What screws & bolts for aluminum boats?

[Black Bass]

What screws, bolts, and nuts do you recommend using with aluminum? Reading this forum, I see that I shouldn't use stainless steel. Will brass work?

[Catt]

Why would you not want to use stainless steel?

[CGH]

Aluminum Screws would be you best best or aluminum Pop ribbits

[RobDar]

stainless and aluminum are fine together. I was told by a mechanic that the only problem with stainless and aluminum is when used on a high stress hull rivet. The stainless is harder than the aluminum...so on a keel rivet or other high stress local, the aluminum hull will fail/ tear before the stainless rivet...This is why the manufactuers use aluminum rivets. It is better to wreck a rivet than the hull. seemed to make sense to me...

[Long Mike]

DE Lee, thanks for some very valuable input.  The stainless vs aluminum connections at stress-points is one I wouldn't have thought about.

[Catt]

Dissimilar Metals: When two different metals are in contact in a corrosive environment, one of the metals experiences accelerated galvanic corrosion while the other metal remains galvanically protected.

Galvanic corrosion can be rapid when aluminium alloy is placed in proximity to stainless steel in a wet environment. To stop Galvanic corrosion between two dissimilar metals a thin layer so some type of sealant is required.

[Black Bass]

DE Lee, so stainless steel and aluminum don't interact being dissimilar metals?

[Way2slow]

Just be darn sure you DO NOT use galvanized or zinc plated fasterners.  You will have a hole the size of the fastner head in no time.  I've never had a problem with with stainless and been messing with boats a long time.  Granted, in the ideal world, you want to use similar metals, but try finding much in the way of aluminum fastners.  Pop rivets and not good because they have a hole in them.  Rivets create problems because you don't have a way to brad them tight enough to be water tight.  Welding can create a whole bunch of problems unless it's done by a professional that knows his stuff.

[Catt]

When you drill a hole in aluminum, the inside of the hole does not have a protective coating, adding a stainless steel fastener will cause corrosion; it could take years to cause a problem or months depending on the quality of the aluminum. The solution is to install the fasteners wet, coated with a sealant such as paint or any thing to stop the galvanic corrosion.

Now some believe its overkill and not necessary but when I build some thing its permanent.  

[RobDar]

DE Lee, so stainless steel and aluminum don't interact being dissimilar metals?

here is what I know...

They are two different types of metal...stainless is ferrous and aluminum non ferrous...and typically the two are compatable...but there are exceptions to EVERY rule. the difference here is that niether stainless or aluminum are magnetic...which is a major contributor to galvonic action ( electricians refer to this as dia-electric).

I am an electrician by trade...and have worked on SEVERAL projects...the waste water tunnels under the city of Chicago, the old coal tunnels under the city, several hospitals, Cargill IOPD refinery, and most of the grain elevators in the area...all of these instances prefer aluminum conduit (for weight mostly) and in every instance whenever we passed through a concrete wall or a hazardous/corrosive environment and had to change to standard glavanized conduit to satisfy Chicago electrical code...THE BONDING BETWEEN THE ALUMINUM AND THE GALVANIZED WAS ALWAYS DONE WITH A STAINLESS STEEL FITTING TO STOP CORROSION.

now this is stuff designed by engineers...not made up by an electrician or maintenance guy. this is high tech, well planned, designs...so I always assumed the engineers knew their stuff.

1.you cannot use steel screws with aluminum...

2. and aluminum screws will break to easily...

3. boats come from the manufacturer with stainless deck screws mounting the deck to aluminum sub-structure

4. electrical equipment manufactures building for Type 2 and above hazardous locations use stainless for their cabinets...for the longevity of the cabinet and so that any and all conduit types, aluminum, pvc, coated, and galvanized can be used.

I assumed the manufacturers know what they are doing...but I cannot say I ever sat down and read the science on this.

the only questions remaining are these...

I do not have any real data on whether the presence of water changes this...freshwater, I would say NO...since, barring contaminants, freshwater is chemically inert. Salt water....I do not know about that...it corrodes about everything eventually.

there are three major alloys/types/grades of stainless steel. I cannot say I know if there is a difference between the alloys and whether one is okay and others are not. I never really paid any attention to the type of stainless being used.

I do not know if the marine alloy reacts differently with stainless than the conduit alloy. The marine alloy has more boxite in it than the electrical alloy...and the electrical alloy has zinc...so I do not know for a fact if there is any differences there.

most welders will tell you that you cannot mix the two...and they are right...YOU CANNOT WELD THE TWO...but there is a big difference a mechanical bond and a weld. there is alot you can do with a mechanical bond that will not work and is not allowed with a weld.

