I've got an interesting problem to figure out how to handle. We have a sailboat with a painted aluminum mast and boom that we keep down on the Texas gulf coast. Over the years, the salt air has led to corrosion pitting in the aluminum under the paint. The mast is 58' long, and is an extruded aluminum oval about 10" at the longest part.
I would like to grind out the pits, and then fill them in and sand them smooth while maintaining the structural integrity of the mast. My thought is to weld the pits back up with aluminum filler, however there is a lot of material to conduct the heat away.
Replace it. The day it breaks will be a day you're in deep doo-doo; If nobody dies you'll be lucky. A flagpole more than 1 flagpole's length from anything valuable, perhaps - a mast, seems like a penny-wise, pound-foolish choice.
If the pits are cosmetic then sandblast the mast to bare metal then sand it with an orbital sander, then paint. Better yet save the cost of paint and step the mast and go sailing, aluminum oxide is great stuff far more durable than paint. There are many masts here in Seattle that have been in the boats for 25+ years with no problems. Here in the NW there are thousands of aluminum hulls that are never painted ( except below the waterline) these things seem to last forever. No matter how good the paint sticks to the mast ( and getting paint to stick to aluminum is not an easy thing to do) all those places where you rivet, or screw fittings moisture will get under the paint, and lift the paint. Be sure and put an insulator between and stainless parts and the mast, and if you can find it bed the stainless screws using Alumalastic, or second best Lanacote. Welding aluminum fittings such as goosenecks, and winch bases to a mast is one of those endless debates I have owned boats with welded fittings and without and have raced them for years without failure. Regular inspections and isolation of dissimilar metals pays off.
That's two votes for replace it. These masts are close to $10K new, and will cost $2K or better used plus a bunch for shipping because of the size. Plus there is no guarantee that a used mast doesn't have any corrosion either unless you personally inspect it before buying, and most of the ones I've seen are in Florida, California, or Louisiana so we're talking an airplane ride or a
10 hour drive each way. I sure hate to scrap the old one just for a few corrosion spots.
I'll look at it closely when we pull it to work on it, but I don't think it's scrap yet. There are not very many visible corrosion bubbles, and they do not go all the way through the base metal on the ones that I have dug out with my knife. The largest bubbles are about 1/2" in diameter. I'm pretty sure that you could drill a hole that big in the mast and not compromise the structural integrity.
My primary concern right now is whether to fill the few corrosion pits with aluminum (tig/mig), or aluminum solder (o/a), or bondo. I'm thinking that welding in aluminum or just using bondo wins because of galvanic corrosion concerns around dissimilar metals if I use the solder.
The mast is probably a 6000 series heat treated alloy. Welding heat will drop the strength, perhaps even to half the original. So welding will probably do more damage than it will help.
Ship hulls are usually made from 5000 series aluminum which has better weldability and is more corrosion resistant than 6000 series, so you can leave a hull unpainted, but you wouldn't want to do the same for the mast.
If the pitting isn't very deep and doesn't cover much of the surface, I'd say you're better refinishing the mast. With aluminum it's a challenge because paint doesn't want to stick to it. Where the existing paint is still adhering well you can scuff it and paint over it, but you'll be sanding out the corrosion to bare metal. Check with Interlux or whoever's paint you want to use for their recommendations. You'll probably need an aluminum primer, followed by an epoxy primer, then an epoxy filler, another layer of primer, then the exterior paint.
If there's just a section of the mast that is badly corroded, you might be able to have it replaced. Find a sparmaker and if they can find more of that extrusion shape, they can join in a new piece and sleeve the inside for reinforcement. In any event, a sparmaker or good rigger would be able to look at your mast and give an opinion of what should be done.
6000 series aluminum alloys and specifically 6061 are precipitation hardening alloys. To get to the T-6 hardened condition the entire piece is subjected to a series of electronically controlled heatings and coolings. This is called, euphemistically, "artificial age hardening", and is the only way to achieve T-6 hardness and above (T-8 is the hardest).
When you weld 6061 that is T-6 hard, the heat affected zone, will drop in hardness to around a T-2 to T-3 hardness.
Over the next few weeks the aluminum will recover some of it's hardness through "natural age hardening". The hardest it can get to is a T-5.
The only way to restore the T-6 hardness is to have the entire piece re-heat-treated.
T-5 is pretty strong.
In order to do weld repairs to the mast you would need to die grind out any corrosion, and then TIG weld the area using a 5356 or 5556 filler rod.
You would need a high amperage AC TIG machine, preferably an inverter, and a mix gas with a high helium content like HE-75. (75% Helium, 25% Argon)
The idea is to do the welds very quickly to reduce the heat affected zone.
MIG is not an option as it would impart severe distortion to the mast.
