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From: "Barry L. Ornitz" <ornitzz@dpnet.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: Stainless/ Aluminum reaction?
Date: Thu, 14 Oct 1999 22:30:36 -0400

Dave Carlson wrote in message <38057BE1.DFBF94C3@newportnet.com>...
>Can anyone tell me if I ought to avoid using stainless screws
>through aluminum in a roof repair I am about to make?
>I'm hoping to avoid the reaction/ galvanic corrosion that I know
>relates to where each resides on the periodic table of the
>elements.


Aluminum is anodic to stainless steel so the aluminum will corrode
preferentially.  [Stainless can be both anodic and cathodic to
itself in certain situations.]

If you want to avoid galvanic corrosion, use aluminum screws or
completely insulate the screw from the aluminum with a plastic
shoulder washer and insert.

        Barry L. Ornitz     ornitzNOSPAM@NOSPAMdpnet.net





From: "Barry L. Ornitz" <ornitzz@dpnet.net>
Newsgroups: rec.crafts.metalworking
Subject: Galvanic Corrosion, was RE: Stainless/ Aluminum reaction?
Keywords: corrosion, galvanic series, electromotive series, dissimilar metals
Date: Sun, 17 Oct 1999 22:11:05 -0400

I sent this out last Friday night, but never saw it posted to the
group.  Another post to the sci.engr.joining.welding did not show
up either but it was archived by DejaNews.  This one was not.
Strange things happen with Usenet's distribution floods!  For
those that might be interested, here it is again - I hope!

        Barry


Several people have touched on the subject of dissimilar metal or
galvanic corrosion.  The subject can be extremely complicated, but
there are several good generalities that can be used as a guide
when picking compatible metals.

First - you need a chart or table of the galvanic series.  You
really need one that lists the alloys you are contemplating
using.  Just saying aluminum or stainless is not enough if you
really want good results.  Different alloys behave quite
differently.  Understand how the tables work too (see below)  Two
of the better tables I have found are listed below.

Second - pick compatible metals based on how close they are in the
galvanic series.  The closer the galvanic potentials are, the less
corrosion that will occur.

Third - the amount of corrosion is proportional to the ratio of
cathode to anode area ratio.  This leads to a number of methods to
minimize corrosion.  Make the anodic metal piece much larger than
the cathodic one.  The corrosion will be spread out over a larger
piece of metal and thus the loss of some atoms from the surface
will likely not result in much weakening.  If you have to use a
different metal as a fastener, use one more cathodic than the
metal in which it is to be used.

Fourth - remember that sometimes the above rules must be bent a
little for other reasons, and that corrosion can be far more
complex than just galvanic corrosion.  Look at the stainless
steels for example.  Most can be both cathodic and anodic to
themselves!  Corrosion while immersed in sea water can be quite
different than corrosion from exposure to pollutants in the
atmosphere.

So before presenting the table, let's look at the original
question - stainless fasteners on aluminum.  First we need to know
what alloy of aluminum and stainless we are talking about.  With
no other information all we can do is note that aluminum is
generally more anodic than any of the stainless steels.  This is
good from the standpoint of having a small fastener.  But note
that zinc or cadmium plating is likely better as they are closer
to aluminum in the galvanic series.  I would probably pick a hot-
dip galvanized screw over a cadmium plated one because of the
thicker coating, but I would pick a cadmium plated one over a
simple zinc plated one (which is likely to be a rather thin
plating).  But I would certainly not pick brass, copper, or
bronze.

This brings up the point of why I listed two tables.  The shorter
one gives a better idea of how far apart the metals tend to be in
the series.  The longer list gives many alloys, but if you view
these with their electromotive potentials (not shown), you will
find many alloy series differ little in their actual placement in
the series.  Thus with aluminum, you might notice little
difference between the corrosion with either 304 or 17-7PH
stainless steel.

Corrosion can be a nearly exact science if everything you work
with is exceptionally pure.  But in the real world, this is not
the case and an experienced metallurgist is a wonderful friend to
have.  I first learned this in a corrosion class, but it really
sank home after I learned it by experience too.

[And on one or two occasions, I have seen my metallurgist friend
scratch his head in confusion too!]

When your choice of materials is limited, sometimes you have to
accept that some corrosion is inevitable and design accordingly.

        Barry L. Ornitz     ornitzNOSPAM@NOSPAMdpnet.net

Take the NOSPAM out before replying directly.  Thanks.

