What type of bolts for aluminum ?

[Marco A. Garcia]

Ok, once again. I have to re-fasten the aluminum rail around the deck-to-hull
joint. I would like to use bolts instead of the rivets it has right now. What
material the bolts (and nuts) should be made of; stainless steel, bronze,
aluminum (?). I need help, wouldn't like to transfrom my boat into a floating
galvanic battery.

Thanks in advance
===========================================
|
Marco A. Garcia /|
i...@sdcc1.ucsd.edu / |\
/__|_\
IATTC --------
c/o Scripps Inst. of Oceanography
8604 La Jolla Shores Dr.
La Jolla, CA 92037
Voice: (619) 546-7190
Fax: (619) 546-7133

[J Gribskov]

In article <it7.21....@sdcc1.ucsd.edu>, i...@sdcc1.ucsd.edu (Marco A.
Garcia) writes:

>I have to re-fasten the aluminum rail around the deck-to-hull
>joint. I would like to use bolts instead of the rivets it has right now.
What
>material the bolts (and nuts) should be made of; stainless steel, bronze,

>aluminum (?).

Use stainless steel bolts that are low in carbon and corrosion resistant.
There are many grades of stainless. "316L" is what you want. If you were
looking for a higher strength bolt, you would have to go to "308" or
similar, but in the toe rail application you don't need the higher tensile
strength. You probably have closely spaced bolt holes to help form a good
seal, so there are lots of bolts to take even local loads.

Buy your bolts at a good marine store, such as West Marine. If they don't
have the grade marked, use a magnet to insure you're getting a low carbon
variety. If the magnet does not stick at all, it's low carbon. By the
way, this is a good test when you're buying hose clamps. Many stainless
hose clamps are made of 308, and sometimes the screw is, even when the
clamp is not. In a wet bilge you will get some corrosion on the 308, and
could get a failure.

--Jon R. Gribskov (jgri...@aol.com)

[g.turner]

In article <318hbc$m...@search01.news.aol.com>,

J Gribskov <jgri...@aol.com> wrote:
>In article <it7.21....@sdcc1.ucsd.edu>, i...@sdcc1.ucsd.edu (Marco A.
>Garcia) writes:
>
>>I have to re-fasten the aluminum rail around the deck-to-hull
>>joint. I would like to use bolts instead of the rivets it has right now.
>What
>>material the bolts (and nuts) should be made of; stainless steel, bronze,
>
>>aluminum (?).
>
>Use stainless steel bolts that are low in carbon and corrosion resistant.
>There are many grades of stainless. "316L" is what you want. If you were

Stainless steel is more noble than aluminum and will cause the aluminum it
contacts to corrode. What kind of bolt is electrochemically closest to
aluminum to minimize this problem? What else can you do?

Thanks,

Gary Turner

[MARTIN David]

g...@anuxv.mv.att.com (g.turner) writes:

Use SS bolts with high density plastic bushings and polyurathane rubber
caulking. This will minimize the metal-metal contact and the corrosion.

-dave.
--
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Dave Martin dav...@engr.mun.ca
Memorial University of Newfoundland, St.John's Nfld
Faculty of Naval Architectural and Ocean Engineering

[dkbe...@bocaraton.ibm.com]

In <318hbc$m...@search01.news.aol.com>, jgri...@aol.com (J Gribskov) writes:
>
>Use stainless steel bolts that are low in carbon and corrosion resistant.
>There are many grades of stainless. "316L" is what you want. If you were
>looking for a higher strength bolt, you would have to go to "308" or
>similar, but in the toe rail application you don't need the higher tensile
>strength. You probably have closely spaced bolt holes to help form a good
>seal, so there are lots of bolts to take even local loads.
>
>Buy your bolts at a good marine store, such as West Marine. If they don't
>have the grade marked, use a magnet to insure you're getting a low carbon
>variety. If the magnet does not stick at all, it's low carbon. By the
>way, this is a good test when you're buying hose clamps. Many stainless
>hose clamps are made of 308, and sometimes the screw is, even when the
>clamp is not. In a wet bilge you will get some corrosion on the 308, and
>could get a failure.
>

Not low carbon! High Chromium. Stainless steel is an alloy of Chromium
and steel. The chromium binds up the electrons in the steel, preventing
their taking a specific spin, and rendering it non-magnetic. Lower
Chromium stainless is stronger, but not as anti-corrosive, and not as
anti-magnetic, since more electrons associated with the iron are free
to be twiddled about with magnets and Oxygen molocules.