Thanks Tim. I'll check with the boat manufacturer to determine the original manufacturer of the spar, and then contact them to try to determine the alloy. If it turns out that it is a heat treated alloy which will lose strength on re-heat, I'll drop the welding idea and go to and epoxy filler for cosmetic repairs.
I just did a little checking online, and all the masts I saw were either 6061-T6, or 6063-T6, so it appears that Tim is right about the alloys. That is giving me some pause for thought, althought I'd think a few small T5 hard spots in the spar would not hurt anything. Something to think about though.
This is pretty much what I was thinking of doing.
Yep. There is a lot of aluminum to pull the heat away.
Check.
Hmm. Why do you say that? These spots are typically less than 1/2" in diameter.
As someone else commented, the spars are probably a 60-series aluminum which is weldable. The question then is whether welding will harm the mast to any significant degree.
It is quite common to see masts with welded on goose neck fittings or winch mounts. My boat has both. Since it would be unlikely that the mast was heatreated and then work hardened to a T6 condition after the welding was done it would seem that a reasonable amount of welding on a spar is a safe practice.
In my own case (40 ft. deck mounted mast) I stripped the paint and then filled what pitting I found by welding. Then re-painted using an etching primer and epoxy paint. To date it hasn't failed. As you quite rightly noted ther are several drilled holes in the mast larger then any of the corrosion.
If you do not TIG weld be sure that any flux deposits are completely removed as the flux deposits seem to accelerate new corrosion.
If all the corrosion is down low , like where it passes thru the deck, you can always just shorten it, move the rigging up and put a reef in the sails. That cheap enough?
It varied quite a bit. The mast had little to no corrosion, the boom quite a bit. I spot cleaned the mast using a stainless wire brush on a
4" angle grinder and spot painted using an etching primer and two part epoxy paint. Not the best solution as epoxy paint "chalks" in sunlight but it works.
After I got the paint off the boom there were a multitude of screw holes where hardware had been removed, re-installed, changed, etc.
My original plan had been to clean up the corrosion and fill the pits with epoxy and repaint but after looking at all the holes I decided to weld up the holes and the major corrosion areas.
I stick welded the boom since that was the equipment I had on hand at the time, TIG would have been better. Once the welding was finished I used a 4 " angle grinder with sanding disks to grind the welds flush and smooth.
I might point out here that the white powdery deposit on the corroded spots is, in itself, corrosive so be sure that you remove it all! Some form of chemical wash would be best but would probably need to be neutralized to ensure no further corrosion.
I'd like to comment that excepting the foot of a mast that lacked good drainage I have never seen corrosion on spars that was so extensive that it couldn't be repaired. Not to say that it can't occur, just that I've never seen it. My present boat is 30-odd years old, crossed the Pacific, etc., and as I said had no corrosion extensive enough that I felt it couldn't be repaired.
MIG on aluminum is a very fast process and causes rather extreme distortion on tubing. The rapid heating and cooling can warp the crap out of any tubular extrusion.
The original post referred to a 50-something foot mast. Believe me this is not tubing.
I'm not that familiar with MIG welding but I have successfully stick welded both masts and boom on a 40 ft. sloop. Bruce in Bangkok (brucepaigeatgmaildotcom)
I dunno Ernie. Everything I've found has said that while some strength is regained by age hardening, the heat affected zone is still considerably weaker than a T6 condition. For example, here's a quote from a Lincoln brochure
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"When T4 or T6 materials are welded, the heat of welding affects the properties in the HAZ, reducing them. Properties are usually not reduced all the way down to the "O" temper. It is difficult to give a general rule regarding the reduction in properties. The specific value depends on the alloy and temper under consideration. However, as an example, 6061-T6 is required to have a minimum utlimate tensile strength of 40 ksi (276 MPa) before welding. In the welded condition, most codes require a minimum tensile stress of 24 ksi (165 MPa), so that the reduction can be significant."
If 6061 naturally age hardened to regain most of the T6 strength, wouldn't the welding codes recognize this?
In any event, building up the corroded spots on this mast by welding is going to be a lot more effort than just using an epoxy filler. All the rest (corrosion removal, scuffing or removing old paint, priming, fairing and painting) will be the same either way. Assuming the corrosion is cosmetic, the mast will look the same in the end.
On the other hand, if the corrosion is bad enough to structurally weaken the mast, welding will replace lost metal, but it may not add strength.
If it's minor pitting and you are trying to fill the pits for cosmetic purposes, I wonder if Lab Metal would work? Strange stuff---I've used it on thermoforming molds with good success.
Well, maybe not. It may be 4 or 5 hours of welding and sanding versus 1 hour of epoxy and sanding. Either way it is about half a day ;-)
True.
Sounds like a reasonable argument to me. One thing to consider is that masts are rigged to be in compression at all times, so the tension specs on the alloy may not be what is important.
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