Brass/bronze screws in Hondo Mahog

[Douglas]

Has anyone else had corrosion problems, with brass or silicon bronze wood screws set in Honduras Mahogony ?

The deck boxes on my BCC are constructed of Hondo, and the silicon bronze wood screws securing the hatch hasp, and the piano hinge, have turned the Hondo a black color, around the screw holes, and the screws themselves have an inordinant amount of blackish corrosion on them.

I also noticed this elsewhere like on the piano hinge screws on the head hatch lid, that same kind of corrosion.

At first I thought that the piano hinges were installed with brass screws, so I changed them to silicon bronze, but the corrosion continues.

Would a fix be to flood the screw holes with epoxy, then re-drill and re-set new silicon bronze screws ?

BTW , I don’t have this corrosion problem on screws set in Teak .

Douglas

[Idon84]

I just removed my bulwarks and found the same issues in my taffrail where the bulwarks attached to it. All of the holes were loose and I was able to remove the bulwarks without removing the screws. The holes in the taffrail were considerably darkened as well as the SB screws.

That said much of the darkened wood still seemed to be in good shape. It is hard and holds a new screw well. I went ahead and removed an area around the old dark holes to dry it all out then to be filled with thickened epoxy. I took a sharp chisel and probed many of the areas to see how soft they were. Only really found 1 place where I just dug until I found hard wood (see pictures). I’ll be painting this wood in the future so the “look” is not as important to me as it would be to someone whom is varnishing. I’ll probably overbore all holes in wood at this point and thicken epoxy to fill.

[Idon84]

I just removed my bulwarks and found the same issues in my taffrail where the bulwarks attached to it. All of the holes were loose and I was able to remove the bulwarks without removing the screws. The holes in the taffrail were considerably darkened as well as the SB screws.

That said much of the darkened wood still seemed to be in good shape. It is hard and holds a new screw well. I went ahead and removed an area around the old dark holes to dry it all out then to be filled with thickened epoxy. I took a sharp chisel and probed many of the areas to see how soft they were. Only really found 1 place where I just dug until I found hard wood (see pictures). I’ll be painting this wood in the future so the “look” is not as important to me as it would be to someone whom is varnishing. I’ll probably overbore all holes in wood at this point and thicken epoxy to fill.

[wfraser]

Bryon, Glad to hear the damage to the bulwark ends was less than first thought. The notion of having to build new ones would be a definite downer. For future reference, are all those marks on the end piece face screw holes?

As for blackened mahogany, I too had my taffrail go black at the ends. However upon removal and light sanding, they were fully restored. I don’t think it blackened because of the silicon bronze fasteners, though, but from water intrusion via leaking ss screw holes.

[Idon84]

Yes, where you see the holes I drilled there was previously a SB Screw holding the bulwark to the taffrail. I just enlarged the holes to get away any soft wood.

[honkinsailor]

All,

I have heard of this problem before with several types of wood. The same can happen with some Oaks and stainless, Birch and brass, Poplar and brass, Mahogany brass/red bronzes, teak and nickel plated steel. I do not know the exact reason, but it’s kind of hit-or-miss. With woods being living things, a bronze screw might discolor the wood from one tree, but not that from its neighbor.

In my W32, I have mahogany ply bulkheads. A previous owner had brass lamps mounted to two bulkheads, both with bronze screws. When I removed the lamp, one had three clean holes, the other three badly discolored holes. Different bulkheads, different sheets of wood, different trees. The same owner put stainless into the red oak headliner in a few places. In that case, all of the holes discolored (seems more prominent in Oaks/stainless).

Anyway, I don’t think simply smearing epoxy into the hole will work. To stop it, the fastener can’t touch the wood, and as much as we want to think bedding will keep the screw from it, chances are that it will wrigle its way into contact at some point. So you’re probably better to either use a different metal, or:

overdrill the hole by about 50%, put some thinned epoxy in the hole and let it setup (but not fully cure as you want the next step to have a chemical bond). Then either fill the hole with thickened epoxy, drilling and tapping it for your fastener once cured, or bed the fastener in it to cure in place. If you do the later, be sure to put a release agent on the screw (wax, lotion, etc.) I suggest the bedding method as it removes the challend of tapping brittle epoxy. If you have to do it that way though, you can heat your tap with a torch to keep it froshipping rhe epoxy (too hot will melt epoxy).

