Mixing metals underwater

In consideration of corrosion control.

I am building a new rudder out of mild steel (saltwater use/older wooden boat). What would be the best choice of metal for the rudder shaft?

Thanks,

Jim

 

Use a metal of the same anodic rating, in this case it would be mild steel, same as the rudder itself.

Now having said that, in practical terms, the use of a stainless steel may be practically a better proposition as it will be easier to look after above the water (cosmetically). Anodes are going to be needed in any case, so you may as well make life easier for yourself and make the whole thing out of stainless steel.
Grade 316 is "suitable". I am still one of those that feel stainless steels do not belong underwater anyhow, I am just answering your question as put.

 

Mild steel will not last very long in salt water, anodes or not. You could try to entomb it in epoxy, but this will likely fail in time as well, though could extend the service period quite a bit. Power coating is an option as is Rhino-Coat which sticks to steel very well. Personally I prefer inert materials for rudders, so you don't have these issues.

What other metals are in the water? Fasteners, struts, shafts, etc.

 

The prop is a type of bronze; the shaft is monel, the rudder shoe is mild steel and I am not sure what to call the cooler but it is made by Walters and appears to be a monel also.
Fasteners are or were galvanized screws. It is these screws rusting that has sparked my interest in doing this rudder project right.

By the way, presently the rudder on the vessel now is mild steel with a stainless steel shaft. In talking with others about this, the consenus seems to be that stainless and mild steel are not a good combination. I stumbled on the forum and thought this would be a good place to get some opinions. Maybe a good coat of something on the stainless rudder shaft would be better than bare metal?

I appreciate the responses.

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Jim,
My fishing vessel has a bronze rudder coupled to a SS shaft. Its 62 years old and will probably last another 62.

 

Suggest making the rudder shaft from stainless steel (316 preferred, otherwise 304), likewise the stub.
Weld the s/s to the mild steel using 316L rods. In addition to the weld itself, wipe (i.e. very light penetration) s/s weld metal approx half an inch onto the mild steel. If required, weld on s/s spacer washers to ensure that the rudder blade never rubs at top or bottom.
After dressing the welds, shot-blast the mild steel, ensuring that the half inch s/s 'overlap' is also blasted to ensure a good key.
Then apply at least three coats of epoxy tar or similar to the rudder, ensuring that the s/s overlap is well coated. Recommend using a plastic bearing between the stub and mild steel rudder shoe.

If there is any chance of the rudder banging against anything and chipping the paint, then wipe that area with s/s weld metal at the fabrication stage too.

If you're rich, then make the whole assembly from stainless steel and galvanically isolate using high density plastic bushings.

If you're concerned about using s/s underwater, then build everything over-size and periodically check.

Colin

 

316 stainless will not be a problem submerged in salt water so long as your boat remains in cool waters (temp around 75 deg F or below). Chloride (salt) corrosion rates have been documented as increasing rapidly as water temperatures rise above 80 deg F.

 

I thought SS was OK as long as new water could pass over it. It's the lack of oxygen in the water that gives corrosion a start.
Boats that bob by the dock go to hell faster than boats that bob on the sea. (a saying my father had). True or no?

 

While I'm not a chemist, Ted, I'm very familiar with the results and somewhat familiar with the process. The chloride corrosion process occurs in vessels used a lot as well as those at rest, and I've observed the results in small and large desalination system piping, in which water flows constantly. Your dad is right about one part of the process, however. What changes in some sheltered waters in the tropics is the salinity of the seawater itself. A combination of increased solar energy and lack of circulation from currents heats the water, increasing the rate of evaporation. Salts don't evaporate, but as more water evaporates, the salts percentage in the remaining water increases. The result can be localized salinity content as high as 39,000 - 45,000 mg/l (milligrams of salts per liter of water), compared to the average open ocean level of 34,500 mg/l. There are places in the Persian Gulf that have been measured at over 50,000 mg/l.


Oversimplified, the corrosion resistance of stainless steels is a result of a chromic oxide passivation layer formed on the surface almost instantaneously in the presence of oxygen. Chloride ions interfere with this process and penetrate the passivation layer. The process is slow enough at cooler temperatures and average salinity to allow a practical lifetime for stainless steel components in marine use. Heat and higher salinity levels accelerate the process, however. I've seen piping failures in less than one year in tropical waters.