I know that this is not really a definitive answer for you...sorry about that. I am not a metallurgist...but my fathers 42 years as a tool and die/ fabricator foreman and my experience in the 20 years electrical industry tells me that you can use stainless with aluminum...

but I do not have any "print outs" or technical data sheets to back up those claims. I never actually sat down a researched the topic.

[Catt]

Here's what I know based on 15 years as a Manufacturing Engineer in the Aerospace Industry and I can give you the technical data if required.

In order for galvanic corrosion to occur, three elements are required.

1) Dissimilar metals

2) Metal-to-metal contact

3) Metals in the same conduction solution (usually called an electrolyte)

If any of these elements is missing, galvanic corrosion cannot occur. If, for example, the direct contact between the two metals is prevented (plastic washer, paint film etc.) there cannot be galvanic corrosion.

Electrical conduit and the extruded aluminum angles Black Bass would purchase from a local lumber yard are light years apart in design and are no comparison.

Now is this to say you boat will corrode to dust in a matter of months? No

I'm not saying you can not use stainless fasteners, what I am saying to be sure to us a protective coating between the two metals.

[FishingBuds]

From Catt: I'm not saying you can not use stainless fasteners, what I am saying to be sure to us a protective coating between the two metals.

[Catt]

In the Aerospace Industry it is known as installing fasteners wet with either a sealant or primer. As long as there is no metal-to-metal contact of dissimilar metals galvanic corrosion cannot occur.

[Peewhy]

If someone is still reading this thread, I would have been interested in knowing what sealant or primer to use. I guess it must resist the pressure of the screw?

[Catt]

4 hours ago, Peewhy said:

If someone is still reading this thread, I would have been interested in knowing what sealant or primer to use. I guess it must resist the pressure of the screw?

 

Zinc chromate primer

 

 Sherwin-William, PlastiKote or Krylon

 

3M or Loctite sealant 

[DaubsNU1]

13 minutes ago, Catt said:

 

Zinc chromate primer

 

 Sherwin-William, PlastiKote or Krylon

 

3M or Loctite sealant 

 

 

^^ Agreed ^^

 

Stripped two old jon boats down to bare metal...zinc chromate primer...camo paint (they are used as decoy boats outside 100% of the time...get lots of use and abuse...

 

Paint and primer have held up well.

[king fisher]

I am not an engineer, so do not claim to have as much knowledge on the subject as others that have posted on this thread.  I can say from experience that stainless steel screws are ok to use on aluminum.  Any other steel will cause dissimilar metal corrosion.  I rebuilt the wings on my Piper cub one winter after severe wind damage.  Most of the screws in the aluminum spars were stainless steel.  Some were regular steel, having been replaced by a previous owner one who was either cheap or ignorant probably both.  The ones that were not stainless showed signs of corrosion to the point that a couple ribs were not even attached.  Just held in place by fabric.  This should have been noticed by a mechanic during annual inspection but was missed until I tore the wings completely apart.  The proper screws installed from the factory for connecting aluminum ribs to the aluminum spar were stainless steel.  The legal recommended screws bought to replace the old ones were also stainless steel.  Again I am not an engineer, but I do remember how expensive those approved stainless steel screws were. and the reason they were worth the money.

[E-rude dude]

Use SS hardware with a silicone sealant, or 3M 5300 on the water exposed side. 
 

The corrosive effect of dissimilar metals is a little overblown. It takes years of exposure and neglect before the aluminum corrodes. I’ve always owned aluminum boats and the worst I’ve seen is the screw head breaks off after several years leaving the hole permanently plugged.

 

Any seam that needs to be repaired should be done with solid aluminum rivets.

[TnRiver46]

That 3M 5xxx stuff worked great for my transom rebuild 

[Ayrrloom]

You should never allow dissimilar metals to contact each other in regards to use as fasteners on an aluminum hull as electrolysis occurs especially in a saline environment. Always use a barrier between the two different types of metals be it rubber backed washers Teflon washers etc. You will find out rather quickly when you loose a ground onboard that it will literally burn holes in the aluminum hull anywhere dissimilar metals are touching. And over a long period the same will occurs. I can show you many instances of the same thing. 

[Ayrrloom]

Classic case of electrolysis occurring due to dissimilar metals. Guy used steel bolts to put his transom together had a grounding issue on board and within a year he brought it to me to fix. When I told him what it was gonna cost he left it and said it’s yours. I am in the process of rebuilding it. I’ll sell it back to him when I’m done.