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
Any thoughts on the best way to go about this?
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,
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
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
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
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
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
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
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
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.
Bruce in Bangkok
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
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.
Bruce in Bangkok
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
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
"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
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 ;-)
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.