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Old 03-10-2008, 07:22   #1
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Stainless & Bronze Underwater -- Problem?

Would there be a potential problem with galvanic corrosion if the only underwater metal on a sailboat was bronze (through hull fittings) and stainless steel (rudder post, gudgeons, pintles)?

Currently, all my through-fittings are marelon, and I was considering whether or not to replace them with bronze.

Thanks.
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Old 03-10-2008, 07:47   #2
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The Bronzes and the Stainless Steels are relatively close on the galvanic series, and would normally be considered compatible.

In a galvanic couple, the metal higher in the series (or the smaller) represents the anode, and will corrode preferentially in the environment. Listed below is the latest galvanic table from MIL-STD-889.
For any combination of dissimilar metals, the metal with the lower number will act as an anode and will corrode preferentially

GALVANIC SERIES in SEAWATER:

Active (Anodic)
1. Magnesium
2. Mg alloy AZ-31B
3. Mg alloy HK-31A
4. Zinc (hot-dip, die cast, or plated)
5. Beryllium (hot pressed)
6. Al 7072 clad on 7075
7. Al 2014-T3
8. Al 1160-H14
9. Al 7079-T6
10. Cadmium (plated)
11. Uranium
12. Al 218 (die cast)
13. Al 5052-0
14. Al 5052-H12
15. Al 5456-0, H353
16. Al 5052-H32
17. Al 1100-0
18. Al 3003-H25
19. Al 6061-T6
20. Al A360 (die cast)
21. Al 7075-T6
22. Al 6061-0
23. Indium
24. Al 2014-0
25. Al 2024-T4
26. Al 5052-H16
27. Tin (plated)
28. Stainless steel 430 (active)
29. Lead
30. Steel 1010
31. Iron (cast)
32. Stainless steel 410 (active)
33. Copper (plated, cast, or wrought)
34. Nickel (plated)
35. Chromium (Plated)
36. Tantalum
37. AM350 (active)
38. Stainless steel 310 (active)
39. Stainless steel 301 (active)
40. Stainless steel 304 (active)
41. Stainless steel 430 (active)
42. Stainless steel 410 (active)
43. Stainless steel 17-7PH (active)
44. Tungsten
45. Niobium (columbium) 1% Zr
46. Brass, Yellow, 268
47. Uranium 8% Mo
48. Brass, Naval, 464
49. Yellow Brass
50. Muntz Metal 280
51. Brass (plated)
52. Nickel-silver (18% Ni)
53. Stainless steel 316L (active)
54. Bronze 220
55. Copper 110
56. Red Brass
57. Stainless steel 347 (active)
58. Molybdenum, Commercial pure
59. Copper-nickel 715
60. Admiralty brass
61. Stainless steel 202 (active)
62. Bronze, Phosphor 534 (B-1)
63. Monel 400
64. Stainless steel 201 (active)
65. Carpenter 20 (active)
66. Stainless steel 321 (active)
67. Stainless steel 316 (active)
68. Stainless steel 309 (active)
69. Stainless steel 17-7PH (passive)
70. Silicone Bronze 655
71. Stainless steel 304 (passive)
72. Stainless steel 301 (passive)
73. Stainless steel 321 (passive)
74. Stainless steel 201 (passive)
75. Stainless steel 286 (passive)
76. Stainless steel 316L (passive)
77. AM355 (active)
78. Stainless steel 202 (passive)
79. Carpenter 20 (passive)
80. AM355 (passive)
81. A286 (passive)
82. Titanium 5A1, 2.5 Sn
83. Titanium 13V, 11Cr, 3Al (annealed)
84. Titanium 6Al, 4V (solution treated and aged)
85. Titanium 6Al, 4V (anneal)
86. Titanium 8Mn
87. Titanium 13V, 11Cr 3Al (solution heat treated and aged)
88. Titanium 75A
89. AM350 (passive)
90. Silver
91. Gold
92. Graphite
Noble (Less Active, Cathodic)

See also “More on Galvanic Table” at:
Corrosion Control - Galvanic Table
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Old 03-10-2008, 08:19   #3
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More than not the problem with galvanic corrosion is stray current more than dissimilar metals. Stray current from improper wiring or corrosion of wiring on the boat and even more so stray current from dockside electrical problems and especially stray current from your neighbor in the next slip.
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Old 03-10-2008, 08:31   #4
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Quote:
Originally Posted by Triton318 View Post
Currently, all my through-fittings are marelon, and I was considering whether or not to replace them with bronze.
Just curious, what is wrong with the marelon fittings to cause you to consider bronze or stainless.