 

The choices on offer for stern gear are few: monel, or a similar naval bronze; mild steel, or stainless steel.

In my experience monel can never be found in scrap yards or industrial stockholders, and can only ever be sourced from specialist marine suppliers. Although it's the best material to use, with a 'marine' tag it's expensive stuff. It's also awkward to DIY-fabricate with.

Used underwater, stainless steel is always at risk from crevice corrosion wherever oxygen is denied to it.
But in life, there's always theory (often soundly based) and it's practice: the technique I've described is standard practice on steel Brent Swain OrigamiBoats, many of which have been continuously in the water for 10 years or more, and many of which spend the lion's share of their time in the tropics. Where a pintle should be (say) 15mm diameter in mild steel, simply use 25mm dia stainless steel as belt and braces. Keep an eye on it every time you antifoul, and simply live with the potential problem. Brent has used the technique of stainless steel fittings (including thru-hulls/stand pipes) welded to mild steel both above and below the waterline on his own boat, which has now had over 20 years of continuous immersion without any sign of a problem.

Suprisingly, mild steel remains an option for stern gear. The Wylo II design (again - all-steel, many with continuous immersion for 20 yrs plus) uses all mild-steel stern gear (including propellor) and so dispenses with the need for sacrificial anodes. Again, the designer employs the technique of using over-sized fittings, especially where corrosion can be predicted, with routine inspection of the gear when anti-fouling. Nick Skeates tells me that he's replaced the rudder pintles etc once in 20 years, and has also built-up the prop shaft with weld metal just once during that time. If memory serves, in a non-electrolysis environment exposed mild steel loses approx 1mm to seawater every 7 years - so a predictable maintenance schedule becomes possible.
But it's not my way of doing things (I don't think I could live with the inevitable rust streaks), so I include it here only as a possible option.

Colin

 

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I am not rich - hence the mild steel. Wish I was..

A couple of things. Isn't 316 Stainless higher on the noble list of metals than 304? Aren't I better off keeping metals closer together on the noble chart if I have a choice? Maybe 304 is higher on the scale, I don't know....

And the reason I asked about painting the stainless and not the mild steel would be to help eliminate the nobler (316) metal electrical superiority (I heard somebody say that paint helps stop galvanic corrosion) so that my mild steel would not corrode as fast (if I am making sense). And to save other metals on the boat - mainly my fasteners. I mean I would paint it all but was thinking of a super-durable coat of something on the stainless so that there was not a big difference in electical values in terms of the two metals involved here, (not to forget my fasteners).

I think my fasteners are okay but I see a couple of them bleeding rust now and then then I have to watch things to make it right. My boat was built in 1942 but it's been rebuilt at least 3 times since. It has had an extra set of ribs added too and is really a stout boat.

I should add that I am from Alaska and the waters I operate in are about 50-55 in the summer months and about 38-42 in winter months here in Southeast Alaska. The boat has frequent use; almost daily furing the summer months.

So I like the epoxy tar idea and I think I want to do that. Thanks for your idea's. I hope I am not babbling too much here.

And thank everyone one else who has responded to me (if you have read down this far). I appreciate it.

 

Hi Jim

Off the top of my head I think 316 (aka A4) and 304 (aka A2) are pretty close on the galvanic scale - but don't quote me on that ...

The practical difference between 'em is that although 316 is less prone to crevice corrosion and 'rust blooming', it's a much softer material to work with. Now that doesn't matter a fig where you're fitting something more-or-less permanently in place, like welding a chunk of it to something else, but where susbsequent movement is going to take place, the harder 304 is often preferred. A good example is what happens between the thread surfaces of a 316 nut and bolt: when a 316 surface presses hard against another 316 surface, what can happen is that a small amount of metal is 'picked-up/ rubbed-up' and tends to form a tiny ball betwen the 2 surfaces, and once this happens the process tends to accelerate, until the 2 surfaces start breaking up and become jammed - effectively locking the nut permanently on the thread. Once it starts, it's a completely unrecoverable process.

In food factories around here, 316 plumbing fittings are used a lot and PTFE tape is used on pipe threads to prevent this from happening.

