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

1. Stray Current Corrosion in Yachts Capt. David Rifkin (USN, Ret.) June 2008 Madrid, Spain MDY ’08INTERNATIONAL SYMPOSIUM ON YACHT DESIGN AND CONSTRUCTION

2. 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. Capt. David Rifkin, Quality Marine Services, LLC Topics at a Glance

3. 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. Capt. David Rifkin, Quality Marine Services, LLC Bottom Line

4. 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. Capt. David Rifkin, Quality Marine Services, LLC What is Corrosion?

5. 1. Anode2. Cathode3. Electrical Path4. Electrolyte Capt. David Rifkin, Quality Marine Services, LLC But… They all require the same 4 items for corrosion to occur!

6. Requires dissimilar metals in electrical contact and in the same electrolyte. Capt. David Rifkin, Quality Marine Services, LLC -275 mv Galvanic (cell) Corrosion e- e- e- -1000 mv Cathode Protected Bronze Zinc Anode Sacrificed Alkali is produced at the cathode Zn++ Zn++ Zn++ Zn++

7. Capt. David Rifkin, Quality Marine Services, LLC Galvanic corrosion over a long period; estimated 12-18 months

8. Capt. David Rifkin, Quality Marine Services, LLC Dezincification of a manganese bronze prop; a weak, brittle copper matrix is left behind.

9. Capt. David Rifkin, Quality Marine Services, LLC Galvanic corrosion; caused by a lack of cathodic protection of the stern drive unit

10. Our focus today Poor design can lead to stray current damage Here’s the basic DC Stray Current mechanism Capt. David Rifkin, Quality Marine Services, LLC -275 mv Cu++ Cu++ Cu++ Cu++ Stray Current Corrosion e- e- e- -1000 mv Anode Sacrificed Cathode Protected Bronze Zinc Alkali, H2 produced at the cathode Acid, O2 produced at the anode

11. Capt. David Rifkin, Quality Marine Services, LLC DC stray current corrosion over several weeks; caused by DC Fault and lack of engine block bonding

12. Capt. David Rifkin, Quality Marine Services, LLC Same shaft as previous slide; note the terminal corrosion that caused the electrical fault

13. Capt. David Rifkin, Quality Marine Services, LLC DC stray current corrosion of shaft; the entire marina was affected in this situation

14. Capt. David Rifkin, Quality Marine Services, LLC DC stray current corrosion over 8 hours; caused by alternator fault and lack of engine block bonding

15. Capt. David Rifkin, Quality Marine Services, LLC Section of shaft from previous slide sectioned at cutlass bearing; note corrosion is external to cutlass bearing

16. Capt. David Rifkin, Quality Marine Services, LLC Misc. Stray Current examples 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

17. From Other Boats e- e- e- e- e- e- e- e- - + Fault Source e- e- e- Capt. David Rifkin, Quality Marine Services, LLC Electrical & Bonding Faults Shore Power Pedestal Corrosion: Grounded dock structures, other boats’ metals, faulted boat prop/shaft X Bonding Bus Hull Bus Ground Bus Engine Hull Fitting To Other Boats Ion Flow Paths ( )

18. Digital Meter - mvdc e- e- - + + Fault Source e- e- Ref Cell (Ag/AgCl) Capt. David Rifkin, Quality Marine Services, LLC Fault Corrosion Potentials Marina Ground Shore Power Pedestal +1.2v X Bonding Bus Hull Bus Ground Bus Engine Hull Fitting Nominal dock potential –0.4 to –0.7vdc

19. Digital Meter - mvdc e- e- - + + Fault Source e- e- Ref Cell Capt. David Rifkin, Quality Marine Services, LLC Fault Corrosion Potentials Marina Ground Shore Power Pedestal +3.2v X Bonding Bus Hull Bus Ground Bus Engine Hull Fitting

20. Digital Meter - mvdc e- e- - + + Fault Source e- e- Ref Cell Capt. David Rifkin, Quality Marine Services, LLC Fault Corrosion Potentials Marina Ground Shore Power Pedestal -2.5v X Bonding Bus Hull Bus Ground Bus Engine Hull Fitting

21. e- e- e- - + Fault Source Capt. David Rifkin, Quality Marine Services, LLC Electrical Fault, Intact Bonding Marina Ground Shore Power Pedestal Bonding Bus Hull Bus Ground Bus Engine Hull Fitting All U/W Fittings at Same Potential Stray Current Corrosion STOPS!

22. e- e- e- - + Fault Source e- e- Capt. David Rifkin Quality Marine Services, LLC Isolated Engine Block FaultNo Engine Block Bonding Shore Power Pedestal Bonding Bus Ground Bus Engine Hull Fitting Anode Corroding Cathode Protected

23. - + Fault Source Capt. David Rifkin Quality Marine Services, LLC Isolated Engine Block FaultWith Engine Block Bonding Shore Power Pedestal Bonding Wire Added Bonding Bus Ground Bus e- e- e- Engine Hull Fitting Potentials in water are equal; Corrosion Stops!

24. 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! Capt. David Rifkin, Quality Marine Services, LLC Stray Current Damage; 2 Faults Required

25. 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. Capt. David Rifkin, Quality Marine Services, LLC Preventing Stray Current Corrosion

26. 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. Capt. David Rifkin, Quality Marine Services, LLC Detecting Stray Current Corrosion

27. 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. Capt. David Rifkin, Quality Marine Services, LLC Does AC cause corrosion?

28. 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. Capt. David Rifkin, Quality Marine Services, LLC Does AC cause corrosion?

29. Capt. David Rifkin, Quality Marine Services, LLC Other General design Considerations for preventing corrosion

30. 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. Capt. David Rifkin, Quality Marine Services, LLC Other Design Considerations

31. Capt. David Rifkin, Quality Marine Services, LLC 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.

32. 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. Capt. David Rifkin, Quality Marine Services, LLC Other Design Considerations

33. 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. Capt. David Rifkin, Quality Marine Services, LLC Other Design Considerations

34. 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. Capt. David Rifkin, Quality Marine Services, LLC Other Design Considerations

35. 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. Capt. David Rifkin, Quality Marine Services, LLC Other Design Considerations

36. 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). Capt. David Rifkin, Quality Marine Services, LLC Other Design Considerations

37. 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. Capt. David Rifkin, Quality Marine Services, LLC Summary

38. Capt. David Rifkin (USN, Ret.) Quality Marine Services, LLC USA, 904-382-7868 Email: qualitymarinesvcs@comcast.net www.qualitymarineservices.net Thanks to James Shafer (www.marinaguard.net) for photo contributions and peer review of this presentation. Capt. David Rifkin, Quality Marine Services, LLC Contact information