Reason I ask is I am considering marelon thru-hulls instead of bronze.
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Old 03-10-2008, 08:55   #5
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Originally Posted by w42 View Post
Just curious, what is wrong with the marelon fittings to cause you to consider bronze or stainless.
Actually, nothing. In fact, as soon as I hit the Submit button, I thought, "I bet someone is going to ask why I'm thinking of replacing the marelon." The only reason I'm considering it is because it seems that a bronze seacock and bronze thru-hull combination is stronger than a marelon combination. Sometimes when I'm turning the handle on the marelon seacock (which is threaded directly to the marelon thru-hull fitting), I wonder if the threaded portion of the thru-hull is ever going to break off.

Now...maybe they make a marelon seacock that has a flange and could be bolted to a backing plate glassed to the inside of the hull. That might be the way to go.
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Old 03-10-2008, 09:14   #6
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Quote:
Originally Posted by Triton318 View Post
...maybe they make a marelon seacock that has a flange and could be bolted to a backing plate glassed to the inside of the hull. That might be the way to go.
Forespar make Marelon Seacocks:

Forespar: Marine Products that Perform
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Old 03-10-2008, 09:39   #7
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Marelon is pretty dang strong. I saw a video of a guy beating on one with a 5lb sledge hammer and he was taking the worst of it as the hammer rebounded off the Marelon.

Throw in that it cannot corrode nor provide a path out of the boat for lightening and it seems to me to have clear advantages.

The only trouble I have ever heard of is when a Marelon seacocks is installed on a bronze through hull. Different expansion rates would clearly be a bad idea.



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Old 03-10-2008, 11:37   #8
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Quote:
Originally Posted by Triton318 View Post
Now...maybe they make a marelon seacock that has a flange and could be bolted to a backing plate glassed to the inside of the hull. That might be the way to go.
ok, I see your reasoning. Makes sense. Yes Forespar makes a flanged thru-bolted seacock. I currently plan on using them for cockpit drains (be a while yet).
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Old 03-10-2008, 13:16   #9
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Marelon Comparison Chart:
http://www.forespar.com/MarelonPlumb...risonChart.pdf
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Old 03-10-2008, 14:37   #10
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The biggest problem with Marelon is that so many people think it means "black plastic" and what they have, isn't Marelon.

It can be broken, it can be melted...but it will never turn bright pink, punk out and disappear on you the way even the best of bronze can.

Buy a bag of damage control plugs, tie one to each fitting as appropriate, and don't worry about it. (You'd have to do that for the finest bronze, too.)
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Old 03-10-2008, 15:30   #11
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It can be broken, it can be melted...but it will never turn bright pink, punk out and disappear on you the way even the best of bronze can.
For me the "jury is in" on Marelon seacocks. They are just as good as bronze seacocks. Now that is DIFFERENT from marelon ball valves on marelon thru-hulls (with backing nuts), which I would never do in bronze either unless it were above the waterline.

Seen a number of boats come thru the yard with marelon underwater fittings. Some have had them for years. No problems.
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Old 03-10-2008, 15:56   #12
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There is also a big choice of rugged industrial ball valves and fittings in reinforced plastic - even the worst of them are rated at well over 200 psi. The valves are normally of much better construction than the ones aimed at the pleasure marine market.
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Old 09-10-2008, 15:33   #13
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A friend of mine is a consultant for McConaghy (one of the premier yacht builders in the world). He told me recently that they use carbon fibre thru-hulls and epoxy them into the (carbon fibre) hull build... heh
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Old 09-10-2008, 15:47   #14
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CF thruhulls?

I wonder what the R&R procedure following a lightning strike is.<G>
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Old 10-10-2008, 03:37   #15
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A friend of mine is a consultant for McConaghy (one of the premier yacht builders in the world). He told me recently that they use carbon fibre thru-hulls and epoxy them into the (carbon fibre) hull build... heh
I've never seen, nor heard of, carbon fibre thru-hulls.
Could you direct me to any sources/resources?

An interesting sidenote:
Do You Really Want a High Tech Carbon Fiber Boat? ~ by Terry Troxell
Do You Really Want a High Tech Carbon Fiber Boat?
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