Brass!!!

Mods and custom builds
3f8
Doator '08, '09, '10, '11, '12, '13, '14, '15, '16, '17
Posts: 177
Joined: Tue Mar 24, 2009 7:12 pm
15

Brass!!!

#1

Post by 3f8 »

Unhappy find. Yes boys and girls that is a brass fitting on an aluminum nipple in a deck bilge.The other pic is the nipple from a line already removed. It was on the unused kicker filter supply. First pic is my main fuel. Can't wait to pull it apart.
bad_brass.jpg
bad_brass.jpg (210.52 KiB) Viewed 3993 times
P1000528.JPG
P1000528.JPG (264.17 KiB) Viewed 3993 times
I don't know the outfitter I'm the second owner. All the Stabicraft factory fittings were done in aluminum. The fitted fuel filter lines is were the brass came in.
"It was the Law of the Sea, they said. Civilization ends at the waterline.
Beyond that, we all enter the food chain, and not always right at the top."
Hunter S. Thompson
capesteve
Posts: 94
Joined: Tue Apr 23, 2013 5:31 am
10

Re: Brass!!!

#2

Post by capesteve »

the joys of owning an aluminum boat
kmorin
Donator 08, 09, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24
Posts: 1735
Joined: Mon Aug 18, 2008 1:37 am
15
Location: Kenai, Alaska

Re: Brass!!!

#3

Post by kmorin »

Pipe threads, no matter how we goop'em up, lube'em or seal them are a pain in the stern in aluminum! Pipe threads are a wedge - they seal by jamming or forcing two twisted path-cut objects; one into the other. So... even with lots of lube, plenty of goop, tape, or putty... there is going to be a metal-to-metal seal- that's how this thread is designed to work.

In carbon steel they work reasonably well, however, in almost all other cases- especially in mixed metals- this port sealing design is poor. That is why pipe threads (National Pipe Thread) is not used in serious (read: aviation, heavy equipment, main industrial, most military.... ) mechanical systems - at all.

Aluminum female threads are about as poor a design as we could find to use this metal! First the fitting has to jam to seal- that is tighten to the point of metal to metal -face to face pressure~! Anyone who's fiddled a bit with aluminum knows how soft it is- so it deforms in threads- and if the male pieces are of aluminum they will simply gall into a fused mess of cold welded parts.

If the male parts are carbon steel, SSteel, any copper alloy or bronze... they're instantly a battery- so the two faces begin to shift molecules from the aluminum to the other metal and the seal is lost... except when that seal is retained by "white corrosion paste" - as shown above.

If SS is passivated it will last a bit longer before eating the aluminum but tightening the SS fitting into or onto aluminum threads will usually sand off, grind off, break down the passivisation layer- and we're back to square one. If the fittings can stay dry of water- not of oxygen rich fuel or oil, then the corrosion can be delayed- and that's much better than the alternative.

The longest lasting, but probably hardest to find is hot dipped galvanized fittings where the threads (both male and female) are coated AfteR they're cut- so the surfaces of the threads are actually galvanized (coated with zinc) too. This is true of bolts too, but even harder to find in many markets are post thread cutting galvanized coatings.

The reason to consider the zinc with aluminum is the very minimal galvanic differences- aluminum threads would be a nice idea using two aluminum parts but.... they gall and fuse 90+% of the time.

If you know that hose fittings with king nipples or barbs will be used?? it works the best to weld these directly to the tank walls, if the fitting area will be wetted - even from dew- as is the case in humid climates.

SS bushings holding copper alloy fittings will simply slow the galvanic cell's progress as the nickel steel is just slightly less different than copper on the galvanic scale. However in my experience repairing boats, some of which were power boats built in the 1960's, just the condensation dripping off brass/bronze/copper alloy tubing, fittings and parts will actually corrode the aluminum onto which those drips land- it just takes time.

The best thread system to use is SAE O-ring fittings, where the threads have a different profile, goop and sealants aren't the sealing method and the O-ring (on top) can be changed out to Buna-N or some other reasonably inert elastomer- while being bedded in something inert like 5200. These fittings can often be found plated in different metals, zinc & cadmium included, vastly reducing their corrosion potential compared to other metals on the scale.

I do realize that conserving costs of hoses, tubing and fittings is important- but at the same time- by switching to a hydraulic hose fitting standard- the reliability goes up along with corrosion resistance and ease of future maintenance. [Yep, I'm spending your money wildly -again!]

If you're using SS, its a good practice to make sure you choose 316L (inactive/passivated) compared to a 304 (active and un-passivated) even if the cost and time to treat these fittings are higher in the case of 316 less readily available- compared to 304- more commonly found.

Many manufacturers- Racor for example, included in my experience- coat their cast aluminum filter head assemblies to reduce corrosion. That includes powder coating, specialized paint systems, heated paints and perhaps some anodize a layer or two as well- however by including NPT female ports cut into the coating systems; this weak link in the "metal isolation chain" seems assured to fail when a pressure/Vac. gauge with brass 1/8" NPT male base is installed on the body? Or when an installer grabs out of the brass pipe fitting bin he uses on the plastic boat's he fits fuel systems into; the result is: even the well prepared cast aluminum body of the filter head assembly becomes a battery with the brass fittings- as shown above.

Good luck finding a reliable system of parts to connect the various systems that have Nation Pipe Thread involved- its always been a frustration in aluminum. Careful buying of alloy fittings, surface passivization, and care in the install will all contribute to longer lasting systems.

Cheers,
Kevin Morin
Kenai AK
kmorin
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