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Nickel
plating is a good way to go, for things like the hatch.
Brian
Hey Frank,
I was wondering
if some one was going to bite. Some time back I was corresponding with
Karl Stanley, and he mentioned that if he ever built another sub he wanted to
have it hot dig galvanized. That got me to doing some checking.
Karl's sub Idabel has a base coat of high zinc epoxy primer, (green primer of
death) and it has been operating in saltwater for years. That basic type of
primer is what many consider to be one of the best steel primers for marine
applications, and if Karl isn't completely happy with it, he might just have a
good reason.
I had data on the last 4 ft OD hyperbaric chamber that my
relative built and powder coated for about $500. But unless your going to
Rhino coat the whole hull inside and out in hopes you don't chip the coating
and the coating is a consistent thin enough thickness to flex and not crack
with the pressures you plan to dive the sub. I don't find it to be the best
option.
Hot spray zinc can be good, but you need to spray it with in
four hours of sandblasting from the data I found, you will not be able to get
it inside of small tubing and blind spots such as the back sides of the
flanges on my K-250's support rings. Not to mention it cost about four times
as much as hot dip galvanizing on average. But for large weldment's that you
are not able or willing to take to a large enough galvanizing plant, hot spray
zinc might be your next best thing.
http://www.galvanizeit.org/images/uploads/publicationPDFs/hdg_vs_met_sheet.pdf
Some types of hot dip
galvanizing will give you a nice smooth finish if the surface finish is smooth
to start with, like you can see on many Simpson Strongtie brackets for holding
joist up and the like. So other then some areas that will have some zinc drips
on them that you will need to sand off you can have a good base to apply your
epoxy paints to and then be able to seal your acrylic viewports, hatches, and
what not against those surfaces.
You will not want to leave the
galvanizing exposed in fresh or saltwater long term, with out a good top
coating and sacrificial anodes, or it will not last long term. I just
got back from a boat trip on Lake Roosevelt in Washington state, and saw
a larger dock that was held up in this fresh water lake by foam that was
covered in galvanized sheet metal boxes. All the galvanized sheet metal that
was below the water line was almost completely rusted away, and the dock
didn't look like it was all that old.
One of my local galvanizing
plants in Spokane, Washington told me that the common hot dip galvanizing
finish that you see that looks like a sort of crystalen structure with all
sorts of pieces of a puzzle, has lead in the mix, and it's called a spangle
finish, and they didn't want to have hot lead in there facility for health
reasons.
That plant has a 7 ft deep tank, that is 5 ft wide and 16 ft
long. They quoted me $350 for dipping my 15 ft sub trail, of which they
would have to dip on edge and one side at a time. They called it progressive
dipping. All tubing and areas that would have air pockets if dipped at a
particular angle, need to have small venting holes drilled into them.
This will be a bit tricky for me since I'm adding a flange to the other side
of my support rings, making them into T's like on the K-350's and the addition
of horizontal weldments between them. I don't know if they can tilt the
hull when it is in the molten zinc for some configurations to make sure all
the air pockets are vented. The original small threaded steel nipple thru
hulls on the heads of my sub will all be removed and the two large ring plate
thru hulls in each head will replace them, and those will drain much
better.
I've heard that some galvanizing plants weigh your weldment
before and after it is galvanized, and charge you by the weight of zinc that
was added to it. So don't bolt on any zinc anode molds and think your going to
get away with it. ;}
My K-250 will have to be progressive dipped with
it's bow up and then down so it can drain properly and fit into the tank that
is here local to me. Unless I have it done in Portland in one of the larger
plants there, or I make the from MBT front attachment means removable, of
which I might do for other reasons as well.
They first acid dip
the steel weldment to remove any oils, mill scale, and light rust, and what
not. Then it's dipped in a flux bath, then on to the galvanizing tank.
From the quote I got for my trailer, I suspect it will cost $350 to $500 to
have my sub and other large sub weldments hot dipped galvanized.
Also
of note, the SS tabs that will be welded onto the hull for bolting on things
to the hull should have now problem with being hot dip galvanized, even though
it's not needed on the SS. It might be best to later remove the galvanizing on
the attachment hole and other unpainted areas of those tabs for corrosion
issues.
The more aggressive the surface profile of the steel the
better the zinc bond. So blasting with copper slag will work better for me
then blasting with silica sand. For those of you with new weldments, the
guys I spoke to at two different galvanizing plants said, you don't have to
worry about removing mill scale and light rust since that is removed with the
acid wash. This would save a lot of time, but for the best results I
would media blast it first.
When I considered the cost of coating my
subs bare hull with a good quality high zinc primer at $75 a gallon, and
I knew I was planning on getting the subs interior wet at depth for wet exit
training, I figured I'd like to go for hot dip galvanizing the whole hull once
I was done all the welding mods.
Regards,
Szybowski
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