUCTION

Capt. David Rifkin (USN, Ret.)June 2008Madrid, Spain

Corrosion in general Stray current corrosion, cause and

prevention Is AC or DC the stray current concern? Other general design considerations to

minimize corrosion problems.

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Capt. David Rifkin, Quality Marine Services, LLC

Prevention of corrosion begins in the design stage and is a key factor in the production process. The best plans not followed precisely in production can

manifest themselves in corrosion problems. Assuming maintenance is sufficient, most all

corrosion problems can be traced to poor design, engineering, and production.

All design plans should receive a deliberate review which focuses on potential corrosion problems.

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All corrosion is based on same basic principles. Corrosion requires the following in all cases:

An Anode (where metal ions are lost causing damage) A Cathode (which becomes the protected metal) Electrical continuity between the Anode and Cathode Both Anode and Cathode in the same electrolyte

We give names to different categories and forms of corrosion based on initiation and specific nature. stray current, galvanic, simple, crevice, pitting,

poultice, erosion, impingement, dealloying, stress cracking, etc. 4


1. Anode2. Cathode3. Electrical Path4. Electrolyte

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Requires dissimilar metals in electrical contact and in the same electrolyte.e- e- e-

-1000 mv -275 mv

Zn++Zn++ Zn++

Zn++

Zinc Bronze AnodeSacrificed

CathodeProtectedAlkali is producedat the cathode

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Galvanic corrosion over a long period; estimated 12-18 months 7


Dezincification of a manganese bronze prop; a weak, brittle copper

matrix is left behind.8


Galvanic corrosion; caused by a lack of cathodic protection of the

stern drive unit9


Our focus today Poor design can lead to stray current damage Here’s the basic DC Stray Current mechanism

Acid, O2 produced at the anode

-1000 mv

-275 mv

Zinc Bronze

Cu++

Cu++Cu++Cu++

e- e- e-

Alkali, H2 producedat the cathode

AnodeSacrificed

CathodeProtected

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DC stray current corrosion over several weeks; caused by DC Fault and lack of

engine block bonding11


Same shaft as previous slide; note the terminal

corrosion that caused the electrical fault

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DC stray current corrosion of shaft; the entire marina was affected in this

situation13


DC stray current corrosion over 8 hours; caused by alternator fault and lack of

engine block bonding14


Section of shaft from previous slide sectioned at cutlass bearing; note corrosion is external to

cutlass bearing

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Prop stray current corrosion Note gas generation and corrosion at the anode

(the hull fitting)

Cathodes white with calcareous deposits, anodes corroded

Corroded battery terminal helped determine boat had sunk 16


+ -

Fault Source

Shore Power

Pedestal

Bonding Bus

Hull BusGround Bus

Engine

Hull Fitting

Ion Flow Paths ( )

X

e-

e-

e-

e-e-

e-

e-

e-

e-

e-

To Other Boats

e-

From Other Boats

Corrosion: Grounded dock structures, other boats’ metals, faulted boat prop/shaft

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Shore Power

Pedestal

Bonding Bus

Hull BusGround Bus

Marina Ground

Engine

Hull Fitting

X

e-

e-

e-

e-

Digital Meter

+ -

Ref Cell (Ag/AgCl)

mvdc

Nominal dock potential –0.4 to –0.7vdc

+ -

Fault Source

+1.2v

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Shore Power

Pedestal

Bonding Bus

Hull BusGround Bus

Marina Ground

Engine

Hull Fitting

X

e-

e-

e-

e-

Digital Meter

+ -

Ref Cell

mvdc

+ -

Fault Source

+3.2v

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Shore Power

Pedestal

Bonding Bus

Hull BusGround Bus

Marina Ground

Engine

Hull Fitting

X

e-

e-

e-

e-

Digital Meter

+ -

Ref Cell

mvdc

+ -

Fault Source

-2.5v

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Shore Power

Pedestal

Bonding Bus

Hull BusGround Bus

Engine

Hull Fitting

Marina Ground

All U/W Fittings at Same Potential Stray Current Corrosion

STOPS!

e-

e-e- + -

Fault Source

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+ -

Fault Source

Shore Power

Pedestal

Bonding Bus

Ground Bus

Engine

Hull Fitting

e-

e-

e-

e-

e-

Anode

Corroding

Cathode

Protected

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Capt. David Rifkin Quality Marine Services, LLC

+ -

Fault Source

Shore Power

Pedestal

Bonding Bus

Ground Bus

Engine

Hull Fitting

Potentials in water are equal; Corrosion Stops!

e-

Bonding Wire Added

e-

e-

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Capt. David Rifkin Quality Marine Services, LLC

It always takes 2 faults to have a stray current corrosion cell. First: There must be an electrical fault to

ground. Second: There must be a break in the

bonding system. If the bonding system is intact, an

electrical fault to ground will not result in stray current damage since all underwater metals will be at the same potential. This is a key point of the presentation!

