Stainless Fasteners through Aluminum - Corrosion?

[Syncrowave]

Hey, all, new guy here. I've been boating a long time but mainly in FG boats.

Just got a new alum jon boat (Lowe L1648) and I am rigging it, and I want to fabricate an aluminum box to attach the outboard control box to. The sides of the hull are at too much of an angle leaning outboard to attach the control box directly, because the lever would lean outside of the boat, so I plan to make the box triangular such that it has a vertical surface to mount the control box to.

I want to attach this box semi-permanently to the hull, and since it will carry some weight and be subject to some force when I move the control lever, I don't really want to use aluminum pop rivets, since they will probably get loose over time.

I'm considering using stainless steel machine screws and locknuts, but I am concerned about galvanic corrosion between the stainless steel and the aluminum hull, especially since I will be using the boat almost entirely in saltwater. Theoretically, since stainless steel is more noble than aluminum on the galvanic series, the aluminum may corrode around where the stainless steel machine screws go through.

What's the collective opinion on how to address this? Is it not an issue (even in saltwater)?

Or, alternately, should I use aluminum pop-rivets and back it up with 3M 5200 marine adhesive sealer to hold everything in place?

I would consider welding the aluminum box to the side of the hull, but I don't want it to be THAT "permanent."

Thanks in advance for any suggestions.

 

[kfa4303]

Hi Sync. If you can't use aluminum pop rivets, stainless steel is the way to go. In fact ss is superior in that it's stronger. While aluminum and ss are dissimilar metals they will not react in such a way as to cause any electrolytic pitting. You can find a decent selection of ss hardware at Lowes/HD/West Marine, but try to buy in bulk as it can get pricey if you buy it bit by bit.

 

[Zum]

I somewhat agree with the above statement.
Aluminum rivets would be best,stainless fasteners second.
I do think there will be alittle corision issues,as in white powder around the stainless.
I used stainless bolts to fasten my other bigger outboard to the boat,I'd check it a few times a year and put grease on the bolts to minimize this corrosion.

 

[PGRChaplain]

I always use stainless fasteners, never any problems. Freshwater use only.

 

[thudpucker]

Bronze below the water line; Stainless above the water line!

Works in Salt or Fresh water. I struggled with Star-craft, Sears, Lund, and etc for years.

 

[Ranchero50]

Ditto.

I also wouldn't hesitate to get the larger peen over aluminum rivets and install them. Kind of a PITA to install but they'l outlast the hull if done right.

 

[sr71]

I use ss bolts and nuts with nylon washers where the two metals contact to avoid corrosion. Also a nice dab of 5200 on the treads of the bolt to secure and seal. Use my tinny in salt water. My 2 cents.......good luck.

 

[Syncrowave]

Thank you all for the replies, folks.

Thudpucker, I'm surprised as heck by your reply about bronze below the waterline. Theoretically, at least, bronze (because it is largely copper) should gobble up aluminum like there's no tomorrow in an electrolytic environment. But I guess your experience shows that theory to be untrue...so much for theory!

I think what I'll do is use 3 or 4 aluminum pop-rivets to hold everything in place (as well as some clamps if I can get them in there) while a bead of 3M 5200 glues everything in place. That miserable stuff sticks like you wouldn't believe!

Thanks again, everyone.

 

[RiverBottomOutdoors]

SS will bond to aluminum in salt water.

 

[kfa4303]

Take it from a lifelong marine boater, SS will NOT rust, seize, or cause pitting and is the standard in all marine applications. The SS and aluminum will get along just fine. You can add a dab of 3M 5200 for good measure, especially on any through hull fittings. Again, I would suggest buying in bulk if at all possible, as the cost of SS hardware can ad up quickly.

