Marrying Stainless and Bronze

BCC Forum
1

Most of us don’t like to see our ss fittings on our boats and rigging get rusty and streak rust stain down the hull or on deck.

Brion Toss told me that under a rust stain on ss, a pit in the surface of the metal is or has formed, and you can see this with a 50 X hand held microscope.

Ok, so I placed my 50 x scope order with Edmund Scientific, and verified what he said, as I checked rust spots on ss.

This led me on a quest to try to remedy the rust forming problems on my ss.

At that time circa 1987, there wasn’t much information available to me and I had to get a bit here and a bit there, but now there is much more information available, like in the sailboat rigging section, (West Advisor) in the West Marine catalog, and many places elsewhere.

The worst rusting areas were on the welded ss stansion bases, around bolts and toggle pin holes on chainplates, and on the welds on the cranze iron and boomkin chainplates.

I found that after cleaning the rust off the ss, with a ss pickling paste, and then paste waxing where I could, this reduced my rusting problem, but I also started experimenting with marrying bronze with the ss, and found that really helped reduce rusting too.

Where it was easy, I replaced ss bolts with silicon brnz bolts, or put silicon brnz washers under ss bolts, or on clevis pins between chainplate tangs and ss toggle cheeks.

I still do get rust spots/pitting on open areas on the ss surfaces, but almost none around the bronze to ss contact areas.

There is a heap of stuff, I haven’t said here, but marrying bronze with ss has helped reduce maintaince on my BCC.

Douglas

2

Doug, Interesting comment. In your research did you find anything out about galvanic action between the two? Silicone Bronze is third from the top of the Galvanic Scale with Mercury on top and Monel second. That means that all metals below them will sacrifice to these metals. Stainless steel is about number 8. This makes the stainless steel the anode and the bronze the cathode. The saltwater environment accentuates this action. I would be concerned that the bolts or clevis pins could sacrifice under the bronze and would not be noticed. However, electrolysis will keep everything clean in appearance.

Since Zinc is at the bottom of the Galvanic Scale, it will sacrifice to all other metals. An idea is to use brass washers. Brass is a combination of copper and zinc. This puts it quite low on the Galvanic Scale. It would sacrifice to the stainless steel. I am not disagreeing with you but only providing another prospective. I would be curious what the more professionals out there have to say.

Roger

3

Hi Roger, T Y , for the reply and info on the galvanic scale.

You know, I never even thought about the galvanic scale when I replaced my boat’s ss toggles, (that were rusting) with the eye jaw bronze toggles, that I purchased from West Marine.

My boat was on a mooring, and it only had salt water wash downs, so I was able to observe, that the bronze toggle to ss chainplate virtually stopped the rusting problem on the ss near the clevis pin holes.

I was also amazed that the bronze toggles came equiped with ss clevis pins, that never rusted, but my ss toggles with ss clevis pins went rusty quite soon, after installation.

When I started replacing ss nuts and bolts on my dodger and lifeline tube end fittings, the rusting problems there, seemed to stop, too.

Since my pre-purchase surveyor told me that as a standard practice, he reccomends replacing all ss structural fittings after 10 years, for safety reasons, I decided that I am willing to replace my bronze structural fittings too, but at least I don’t have to fight a rusting problem in and around the connections, anymore .

B T W , didn’t the Morse Co., use silicon bronze bolts on the lower gudegons attached to the dead wood area, for the rudder ? I noticed that these bolts were turning pink , so I replaced them with new silicon bronze bolts, but if silicon bronze has no zinc in it, how can it turn pink by losing an alloy it doesn’t have ?

Douglas

4

Hi Doug. Since the gudgeons and pintles are all cast in silicone bronze, we only used silicone bronze mounting bolts (I have no idea what was used before my watch). If “only” the bolts turned pink, that is electrolysis. Which means that the bolts were not silicone bronze. If the whole gudgeon and bolt turned pink then there is some stray electricity in the water. There have been many boats that have had severe damage to metal under the hull as a result of insulated electrical wires submerged next to the boat. That said, your bolts could have have been manganese bronze without knowing. Manganese bronze is lower on the galvanic scale than silicone bronze. The problem is that a person can order silicone bronze and might even get aluminum bronze which is even worse. Even worse yet would be to use brass (copper and zinc). There is no way for the “layman” to know the difference just by looking. Sometimes they are lighter in color or yellow like brass but if they have been setting out for awhile, they all turn that brown color.

Just because a metal does not have zinc does not change the electrical charge of the molecules of the metal. All metal molecules have a charge; some are more severe than others. At the extreme ends are zinc and monel. Anytime any dissimilar metals are in contact there is “some” galvanic action that takes place. Galvanic action on metals without zinc can be seen everywhere. Look what happens if you leave uninsulated stainless steel fastenings in an aluminum mast. To use copper or its alloy would be worse. Have you ever used brass (copper and zinc) fastenings against stainless or other metals on the exterior of your boat? If you have, you know that the brass with soon turn pink. When you try to remove it, it may break. If not, it will show signs of deep pitting. This is the zinc leaving the copper. Have you ever bit on a piece of aluminum foil and have it contact your filling?

