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Posted by on Aug 18, 2012 in Ask the Expert, Boat Maintenance, How To, Powerboating, Sailing, UK, US |

Keep Aluminum and Brass Separated in the Marine Environment

Thread sealer alone won't keep these two metals apart, and they're far enough from each other on the galvanic scale for corrosion to occur if exposed to seawater.

Question: My boat has an aluminum fuel tank. The other day I was checking out the fuel lines connected to the tank because I could faintly smell diesel inside my boat and I wanted to make sure the connections were tight and that none of the hoses were leaking. Although I didn’t find a leak anywhere, I did notice that the brass fitting in the middle of the photo was screwed directly into the aluminum. It looks like it had some kind of thread sealer on it (the brown stuff). The other fittings you can see look like they have stainless steel bushings that the fittings are threaded into. Those look right to me. I’ve been told that aluminum and brass don’t mix in the marine environment. Any tips on this?

The brass fitting in the middle is screwed directly into the aluminum. This is asking for trouble if salt water enters the mix.

Answer: Nice work, you have made an observation that the average boater would never have made. You are quite correct, aluminum and brass should never be in direct contact with one another in the marine environment. The mix is a guaranteed prescription for corrosion, and in the case of fuel fittings, probable breakage of either the tank or the fitting when you attempt to disassemble the connection.

The problem is that the two metals are just too far apart on the galvanic scale. The farther apart metals are from one another on this scale, the more dissimilar they are and the more likely they are to corrode if connected. One will become anodic and the other cathodic. In this case, the aluminum will become the anode, and as most boaters understand, anodes dissolve in seawater.

In your case, the fittings with the extra bushings shown are done correctly and as the ABYC would recommend in its standards for this type of assembly. Whoever the installer was here may have assumed that the thread sealer would provide adequate separation of the metals, but that is simply not the case. Once the fitting gets tightened properly, all of the sealer will be squeezed out of the threads, putting the brass in direct contact with the aluminum.

Now, in truth this may not be a problem because one additional thing is needed to induce corrosion and that is an electrolyte — seawater. So, if the sealer as shown keeps seawater from coming in contact with the threads then all might be well, but I wouldn’t count on it. Get another bushing like the ones visible on the other fittings, and make sure it is made of 300-series stainless steel to ensure a proper interface between the brass and aluminum.

- Ed Sherman