*********************************************************

First Table:

A Galvanic Series of Certain Metals and Alloys Arranged In Order
of Corrosivity

ANODIC (Least Noble) End Material
    Magnesium
    Magnesium alloys
    Zinc
    Aluminum 25
    Cadmium
    Aluminum 17ST
    Steel or iron
    Cast iron
    Chromium-iron (active)
    Ni-Resist
    18-8 Chromium-nickel-iron (active)
    18-8-3 Chromium-nickel-molybdenum-iron (active)
    Lead-tin solders
    Lead
    Tin
    Nickel (active)
    Inconel (active)
    Hastelloy C (active)
    Brasses
    Copper
    Bronzes
    Copper-nickel alloys
    Monel
    Silver Solder
    Nickel (passive)
    Inconel (passive)
    Chromium-iron (passive)
    18-8 Chromium-nickel iron (passive)
    18-8-3 Chromium-nickel-molybdenum-iron (passive)
    Hastelloy C (passive)
    Silver
    Carbon and graphite
    Platinum
    Gold
 CATHODIC (Most Noble) End material


Second Table from MIL-STD-889:

Active (Anodic) End
------------------------------------
Magnesium
Mg alloy AZ-31B
Mg alloy HK-31A
Zinc (hot-dip, die cast, or plated)
Beryllium (hot pressed)
Al 7072 clad on 7075
Al 2014-T3
Al 1160-H14
Al 7079-T6
Cadmium (plated)
Uranium
Al 218 (die cast)
Al 5052-0
Al 5052-H12
Al 5456-0, H353
Al 5052-H32
Al 1100-0
Al 3003-H25
Al 6061-T6
Al A360 (die cast)
Al 7075-T6
Al 6061-0
Indium
Al 2014-0
Al 2024-T4
Al 5052-H16
Tin (plated)
Stainless steel 430 (active)
Lead
Steel 1010
Iron (cast)
Copper (plated, cast, or wrought)
Nickel (plated)
Chromium (Plated)
Tantalum
AM350 (active)
Stainless steel 310 (active)
Stainless steel 301 (active)
Stainless steel 304 (active)
Stainless steel 430 (active)
Stainless steel 410 (active)
Stainless steel 17-7PH (active)
Tungsten
Niobium (columbium) 1% Zr
Brass, Yellow, 268
Uranium 8% Mo.
Brass, Naval, 464
Yellow Brass
Muntz Metal 280
Brass (plated)
Nickel-silver (18% Ni)
Stainless steel 316L (active)
Bronze 220
Copper 110
Red Brass
Stainless steel 347 (active)
Molybdenum, Commercial pure
Copper-nickel 715
Admiralty brass
Stainless steel 202 (active)
Bronze, Phosphor 534 (B-1)
Monel 400
Stainless steel 201 (active)
Carpenter 20 (active)
Stainless steel 321 (active)
Stainless steel 316 (active)
Stainless steel 309 (active)
Stainless steel 17-7PH (passive)
Silicone Bronze 655
Stainless steel 304 (passive)
Stainless steel 301 (passive)
Stainless steel 321 (passive)
Stainless steel 201 (passive)
Stainless steel 286 (passive)
Stainless steel 316L (passive)
AM355 (active)
Stainless steel 202 (passive)
Carpenter 20 (passive)
AM355 (passive)
A286 (passive)
Titanium 5A1, 2.5 Sn
Titanium 13V, 11Cr, 3Al (annealed)
Titanium 6Al, 4V (solution treated and aged)
Titanium 6Al, 4V (anneal)
Titanium 8Mn
Titanium 13V, 11Cr 3Al (solution heat treated and aged)
Titanium 75A
AM350 (passive)
Silver
Gold
Graphite
----------------------------------
Passive (Cathodic) End





From: "Barry L. Ornitz" <ornitzz@dpnet.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: Galvanic corrosion - marine battery terminals
Date: Tue, 19 Oct 1999 01:05:28 -0400

Mike Graham wrote in message
+ADwAXw-8LO3.6088+ACQ-K95.235600@newscontent-01.sprint.ca>...
>  Something just caught me as odd.  I believe that cranking battery
>terminals are generally lead.  If this is the case then wouldn't the best
>material to avoid electrolytic corrosion be lead terminal clamps?  Marine
>specs call for brass clamps, though, right?  I'm just curious why this is.
>Is it so the terminal is sacrificial and the clamp stays clean?


Hi Mike,

I think this is a case of having to ignore galvanic corrosion in favor of
other constraints.  Lead is not a particularly good conductor of
electricity.  Copper, brass or bronze are pretty good in this regard.  They
are also stronger mechanically.   One common approach is to have the copper
or brass clamp lead plated or tinned with lead-tin solder.  With cranking
currents often in the hundreds of amperes, a little resistance goes a long
way in reducing the voltage to the starter.

        Barry

 



































































































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