However, if the metals are fairly close to each other and are in an open environment, the damage would hardly be noticed. However, in a salt water environment, it could be more rapid and more severe. To be honest with you, I could be entirely wrong on this. However, this is what I thought to be the case with all metals. What I do not know, does bronze and stainless steel react to one another to the point to be a concern…where are the metallurgist when you need one.

Normally, I would take this subject to our private email box. However, this is an important subject and others might be interested in the results. Also, I hope some of the more professional engineers might step up and put their thoughts to the subject.

Roger

5

Doug & Roger:

I am enjoy this thread. If you plug into shore power, unless you have an isolator, you are connected to all boats in a marina that are plug into shore power. Hence, your zinc anodes are protecting all the boats. Once the zinc is gone, then the bronze becomes the anode.

Silicone bronze is not alloyed with zinc but many bronzes contain besides tin, lead and zinc.

It has been ages since I studied electro-chemistry in my analytical chemistry class but I believe the potential between two metals has to be at a specific difference for galvanic corrosion to occur.

Rod

6

Hi Roger and the group,
Our 1981 (#59) BCC didn’t have bushings on the pintles and after about 10 yrs, there was enough wear to cause an annoying noise if the rudder encountered even small chop from the side. I asked Sam what they were made of, and he said “naval bronze”. When I looked up naval bronze, it turns out to be a brass, as it has some zinc in it. The machine shop ordered me a piece of 1" rod for the pintle, and they pressed that in and and also made up delrin (?) bushings. It would be interesting to know when the switch from naval bronze to silicon bronze was made. Our original prop shaft was also naval bronze according to Sam, and that lasted about 10 yrs. It was replaced with a modern stainless alloy one.

The rudder fittings have worked great for the last about 17 years, with no sign of metal loss. Although I have fastened a small zinc to the bottom gudgeon and pintle, and to the middle pintle, I don’t know if it is necessary. Jack Seipel on Spicer (#58?) didn’t use zincs on his gudgeon and pintles for about 20 years and said he had no corrosion problems with them.

As has been mentioned, being in a marina with faulty nearby wiring can cause big problems quickly. It seems to happen very rarely though.

Dan

7

Hi everyone, I think that I would have the galvanic corrosion problems that Roger refered to, it I tried the marrying of the two metals under water especially sea water.

In open salt sea air , I have noticed the bronze to corrode or deteriorate more rapidly, going beyond just a green patina, to a more flaky green sluff off, when in contact with ss .

On my BCC ss cranze iron, the whisker stays were attached with bronze eye jaw toggles, which did show on their surface, more corrosion, than like what my port holes showed, but never the less, there was less rusting on the ss around them .

For me that was good, meaning less maintaince/polishing/waxing on the ss cranze iron.

As for my rudder lower gudeon it is vintage 1985 or before, sooo Naval Brass, it could be made of, as I did notice some open areas on the surface between the bolts, that were turning pink, too.

Worring that de-zincification was taking place, I did tap and screw a zinc on it too. Hope this stops the pink from spreading, next haul out will tell !

Douglas

8

Douglas:

IDUNA has a zinc attached to the lower “bronze” pintel with a length of SS all-thread. The SS all-thread is also used to secure the pintel to the rudder. I have not noted any pink coloration on any of the pintels or gudgeons.

We also have two zincs on the transom just at the waterline. These are located on each side of the rudder and are bonded to the engine and one dynaplate. The second dynaplate is the SSB antenna counterpoise and is not bonded to the boats ground system I have never seen discoloration on IDUNA’s “bronze” prop, stern bearing or dynaplates. IDUNA’s hardware, fittings and fasteners are “bronze.”

IDUNA have never had a shore power hookup nor will she. If I were to install shore power, I would install a galvanic isolator in the circuit to prevent DC current in the green ground wire.

Hope that informaton helps.

Rod

9

Roger: Hi!

Silicone Bronze is third from the top of the Galvanic Scale with Mercury on top and Monel second. That means that all metals below them will sacrifice to these metals. Stainless steel is about number 8. This makes the stainless steel the anode and the bronze the cathode. The saltwater environment accentuates this action. I would be concerned that the bolts or clevis pins could sacrifice under the bronze and would not be noticed.

I think something is wrong with your copy of the Galvanic Scale. Your copy is muddling 316 ss with other, more anodic, ss flavours.

If you measure electric potentials in coastal seawater and use a silver/silver chloride half cell as your reference, then passive 316 stainless steel (with a corrosion potential of between 0.0 and -0.10 volts) is more noble than silicon bronze (cp of -0.26 to -0.29 volts) which is more noble than active 316 stainless steel (cp of -0.43 to -0.54 volts) and zinc (cp of -0.98 to -1.03 volts).

A passive 316 ss propeller shaft is more noble than a bronze propeller, according to the galvanic scale. The bronze prop is the anode, the ss prop shaft the cathode. The bronze will be sacrificially lost to ‘protect’ the ss.

