TOPIC: compatible screws for aluminum

The deck on my Glastron is attached to the hull by approximately 80 1" #8 flat head machine screws. These are screwed through the deck, hull and a piece of aluminum trim. The screw heads are countersunk in the aluminum trim. The originals are bronze. Several of them broke trying to take them out, so my plan is to put new ones in when I put it back together. My question is what material should I use? There was no corrosion with the bronze, but I have seen very few applications with bronze and aluminum. Stainless can cause corrosion on the aluminum after a while and there is no way to insulate it with the countersink. Any other thoughts? I do live in a salt air environment, so that may play into it.

the tudor,

MONEL screws seem to work fine on aluminum.

also, NON-magnetic stainless seems OK too.

yours, satx

I can't find Monel screws except as special order and almost don't want to know the price. The best deal money-wise is 8-18 stainless. The scews will be covered over by a rubber insert (assuming I can find the right size in white), so I may be over-thinking this. But then again, that's what I do......

the tudor,

take a magnet in your pocket to Lowe's or Home Depot & test their screws for the ones that are NOT magnetic. - both places seem to dump all the SS screws into the same bins.

GOOD LUCK.

yours, satx

Todd, you might give the gang at McMaster-Carr a call, their number (800) is listed on their website. I feel sure they could recommend the right stuff. They also have all the hardware (bolts, screws, etc.) you could imagine and then some! (Pretty reasonable prices too, IMHO).
www.mcmaster.com/#

SS is a NO NO aluminum. It will start an electrolytes action and and the holes will soon be larger. A white power will start to form and fall out. then the pieces that they are holding together will start to work. Good plated screws is OK to use, however if you can rivet them with aluminum rivets that is what I would do. You can get counter sunk rivets and even get blind end rivets...Closed end as to keep water out. Ever need to remove them drill them out

www.hansonrivet.com/w22.htm

I did a little research on this. The corrosion is caused by galvanic reaction between the 2 metals. Water, especially sea water, enhances this. One material is the donor (anode) and the other is the collector (cathode). Metals are classified as being more or less "noble", which defines their corrosion resistance. The greater the spread between "nobility" of two metals, the more likely for corrosion to occur. The corrosion will always occur on the least noble metal.

The trick to decrease corrosion seems to be have the two metals being as close as possibly on the nobility scale, thus decreasing the galvanic action (battery effect). Here are some values based on a table I found on line:

Aluminum = -.75 to -1.0
304 Stainless Steel (the typical stuff) = .-.05 to -.15
316 Stainless Steel = 0 to -.10
Nickle Chromium 600 = -.10 to -.15
Silicon Bronze = -.25 to -.30
Naval Brass & Copper = -.325 to -.40
Aluminum Bronze = -.325 to -.425

This could possibly explain why the aluminum trim piece attached with silicon bronze showed very little if any corrosion. This may be the way to go, especially if covered. Perhaps some sort of coating over the countersinks in the trim piece would help as well.

There are probably some out there with way more metallurgy experience than I, but this is what I came up with and the conclusion I reached.

No one mentioned aluminum screws. They are difficult to find but I had some here. They are about 3/4 inch long. They are for aluminum storm doors. Good stainless should not cause a problem. If in doubt take a magnet to it. There are different grades of stainless.. Of course we have brass too.

What I have are machine screws, which is actually a type of bolt. I wouldn't use an aluminum bolt and I think brass might be a little too soft.

I checked and only have a few aluminum screws left. NUTS They are handy.

You might try coating the screws with anti-seize. I built a boat lift out of aluminum square tube held together with stainless bolts coated with anti-sieze some years ago. It is in frequent contact with salt water, and hangs right over top of it, and so far there do not seem to be any problems. I've also heard that's commonly done when putting stainless into aluminum on motorcycles, although obviously they are not regularly in the water. Good luck. Shannon

I had been thinking about some sort of coating over the aluminum, but had not thought os anti-sieze. My original thought was some sort of spray on or paint on clear coat.