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Old 15-03-2015, 23:08   #1
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Bronze rudder hardware on stainless rudder stock?

My boat's rudder is skeg hung, and the stainless mounting hardware (gudgeons?) that connect the rudder stock to the skeg has suffered from corrosion on the inside surfaces.

I'm about to pull the trigger on having some silicon bronze replacements machined, but I'm a bit worried about the stainless rudder stock having a galvanic interaction with the bronze hardware.

The rudder hardware clamps down on either side of the rudder stock, with virtually no play between them. The pieces that clamp together look a bit like these: Imgur

I was thinking about making them a bit looser around the shaft so that I could somehow get a delrin tube between the two surfaces to isolate them, but I'm not exactly sure how to go about that, and don't know if its neccessary. I could try painting the bronze with some kind of low friction paint, but I'm not sure what I could use in this application of how long it would hold up.
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Old 16-03-2015, 00:03   #2
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Re: Bronze rudder hardware on stainless rudder stock?

Maybe I'll buy some of this teflon powder, mix it with epoxy, and paint it on the silicon bronze surface. I'll leave the stainless shaft bare so that it can get some oxygen and stay passivated. If the epoxy eventually chips off I'm no worse off than if I did nothing. Teflon seems like itd be better in this case than graphite since graphite is electrically conductive so doesn't seem like it would do a good job of isolating the metals. PTFE Powder Lubricant for Grand Piano Knuckles | eBay
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Old 16-03-2015, 15:48   #3
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Re: Bronze rudder hardware on stainless rudder stock?

Red point,

Bronze and stainless are close enough galvanically that it isn't enough to know the generally alloy used you need to know exactly which alloy number you have to tell you if there is even a possibility of a problem. Technically it is possible, but it would be very minor if there is one.

Siliconized bronze runs around -.15v
316 stainless is between 0 and -.1v

The general threshold for galvanic compatability is within .15v.


The more likely issue is you are seeing poltice corrosion, and there isn't much that can be done to stop that except replace it timely.
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Old 19-03-2015, 01:11   #4
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Re: Bronze rudder hardware on stainless rudder stock?

The silicon bronze is CDA 655 or C6550. There seems to be two numbering schemes. Hopefully in the worst case I might get some minor interactions over a long time period. Probably overthinking it but I think I'll end up painting the bronze surface that touches the steel with some nonconductive material.
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Old 19-03-2015, 01:21   #5
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Re: Bronze rudder hardware on stainless rudder stock?

With that alloy I don't think you will have an issue, but it is always worth isolating contact where you can. While it's available re-pasivation the stainless is also worth doing if you can.
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Old 19-03-2015, 06:51   #6
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Re: Bronze rudder hardware on stainless rudder stock?

put in a local zinc anode
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Old 19-03-2015, 07:14   #7
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Re: Bronze rudder hardware on stainless rudder stock?

If you need a few pieces of bronze plate, let me know... I've got 10 pieces left over that are 2 inches wide, 14 inches long and 1/4 thick in 655.

I was going to have gudgeons and pintles machined, out of rod stock and then have the plate tig welded to the gudgeons and pintles as the raw cost was lower than machining from bar stock.

Cheers,

Zach
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