Galvanic Corrosion by Fasteners

[Target9000]

As our refit continues, I find myself often put in situations where I would either have to go shopping online and wait for long periods or use what I have on hand to continue.

One thing that has bothered me is the use of dissimilar metals, especially fasteners. For instance, I used SS screws to secure the bronze mounting brackets for our raw water strainer.

In other cases I'm using aluminum brackets secured by stainless bolts and screws.

Our dual filter setup uses a SS manifold with a steel fitting then to a brass fitting. Our return and supply selector valves are brass secured with SS fasteners.

I know that this all introduces the possibility of galvanic corrosion. How bad will this be? What type of life expectancy can I expect on these sorts of things and how do you even fasten things like aluminum. I'm not aware of aluminum screws available anywhere near by. What about brass? Sure I could order some bronze fasteners for the water strainer but is it worth it?

Guidance please.

[44'cruisingcat]

You need to use a product such a Duralac to keep the metal of the fasteners seperated from the other metal. If you don't you will almost certainly have galvanic corrosion, particularly where you fasten aluminium with stainless, and it could be very destructive.

:: Llewellyn Ryland :: Bringing Colour To Life ::

[roverhi]

Are we talking above or below the water line?? Below the water line, it would be best to use similar metals if possible. We used Stainless machine screws to fasten our bronze seacocks and bronze pins in the stainless gudgeons and pintles on our rudder without a problem.

Brass should never be used where it will be constantly immersed in sea water. Brass is an alloy of copper and zinc. ZINC, the stuff you put on your under water metal as a sacrificial anode. The same thing happens to the zinc in Brass leaving just the copper.

I also would not use aluminum below the water line except on an all aluminum boat. Aluminum is just too susceptible to galvanic corrosion when immersed in salt water.

SS fasteners in aluminum abpve the water line seems to work okay. You should use a sealant/insulator like TefGel or lanolin on the threads if you want to have an easy time of undoing the fasteners at a later date. I was able to remove the Fasteners from my 40 plus year old aluminum mast and the threads in the mast were still good reusable.

Never ever use bronze or brass fasteners or put bronze like a winch base directly against aluminum. In the case of winches, use something like vinyl window material or electrical tape to keep the bronze from actually contacting the aluminum

[Target9000]

Basically all of this stuff is inside the boat in the engine room. Nothing in the water, but of course still subject to getting wet here and there.

[David M]

Having an aluminum boat myself with dissimilar metals all over it, that is excellent advice from roverhi.

One thing to add is to use Tef-Gel wherever possible. The stuff seems to last forever, more so than Lanocoat.

I use Delrin or sheet rubber when I need to electrically isolate dissimilar metals above the waterline...such as how roverhi describes mounting a winch to aluminum.

[GordMay]

Generally, if you cannot avoid dissimilar metals, you want the larger part (Al. mast) to corrode preferentially to the smaller part (S/S fastener)*.

Stainless steel is generally less active (cathodic & more noble) and higher on the the Galvanic series than is aluminum (anodic & less noble). This is an acceptable solution, especially if an isolating coating is used on the connection.

* To avoid corrosion, avoid a small anodic area relative to the cathodic area.

- Use LARGE ANODE AREA (mast) & SMALL CATHODE AREA (fastener).
The larger the relative anode area, the lower the galvanic current density on the anode, the lesser the attack.

- The amount of galvanic corrosion may be considered as proportional to the Cathode/Anode area ratio.
Design for a SMALL Cathodic/Anodic Ratio (CAR). (When designing, remember your small CAR.)

- The same metal or more noble (cathodic, higher number in the table) metals should be used for small fasteners and bolts.

➥ Galvanic Corrosion Series Chart*of Metals Seawater

[Cheechako]

Stainless and bronze are great together. Aluminum and anything but Aluminum not so great... but for little things you just use stainless anyway. Mixed metals in the fuel system (copper, brass, stainless, steel, racor pot metal) should be good for a long time... I've never seen them corrode much with diesel in them... on the mast use stainless (never bronze) and coat the screws with lanolin (or Lanocote)