Negatives of using stainless screws.

[Alan Wheeler]

I have been slowly repairing and replacing my windows around the boat. You may remember and have seen some photos of the brass surrounds I have been manufacturing. These have been replacing timber surrounds around my windows, of which some have been rotting away. Well I discovered a very interesting reason why these wooden frames had bee rotting. I have been perplexed for a while. I would remove a frame by chiseling the timber away and exposing the heads of the crews. I would SST screws, all shiny and new looking as the day they went in, neatly holding the frame to the cabin side. The heads are sunk in a good 1/4" or more and sealed over with epoxy sealer and the timber in good condition. Then I would come across a frame that started me off on the whole excersise. The frame is rotting away. Upon cutting away the wood and exposing the screw heads, I would find the builder had used plated screws. Or so I thought. These screws were rusted away and had become wicks that had drawn water and rust deep into the timber and cabing side timbers. Thinking nasty things about the idiot that had taken some short cut of not buying a few bucks worth of screws. Then I came across a frame that had a mixture of these screws. What the?!? But wait, whats this? a half SST and have rusted screw?!?!?! It suddenly clicked. All along, these screws WHERE in fact SST. But being totally vod of oxygen, they didn't do their job. so if a little moisture had found it's way to the screw, it would start to rust. The rust and seemingly a wicking effect would then take the water on in further. then the wood would breakdown and the screws along from this problem would rust and the frame was made useless.
So using SST screws on the outside of a boat is not good practice in my view. Maybe this is already known in the pro boat builder circles, but I didn't. Silicon bronze screw would have been the better choice, although the price hurts.

[delmarrey]

I believe your right about wanting to use S. Bronze screws for your frames. But, I don't believe that the SS screws were the problem with the wood, just the opposite. The rotting wood eats away at the SS. Rotting wood creates acids, which can eat most metals, even SS. Even if those SS screws had not been encapsulated they'd still been rotten at the threads.

S. Bronze is quite resistant to acids. That’s why they are used on planked hulls rather then SS. Pound for pound S. Bronze is much more expensive then SS. At first the S.B. screws were slotted head, then went to Phillips head (bad idea) and now are a square drive, which seems to work real good for extracting old screws.

The main thing, I think, is the frames were not sealed well and allowed the water to get trapped between the surfaces, creating the rot, which took out both wood and screws.


http://www.corrosion-doctors.org/Microbial/Bacteria.htm

http://www.corrosion-doctors.org/Mat...corrbronze.htm

http://www.corrosion-doctors.org/Mat...stainsteel.htm

Enjoy the reading........................................... ........_/)

[Rick]

Hey Wheels! I have seen the same problem and switched to bornze like you are considering. There just may be a better solution, tho.

Consider this:
Ceramic Coated Deck Screws
These ceramic flouropolymer coated fasteners are designed for pressure treated lumber. Great for pressure treated decks and docks where a non-corrosive and strong screw is desired.

My experience with these screws and wood is that they just don't rust! I DO, however, recommend that you buy only the Robertson square drive heads or the combo heads which take either #2 Phillips or the Robertson square drive. I had a heck of a time if I used a short #2 Phillips bit because if I drove the screw too far off-axis the sqrew head would break off one or more flukes of the driver bit. In a few cases I couldn't even hammer off the dirver piece embedded into the screw head. When I was able to remove the piece of the bit from the screw I could observe NO evidence of damage to the screw head finish. Using longer driver bits helped a lot yet the real answer was merely to use the square drive bits.

You might also consider some of the deck screws which also have a very durable finish. Drive several screws in and out of hardwook and observe the finish after the "abuse" to see if you can count on being able to drive such fasteners into your workpieces with confidence that they leave no part of a surface exposed to the "outside" of the steel.

It just could be that the ceramic screw finishes will make the way into a wider variety of fastener sizes for marine use and the problems of corrosion and stress/strain failure will all but go away.

Rick

[Alan Wheeler]

Hi Rick, now that sounds interesting. I bet they are expensive though. I can just imagine buying a box of those and getting the space shuttle thrown in as a promo freebie.

Actually I wouldn't mind a few thoughts here. I am actually screwing the Brass frames in with Brass screws. The frames are laqured and I was intending to laquer the screw heads as well , once screwed home. The screw heads are exposed by the way. The reason for using Brass screws, I was conserned that Silicon bronze and the brass frames might not like each other. Any thoughts?????

[Rick]

Yep, I've been through the problem of "matching" brass and bronze. First of all, I feel that you should avoid yellow brass fasteners at all costs. Lacqured or not, you just cannot see where under the head that the lacquer has been removed. Once sea water gets into yellow brass the fastener can rapidly erode away. Yellow brass screws only belong in non-sea water environments. I'm guessing that your brass frames are not yellow brass but naval brass (still not a great resistant metal to marine corrosion but better than yellow brass).

Here is how I "match" different alloys which are allowed to come in contact with each other in the presence of sea water. Dip up a cupful of clean seawater and partially immerse your test metals (fasteners, or whatever). Use your DVM to measure the relative potential between any two metals. I feel that if you obtain readings around 60mV or less then you will not experience rapid degreadation between the two. Of course if you are in the Carribean then you have saltier sea water than we do on the Pacific and you would want to strive for a smaller milli-Volt reading.

Like me you will probably be surprised at the variation that you will observe (as well as the lack thereof with a good "match") I've been able to match some bronze alloys with 304 or 316 S/S with less than 30mV difference, for example.

You can buy a pound of coated deck screws for six bucks. You can pay more by buying coated S/S deck screws buy why? In that particular case I think that the buyer thinks that he is getting a more robust fastener yet, if you think about it, either the coating is not necessary or it is. If the coating is necessary then why use S/S when stronger steels are available which are vastly cheaper to manufacture? As long as the barrier coating does not fail you get a better alloy for a fastener than S/S. Remember that S/S is not particular strong or hard or forgiving in the stress/strain failure curve as compared even with mild steel. Most people confuse the toughness (truly a metallurgical term) with hardness. You would never, for example, want to fabricate a motor mount from S/S you would use mild steel which is very tolerant of vibration without stress fracturing as is S/S.

I am using Phillips 11 KwickTap 3/16" X 3" flathead fasteners (normally used for concrete/masonry fastening) to attach teak handrails because the heads just fit inside a 3/8" counterbore (for the plugs) yet they are rated at 90lb load each with a 4:1 safety factor. To get that rating with S/S I would have to use larger plugs and fasteners (#14). I don't know what is the coating on these babys yet so far I have not scraped or dinged the coating off of them. I've used them on metal and treated wood exposed outside near the seawater with good results so far. THESE screws are NOT cheap...they are 9 to 10 bucks for 25 of them. The threads look good for both wood and fiberglass and, yes, I do still drill pilot holes even for those screws claiming not to need them. Why unnecessarily stress hardwood?

What do you think?
Rick