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Key design consideration: Keep all underwater metals at the same

electrical potential. This will prevent any current flow between metals, meaning no stray current corrosion.

Required Specification: Bond all underwater metals together to a

common bus which is then connected to engine negative or the DC negative bus. Load currents must not be allowed to flow in the bonding wires to underwater equipment.

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Presence of DC stray currents in the water can be detected by hull potential measurements. These are made with a multimeter using a reference

cell (Silver/Silver Chloride and Zinc are common types).

Hull potentials which are strongly negative or positive may suggest stray currents are causing corrosion.

Sources can be identified by switching on DC equipment while measuring hull potential. A shift in potential indicates equipment which may be causing stray current damage.

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Yes! But it’s not a common field problem. Many gas oil pipeline studies show AC

causes corrosion on the alloy steel pipeline. Current densities <40A/m2 cause negligible

corrosion Other studies, including our own, have

shown that corrosion by stray AC current in stainless steel, mild steel, lead, copper alloys, iron and zinc, even at extremely high current densities (>1000A/m2), is negligible.

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The exception appears to be aluminum. Aluminum will corrode at 40% the rate

of a like value of DC current. Other metals mentioned will corrode at less than 1% of a like value of DC current.

In the field, it will likely require 10s of amps of continuous AC current to cause damage to aluminum sterndrives. The greater the surface area of the exposed metal, the more current is needed to reach damaging current densities. 28


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Select materials compatible with the application. Stainless steel is a poor choice for continuously immersed metals (example: seawater piping, strainers, hull fittings, valves, fasteners). Copper alloys such as copper nickel or bronze are much better. If stainless must be used below the water line, it should be at least 316 grade or better, coated, and cathodically protected if possible.

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Pipe leakage caused by pitting corrosion inside

stainless seawater piping; most leaks were at or

near weld seams where chromium carbide is

produced in heat affected zones. Stainless steel is

subject to pitting corrosion where water is

stagnant.

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Select materials galvanically compatible with each other. If they are within 200mv on the galvanic scale, the

corrosion rate will be small. It’s always best, however, to isolate dissimilar

metals to prevent galvanic corrosion. Fasteners should always be the same or more

noble than the item they are fastening. This ensure fasteners, with relatively small mass,

will not sacrifice themselves to protect the base metal. This will compromise structural integrity.

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Design should prevent accumulations of standing water on metal surfaces. Standing water promotes simple corrosion of

the base metal. Aluminum must be allowed to breath with

no possibility for water to accumulate/stagnate against the metal. Alternately, the metal must be completely encapsulated by coating. Wet materials (foam, lagging, wood) in contact

with bare aluminum causes poultice corrosion. Uncoated aluminum depends on its natural

oxide coating for protection in air.33


Apply a tenacious coating to metal hulls, and other underwater metals as practical. This reduces the demand on cathodic

protection systems. Install a cathodic protection system

appropriate for the use and economics of the vessel. This, along with good coatings, will protect

underwater metals from corrosion.

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Choose the correct sacrificial anode material based on the boat’s location and materials. Aluminum works in all types of water and is available

for all common outdrive and running gear applications.

Use zinc in saltwater only. Magnesium is limited to freshwater only.

Use enough sacrificial anode to protect the most anodic underwater metal, and to last until the next inspection or maintenance period. Ensuring the hull potential is 200mvdc more negative

than the freely corroding potential of the most anodic metal will protect all the underwater metals.

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Isolate the boat and dock grounding systems with failsafe galvanic isolators. This will prevent a boat’s sacrificial anodes from

protecting other boats connected to the dock grounding system.

Using a failsafe design is critical in maintaining the safety grounding connection between shore and boat.

In “isolated engine block” designs, the block should be bonded with a single conductor at least as large as the alternator positive conductor. The block should not be a current carrying conductor

(the intention of the engine manufacturer). Simply treat the block as one large hull fitting and

bond it as if it were a hull fitting (with single bonding conductor).

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Considering corrosion at the design stage is critical in material performance.

A properly bonded boat, with all underwater metals held at the same potential, is least likely to suffer from stray current corrosion.

DC is the primary contributor to stray current corrosion damage in the field. Aluminum is susceptible to damage from AC stray currents but only at relatively high current densities.

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Capt. David Rifkin (USN, Ret.) Quality Marine Services, LLC USA, 904-382-7868 Email: [email protected] www.qualitymarineservices.net Thanks to James Shafer

(www.marinaguard.net) for photo contributions and peer review of this presentation.


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capt. david rifkin (usn, ret.) june 2008 madrid, spain

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