T&T: Mixing stainless steel and brass

[Albin43SDtr]

'Lo All,

Corrosion between dissimilar metals requires an electrolyte of some 
sort - impure/salt water is the type of electrolyte we are usually 
concerned with when discussing the mixing of different metals in 
contact with each other.

Based on my observations, albeit non-scientific, of boats, some as 
old as I am that I have worked on, as well as my own boats, the 
conclusion I have come up with is that there will be no/little 
corrosion if SS bolts and screws are used to attach bronze fittings 
if assembled using a marine sealant. The sealant keeps the threads 
and shanks "dry" and if the sealant has squished out around the head, 
and is carefully wiped off, leaving something like 1/32- 1/16 of 
sealant around the edge of the fastener (easy when using countersunk 
fasteners), there will be no corrosion, even after years of  service 
in Florida's salt water environment. Of course, if the fitting was 
subjected to movement, which may break the isolation provided by the 
sealant, then all bets are off.

One boat I worked on and supervised yard work on was an old Army boat 
that was converted to a commercial dive boat. She had huge twin 
6-110s in her. Most of the original bolts were made of silicone 
bronze and often seized and broke when being removed. Others that had 
been replaced with SS never gave us any problems and never showed 
signs of mixed-metal corrosion.

A hurricane damaged the rudder on my old sailboat. In order to remove 
the rudder, the gudgeon plate had to be removed. It was solid cast 
bronze, held on with 8 5/8" SS bolts that had been assembled using a 
sealant. There was absolutely no corrosion evident after being in 
saltwater for over 16 years.

I have removed and reinstalled bronze exterior strainers on several 
boats that had SS fasteners. I do not recall ever seeing any 
corrosion other than that caused by stray electrical currents which 
"dezinced" the bronze metal on a couple of boats, but the area around 
the fasteners, especially under the fasteners' heads were like new or 
at least no worse than the rest of the surfaces.

So...when installing an underwater strainer or whatever, I always use 
a marine sealant (not an adhesive) - usually 3Ms stuff - and 
thoroughly coat the entire bolt or screw, then assemble it. I wipe 
the head so that a small ring of sealant is visible around the head. 
Again, I have never observed a problem, but as usual, YMMV.

Just MVHO based upon my observations and experience.

Take care and be safe.

Wayne
M/V Celestial
Albin43 Sundeck
Panama City, FL area