If you’re concerned about the look of the epoxy plug (white when thickened with colloidal silica or mishmash), add a few drops of brown, and one of red, acrylic paint, and/or do half of the thickening with mahogany sanding dust (but still use some glass fibers for strength) This will help it blend in.

If you just paint the wood with a fastener in it that is causing the discoloration, it will likely cause the paint to chip or bubble in the area. Someone told me that the process creates some sort of acid that promotes gassing.

Hope this helps!
Aaron N.

[Douglas]

Ahoy , Idon 84 and honkinsailor , T Y , for your reply, I need to hear, your imput !

On topsides , I am trying to convert all my Hondo, to Teak , because , I have that time, to do so , and I need, that reserve of negledgence.

I am Not a long time boat builder, but I need my boat to stand-up to the elements, which includes termites and insects, while at anchor. (You Wouldn’t Believe Me)

My boat means the world to me, and I want to do her proper justice, in port, as well as at sea.

This bronze screw corrosion problem is new to me , and I shared a discussion about this problem with BCC Zygote, but I think that Zygote, hasn’t, had this problem, yet !

I am looking for a “fix” , to this problem , any ideas, out there ?

Douglas

[svshaula]

Douglas,
Almost all of Shaula’s exterior wood is honduras. We’ve had her 27 yrs, and had lots of the black discolorization during our varnish yrs. I agree with Warren–water getting under the varnish is the problem. I suspect that mold is involved. Our port bulwarks were continuously splashed on our passage from HA to Seattle (close-hauled in the NE trades for 4 or 5 days). Our beautiful varnish job was a mass of black discolorization when we got to Seattle. I stripped it all off, and some light sanding restored the gorgeous reddish grain.

Our ash tiller also discolors if there’s a nick or crack in the varnish, and that black is a lot harder to get rid of. I eventually sealed the bulwarks and the tiller with epoxy. The bulwarks were then primed and painted, and the tiller varnished. The tiller lives in an acrylic cover when we’re not using it.

Our head piano hinge (on varnished mahogany) also had the black problem, no doubt due to saltwater. In addition to the black, the SB screws corroded away and were replaced with SS. It took some years before I got the fore hatch really well gasketed. The gap between the wooden hatch and the fiberglass wasn’t constant.

When copper oxidizes in air, it goes green, but if it’s wet and salty with little or no oxygen, it turns black. If salt water got under your plugs, the copper in your SB screws was probably turning to black copper oxide.

Hope this helps.
Dan Shaula BCC 59

[Bil]

Zygote has had the same problem of an incompatibility between bronze fixings and Honduras mahogany that has been moistened with sea water.

I’ve found three bronze wood screws that lost their holding power because they had been eroded by copper leaching. And at least one bronze through-bolt that had a reduction in its diameter (and hence its tensile strength) at a spot way inside the timber through which it was passing.

I agree that Hondo mahogany + salt water conditions + bronze fixings is an imperfect combination. Replacing the bronze fixings with stainless steel has been my work around.

Cheers

Bil

[svshaula]

Bill, Shaula has also had serious corrosion on the through-bolts that fasten a flanged seacock to the hull. In one case it was a silicon bronze bolt and in the other, a ss bolt. In both cases, the polyurethane sealant failed to keep the salt water away from the shaft of the bolt.

Speaking of seacocks, I never saw the flanged type in Australia. There, the through-hull is secured with that big nut, and the seacock screwed on the end of the through-hull. If the wood backing piece deteriates at all, things start moving, that shouldn’t!

So Douglas, my advice is to try and keep the water out of contact with metals in low oxygen sites.
Dan
BCC 59
Shilshole Marina, Seattle