 

[RiverBottomOutdoors]

kfa4303 » 49 minutes ago[/url]"]Take it from a lifelong marine boater, SS will NOT rust, seize, or cause pitting and is the standard in all marine applications. The SS and aluminum will get along just fine. You can add a dab of 3M 5200 for good measure, especially on any through hull fittings. Again, I would suggest buying in bulk if at all possible, as the cost of SS hardware can ad up quickly.

SS will bond to aluminum in saltwater. It's usually not that significant of a bond...but it can present a problem depending on the application.

Example: SS retention pins for outboard jet grates. It's recommended that these pins be treated with non-seize product for saltwater applications to prevent them from bonding to the foot housing. This helps prevent flaring the pin when punching it out.

 

[earl60446]

I would use the pop rivets, good size ones and back it up with the 5200 like you said. I would also
use like 6-8 rivets, not 3-4. I got that triangle piece on my boat from the factory, they left the
front and back open to run the steering and other (non-control) cabling thru. Just thought I would mention that as a possible way to do it.
Tim

 

[wyatts dad]

I worried about the same thing, reading that chart about the ratings and what is noble etc will drive you nuts, one point was on that chart you may not have noticed, the most minor part if a weak link will suffer, plain steel or galvanized fasteners thru aluminum, the steel will rust and leave the aluminum alone because it is dominant, ever see a roofing nail in aluminum fascia, it wont last long, but the fascia will still be there flapping around, in the sheet metal trade we see this all the time, things fail when cheap plain or cheap plated steel fasteners are used, the stainless may bond but it will not going anywhere, another truth is when you tighten down the stainless fasteners the aluminum will crush around it almost like a rivet, actually helps with leaks issues, btw make sure you use washers on the nut side, do you really think you will ever remove any of those bolts anyway, naw, bet not, but the real problem is that the salt eats everything, check out aluminum boat windshield framing and cast aluminum stuff on a car or truck that had a bad winter, pit city, marine motors, sure the fasteners will seize, even land yachts use antiseize especially on head bolts, just have some fun, keep it all in perspective, don't be too serious, the little boat with stainless fasteners will last as long as your infatuation with it does, then it is down the road, take it from me, I have been struggling to get my project done for too long, close, but not wet yet, maybe in a week or so.

 

[kfa4303]

RiverBottomOutdoors » Yesterday, 12:42[/url]"]

kfa4303 » 49 minutes ago[/url]"]Take it from a lifelong marine boater, SS will NOT rust, seize, or cause pitting and is the standard in all marine applications. The SS and aluminum will get along just fine. You can add a dab of 3M 5200 for good measure, especially on any through hull fittings. Again, I would suggest buying in bulk if at all possible, as the cost of SS hardware can ad up quickly.

SS will bond to aluminum in saltwater. It's usually not that significant of a bond...but it can present a problem depending on the application.

Example: SS retention pins for outboard jet grates. It's recommended that these pins be treated with non-seize product for saltwater applications to prevent them from bonding to the foot housing. This helps prevent flaring the pin when punching it out.

"recommended" perhaps, but not required. If you use marine grade (316) stainless steel it will not gall or rust. Just check out any boat built for marine use and see what hardware they use.

 

[RiverBottomOutdoors]

I'd be willing to bet that any SS hardware that requires servicing or removal for servicing of parts is treated with non-seize. I agree, with permanent attachments, non-seize is not needed.

 

[Zum]

thudpucker » Sun Aug 11, 2013 9:07 pm[/url]"]Bronze below the water line; Stainless above the water line!

Works in Salt or Fresh water. I struggled with Star-craft, Sears, Lund, and etc for years.

I don't think bronze is good on aluminum...high grade stainless /aluminum...
"Except for sacrificial anodes, don't mount any metal to the hull other than aluminum or high quality stainless steel. If you use fasteners they should be plastic or stainless, not zinc, brass, copper or iron based. Inside the boat, don't let any metal sit in the same spot in the bilge for long. Lead sinkers, fish hooks as mentioned earlier copper even a penny sitting in one spot can be a starting point for electrolysis."