Crevice Corrosion

[Triton318]

Does anyone know if silicon bronze is less susceptible to crevice corrosion than stainless steel? (This is for fittings constantly submerged in saltwater.)

[roverhi]

Yes it is. In fact think it's impervious to crevice corrosion. Just saw a movie about Salvage operations on a ship that went down in 1865. The bronze gudgeons and pintles looked as new after 140 years on the bottom.

Aloha
Peter O.

[David M]

There are different grades of stainless. Use silicon bronze if you are not concerned about it corroding less nobel metals such as aluminum. I have stainless steel ball valves on an aluminum hull and I have never see any metal loss on the stainless valves.

[delmarrey]

Bronze is always better below the waterline then stainless. At one time they use to make prop shafts from bronze but stainless has proved to be a stronger, more ware resistant then bronze. But you have to keep the zinks on the SS.

Now with the new aqualoy's SS will last even longer. But thruhulls are better bronze or Marelon!

[MidLandOne]

Originally Posted by delmarrey

But you have to keep the zinks on the SS.

Well, not really, there is no reason why the ss has to be protected, nor in fact the prop if they are compatible.

As one example, our own boat has ss shaft and manganese bronze prop and has never in its 12 year life had nor needed an anode on the shaft or prop as both are happy together. Just needs a bit of corrosion knowledge to achieve and not taking too much notice of advice on forums .

Another example, how many have anodes on their ss rudder stock.

I'm with David on this, ss of the correct grades is fine underwater and is widely used in high quality vessels.

[delmarrey]

Originally Posted by MidLandOne

Well, not really, there is no reason why the ss has to be protected, nor in fact the prop if they are compatible.

As one example, our own boat has ss shaft and manganese bronze prop and has never in its 12 year life had nor needed an anode on the shaft or prop as both are happy together. Just needs a bit of corrosion knowledge to achieve and not taking too much notice of advice on forums .

That maybe true for many boats but not for all. Wouldn't it be better to play it safe rather then discount it totally? If that were the case then why do they even make shaft zinks?

I have machined prop shafts that had worm holes in them running longitudinal from erosion/electrolysis. And that was enough to convince me. As well, my old bronze prop was full of pit holes after 20 years and that was with zinks. So, I will continue to use zinks for as long as I have any metal below the waterline.

Isn't your boat steel?? Maybe your hull is acting as an anode??

[MidLandOne]

Was only the inference that one must have zincs on ss that I was referring to - proper selection of materials will not require that. Where poor materials or a poor combination of metals has been specified then anodes will be necessary. In some few cases vulnerable mixed metals come with the territory eg sail drives, variable props, etc and anodes will be necessary.

As I said, for the shaft and a bronze prop in a well specified boat it should not be necessary to use anodes on them unless to protect vulnerable mixed metals in a variable prop.

The common situation where anodes are fitted in association with ss is on alumnium vessels but in that case the anodes are to protect the aluminium.

Yes our boat is steel. Apart from the prop shaft (with manganese bronze, which is actually a brass, prop) and rudder stock we have some other ss underwater welded to the hull itself - the only care needed is to treat the transition at the weld from ss to the mild steel as mild steel for a short distance over the ss due to carbon migration from the mild steel into it, but that is just a matter of extending the hull plating coating system an inch or so over the ss part.

We only have one light anode on the boat, that on one side of the fin keel and it lasts easily 2 years and we have no galvanic or electrolytic corrosion problems whatsoever. The anode loss is probably mostly at its natural corrosion rate in seawater so it is in fact likely redundant.

I think it would be quite possible to build an alumnium boat without anodes (and I believe our own steel one would be fine without) but due to the vulnerability of aluminium is probably not a secure approach.

So it is quite possible to specify a boat, even of steel, that does not sail around with a crop of anodes connected to the other underwater metals, turning the whole boat into a zinc/bronze/ss etc, salt water electrolyte battery with short circuit currents flowing around it all (the anodes and the "protected" metals form short circuited batteries through their bonding conductors). One just has to know what one is doing and not believe everything one reads about corrosion in forums, because it is mostly incorrect.