If you're constantly rubbing 316 against a plastic or rubber bearing (or lip seal), the 316 will *eventually* wear away or groove as it's a softish metal. Always a trade-off. Takes many years, but nothing lasts forever.

Ok next point - there are 2 processes at work when you immerse metals within salt-water. The first is straight-forward oxidation, or rusting. Imagine hanging a piece of unprotected mild steel into salt-water on the end of a length of fishing line. It will rust away, but at a relatively slow rate (about 1mm every seven years).

Then there's electrolysis, where 2 dissimilar metals are in contact within a conductive liquid environment. This situation requires a *circuit* to be completed, just as with any other electrical arrangement (light bulb & battery etc). Welding 2 dissimilar metals together provides one 'connection' of the circuit - what is then required is to connnect the 2 metals at another place to complete that circuit so that current will flow. Unless some kind of protection is in place, salt-water will happily provide that secondary connection and thus complete the circuit.

Applying paint is the equivalent of inserting an insulator (or an even better analogy, a switch) into the circuit, which will stop the current of electrons from flowing. It doesn't then matter which surface is painted - either will prevent the circuit from being completed. But as stainless is more-or-less non-corrodable, doesn't it make more sense to paint the mild steel - which at the very least would otherwise passively rust away ?

But why do I say a 'switch' is a better analogy for paint in this situation ? Simply because the 'off' switch will only exist as long as the paint is intact - even a small paint chip is the equivalent of flicking the switch 'on' to allow a few electrons through, thus completing the circuit and starting the destructive process. Therefore it's important to keep on top of the paint job, and epoxy tar is considered one of the best marine coatings for blasted steel. Use several coats to ensure that any 'pin-holes' are eliminated. Within reason, the more coats the merrier. There are numerous other marine products (polyurethanes, Wasser products etc) on the market for covering steel, but discussing the pro's and con's of marine paints would take a thread of it's own !!

In terms of rust bleeding from fasteners - yes isn't it a pain in the butt ? If it's serious, then do check to make sure they're not galvanised and that the zinc coating hasn't been eaten through. If they are 304 stainless (usually a very slightly magnetic steel), then a scrub with something mildy abrasive might do the trick. If it persists than a coating of fish-oil might help.

Hope at least some of the above helps - good luck
Colin

Almost forgot - in my first post, the reason I suggested wiping a stainless rod on any areas at risk from chipping, is that the salt water will only 'see' a circuit from stainless to stainless - and thus no current will flow and the rudder will continue to be protected, despite the chip in the paint.

 

SAE140, your quote:
"A good example is what happens between the thread surfaces of a 316 nut and bolt: when a 316 surface presses hard against another 316 surface, what can happen is that a small amount of metal is 'picked-up/ rubbed-up' and tends to form a tiny ball betwen the 2 surfaces, and once this happens the process tends to accelerate, until the 2 surfaces start breaking up and become jammed - effectively locking the nut permanently on the thread. Once it starts, it's a completely unrecoverable process."

This process is what is known as galling, just for those that have not been introduced to the term, you will know about it practically one day if you have not met it yet!

The solution to the problem is to not use air tools for fastenings, lubricate the threads, and turn the nut slowly, all should help to prevent galling.

Now having said that, and the nut does gall, there is a great 1mm cut off disc available for 4" angle grinders, cut the nut /bolt combo in half along the centreline of the bolt, the nut of course will split in half, and the fastening can then be replaced, no need to try to cut off the nut nicely at 90deg to the bolt, this usually results in damage to the job where the disc touvhes the finished surface.

 

Landlubber - thanks for that - I suspected that there must be a proper name for it ...

I first came across this when a friend inserted a s/s pipe into a ball-valve fitting - "just to see if the thread was ok", as it looked a bit rough. Big mistake.

Much sweating and cursing later, the fitting avec pipe came flying out of the workshop door followed by a torrent of verbal abuse.
Now that ball-valve was damned expensive, but I managed to save the day by dismantling it (to save the seals) and welding the pipe directly onto the valve. Bit of bodge, but I learned a lot that day - well, except the right name for that snafu !
Cheers, Colin

 

I would be too embarrassed to explain my first encounter with the phenomenon, me thinks you got off easy!