Re: [Paddlewise] aluminum and salt water

rdiaz_at_ix.netcom.com wrote:
> 
> DANJW_at_aol.com wrote:
> >
> > All this talk of metals and saltwater and corrosion causes me to wonder what
> > the metallic frames of folding kayaks are made of , and how do they defeat the
> > corrosion problem?
> 
> I hope someone with more metallurgical knowledge that I have will pipe
> up on this.  Feathercraft uses two different types of aluminum.  The
> K-Light uses 6063-T832 extruded aluminum/magnesium whereas its bigger
> brothers use a heftier 6061-T6 aluminum/magnesium.  In both cases they
> are heavily anodized against corrosion.  The last I talked to them about
> it, they had it anodized in the US even though they have plenty of
> aluminum up in Canada.  It may be price or it may be quality but knowing
> Feathercraft's high standards, I would suspect the latter as they go out
> of their way to source their hypalon from France some 7 thousand miles
> away from them rather than source in the US or Canada.
> 
> I don't know what Folbot uses in its aluminum but it is anodized.  The
> frames do seem to hold up well though.  [big snip]

> There isn't anything that doesn't suffer in a marine environment.

I've kept my mouth shut during most of this corrosion discussion because I
know I'm definitely not Ralph's desired expert.  Maybe the best
contribution I can make is that almost every statement I've read so far
should be followed with the rejoinder ... "Well, that's generally true, but
it also depends on [fill in the blank]."  Maybe a story will help explain.

Out here, they make gill net boats (for saltwater service) out of one of
the 5000-series aluminum alloys (don't know which one), so I scurried off
to my buddy who makes those boats when I wanted a chunk of
corrosion-resistant aluminum to make a skookum rudder horn for my Folbot. 
Much to my surprise, if the horn is left, wet, in contact with the
seemingly-cheesy Folbot aluminum castings, it erodes badly at the contact,
and the castings *don't* corrode.  Why?  I don't know.  I could speculate
about it, but the value of the speculation would be exactly equal to what
the readers of this list paid for it.  The gill net boats hold up well in
marine service (years and years).  Go figure.

There's only one feature of Ralph's dissertation I'd like to play with: 
anodizing.  Good idea for the protection it provides for the anodized
surfaces.  **Maybe** not so good an idea if the aluminum is riveted,
bolted, screwed, or attached in some other way to a different metal. 
Here's why:  the anodizing prevents corrosion on the bulk of the aluminum,
but at the contact with dissimilar metal, there *may* be *accelerated*
corrosion, owing to the innate tendency of the dissimilar metals to form a
galvanic couple.  If the corrosive effect of the galvanic couple is
concentrated at the fastener, it is possible that the fasteners will
corrode the aluminum (or, vice versa), and ... boom! ... the fastened
joints give way.  OTOH, if the aluminum part is **not** anodized, the
galvanic corrosion is often spread out over a greater surface area,
prolonging the lifetime of the fastened joint.  Scuttlebutt over on the
Folbot Web site is that there was a bad batch of (aluminum?) rivets punched
into boats a couple years back, which corrode out and leave some of the
frame pieces detached from the longerons.  Not happening on my boat, but
could, and would probably happen less rapidly if the frame parts were not
anodized.  (I hit the frame parts now and then with a version of Boeshield,
and it seems to help.)

Bottom line:  if the anodizing completely enshrouds *all* of the assembly
from corrosive contact (i.e., salt water), then it is incredibly
long-lasting.  If there are gaps, there *may* be accelerated corrosion at
the gaps if there is contact with a dissimilar metal.  

Oh, one other thing, anodizing usually produces some very toxic wastes,
which cost a lot of money to deal with properly.  Feathercraft may have the
anodizing done someplace special because the diseconomy of a small-scale
operation (which has to deal with proper disposal of those wastes just like
a larger operation) has driven a lot of smaller anodizing places out of
business.  Feathercraft is not the kind of firm to have it done someplace
which is cheaper because the anodizer deals improperly with the wastes.

As a chemist who deals with waste disposal issues in a very small academic
lab, I'm very conscious of the impact correct disposal has on the cost of
my operation.  Sometimes costs way more than the raw ingredients we buy.

-- 
Dave Kruger
Astoria, OR


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