When I go shopping for Stainless Flatware, I always bring a magnet. I
don't really need high strength, but I do demand low corrosion.

Duane Becker

[Geoffrey H. Campbell]

This thread touches on a couple of very complex subjects: Galvanic
corrosion and stainless steel. If I tried to do these subjects justice,
then this reply would be too long and boring for most people to tolerate.
I'll just try to clarify a couple of issues that have been raised.

Galvanic corrosion is a widespread and insidious problem. A potential
difference will exist between to dissimilar metals when they are in
contact. This occurs because the Fermi energy of the metals depends on the
composition of the metal. The Fermi energy can be thought of as the average
energy of the electrons in the metal. When two dissimilar metals are
brought into contact the electrons will want to flow from the higher energy
states in the one metal to the lower energy states in the other metal. This
difference in Fermi energy is what creates the electro-motive force (EMF)
series that you might remember from high school chemistry.

The effect of the potential difference is mostly on the speed of the
corrosion rate. A larger difference will give a faster rate of corrosion of
the less noble metal and a slower rate of corrosion for the more noble
metal, as compared to the behavior of these metals when they are not in
contact. Even different alloys of the same metal, such as 1100 aluminum and
6061 aluminum or 304 SS and 316 SS, will develop a small potential
difference because their compositions are different. These differences are
comparatively small but none-the-less could give rise to noticeable
effects.The best solution is to work with materials that have inherently
good corrosion resistence to begin with.

For the application of bolting on an aluminum rub rail, aluminum bolts
might work depending on the composition of the alloy. High strength
aluminum alloys are usually high in copper content which decreases their
corrosion resistence. The best advise is that offered by Mr. Martin above,
which is to electrically insulate the bolts with polymeric material. With
the circuit broken, there shouldn't be a galvanic effect. (Because
saltwater is an ionic conductor, there is no way to get the electrons
between the metals)

Now on to stainless steel and magnetism. What enables a steel to be
magnetic is its crystal structure. The magnetic phase is body centered
cubic (bcc) and the non-magnetic phase is face centered cubic (fcc). Pure
iron is bcc at room temperature but transforms at high temperature (about
where it's a yellowish-red color) to the fcc phase. Adding other elements
to the iron can stabilize the fcc crystal structure all the way down to
room temperature. It's these compositions which create non-magnetic steels.

It so happens that the same elements added to steels to increase its
corrosion resistence also stabilize the fcc structure, usually nickel and
chrome. But during the fabrication of stainless steel, the alloying
elements may be unevenly distributed and so some fraction the metal will
transform to bcc upon cooling from the processing temperature (usually the
liquid state). All commercial grades of stainless steel have enough
alloying elements in them to become completely non-magnetic if they are
processed correctly! They must be well annealed and homogenized while in
the fcc state at high temperature.

The situation becomes more complicated when we consider the effects of
carbon. Carbon likes to stabilize a phase of steel called martinsite (this
is close to the bcc phase, actually body centered tetragonal). The effect
is especially noticeable in type 304 stainless, where martinsite can be
introduced in the metal by simply cold working it (deforming it at room
temperature).

So the flip side of magnitization question is that 304 and 316 can both be
magnetic or non-magnetic, but 304 has a greater tendency towards being
magnetic. The only grade of stainless which is more or less guaranteed to
be non-magnetic (completely austinitic) is 310. But 310 has the highest Ni
and Cr contents (both over 20%) so it's more expensive and more corrosion
resistent.

For the corrosion resistence of stainless steel there are some general
trends with composition. Increasing the amounts of nickel, chrome, and
molybdenum increases the corrosion resistence while increasing the carbon,
sulfer, or silicon content decreases the corrosion resistence. Nitrided
steels also have better corrosion resistence.

Stainless steels don't corrode in the normal sense but rather they form
deep pits. Pitting is a complicated and not completely understood mechanism
of corrosion. The chemistry of the water in the pit can change and even
become very acidic as compared to out side the pit and this exacerbates the
corrosion rate. The only materials my handbook gives which resist this form
of corrosion in seawater is Hastelloy C and titanium. Hastelloy is a
trademark of Union Carbide and it's a nickel, chrome, molybdenum alloy (no
iron).

Dr. Geoffrey H. Campbell
Lawrence Livermore National Laboratory
Mailstop L-356
Livermore, CA 94550
camp...@llnl.gov