I can back the theory with practice: I recently helped the owner of a catamaran who did not have zinc anodes on his two ss prop shafts. Both of his bronze props were junk (one had two blades, the other had one - the bronze had lost strength and the missing blades broke off).

As long as we’re talking passive 316 ss, bronze (regardless of whether it’s silicon bronze or an alloy that should more strictly be called a brass) will act as a sacrificial anode to it.

For an authoritative statement of the galvanic scale, plus a practical guide to using a multimeter and a silver/silver chloride half cell to check out your boat and marina dock wiring, I recommend Nigel Calder’s “Ground Fault, Interrupted” in Professional Boat Builder, April/May 2006, pp. 56 - 79 (http://www.proboat-digital.com/proboat/200604/).

Cheers

Bil

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Galvanic Series:

http://corrosion-doctors.org/Definitions/galvanic-series.htm

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Ah-Ha !!! Gosh, Thank You, Bil and Rod, at last I understand why my Silicon Bronze 655 series, corrodes faster as the anode, and reduces rusting corrosion on my ss fittings.

Now my attention turns to how to discover, when my ss fittings are “active” or “passive” ?

I have read that “active” ss, is when corrosion including rust corrosion is taking place, as it is easy to see rust on the ss surface.

Could I safely assume that if my ss fittings are bright and shiny, with no other signs of corrosion or surface contamination, that they are passive ?

When I use a ss pickling paste (nitric-hydroflouric-phosphoric acid combo) on my ss fittings, they turn more nickel-gray and dull, then I have to polish them shiny again, at this point, can I assume that I have passivated the ss ??? at least until I see rust corrosion happening again ?

Douglas

12

Does polishing remove the passive layer left by the chemical treatment?

We also need to give much credit to Roger. He set the wheels in motion regarding galvanic scale. I suspect the galvanic scale he used did not cover all metals.

The subject delves into electro-chemistry and what knowledge I had from my electro-chemical analytical course is buried somewhere beyound my mental reach. Although electro-chemistry is science, it is also industrial art. Hence, I understand the rudiments of the science but would never consider myself knowledgeable or and expert on the subject.

There are three steps to invention, conception, development and implimentation. This is well demonstrated in this forum thread.

Now to bicycle to the boat. By-the-by, the gas mileage for the engine in my bike is 700 mpg.

Cheers,

Rod

P.S. America is going from the car to the bike and China is going from the bike to the car.

13

Thanks to Doug for bringing up a good question and to the others for their excellent responses. I found the “Corrosion-Doctors” list of galvanic metals most interesting. It certainly expands my list. To conclude my input, I doubt that Doug would have any problem above the waterline but…just in case, using brass would be safer if it has the same cleaning effect. However, do not use brass below the waterline.

Thanks again for all the input…I learned a lot.
Roger

14

Douglas: Hi!

I have read that “active” ss, is when corrosion including rust corrosion is taking place, as it is easy to see rust on the ss surface.

One way that ss gets activated is when it’s starved of oxygen, eg trapped in stagnant seawater in a crevice. A handprint, with natural oil and grease trapping salty sweat on the surface of the ss, can do it too - the natural greases and oils stop air getting to the ss, the sweat provides the Cl ions and water to corrode the ss. That’s why we need to keep polishing ss rails and stanchions.

Sealing passive ss so neither water nor oxygen can get to it, such as coating the ss with lanolin grease, preserves the ss.

When I use a ss pickling paste (nitric-hydroflouric-phosphoric acid combo) on my ss fittings, they turn more nickel-gray and dull, then I have to polish them shiny again, at this point, can I assume that I have passivated the ss ??? at least until I see rust corrosion happening again ?

Last I read the literature, both polishing and pickling passivate the ss, working in different ways to remove microscopic lands that are Fe-rich.

I suspect that one treatment (either pickling or polishing) should be enough and that Rod is correct that polishing after pickling negates the effect of the pickling, but then again passivates the ss.

I never have enough time on my hands to do both. And pickle the parts that I cannot easily polish.

However, all experts say that clean and mirror-shiny ss is more likely to stay passive than ss that has the tiniest hollow or scratch.

Finally, at the risk of boring you to annoyance, I really really recommend that you (1) read Nigel Calder’s article (see above for reference; yes, I know that ProBoatBuilder is hard to read on the Net) and (2) buy an Ag/AgCl half cell to add to your multimeter. With an Ag/AgCl half cell dangling in the water, you can use your multimeter to measure the corrosion potential of your underwater parts (that’s why Calder’s article has the cp, in volts, for each metal - you can tell if your zinc anode needs to be replaced or resized; you can tell if your ‘bronze’ thru-hull is bronze or not).

Cheers

Bil

15

An afterthought …

If you would like an MS Word document (160 KB, text only - no diagrams) of selected key parts of Calder’s article on testing wiring of your dock and boat (for academic use only, of course), send me a PM with your e-mail address.

Cheers

Bil