Brass - Stainless Compatibility

[Passages]

Need to reattach a bronze (brass?) hold down plate in the bilge. It's not submerged in seawater. Originally secured with a bronze 1/4' bolt & nut. The nut vibrated off and is now lost. My neighborhood hardware store does not carry brass fasteners but said stainless & Bronze are compatible. I can either go with SS locking nuts or SS bolt & nut without issue.

Thought I'd run it by the forum. Is brass/SS OK?

 

[REBrueckner]

In general brass is more subject to corrosion than bronze. You should be able to get a bronze nut from a marine supply store. It's preferable to retain all bronze with bronze components if that's what was original. You can use marine stainless, but that's less desireable with bronze components. Defender and Hamilton Marine online likely sell marine bronze nuts...you want silicon bronze....as original.

 

[REBrueckner]

I should have mentioned: If the hold down plate is not regulalrly in bilge nor other water likely a brass nut will last many,many years....

 

[Nonchalant1]

Bronze, brass and stainless are all compatible. Your prop shafts are stainless, your props are bronze or brass. They connect directly to each other.
The concern is where there can be a current passing through or between them if they're in salt water...then galvanic action will corrode the more susceptible of the two metals. Particularly since yours is not regularly in salt water, any of them will do just fine.

Doug Shuman

 

[luckydave215]

Brass (bronze, whatever, it's made of copper) and stainless are NOT compatible in an electrolyte. We get away with it re our props and shafts only because there is a sacrificial anode (zinc) that is less noble than either of the other metals, and sacrifices (ionic flow to the more noble metal) thereby protecting the shaft/prop. If you combine ANY unlike metals in an electrolyte without a sacrificial anode you're asking for trouble. Even combining two different alloys of stainless (304/316/316L) in seawater is a problem.
Stick with bronze nuts on bronze bolts/parts.

 

[Nonchalant1]

He said it's not in seawater.

 

[luckydave215]

For the purposes of corrosion, anything on a boat must be viewed as if it were in seawater to some extent, as it lives in a very humid invironment, so there is always some ionic flow between the dissimilar metals. That's why the connectors on the back of your salon TV corrode.

 

[ELECTRA VI]

I am little confused on this subject. Recently, I changed out the impellor on my raw water pump (Johnson pump). When I went to put the cover plate (which is stainless steel) back on the pump (which is bronze), I broke the head off one of the screws and could not get a replacement in bronze. I did replace it with stainless steel. My question is, if the pump cover plate is stainless and the pump itself is bronze, is there a problem with stainless screws holding the cover plate to the pump? BYW, I am in saltwater.
George

 

[luckydave215]

Yes, there's a problem with it, but if you assemble the parts with grease smeared on everything, it will likely be ok even though it's wrong. Remember, almost nothing in this world is really done "right".

 

[fixin-to-cruise]

The corrosion resistance is highest with stainless, then bronze with brass last. I'd go with the stainless fastener. If there is any corrosion it will be on the plate, which is probably thick enough that it won't be a problem. A brass nut will corrode off in no time at all. Notice how brass always needs polishing but bronze statues are around forever? A brass nut will turn into a green crusted crumbly copper nut real quick. If the parts were under water I'd spend some time looking for bronze but stailess is ok for this.

 

[luckydave215]

Not true. If you mix stainless bolts with bronze hardware under water (without a sacrificial anode) the bronze and stainless will form a battery with undesireable results. The nobility chart isn't the only thing to go on-it also depends on how much mass of each metal is present.

 

[fixin-to-cruise]

He said "It's not submerged underwater" so I stand by that "If the parts were under water I'd spend some time looking for bronze but stainless is ok for this" (nuts).

Hatteras uses stainless nuts inside the hull on bronze rudder thru-hull castings and the steering linkage - just looked in my bilge.

I'm with you that underwater metals should not be mixed unless unavoidable, i.e. bronze prop on stainless shaft.

 

[luckydave215]

Sometimes there is no choice but to mix metals. The stuffing boxes are a good example. Bronze bolts and nuts aren't strong enough to do the job, and the threads are prone to galling, so stainless it is. But if you have a choice..............